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Difference between Two-Stroke and Four-Stroke Engines

Difference between Two-Stroke and Four-Stroke Engines

4-stroke engine

A 4 stroke engine takes two complete crankshaft revolutions and goes through four stages to complete one power stroke. It is heavy but fuel-efficient and environment-friendly. They operate on the following steps-
1. Intake – the fuel is drawn in from the intake valve with a downward stroke.
2. Compression – the fuel is compressed as the piston moves upward.
3. Power – it is ignited once the fuel is compressed to produce the engine’s power.
4. Exhaust – the exhaust gases exit the 4-stroke engine chamber as the exhaust valve is opened.

The four-stroke engine is the most commonly used, especially the one that explicitly uses gasoline as fuel. Vehicles like cars, trucks, and some motorcycles have four-stroke engines. Other applications are small propeller aircraft, formula one, small motor-powered boats, auto-rickshaw, water spray systems, etc. they all have a four-stroke engine.

2-stroke engine

The 2-stroke engine is more efficient than the 4-stroke engine. It takes two stages and one crankshaft revolution to complete its one cycle. As a result, it works twice as the 4-stroke engine and produces double power with the same fuel input. Moreover, its weight is also less. Its two-step process includes-

1. Upstroke (compression/ignition) – the air and fuel enter as the piston goes up, allowing the engine to take the input. The      air and fuel are mixed and compressed. After that, they are ignited to produce power.
2. Downstroke (exhaust/power) – after producing power, the exhaust opens, allowing the exhaust gases to go out. This is          done when the piston is moved to let the gases escape

Two-stroke engines are used in scooters, motorcycles, military tanks, and also in ship propulsion.

Both the engines have their pros and cons. Stroke is the distance traveled by a piston. The phase of the combustion cycle is represented by the sequential strokes that a piston delivers right from the intake to exhaust. The 4-stroke engine lasts longer and is more efficient than the 2-stroke engine. However, the 2-stroke engine is lighter in weight and works faster than the 4-stroke engine. So which engine a person wants depends upon the type of work they want to do.

Difference between 2-stroke engine and 4-stroke engine

Two stroke engine

Four stroke engine

It undergoes one crankshaft revolution to produce a unit power stroke, i.e., one power stroke per 360 degrees rotation of the crankshaft.

It undergoes two crankshaft revolutions during one power stroke, i.e., one power stroke in every 720 degrees rotation of the crankshaft.

It generates high torque. 

It generates less torque. 

The inlet and outlet of fuel is done through ports. 

The inlet and outlet of fuel is done through valves. 

The thermal efficiency of this engine is less. 

It has a high thermal efficiency. 

It generates more torque at higher rpm. 

It generates low torque at higher rpm. 

The power to weight ratio of this engine is larger as compared to 4-stroke engines. 

The ratio of power to weight is lesser than 2-stroke engines. 

They generate more smoke and are less efficient. 

They generate less smoke and are more efficient. 

They require more lubricating oil as some oil burns with the fuel. 

They require less lubricating oil. 

Due to poor lubrication, more wear and tear occurs. 

Due to proper lubrication, less wear and tear occurs. 

2-stroke engines are cheaper and simpler in manufacturing. 

4-stroke engines are expensive due to heavy lubrication and valves. These are tough to manufacture. 

These are lighter and noisy. 

These are heavier because of the component attached to it, called flywheel. They are 50% less noisy as compared to 2-stroke engines. 

The charge is partially burnt and it gets mixed with the burnt gases during inlet. 

The charge is fully burnt and does not get mixed with the gases inside the cylinder.

Frequently asked questions on 2-stroke and 4-stroke engines

  • Why do we need to change the oil in a 2-stroke engine?

Periodical changing of oil is necessary to cool down the piston and cylinder. Without lubrication, the metal will grate against each other or burn due to enormous heat produced during its functioning. Also, without oil, wear and tear will be more due to high frictional values. This could even seize the engine. Therefore, it is best to keep changing oil for the smooth and proper functioning of the 2-stroke engine.

  • Does a 3-stroke engine exist?

For the functioning of a three-stroke engine, we need suction stroke, compression, expansion, and exhaust stroke to work altogether. However, in a 3-stroke engine, only three of them work, where the last chamber is left unused. Due to this, power cannot be generated. Hence, a 3-strokes engine does not exist.

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