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Frequently Asked Questions

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Compression System

What components are included in the compression system? How does it function?

Compression System

The inventors of the first internal combustion engines discovered that fuel burns more efficiently if you compress it in a sealed chamber before burning it. Compression of the air-fuel mixture in the small engine begins as the intake valve closes. The trapped vapors are pushed toward the cylinder head by the piston and compressed into a space about one-sixth their original volume. The exact amount of compression is an indicator of an engine's efficiency. That's why a tightly sealed combustion chamber is so important for good engine performance.


Valves located in the combustion chamber let fuel vapors and air enter the cylinder and let exhaust gases exit at precisely timed intervals. A typical four-stoke cycle small engine contains one intake valve and one exhaust valve per cylinder.

Most small engines have one cylinder and use an L-head (or flathead) design. The valves are installed in a valve chamber next to the piston. Overhead valve (OHV) designs offer greater efficiency, however, and are increasingly popular with consumers. The valves are located in the cylinder head directly in line with the piston and are moved by pivoting rocker arms.


The piston rides through the cylinder, much as a plunger rides through the chamber in a hand operated air pump. At the appropriate moment, the cylinder is sealed so that the air-fuel mixture is compressed as the piston moves toward the cylinder head. When the mixture is ignited, rapidly expanding gases force the piston back down through the cylinder.


The piston diameter is narrow enough to permit a thin space around it for a coating of oil. Flexible piston rings, installed in grooves in the piston, work in concert with the oil to create a seal between the piston and the cylinder wall. This ensures good compression.

As the piston is pushed down through the cylinder by expanding gases, a connecting rod transfers the force of those gases to the flywheel. It's the flywheel's momentum that perpetuates the engine's four-stroke cycle.

Compression Release System

Some small engines incorporate a compression release system to decrease operator effort when pulling a rewind starter. A compression release system is a system that relieves excess pressure during the compression event by allowing a small amount of compressed gas to be released through the muffler or carburetor.

The compression release system lifts either the exhaust or the intake valve slightly off its seat during the compression event (this is why the piston is positioned past top dead center to about 1/4" down from the top of the cylinder when adjusting valve clearances). This releases pressure and reduces the force required to pull the starter rope or load on the starter motor. A compression release is used on most Briggs & Stratton engines and does not affect engine performance above engine starting rpm.

Compression Problems

Too little or too much compression can damage pistons, rings, valves, valve guides, valve seats and the cylinder wall. If an exhaust valve leaks, exhaust can back up into the cylinder, causing premature wear. Too much compression can cause the air-fuel mixture to burn too fast, causing knocking or pinging. Excess compression can leave carbon deposits that further aggravate problems.

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