Internal-combustion engines are used extensively in the Navy. They serve as propulsion units in a variety of ships and boats. Internalcombustion engines are also used as prime movers (drive units) for auxiliary machinery. Because they have pistons that employ a back-and-forth motion, gasoline and diesel engines are also classified as reciprocating engines.

This chapter provides you with the general construction features and operating principles of various types of internal-combustion engines. After reading this chapter, you will have a basic understanding of the components that make up an internal-combustion engine and how these components work together to develop power.

RECIPROCATING ENGINES

The internal-combustion engines (diesel and gasoline) are machines that convert heat energy into mechanical energy. The transformation of heat energy to mechanical energy by the engine is based on a fundamental law of physics. Gas will expand when heat is applied. The law also states that when a gas is compressed, the temperature of the gas will increase. If the gas is confined with no outlet for expansion, the pressure of the gas will be increased when heat is applied. In the internal-combustion engine, the burning of a fuel within a closed cylinder results in an expansion of gases. The pressure created on top of a piston by the expanding gases causes it to move.

The back-and-forth motion of the pistons in an engine is known as reciprocating motion. This reciprocating motion (straight-line motion) must be changed to rotary motion (turning motion) to perform a useful function, such as propelling a boat or ship through the water or driving a generator to provide electricity. A crankshaft and a connecting rod change this reciprocating motion to rotary motion (fig 7-1).

Figure 7-1.-Cylinder, piston, connecting rod, and crankshaft for one cylinder of an engine.

All internal-combustion engines are basically the same. They all rely on three things-air, fuel, and ignition.

Fuel contains potential energy for operating the engine; air contains the oxygen necessary for combustion; and ignition starts combustion. All are fundamental, and an engine will not operate without all of them. Any discussion of engines must be based on these three factors and the steps and mechanisms involved in delivering them to the combustion chamber at the proper time.

GASOLINE VERSUS DIESEL ENGINES

There are two main differences between gasoline and diesel engines. They are the methods of getting the fuel into the cylinders and of igniting the fuel-air mixtures. In the gasoline engine, the air and gasoline are mixed together outside the combustion chamber. The mixture

Figure 7-2.-Comparison of events in diesel and gasoline four-cycle engines.

then passes through the intake manifold, where it starts to vaporize. Then the mixture enters the cylinder through the intake valve. Here it is completely vaporized by the heat of compression as the piston moves upward on the compression stroke. When the piston is near the top of its stroke (top dead center or TDC), the mixture is ignited by a spark from the spark plug.

The diesel engine uses neither spark plugs nor a carburetor. On the intake stroke, only fresh air is drawn into the cylinder through the intake valve and manifold. On the compression stroke, the air is compressed and the temperature in the cylinder rises to a point between 700 F and 1200 F. At the proper time, the diesel fuel is injected into the cylinder by a fuel injection system. When the fuel enters the cylinder, it ignites Figure 7-2 shows the comparison of the four strokes of four-cycle diesel and gasoline engines.

The speed of a diesel or gasoline engine is controlled by the amount of fuel-air mixture that is burned in the cylinders. The primary difference is the method in which the fuel and air enter the combustion chamber. In a diesel engine, the fuel is injected directly into the combustion chamber, where it mixes with air. In a gasoline engine, the fuel and air are mixed in the intake manifold and then drawn into the combustion chamber.

Mechanically, the diesel engine is similar to the gasoline engine. The intake, compression, power, and exhaust strokes occur in the same order. The arrangement of the pistons, connecting rods, crankshaft, and engine valves are also the same.