The basic piston engine is a metal block containing a series of chambers. The upper engine block is usually an iron or aluminum alloy casting, consisting of outer walls, which form hollow jackets around the cylinder walls. The lower block provides a number of rigid mounting points for the bearings which hold the crankshaft in place, and is known as the crankcase. The hollow jackets of the upper block add to the rigidity of the engine and contain the liquid coolant which carries the heat away from the cylinders and other engine parts. The block of an air cooled engine consists of a crankcase which provides for the rigid mounting of the crankshaft and for studs which hold the cylinders in place. In a water cooled engine, only the cylinder head is bolted to the top of the block. The water pump is mounted directly to the block.
The crankshaft is a long, iron or steel shaft mounted rigidly in the bottom of the crankcase, at a number of points (usually 4-7). The crankshaft is free to turn and contains a number of counterweighted crankpins (one for each cylinder) that are offset several inches from the center of the crankshaft and turn in a circle as the crankshaft turns. The crankpins are centered under each cylinder. Pistons with circular rings to seal the small space between the pistons and wall of the cylinders are connected to the crankpins by steel connecting rods. The rods connect the pistons at their upper ends with the crankpins at their lower ends.
When the crankshaft spins, the pistons move up and down in the cylinders, varying the volume of each cylinder, depending on the position of the piston. Two openings in each cylinder head (above the cylinders) allow the intake of the air/fuel mixture and the exhaust of burned gases. The volume of the combustion chamber must be variable for the engine to compress the fuel charge before combustion, to make use of the expansion of the burning gasses and to exhaust the burned gasses and take in a fresh fuel mixture. As the pistons are forced downward by the expansion of burning fuel, the connection rods convert the reciprocating (up and down) motion of the pistons into rotary (turning) motion of the crankshaft. A round flywheel at the rear of the crankshaft provides a large, stable mass to smooth out the rotation.
The cylinder heads form tight covers for the tops of the cylinders and contain machined chambers into which the fuel mixture is forced as it is compressed by the pistons reaching the upper limit of their travel. Each combustion chamber contains one intake valve, one exhaust valve and one spark plug per cylinder. The spark plugs are screwed into holes in the cylinder head so that the tips protrude into the combustion chambers. The valve in each opening in the cylinder head is opened and closed by the action of the camshaft. The camshaft is driven by the crankshaft through a chain or belt at 1 / 2 crankshaft speed (the camshaft gear is twice the size of the crankshaft gear). The valves are operated either through rocker arms and pushrods (overhead valve engine) or directly by the camshaft (overhead cam engine).
Lubrication oil is stored in a pan at the bottom of the engine and is forced fed to all parts of the engine by a gear type pump, driven from the crankshaft. The oil lubricates the entire engine and also seals the piston rings, giving good compression.