Camshaft

The camshaft is one of the defining components of the four-stroke engine, a rotating shaft fitted with carefully shaped lobes that push the valves open at exactly the right instant in each combustion cycle. Its job is to coordinate the breathing of the engine, admitting fresh air or air-fuel mixture into the cylinders and releasing burnt gases afterwards, all in precise step with the rising and falling pistons. Without this timed orchestration the valves and pistons would have no fixed relationship, and the engine simply could not run.

Mechanically, the camshaft is driven by the crankshaft and turns at exactly half engine speed, because each valve needs to open only once every two crankshaft revolutions in a four-stroke cycle. As the shaft rotates, each egg-shaped lobe presses against a follower, tappet or rocker arm, which in turn pushes the valve down against a strong return spring; once the lobe rotates past, the spring snaps the valve shut. The drive itself comes from a timing belt, chain or set of gears that keeps the camshaft and crankshaft locked in synchronisation, and any slippage of that link throws the whole engine out of time.

The exact shape of each lobe — its profile — has a profound effect on how the engine performs. A gentle lobe that opens the valve modestly and briefly favours strong low-speed torque and smooth, economical running, while an aggressive profile that opens the valve wider and holds it open longer lets the engine breathe deeply at high revs, trading low-down tractability for top-end power. This is the essence of camshaft design, and tuners have long swapped camshafts to reshape an engine's character to suit racing or road use.

Engines are configured with one or two camshafts per cylinder bank. A single overhead camshaft layout, abbreviated SOHC, uses one shaft to operate both intake and exhaust valves, whereas a double overhead camshaft, or DOHC, dedicates one shaft to each, allowing more valves per cylinder and freer breathing at high speed. Many modern engines add variable valve timing, which can advance or retard the camshaft's phase relative to the crankshaft, broadening the useful rev range so a single engine can be both docile at low revs and energetic when pressed.

In practical terms the camshaft is a durable, long-lived part, but the components that drive and follow it deserve attention. A worn timing belt that fails can let the valves collide with the pistons in an interference engine, causing severe damage, which is why scheduled belt replacement is critical. Followers, hydraulic lifters and variable-timing actuators can also wear or clog with degraded oil, so clean, correct-grade oil is the camshaft's best protection against premature wear and the loss of precise valve timing.