valvetrains

Valvetrain, or valve train, is the mechanism that operates the intake and exhaust valves of an internal combustion engine. It controls when valves open and close, how much they open (valve lift), and for how long they stay open (duration). Together with the timing of the crankshaft, the valvetrain determines engine breathing, efficiency, and power. The plural form valvetrains is sometimes used to refer to multiple engines' valvetrain assemblies.

Key components include one or more camshafts, lifters or followers, pushrods (in some designs), rocker arms, valve springs, valve guides and seals, and the valves themselves. The camshaft bears lobes that, as it rotates, actuate lifters or rocker arms to push valves open against spring force. Timing is synchronized with the crankshaft by a belt, chain, or gears.

Configurations vary. In overhead camshaft (OHC) designs, the camshaft is located in the cylinder head. In SOHC engines there is one camshaft per head; in DOHC engines separate cams operate intake and exhaust valves. In OHV designs a camshaft in the engine block operates pushrods and rocker arms. Modern engines commonly use multiple valves per cylinder, often four per cylinder (two intake, two exhaust).

Variations and modern technologies include hydraulic lifters that automatically adjust lash, solid lifters that require periodic adjustment, and desmodromic systems used on some motorcycles. Many engines employ variable valve timing or phasing, and some systems also alter valve lift or use electronic actuation for improved efficiency and performance.

Maintenance considerations include wear of cams, lifters, rockers, springs, and timing components, which can lead to timing drift, reduced performance, noise, or valve damage if lubrication or alignment fails.