Fuel in minus fuel out


Fuel monitoring is often a contentious issue when it comes to diesel engines. There are many ways to monitor consumption, some being accurate, while others not so much.

Diesel engines have a circular fuel flow system. Fuel is fed into the engine, part of that fuel is consumed (but not all of it), and the remaining fuel returns to the tank. This flow sequence continues until there is no more fuel remaining in the tank.

When MarineDiesel designs engines, we publish a fuel consumption graph that details how much fuel the engine consumes at different speeds. This consumption is determined by both testing and the use of mathematical formulas, under well-defined conditions. ISO standards are the defined conditions used (several are required), and we are required under EU law to adhere to these conditions.

So why is fuel consumption sometimes controversial?

The answer is simple. Under use, conditions change. All of the following impact fuel consumption:

  • Fuel quality
  • Fuel temperature
  • Air temperature
  • Water temperature
  • Fuel pressure
  • Exhaust pressure

Note that all of these variables can vary significantly from location to location. The fuel consumption figures we provide are based on static and defined conditions that rarely, if ever, are matched exactly in the real world. This is why we sometimes hear feedback from customers that their fuel consumption is more or less than advertised. This is why, on any online boating forum, there is always a lot of discussion on fuel use. Given the fact that fuel consumption is the biggest expense of operating any engine, small variations can have a big financial impact.

So, what is the “real” fuel consumption?

To start, we can give data that is best used as a starting point. What you can expect to see if you are in lab conditions. The variation in consumption could be as high as 10% or even more, depending on all of the criteria listed above and their variation from our test cell baseline.

The best way to determine actual consumption is by doing a combination of the following measurements:

  1. Tank levels. This is somewhat inaccurate, since tanks are usually odd-sized or odd-shaped. If the volume is known, the level is more useful.
  2. Metering. There are dozens of different meters in the market. Any that are well-calibrated will work, and some are more accurate than others. Monitoring the fuel in and fuel return flows, and calculating the difference is accurate. Fuel in minus fuel out equals consumption.
  3. Mathematically. There are various monitoring systems on the market that take sensor data and, using algorithms, calculate fuel consumption.
  4. Fuel receipts. Some commercial users use fuel receipts to determine consumption. This is not usually accurate unless there is ample historical data. For instance, a vessel was re-powered and the same operating patterns are present. We see this in small passenger ferries and water taxis where there are fixed routes and numbers of trips.

MarineDiesel, as a manufacturer, tries to bring the greatest value to our customers. Therefore, on all MarineDiesel engines, we have provided different data outputs using both or either  J-1939 and NMEA2000 protocols, so that customers may easily use most commercial fuel monitoring systems that are on the market.