In a four cycle engine, four strokes of the piston are required to complete one cycle; Intake, Compression, Power, and Exhaust. With only one stroke providing power, a heavy rotating flywheel keeps the crankshaft turning through the other three strokes.
When the intake valve (8/4)
is open and the piston (7/15)
is pulled downward in the cylinder (7/27)
, air is drawn into the cylinder and fills the volume above the piston (intake stroke).
At the bottom of the stroke, the intake valve closes and the piston is pushed up toward the cylinder head. The trapped air is compressed into a small volume (the cup in the top of the piston) and heat is generated due to this compression. The smaller the volume the more heat is generated. When the heat in the compressed air can be concentrated in the piston cup so that it cannot escape into the cylinder walls, etc., the temperature of the compressed air will get very high.
At the top of the compression stroke the fuel is sprayed into the very hot compressed air and ignites. As the fuel burns, the hot combusted gases expand, forcing the piston down. The force applied to the top of the piston in the downward motion is converted into rotary motion by the connecting rod and the crankshaft. This is called the power stroke.
At the bottom of the power stroke, the exhaust valve (8/11)
is opened and as the piston comes back up, the burned gases are expelled into the exhaust elbow (16/6)
. When the piston is back at the top, the exhaust valve closes, the intake valve opens, and the whole cycle is repeated. This is called a four cycle engine.