Know its difference: 2Stroke and 4Stroke engines

Both gasoline and diesel engines are available in 2 stroke or 4 stroke versions. Both versions convert chemical energy into mechanical energy. They differ in the way the engine works and the construction of the engine itself.

Let us understand their difference

Four stroke engine cycles:

(We will consider a 4 stroke petrol engine for discussion. To know the difference between how petrol and diesel engine works click here)

1. Intake or suction stroke:

The piston moves down and the inlet valve opens to allow air-fuel mixture into the combustion chamber.

2. Compression stroke:

The piston now moves up compressing the air-fuel mixture, increasing their temperature considerably.

3. Combustion or power stroke:

When the piston reaches the top of the cylinder, spark plug ignites the mixture and combustion takes place. The expansion of the mixture moves the piston downwards rotating the crank shaft.

4. Exhaust stroke:    

The piston once again moves up pushing the remains of the combustion through the exhaust valve.

Two stroke and 4 stroke petrol engines

Now let us see how a two stroke engine works

Two stroke engine cycles:

As the name suggests, it has only two strokes

1. Compression stroke:

The piston moves up compressing the air-fuel mixture.

2. Power stroke:

Fuel is ignited and the piston moves down. The moving piston not only turns the crank shaft but also pressurizes the mixture inside the crank case. When the piston reaches near the bottom, outlet opens and exhaust gases escape. When the piston moves even near the bottom inlet opens and lets the pressurized mixture inside. The entering mixture pushes the remaining exhaust gases out. As you can see for sometime both outlet and inlet are open and there is a chance for the fresh charge entering the cylinder to escape without combustion. This increases the pollution level and also decreases the fuel efficiency.

A two stroke engine fires every revolution whereas a four stroke engine fires every other revolution, thus producing twice the power as a four stroke engine for the same cycle.

Since the air-fuel mixture enters the crankcase, in order to keep it clean and lubricated, special oil needs to be circulated through the crank case.

Unlike a four stroke engine, two stroke engines does not have valves, instead has ports. This considerably reduces the number of parts inside the engine.

Most cars use only four stroke engines because of the disadvantages a two stroke engine presents (as discussed earlier). But they find usage in some motor bikes, lawn movers etc.

Difference between 2 stroke and 4 stroke engines

• Two stroke engines produce twice as much power as a four stroke engine for the same cycle.
• Two stroke engines have ports instead of valves.
• 2 engines have fewer parts compared to a 4 stroke engine.
• 2 stroke engines are much polluting.
• Fuel efficiency is less for 2 stroke engine.

The above images were obtained from http://commons.wikipedia.org