TM 5-4240-501-14&PTHEORIES OF OPERATIONCompressionFigure 1.First is the intake stroke. With the exhaust valve closedand the intake valve open, the piston moves downwardand the air-fuel mixture is drawn into the cylinder. (A -Fig. 1)Then, the intake valve closes, and the piston movesupward on the compression stroke. The air-fuel mixturebecomes greatly compressed in the small spacebetween the top of the piston and the cylinder head. (B -Fig. 1)Figure 2.The spark occurs, igniting the mixture, and the force ofthe expanding gases push the piston down. This is thepower stroke. (C - Fig. 2)The exhaust valve opens, and the upward movement ofthe piston on the exhaust stroke forcesthe burnt gases out of the cylinder. (D - Fig. 2) Then theexhaust valve closes, the intake valve opens, and theengine is ready to repeat the cycle just described. Thusfour strokes complete the cycle.214
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