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 of most engines 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 (through these are rarely seen in automotive applications today) consists of a crankcase which provides for the rigid mounting of the crankshaft and for studs which hold the cylinders in place. The cylinders are individual, single-wall castings, finned for cooling, and are usually bolted to the crankcase, rather than cast integrally with the block. In a water-cooled engine, only the cylinder head is bolted to the top of the block. The water pump is usually 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, equipped 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 to the crankpins at their lower ends.
When the crankshaft spins, the pistons move up and down in the cylinder. Two openings in each cylinder head (above the cylinders) allow the intake of the air/fuel mixture and the exhaust of the burned gases. The volume of the combustion chamber must be variable. This allows the engine to compress the fuel charge before combustion, to make use of the expansion of the burning gases and to exhaust the burned gases (in order to take in a fresh fuel mixture). As the pistons are forced downward by the expansion of burning fuel, the connecting 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 head forms a tight cover for the tops of the cylinders and contains 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 at least one intake valve, one exhaust valve and one spark plug per cylinder. Some modern engines are equipped with multiple intake and/or exhaust valves. 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 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 through a valve train which may take various forms. Many older engines utilize lifters, pushrods and rocker arms. Overhead camshaft engines may utilize rockers arms (with or without adjustment shim tappets) or may actuate the valves directly through lifters or shims.
Lubricating oil is stored in a pan at the bottom of the engine and is force 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.