CAMSHAFT

A camshaft is a shaft comprising of multiple cam lobes which operate the valves either directly or through a linkage of pushrods and rockers found inside an internal combustion engine. Direct operation involves a simpler mechanism and leads to fewer failures, but requires the camshaft to be positioned at the top of the cylinders. In modern petrol engines the over head cam system is where the camshaft is positioned on top of the cylinder head.

Since the valves control the flow of the air/fuel mixture intake and exhaust gases, they must be opened and closed at the appropriate time during the stroke of the piston. For this reason, the camshaft is connected to the crankshaft either directly, via a gear mechanism, or indirectly via a belt or chain. In some designs the camshaft also drives the distributor and the oil and fuel pumps.

The timing of the camshaft can be advanced to produce better low RPM and torque, or retarded for better high RPM power. Either of these moves the overall power produced by the engine up or down the RPM scale respectively.

Duration is the number of crankshaft degrees of engine rotation during which the valve is off the seat. As a generality, greater duration results in more horsepower. The RPM at which peak horsepower occurs is typically increased as duration increases at the expense of lower rpm efficiency (torque).

A secondary effect of increase duration is increasing overlap, which is the number of crankshaft degrees during which both intake and exhaust valves are off their seats. It is overlap which most affects idle quality, as with the ‘blow-through’ of the intake charge which occurs during overlap reduces engine efficiency, and is greatest during low RPM operation. In reality, increasing a camshaft's duration typically increases the overlap.

The further the valve rises from its seat the more airflow can be achieved which is beneficial. Greater lift has some limitations. Firstly, the lift is limited by the increased proximity of the valve head to the piston crown and secondly greater effort is required to move the valve's springs to higher state of compression.

Higher lift allows accurate timing of airflow; although even by allowing a larger volume of air to pass in the relatively larger opening, the brevity of the typical duration with a higher lift cam results in less airflow than with a cam with lower lift but more duration. On forced induction motors this higher lift could yield better results than longer duration, particularly on the intake side. Notably though, higher lift has more potential problems than increased duration, in particular as valve train rpm rises which can result in more inefficient running or loss of torque.

Cams that have too high a resultant valve lift, and at high rpm, can result in what is called ‘valve bounce’. This could also be as a result of a very steep rise of the lobe and short duration, where the valve is effectively shot off the end of the cam rather than have the valve follow the cams’ profile.

For more information  CLICK HERE  “Camshafts / Cylinder Heads / Engineering”  or  Call Centre : 0861 7777 22