No matter the size of your gas-powered lawn mower engine, the combustion chamber produces high temperatures. Those hot gases are what drive the engine to rotate the cutting blade on push mowers, and power the engine on riding lawn mowers. To relieve air pressure, lawn mowers are equipped with breather systems.
To understand how lawn mower breathers work, you first must understand the combustion system. The carburetor mixes fuel and air to create an ignitable vapor. That vapor is released into the combustion chamber. Once the spark plug releases an electrical discharge, the vapor ignites and pushes the pistons. The pistons generate power by pushing down to compress the hot air, then pressing up to release that air. When the piston goes down again, the spent hot gases are released into the breather system.
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The breather tube is the key component of the breather system in a lawn mower. Instead of allowing the hot gases to move freely, the breather tube directs that air toward the exhaust system and muffler. Along with the oil, this system provides cooling for the engine. Breather tubes also separate the passing oil by redirecting it back to the crankcase.
Depending on the engine model, there are additional parts to the breather system. The positive crankcase ventilation (PCV) valve is a one-way valve that helps redirect hot gases to the exhaust system as well as clean the emissions by trapping oil mist. A breather reed separates the air from the oil. Air fins provide additional cooling by directing the cool air in or redirecting hot air out, depending on their locations. The air filter helps reduce contamination into the combustion chamber.
There are problems that can occur if the breather system isn't working properly. A clogged air filter or breather tube can pressurize the gases in the combustion chamber, causing gaskets and seals to blow. This results in an oil leak that can pass into the exhaust system and cause heavy smoking. If the breather reed breaks, oil does not pass back into the crankcase. The excess oil causes overheating or smoking if it leaks into the combustion chamber.