Two-stroke engine
A two-stroke engine is a type of internal combustion engine where the fuel is brought into the combustion chamber, ignited, and the exhaust is expelled in two strokes. These engines do not have separate oil systems and they burn a mixture of fuel and motor oil. As a result they generate much more air pollution than equivalent four-stroke engines. They also tend to be noisier. One advantage is that they can be made lightweight, but this has diminished over the years as four-stroke designs have improved.