Here at MarineDiesel, we recognize that engines are complex devices. We also know that the jargon and terminology can often be a bit confusing. Please see the dictionary below as our effort to help clarify some of the terms used on this site and in the industry.



ABDC – Abbreviation for after bottom dead center.

Absolute Pressure – Gauge pressure plus atmospheric pressure.

Absolute Temperature – The temperature measured using absolute zero as a reference. Absolute zero is -469.69° F (-273.16° C) and is the lowest point of temperature known.

Accelerate To increase the speed of movement, such as increasing the speed of a piston or flywheel.

Accumulator – A device used for storing liquid under pressure (sometimes used to smooth out pressure surges in a hydraulic system).

Active Regeneration – If conditions for passive regeneration within an exhaust filter cannot be reached, PM must be removed using active regeneration, an automatic cleaning process. This requires injecting a small quantity of fuel into the exhaust stream and elevating exhaust temperatures to clean the filter.

Additive – A compound which is added to improve fuel.

Advance – A device which advances the timing of the injection pump or injectors.

Aftercooler – A device used on turbocharged engines to cool air which has undergone compression.

Aftertreatment Devices – Devices which remove pollutants from exhaust gases after the gas leaves combustion chamber (e.g., catalytic converters or diesel particulate filters). The term “exhaust gas aftertreatment” is considered derogatory by some in the emission control industry, but there is no consensus on the use of such alternatives as “post-combustion treatment” or “exhaust emission control”.

Air Bleeder – A device used to remove air from a hydraulic system. Types include a needle valve, capillary tubing to the reservoir, and a bleed plug.

Air Cell – A small receptacle communicating with an engine cylinder into which some of the compressed air is forced, and from which air later flows back into the cylinder.

Air Cleaner – A device (filter) for removing unwanted solid impurities from the air before the air enters the intake manifold.

Air Gap – A space for air flow between two components.

Air Injection – The system of injecting fuel into the combustion chamber of a diesel engine by means of a blast of highly compressed air.

Airless Injection – A general term describing all methods of injecting fuel without the use of compressed air.

Air Lock – The presence of air in a pump or pipes which prevents the delivery of liquid.

Air Pollution – Contamination of the earth’s atmosphere by pollutants such as smoke, harmful gases, etc.

Air Signal Attenuator – A device that reduces both exhaust smoke and noise by slowing engine acceleration slightly until adequate turbocharger boost pressure is present.

Air Starting Valve – A valve which admits compressed air to the air starter for starting purposes.

Air – to – Air Aftercooling – Air-to-air aftercooled aspiration lowers in-cylinder temperatures, reducing nitrogen oxide (NOx) and increases low-speed torque and power density

Air Toxics – Toxic air pollutants, as classified by pertinent regulations. Examples of substances classified as air toxics by the US Clean Air Act include acetaldehyde, benzene, 1,3-butadiene, formaldehyde, and polycyclic organic matter (POM). California air toxics regulations also classify diesel exhaust particulates as a toxic air contaminant.

Align – To bring two or more components of a unit into the correct positions with respect to one another.

Allowance – The difference between the minimum and the maximum dimensions of proper functioning.

Alloy – A mixture of two or more different metals, usually to produce improved characteristics.

Alnico Magnet – A magnet composed of aluminium (AI), nickel (Ni), and cobalt (Co).

Alternating Current (AC) – An electric current that changes polarity.

Alternative Fuel – Fuel other than petroleum diesel or gasoline.

Alternator – An electromechanical device which produces alternating current.

Ambient Temperature – Surrounding air temperature.

Ammeter – An instrument used to measure the rate of current flow in amperes.

Ampere (A) – A unit of measurement defined as the current that 1 V can send through 1 Ω resistance.

Ampere-Hour Capacity (Ah) – A measurement of the battery’s capacity to deliver a specified current over a specified length of time.

Analogue Sensor – A sensor which sends a range of voltages in DC to the ECM to describe, for example, coolant temperature, engine oil pressure, or atmospheric pressure.

Aniline Number – The lowest temperature at which equal parts of aniline and a sample of oil are completely miscible, or the temperature at which the mixture becomes turbid or cloudy.

Anneal – To toughen metals by heating and then cooling.

Annular – Ring shaped.

Antifreeze – A chemical added to the coolant in order to lower its freezing point.

API – Abbreviation for American Petroleum Institute.

API° Gravity – Gravity expressed in units of standard API degrees (hydrometer).

AQMD – Abbreviation for Air Quality Management District in the USA

Arc – Portion of a curved line or circle.

Arcing – Electrons leaping the gap between negative and positive terminals.

Armature – The movable part of a relay, regulator, or horn or the rotating part of a generator or starter.

Articulated Pistons – Two-piece pistons incorporating an entirely separate piston crown or dome with a separate skirt, and linking the two together with the piston pin. Many 1994 and later engines incorporate steel crown/aluminium skirt articulated pistons.

Asbestos – A heat-resistant and nonburning organic mineral.

Aspirate – To draw out gas by suction.

ASTM – Abbreviation for American Society of Testing and Materials

ATDC – Abbreviation for after top dead center.

Atmosphere – The layer of air surrounding the earth.

Atmospheric Pressure (barometric pressure) – The pressure exerted by the atmosphere, averaging 14.7 psi at sea level with a decrease of approximately 1/4 lb per
1,000 ft of altitude gained.

Atom – The smallest particle of an element.

Atomizer – A device which disperses fuel into fine particles. .

Attrition – Wearing down by rubbing or by friction.

Automatic Advance – A device that advances the timing of the fuel injection.

Automatic Timer -An electrical or mechanical device that retards fuel injection timing at low rpm and advances injection at higher rpm.

Automatic Valve – A valve assisted by a spring, which is opened by a difference of pressure acting in one direction and closed by a difference in pressure acting in the opposite direction.

Auxiliary – An aid to the main device which may only be used occasionally.

Auxiliary Air Cartridge Breech – A steel container which contains a high-pressure air-pack cartridge as an emergency supply of air to the air tanks.





Babbitt – An antifriction metal used to line bearings, thereby reducing the friction of the moving components.

Back Pressure – A pressure exerted in the opposite direction from the main flow.

Backlash – The play between two movable components.

Baffle – A device which slows down the flow of gases, liquids, or sound.

Balanced Valve – A valve in which the fluid pressure is equal on both sides (i.e., the opening and closing directions).

Ball Bearing – A bearing using steel balls as its rolling element between the inner and outer race.

Ball Check Valve – A valve consisting of a ball held against a seat by a spring, used to check flow or limit pressure.

Barometer – An instrument which measures atmospheric pressure.

Basic Size – The theoretical or nominal standard size from which all variations are made.

Battery – An electrochemical device that stores electric potential charge.

Battery Warmer – An AC heater which brings battery temperature up to increase battery cranking power.

BBDC – Abbreviation for before bottom dead center.

BDC – Abbreviation for bottom dead center.

Bearing – The contacting surface on which a revolving part rests.

Bearing Clearance – The distance between the shaft and the bearing surface.

Bedplate – Lower part of the engine resting on the foundation.

Bell Housing (Clutch Housing) – The metal covering around the clutch or torque converter assembly.

Bendix Type Starter Drive (Inertia Starter Drive) – A type of starter drive that causes the gear to engage when the armature starts rotating and to automatically disengage when it stops.

Bernoulli’s Principle – Given a fluid flowing through a tube, any constriction or narrowing of the tube will create an increase in that fluid’s velocity and a decrease in pressure.

Bimetal Arm – A thin sandwich strip of two metals with very different heat expansion rates that bends to the left or the right due to heat caused by resistance to voltage.

Biodiesel – The mono alkyl esters of long chain fatty acids derived from renewable lipid feedstocks, such as vegetable oils and animal fats, for use in compression ignition (diesel) engines. Manufactured by transestrification of the organic feedstock by methanol.

Blowby – Exhaust gases which escape past the piston rings.

Blower – A low pressure air pump, usually of either rotary or centrifugal type.

Boiling Point – The temperature at which a liquid begins to boil.

Bond – The holding together of different parts.

Bonded Electrons – Inner orbit electrons around the nucleus of the atom.

Bore – A cylinder, hole, or the inside diameter of the cylinder or hole. Also, the diameter of an engine cylinder.

Boring – Enlarging the cylinders by machining them to a specified size.

Boring Bar – A machine used to renew and enlarge engine cylinder bores.

Bosch Metering System – A metering system with a helical groove in the plunger which covers or uncovers ports in the pump barrel.

Bound Electrons – The inner-orbit electrons around the nucleus of the atom.

Boyle’s Law – At a constant temperature, the pressure and volume of a gas are inversely proportional.

Brake Horsepower (bhp) – The usable power delivered by the engine.

Brake Mean Effective Pressure (bmep) – Mean effective pressure acting on the piston which would result in the given brake horsepower output, if there were no losses due to friction, cooling, and exhaustion. Equal to mean indicated pressure times mechanical efficiency.

Brake Specific Fuel Consumption – BSFC is the ratio of the engine fuel consumption to the engine power output (as measured at the flywheel). BSFC has units of grams of fuel per kilowatt-hour (g/kWh) or pounds mass of fuel per brake horsepower-hour (lb/bhp-hr). BSFC is a measure of engine efficiency.

Brake Thermal Efficiency – Ratio of power output in the form of brake horsepower to equivalent power input in the form of heat from fuel. Typical brake thermal efficiency ranges from thirty to forty-two percent.

Branch Circuit – A circuit which has branches of both series and parallel arms.

Brazing – The fastening of two pieces of metal together by heating the edges and then melting brass or bronze on the area.

Break-In – A period in which engine surfaces conform to mating surfaces.

Breather Pipe – A pipe opening into the crankcase to assist ventilation.

Brinen Hardness – The surface hardness of a metal, alloy, or similar material according to l.A. Brinell’s method of measurement. A metal’s surface is struck at a given force by a rigid steel ball of a given diameter, and the indentation is measured.

British Thermal Unit (BTU) – The amount of heat required to raise 1 lb of water 10 F.

Brush – The pieces of carbon or copper that make a sliding contact against the commutator or slip rings.

BTDC – Abbreviation for before top dead center.

Buoyancy – The upward lifting force exerted on a body by a fluid.

Burnish – To polish or shine a surface with a hard, smooth object.

Bushing – A metallic or synthetic lining for a hole, which reduces or prevents abrasion between components.

Butane – A hydrocarbon gas which becomes a liquid when under pressure.

Butterfly Valve – A valve in the venturi unit which controls airflow.

Bypass Filter – An oil filter that only filters a portion of the oil flowing through the engine lubrication system.

Bypass Filtration – Filtered lube oil which flows back to the oil pan rather than lubricating a part of the engine.

Bypass Valve – A valve that opens when the set pressure is exceeded. This allows the fluid to pass through an alternate channel.





Cage – A housing in which a bearing’s rolling elements are held.

Calibrate – To make an adjustment to a meter or other instrument so that it will indicate accurately its input.

