# How to Calculate Approximate Fuel Consumption from Engine Specs

I have a marine diesel engine, and I'd like to calculate the approximate fuel consumption based on the size of the engine, at a given number of RPM - specifically for the cruising recommended cruising RPM of 2880 - 3240.

Guidance online varies from the super generic and unclear to PhD abstracts that are way over my head. I'm not sure exactly what details are needed but wouldn't it be the size of the cylinder (bore and stroke) x cylinder size x RPM?

My engine is a 3 cylinder Nanni:

• Max Power: 21.3 kW / 29HP
• Compression rate 22:01
• Bore & Stroke (mm): 78 x 78.4
• Recommended cruising (RPM): 2880 - 3240

P.S. happy to take advice using non-metric, but if so please just be precise in terms of what you're using (e.g. people talk online about "gallons" but not whether it's "US Liquid Gallon" or "Imperial").

• Manufacturer will give info like fuel consumption figures - have you looked for that info? Mar 28, 2022 at 7:40
• RPM is insufficient to provide what you desire. It can only be done as the vehicle system, with a screw, for a specific speed through the water. This is because an engine can run at a specific rpm at no load, half load, full oad, etc. Mar 28, 2022 at 17:28
• I had gone through the manual on the boat - which didn't have the fuel consumption data; but after posting this I eventually found some sales info online which did have it, for different RPM. Mar 28, 2022 at 21:32

## 1 Answer

In short, it's complicated.

In long:

What you tried is the cylinder volume times revolutions per minute. Indeed, this gives the per minute volume of air-fuel mix entering each cylinder of the engine. The point is that the air-fuel mix is not all fuel. If you can somehow get an indicative figure of the air-fuel ratio then you can calculate fuel consumption from there... but I'm not sure you can find that.

Engine fuel consumption is something that is measured through experiments and recorded as a function of speed (RPM) and load (torque). It is called Break Specific Fuel Consumption (BSFC) and it may be written in grams per kilowatt-hours of work done by the engine. BSFC plots are like topographic maps: small round shapes inside larger round shapes, with each shape having a constant BSFC value. And the axes of the plot are speed and load. Suppose we're looking for optimum fuel consumption. Then we need that plot, and we need to look at the smallest round shape that is inside all other, larger, round shapes. Note that it is associated with a unique speed and load combination.

Then, an interesting problem needs to be solved: with the said speed (RPM) and load (torque), how fast can your boat go and carrying what load!

• Thanks @saeed, the explanation of BSFC was very useful too. Mar 28, 2022 at 21:34
• @AdrianK You're very welcome, glad that it helped. Mar 29, 2022 at 1:06