WHAT IS A CAM, ITS FUNCTION, AND TYPES USED IN DIESEL ENGINES

A cam is a mechanical element, usually a hardened profiled disc or block, that rotates and, through a cam follower/ roller and linkage, imparts a precise reciprocating motion to a valve, pump or other component. Its function is to convert rotary motion of the camshaft into a controlled linear motion (lift) of the follower, with a specific timing, lift, and velocity/ acceleration profile, to operate components at the correct point in the engine cycle.

In diesel engines, cams are driven from the camshaft (driven from the crankshaft at the same speed for two-stroke, half speed for four-stroke). The cam profile determines the timing, lift, dwell and rate of operation of the driven component.

Types of cam used for each application:

• A cam with a rise (flank) that displaces the pump plunger to build fuel pressure for injection. The profile gives a rapid rise to build injection pressure quickly, a dwell at full lift, and a fall. The number of lobes and the profile determine the injection timing and rate. A tangent/ convex cam is used, giving the required plunger velocity.

• A cam that operates the indicator/ engine performance recording device. It is typically a small cam with a short lift that opens the indicator valve at the correct point in the cycle to draw the indicator diagram. A simple cam with a short dwell is used.

• A cam that opens the inlet valve to admit charge air. The profile gives a smooth rise, a dwell at full lift, and a smooth fall, with the timing set to open before TDC and close after BDC. A convex/ tangent cam with a dwell is used to give smooth, quiet operation.

• A cam that opens the exhaust valve to release the exhaust gas. The profile gives a rise, dwell and fall, with timing set to open before BDC and close after TDC. A convex cam with a dwell is used; in large two-stroke engines with hydraulic/ air-spring exhaust valves, a negative/ long-dwell cam may be used (see the negative cam question).

• A distribution disc/ cam with a series of lobes around its circumference that opens the pilot valves of the air-start valves in the correct sequence with the cylinder firing order. The number of lobes equals the number of cylinders; the profile size sets the duration of the air-admitting window. A cam with short, evenly spaced lobes is used.