Overhead Camshaft - Overview

Overview

Compared to OHV pushrod systems with the same number of valves, the reciprocating components of the OHC system are fewer and have a lower overall mass. Though the system that drives the camshafts may be more complex, most engine manufacturers accept that added complexity as a trade-off for better engine performance and greater design flexibility. The fundamental reason for the OHC valvetrain is that it offers an increase in the engines' ability to exchange induction and exhaust gasses. (This exhange is sometimes known as 'engine breathing'. ) Another performance advantage is gained as a result of the better optimised port configurations made possible with overhead camshaft designs. With no intrusive pushrods, the overhead camshaft cylinder head design can use straighter ports of more advantageous crossection and length. The OHC design allows for higher engine speeds, which in turn will increase power output for a given torque.

The OHC valvetrain system may be driven using the same methods as an OHV system, but in practice (and depending on the application), lighter weight and maintenance-free methods are more commonly used. These include using a rubber/kevlar toothed timing belt, and double overhead camshaft (DOHC) roller chains, or in less common cases, gears. Early Ducati motorcycle engines used shafts with bevel gears to drive the camshafts in their OHC engines.

In conjunction with multiple valves (three, four or five) per cylinder, many OHC engines today employ variable valve timing to improve efficiency and power. OHC also inherently allows for greater engine speeds over comparable cam-in-block designs, as a result of having lower valvetrain mass.