The two stroke engine

The two stroke internal combustion engine is an engine which completes a working cycle in two strokes. The crankshaft rotates one circle, and the two stroke engine outputs external kinetic energy once.

Two stroke engine

We will focus on these aspects:

brief introduction

First stroke: the piston completes from the top dead center to the bottom dead end, and completes the two movement.

  •  Spark plug ignition, work and exhaust
  • Close the intake port, press the mixture from the bottom of the piston into the crankcase, and then enter the cylinder from the upper part of the piston.

The second stroke: the piston moves from the bottom dead center to the top dead center. Complete two movements.

  • Close all intake ports and vent holes.
  • Compress the mixture.

To complete a cycle of work, the piston needs two strokes, so it is called the two stroke engine.

Working principle

There are three blowholes on the engine cylinder, that is, air intake, exhaust holes and air holes.

The three holes are opened or closed by pistons at a certain time. The work cycle contains two step:

Up stroke

1. first stroke:

The piston moves upward from the bottom dead center.

After the three holes are closed, the mixture entering the cylinder is compressed.

When the intake hole is exposed, the combustible mixture flows into the crankcase.

2. The second stroke:

When the piston is compressed near the top dead center, the spark plug ignites the combustible mixture, and the gas expansion in the cylinder drives the piston down to work.

Down stroke

When the inlet air hole is closed, the combustible mixture enclosed in the crankcase is compressed.

When the piston approaches the bottom dead center, the exhaust port opens and the exhaust gas rushing out.

Then the air vent opens, and the pre-compressed combustible mixture rushes into the cylinder to drive out the exhaust gas and perform the air exchange process.


  • The two stroke engine has no valves, which greatly simplifies their structure and reduces their own weight.
  • The two-stroke engine ignites every turn, while the four-stroke engine ignites every other turn. This gives the two stroke engine important power base.
  • The two stroke engine can operate in any direction, which is very important in some equipment such as chain saws.

The standard four stroke engine may fail when the fuel is shaking, unless it is upright. Solving this problem will greatly increase the flexibility of the engine.

These advantages make the two stroke engine lighter, simpler and cheaper to manufacture.

A two-stroke engine also has the potential to double its power with the same space, because it has a double power stroke per turn.

The combination of lightness and double power makes it an amazing “thrust-to-weight ratio” compared with many four-stroke engines.


  • The two stroke engine can’t last as long as a four stroke engine. The lack of precision lubrication system means that the components of the two stroke engine consume more quickly.
  • Two stroke lubricating oil is very expensive. You need four ounces of lubricant for every gallon of petrol. If you use a two-stroke engine in a car, you burn off a gallon of oil every thousand miles.
  • The fuel consumption rate of the two stroke engine is high. With a low
    engine efficiency, you can’t run a few miles per gallon.
  • Two stroke engines cause a lot of pollution, so much that you may not see pollution around you. Pollution comes from two aspects.

The first is the burning of lubricating oil.

To some extent, lubricants diffuse smoke from all two-stroke engines, and a heavily worn two-stroke engine releases large masses of oily smoke.

The second reason is not very obvious.

When a lot of new air / fuel is injected into the combustion chamber, some of them leak out from the vent. That’s why you see luster oil around any two stroke motorcycle. The release of hydrocarbons from new fuels mixed with leaking lubricants poses a major environmental problem.

These deficiencies mean that two-stroke engines can only be used in situations where motors are infrequently used and where unit efficiency is important.