. . . So in a
four-stroke gasoline engine,
fuel is burned in the
combustion chamber every other upstroke. The resulting
explosion is what propels the
piston back down, which is what makes your
car move. During the alternate upstroke, the remnants of
combustion (otherwise known as
exhaust gases) are pushed out of the combustion chamber by the rising piston, then as the piston falls again it draws unburned
fuel-air mixture back into the piston, rises, burns, and there you go.
So there have to be holes in the combustion chamber for fuel-air mixture to come in through, and different holes for exhaust to leave by. And they have to be able to be opened and closed, very fast, because gasoline engines turn over at thousands of revolutions per minute.
These valves are trumpet-bell-shaped pieces that fit into holes in the top of the combustion chamber. When they're pushed down, the holes are open. When they're lifted up, the holes are closed.
So these little trumpet-shaped bits have to get rattled up and down very fast, very precisely, for everything to work. To do this, a camshaft is used. The camshaft is a rod of metal punctuated with oval protrusions (cams) up and down its length. This is hard to describe, but if you can find a picture of one, all will become clear. A camshaft is a work of art.
Anyway, those little oval bits are the key. They are cams. Cams are eccentrically-shaped rotors that convert rotational movement into linear movement. The rotation is the spinning crankshaft of the motor; the linear movement is what actuates the valves. As the cams rotate, they push against whatever's actuating the valves, closing them. The valve springs snap the valves back into the open position. Desmodromic engines have no valve springs, but that's outside the scope of this discussion.
So cams open valves. But how? And where do you put the camshaft? There are essentially three camshaft configurations used in modern engines:
- Pushrod
- Single Overhead Cam (SOHC)
- Double Overhead Cam (DOHC)
In a pushrod motor, the camshaft is right next to the
crankshaft, making it easy to drive. Usually the drive mechanism is a
toothed gear. The
crankshaft spins the
camshaft, whose cams act on pushrods running up the side of each cylinder. These pushrods actuate
rocker arms at the cylinder head, opening the valves. Simple, yes, but the number of
oscillating parts is a bit highter than one would like, so pushrod engines can't rev as high as their
SOHC and
DOHC counterparts.
In a single overhead cam engine, there's a (no, really) single cam above the cylinder head. Its cams directly actuate the rocker arms to open valves. This is a compromise between pushrod and DOHC configurations; you only have to go to the trouble of spinning one camshaft. This is accomplished by a timing belt or chain that couples the camshaft to the crankshaft.
The configuration most commonly associated with high performance is double overhead cams. In this layout there are TWO camshafts - one for intake valves, and one for exhaust. The cams act directly on the valves themselves, with no intervening pushrods or rocker arms. Because of this, DOHC engines can rev quite a bit faster than the alternatives, but you still have to figure out how to spin twice the number of camshafts. Usually, the solution is two timing belts, one for each camshaft, but occasinally gears are used.
Now you know. Also, here's something to ponder. If the timing chain, belt, whatever slips, the valves will spring into the open position, pushed down into the cylinder, but the piston (traveling several hundred feet per second) will keep going, colliding with the valves. Can you say catastrophic engine failure? Sure you can. Timing failures are nearly the worst thing that can happen to an engine.
As a postscript, the electromagnetic actuation that The Custodian mentions below is a very intruiging idea. By doing away with the camshaft, it eliminates a significant amount of rotating and oscillating mass, allowing for greater efficiency (no need to expend energy spinning those camshafts around) and more revolutions per minute. This, in addition to the benefits of variable virtual cam profiles (through the software he describes) would mean significant performance gain, not to mention serious geek drool potential.