Timing gears keep the crankshaft and camshaft turning in proper relation to one another so that the valves open and close at the proper time. Some engines use sprockets and chains.

The gears or sprockets, as the case may be, of the camshaft and crankshaft are keyed into position so that they cannot slip. Since they are keyed to their respective shafts, they can be replaced if they become worn or noisy.

With directly driven timing gears (fig. 12-27), one gear usually has a mark on two adjacent teeth and the other a mark on only one tooth. Timing the valves properly requires that the gears mesh so that the two marked teeth of one gear straddle the single marked tooth of the other.

We have discussed the main parts of the engine proper; but other parts, both moving and stationary, are essential to engine operation. They are not built into the engine itself, but usually are attached to the engine block or cylinder head.

electrical system is provided to supply power for starting the engine and for igniting it during operation. The operation of an internal combustion engine requires an efficient cooling system. Water-cooled engines use a water pump and fan while air-cooled engines use a blower to force cool air around the engine cylinders.

This chapter explained briefly the following operational principles and basic mechanisms of the internal combustion engine:

The power of an internal combustion engine comes from the burning of a mixture of fuel and air in a small, enclosed space.

The movement of the piston from top to bottom is called a stroke.

To produce sustained power, an engine must repeatedly accomplish a definite series of operations. This series of events is called a cycle.

Engine classifications are based on the type of fuel usedgasoline or diesel.

Design and size must be considered before engine construction.

Engines require the use of auxiliary assemblies such as the fuel pump, the carburetor, and an electrical system.