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Basics of I C Engine

 Q.1. Explain working of Four Stroke SI(Petrol) Engine

 1. Suction stroke:-  During this stroke the crank moves from TDC to BDC. The piston moves downward. inlet valve is open and exhaust valve is closed. The charge is sucked into cylinder. Stroke continues till  piston reaches  BDC ,as shown in fig

2. Compression stroke:- In this stroke both inlet and exhaust valves are closed and charge is  compressed as the piston moves upward from BDC to TDC. As result of compression, the pressure and temperature of charge is increased considerably. This completes one revolution of crankshaft. The compression stroke is shown in fig.

3. Expansion or power stroke:- Shortly before the piston reaches TDC the charge is ignited with the help of spark plug. It suddenly increases temperature and pressure in the cylinder with constant volume. That pressure pushes piston downward. Hot burned gases expands due to high speed of piston. Thus the heat energy is converted into mechanical energy. In this stroke both valves are closed and piston moves from TDC to BDC.

4. Exhaust stroke:-  In this stroke exhaust valve is open and piston moves from BDC to TDC. This movement of piston pushes out products of combustion from cylinder. This completes the second revolution and one cycle .





Q.2 . Explain working of Four Stroke CI (Diesel) Engine



Spark plug is replaced by Injector, Charge(air+fuel) is replaced by air only

Spark ignition is replaced by Fuel Injection

Q.3. Compare SI engine with CI engine


Petrol engine (S.I)

Diesel engine (C.I)

1. Air fuel

Air fuel mixture is inducted into cylinder

Only air is inducted into cylinder

2. Fuel supply

Carburetor is used to mix air and petrol in the required proportion and to supply to the engine during

The injector is employed to inject fuel at the end of compression stroke

3. Pressure

Pressure at the end of compression is about 10 bar

Pressure at end of compression is about 35 bar

4. Ignition

The charge is ignited with the help of spark plug

The fuel is injected in the form of fine spray. Which gets auto-ignited.

5. Compression ratio

6 to 10

15 to 25

6. Weight

As compression ratio is low weight of engine is less.

As the compression ratio is high the diesel engines are heavier.

7. Running cost

The running cost of petrol engine is high because of higher cost of petrol

The running cost of diesel engine is low because of lower cost of diesel

8. Thermal efficiency

The thermal efficiency is about 26%

The thermal efficiency is about 40%

9.Noise and vibration

Less noise and vibration

More noise and vibration


It is used in motorcycles and light motor vehicles,sporting vehicles

It is used in heavy duty vehicles and earth moving equipments and for power plants.

Q.4. Compare two stroke and four stroke engine.


Four stroke

Two stroke

1. No. of strokes

Cycle is completed in four strokes  (Two Rev)        

cycle is completed in two strokes (1rev.)


2. Power stroke

One power stroke every two rev.

One power stroke in each revolution


3. Valves    

It has  valve  mechanism   

It has no valves.( it has ports)

4. Weight

It is heavier that 2 stroke

It is light  weight

5. Piston

Piston is flat or dome shaped

Piston has deflector

6. Flywheel

Heavy flywheel is required because of more fluctuation.

Lighter flywheel is required due to less energy fluctuation.

7. Thermal efficiency

High thermal efficiency

Low thermal efficiency


Maintenance is complicated

Maintenance low

9. Cost

Higher cost

Lower cost

10. Applications

Used where efficiency is important

ex. Trucks,buses,bikes

Used where more power is required in small size ex. Scooter, Motor boats