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All small engines generally work the same way. But there are minor differences. A four-cycle engine generally is found in larger yard and workshop tools. Engines may take on different appearances, but the four-cycle engine has a vertical crankshaft typically on a riding lawn mower. Tillers often have horizontal crankshafts. The bottom line on how a four-cycle engine works is that a carburetor allows a mixture of air and gasoline into a cylinder, and a spark plug ignites the fuel, causing it to expand. When the burning fuel expands, the pistons move faster and cause the gears the shafts to turn at a high rate of speed. The used fuel leaves via an exhaust valve and new fuel enters the cylinder through the carburetor.

· The intake valve opens up when a cam secured on a rotating camshaft forces it to open. The piston moves downward heftily in a cylinder and pulls an air/gasoline mix out of the carburetor. The camshaft turns half as fast as the crankshaft does.
· The intake valve closes as a camshaft pushes a spring to tighten the valve. The piston moves back up the cylinder, and the air and gasoline mixture is forced to the top of the cylinder, as well. The valves are so tight on the engine that the gases can’t escape.
· The spark plug creates a spark and lights the air and gasoline, and the fuel begins to burn. The vapors get larger and the cause the piston to be pushed back down into the cylinder at a high rate of speed. This rapid fall causes the crankshaft, flywheel, and camshaft to turn even faster.
· The piston gains momentum and is forced back and forth, up and down the cylinder. Burned gases are forced out through an exhaust valve, and more unused air and gas has been allowed into the piston.