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Introduction-How a Two Stroke Engine Works

The two-cycle engine is used almost exclusively in hand held gasoline power equipment. The DH65 saw is of typical design, yet incorporates unique features which maximize performance to meet the heavy demands of the construction trade.

The carburetor performs the same function as with other engines. It mixes the fuel with air in a fixed ratio to assure the most efficient combustion of the fuel. The fuel vapor travels through the crankcase on its way to the combustion chamber and provides a cooling effect as well as critical lubrication to the engine components. Oil is mixed with the gas to provide this lubrication.

□ 1 The fuel vapor is drawn from the carburetor through the reed valve. Negative pressure created inside the crankcase is the drawing force that opens the reed valve. The negative pressure, or vacuum, is created in the crankcase by two actions: the upward movement of the piston, and the scooping action of the crankshaft counterweight. 

□ 2 At or near TDC (Top Dead Center), the ignition module provides the current (spark) to the spark plug. Starting combustion. The burning and expanding gases push the piston down, providing the power “stroke” needed to turn the crankshaft.

□ 3 As the piston moves downward, the exhaust port is exposed by the descending piston, scavenging (releasing) the hot expanded gases (know as exhaust) out through the muffler.

□ 4 As the downward movement of the piston continues, the intake port is exposed, allowing new fuel vapor to enter the combustion chamber. Positive pressure, created in the crankcase by the downward movement of the piston, forces the vapor into the combustion chamber. Note that this positive pressure in the crankcase also closes the reed valve.

As the piston begins its upward motion, beginning the compression part of the cycle, the intake port is closed before the exhaust port. Because of this, some unspent fuel vapor is expelled through the exhaust port.

The cycle is now complete, and starts over again. Since complete engine cycle takes two “strokes” of the piston, this type of engine is known as “two stroke” or “two cycle”. 


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