How Does a Lawn Mower Engine Work?
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Starting the Mower
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The first part about working a lawn mower engine is the starter. For a gas lawn mower engine, it runs much like a car engine's combustion system. Fuel, air and spark must all come together in order to get the engine running. The lawn mower engine's starter is responsible for setting off this chain of events. The starter consists of a pull cord, flywheel and clutch. When the operator pulls on the pull cord, it engages a clutch inside the mower. The clutch spins the mower to allow air and fuel into the combustion chamber. It also causes the spark plug to fire.
Carburetor
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Many gas powered lawn mower engines come with a carburetor, which controls the mix of air and fuel. The throttle is attached to the carburetor as well. Before pulling the starting cord, mower operators usually have to depress a button to "prime" the fuel. This button actually forces a small amount of fuel into the float chamber of the carburetor. If extra fuel is needed because of a cold weather start, the choke function on the carburetor allows more fuel and less air to enter the combustion chamber.
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Combustion
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Combustion occurs in the engine's piston area. The piston compresses the air and fuel on the up-stroke just as the spark plug fires. In lawn mowers with four-stroke engines, the spark plug firing occurs only on the piston's upstroke. With two-stroke engines, the spark plug fires on the up and the down stroke. Once the combustion occurs, the energy created pushes the crankshaft in the mower. The crankshaft then rotates the engine blade at the bottom of the mower.
Operation
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Once the lawn mower engine gets a full start, the operator may engage the clutch further by depressing on a horizontal bar or gear switch that activates the throttle. The throttle allows the engine to receive more air and fuel at a higher velocity, allowing the blade to rotate faster. For fully-powered motors, this also engages the rear wheels that move the push mower forward.
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