Calliper – A tool for measuring diameter.

Calorie – The amount of heat required to raise 1g of water from 17 to 18° C.

Calorific Value – The amount of heat produced by burning one pound of fuel. (See heating value.)

Cam – A rotating component of irregular shape. It is used to change the direction of the motion of another part moving against it, e.g., rotary into reciprocating or variable motion.

Cam Follower (Valve Lifter) – A part which is held in contact with the cam and to which the cam motion is imparted and transmitted to the pushrod.

Cam Lift – The distance from the peak of the lobe of a cam to its axis minus the distance from the back of the cam to its axis. Another description would be the distance the valve opens plus the valve lash or tappet clearance measurement.

Cam Nose – That portion of the cam that holds the valve wide open. It is the high point of the cam.

Cam-Ground Piston – Oval shaped piston head design used by most large-diameter aluminium pistons, which expands in a nonuniform manner when hot to attain a round shape.

Camshaft – A shaft to which a cam is fastened or of which a cam forms an integral part

Camshaft Gear – The gear that is fastened to the camshaft.

Camshaft Journals – The load bearing areas of a camshaft.

Camshaft Lift – Radial difference between the camshaft lobe base and the nose.

Capacitor (Condenser) – An arrangement of insulated conductors and dielectrics for the accumulation of an electric charge.

CARB – A state regulatory agency charged with regulating the air quality in California.

Carbon – One of the nonmetallic elements constituting fuel and lubricating oil.

Carbon Dioxide (CO2) – A colorless, odorless gas which results when carbon is burned completely.

Carbon Monoxide (CO) – A colorless, odorless, poisonous gas which results from the incomplete burning of carbon.

Carbon Pile – Carbon disks or plates capable of developing high resistance.

Carbon Residue – A deposit left in the combustion chamber because of inefficient combustion or poor lubricating oil performance.

Carbon Tetrachloride – A colorless liquid, the fumes of which are toxic. Used in fire extinguishers.

Carburizing – To add carbon to a metal alloy for increased hardness.

Case-Harden – To harden the outer surface of metal to a given case or shell depth, while leaving the inner portion soft to absorb shocks and allow bending.

Catalyst – A substance which influences the rate of a chemical reaction but is not one of the original reactants or final products, i.e. it is not consumed or altered in the reaction. Catalysts are used in many processes in the chemical and petroleum industries. Emission control catalysts are used to promote reactions that change exhaust pollutants from internal combustion engines into harmless substances.

Cavitation – The formation of air bubbles.

Cell Connectors – The lead straps connecting the cell groups in a battery.

Cells (Battery) – The individual (separate) compartments in the battery which contain positive and negative plates suspended in electrolyte.

Celsius (Centigrade) – Thermometer scale on which the freezing temperature of water is 00 C and the boiling temperature is 1000 C.

Center of Gravity – The point of a body which can be thought of as the point on which gravity acts.

Centrifugal Force – A force exerted on a rotating object in a direction outward from the center of rotation.

Centrifugal Governor – A governor which uses flyweight force to sense speed in order to control the amount of fuel supplied to the combustion chambers.

Centrifugal Pump – (See Impeller.)

Cetane Index – A calculated value, derived from fuel density and volatility, giving a reasonably close approximation to cetane number.

Cetane Number – A number which expresses how well a diesel fuel ignites.

CFR – Abbreviation for Cooperative Fuel Research. A single cylinder, overhead valve, variable compression ratio engine used for measuring octane or cetane quality.

Chamfer (Taper Lead) – The taper at the thread end of a tap or the throat of a die, made by cutting away the crests of the first few threads. This distributes the work of cutting over several threads and acts as a guide in starting the tap or die. The chamfer is relieved to facilitate cutting.

Charging Rate – The rate at which a battery is charged.

Charles’s Law – The physical law of gases which states that an increase in temperature will cause an increase in volume at constant pressure.

Check Valve – A valve which permits only one direction of flow.

Chemical Change – A change which alters the composition of the molecules of a substance, producing new substances with new properties.

Circuit (Electric) – A path for current flow with one or more resistant units.

Circuit breaker – A device that opens the circuit when the current draw becomes excessive and closes the circuit when the current flow is reduced.

Circulating Pump – Oil cooling pumps which circulate fluid.

Circumference – The distance around the perimeter of a circle. ) ( π times the diameter.)

Class A Fires – Fires in which the combustible material is wood, paper, fabric, rubber, etc.

Class B Fires – Fires in which the combustible material is a liquid such as gasoline, fuel, or paint.

Class C Fires – Fires in which the combustible materials are electrical , components such as motors, generators, or switch panels.

Clean Air Act – In the U.S., the fundamental legislation to control air pollution. The original Clean Air Act was signed in 1963. The law set emissions standards for stationary sources, such as factories and power plants. Criteria pollutants included lead, ozone, CO, SO2, NOx and PM, as well as air toxics. The CAA was amended several times, most recently in 1990. The Amendments of 1970 introduced motor vehicle emission standards for automobiles and trucks.

Clean Fuel Vehicle – A vehicle that has been certified to meet clean-fuel standards of the Clean Air Act Amendments of 1990.

Clean Room – A dust free environment used in fuel injector manufacture and service.

Clearance – The space between two components.

Clearance Volume – The volume of a cylinder space with the piston at TDC.

Closed Cooling System – A cooling system which is not exposed to the atmosphere.

Closed Nozzle – A fuel nozzle having a valve between the combustion chamber and the fuel chamber.

Cloud Point – A measure of the ability of a diesel fuel to operate under cold weather conditions. Defined as the temperature at which wax first becomes visible when diesel fuel is cooled under standardized test conditions (ASTM D2500).

Clutch – A device used to connect or disconnect the power input to the power output.

Clutch Pilot Bearing – A small bushing or ball bearing positioned in the crankshaft or flywheel.

Coil Spring – A spring-steel wire wound in a spiral pattern.

Cold Chisel – A forged steel tool with a wedge shaped cutting edge.

Cold Filter Plugging Point – A measure of the ability of a diesel fuel to operate under cold weather conditions. Defined as the lowest temperature at which diesel fuel will pass through a fine wire mesh screen of the test apparatus.

Collector – The transistor lead for voltage out.

Color Code – A method for quick recognition of different electric circuits by the color of the wires.

Combustion – The process of burning.

Combustion Chamber – The chamber in which combustion mainly occurs.

Combustion Chamber Volume – The volume of the combustion chamber (when the piston is at TDC) measured in cubic centimeters.

Combustion Cycle – A series of thermodynamic processes through which the working gas passes to produce one power stroke. The full cycle is: intake, compression, power, exhaust.

Common Rail Injection – A diesel fuel injection system employing a common pressure accumulator, called the rail, which is mounted along the engine block. The rail is fed by a high pressure fuel pump. The injectors, which are fed from the common rail, are activated by solenoid valves. The solenoid valves and the fuel pump are electronically controlled. In the common rail injection system the injection pressure is independent from engine speed and load. Therefore, the injection parameters can be freely controlled. Usually a pilot injection is introduced, which allows for reductions in engine noise and NOx emissions.

Commutator – A number of copper bars connected to the armature windings but insulated from each other and from the armature.

Compound – Two or more elements that are chemically combined.

Compressed Air – Air from a shop air compressor.

Compressibility – The property of a gas or liquid to become denser with increase in pressure.

Compression – The process by which a confined gas is reduced in volume through the application of pressure.

Compression Check – A measurement of the compression of each cylinder at cranking speed or as recommended by the manufacturer.

Compression Fitting – A metal tube connection made by compressing a metal ring around the tube and into its fitting’s seat.

Compression Gauge – A test instrument used to test the cylinder compression.

Compression Ignition – The ignition of fuel through the heat of compression.

Compression Pressure – Pressure in the combustion chamber at the end of the compression stroke before any fuel is burned.

Compression Ratio – The ratio between the total volume in the cylinder when the piston is at BDC and the volume remaining when the piston is at TDC.

Compression Release – A device which holds open the intake or exhaust valve, thereby permitting the engine to be turned over without compression. This is one method of engine braking where by the power stroke is cancelled.

Compression Ring – The piston rings used to reduce combustion leakage to a minimum.

Compression Stroke – That stroke of the operating cycle during which air is compressed into a smaller space, creating heat by molecular action.

Compressor – A mechanical device which increases air pressure.

Concentric – Having the same center of rotation.

Condensation – The reduction of a vapor or gas to a liquid state.

Condense – To reduce from gas or vapor to liquid.

Condenser – Also called a capaciator, this electronic component CI stores up electric charge until a set voltage is reached; then a high but rapidly decaying current flows through the circuit until finally the voltage difference between the two leads reaches zero.

Conduction – Heat transfer through solids, especially metals, and into adjacent liquids. Also, any material or device forming a path for the flow of electrons.

Conformability – The ability of an engine bearing to conform to irregularities of the crankshaft surface.

Connecting Rod – The rod joining the piston with the crankshaft.

Connecting Rod Alignment – The straightness of a connecting rod.

Connecting Rod Bearing – The bearing used in the connecting rod bore.

Connecting Rod Resizing Grinder – A specialized precision grinding instrument for reaming and honing of connecting rods.

Constant-Pressure Combustion – Combustion which occurs without change in pressure. In an engine, this is obtained by a slower rate of burning than with constant volume combustion.

Constant Volume Combustion – Combustion in a cylinder so fast that there is no change in volume. Many high-speed diesel engines have practically constant-volume combustion.

Contamination – The presence of harmful foreign matter in a fluid or in air.

Contour – Outline.

Contract – To reduce in size.

Control – To regulate or govern the function of a unit.

Control Rack – A mechanism for changing linear governor motion to rotary motion at the fuel-injection plunger.

Controlled Port Scavenging – Scavenging method using ports which are controlled by valves in addition to the power piston.

Convection – Heat transfer through currents of liquid and gas.

Conventional – According to the most common or usual mode.

Converge – To incline to or approach a certain point; to come together.

Convolution – One full turn of a screw.

Coolant – A liquid used as a cooling medium.

Cooling System – The complete system for circulating coolant.

Cordierite – A ceramic material of the formula 2MgO-2Al2O3-5SiO2 which is used for automotive flow-through catalyst substrates and ceramic wall-flow diesel filters.

Core – The central or innermost part of an object.

Corrosion – The slow destruction of material by chemical agents and electrochemical reactions.

Counterbalance – A weight, usually attached to a moving component, that balances another weight.

Counterbore – A cylindrical enlargement of the end of a cylinder bore or bore hole.

Counter Electromotive Force (CEMF) – The electromotive force (voltage) that opposes the applied voltage.

Countersink – An oversized hole for flush screw mounting.

Counterweight – Weights that are mounted on the crankshaft opposite each crank throw. These reduce vibration and bearing loads.

Coupling – A device used to connect components.

Crank Throw – One crankpin with its two webs.

Crankcase – The casing which surrounds the crankshaft.

Cranking Speed – The speed of rotation of a cranking motor.

