How 2 Cycle Engines Work

All piston engines have the same basic operating principles or cycles: intake, compression, combustion and exhaust.   A two-stroke engine combines those cycles into pairs: intake and compression, and combustion and exhaust.

Two things you need to know:

• A stroke is the movement of the piston either up or down the cylinder.
• There are three ports: exhaust, transfer and intake.   The exhaust port is slightly higher than the transfer port, and both are near the bottom of the cylinder.   The intake port is in the crankcase.

As the piston rises in the cylinder, the air/fuel mixture is compressed.   While this is happening, the bottom of the piston is leaving the crankcase; this lowers the pressure in it allowing the air/fuel mixture to enter through the intake port.

Near the top of the piston's upward stroke, the spark plug fires causing the air/fuel mixture to explode and propel the piston down the cylinder.   This provides power by spinning the crankshaft.

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On its way down, the piston passes the exhaust port allowing the hot, expanding gasses to vent.    As the piston descends, the intake port is closed allowing pressure to build up in the crankcase.   The piston now passes the transfer port allowing the pressurized fuel/air mixture to enter the cylinder.

The piston's shape directs the fresh fuel/air mixture to the top of the cylinder aiding in removing the last of the exhaust gasses.

The piston again begins its upward travel, sealing off the exhaust and transfer ports and opening the intake port.

Now as you can imagine, fuel and air flowing through the crankcase, where all the bearings for the moving parts are, would introduce a bit of a lubrication problem.   This is why all two stroke engines require adding special oil to the fuel - the air fuel mixture is also lubricating the motor!

Two stroke engines are more powerful and responsive pound for pound than their four stroke brothers.   They produce power every time they are on the down stroke, whereas a four stroke only produces power every second down stroke.   Two strokes also have fewer parts, not requiring valves and valve train.