Difference Between Two Stroke and Four Stroke Engines

In the context of an internal combustion engine, the stroke is defined as the process in which the cylinder’s piston moves up to the top and down. 

The cycle starts with the intake stroke in which a fresh mixture of air-fuel enters the engine’s cylinder by the downward expanding motion of the piston. 

Stroke is the length of the distance traveled by the piston that means it is a factor used in calculating the engine displacement. The phase of the combustion cycle is designated by the sequential strokes of the piston starting from intake, compression, expansion to the exhaust. 

2 Stroke Engine

The two stroke engine finishes two piston movements(one crankshaft revolution) in order to yield power.

The engine is able to create power after one cycle because the intake of gas inside the cylinder and the exhaust of the residual gases occur simultaneously.

There is a valve for the intake stroke that opens and closes due to varying pressures.

However, due to its prevalent contact with moving parts, the fuel is blended with oil to add lubrication, allowing seamless strokes.

What is Four Stroke Engine?

A four stroke engine has four sequential steps starting from intake, compression, power, to exhaust. Each equates to one full stroke of the piston. Therefore, it supplies one power stroke for every two cycles of the piston (or four piston strokes) and a complete cycle requires two revolutions of the crankshaft. 

The 4 stroke engine has four steps that are described below

Step1: Intake Stroke

It draws an air-fuel mixture into the combustion chamber. The piston descends in the cylinder bore to empty the combustion chamber. When the inlet valve opens, atmospheric pressure pushes the air-fuel charge into the evacuated chamber.

Step2: Combustion Stroke

The cylinder is completely filled with the maximum mixture and the intake valve seals the mixture and the piston moves upward. 

The compression occurs between the piston and the cylinder head.

Step3: Power Stroke

After the completion of the compression stroke, the spark ignites the air-fuel mixture and forces the piston back down the cylinder bore generates torque in the crankshaft.

The pressure on the piston determines the amount of torque generated.

Step4: Exhaust Stroke

The exhaust stroke occurs when residual gases expelled from the combustion chamber are liberated into the atmosphere. The exhaust stroke is the concluding stroke and takes place when the exhaust valve is open and the intake valve is closed. Piston movement liberates exhaust gases into the atmosphere.

Applications of 4 Stroke Engine

The four-stroke engine is the most commonly used engine at present, especially the one that specifically 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.

Difference Between 2 Stroke and 4 Stroke Engine

2 Stroke

4 Stroke

It can generate one revolution of the crankshaft within one power stroke, i.e.one power stroke per 360 degrees rotation of the crankshaft.

It can generate two revolutions of the crankshaft between one power stroke i.e., one power stroke in every 720 degrees rotation of the crankshaft.

Uses port for inlet and outlet of fuel.

Uses valve for inlet and outlet.

It requires a lighter flywheel to cause a more balanced force due to one revolution for one power stroke.

It requires heavy flywheel because it gives rise to unbalanced forces due to two revolutions for one power stroke.

Cheaper in price as they require less effort in manufacturing and are light by weight.

Hard to manufacture due to the heavy flywheel and valve mechanism and are expensive due to the valve and lubrication mechanism.

Generates more torque at a higher rpm. 

Generates a higher torque at a lower rpm.

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

The charge is fully burnt and doesn’t get mixed with the gases inside the cylinder.

More power generation.

Less power generation.

Produces more heat so it requires greater cooling and lubrication.

Generates less heat.

Lower thermal efficiency 

Higher thermal efficiency

Requires more lubrication while functioning as some oil burns with the fuel as due to poor lubrication more wear and tear occurs

Comparatively smooth at functioning and requires less lubrication (less wear and tear occurs).

Generates a lot of smoke due to poor efficiency.

Generates less smoke due to greater efficiency.

Lower volumetric efficiency as the time of induction of charge is less.

High volumetric efficiency due to a greater time for induction of charge.

Very noisy.

Produces less noise.

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

Four stroke engines are used in cars, motorboats, modern bikes, aircraft.

FAQ (Frequently Asked Questions)

1. Why does oil in a 2 stroke engine need to change?

Ans: It is necessary to change the oil because the oil helps cool the piston and cylinder by keeping them evenly lubricated. Without lubrication, the metals can melt and potentially grate against each other, transferring metal to and from one another and permanently distorting them causing wear and tear of the components due to high friction This could seize the engine. Therefore, the better option is to keep changing oil for the smooth functioning of the engine.

2. Why don't we have a three stroke engine?

Ans: You need to perform four important operations of the suction stroke, compression stroke, expansion stroke, and exhaust stroke to generate work from an engine. For a 3 stroke engine, you need to choose only three of them leaving any one stroke. Practically it is not possible to generate power by executing the 4 operations in only 3 strokes.

3. Is 2 stroke or 4 stroke quieter?

Ans:  At present, four strokes remain up to 50% quieter than most two stroke engines and they are more efficient than two stroke engines.