Crankpin – The portion of the crank throw attached to the connecting rod.

Crankpin Bore – On the connecting rod, the larger hole which is fastened to the crankshaft connecting-rod journal.

Crankshaft – A rotating shaft for converting rotary motion into linear motion.

Crankshaft Bearing Clearance – The oil clearance between the surface of the crankshaft and the surface of the bearing.

Crankshaft End Play – The allowable linear movement of the crankshaft resulting from oil clearance between the crankshaft thrust bearing and the block.

Crankshaft Gear – The gear that is mounted to the crankshaft.

Crest – The top surface joining the two sides of a thread.

Crest Clearance – On a screw, the space between the top of a thread and the root of its mating thread.

Crimping Terminals – A mechanically crushed terminal connection method.

Critical Compression Ratio – Lowest compression ratio at which any particular fuel will ignite by compression. The lower the critical compression ratio the better ignition qualities the fuel has. (Gasoline engine, 4:1; oil engine, 7:1; diesel engine, 12.5:1.)

Critical Speeds – Speeds at which the frequency of the power strokes synchronize with the crankshaft’s natural frequency. If the engine is operated at one of its critical speeds for any length of time, a broken crankshaft may result.

Crocus Cloth – A very fine abrasive polishing cloth.

Crosshatch Pattern – The pattern made by the rotating abrasives of the cylinder hone in the cylinder bore.

Crowned – A very slight curve in a surface (e.g., on a roller or raceway).

Crude Oil – Petroleum as it comes from the well (unrefined).

Crush – Pre-stress on engine bearings’ shells to hold them in place during operation.

Current – The flow of electrons passing through a conductor. Measured in Amperes.

Cycle – One complete performance of a repeating process.

Cylinder – The piston chamber of an engine.

Cylinder Head – The replaceable portion of the engine that seals the cylinder at the top. It often contains the valves, and in some cases, it is part of the combustion chamber.

Cylinder Hone – A tool used to bring the diameter of a cylinder to specification and at the same time smooth its surface.

Cylinder Liner – A sleeve which is inserted in the bores of the engine block which make up the cylinder wall.

Cylinder Liner Protrusion – Distance the cylinder liner protrudes above the cylinder block.





Dashpot Governor – A dashpot which uses a hydraulic shock absorber to eliminate spikes in engine rpm.

Dead Center – Either of the two positions when the crank and connecting rod are in a straight line at the end of the stroke. The total piston travel is the distance between the two dead center positions. (See TDC and BDC.)

Deceleration – Opposite of acceleration. Also called negative acceleration.

Deflection – Bending or movement away from the normal position, due to loading.

Deglazer – A tool used to remove the glaze from cylinder walls.

Deglazing – Removing (by honing) the glaze from cylinder walls during rebuilding.

Degree (circle) – 1/360 of a circle.

Degree Wheel – A wheel marked in degrees to set the lifter height.

Delivery Valve – A check valve on the outlet of the injection pump that keeps the high pressure fuel lines full of fuel.

Density – The weight per unit volume of a substance.

Depth of Engagement – The depth of a thread in contact with two mating parts. It is the radial distance by which their thread forms overlap each other.

Detergent – A chemical with cleansing qualities added to the engine oil.

Detonation – Burning of the fuel in the combustion chamber at a rate faster than desired (knocking).

Diagnostic Fault Code – Coded ECM output to a data link display that indicates a problem or event in the unit.

Dial Indicator (Dial Gauge) – A precision measuring instrument.

Diaphragm – Any flexible dividing partition separating two compartments.

Die – A thread cutting tool.

Diesel Engine – An internal combustion engine having fuel injected into the combustion chamber near the end of the combustion stroke. The fuel is ignited by the heat of compression only.

Diesel Exhaust Fluid – Selective catalytic reduction (SCR) systems utilize DEF to reduce NOx. DEF is made up of 32.5 percent urea and 67.5 percent de-ionized water. The ammonia in the urea mixes with engine exhaust gases in the SCR catalyst to convert NOx to nitrogen and water vapor.

Diesel Exhaust Catalyst – A DOC is a flow-through after-treatment device that reacts with exhaust gases to reduce carbon monoxide, hydrocarbons and some particulate matter (PM).

Diesel Index – A rating of fuel according to its ignition qualities. The higher the diesel index number, the better the ignition quality of the fuel.

Diesel Particulate Filter – A device which physically captures diesel particulates preventing their discharge from the tailpipe. Collected particulates need to be removed from the filter, usually by continuous or periodic oxidation in a process called “regeneration”.

Diesel Particulate Matter – Sub-micron size particles found in diesel exhaust. Most emission regulations specify DPM measurement methods in which particulates are sampled on filters from cooled exhaust gas. The cooling causes condensation of vapors in the gas sampling train. Thus, the DPM is composed of both solid and liquid particles and is generally classified into three fractions: (1) inorganic carbon (soot), (2) organic fraction (often referred to as SOF or VOF), and (3) sulfate fraction (hydrated sulfuric acid).

Differential Pressure Fuel Valve – A normally closed fuel valve which is lifted by fuel pressure.

Diffuser – Turbocharger component which converts exhaust air velocity to increased air pressure at turbocharger outlet.

Digital Sensor – A sensor which uses duty cycles of two voltages in timed pulses of AC to relay such information as speed, throttle position, and engine timing.

Dilution – Thinning, such as when fuel mixes with the lubricant.

Diode – A component which allows current to flow in just one direction.

Dipstick – A device to measure the quantity of oil in the reservoir.

Direct Cooled Piston – A piston which is cooled by internal circulation of oil from an oil jet.

Direct Current (DC) – Current that flows in one direction only.

Direct Injection – In diesel engines with direct injection the combustion chamber is not divided and fuel is injected directly to the cylinder.

Directional Control Valve – A valve which selectively directs flow to or from specific channels. Also referred to as selector valve, control valve, or transfer valve.

Discharge – A draw of current from the battery.

Displacement – In a single acting engine, the volume swept by all pistons in making one stroke each. The displacement on one cylinder in cubic inches is the circular area (in square inches) times the stroke (in inches).

Distillation – Heating a liquid and then condensing the vapors given off by the heating process.

Distributor Head – The port through which fuel is discharged on the high pressure side of the injection pump.

Distributor Rotor – The metering mechanism of the distributor-type fuel-injection system.

Distributor Type Fuel-Injection Pump – A fuel-injection pump with the high pressure outlet fuel fittings in a circular pattern like the distributor cap on a gasoline engine.

Division Plate – A diaphragm surrounding the piston rod of a crosshead type engine, usually having a wiper ring to remove excess oil from the piston rod as it slides through. It separates the crankcase from the lower end of the cylinder.

Double Flare – A metal tube flared in a two step process to give a more durable flare.

Dowel – A pin, usually of circular shape like a cylinder used to pin or fasten something in position temporarily or permanently.

Drill – A tool used to bore holes.

Drill Press – A fixed machine to drive a tool in rotary motion.

Drive Fit – See Press fit.

Droop Speed – Speed reduction from high idle after a load is placed on the engine.

Drop Forged – Formed by hammering or being forced into shape by heat.

Dry Battery – Primary batteries which convert chemical energy into electric energy and which usually can’t be charged.

Dry Cell – A battery that uses no liquid electrolyte.

Dry Charged Battery – A battery in a charged state but without electrolyte. The electrolyte is added when the battery is to be placed in service.

Dry Sleeve – A cylinder liner which is supported over its entire length, and in which coolant does not touch the sleeve itself.

Dry-Type Air Cleaner – Air filters which use a combination of dry filter media and/or turbulent flow to remove dust particles from the engine air.

Dual Valves – Refers to cylinders having two valves performing one function, i.e. two intake valves or two exhaust valves.

Duty Cycle – An AC signal in which the information is coded as the time the signal is at high voltage even as a percent of the total cycle.

Dwell – The angle that the valve remains in the fully open position. The profile of the lobe of the cam causes the valve to open until the lobe flattens out. The valve stays in this fully open position which is the angle of dwell until the other side of the lobe is reached when the valve starts to close.

Dynamic Balance – Condition when the weight mass of a revolving object is the same as the centerline of the object. This occurs when rotating element has a counter-balancer.

Dynamic Pressure – The pressure of a fluid resulting from its motion, equal to one half the fluid density times the fluid velocity squared.

Dynamic Seals – Seals formed between two moving parts.

Dynamometer – A device for measuring engine torque and horsepower.





Eccentric – Rotating elements which do not have the same center.

Edge Filter – A filter which passes liquid between narrowly separated disks or wires.

Efficiency – In general, the proportion of energy going into a machine which comes out in the desired form, or the proportion of the ideal which is realized.

Electric Circuit – A path for current flow with one or more resistant units. A circuit can include wires, batteries, circuit boards, switches, and other elements.

Electrolyte – A solution of sulfuric acid and water used in batteries.

Electromotive Force (EMF) – Forces that move or tend to move electricity.

Electronic Control Module (ECM) – A microprocessor that determines the beginning and end of each injection cycle on every cylinder. The ECM determines both fuel metering and injection timing in response to such parameters as engine crankshaft position and rpm, engine coolant and intake air temperature, and absolute intake air boost pressure.

Electronic Engine Control (EEC) – Microcomputer coupled with sensors and controls for fuel, coolant, and other on board systems, as well as self diagnostic ability.

Electronic Unit Injector (EUI) – An electronically controlled injector which depends on a cam and rocker arm mechanism to produce fuel injection pressure.

Embedability – The quality of an engine bearing that lets small pieces of debris embed in the bearing to prevent scratching of the crankshaft surface.

Emergency Brakes – A system that uses the service and parking brakes to stop the vehicle in an emergency.

Emitter – The transistor lead for voltage in.

Emulsify – To suspend oil in water in a mixture in which the two do not easily separate.

End Play – The amount of axial movement in a shaft that is due to clearance in the bearings or bushings.

Energize – To provide with electricity.

Energy – Capacity for doing work.

Engine Brakes – A device which increases the retarding force of an engine.

Engine Counterbalancer – A rotational element which dampens and counteracts vibrations from acceleration and deceleration in engine speed.

Engine Displacement – The volume displaced by all pistons total during one stroke.

Erode – To wear away.

Ether – A fluid that remains combustible at low temperatures.

Ethylene Glycol – A compound added to the cooling system to reduce the freezing point.

Evaporative Cooling System – A cooling system in which the heat passes to the atmosphere by evaporation. This system may be either open or closed.

Evaporative Emissions – Hydrocarbon vapors that escape from a fuel storage tank or a vehicle fuel tank or vehicle fuel system.

Excess Air – Air present in the cylinder over and above that which is theoretically necessary to burn the fuel.

Exhaust Analyzer – A test instrument used to measure the density of the exhaust smoke to determine the combustion efficiency.

Exhaust Brake – A device which restricts exhaust airflow to retard engine speed.

Exhaust Filter – An exhaust filter is an after-treatment device consisting of a diesel oxidation catalyst (DOC) combined with a diesel particulate filter (DPF).

