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

[lapuwali]

If you look at typical cam timings, the exhaust valve usually opens well before the piston has even reached BDC on the power stroke. By the time the piston has reached about 80% of the way down the stroke, no more energy is going to be transferred to the crank by the still-expanding mixture, mostly due to geometric reasons (the big end of the rod is now moving as much sideways as down), and for simple thermodynamic reasons. But, as stated, the pressure in the cylinder is still quite high, so opening the exhaust valve early helps to evacuate the cylinder that much faster.

With perfect timing (which, with any fixed valve timing, will only happen over a narrow rev range), you'll actually have a slight vacuum in the cylinder by the time the intake valve opens while the piston is still rising. The inertia of the exhaust gases will draw the pressure down below atmospheric, and will help to draw the incoming charge in through the intake valve. The additional vacuum created by the piston falling will help, as well, though the intake vacuum is still far smaller than the starting pressure of the exhaust gasses just before the exhaust valve opens.

Since the "force" pushing the intake mixture into the cylinder are so much less than the forces expelling the exhaust gasses, and since you have a limited amount of space to use for valves, you bias the valve sizes towards the intake side.

Again, when the timing works perfectly, the inertia of the incoming charge will allow the cylinder to be pressurised slightly ABOVE atmospheric pressure, and you can shut the intake valve at just the right time, before the charge reverses direction and tries to flow back out the intake valve. This is why you can get such big power gains with just the right cam, but only over a narrow powerband. At the very extreme ends of this were engines like the Honda RC115 4-stroke 50cc GP bike engine of 1965, which made as much as 17hp (340hp/liter, or more than a modern F1 engine), but had a powerband so narrow it required a 17-speed gearbox. Riders talked of needing to make 9 gearchanges in a simple chicane.