Valve timing

By: Ali, Alex, and Gareth

 

In four-stroke, and some two-stroke cycle engines, the valve timing is controlled by the camshaft, and can be varied by the cam lobes being modified slightly, or it can be varied during engine operation by the relatively new technology of variable valve timing.

An example of valve timing:

 

It is also affected by the adjustment of the valve mechanism, and particularly by the tappet clearance, although, this variation is normally unwanted. In many two-stroke engines, there are no camshafts or valves, and the port timing can only be varied by machining the ports and/or modifying the piston skirt, unlike supercharged two-stroke diesel engines. These engines have a cylinder head and camshaft similar to a four-stroke engine. 

A video explaining 4 stroke and 2 stroke engines

A two stroke Diesel cycle:

Why valve timing is important and valve overlap:

Valve overlap is the transition between the exhaust and inlet strokes and is a practical necessity for the efficient running of any internal combustion engine. It is necessary to begin opening the inlet valve before the piston reaches Top Dead Centre (TDC) on the exhaust stroke. Likewise, in order to effectively remove all of the combustion gases, the exhaust valve remains open until after TDC. Thus, there is a point in each full cycle when both exhaust and inlet valves are open.