Exhaust Gas – The products of combustion in an internal combustion engine.

Exhaust Manifold – A device which connects all the exhaust ports to one outlet.

Exhaust Port – The opening through which exhaust gas passes from the cylinder to the manifold.

Exhaust Valve – The normally closed valve which, when opened, allows the exhaust gas to leave the cylinder.

Expansion Ratio – Ratio of the total volume when the piston is at BDC to the clearance volume when the piston is at TDC.

Eye Bolt – A bolt threaded at one end and bent to a loop at the other end.





Fahrenheit (F) – A designated temperature scale in which the freezing temperature of water is 320F and boiling point 2120 F at standard atmospheric pressure.

Fahrenheit Thermometer – A thermometer using a Fahrenheit scale.

Fatigue – Deterioration of material caused by cycles of loading.

Fatigue Strength – The property of a material to withstand a large number of stress cycles without breaking.

Feeler Gauge – A strip of steel ground to a precise thickness used to check clearance.

Field – The area affected by magnetic lines of force.

Field Coil – An insulated wire wound around an iron pole.

Fillet – A curved joint between two straight surfaces.

Filter – A device for cleaning or purifying fluid or air.

Finishing Stone (Hone) – A honing stone with a fine grit.

Fire Point – Lowest temperature at which an oil heated in standard apparatus will ignite and continue to burn.

Firing Order – The order in which the cylinders deliver their power stroke.

Firing Pressure – The highest pressure reached in the cylinder during combustion.

Fit – The closeness of contact between machined components.

Fixed Displacement Pump – A type of pump in which the volume of fluid per cycle cannot be varied.

Fixed Geometry Turbocharger – A fixed geometry turbocharger is sized for a specific power range, allowing for higher performance results across the entire torque curve. Fixed geometry turbochargers are also designed to maximize fuel economy between the engine’s rated speed and peak torque.

Flange – A metal part which distributes stress to a wider area.

Flank – The straight part of the thread which connects the crest with the root.

Flank Angles – The angle between a specified flank of a thread and the plane perpendicular to the axis (measured in an axial plane).

Flare – To open or spread outwardly. The flare of a section of tubing is measured in degrees from the longitudinal tube axis.

Flaring Tool – A tool used to form a flare on a tubing.

Flash Point – The temperature at which a fuel will give off a vapor that will burn momentarily when exposed to a flame or spark.

Flat Crank – A crankshaft in which one of the bearing journals is not round.

Flexible Tubing – Nylon, neoprene, or other synthetic material used for tubing whose advantages are adaptability and vibration resistance. This type of tubing or hose is usually reinforced with a metal and/or cloth braid.

Flow Control Valve – A valve which is used to control the flow rate of fluid.

Flowmeter – An instrument used to measure the flow rate of a fluid in motion.

Fluctuating – Wavering, unsteady, and not constant.

Fluid – Usually a liquid, but also can refer to a gas or a mixture of liquid and gas.

Fluid Flow – The stream or movement of a fluid; the rate of a fluid’s movement.

Fluid Friction – Resistance to flow in fluids.

Fluid Power – Power transmitted and controlled through the use of fluids, either liquids or gases, under pressure.

Flute – The grooves of a tap that provide the cutting rake and chip clearance.

Flux (Magnetic) – Magnetic force.

Flyback Governor – Conventional type of centrifugal governor commonly called a mechanical governor.

Flywheel – An engine component for maintaining rotational inertia of the crankshaft.

Flywheel Reaction Face – The flat disk section of the flywheel which drives the engine powered machinery.

Flywheel Ring Gear – The outer toothed gear that is shrink fitted to the flywheel.

Foot-Pound (FT.LB) – The amount of work accomplished when a force of one Ib. produces a displacement of one ft.

Force – The action of one body on another tending to change the state of motion of the body acted upon. Force is usually expressed in pounds (kilograms).

Force-Feed Lubrication – A lubricating system in which oil is pumped to the desired points at a controlled rate by means of positive displacement pumps.

Forged – Shaped with a hammer or machine.

Foundation – The structure on performs one or more of the following functions: holds the engine in alignment with the driven machine; adds enough weight to the engine to minimize vibration; adds rigidity to the bed plate.

Four-Stroke-Cycle – Cycle of events which is completed in four strokes of the piston, or two crankshaft revolutions.

Frame – The main structural member of an engine.

Free Electrons – Outer shell electrons of an unfilled shell which are free to conduct electricity.

Free Flow – Flow which encounters little resistance.

Friction – The resistance to motion of two surfaces in contact moving relative to each other.

Front Crankshaft Pulley – The pulley on the front of the crankshaft that drives the fan belts.

Fuel Cycle – The processes involved in extracting a fuel in its native form, converting it to a useful product, transporting it to market, and consuming it at its final destination.

Fuel Injection – This is the manner by which the fuel is introduced into the cylinder at the proper time during the compression cycle, resulting in combustion. Some engines use multiple injections to lower engine noise, improve engine performance and reduce emissions.

Fuel Knock – A noise produced in the cylinder of a diesel engine during combustion, usually when the fuel oil has a low ignition quality.

Fuel Level Indicating Circuit – The electric circuit that indicates fuel level.

Fuel Mixture – A ratio of fuel and air.

Fuel Transfer Pump – A mechanical device used to transfer fuel from the tank to the injection pump.

Fuel Valve – A valve admitting fuel to the combustion chamber. In a more general sense, this term may also apply to any manual or automatic valve controlling flow of fuel.

Fulcrum – The pivot point of a lever.

Full Flow Filtration – Oil filtration in which all the oil in the system passes through the oil filter and on to components for lubricating purposes.

Full-Floating Piston Pin – A piston pin free to turn in the piston boss of the connecting-rod eye.





Gallery – Passageway inside a wall or casting.

Gallon (British Gallon or Imperial Gallon) – A gallon measurement of 277.4 in.3

Galvanic Action – When two dissimilar metals are immersed in certain solutions, particularly acid, electric current will flow from one to the other.

Gas – Matter that has no definite form or volume, but instead tends to expand indefinitely unless contained.

Gasket – A layer of material used between machined surfaces in order to seal them against leakage.

Gassing Period – The rapid voltage increase phase of battery charging in which explosive hydrogen is being produced.

Gate Valve – A common type of manually operated valve in which a sliding gate is used to obstruct the flow of fluid.

Gauge Pressure – Pressure above atmospheric pressure.

Gauge Snubber – A pressure regulating device that protects the gauge from pressure surges.

Gear Backlash – Amount of back rotation in the camshaft gears before crankshaft gear movement is detected.

Gear Type Pump – An oil pump with side by side drive gear and driven gear whose function is to increase oil pressure and create oil flow.

Genset – Generator set, a generating system comprising a combustion engine driving an electrical generator.

Glaze – A smooth, glassy surface finish.

Glow Plug – An intake-air heater that runs on DC power.

Governor – A device for controlling the speed of an engine.

Gravity – The force which pulls objects toward the center of the earth.

Grid (Battery) – The lead frame to which the active material is affixed.

Grinding – Removing metal from an object by means of a revolving abrasive wheel, disk, or belt.

Grinding Compound – Abrasive for resurfacing valves, etc.

Ground (Electric) – The battery terminal that is connected to the engine or the framework.

Grounded Circuit – A circuit correctly connected to ground, or mistakenly grounded in the middle of the designed circuit.

Growler – A test instrument used for testing the armature of a starter or generator for open, short, and grounded circuits.





Half-Moon Key – A fastening device in a shape somewhat similar to a semicircle. (See Key.)

Hand Taps – A set of tools for forming threads.

Heat – A form of energy.

Heat Exchanger – A device used to cool by transferring heat.

Heat Sink – A metal component which conducts heat away from temperature sensitive components and gets rid of it through radiation and conduction to the air.

Heating Value – Amount of heat produced by burning 1 Ib of fuel.

Helical Gear – A gear in which the teeth are cut across the face at an angle.

Helix – The spiral curve shape used on plungers.

High Pressure Common Rail Fuel System – See Common Rail

High Resistance – An excess of opposition to current which can be caused by corroded, poor, or loose connectors or too small of a conductor.

Hone – A cylinder finishing tool using an abrasive stone for removing metal.

Honing – Conditioning of the engine cylinder with a rotating abrasive.

Horsepower (HP) – A unit of power equivalent to 33,000 fNb of work per minute (75 kg.m/s). (See Brake horsepower and Indicated horsepower. )

Horsepower-Hour – A unit of energy equivalent to that expended in 1 hp applied for 1 hour. Equal to approximately 2,545 Btu.

Hose Ends – An adapter used to connect a hose to another adapter or component.

Human Factor – Engine problems which may result from operator performance issues rather than the fault of the engine components. Technicians should be aware of the human factor when diagnosing problems.

Hunting – Alternate overspeeding and underspeeding of the engine caused by governor instability.

Hydraulic Electronic Unit Injector (REVI) – An electronically controlled injector that uses hydraulic pressure acting on an intensifier mechanism to produce fuel injection pressure.

Hydraulic Governor – A governor using fluid to operate the fuel control.

Hydraulic Head – Holds and contains all high-pressure pumping components of the injector pump.

Hydraulic Retarder – An engine retarding device which converts kinetic energy from the engine into heat energy in a fluid.

Hydraulics – The engine systems that use liquids forced through tubes and orifices to operate various mechanisms.

Hydrocarbon (HC) – A compound of hydrogen and carbon. Petroleum and its derivatives are mixtures of various hydrocarbons.

Hydrogen – One of the elements constituting fuel and lubrication oil.

Hydromechanical Injection – An injection system in which mechanical parts work through hydraulic pressure to meter and time the injection of fuel. No electronics are incorporated into hydromechanical injection systems.

Hydrometer – A device which measures the specific gravity of a liquid.






Idler Gear – Any gear or gears that transfer rotation between the camshaft and the crankshaft timing gears and any other driven member.

Idling – An engine running without load.

Ignition – The start of combustion.

Ignition Delay – See Ignition Lag

Ignition Lag – The time between start of ignition and ignition.

Immersed – To be completely under the surface of a fluid.

Impact Wrench – An air wrench or electrically driven wrench. impeller A wheel or disk with fins.

In-Line Engine – An engine in which all the cylinders are in a straight line.

Indicated Horsepower (IHP) – The power transmitted to the pistons by the gas in the cylinders.

Indicated Thermal Efficiency – The ratio of indicated horsepower to equivalent power input in the form of heat from fuel.

Indicator – An instrument for recording the variation of cylinder pressure during the cycle.

Indicator Card – A graphical record of the cylinder pressures made by an indicator.

Indirect Ignition – In diesel engines with indirect injection the fuel is injected to an auxiliary pre-chamber. Combustion starts in the pre-chamber and propagates to the cylinder.

Indirectly Cooled Piston – A piston cooled mainly by the conduction of heat through the cylinder walls. (See also Direct cooled piston)

Induction – Using a magnetic field to impart electricity into an object which is not otherwise connected to the circuit. (See Relay, Solenoid, and Magnetic switch.)

