Internal combustion engine plants operate with the combustion of fuel taking place within the prime mover. The fuels commonly employed include natural gas, gasoline, petroleum and its distillates, alcohol, and by-products of industrial and other operations.

The fuels used may be in a gaseous state originally; they may be liquid fuels gasified by evaporation; or they may be heavy fuel oils injected into the cylinders of the (Diesel) engines by means of a fuel pump or air jet.

The combustion of the fuel may take place within one or both ends of its one or more cylinders. Although their utilization of the heat energy of the fuel is usually better than that found in steam plants (as the intermediate steps with the heat losses are not necessary), their initial cost, maintenance charges and depreciation restrict their employment to relatively small units for emergency purposes.

Internal combustion engines may be classified as to:

1. Cycles of combustion: Regular or constant volume; and, heavy oil (Diesel) or constant pressure.
2. Cycles of operation: Two stroke and four stroke cycles.
3. Cylinders: Number (one, two, four, etc.); arrangement (vertical, horizontal, V, single-acting, double-acting).
4. Fuel handling: Vaporizer, carburetor, solid injection, air injection.
5. Cooling: Water, liquid compounds, air.
6. Power output (in horsepower) and speed (in revolutions per minute).

Cycles of Combustion
The combustion of fuel in an internal combustion engine may be accomplished in two ways:

1. The fuel in gaseous or vaporized form is intimately mixed with a suitable quantity of air and the mixture compressed in the cylinder. It is then ignited by an electric spark and the resultant explosion causes a rise in pressure that is practically instantaneous, with a nearly constant volume.

2. Air only is compressed in the cylinder at a very high pressure so that its temperature is sufficient to ignite the fuel without the aid of an electric spark.

At the instant this high compression pressure is attained, the fuel is sprayed or injected into the cylinder, at the same time the piston begins to move on its power stroke.

The ignition of the fuel produces a further increase in temperature and a practically constant pressure in maintained during this portion of the stroke of the piston; fuel injection can continue and pressure maintained until all the oxygen available is consumed.

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