Some research I’ve done suggests that a small idling engine should consume about 0.2 - 0.3 gal/h, which is about 0.8 - 1.1 L/h. However, the following calculations I’ve done are off by a mile.
At idle, engines typically operate in a rich-running condition, which implies a best-case air-fuel ratio of roughly 14.7:1 (i.e. 1 Lfuel / 14.7 Lair).
According to this article, engines typically have a volumetric efficiency of 15% at idle. This means the volume of fresh air drawn into each cylinder is about 15% of the cylinder’s volume.
A 4-stroke engine sucks air into all cylinders in two revolutions.
Using this information, I’ve calculated the idle fuel consumption for a 1.1L engine at 800 RPM as follows:
rate = (800 rev / min) * (15% * 1.1 Lair / 2 rev) * (1 Lfuel / 14.7 Lair)
This comes out to roughly 4.5 Lfuel / min, or 269 Lfuel / h. What gives?
I think your 15% is way off.
Look at it another way. Your 1.1L engine makes what… 90-100hp? Let’s just say 100 for easy math, that means it takes 15hp just to get it to idle? No way.
The idle power percentage will likely be a function of the number of cylinders and displacement, and will be different for every engine. But I would guess probably on the order of 2hp for your engine. That’s 2%, which if your engine makes 80hp, is 2.5%.
If you rerun everything at 2-2.5% the number drops to 35L/h
I would also guess that your engine should be running much leaner at idle.
Sorry but that’s still way off.
I’m confused. How can you compare peak horsepower with volumetric efficiency?
My engine makes 68 HP and runs at 14.7:1 at idle according to the ECU