Inertia – The property of matter which causes it to tend to remain at rest if already motionless or to continue in the same straight line of motion if already moving.

Inhibitor – Any substance which retards or prevents such chemical reactions as corrosion or oxidation.

Injection Period – The time, measured in degrees of crankshaft rotation, between the beginning and end of injection. On engines with hydromechanical injection systems, it is controlled by the opening and closing of ports in the injector body or by the action of a plunger forcing fuel out of a cup. On electronic injection systems, it is determined directly or indirectly by the action of a solenoid valve.

Injection Pump – A high-variable-pressure pump delivering fuel into the combustion chamber.

Injection System – The components necessary for delivering fuel to the combustion chamber in the correct quantity, at the correct time, and in a condition satisfactory for efficient burning.

Injector – A device used to bring fuel into the combustion chamber.

Injector Sleeve – A tube that keeps engine coolant away from the injector.

Injector Synchronization – Adjustment of the injectors so that the fuel delivery rate is the same for each injector in each cylinder.

Inline Injection Pump – An injection pump with a separate cylinder and plunger for each engine cylinder. Each plunger is rotated by a rack to determine metering via ports in the body of the pump and helical cuts on the pump plungers. The plungers are driven off a camshaft, which usually incorporates a centrifugal or electronically controlled timing advance mechanism.

Input – Any information sent to the electronic control module (ECM) from a sensor.

Insert Bearing – A removable, precision-made bearing.

Insulator (Electrical) – A material that, under normal conditions, will not conduct electricity.

Intake Manifold – A port or component that brings air to the cylinder. Also, a connecting casting between the air filter or turbocharger.

Intake Valve – The valve which allows air to enter into the cylinder.

Intercooler – Heat exchanger for cooling the air between stages of compression.

Internal Combustion Engine – An engine that burns fuel within itself as a means of developing power.

Internal Gear Pump – An oil pump with an off-center internal
drive gear and external driven gear used to increase oil pressure and create oil flow.

In-Use Deterioration – The effect of time and use on vehicle performance and emissions.

Isochronous – Maintaining constant engine speed.

Isochronous Governor – A governor having zero speed droop.





Jerk Pump – A positive displacement pump with a close fitting piston.

Jet – A small hole in a carburetor passage to measure the flow of gasoline.

Jet Cooling – A method of passing cooling oil to the underside of the piston by means of a jet or nozzle.

Journal – The portion of a shaft, crank, etc., which turns in a bearing.





Keel Cooling System – A marine engine cooling system taking advantage of cold seawater temperatures to lower engine coolant temperature.

Keeper  – A dowel or pin used to keep piston rings from moving from an assigned position.

Kelvin Scale (K) – A temperature scale having the same size divisions as those between Celsius degrees, but having the zero point at absolute zero.

Key – A fastening device wherein two components each have a partially cut groove, and a single square is inserted in both to fasten them together.

Keyway – The groove cut in component to hold the key.

Kilometer (km) – A metric measurement of length equal to 0.6214 mi.

Kilowatt (kW) – A unit of power equal to 1,000 W.

Kilowatt hour (kWh) – A unit of electric energy.

Kinetic Energy – The energy of an object in motion.

Knocking – A sharp pounding sound occurring regularly in an engine.

Knurling – A method of placing ridges in a surface, thereby forcing the areas between these ridges to rise.




Lag – To slow down or get behind; time interval as in ignition lag.

Laminar Flow – A smooth flow of fluid without turbulent disturbance.

Land – The projecting part of a grooved surface; for example, that part of a piston on which the rings rest.

Lanova Cell – A special combustion chamber also called energy cell, for diesel engines of high rotary speeds.

Lap (Lapping) – A method of refinishing (grinding and polishing) the surface of a component.

Leakoff – Leakage in an injector which can keep opening pressure from building up or keeps the injector from maintaining pressure difference. Slight leakoff is acceptable.

Lift Pump – A mechanical or electric supply pump that sends fuel from the tank to the injection pump

Line – A tube, pipe, or hose which is used as a conductor of fluid.

Liner – A replaceable sleeve within the cylinder bore which forms the cylinder wall.

Liner Failure – Failure of the cylinder liner.

Liner Seal Rings – The O-ring seals that contain the coolant on the outside of wet cylinder liners.

Linkage – A movable connection between two units.

Liquid – Matter which has a definite volume but takes the shape of any container.

Liquid Smoke – A name given to gray or white exhaust carrying uncombusted fuel.

Liter (L) – A metric measurement of volume equal to 0.2642 gal (U.S.).

Live Wire – A conductor which carries current.

Load – The power that is being delivered by any power-producing device. The equipment that uses the power from the power-producing device.

Load Factor – The mean load carried by an engine, expressed in percent of its capacity.

Load Line – A center line indicating the points of contact where the load passes within the bearing.

Load-Line Angle – The angle of a load line with respect to the shaft center or bearing radial centerline.

Lobe – The projecting part, usually rounded, on a rotating shaft. The elongated section of a cam.

Logged Event Codes – Any special or critical event which is stored in the ECM until it is viewed and cleared by entering a password.

Lubricant – A substance to decreases the effects of friction, commonly a petroleum product (grease, oil, etc.).

Lubricator – A mechanical oiler which feeds oil at a controlled rate.

Lug (Engine) – Condition when the engine is operating at or below its maximum torque speed.





Magnaflux – The magnetic field fluorescent method for locating fine fatigue cracks.

Magnetic Field – Magnetic lines of force that travel from north to south poles of a magnet and which can be induced by a wire carrying an electric current. Also, the affected area of the magnetic lines of force.

Magnetic Switch – A mechanical a switch that is operated electromagnetically.

Main Bearing – A bearing supporting the crankshaft on its axis.

Mandrel – A mounting device for a stone, cutter, saw, etc.

Manual Valve – A valve which is opened, closed, or adjusted by hand.

Matter – Any substance which occupies space and has weight. The three forms of matter are solids, liquids, and gases.

Mean Effective Pressure (mep) – The difference between compression pressure and expansion pressure.

Mean Indicated Pressure (mip) – Net mean gas pressure acting on the piston to produce work.

Mechanical Advantage – The ratio of the resisting weight to the acting force. The distance through which the force is exerted divided by the distance the weight is raised.

Mechanical Efficiency – (1) The ratio of brake horsepower to indicated horsepower, or ratio of brake mean effective pressure to mean indicated pressure. (2) An engine’s rating which indicates how much of the potential horsepower is wasted through friction within the moving parts of the engine.

Mechanical Filter – Screens or strainers which are surface oil filters. Particles are strained or filtered out based on their size. Mechanical filters mayor may not have moving parts.

Mechanical Injection – Mechanical force pressurizing the metered fuel and causing injection.

Mechanical Variable Timing – The use of flyweights to advance or retard fuel injection timing according to engine rpm. Other types of timing include electronically controlled and hydraulically controlled.

Mechanically Operated Valve – A valve which is opened and closed at regular points in a cycle of events by mechanical means.

Mercury Manometer – A pressure gauge using two columns of mercury.

Metal Fatigue – When metal develops cracks and is in jeopardy of breaking because of vibration or repeated cycles of twisting, bending, or loading. Fatigue cracks can be very fine and if suspected they will be detectable through the magnaflux method. (See Magnaflux.)

Metering Fuel Pump – A fuel pump delivering a controlled amount of fuel per cycle.

Metric Size – Size of a component, part, etc., in metric units of measurement (e.g., meters, centimeters).

Mexican Hat – The inverted cone-shaped center of the piston in some open type combustion chambers.

Micrometer (mike) – A precision measuring tool that is accurate to within one one-thousandth of ; an inch or one one-hundredth of a millimeter

Micrometer (Mm) – One one-millionth of a meter or 0.039370 in.

Milling Machine – A machine used to remove metal, cut splines, gears, etc., by the rotation of its cutter or abrasive wheel.

Misfiring – When the pressure of combustion of one or more cylinders is lower than the remaining cylinders, one or more cylinders have an earlier or later ignition than the others.

Mixed Cycle – Where fuel burns partly at constant volume and partly at constant pressure. Sometimes applied to the actual combustion cycle in most high-speed internal combustion engines.

Molecule – The smallest portion to which a substance may be reduced by subdivision and still retain its chemical identity.

Motor – An actuator which converts fluid power or electric energy to rotary mechanical force and motion.

Multihole Nozzle – An injector nozzle with more than one injector hole.

Multiviscosity Oil – Oil with different viscosities at different temperatures meeting SAE requirements.





Naturally Aspirated – Any engine intake system which does not employ a turbocharger or other supercharging mechanism.

Needle Bearing – A roller-type bearing in which the rollers are smaller in diameter than in length proportional to the race.

Needle Lift – On a needle valve, the distance from the shoulder to the face of the nozzle body. . .

Negative Terminal – A terminal from which the current flows back to its source.

Neoprene – A synthetic rubber highly resistant to oil, light, heat, and oxidation.

Neutral Start Switch – Safety mechanism mounted on the transmission or transmission linkage which insures the vehicle will not start unless the gear shift is in neutral.

Neutron – An uncharged particle within an atom.

Newton’s Third Law – For every action there is an equal, opposite reaction.

Nitrogen Oxide (NOx) – The combination of nitrogen and oxygen that is released during the combustion process.

Nonferrous Metals – Any metals not containing iron.

NOx absorber catalyst (NAC) – A NAC, sometimes called a Lean-NOx trap (LNT), is an after-treatment device that acts as a molecular sponge, trapping NOx. When the trap is full, the NAC must be cleaned or regenerated. A clean NAC can reduce NOx by up to 90 percent.

North Pole (magnet) – The pole from which the magnetic lines of force emanate.

Nozzle – The component containing the fuel valve and having one or more orifices through which fuel is injected.





OEM – Abbreviation Original Equipment Manufacturer. Manufacturers of equipment (such as engines, vehicles, etc.) that provide the original product design and materials for its assembly and manufacture. OEMs are directly responsible for manufacturing and modifying the products, making them commercially available, and providing the warranty.

Ohm (0) – The unit of electrical resistance.

Ohm’s Law – The number of amperes flowing in a circuit is equal to the number of volts divided by the number of ohms.

Ohmmeter – An instrument for measuring the resistance in a circuit or unit in ohms.

On Analysis – Testing of the filtered oil to get an early indication of abnormal engine wear.

On Bath Air Cleaner – An air filter that utilizes a reservoir of oil to remove the impurities from the ail before it enters the intake manifold,

On Board Diagnostics – A system on board of the vehicle that monitors emission control components and alerts the driver (e.g., by a dashboard light) if malfunctions or emission deterioration occurs. The OBD system involves a number of sensors and a data processor, which is typically integrated with the vehicle’s electronic management system.

Oil Clearance – The normal operating clearance between metal parts! that is filled by lubricating oil.

