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Smaller yard and workshop tools generally use two-cycle engines, opposed to four-cycle ones. You’ll find two-cycle engines in many trimmers, chainsaws, and power saws. These are devices that simply don’t need as much power and force that larger tools that require four-cycle engines. 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 and 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. Here’s a more detailed explanation of the two different cycles of the two-cycle engine.

· A piston moves upward and pushes the air/gasoline combination into the cylinder. A vacuum is formed in the crankcase due to the piston’s rising motion. That vacuum forces more air and gasoline into the reed valve from the carburetor.
· The pressure neutralizes in the crankcase and the reed valve shuts. The air and gasoline mixture stops flowing from the carburetor, and the spark plug ignites the fuel and forces the piston downward. That is cycle number one.
· When the piston moves downward, the crankshaft turns and compresses the air and gasoline mixture in the crankcase. When the exhaust port starts to be uncovered by the crankcase, the exhaust leaves the system. That is cycle number two.
· The piston uncovers all the ports and the air and gasoline mixture seeps quickly into the cylinder. That causes the burned gases out through the exhaust port and into the atmosphere.
· The system starts all over again by having the piston move upward, causing the air and gasoline combination rush into the cylinder and the vacuum once again formed in the crankcase.