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?
My first thought is the air/fuel ratio is for fuel vapor n air, not liquid fuel, so you have to compensate for the difference in gas/vapor volume at operating temperature and pressure vs. liquid volume. In other words, 1 L of liquid, when evaporated, would yield more than 1 L vapor at standard pressure.
Oh, right, the ratio of 14.7:1 compares air and fuel’s masses, not their liquid volumes. That makes things a whole lot more complicated