Basic Engine Strokes

Figure 12-3.-Relationship of piston, connecting rod, and crank on crankshaft as crankshaft turns one revolution. Since similar action occurs in all cylinders of an engine, we will describe the use one cylinder in the development of power. The one-cylinder engine consists of four basic parts: cylinder, piston, connecting rod, and crankshaft (shown in fig. 12-2). The cylinder, which is similar to a tall metal can, is closed at one end. Inside the cylinder is the piston, a movable metal plug that fits snugly into the cylinder, but can still slide up and down easily. This up-and-down movement, produced by the burning of fuel in the cylinder, results in the production of power from the engine. You have already learned that the up-and-down movement is called reciprocating motion. This motion must be changed to rotary motion to rotate the wheels or tracks of vehicles. This change is accomplished by a crank on the crankshaft and a connecting rod between the piston and the crank. The crankshaft is a shaft with an offset portion-the crank— that describes a circle as the shaft rotates. The top end of the connecting rod connects to the piston and must therefore go up and down. Since the lower end of the connecting rod attaches to the crankshaft, it moves in a circle; however it also moves up and down. When the piston of the engine slides downward because of the pressure of the expanding gases in the cylinder, the upper end of the 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; in turn, the crank rotates the shaft. This rotation is the desired result. So remember, the crankshaft and connecting rod combination is a mechanism for changing straight-line, up-and-down motion to circular, or 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, it is said to be at top dead center. When the piston is at the bottom of a stroke, it is said to be at bottom dead center. These positions are rock positions, which we will discuss further in this chapter under “Timing.” See figure 12-3 and figure 12-7. The basic engine you have studied so far has had no provisions for getting the cylinder or burned gases fuel-air mixture into the out of the cylinder. The 12-3