CAMSHAFTS EVERYTHING YOU NEED TO KNOW
Ever wondered why these strange metallic sticks have such an impact on how your classic runs? Fuzz explains how they work
Essential to the operation of the Otto, or four-stroke cycle, the camshaft is one of the most interesting and physically manipulable components contained within a reciprocating piston engine.
It is connected and synchronised to the crankshaft by means of a chain, gears or a belt and this synchronisation is key to the correct operation of the engine. Indeed, an incorrectly synchronised camshaft can cause not only rough running, but collision between the valves and the pistons.
The main function of the camshaft is to control the timing and extent of the opening and closing of the inlet and exhaust valves. In this way, it controls the ‘ breathing’ of the engine and does so by the use of machined elliptical lobes, formed around a central shaft core.
These lobes turn the even rotation of the core of the shaft into an eccentric, lifting and falling action which, when transferred to the engine’s valves, causes them to open and close. The exact shape of the lobes controls the rate and amount of lift and fall of the valves, which then determines the gas flow through each cylinder.
Cam shapes for everyday car engines generally feature a lower overall rise, giving lower and smoother valve opening, which provides for a more manageable engine for ordinary driving conditions.
High-rise cams offer increased valve openings, both in lift and time which, on inlet valves and when used in conjunction with a forced induction method (such as a supercharger) gives the opportunity to induce far greater amounts of air and fuel into the combustion chamber and thus for the engine to produce more power.
Conversely, the shape of the cam operating the exhaust valve can be formed to allow large amounts of spent combustion gases to be rapidly expelled.
Camshafts are generally made from chilled cast iron, which gives durable running surfaces for cam followers to operate upon and superior wear characteristics.
Billeted camshafts are machined from solid, round steel bar and are time-consuming and therefore expensive to produce. To provide a hardened surface to each cam, the shaft is nitrided, which is a heat treatment process. Nitrogen is diffused into the surface of the steel to provide a ‘case-hardened’ surface, on which the cam-followers operate.
Worn cast camshafts can take a small amount of regrinding, but the nitrided case-hardening of billeted cams would be ground away if machined and so generally this latter type must be replaced if it shows signs of wear.
It is not only the inlet and exhaust valves that are operated by the camshaft, however. On most vehicles built up until the late 20th century, the distributor is often driven from a worm gear machined into the camshaft, or its drive is taken directly from the end of the camshaft, the latter especially on overhead cam engines.
Also, mechanical fuel lift pumps are normally operated by another cam, formed into the main camshaft and so this automotive part is one of the most important components in a fourstroke engine.
‘This is one of the most important components in a four-stroke engine’