The Four Stages of Engine Operation

FIGURE 13-The parts of the four-stroke engine are clearly labeled for you in this illustration. During the intake stage, the piston lowers and causes the airand- fuel mixture to be sucked into the cylinder. The intake valve is open at this time, and the exhaust valve is closed.

Now, let's take a closer look at the operation of the four-stroke engine. A simplified drawing of a four-stroke engine is shown in Figure 13. Note the position of the piston, the cylinder, and the crankshaft. The crankshaft is connected to the piston by the connecting rod. The spark plug is positioned at the top of the engine over the combustion chamber.

You'll also notice that we've added two valves to the illustration of the cylinder head. A four-stroke engine will contain at least two mechanical valves: an intake valve and an exhaust valve. These valves are moved up and down to open and close during engine operation. The intake valve opens to allow the air-and-fuel mixture to enter the cylinder. The exhaust valve opens to allow the exhaust gases produced by the burning mixture to exit the cylinder.

In order to burn properly in an engine, fuel must be mixed with air. The engine part that mixes the two together is the carburetor. Fuel moves from the vehicle's fuel tank to the carburetor, where it's vaporized and mixed with air. The air-and-fuel mixture is then delivered to the cylinder through the intake valve. (Note that the carburetor isn't shown in Figure 13.)


FIGURE 14-During the compression stage, both the intake and exhaust valve are closed. As the piston rises, it compresses the air-and-fuel mixture in the sealed combustion chamber.

As you've learned, a four-stroke engine completes the four stages of engine operation in four piston strokes. During the intake stage, the intake valve opens and the piston moves down in the cylinder. As the piston moves down in the cylinder, a vacuum is created that sucks the air-and-fuel mixture into the cylinder through the open intake valve. (This vacuum is similar to that created when you use a drinking straw to suck a beverage out of a glass.) The intake stage continues until the piston reaches bottom dead center. During the intake stage, the engine's exhaust valve remains closed. The intake stage is illustrated in Figure 13.

When the piston reaches bottom dead center, the intake stage is completed and the compression stage begins. During this stage, both the intake and the exhaust valves are closed. The air-and-fuel mixture is now trapped inside the sealed combustion chamber. At this point, the piston begins to rise, compressing the air-and-fuel mixture tightly. The compression stage is illustrated in Figure 14. Compressing the airand- fuel mixture tightly will make it ignite easier and burn more efficiently, and thus produce more power.

The compression stage continues as the piston rises to the top of the cylinder. As the piston approaches top dead center, the engine's ignition system causes the spark plug to create a spark. That is, the ignition system causes the spark plug to fire. Naturally, when a spark is applied to a compressed mixture of fuel and air, an explosion immediately occurs. The process of burning the air-and-fuel mixture in the cylinder is called combustion.


FIGURE 15-When the piston reaches top dead center, the spark plug fires and ignites the air-and-fuel mixture. The force created by the exploding fuel gases pushes the piston down in the cylinder. This is the power stage.

When the air-and-fuel mixture is ignited, the burning gases expand rapidly with great force. Therefore, the contained explosion inside the cylinder forces the piston down hard and fast. Since the piston is connected to the crankshaft through the connecting rod, the piston's downward movement causes the crankshaft to turn around (just like pushing down on the pedals of a bicycle). This stage of burning the mixture in the cylinder and forcing the piston downward is called the power stage, and is illustrated in Figure 15.

The power stage continues until the piston reaches the bottom of the cylinder (BDC). At this point, the piston begins to move back upward in the cylinder, and the exhaust valve opens. However, the intake valve remains closed at this time. As the piston moves upward in the cylinder, it forces the remaining burned gases in the cylinder out through the open exhaust valve. This is the exhaust stage, which is illustrated in Figure 16.

The exhaust stage continues until the piston reaches the very top of the cylinder (TDC). Once the exhaust stage is completed, the four stages of engine operation are finished, and the cycle begins all over again. The intake valve opens, and the piston moves downward to begin a new intake stage.


FIGURE 16-During the exhaust stage, the piston rises and the exhaust valve opens. The rising piston pushes the remaining burned gases in the cylinder out through the open exhaust valve.

The four stages of operation will continue as long as the engine is operating. Also, keep in mind that these cycles are repeated at a very high rate of speed. A typical automobile crankshaft will complete anywhere from 500 to 6,000 revolutions every minute. It's really amazing how fast an engine's components move while it's in operation!