What is the Valvetrain of an Engine?

[WapCar] The valvetrain in an engine controls the opening and closure of the intake and exhuast valves of every cylinder, in the sequence of the working cycle of the engine, to let fresh air or mixed air flow into the cylinder, while the spent exhaust gasses flow out.

The valvetrain configuration for most cars is the overhead camshaft(s). The camshaft controls the timing and lift profile of the valves. According to the number of camshaft(s) in the valvetrain, the overhead camshaft(s) can be catergorized into single overhead camshaft (SOHC) and double / dual overhead camshafts (DOHC).

Single Overhead Camshaft (SOHC)

The layout of SOHC is to intall only 1 camshaft in the cylinder head per bank. Engines with in-line cylinders need only 1 camshaft installed above the cylinders, while the V engines need 2 - 1 for each cylinder bank.

The SOHC layout can significantly decrease the amount of components in reciprocating motion and the total weight of the engine, compared to pushrod engines. Hence, the SOHC layout can increase engine revs,  thereby increasing engine power, with the same amount of torque delivered. The camshaft in the SOHC layout controls the opening events of the valves directly by a rotating shaft, rather than transferring motion through lifters, pushrods and rocker arms to the valves in the cylinder head, in pushrod engines.

Comparing to pushrod engines, SOHC engines are structurally more compact (mainly the valvetrain). This advantage is especially significant on multi-valve (more than 2 valves per cylinder) engines. But the SOHC has its weakness - the precision of the valve timing is affected, due to the difference in positions of the intake and exhaust valves in the air way.

Double / Dual Overhead Camshafts (DOHC)

The DOHC layout has 2 camshafts in the cylinder head for each bank, and they control the intake valves and exhaust valves seperately. Depending on the different structures (mainly the different layouts of cylinders) of engines, a common DOHC engine may have 2 or 4 camshafts.

A DOHC engine does not necessarily have 2 or more intake and exhuast valves. But if the valves of a multi-valve engine need to be controlled directly (although they are usually controlled by tappets), the DOHC becomes necessary. Although not all DOHC engines are muti-valve ones, since 2-valve engines were oftenly equipped with 2 camshafts before the multi-valve technology becomes popular, DOHC is equated with multi-valves nowadays, because every cylinder in almost every new DOHC engines, has 3 to 5 valves.

Cam-in-block

In addition, some of the american big engines, uses a cam-in-block layout, where the camshaft is placed near the bottom of the cylinder block, when the valves are at the top. With 2 valves for each cylinder, this type of engines are efficiently aspirated at low and middle rpms, thereby producing decent power even at these ranges.