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Basic 2 Cycle Theory

Intake/Compression Stroke

A 2 cycle engine actually accomplishes two things per cycle while running, whereas a 4 cycle completes one task per cycle. This is possible by making use of both the combustion side and the crankcase side of the piston. Following the flow of fuel, we’ll start with the engine intake from the carburetor. As the crankshaft rotates, the piston is pushed upwards. This does two things. One, the rising piston generates a vacuum in the sealed crankcase which pulls a fresh air/fuel charge from the carburetor into the crankcase (thus the reason for having to add lubricating oil to the gas for 2 cycle engines). A small reed valve (a one way valve) allows the mixture to enter the crankcase, but not leave by the same path. At the same time, the rising piston is also compressing the previous charge in the combustion chamber. As the piston nears the top of the stroke, the ignition system ignites the charge in the combustion chamber.

Intake/Compression Stroke

Once the air/fuel charge is ignited, the pressures generated by the burning mixture forces the piston down, again accomplishing two things. The downward motion of the piston turns the crankshaft, providing power output. In addition, the downward motion is pressurizing the air/fuel mixture previously loaded in the crankcase. As the piston continues it’s travel down the cylinder bore, it will first expose the exhaust port in one side of the cylinder. The spent gases from the burning mixture are then released to the exhaust system. A small amount of further piston travel down the cylinder now exposes the intake port. At this point, the now pressurized mixture in the crankcase is released into the combustion chamber through the intake port. A piston dome design, unique to 2 cycle engines, is used to help ‘sweep’ the chamber clean of spent exhaust gases by directing the incoming rush of air/fuel up towards the cylinder head and then down towards the exhaust port. As the crankshaft continues it’s rotation, the piston will eventually reach the bottom of it’s stroke and return to the top, covering the intake and exhaust ports in the process, and repeat the Intake/Compression cycle. It can be seen by the overall design that, unless the reed valve is mechanically operated, i.e. is actually a rotary valve used to accomplish the same one-way function, that the crankshaft can rotate in either direction making no difference in how the engine runs. Also notable, is the fact that since the piston itself controls the flow of air/fuel/exhaust by way of the intake & exhaust ports, that no valve train or cam is required.

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