Comparison Between 2-Stroke & 4-Stroke Engine

On the basis of-

I. Working principle

II. Engine Design

III. Engine Performance

IV. Cooling and Lubrication System

Comparison On the basis of-working principle

4-Stroke Engine Following are the four stroke:-

1. Intake/Suction Stroke

2. Compression Stroke

3. Expansion Stroke

4. Exhaust Stroke

1 – Intake stroke

In suction stroke piston starts at Top Dead Center (TDC) of the cylinder and moves to the Bottom Dead Center (BDC). Outlet valve will be closedand inlet valve will be open to allowing the fresh charge of mixed fuel & air into the cylinder.

2 – Compression stroke

In compression stroke,Once piston reaches BDC & moves back TDC,inlet valve will be closed,As the piston moves towards TDC,It compress air fuel mixture inside the cylinder & compression takes place,Hence it is called compression stroke.

3 – Expansion stroke

In expansion stroke,Both the valves are closed,When piston reaches top of its stroke the fuel mixture is ignited by spark plug due to spark high temperature & pressure generated inside the cylinder & push down the piston to BDC,Hence it is known as expansion stroke.

4 – Exhaust stroke

In this stroke exhaust valve is opened,when piston reaches to BDC & moves to upward.

Piston pushes out the burnt gases to the atmosphere through the exhaust valve. Hence called exhaust stroke & the engine is ready to begin the cycle again.

Working principle of two Stroke Engine

Two stroke engine is widely used employed where small power required for motor cycle like auto rickshaw, scooter.

Compression Stroke

When piston moves from BDC to TDC,it first closes the transfer port & then exhaust port,The charge of air-fuel mixture in the chamber is pressurised & creates high temperature. At the same time,

Compression Stroke

a vacuum is developing in the crankcase,so the fresh charge of fuel mixture is being drawn into the crankcase from the carburettor. As the piston moves towards TDC ,a high voltage discharge from the spark plug & ignites the air-fuel mixture in the combustion chamber.

Combustion Stroke

In this stroke both the ports still closed condition,the pressure of the expanding gases forces the piston towards BDC.the pressure in the crankcase is already rising. Later in down stroke exhaust port will be open & forced out the burnt gases,Very shortly after that the

Combustion Stroke

Inlet / Transfer port will also open to intake the fresh charge of fuel mixture,and engine is ready to start of the cycle.

Comparison On The Basis Engine Design

4-Stroke

4-stroke engine is heavy in weight & required more space. Because of one power stroke for two revolution.

4-stroke engine contains valves and valve mechanism.1.Inlet Valve & 2.Exhaust Valve. Exhaust valve shape is larger than the inlet valve.

2-Stroke

2-stroke engine is more compact and due that it is light in weight. It is because a 2-stroke engine will develop twice power of a comparable 4-stroke engine because of one power stroke every revolution.

2-storke engine have no valves but only ports.1.Inlet Port 2.Outlet Port & 3.Transfer port.

Comparison On The Basis Engine Design

4-Stroke

Shape Of piston Top in this engine has Flat shape.

Heavier size flywheel is needed since turning moment is not so uniform.

2-Stroke

The shape of piston is deflector type or crown shape. Top surface is slant surface or inclination surface.

Lighter Flywheel is needed. Since more uniform turning moment is produced.

Comparison On Basis Of Engine Performance

4-Stroke

The effective Compression Ratio is High.

Because of Higher Compression ratio, The Thermal Efficiency of an engine is high.

Power Produced for same cylinder dimensions and speed is less in 4-stroke.

2-Stroke

The effective compression ratio in the case of 2-stroke is less than that for 4-stroke engine for the same stroke(actual) & clearance volume.

Thermal efficiency is low since some fresh charge escapes during scavenging process.

Power Produced for same cylinder dimensions and speed is higher. It is because, some of the power is used for compressing the charge in the crank case and effective stroke is less than the actual stroke.

Comparison On Basis Of Engine Performance

4-Stroke

Volumetric Efficiency Is High Due To More Time Available For Induction.

This engines are less noisy. This engines are less pollute due

to complete scavenging operation compare with 2-stroke.

The actual efficiency is higher & better than the 2-stroke engine.

2-Stroke

Volumetric Efficiency Is Low Due To Less Time Available For Induction.

Sudden release of the gases makes the exhaust more noisy.

The scavenging is not complete particularly in high speed engines as very short time is available for exhaust & hence the fresh charge is highly polluted.

The actual efficiency of the 2-stroke cycle is less than 4-stroke cycle engine because greater overlapping of the ports is necessary in this for effective scavenging.

Comparison On The Basis Of Cooling And Lubrication System

4-Stroke Lesser Cooling &

Lubrication is needed compare with 2-stroke because of 4-stroke has only one power stroke per revolution of crankshaft.

Generally water-cooled system is used in 4-stroke.

Pressurized Lubrication System is used.

2-Stroke More & efficient cooling &

lubrication is required because of the power strokes per minute are twice the power stroke of 4-stroke.

Normally Air-cooled system is used in 2-stroke.

Lubricating oil is mixes with fuel of the engine.

The consumption of lubricating oil is sufficiently large because of high operating temp.

Conclusion

Due to the excessive loss of combustible mixture with exhaust gases and resultant high fuel

consumption, 2-Stroke cycle engines are not widely used except in the case of outboard

motors,scooters,motor-cycles and light vehicles. It is possible to get expected performance of 2-stroke engine only if the engine is built for a large

output and good design.

Prepared By:-1. Dhananjay Babariya-732. Shasank Patel-743. Nimesh Chaudhary-754. Jayesh Nair-765. Kiran Vasava-776. Kamlesh Vasava-78