Valve Configuration

In a four stroke engine there are two types of valves : the intake valves that open during the intake stroke, allowing the air-fuel mix into the engine, and the exhaust valves that open during the exhaust stroke, releasing the waste gases. The valves are located in the cylinder head, and play an important role in connecting and blocking off the combustion chamber.
 

Modern engines typically have camshaft at the top of the engine, which allow more reliable valve movement. Most modern engines have 4 valves per cylinder with 2 intake valves and 2 exhaust valves, but engines that focus on combustion efficiency at the low RPM range with 2 valves per cylinder, consisting of 1 intake valve and 1 exhaust valve, will probably be making a comeback in the future.

The latest trend is towards variable valve timing. Initially, this allowed valves two timing – one for low revs, and one for high revs – but more recent development allow valve timing and lifted to be varied continuously to match the engines’s revs. In the latest valve mechanism of the “valvetronic” BMW engine, power adjustment is accomplished without using the throttle valve, achieving greater efficiency.

Types of Valve Configuration

DOHC (Double Overhead Camshaft)

In a DOHC engine, one camshaft operates the intake valves and one camshaft operates the exhaust valves. Other than ensuring stable operation by sharing the work over two camshafts, it also means that there is less reciprocating mass (inertia) in the valvetrain, and this make it possible to achieve higher RPMs with the engine. This is turn allows better power output, which is why this layout has adopted in most of today’s high-performance engines.

·         

    SOHC (Single Overhead Camshaft)

A single overhead camshaft is engine with a single camshaft operating the intake and exhaust valves. Depending of the type of combustion chamber, the camshaft can either directly operate the valves, or it can operate the valves through rocker arms. Compared to an OHV engine, valve movements are more reliable, and higher revs are possible. Compared to a DOHC engine, valve movement are not as smooth, but high rpm SOHC engines exist, so they are not also inferior.

· 

         OHV (Overhead Valve)

An overhead valve is, as its name suggest a system where the valves are mounted on the cylinder head. Unlike an SOHC or DOHC setup, the camshaft is located to the side of the cylinders and the camshaft operates the valves with long arms called “pushrods”. This structure is simple, and it is easily maintained. However, valves operations of this types of engines are not as reliable at high RPM, and is not generally suited for high power.