ENGINE CYCLE

When the piston is at the highest point in the cylinder, it is in a position called top dead center (TDC). When the piston is at its lowest point in the cylinder, it is in a position called bottom dead center (BDC) (fig. 1-4). As the piston moves from top to bottom or from bottom to top, the crankshaft rotates exactly one half of a revolution. Each movement of the piston from top to bottom or from bottom to top is called a stroke; therefore, the piston completes two strokes for every full crankshaft revolution.

For an engine to operate, the following sequence of events must occur:

1. INTAKE: A combustible mixture is pulled into the cylinder.

2. COMPRESSION: The combustible mixture is compressed into a smaller space.

3. POWER: The compressed combustible mixture is ignited causing it to expand, producing power.

4. EXHAUST: The burnt gases are removed from the cylinder.

The engine repeats this sequence of events over and over again to produce sustained power. One complete series of these events in an engine is called a cycle. Engines have either a four-stroke cycle or a two-stroke cycle; most engines operate on the four-stroke cycle.

Four-Stroke Cycle Gasoline Engine

In the four-stroke cycle gasoline engine, there are four strokes of the piston in each cycle: two up and two down (fig. 1-5). The four strokes of a cycle are as

Figure 1-5.-Four-stroke cycle operation.

follows: intake, compression, power, and exhaust. A cycle occurs during two revolutions of the crankshaft.

INTAKE STROKE. - The intake stroke begins at top dead center, and as the piston moves down, the intake valve opens. The downward movement of the piston creates a vacuum in the cylinder, causing a fuel and air mixture to be drawn through the intake port into the combustion chamber. As the piston reaches bottom dead center, the intake valve closes.

COMPRESSION STROKE.- The compression stroke begins with the piston at bottom dead center and rising up to compress the fuel and air mixture. Since both the intake and exhaust valves are closed, there is no escape for the fuel and air mixture, and it is compressed to a fraction of its original volume. At this point, the fuel and air mixture is ignited.

POWER STROKE.- The power stroke begins when the fuel and air mixture is ignited, burns and expands and forces the piston down. The valves remain closed so that all the force is exerted on the piston. The power stroke ends as the piston reaches bottom dead center.

EXHAUST STROKE.- The exhaust stroke begins when the piston nears the end of the power stroke and the exhaust valve is opened. As the piston moves upward towards top dead center, it pushes the burnt gases, resulting from the ignition of the fuel and air mixture, out of the combustion chamber and through the exhaust port. As the piston reaches top dead center, ending the exhaust stroke, the exhaust valve closes, and the intake valve opens to begin the intake stroke for the next cycle.