Two Stroke Engines and Four Stroke Engines

You should be aware that there are two basic types of gasoline engines: the two-stroke engine and the four-stroke engine. Each engine type is classified according to the number of strokes its piston takes to complete one full engine cycle. (Remember that a piston stroke is the total distance that the piston moves from the top of the cylinder to the bottom of the cylinder. An engine cycle is a complete run through all four stages of operation.)

In order for any engine to operate, it must run through all four stages of operation. However, two-stroke engines and four-stroke engines accomplish this in different ways. In a two-stroke engine, the piston takes only two strokes to complete one full engine cycle of intake, compression, power, and exhaust. In a four-stroke engine, the piston requires four strokes to complete one full engine cycle.

Virtually all automobiles today contain four-stroke engines. In a fourstroke engine, the four strokes of the piston that are required to complete the four stages of operation are as follows:

Piston stroke 1: Intake stage

Piston stroke 2: Compression stage

Piston stroke 3: Power stage

Piston stroke 4: Exhaust stage

After the completion of the fourth piston stroke, the engine has gone through all four stages of operation, and the cycle will begin again.

In contrast, in a two-stroke engine, only two strokes of the piston are needed to complete the four stages of operation, as follows:

Piston stroke 1: Intake stage/Compression stage

Piston stroke 2: Power stage/Exhaust stage

So, in a two-stroke engine, the intake and compression stages are combined and completed by one piston stroke. The power and exhaust stages are combined and completed by a second piston stroke. Note that the two-stroke engine must still complete all four stages of operation it just does this in two piston strokes instead of four.

Two-stroke engines aren't used in modern automobiles, but they're commonly seen in motorcycles. The reason for this is that two-stroke engines aren't very fuel efficient and tend to produce more pollution than four-stroke engines. You should be aware that two-stroke engines exist, but we won't cover them in this program. You'll be concentrating all your study efforts on four-stroke engines.