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

[Brett W]

There is no vaccum in the cylinder. Nothing sucks air into the cylinder. There is a low pressure area created by piston movement. By forcing the air out of the cylinder on the exhaust stroke there is a low pressure area left when the piston moves back to BDC. This low pressure area is much lower than atmospheric pressure. When the intake valve opens pressure in the intake tract will force the mixture into the cylinder. Ram tuning occurs at certian rpms when the wave created by the opening an closing of the intake valve bounces back and forces more mixture into the cylinder. This only happens at certian rpms. Look at the old Chrysler manifolds from back in the 60s.

The NASCAR boys have done quite a bit of testing to deterimine valve sizes and such. They have been increasing the intake size and skrinking the exhaust until they can run the smallest exhaust valves possible.

Look at a diesel engine. They are perhaps the most efficent engines right now. The lack of a throttle plate of any kind makes for very low pumping losses. The high compression ratio helps also.