Lesson 1/Learning Event 1
THE FOUR-STROKE CYCLE
The simple engine that we have just built operates on the same principles that Dr. Otto, a German scientist,
used in 1876. Common names that engines are called when using these principles are Otto cycle, four-cycle, and
four-stroke-cycle engines. We will call them four-stroke-cycle engines in this lesson.
The term "four-stroke cycle" refers to the number of times the piston moves up and down between power
(combustion) strokes. The four strokes are called intake, compression, power, and exhaust. We will discuss
them in the order in which they were just named. But before we do, let's define some common terms that will
be used often.
Cranking. The engine is not self-starting. The piston has to be moved in the cylinder by some outer force to
get it started. This is called cranking and is done in a lot of different ways. For example, to crank a lawnmower
you pull on a rope that is wrapped around a pulley which spins the crankshaft. On a modern car, the engine is
cranked by an electric starter motor that turns the crankshaft.