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How Chain Saws Work


The Basics

Here is a basic 16-inch chain saw:

This chain saw has the following features:

  • It has a 16-inch (40.6-cm) cutting bar. The chain runs in a groove around this bar, and on the chain are the cutting teeth.
  • It has an air-cooled two-stroke gasoline engine.
  • The chain is driven by a centrifugal clutch. When the engine is idling, the clutch is disengaged. When the engine speeds up, the centrifugal plates in the clutch spin outward to engage the clutch, and the chain begins running.
  • The engine displaces 3 cubic inches (49 cc).
  • The engine, consisting of the cylinder, piston, connecting rod, crankshaft, carburetor and magneto, weighs just 4 pounds (1.8 kg).
  • The engine develops approximately 3 horsepower.
  • The engine is started with a pull starter.
  • The ignition is powered by a magneto connected to a spark plug.
  • Gas/oil/air are mixed with a carburetor and drawn into the engine by crankcase vacuum.
  • Exhaust flows out through a simple spark arrestor.
  • There is a small oil pump and a separate chain-oil reservoir to oil the chain and keep it from binding on the bar.
  • The gas tank holds about 1 pint (0.5 liters) of gasoline mixed with two-stroke engine oil.

One of the most amazing things about the engine is the horsepower that it develops for its weight. It approaches 1 horsepower per pound of engine weight! This is possible because the engine is air-cooled (eliminating the weight of the radiator, water pump and water) and two-stroke. A two-stroke engine, as explained in How Two-stroke Engines Work, has no valves or cams and generates power on every rotation of the crankshaft (twice as often as a four-stroke engine). This feature of the two-stroke engine means that it can generate twice the power of a four-stroke engine of the same displacement.