A two-stroke engine is the little brother of the four-stroke engine. Whereas in a four-stroke engine the spark plug arcs every other revolution, a two-stroke engine produces power from every revolution.

Start with the cylinder at the bottom of the stroke. Positive pressure in the area surrounding the cylinder (the "crankcase") forces a fuel/air/oil mixture into the cylinder. Note there are no valves in this type of engine. The piston drives upward, and the spark plug arcs, as in a four-stroke engine. In the design of the cylinder, however, the exhaust port is higher up in the cylinder than the injection port. So, as the piston is pushed down, it uncovers the exhaust port; the pressure inside the cylinder expels most of the exhaust. Also, the downward motion of the piston pressurizes the crankcase, which pushes fuel into the cylinder once the injection port is uncovered. The cycle then stars over again.

Two-stroke engines have some major advantages. First, they are very simple and lightweight, which makes them cheap to produce and ideal for small devices, such as lawnmowers, gas-powered trimmers, go-karts, and jet-skis. Also, because fuel is ignited every revolution, rather than every other revolution, the two-stroke engine produces much more power.

Unfortunately, there are some very large disadvantages to the two-stroke engine. In other types of engines, the crankcase is filled with oil for the purposes of cooling and lubrication. However, because the crankcase is being used to pressurize the fuel mixture, oil cannot be added there. Instead, it is mixed with the fuel. Two-stroke oil can be very expensive, and because it is ignited with everything else, produces a lot of pollution. Also, the lubrication system is not as effective with the oil mixed with the fuel, so two-stroke engines do not last very long.

Fuel efficiency is also a problem. In addition to other things, the lack of valves in the cylinder means that, during the compression stage, some of the mixture leaks out the exhaust port. This decreases efficiency and contributes to pollution.