Camshaft
A camshaft is defined as "a rotating body with an eccentric protuberance, which, as it moves, imparts a linear or angular movement of cyclic nature to some other component of a machine." Huh? How about we just say the camshaft, or cam for short, is a shaft that has ramps on it that open and close the valves at the proper time as it rotates.
Cam location varies according to engine design. Pushrod engines, like the Yamaha XV1700 Road Star and Harley-Davidson FLs and XLs, locate their cams in the crankcase adjacent to the crankshaft and operate the valves through pushrods and rocker arms. Single-overhead-cam engines, like those used in the Victory Kingpin and Honda VTX1300, position their cams in the head, directly between the intake and exhaust valves. The hot rod Suzuki M109 uses a double-overhead-cam engine (that's four cams total, two per cylinder), with each cam being positioned directly over the valves it operates, as does the Yamaha V-Max. It's a complicated way to go, but the added performance makes it worth the trouble.
Because the valves are only open during two of the four strokes, the camshaft is always driven by the crankshaft at exactly half the engine's rpm, regardless of location, design or the number of cams.
Valve Train
The valve train transfers the cam's actions to the valves; its design varies according to cam location and normally encompasses several components.
In a pushrod engine, these include valve tappets, which run directly against the cam and pushrods, which transfer motion from the tappet to the rocker arms. And, of course, the rocker arms, which transfer motion from the pushrod to the valves. Single-overhead-cam mills also use rocker arms to operate the valves, while double-overhead-cam engines generally run their cams either directly against the valves, through an interposed shim or by using a short rocker arm.
Critics of the pushrod engine are quick to condemn the admittedly dated design, complaining that the system is complicated, heavy and flex-prone, all of which limits performance. While they have a point, I'd point out that NASCAR and the NHRA boys build some pretty powerful pushrod engines and that Yamaha and HD twins are no slouches, either.
The current trend is to fit any piece that bears directly on the camshaft (for instance, the tappets in a pushrod motor or the rocker arms in an OHC design) with a ball bearing to reduce friction. Since the end result is less wear and more performance, I'm all for it.
Making Noise
So now that we have some idea of what everything does on its own, let's see what happens when they act in concert.
Technically, what we're looking at is called the four-stroke-cycle engine, not because it's installed into a motorcycle but because it takes four strokes of the piston (two up, two down) to create one power-producing cycle. For the sake of brevity, we're going to assume that valve and ignition timing events begin and end at top dead center and bottom dead center; in real life, an engine set up that way wouldn't make much power, but it does simplify the explanation.
Intake Stroke
The intake stroke starts with the piston at the top of the cylinder, which is called top dead center position, or TDC for short. As the piston descends toward the bottom of the cylinder, the cam opens the intake valve. As the piston moves down, it creates a low-pressure area in the cylinder directly above it. Atmospheric pressure forces the mix of fresh air and fuel in through the open intake valve, filling the cylinder. When the piston reaches the bottom of the cylinder, or bottom dead center (BDC), the intake closes.