a technical discussion, engine efficiency Options

[r_towle]

so, I have been thinking (not always a good thing)
If an engine is basically an air pump, and the objective is to make it as close to 100 percent efficient, why is the exhaust valve so much smaller than the intake valve?

Forget emmisions, forget fuel delivery, just want to understand why this is so...it is so on most engines...

Rich

[dmenche914]

generally engines are most efficient at wide open throotle, and low rpms, Wide open throtle means less restriction on your "air pump", low rpms mean less momentum change energy losses, and heat losses.

That is way generally a smaller displacment motor gets better mileage, as it tends to be ran at wide open throttle all the time, add an overdirve gearing,a nd you get the rpms down low to help reduce friction type heat waste. (think of an old 1100cc 36 HP Bug motor, it is driven wide open throtle almost all the time, the larger 1600 cc engines got worse mileage, dispite more favorable gearing (more overdrive) in part because the engines did not need be at wide open throotle for cruising speed (ie 65 mph) also the later Bugs weighed more, but the engine was just plain bigger than needed to move the car at speed.

what you gain with the bigger engine is the ability to accelerate faster (thats when you do have it at wide open) but at a fixed speed or say 65 mph, the smaller engine should give better economy, all other things being equal.