On Cooler – A device that cool! engine oil, usually with engine coolant.

Oil Dilution – Engine oil contaminated with diesel fuel.

Oil Filter – A device for removing impurities from oil.

Oil Gallery – A drilled or cast passage in the cylinder head block and crankcase that is used to carry oil from the supply to an area requiring lubrication or cooling.

Oil Pan – The lower section of the crankcase used as a lubricating-oil reservoir on an internal combustion engine

Oil Pressure Indicating Circuit – The electric circuit that indicates oil pressure.

Oil Pump – A mechanical device to pump oil (under pressure) into the various oil galleries.

Oil Seal – A mechanical device used to prevent oil leakage, usually past a shaft.

Oil Slinger – A rotational element whose purpose is to keep oil away from lip type seals.

On-Off Sensor – A sensor that gives the ECM only one of two voltages in DC form, such as parking brake depressed/not depressed, cruise on/off, and clutch engaged/ not engaged.

Open Circuit – A circuit in which a wire is broken or disconnected.

Open Combustion Chamber – An open combustion chamber is formed in the piston head and is always accompanied by direct fuel injection.

Opposed Piston Engine – An engine having two pistons operating in opposite ends of the same cylinder, compressing air between them.

O-Ring – Stainless steel wire that’s planted into a groove cut around the combustion area of the cylinder head(s) and provides better sealing of the combustion event

Orifice – An aperture or opening.

Oscillate – To swing back and forth like a pendulum; to vibrate.

Oscilloscope – A device for recording waveforms on a fluorescent screen proportional to the input voltage.

Output – An AC or DC signal that the ECM sends out to activate a solenoid, warning lamp, or data link.

Output Shaft – The shaft which delivers the power.

Overhead Camshaft – A camshaft which is mounted above the cylinder head.

Overrunning Clutch Starter Drive – A mechanical device that locks in one direction but turns freely in the opposite direction.

Overspeed – Engine speed above high idle.

Overspeed Governor – A governor that shuts off the fuel or stops the engine only when excessive speed is reached.

Oversquare Engine – An engine that has a larger bore diameter than the length of its stroke.

Oxidation – That process by which oxygen unites with some other substance causing rust or corrosion.





Packing – A class of seal of flexible material used to seal two parts which move in relation to each other.

Paper Air Cleaner – An air filter with a special paper element through which the air is drawn.

Parallel Circuit – An electric circuit with two or more branch circuits. It is wired to allow current to flow through all branches at the same time.

Parent Bore – A cylinder type machined directly into the engine block and having no replaceable cylinder liner.

Parking Brakes – The brake system which keeps the vehicle stopped when it is not in use.

Particulate Matter – Particles formed by incomplete combustion of fuel. Compression ignition (diesel) engines generate significantly higher PM emissions than spark ignited engines. The particles are composed of elemental carbon, heavy hydrocarbons (SOF), and hydrated sulfuric acid (“sulfate particulates”).

Pascal’s Law – Pressure applied anywhere to a body of confined fluid is transmitted undiminished to every portion of the surface of the containing vessel. All hydraulic engine systems utilize this principle In practice, it means that a piston with small area can exert an equal pressure on a larger area actuator.

Passive Regeneration – Passive regeneration is a natural cleaning process where engine exhaust temperatures are sufficient to oxidize the particulate matter (PM) trapped in the exhaust filter. This process occurs during normal engine operation conditions.

Peen – The thin end of a hammer head (opposite to the face). Also, a surface finishing process for added strength of a metal.

Peening – Flattening the head of a rivet, etc., using the force of a hammer.

Penetrating Oil – A special oil that aids removal of rusted parts.

Perforate – To make full of holes.

Periphery – The external boundary or circumference.

Petroleum – An oil-liquid mixture made up of numerous hydrocarbons chiefly of the paraffin series.

Phosphor-Bronze – A bearing material composed of tin, lead, and copper.

Physical Change – A change which does not alter the composition of the molecules of a substance.

Piezo Injector – A common-rail fuel injector that’s activated using very-fast-reacting piezoelectric crystals; these injectors use electricity to precisely introduce fuel into the combustion chamber and can open and close up to seven times during a single combustion event

Pilot Shaft – A shaft position in or through a hole of a component as a means of aligning the components.

Pilot Valve – A valve used to control the operation of another valve.

Pintle Type Nozzle – A closed-type nozzle having a projection on the end of the fuel valve which extends into the orifice when the valve is closed.

Pipe – In diesel applications, that type of fluid line which is designated by nominal inside diameter.

Piston – A cylindrical plug which slides up and down in the cylinder and which is connected to the connecting rod.

Piston Boss – The reinforced area around the piston-pin bore.

Piston Cooling Jet – An oil jet which sprays the bottom side of the piston to cool the piston.

Piston Crown – The part of the piston above the rings.

Piston Displacement – The volume of air displaced by a piston when moved from BOC to TOC.

Piston Head – The portion of the piston above the top ring.

Piston Lands – That space of the piston between the ring grooves.

Piston Pin (wrist pin) – A cylindrical pin that passes through the piston bore and is used to connect the connecting rod to the piston.

Piston Pin Bore – The smaller of the two holes of the connecting rod which connects to the piston by the piston pin.

Piston Pin Bushing – A replaceable wear bushing in the small bore of the connecting rod.

Piston Ring – A split ring of the expansion type placed in a groove of the piston to seal the space between the piston and the cylinder wall.

Piston Ring End Gap – The clearance between the ends of the ring (when installed in the cylinder).

Piston Ring Groove – The grooves cut in the piston into which the piston rings are installed.

Piston Ring Side Clearance – The clearance between the outside edge of the ring and the ring lands.

Piston Rings – Metal rings that form the seal between the piston and cylinder wall.

Piston Skirt – The portion of the piston which is below the piston bore.

Piston Slap – A noise or knock caused by piston contact with the cylinder wall.

Piston Speed – The total distance travelled by each piston in one minute. The formula is Piston speed = stroke (ft) x 2 or stroke (in.) x rpm/6.

Pitch – Number of threads per inch.

Pitting – A surface fault often due to corrosion that can occur on a piston or a cylinder.

Pivot – The pin or shaft on which a component moves.

Plate (battery) – A flat square rigid body of lead peroxide or porous lead.

Play – The movement or slack between two components.

Plunger – In a unit injection system, the plunger pressurizes and meters the fuel.

Plunger Pump – A pump which displaces fluid by means of a plunger.

Pneumatics – Any system which uses gas pressure as the actuator.

Polar Timing Diagram – The name given to the diagram showing engine timing events around a circle representing crankshaft rotation.

Polarity – Refers to the grounded battery terminal or to an electric circuit or to the north and south pole of a magnet.

Polarizing – To develop polarization of the pole shoes in respect to battery polarity.

Pole (magnet) – Either the North or South end of a magnet.

Pole Shoe – A soft iron piece over which the field coil is placed.

Port and Helix – A metering system used by fuel injectors.

Port Bridge – The portion of a cylinder or liner between two exhaust or scavenging ports.

Port Scavenging – Introducing scavenging air through ports in the cylinder wall when they are uncovered by the power piston near the end of the power stroke.

Ports – Openings in the cylinder block and cylinder head for the passage of oil. and coolant. Also, the exhaust-intake connection and valve openings are ports.

Positive Terminal – The terminal which has a deficiency of electrons.

Potential Energy – The energy possessed by a substance because of its position, its condition, or its chemical composition.

Pour Point – The lowest temperature at which an oil will flow.

Power – The rate of doing work.

Power Setting – The maximum fuel delivery and therefore maximum power setting for the engine.

Precision Insert Bearing – A precision type of bearing consisting of an upper and lower shell and a replaceable wear surface.

Precombustion Chamber – A portion of the combustion chamber connected to the cylinder through a narrow port. Fuel is injected into and is partly burned in the precombustion chamber. Heat released by this partial burning causes the contents of the precombustion chamber to be ejected.

Preloading – Installing tapered roller bearings so that the rollers are under a mild pressure or crush.

Prelubrication (pre-lube) Pump – An oil pump which develops oil flow through the engine before it is turned on.

Premixed Compression Ignition (PCI) – With PCI, multiple fuel injections lower combustion temperatures, reducing NOx.

Press Fit – A fit between two components whose tolerance is so small that the two parts must be pressed or driven together.

Pressure (P) – Force exerted per unit area.

Pressure Cap – A special radiator cap with a pressure-relief valve and vacuum valve.

Pressure Differential – The difference in pressure between any two points of a system or a component.

Pressure Hose – Hose connections for high pressure applications above 100 psi (687 kPa). It is constructed of a number of layers of metal braid or cloth braid, and neoprene or rubber.

Pressure Lubrication – A lubricating system in which oil at a controlled pressure is brought to the desired point.

Pressure Relief Valve – A valve that limits the maximum system pressure.

Pressure Test, Compressed Air – A method of testing the cylinder block or other components for cracks by filling them with compressed air to 80 psi (551 kPa) and
immersing them in a water bath at 1800 F (820 C).

Pressure Timing – The use of an orifice at the injector that allows the fuel pump governor to control engine rpm by controlling fuel rail pressure.

Printed Circuit – An electric circuit where the conductor is pressed or printed in or on an insulating material (panel) and at the same time is connected to the resistors, diodes, capacitors, and electronic components such as the personality module and ECU.

Prony Brake – A friction brake used for engine testing.

Proton – The positively charged particle in the nucleus of an atom.

Protractor – A tool for measuring angles.

Prussian Blue – A blue pigment, obtainable in tubes, which is used to find high spots in a bearing or the extent of mating of two surfaces.

Prybar – A lever arm.

Pulse Turbocharger – Turbocharger with separate exhaust manifolds to maintain precise boost pressure application to the timing of each cylinder’s scavenging phase.

Pulse Width – The length of time of an injection event, also known as duration or injector “on time”

Pulverize – To reduce to powder or dust.

Pump – A device for pressurizing and moving fluids.

Pump-Line-Nozzle Fuel System – A fuel system using a single injection pump driven off the geartrain on the front of the engine that also drives the camshaft. The central injection pump feeds a separate injection nozzle located in the cylinder head above each cylinder. Lines which must be of exactly equal length link each pump plunger with the associated nozzle. Each nozzle incorporates a needle valve and the orifices which actually handle atomization.

Pump Scavenging – Using a piston type pump to pump scavenging air.

Pumping Loss – The power consumed by replacing exhaust gas in the cylinder with fresh air.

Punch – A forged steel tool for forcing a part in or out of a hole.

Purge Test – A test used to determine if fuel vapors are properly drawn from the evaporative canister and the fuel tank into the engine for combustion. If the purge system is not working properly, the evaporative canister can become saturated and vent hydrocarbons into the atmosphere.

Push Fit – The part of the bearing that can be slid into place by hand if it is square with its mounting.

Push Rods – The linkage between the cam and the shorter side of the rocker arm.

Pyrometer – A temperature indicator used for indicating exhaust temperature.





Quench – To cool heated steel or iron by thrusting it into water.

