Cranking up pistons to propel the engine

Deep inside every petrol or diesel engine lives a substantial chunk of metal, with a contorted shape. It is one of the most highly stressed components of an engine.

Parts of it will be machined with extreme precision to tolerances of half the thickness of the paper on which this is printed. It will be spinning at anything between 600 and 6000 revolutions per minute whenever the engine is running. Yet so refined is its design and so good its balance on modern engines that the average owner will probably never be aware of its existence.

We are talking of the crankshaft. Its purpose is to transform the up-and-down movement of the pistons to a rotational movement which can be transmitted by rotating shafts through the drive train to the wheels.

We see the same principle in operation when we look at a cyclist pedalling a bicycle. The up-and-down movement of the cyclist's upper legs (corresponding to the pistons in an engine) is carried by the lower legs (connecting rods) to the pedals (conrod journals) which are part of the crank supported by the bottom bracket (main bearings). In this way the reciprocating movement of the upper legs is transformed to the rotation of the drive sprocket.

A crankshaft starts its life on the drawing boards or, nowadays, computer screens of an engineering division after every dimension has been calculated mathematically. The uncanny smoothness of modern engines is because of careful balancing of the crankshaft-conrods-pistons assembly, made possible by high-power mathematical modelling, and sometimes involving the use of balance shafts.

When it comes to actually manufacturing a solid chunk of metal with the complicated shape of a crankshaft, three options exist:

l It can be formed by pouring molten metal, usually a special kind of cast iron, into a mould where it cools and solidifies. This process, called casting, is cheap, the tooling is long lasting, and the raw casting springs from the mould very close to the exact shape desired, minimising the final machining requirements.

l It can be produced by forging, that is, by placing a hot chunk of rolled steel between heavy dies that have the pattern of a crankshaft, and then squeezing the metal into the crank's basic shape under extreme pressure supplied by a forging press. When a high-grade alloy steel is used, a forged crank will be stronger than a cast one, but it's also more expensive to produce because the tooling is more complex and less durable, and there is likely to be more excess material to be machined from the raw forging to create a finished crankshaft.

l The top end of the high-performance crankshaft scale belongs to billet cranks. These start as massive logs of premium steel, called billets, which are machined into finished crankshafts by whittling away all the material not wanted in the final product. It's a time-consuming, expensive process requiring serious equipment, but the result is a crank with a superior, consistent linear grain structure. This option is sometimes used for development work on a new engine.