I was listening a another car talk kind of show (no, not this Car Talk) it was a local show. Anyway, someone was discussing how much air is used vs. fuel in an 11 to 1 compression engine, and he said it was 11 gallons of air to one gallon of fuel. I always believed it to be a heck of a lot more than that. Comments? Rocketman
You’re right, it’s a mass ratio, not a volume ratio. The ‘stochiometric’ ratio (enough air to burn all the gas, but no more) for gasoline is about 15:1. It has nothing to do with compression ratio, either, so that person was doubly mixed up!
A perfect ratio is called stoichemetric and it is 14.7 to 1, theoretically. More air say 16:1 is lean and less say 13:1 is rich. Rich makes more power up to about 12.7:1 depending on lots of things but makes lots more pollution.
Compression ratio is how much the piston squeezes the air/fuel mixture, or these days air only in direct injection engines, as the piston goes from the bottom dead center to top dead center.
The fuel ratio of 14.7 : 1 is by the weight or mass of the air, not the volume of the air.
Figure that a 232 CID engine displaces almost exactly a gallon. Once each cylinder has completed its intake stroke, a gallon of air has been drawn in. Obviously that’s not enough air to burn 1/11 of a gallon of gas.
1 gallon of gasoline weighs 6.073 lb
multiply by 14,7 to get 89.27 lb of air.
Density of air varies with temperature and pressure, but I’ll use 0.0765 lbm/ft3 (sea level and at 15°C) to get 1167 cubic feet, which is 8730 gallons
So (if my math and physics are correct) 1 gallon of gasoline requires 8730 gallons of air.
It’s a good thing 80% of the air that goes into an engine comes right out of the tailpipe otherwise we’d have an air shortage along with water and gas shortages.
It’s a good thing he’s so wrong.
A 5 litre engine running at 5,000 rpm inhales 12,500 litres (3,300 gallons) of air every minute. If it inhaled 1 gallon of gas for every gallon of air, it would be inhaling 275 gallons of gas every minute. Man, would you need a BIG gas tank!
And for anyone checking my arithmetic (PLEASE check my arithmetic!), remember that it only inhales every second rotation.
If you want to do a little puzzler, try modeling gasoline as a long string of … C-H2 's … , which an individual segment reacts with the oxygen in air as
CH2 + (3/2) O2 = CO2 + H2O
See if you can calculate the air to fuel ratio (by mass) using the respective atomic masses, approximately as
And that the percentage (by mass) of O2 in air is 21% .