Spark Ignition Engine:

➢ A spark-ignition engine (SI engine) is an internal combustion engine, generally a petrol engine, where the combustion process of the air-fuel mixture is ignited by a spark from a spark plug.

➢ In this engine air fuel mixture is supplied to combustion chamber in homogenous mixture form.

➢ Air fuel mixture supplied by carburetor.

Stages of combustion in SI engine :

Ignition lag stage:

✓ There is a certain time interval between instant of spark and instant where there is a noticeable rise in pressure due to combustion. This time lag is called IGNITION LAG.

✓ Ignition lag is very small and lies between 0.00015 to 0.0002 seconds.

✓ This is a chemical process depending upon the nature of fuel, temperature and pressure, proportions of exhaust gas and rate of oxidation or burning.

Flame propagation stage:

✓ Flame propagation refers to the propagation of the reaction zone or “combustion wave” through a combustible mixture.

✓ Pressure rises to maximum in the end of this stage.

After burning:

✓ Combustion will not stop after attaining peak pressure and this combustion is known as after burning.

✓ This generally happens when the rich mixture is supplied to engine.

Compression Ignition Engine:

➢ A compression-ignition engine (CI engine) is an internal combustion engine, generally a Diesel engine, where the combustion of the air-fuel mixture is start due to heat produced in compression process.

➢ In this engine first air is supplied to combustion chamber then fuel is injected in the chamber so Heterogenous mixture form.

➢ Fuel is injected by fuel injectors.


1.In diesel engine, the suction contains
(A) Air only
(B) Fuel only
(C) Air- fuel Mixture
(D) Air or Fuel

2.In a diesel engine, the fuel is ignited by:
(A) Spark
(B) Injected fuel
(C) Heat resulting from compressing air that is supplied for combustion
(D) Ignitor
3.The combustion in a compression ignition engine is _____ .
(A) Heterogeneous
(B) Homogeneous
(C) Laminar
(D) Turbulent

Stages of combustion in CI engine :

Ignition delay:

✓ The time interval between the injection of the fuel and the start of

the self-ignition of the fuel is known as ignition lag or Ignition delay.

✓ The fuel does not ignite immediately upon the injection of fuel into the combustion chamber.

✓ Physical delay due to the complete injection of fuel, atomization, vaporization and mixing of air and fuel and raised to its self-ignition point.

✓ The chemical delay due to the burning slowly starts and then accelerates until the complete ignition takes place.

Rapid combustion:(uncontrolled)

✓ This rapid combustion will starts right After the ignition delay period ends.

 ✓ During this period the heat release is maximum.

 ✓ The pressure released during this period depends on the ignition delay period. If the ignition delay period is more, then the pressure rise is more due to the more fuel will be accumulated during the delay period.

Controlled combustion:

✓ The rapid combustion followed by the third stage called the controlled combustion.

✓ At the point at where it reaches the maximum cycle pressure the rapid combustion ends and the controlled combustion starts. The period of the controlled combustion is assumed to end at the maximum cycle temperature.

After burning:

✓ The combustion process will not stop right after the completion of the injection process.

✓ The unburnt particles left in the combustion particles will start burning as soon as they get in contact with the oxygen.

✓ This process continued for a certain period amount of time called the after burning

Two Stroke IC Engine:

➢ In two stroke cycle engines, the whole sequence of events suction, compression, power and exhaust are completed in two strokes of the piston (one revolution of the crankshaft).


➢ There is no valve in this type of engine.

➢ Gas movement takes place through holes called ports in the cylinder.

Working of Two Stroke IC Engine:


 1.In diesel engines, the duration between the time of injection and ignition, is known as:

(A) pre-ignition period

(B) Delay period

(C) Ignition period

(D) Burning period


2.Increasing the compression ratio beyond a certain limit in SI engines, results to _____ .

(A) Delay in fuel burning

(B) Engine knocking

(C) Excessive heat in the engine

(D) High fuel consumption


3.In an IC engine, the connecting rod connects which of the following pair of parts?

(A) Piston and crankshaft

(B) Cylinder and crankshaft

(C) Cylinder and piston

(D) None of these


4.The crank radius of a single cylinder IC engine is 60 mm and the diameter is 80 mm.The swept volume is cm2 is

(A) 48

(B) 96

(C) 302

(D) 603

 5.Which of the engine has a spark plug used for igniting the fuel?

(A) Petrol

(B) Diesel

(C) Gas

(D) Steam


6.Function of carburetor is to supply :

(B) Air and diesel mixture

(C) Only petrol

(A) Air and petrol mixture

(D) Petrol and diesel mixture