Lawn mowers that run on internal combustion engines burn gasoline as fuel, and like any engine with metal parts, your lawn mower requires oil to lubricate the pistons and other moving parts. The oil reduces friction between moving metal parts and protects the life of your engine. But unlike an automobile or other larger engines, in a lawn mower there are two ways that engine oil can be delivered.
Two-Stroke vs. Four-Stroke Engines
Lawn mower engines come in two different basic designs: two-stroke and four-stroke engines.
In a two-stroke engine, the complete combustion cycle of gasoline vapors being compressed, ignited, and exhausted is completed in just one back-and-forth movement of the piston. A compression stroke in which the piston squeezed the gas vapor happens at the same time the spark ignites the mixture. During the return stroke of the piston, the exhaust gases are let out of the piston, and fresh fuel vapor mixture enters the cylinder. Two-stroke engines require that you mix the oil right in with the gasoline; there is not oil crankcase to fill or maintain.
In a four-stroke engine, the piston goes through two complete cycles during each revolution of the engine:
- During the first stroke, which is the intake stroke, an air-fuel mixture is drawn into the engine cylinder.
- For the second stroke, known as the compression stroke, the piston compresses the fuel-air mixture. At the point of full compression, the spark plug ignites the mixture in the cylinder.
- The third stroke is the power stroke, in which the exploding fuel mixture powers the piston down to the bottom of the cylinder.
- The fourth stroke is the exhaust stroke, in which the piston moves back inside the piston to the same position as the ignition stroke, but this time valves open up to allow exhaust fumes to exit the cylinder.
Four-stroke engines require a reservoir for engine oil to lubricate the connecting rods and other engine parts.
Pound for pound, two-stroke engines have more power than four-stroke engines, since there is a power stroke on every revolution of the engine. They can, therefore, be lighter than four-stroke engines to deliver the same amount of power. However, four-stroke engines are gradually becoming more popular, due largely to the fact that two-stroke engines burn a small amount of oil inside the cylinders with each ignition revolution. In interests of minimizing pollution emissions, manufacturers are gradually shifting to four-stroke engine designs.
Therefore, checking, adding, and changing oil is done only on a four-stroke lawn mower engine. With two-stroke engines, providing motor lubrication is done by properly mixing two-stroke oil with the gasoline used to run the mower.
Four-stroke lawn mower engines come in several variations. Simple, inexpensive mowers may not even have a dipstick to check the oil level; you just need to eyeball the level in the crankcase when you remove the filling plug. Elaborate, expensive lawnmowers or riding mowers may more closely resemble motorcycle engines, complete with oil-level dipstick and oil filters. Check your manufacturer’s owner manual to learn the details of how to check oil levels and how to add or change the oil in the crankcase.
Why Oil Level Matters for Lawn Mowers
You have to be very careful when filling an engine with oil, whether in the process of changing the oil altogether or simply adding oil. Unfortunately, getting the level wrong (whether it be too much or too little) can have seriously bad consequences.
A few things can go wrong when you underfill the engine:
- The engine can overheat, causing damage to the piston and cylinder walls.
- The engine can seize up since there is no longer any lubrication between the parts. An engine that seizes will require serious rebuilding, and there’s a chance it can’t be saved at all.
But overfilling the engine can also cause problems. The excess oil has to go somewhere. It will inevitably end up seeping into parts where it does not belong, potentially causing them to malfunction.
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