The power of an internal-combustion engine comes from the burning of a mixture of fuel and air in a small, enclosed space. When this mixture burns, it expands greatly. The push or pressure created is used to move the piston. The piston then rotates the crankshaft. The rotating crankshaft is then used to perform the desired work.

Since the same actions occur in all cylinders of an engine, we will discuss only one cylinder and its related parts. The four major parts consist of a cylinder, piston, crankshaft, and connecting rod (fig 7-1).

First we must have a cylinder that is closed at one end. The cylinder is stationary within the engine block.

Inside this cylinder is the piston (a movable metal plug) that fits snugly into the cylinder but can still slide up and down easily. Movement of the piston is caused by the burning fuel-air mixture in the cylinder.

You have already learned that the back-andforth movement of the piston is called reciprocating motion, which must be changed to rotary motion. This change is accomplished by a throw on the crankshaft and a connecting rod that connects the piston and the crank throw.

The number of piston strokes occurring during any one series of operations (cycles) is limited to either two or four, depending on the design of the engine.

When the piston of the engine slides downward because of the pressure of the expanding gases in the cylinder, the upper end of the

Figure 7-3.-Piston stroke.

connecting rod moves downward with the piston in a straight line. The lower end of the connecting rod moves down and in a circular motion at the same time. This moves the crank throw and, in turn, rotates the shaft. This rotation is the desired result. So remember, the crankshaft and connecting rod combination is a mechanism for the purpose of changing back-and-forth (reciprocating) motion to circular (rotary) motion.

BASIC ENGINE STROKES

Each movement of the piston from top to bottom or from bottom to top is called a stroke. The piston takes two strokes (an upstroke and a downstroke) as the crankshaft makes one complete revolution. When the piston is at the top of a stroke (fig 7-3 view A), it is said to be at top dead center (TDC). When the piston is at the bottom of a stroke (fig 7-3 view B), it is said to be at bottom dead center (BDC).

In the basic engine you have studied so far, we have not considered provisions for getting the fuel-air mixture into the cylinder or burned gases out of the cylinder. There are two openings in the enclosed end of a cylinder. One of the openings,

Figure 7-4.-Four-stroke diesel engine.

or ports, permits an intake of air, or an intake of a mixture of fuel and air, into the combustion area of the cylinder (intake valve). The other opening, or port, permits the burned gases to escape from the cylinder (exhaust valve). The two ports have valves in them. These valves close off either one or the other of the ports, or both of them, during various stages of engine operation. The camshaft (a shaft with a number of cam lobes along its length) opens the valves and holds them open for short periods during the piston stroke. The camshaft is driven by the crankshaft through timing gears or by a timing chain. On a four-stroke engine, the camshaft turns at one-half crankshaft speed. This permits each valve to open and close once for every two turns of the crankshaft.

The following sections give a simplified explanation of the action that takes place within the engine cylinder. For the purpose of explanation, we will show the action of a fourstroke diesel engine. This type of engine is referred to as a four-stroke engine because it requires four complete piston strokes to complete one cycle. These strokes are known as the intake stroke, the compression stroke, the power stroke, and the exhaust stroke.

In a four-stroke engine, each piston goes through four strokes, and the crankshaft makes two revolutions to complete one cycle. Each piston delivers power during one stroke in four, or each piston makes one power stroke for each two revolutions of the crankshaft.

We will take one cylinder and trace its operation through the four strokes that make up a cycle (fig 7-4). The engine parts shown in this figure include a cylinder, a crankshaft, a piston connecting rod, and the intake and exhaust valves. The valve-operating mechanism and the fuel system have been omitted.

During the intake stroke shown in view A, the intake valve is open and the exhaust valve is closed. The piston is moving downward and drawing a charge of air into the cylinder through the intake valve.

When the crankshaft has rotated to the position shown in view B, the piston moves upward to almost the top of the cylinder. Both the intake and exhaust valves are closed during this stroke. The air that entered the cylinder during the intake stroke is compressed into the small space above the piston. This is called the compression stroke.

The high pressure, which results from the compression stroke, raises the temperature of the air far above the ignition point of the fuel. When the piston nears the top of the compression stroke, a charge of fuel is forced into the cylinder through the injector, as shown in view C. The air, which has been heated by compression, ignites the fuel.

NOTE: The injection portion of a cycle is not considered a stroke.

During the power stroke (view D), the intake and exhaust valves are both closed. The increase in temperature resulting from the burning fuel greatly increases the pressure on top of the piston. This increased pressure forces the piston downward and rotates the crankshaft. This is the only stroke in which power is furnished to the crankshaft.

During the exhaust stroke (view E), the exhaust valve is open and the intake valve remains closed. The piston moves upward, forcing the burned gases out of the combustion chamber through the exhaust valve. This stroke, which completes the cycle, is followed immediately by the intake stroke of the next cycle, and the sequence of events continues.

The four-stroke gasoline engine operates on the same mechanical, or operational, cycle as the diesel engine. In the gasoline engine, the fuel and air are mixed in the intake manifold; and the mixture is drawn into the cylinders through the intake valve. The fuel-air mixture is ignited near the top of the compression stroke by an electric spark that passes between the terminals of the spark plug.

Two-stroke diesel engines are widely used by the Navy. Every second stroke of a two-stroke cycle engine is a power stroke. The strokes between are compression strokes. The intake and exhaust functions take place rapidly near the bottom of each power stroke. With this arrangement, there is one power stroke for each

Figure 7-5.-Two-stroke diesel engine.

revolution of the crankshaft, or twice as many as in a four-stroke cycle engine.

NOTE: A two-stroke engine does not have intake valves. It has intake ports (fig 7-5)

The steps in the operation of a two-stroke diesel engine are shown in figure 7-5 . In view A, the piston is moving upward on the compression stroke. The exhaust valve and the intake ports are closed, and the piston is compressing the air trapped in the combustion chamber. At the top of the stroke, with the piston in the position shown in view B, fuel is injected (sprayed) into the cylinder and ignited by the hot compressed air.

In view C, the piston is moving downward on the power stroke: The exhaust valves are still closed; and the increased pressure, resulting from the burning fuel, forces the piston downward and rotates the crankshaft.

As the piston nears the bottom of the power stroke (view D), the exhaust valves open and the piston continues downward to uncover the intake ports. Air is delivered under pressure by a blower for two-stroke diesel engines. In a two-stroke gasoline engine, air comes from the crankcase through the intake ports; and the burned gases are carried out through the exhaust valve. This operation (referred to as scavenging air) takes place almost instantly and corresponds to

Figure 7-6.-A two-stroke diesel engine cylinder with the piston at the bottom of the power stroke.

the intake and exhaust strokes of the fourstroke cycle.

You might expect a two-stroke engine to develop twice as much power as a four-stroke engine of the same size and to operate at the same speed. However, this is not true. With two-stroke diesel engines, some of the power is used to drive a blower (fig 7-6) that forces the air charge into the cylinder under pressure. Also, the burned gases are not completely cleared from the cylinder, reducing combustion efficiency. Additionally, because of the much shorter period the intake ports are open (as compared to the period the intake valve in a four-stroke cycle is open), a smaller amount of air is admitted. Therefore, with less air being mixed with the fuel, less power-perpower stroke is produced. Nevertheless, twostroke diesel engines give excellent service.