Quicksilver – Metallic mercury, the fumes of which are poisonous.





Race (bearing) – The inner or outer groove or channel bearing ring.

Raceway – The surface of the groove or path which supports the balls or rollers of a bearing race.

Radial – A direction pointing from the center of a shaft outward.

Radial Clearance (radial displacement) – The clearance within the bearing and between the balls and races, perpendicular to the shaft.

Radial Load – A “round-the-shaft” load, that is, one that is perpendicular to the shaft through the bearing.

Radiation – Heat transfer by individual vibrating molecules giving off radiative waves of heat energy.

Radiator – A heat exchanger in which coolant gives up heat to the outside air without coming into direct contact with it.

Radius – The distance from the center of a circle to its outer edge or the straight line extending from the center to the edge of a circle.

Rail Pressure – The amount of fuel pressure in the rail of a common-rail injection system

Ratio – The numerical relationship between two distances.

Rebore – To bore a cylinder to a size slightly larger than the original.

Reciprocating Action – An alternating (back and forth) linear movement. The type of movement of a piston within a cylinder.

Rectifier – A device used to convert alternating current to direct current.

Rectifier Bridge – An electronic circuit which turns positive and negative signals into positive signals only.

Regulator (electrical) – An electromagnetic or electronic device used to control generator voltage.

Relay – An electromagnetic switch which utilizes variation in the strength of one electric circuit to effect the operation of another circuit.

Relief Valve – An automatic valve which is held shut by a spring of correct strength. Excess pressure opens the valve and releases some of the gas or liquid. This valve is for protecting filters, air tanks, etc., from dangerous pressures.

Reservoir – An air supply tank for airbrake systems, or a fluid supply container.

Resistance (electrical) – The opposition offered by a body when current passes through it.

Resistance to Corrosion – The property of materials to resist corrosion.

Resistor – A device placed in a circuit to lower the voltage, to reduce the current, or to stabilize the voltage. The ohm value of a resistor is given by color coded bands around the circumference of the resistor.

Retard (injection timing) – To set the timing so that injection occurs later than TDC or is advanced a few degrees before TDC.

Reverse Flush – To pump water or a cleaning agent through the cooling system in the direction opposite to normal flow.

Rheostat – A variable resistor (potentiometer) which regulates current flow by varying the resistance in the circuit.

Ring Expander – A type of spring which is placed between the ring and ring groove to hold the ring with fixed force against the cylinder wall.

Ring Gap – The opening between the ends of a piston ring as measured in the cylinder bore.

Ring Gear – A steel ring with external teeth which is shrink-fitted to the outer circumference of the fly- wheel whose outer teeth mesh with the starter drive for the purpose of starting the engine.

Ring Groove – A groove machined in the piston to receive the piston ring.

Ring Job – The service work on the piston and cylinder including the installation of new piston rings.

Ring Land – The two sides of a ring groove.

Rivet – A soft metal pin having a head at one end used for joining sheet metals.

Rocker Arm – A lever which transmits pushrod motion to valve stem motion.

Rocker Arm Shaft – A shaft that translates camshaft lift to linear valve motion.

Rockwell Hardness – A measurement of the degree of surface hardness of a given object. (See also Brinell hardness.)

Rod – Refers to a connecting rod.

Roller Bearing – An antifriction bearing using straight (cupped or tapered) rollers spaced in an inner and outer ring.

Roller Tappets (roller lifters) – Refers to valve lifters having a roller at one end which is in contact with the camshaft and is used to reduce friction.

Roots Type Blower – A device driven by the engine to increase intake air pressure and flow volume.

Rope Brake – A friction brake used for engine testing.

Rotary Blower – Any blower in which the pumping element follows rotary motion, as opposed to centrifugal blowers.

Rotary Injection Pump – A lower-cost injection pump used with pump-line-nozzle systems. The pump has a central plunger system (usually consisting of two opposing plungers) that provides fuel to every cylinder during the required injection period. A plate located near the top of the pump rotates, opening an appropriate orifice at the right time for distribution to each cylinder’s injection nozzle through a separate line. It is usually used with automotive or agricultural engines that have lower performance and durability requirements than the heavy-duty truck diesels.

Roughing Stone (hone) – A coarse honing stone.

Rubber-Element Damper – An inner iron-alloy element bonded to an outer cast-iron element by a rubber compound, used to dampen vibration by lagging behind sudden changes in rotational speed.

Running Clearance – Clearance between two parts to allow for thermal expansion and oil clearance at operating temperature.

Running-Fit – Any machine fit with running clearance to provide for expansion and lubrication.

Run-in – The initial break-in starting sequence of a new engine

Runout – A detectable dimension that indicates a shaft is not straight.





S-cam Foundation Brakes – Drum brakes whose actuator is a cam in the shape of an “S.” –

SAE – Abbreviation for Society of Automotive Engineers.

SAE Horsepower (rated horse-power) – Formula to determine power: bore diameter x number of cylinders/2.5 = hp.

SAE Viscosity Numbers – Simplified viscosity ratings of oil based on Saybolt viscosity.

Safety Factor – Providing strength beyond that needed as an extra margin of insurance against parts failure.

Sandblast (glass blast) – A cleaning method using an air gun to force the sand at low pressure (about 150 psi) against the surface to be cleaned. ”

Saybolt Viscosimeter – A container with a calibrated outlet tube for determining the viscosity of liquids. (This method is now obsolete.)

Saybolt Viscosity – The number of seconds necessary for 60 mL of liquid to pass through the outlet tube of a Saybolt viscosimeter under standardized test conditions.

Scale – Precipitated mineral deposits from water.

Scavenger Pump – An oil pump which moves oil toward the oil pump pick-up tube, used especially in earthmoving equipment or other applications where level operation is not constant.

Scavenging – Moving a slightly excess amount of fresh air into the cylinder and out the exhaust valves just before the compression stroke.

Scavenging Air – The air which is pumped into a cylinder to displace exhaust gas.

Scavenging Blower – A device for pumping scavenging air.

Scavenging Pump – A piston type pump delivering scavenging air to an engine.

Scraper Ring – An oil-control ring.

Screw Extractor – A device used to remove broken bolts, screws, etc., from holes.

Sealed Bearing – A bearing which is lubricated and sealed at the factory and which cannot be lubricated during service.

Seat (rings) – Rings fitted or seated properly against the cylinder wall.

Sediment – Solid impurities in a liquid.

Selective Catalytic Reduction (SCR) – This after-treatment technology reduces NOx emissions by using a urea-based additive, sometimes referred to as DEF. The ammonia in the urea mixes with engine exhaust gases in the SCR catalyst, converting NOx to nitrogen and water vapor.

Semiconductor – An element which is neither a good conductor nor a good insulator.

Semifloating Piston Pin – A piston pin which is clamped either in the connecting rod or piston bosses.

Sender – A variable resistor that indicates to the gauge in the instrument panel what the measured value is (e.g. water temperature, oil pressure, or oil temperature).

Separator (battery) – A porous insulation material placed between the positive and negative plates.

Series Circuit – An electric circuit wired so that the current must pass through one unit before it can pass through the other.

Series-parallel Circuit – A circuit with three or more resistance units in a combination of a series and a parallel circuit.

Series Turbochargers – In series turbocharging, fresh air is drawn into the low-pressure turbocharger, where air pressure is boosted. This pressurized air is then drawn into the high-pressure turbocharger, where air intake pressure is further raised. The high-pressure air is then routed to an air-to-air cooler and is cooled before getting routed to the engine’s intake manifold. By splitting the compression of the air between two turbochargers, both can operate at peak efficiency and slower rotating speeds.

Service Brakes – The brake system which applies and releases the brakes during normal operation.

Shaft Horsepower – Power delivered at the engine crankshaft. This term is commonly used instead of brake horsepower to express output of large marine engines.

Shim – Thin, flat pieces of brass or steel used to increase the distance between two components.

Short Circuit – A circuit that goes to ground before it should in the circuit.

Shrink Fit – A fit between components machined to interfere in diameter which can only be fit together by heating the outer component so that it will expand and fit over the inner component. As the outer component cools, it shrinks and thereby fits tight to the inner component.

Shroud – The enclosure around the fan, engine, etc., which guides the airflow.

Shunt – A parallel circuit where one resistance unit has its own ground.

Shutoff Valve – A valve which opens and thereby stops the flow of a liquid, air, or gas.

Sight Glass – A clear viewing window or port in the suction side of the fuel system to see if air has entered the fuel system.

Silencer – A device for reducing the noise of intake or exhaust.

Single-acting Cylinder – An actuating cylinder in which one stroke is produced by pressurized fluid, and the other stroke is produced by some other force, such as gravity or spring tension.

Sleeved Engines – Engines with replaceable cylinder liners.

Sludge – Deposits inside the engine caused by dust, oil, and water being mixed together by the moving components.

Snap Ring – A fastening device in the form of a split ring that is snapped into a groove around a shaft or inside a bore.

Soldering Paste – A paste which insures a clean solder bond.

Soldering Terminals – A method of connecting terminals by means of heating a low melting point metal called solder.

Solenoid – An electrically magnetic device used to do work. Schematically, an electromagnet with one or two coil windings wound around an iron tube that also serves as the bushing of the movable iron core.

Spark Arrester – A device which prevents exhaust gases from igniting any flammable materials that are near the engine.

Specific Gravity – The ratio of the weight of a given volume of any substance to that of the same volume of water.

Spline – The land between two grooves.

Spool Up – The stage when a turbocharger is building boost but has not yet reached maximum boost

Spool Valve – A hydraulic directional control valve in which the direction of the fluid is controlled by means of a grooved cylindrical shaft (spool).

Spray Cone – The cone-shaped pattern of atomized fuel which the fuel injector sprays into the combustion chamber.

Spur Gear – A toothed wheel having external radial teeth.

Squish Area – The area confined by the cylinder head and flat surface of the piston when on compression stroke.

Stability – The resistance of a fluid to permanent change such as that caused by chemical reaction, etc.

Stages (I, II, III A, III B, IV) The European Union’s (EU) term for the phased-in implementation of increasingly stringent diesel engine emissions regulations. The four main regulated emissions are carbon monoxide, hydrocarbons, particulate matter and nitrogen oxides.

Starter Drive – The mechanical device that engages the flywheel teeth and transmits the starter force to the flywheel.

Starting Air – Compressed air used for starting an engine.

Starting Air Valve – A valve which admits compressed starting air to the cylinder.

Starting Fluid Leak Test – The air intake system can be tested by spraying all joints with starting fluid while the engine idles. Changes in idle speed indicate leaking connectors.

Starting Notch – A slot on top of the plunger that provides 20 percent more fuel than is used at maximum torque speed, used at start up and overridden by governor control once low-idle speed is reached.

Static Electricity – Electric charge that builds on surfaces and which can damage electronic components.

Static Seals – Seals formed between surfaces which do not move with respect to one another.

Staybolt – A stress bolt running diagonally upward from the bedplate to the opposite side of the frame.

