Engine Operation - Two-Stroke Cycle

Wheeled Vehicle Engines
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The exhaust port is located slightly higher in the cylinder than the intake port. The intake port is connected to

the crankcase by an internal passageway in the engine block.

Most two-cycle engines in use today have a fuel-air mixture intake valve located in a mixture inlet into the

crankcase. The intake valve or reed valve, as it is sometimes called, will allow mixture to flow into the

crankcase. However, if the mixture tries to flow in the opposite direction, the valve closes and seals the

crankcase.

Because the two-stroke-cycle engine needs less parts, it can be made lighter and smaller than the four-stroke-

cycle engine. Therefore, two-stroke-cycle engines are very common where light weight and small size are

needed; such as on chain saws, outboard motors, and lawnmowers.

Engine Operation - Two-Stroke Cycle

Now let's take a look at how the two-stroke-cycle engine operates. Imagine that the cylinder is already full of

fuel-air mixture and the piston is at the bottom of the cylinder. The engine is cranked; first, the piston moves

up past the intake and exhaust ports to seal the cylinder, then it begins to compress the mixture. At the same

time, the rising piston increases the space inside the crankcase creating low pressure or suction in that area.

This low pressure draws fresh fuel-air mixture past the intake valve into the crankcase.