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?

  • gordon@lemmy.world
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    1 year ago

    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.

    • glibg10b@lemmy.mlOP
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      1 year ago

      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