Steady Flow – A fluid or gas flow which is not turbulent.

Stethoscope – A device for amplifying engine noises.

Straightedge – A flat stiff edge for checking straightness.

Streamline Flow – A nonturbulent flow, essentially fixed in pattern.

Stresses – The forces to which parts are subjected.

Stroboscope (timing light) – An instrument used to observe the periodic motion of injection visible only at certain points of its path.

Stroke – The distance in piston movement up and down.

Stroke-to-Bore Ratio – The length of the stroke divided by the diameter of the bore.

Stud – A rod having threads on both ends.

Stud Puller – A device used to remove or to install stud bolts.

Stuffing Box – A chamber having a manual adjustment device for sealing.

Suction – A low pressure area or partial vacuum which pulls higher pressure fluids in.

Suction Valve – Often used interchangeably with intake valve.

Sulfur – An undesirable element found in petroleum in amounts varying from a slight trace to 4-5%

Sump – A receptacle into which liquid drains.

Sump Pump – A pump which removes liquid from the sump.

Supercharger – An air pump driven by the engine which fills the cylinders with a higher pressure than atmospheric pressure.

Supercharging – A method of raising manifold intake pressure of any four-stroke-cylinder engine above atmospheric pressure.

Supply Line – A line that conveys fluid from the reservoir to the pump.

Surge – A momentary rise and fall of pressure or speed in a system or engine.

Swirl Combustion – A combustion chamber configuration which uses curved mixing ridges in the intake ports and/or a re-entrant piston bowl (a bowl whose top edges curve inward). Some swirl combustion chambers have a larger rim around the outside of the piston and a more compact combustion chamber or bowl. The swirl is used to reduce particulate emissions.

Synchronize – To make two or more events or operations occur at the proper time with respect to each other.

Synchronous – Happening at the same time.

Synthetic Material – A complex chemical compound which is artificially formed by the combining of two or more compounds or elements.

Synthetic Oil – Engine oil which remains low in viscosity even at cold temperatures.





Tachometer – An instrument which indicates rpm of any rotating element such as a crankshaft.

Tap – A cutting tool used to cut threads in a bore. (See Chamfer.)

Tap and Die Set – A set of cutting tools used to cut internal and external threads.

Tapered Roller Bearing – (See Roller bearing.)

Tappet – The rocker arm.

Tappet Noise – The noise caused by excessive clearance between the valve stem and the rocker arm.

TDC – Abbreviation for top dead center.

Temper – The condition of a metal with regard to hardness achieved through heating and then suddenly cooling.

Temperature Indicating Circuit – The electric circuit that indicates temperature.

Temperature of Compression – The temperature of the compressed air charge in a power cylinder at the end of the compression stroke before combustion begins.

Temporary Hardness – Dissolved substances which precipitate out when water is heated.

Tension – Stress applied by a pulling force on a part.

Terminal – The connecting point (post) of a conductor.

Theory – A scientific explanation tested by observations and experiments.

Thermal Efficiency – The amount of fuel energy used to do work compared to all available fuel energy. (See also Brake thermal efficiency and Indicated thermal efficiency.)

Thermal Expansion – The increase in volume of a substance caused by temperature change.

Thermocouple – The part of a pyrometer which consists of two dissimilar metal wires welded together at the inner end and held in a protective housing.

Thermometer – An instrument for measuring temperature.

Thermostat – A temperature responsive mechanism used for controlling heating systems, cooling systems, etc., usually with the object of maintaining certain temperatures without further personal attention.

Three-wire System – The most common transistor arrangement, in which each part of the PNP or NPN emitter, base, collector sandwich has its own metal lead.

Throttle-delay Mechanism – A piston which blocks oil flow into the reservoir during full-fuel operation to reduce exhaust smoke during acceleration.

Throttling – Reducing the engine speed by reducing the flow of fuel.

Throw – The part of a crankshaft to which the connecting rod is fastened.

Thru-bolt – Term usually applied to the stress rod passing through the engine frame to carry combustion stresses.

Thrust Bearing (washer) – A bearing or washer of bronze or steel which restrains endwise motion of a turning shaft, or withstands axial loads instead of radial loads as in common bearings.

Thrust Load – A load which pushes or reacts through the bearing in the axial direction of the shaft.

Thrust Side of Piston – The side of the piston that is thrown hard against the cylinder wall during the power stroke.

Tiers (I,II,III,IV) – The Environmental Protection Agency’s (EPA) term for the phased-in implementation of increasingly stringent diesel engine emissions regulations. The four main regulated emissions are carbon monoxide, hydrocarbons, particulate matter and nitrogen oxides.

Time Lag of Ignition – (See Ignition lag.)

Timing Gears – Gears attached to the crankshaft, camshaft, idler shaft, or injection pump to provide a means to drive the camshaft and injection pump and to regulate the speed and performance.

Timing Marks – The marks located on the vibration damper, flywheel, and throughout an engine to check injection and valve opening timing. These alignment lines must match up during installation of engine timing gears.

Tolerance – A fractional allowance for variations from the specifications.

Torque – A force or combination of forces that produces or tends to produce a twisting or rotary motion.

Torque Wrench – A wrench used to measure the turning force being applied.

Torsional Vibration – The vibration caused by twisting and untwisting a shaft.

Traction – The ability of the tire surface to maintain contact with the road surface and prevent slipping.

Transfer Pump – A mechanical device for moving fuel from one tank to another or bringing fuel from the tank to the injection pump are one unit. The unit injector also contains a means of metering the fuel.

Transient Response Time – The time required for an engine to recover the set speed after a load is imposed. This is an engine’s “zero to 60” factor.

Turbine – A rotary machine which extracts mechanical shaft power from the working fluid (gas or liquid) using rotor vanes. The exhaust-side wheel and housing of a turbocharger

Turbocharging – This is a form of aspiration. A turbocharger, which consists of a turbine and a compressor, puts air into the cylinder more forcefully. The exhaust spins the turbine, which is joined by a shaft to the compressor, thereby pressurizing the intake air.

Turbo Lag – The time delay between injecting fuel to accelerate and delivering air to the intake manifold by the turbocharger. This phenomenon may cause black smoke emissions in some turbocharged diesel engines during acceleration.




Unit Injector – An injector which is camshaft-driven and incorporates a plunger. The plunger works in conjunction with orifices in the injector body to determine the beginning and end of injection. The plungers has a helix and is rotated by a rack so the beginning and end of injection can occur closer together or farther apart, thus shortening or lengthening the injection period and changing metering. A special pump which supplies fuel through an orifice to the injector operates at a pressure which is precisely controlled depending on the changes in engine speed.





Vacuum – A pressure less than atmospheric pressure.

Vacuum Gauge – A gauge used to measure the amount of vacuum existing in a chamber or line.

Valve – Any device or arrangement used to open or close an opening to permit or restrict the flow of a liquid, gas, or vapor.

Valve Duration – The time (measured in degrees of engine crankshaft rotation) that a valve remains open.

Valve Float – A condition where the valves are forced open because of valve-spring vibration or vibration speed.

Valve Grinding – Resurfacing the valve face by a special grinding machine.

Valve Guide – A replaceable cylindrical guide for the valve.

Valve Keeper (valve retainer) – A device designed to lock the valve spring retainer to the valve stem.

Valve Lift – The distance a valve moves from the fully closed to the fully open position.

Valve Lifter – (See Cam follower.) –

Valve Margin – The distance between the edge of the valve and the edge of the face.

Valve Oil Seal – A sealing device to prevent excess oil from entering the area between the stem and the valve guide.

Valve Overlap – A part of the piston stroke in which both intake and exhaust valves are open for the purpose of scavenging. It is measured in degrees of crankshaft

Valve Rotator – A mechanical device locked to the end of the valve spring.

Valve Stem – The surface which forces the valve to rotate about 5° with each rocker arm action.

Valve Seat – The surface on which the valve rests when closed.

Valve Seat Insert – A ring of material, sometimes of heat and wear resistant metal used to form a valve seat.

Valve Stem – The long central section of the valve.

Valve Timing – The positioning of the camshaft (gear) to the crankshaft (gear) to ensure proper valve opening and closing.

Vaporization – The process of converting a liquid into vapor.

Variable Geometry Turbocharger (VGT) – A VGT places variable angle vanes in an annular ring around the turbine. Varying the angle of these vanes can boost compressor rotation at low speeds or prevent overspinning at high speeds. Coupling VGTs with electronic controls enables improvements in engine performance and emissions reductions.

Venturi – A specially shaped tube with a small or constricted area used to increase velocity and reduce pressure.

Vibration Damper – A device which dampens axial vibration to reduce fatigue wear and engine roughness.

Vibration Damper, rubber-element – A rotational damping element composed of an inner and outer weight bonded by a flexible rubber seal which resists change in rotational speed.

Viscosity – The property of an oil (or any fluid) by virtue of which it offers resistance to flow.

Viscosity-Index As temperature goes down, oil becomes more viscous. The measure for how fast viscosity changes with temperature is called the viscosity index.

Viscous Damper – A two-piece component whose inner cavity is filled with a small amount of a grease-like fluid.

Volatile Organic Compounds (VOC) – Hydrocarbon-based emissions released through evaporation or combustion. The term VOC is usually used in regard to stationary emission sources.

Volatility – The ability of a fluid to vaporize.

Volt (V) – A unit of electromotive force that will move a current of one ampere through a resistance of 1Ω.

Voltage – Electrical potential expressed in volts.

Voltage Drop – Voltage loss due to added resistance caused by undersized wire, poor connection, etc.

Voltmeter – A test instrument for measuring the voltage or voltage drop in an electric circuit.

Volume – The amount of space within a given confined area.

Volumetric Efficiency – The ratio of the air that enters the cylinder compared to the actual volume of the cylinder.





Water Brake – A device for engine testing in which the power is dissipated by churning water.

Water Jacket – The enclosure directing the flow of cooling water around the parts to be cooled.

Water Manometer – A pressure gauge using two columns of water.

Water Pump Impeller – A volute housing pump with a central inlet and outer outlet used as a coolant pump.

Wategate – A valve mounted in the exhaust system designed to release excess exhaust pressure

Wastegated Turbocharger – A wastegated turbocharger is designed to develop more airflow at lower engine speeds, improving low-speed torque. The wastegate control device unloads a portion of the exhaust flow at higher engine speeds. WGTs deliver improved transient response and a higher peak torque without compromising engine envelope size.

Wet Sleeve – A cylinder liner which is about 70 percent exposed to the coolant.

Wind-Chill Index – A measure of how rapidly heat is lost due to convection with the outside air.

White Smoke – The smoke emitted during a cold start from a diesel engine, composed mainly of unburnt fuel and particulate matter.

Wrist Pin – A stud or pin that forms a journal (as in a crosshead) for a connecting rod





Yoke – A link which connects two points.




Zener Diode – A diode which conducts at a specified voltage in the reverse direction, without burning out, over a wide current range.