Push Mower Motor Basics
Push mowers use single-cylinder gasoline engines, similar to the one used in a car or motorbike. The piston turns a crank, which spins a driveshaft at a high rate of speed. Unlike in a car, push motor motors have no gearbox. Instead, the driveshaft is mounted vertically under the motor and a horizontal rotating blade is attached to the driveshaft via a clutch. Push mowers can use either light two-stroke motors like some motorbikes use, or heavier but cleaner-burning four-stroke motors like the engines used in cars.
A two-stroke engine is called that because it only takes two strokes every combustion cycle. On the up stroke, the piston rises and compresses a mixture of gas, oil and air in an upper chamber. At the same time, the piston creates a vacuum in the crankcase below it, pulling a new mixture of fuel, oil and air into the area beneath it. When the piston reaches the top of its stroke, a spark plug fires, igniting the mixture. This creates an explosion of hot gasses, which pushes the piston down. As the piston descends, it compresses the fresh fuel air mixture below it, propelling that mixture through a valve into the upper chamber. The new fuel mixture pushes out the hot gasses, which leave as exhaust. The piston then starts the cycle again, compressing the new mixture of fuel, oil and air. As it goes up and down, the piston turns a crank, which powers the driveshaft.
Many push mowers use four-stroke engines instead of two-stroke engines. Four-stroke engines are larger and heavier, but are also cleaner, since they do not burn oil. A four-stroke engine has both an intake and exhaust valve on the top of the cylinder. When the piston moves up during its first stroke, it compresses a mixture of gas and air. A spark plug ignites the mixture, driving the piston back down. The piston then travels up again and the exhaust valve opens, venting exhaust gasses out of the engine. Finally, the piston travels down again and the intake valve opens, supplying it with more gasoline and air.