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 anengine, we will describe the use one cylinder in thedevelopment of power. The one-cylinder engineconsists of four basic parts: cylinder, piston, connectingrod, and crankshaft (shown in fig. 12-2).The cylinder, which is similar to a tall metal can, isclosed at one end. Inside the cylinder is the piston, amovable metal plug that fits snugly into the cylinder, butcan still slide up and down easily. This up-and-downmovement, produced by the burning of fuel in thecylinder, results in the production of power from theengine.You have already learned that the up-and-downmovement is called reciprocating motion. This motionmust be changed to rotary motion to rotate the wheelsor tracks of vehicles. This change is accomplished by acrank on the crankshaft and a connecting rod betweenthe piston and the crank.The crankshaft is a shaft with an offset portion-thecrank— that describes a circle as the shaft rotates. Thetop end of the connecting rod connects to the piston andmust therefore go up and down. Since the lower end ofthe connecting rod attaches to the crankshaft, it movesin a circle; however it also moves up and down.When the piston of the engine slides downwardbecause of the pressure of the expanding gases in thecylinder, the upper end of the connecting rod movesdownward with the piston in a straight line. The lowerend of the connecting rod moves down and in a circularmotion at the same time. This moves the crank; in turn,the crank rotates the shaft. This rotation is the desiredresult. So remember, the crankshaft and connecting rodcombination is a mechanism for changing straight-line,up-and-down motion to circular, or rotary, motion.BASIC ENGINE STROKESEach movement of the piston from top to bottom orfrom bottom to top is called a stroke. The piston takestwo strokes (an upstroke and a downstroke) as thecrankshaft makes one complete revolution. When thepiston is at the top of a stroke, it is said to be at top deadcenter. When the piston is at the bottom of a stroke, it issaid to be at bottom dead center. These positions are rockpositions, which we will discuss further in this chapterunder “Timing.” See figure 12-3 and figure 12-7.The basic engine you have studied so far has had noprovisions for getting thecylinder or burned gasesfuel-air mixture into theout of the cylinder. The12-3
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