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| Diagram of an overhead camshaft spark ignition (petrol) engine showing main components |
1. The cylinder block - This contains the cylinders, pistons, connecting rods and crankshaft, oilways of the lubrication system, coolant passages of water-cooled engines and in the case of overhead and side-valve engines, the camshaft and pushrods. The cylinder block and crankcase can be separate but are usually cast as one, and usually from either cast iron or aluminium alloy, which is lighter and a better conductor of heat. In water-cooled applications, the water jacket, surrounds the cylinders and are designed to and contains passages through the block which cooling water passes through in order to maintain the engine at optimum temperature and is linked with that of the cylinder head via corresponding passages. Core plugs are provided in the block and are designed to dislodge in the event of the coolant expanding due to freezing, which could crack the cylinder block. The core plugs also seal holes created during the casting of the block and various mountings are provided for components such as water pumps and distributors etc.
2. The cylinder head - This contains the valves, valve ports and the relevant operating gear which includes the camshaft and followers in an overhead camshaft design and the rocker gear for opening the valves on an overhead valve design. Again, cast iron and aluminium are typical materials used in cylinder head construction with aluminium being most popular for performance and race engines due to its lightweight and ability to conduct heat. However, inserts are required to harden and therefore strengthen valve seats and guides in heads using the latter material and where an alloy head and cast iron block are used, an effective seal between the two can be hard to achieve given that the two metals heat, cool, expand and retract at different rates. The faces of both the cylinder block and head are machined flat in order to mate properly and a gasket is normally used in between the two however some designs rely on perfectly flat faces to provide a reliable fluid tight seal. Any distortion caused by overheating of either head or block faces can lead to gasket failure, resulting in the leakage of oil, gas or/and water and the three mixing together inside the engine, Combustion chambers and exhaust ports are the hottest areas of the cylinder head and therefore must be adequately cooled.
3. The crankshaft - The main shaft of the engine which is carried in bearings in the crankcase and has crankpins offset from the main journals, which carry the connecting rods (often abbreviated to con-rods) and is free to rotate
4. The cylinders - Simply, tubes formed in the cylinder block in which the pistons reciprocate
5. The pistons - Cylindrically shaped and fit tightly into the cylinders, forming an airtight seal
6. The connecting rods - These connect the pistons to the crankshaft and are held in places by "gudgeon pins" formed in the pistons at the "small-end" and Big end bearings in which the crankpins rotate. It is these connecting rods that allow the reciprocating movement of the pistons to be transferred into the rotational movement of the crankshaft that passes on the engine power to the drivetrain in order to propel the car.
7. The camshaft - The camshaft is located in the cylinder head (overhead camshaft) or in the cylinder block (overhead valve) and is responsible for the opening of the valves in turn at the correct time and the durstion of which the valves remain open.
