Air vs fuel consumption

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

C 12
O 16
H 1

And that the percentage (by mass) of O2 in air is 21% .