shows the motion of a piston in its cylinder. The piston is connected to the rotating crankshaft by a connecting rod. In view A of figure 2-4, the piston is at the beginning or top of the stroke. As the crankshaft rotates, the connecting rod pulls the piston down. When the crankshaft has rotated one-half turn, the piston is at the bottom of the stroke. Now look at view B of figure 2-4. As the crankshaft continues to rotate, the connecting rod begins to push the piston up. The position of the piston at the instant its motion changes from down to up is known as bottom dead center (BDC). The piston continues moving upward until the motion of the crankshaft causes it to begin moving down. This position of the piston at the instant its motion changes from up to down is known as top dead center (TDC). The term dead indicates where one motion has stopped (the piston has reached the end of the stroke) and its opposite turning motion is ready to start. These positions are called rock positions and discussed later under "Timing."

The following paragraphs provide a simplified explanation of the action within the cylinder of a four- stroke-cycle gasoline engine. It is referred to as a four- stroke cycle because it requires four complete strokes of the piston to complete one engine cycle. Later a two- stroke-cycle engine is discussed. The action of a four- stroke-cycle engine may be divided into four parts: the intake stroke, the compression stroke, the power stroke, and the exhaust stroke.

Intake Stroke

The first stroke in the sequence is called the INTAKE stroke (figs. 2-5 and 2-6). During this stroke, the piston is moving downward and the intake valve is open. This downward movement of the piston produces a partial vacuum in the cylinder, and the air-fuel mixture rushes into the cylinder past the open intake valve. This is somewhat the same effect as when you drink through a straw. A partial vacuum is produced in the mouth and the liquid moves up through the straw to fill the vacuum.

Compression Stroke

When the piston reaches bottom dead center (BDC) at the end of the intake stroke and is therefore at the bottom of the cylinder, the intake valve closes. This seals the upper end of the cylinder. As the crankshaft continues to rotate, it pushes up through the connecting rod on the piston. The piston is therefore pushed upward and compresses the combustible mixture in the cylinder; this is called the COMPRESSION stroke (figs. 2-5 and 2-6). In gasoline engines, the mixture is compressed to about one eighth of its original volume; this is called 8 to 1 compression ratio. This compression of the air-fuel mixture increases the pressure within the cylinder. Compressing the mixture makes it even more combustible; not only does the pressure in the cylinder increase, but the temperature of the mixture also increases.

Power Stroke

As the piston reaches top dead center (TDC) at the end of the compression stroke and therefore has moved to the top of the cylinder, the compressed air-fuel mixture is ignited. The ignition system causes an electric spark to occur suddenly in the cylinder, and the spark ignites the air-fuel mixture. In burning, the mixture gets very hot and tries to expand in all directions. The pressure rises between 600 to 700 pounds per square inch. Since the piston is the only thing that can move, the force produced by the expanded gases forces the piston down. This force, or thrust, is carried through the connecting rod to the crankshaft throw on the crankshaft. The crankshaft is given a powerful push This is called the POWER stroke (figs. 2-5 and 2-6). This turning effort, rapidly repeated in the engine and carried through gears and shafts, turns the wheels of a vehicle and causes it to move.

Exhaust Stroke After the air-fuel mixture has burned, it must be cleared from the cylinder. This is done by opening the exhaust valve just as the power stroke is finished, and the piston starts back up on the

EXHAUST stroke (figs. 2-5 and 2-6). The piston forces the burned gases out of the cylinder past the open exhaust valve.

TWO-STROKE-CYCLE ENGINE

In the two-stroke-cycle engine (fig. 2-7), the same four events (intake, compression, power, and exhaust) take place in only two strokes of the piston and one complete revolution of the crankshaft. The two piston strokes are the compression stroke (upward stroke of the piston) and power stroke (the downward stroke of the piston). Remember that a diesel engine has six events that must happen to complete a cycle of operation. To better understand the cycle of operation that happens inside the cylinders of a two-stroke diesel engine, refer to the chart below while reviewing figure 2-7.