Peter Hunn - Small Engines and Outdoor Power Equipment: A Care & Repair Guide for: Lawn Mowers, Snowblowers & Small Gas-Powered Implements

Except for most chain saws, string trimmers, leaf blowers, and light-duty snowblowers that use 2-stroke (also known as 2-cycle) engines (see 2-Stroke Power Equipment on ), the majority of small power plants are of the 4-stroke (or 4-cycle) variety. These diminutive 4-strokes follow the same essential operating principles as automotive engines. To use a bakery metaphor, the little mills on lawn mowers would be considered cupcakes compared to the far-larger, multitiered confection created for a fancy wedding reception. No matter the intended application, however, here, in its simplest form, is how a 4-stroke engine works:

When you pull the rope, known as a rewind cord, or use your electric starter, precise amounts of fuel and filtered air mix together in the carburetor. The mixture rushes into the engine to be compressed, ignited, and burned in a controlled process known as internal combustion. This produces hot gases. As these gases expand, they push a smooth, well-lubricated cylindrical component, known as the piston. The piston, in turn, drives the crankshaft, the arm that spins a blade or performs other work. Moving valves on a 4-stroke engine (and stationary ports in the cylinder wall on a 2-stroke motor) let air and fuel into the combustion chamber above the piston and allow spent gases to exit through the muffler. Pulling vigorously on that cord or turning the starter key gets this process moving. When it works correctly, it coaxes the fuel-air atomization, fuel-air compression, ignition/spark, and exhaust of burned gases to work on its own. It is designed to become self-sustaining from the time the engine starts until the moment it stops. Timed electrical surges cause the spark plug to fire repeatedly inside the combustion chamber, igniting each fresh supply of air and fuel and producing gases that continually drive the piston and crankshaft. All the while, oil from the crankcase and air circulate to keep engine temperatures within an acceptable range, and a governor monitors changes in the workload and adjusts engine speed accordingly. Theres a lot being asked of a small engine when the starter is suddenly engaged. Thats why an engine might sound like its taking a second or two to fully come to life, the way somebody might when roused out of a warm, comfortable bed when the clock radio alarm fires up on a winters morning. Two-cycle engines complete this wake-up routine in two piston strokes (one up, one down, or one 360-degree rotation of the crankshaft). For a 4-cycle engine, the whole process involves four motions of piston travel (two up strokes and two down strokes, or two full crankshaft rotations.