Figure 12-6.-Crankshaft for a six-cylinder engine.MULTIPLE-CYLINDER ENGINESThe discussion so far in this chapter has concerneda single-cylinder engine. A single cylinder provides onlyone power impulse every two crankshaft revolutions ina four-cycle engine. It delivers power only one-fourthof the time. To provide for a more continuous flow ofpower, modem engines use four, six, eight, or morecylinders. The same series of cycles take place in eachcylinder.In a four-stroke cycle, six-cylinder engine, forexample, the cranks on the crankshaft are set 120degrees apart. The cranks for cylinders 1 and 6, 2 and 5,and 3 and 4 are in line with each other (fig. 12-6). Thecylinders fire or deliver the power strokes in thefollowing order: 1-5-3-6-2-4. Thus, the power strokesfollow each other so closely that a continuous and evendelivery of power goes to the crankshaft.TIMINGIn a gasoline engine, the valves must open and closeat the proper times with regard to piston position andstroke. In addition, the ignition system must produce thesparks at the proper time so that the power strokes canstart. Both valve and ignition system action must beproperly timed if good engine performance is to beobtained.Valve timing refers to the exact times in the enginecycle that the valves trap the mixture and then allow theburned gases to escape. The valves must open and closeso that they are constantly in step with the pistonmovement of the cylinder they control. The position ofthe valves is determined by the camshaft; the positionof the piston is determined by the crankshaft. Correctvalve timing is obtained by providing the properrelationship between the camshaft and the crankshaft.When the piston is at top dead center, the crankshaftcan move 15° to 20° without causing the piston to moveup and down any noticeable distance. This is one of thetwo rock positions (fig. 12-7) of the piston. When thepiston moves up on the exhaust stroke, considerablemomentum is given to the exhaust gases as they pass outthrough the exhaust valve port. If the exhaust valvecloses at top dead center, a small amount of the gasesFigure 12-7.-Rock position.12-7
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