Lesson 1/Learning Event 1
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
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.