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I thought that the formula for full throttle fuel economy that I gave in the last message (mpg(full throttle)=v1k/d) gave results that were a little low so I did some more calculations. I remembered reading that one pound of gasoline makes 210 cubic feet of air and fuel mixture. From that I was able to calculate that one gallon of gasoline makes 2,268,000 cubic inches of mixture.
6.25 lb/gal x 210 cubic feet/lb x 1728 cubic inches/cubic foot
= 2,268,000 cubic inches
A one liter engine running at 1000 rpm at 100% VE (volumetric efficiency) would consume 1,830,000 cubic inches of fuel mixture each hour.
1 Liter x 1000 rpm x 61 cubic inches/liter x 60 min/hr x 0.5 intake strokes/rev
= 1,830,000 cubic inches of fuel mixture per hour
So a 1 liter engine running at 1000 rpm with 100% volumetric efficiency should use
1,830,000 cubic inches/hr / 2,268,000 cubic inches/gal
= 0.807 gal/hr
So the proper formula for full throttle mpg should be
mpg = v1k/(d x 0.807)
or
mpg(full throttle) = 1.24 x v1k / d
This assumes normal atmospheric pressure at sea level, normal temperature about 70 degrees Fahrenheit, and plain gasoline without alcohol and a chemically correct fuel mixture (14.7 parts air to 1 part fuel). The original formula might be close to correct if using a rich fuel mixture (about 12.5 to 1) and 10% ethanol mixed with the gasoline.
The formula I made for constant speed fuel economy (mpg=4*v1k/d) would imply a volumetric efficiency of 31%. If you think your car can cruise with the engine running at 25% VE (maybe you have a high compression engine) you can use this formula:
mpg = 4.96 x v1k / d
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