Characteristics of Different Fuels



Different fuels have inherently different combustion characteristics (They burn in different ways and hold different amounts of energy). When considering alternative fuels, it is important to keep these characteristics in mind, over and above merely considering cost of fuel and cost of operation. The chart below demonstrates different fuels and gives an easy “at a glance” comparison between fuel types:

fuel characteristics


Comparison of Auto-Ignition Temperature

The auto-ignition temperature is the temperature at which fuel will ignite without a flame or spark. In respect to the auto-ignition temperatures, LPG, CNG, and LNG are safer than gasoline or diesel.because the auto-ignition temperature is much higher.

Comparison of Peak Flame Temperature

The flammability range is the difference between the leanest (LEL) to the richest (UEL) mixture of fuel and air that will burn. Fuel with narrower ranges are safer to work with, but are less versatile because they offer less choice of air / fuel ratios. CNG has a peak flame temperature of 1,790 deg C / 3,245 deg F, which is 187 deg C / 337 deg F cooler than peak flame temperature of gasoline at 1,977 deg C / 3,591 deg F. The peak flame temperature of LPG at 1,991 deg C / 3,614 deg F is only 13 deg C / 23 deg F (Less than 1%) higher than gasoline.

Comparison of Energy Content

Energy content per unit of fuel (energy density) is an important factor affecting range and power output of internal combustion engines. The higher the energy content of the fuel, the more power the engine will make.

Volumetric Efficiency

The amount of air entering an engine at a particular throttle angle and load is fixed. Any fuel added to the air before it enters the cylinder will displace an equal volume of air and will reduce the volumetric efficiency and power output of the engine. Reductions are as follows:

  • Diesel – Less than 1% (approx.)
  • Gasoline – 1-2% (approx.)
  • LPG – 4% (approx.)
  • CNG – 9% (approx.)

What is LPG?

LPG is “Liquefied Petroleum Gas” Commonly known as propane C3H8, a combustible hydrocarbon based fuel. It comes from the refining of crude oil and natural gas. At normal pressure and temperatures, propane remains in its’ gaseous form. At lower temperatures and / or  higher pressures, propane becomes a liquid.

Propane is odorless and colorless. For safety reasons, propane is required to be odorized. There are currently three grades of propane available: HD5 for internal combustion engines, commercial propane, and propane / butane mixture for other uses. The exact composition of propane varies slightly between different parts of the world and different refineries. Compared to gasoline, the energy content of LPG is 74%.

What is CNG?

CNG is “Compressed Natural Gas”. Natural gas (CH4) is a naturally occurring mixture of combustible hydrocarbon gases found in porous formations beneath the earth’s surface. Natural gas is created by the decomposition of plant and animal remains, under great heat and pressure, over very long periods of time.

Natural gas can be found as:

  • Non-associated gas: Free gas not in contact with significant amounts of crude oil in the reservoir.
  • Associated gas: Free gas in contact with crude oil in the reservoir.
  • Dissolved gas: Gas in solution with crude oil in the reservoir.

For safety reasons, CNG is required to be odorized. Compared to gasoline, the energy content of CNG is 25%.