20-09-2008, 10:47 PM
The present day requirement for high performance from a medium power engine has resulted in generally higher maximum crank shaft speeds, typically in the region 5500 ? 6000 rpm. This can be explained by recalling the factors governing the power output of an engine since, if neither piston displacement nor mean effective pressure can be further increased. Then the only other way to raise the maximum power is to permit higher crank shaft speeds. For this reason it becomes increasingly more important to avoid erratic operation of valves. As a result, in automobiles we use multi valved engines which are operated directly from an over head cam shaft.
In the engine terminology, the designation ?multiple valves? signifies that each cylinder has more than one inlet and one exhaust valve. Since the early 1980?s, there has been a trend in combustion chamber design towards providing two inlets and two exhaust valves for each cylinder, instead of the customary one of each. Four- valved combustion chambers were actually race proven even before world war I and thereafter were mainly used in application to racing car and few high- performance sports car engines, the reason for their use being that of obtaining maximum power output.
What then is the reason for now using four-valve combustion chambers in the newly produced cars, especially considering the additional application and cost incurred with such an arrangement. The answer to this question has to be more comprehensive and lies in the present day requirement for high performance from a medium power engine, which has resulted in generally higher maximum crankshaft speeds, typically in the region of 5500-6000 rpm. This can be explained by recalling the factors governing the power output of an engine since, if neither piston displacement nor mean effective pressure can be further increased then the only other way to raise maximum power is to permit higher crankshaft speeds. For this reason it becomes increasingly more important to avoid erratic operation of the valves. As a result, in automobiles, we use multi-valved engines, which are operated directly from an overhead camshaft. .
