How Gas-Powered Self-Propelled Lawnmowers Work

Starting System

• The ignition system on a self-propelled lawnmower uses magnetic energy to gather and build a high voltage spark. Most self-propelled lawnmowers are equipped with a recoil starter system, where the familiar cranking on the starter rope actually allows the operator to mimic the normal movements of the engine. As the rope pulls out, the recoil rotor spins the flywheel; and as the flywheel spins, it helps build magnetic energy in the ignition coil. Once the flywheel spins fast enough, the energy is released up the lead wire to the spark plug, where it ignites the fuel.

Engine Basics

• After combustion takes place, the engine does the rest on its own. When the fuel ignites in the cylinder, the explosion drives the piston downward, pushing the exhaust out and turning the crankshaft. As the crankshaft spins, it pushes the piston back up, and this pulsing also drives the diaphragm in the carburetor's fuel pump, pulling more fuel into the carburetor. After fuel reaches the carburetor, it gets mixed with air and pushed into the cylinder, where it gets ignited, and the whole process begins again.

Cutting

• Once combustion gets going, it will power a drive shaft that starts spinning the lawnmower blades, allowing the mower to cut grass. Most self-propelled mowers will run around 2,800 to 3,300 rpm when the blades are engaged. Blades range in length from 18 to 22 inches on most self-propelled lawnmowers. However, as the blades get longer, their cutting surface also speeds up which, ultimately, requires more power from the engine to spin the blades.

Drive System

• At the end of the crankshaft, near the base of the mower, generally, two pulleys connect the crankshaft and the wheels via a drive belt. As the crankshaft starts spinning, it turns the pulley which, in turn, spins the belt and the other pulley. This second pulley often has a pinion gear in its inner ring that connects to the front wheels of the mower. So, when you press down on the drive lever, it puts tension on the belt, tightening it against the pulleys and causing the lawnmower to move forward.