The power of an internal combustion engine comes from burning a mixture of fuel and air in a small, enclosed space. When this mixture burns, it expands significantly; building pressure that pushes the piston down, in turn rotating the crankshaft. Eventually this motion is transferred through the transmission and out to the drive wheels to move the vehicle.
Since similar action occurs in each cylinder of an engine, let’s use one cylinder to describe the steps in the development of power. The four basic parts of a one-cylinder engine is: the cylinder, piston, connection rod, and the crankshaft, as shown in Figure 1-1.
Figure 1-1 – Cylinder, piston, connecting rod, and crankshaft for a one-cylinder engine.
First, there must be a cylinder that is closed at one end; this cylinder is similar to a tall metal can that is stationary within the engine block.
Inside this cylinder is the piston—a movable plug. It fits snugly into the cylinder but can still slide up and down easily. This piston movement is caused by fuel burning in the cylinder and results in the up-and-down movement of the piston (reciprocating) motion.
This motion is changed into rotary motion by the use of a connecting rod that attaches the piston to the crankshaft throw.
The throw is an offset section of the crankshaft that scribes a circle as the shaft rotates. Since the top of the connecting rod is attached to the piston, it must travel up and down. The bottom of the connecting rod is attached to the throw of the crankshaft; as it travels up and down, it also is moved in a circle. So remember, the crankshaft and connecting rod combination is a mechanism for the purpose of changing straight line, or reciprocating motion to circular, or rotary motion.
Each movement of the piston from top to bottom or from bottom to top is called a stroke. The piston takes two strokes (an up stroke and a down stroke) as the crankshaft makes one complete revolution. Figure 1-2 shows the motion of a piston in its cylinder.
Figure 1-2 – Piston stroke technology.
The piston is connected to the rotating crankshaft by a connecting rod. In View A, the piston is at the beginning or top of the stroke. When the combustion of fuel occurs, it forces the piston down, rotating the crankshaft one half turn. Now look at View B. 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.
The intake stroke draws the air-fuel mixture into the cylinder. 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.
The compression stroke begins when the piston is at bottom dead center. As the piston moves upwards, it compresses the fuel and air mixture. Since both the intake and exhaust valves are closed, the fuel and air mixture cannot escape. It is compressed to a fraction of its original volume.
The power stroke begins when the piston is at top dead center (TDC). The engine ignition system consists of spark plugs that emit an electrical arc at the tip to ignite the fuel and air mixture. When ignited, the burning gases expand, forcing the piston down. The valves remain closed so that all the force is exerted on the piston.
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. The piston forces the burned gases out of the cylinder past the open exhaust valve. Figure 1-3 shows the operations of a four-stroke-cycle gasoline engine.
Figure 1-3 – Four-stroke cycle gasoline engine in operation.
Figure 1-4 depicts the two-stroke- cycle engine. 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).
As shown, a power stroke is produced every crankshaft revolution within the two-stroke- cycle engine, whereas the four- stroke-cycle engine requires two revolutions for one power stroke.
Figure 1-4 — Two-stroke-cycle engine.