How Do Self-Propelled Lawn Mowers Work?

The Combustion Process

• The internal combustion engine on a self-propelled lawn mower works through a piston and crankcase system similar to an automobile's. When fuel gets ignited in the cylinder, the force of the explosion drives the piston down, which turns the crankcase. As the crankcase moves, it helps generate the compression needed for the fuel pump in the carburetor to suck more fuel in. When more fuel reaches the carburetor, it gets diluted with air and pushed off to the cylinder, starting the whole process again.

Starting it Up

• However, for the combustion process to start on a self-propelled lawn mower, the operator needs to manually crank the engine to get the internal processes moving. The recoil starter, that familiar pull rope, is pulled and helps spin the recoil rotor. The recoil rotor connects to the flywheel, which, with the power of magnetic energy, helps build the spark inside the ignition coil. Once the charge reaches full power, the spark travels up the lead wire to the end of the spark plug, igniting the fuel.

The Cutting

• The cutting blades on a self-propelled lawn mower are usually attached to the drive shaft, which is in turn connected to the crankshaft. As the crankshaft turns, it also sends power to the cutting blades, allowing the mower to cut grass. The engine runs around 3,500 rpm when the blades aren't cutting grass, and drop down to around 3,000 rpm when the blades start cutting grass. The blades on a self-propelled mower usually run from 18 to 22 inches in length.

The Drive System

• At the base of the engine, also near the crankcase, a system of two pulleys and a belt sit. This belt also runs off the spinning of the crankshaft along with the first pulley. The second pulley usually sits nearer to the front wheels and a pinion gear inside the pulley connects the pulley to the front wheels. When the drive lever gets pushed down, a spring helps tighten the belt onto the pulley. Once the belt gets tightened onto the pulley it will also help power the second pulley and the mower will start moving forward.