The Mechanical post welcomes you back ! today we will see What is Spark Ignition engine and Compression Ignition (CI) Engine and their differences, starting off with their definition.

Spark ignition (SI) vs Compression ignition (CI) engine

Definition of Spark Ignition engine


Spark Ignition engine is a type of engine where the burning of the fuel happens with the assistance of Spark Plug. 

Definition of Compression Ignition (CI) Engine


It is the type of engine which works by compressing the air such that its temperature increases to a point that the fuel can ignite easily and combustion can be achieved.

A heat engine makes use of combustion of fuel, which causes expansion in the cylinder and thus work is obtained. 

The engine cannot itself ignite the fuel, thus different methods are used to cause the burning or combustion of fuel. 

This can  be done by using a sparking element like a spark plug or by heating the air in the cylinder to a high temperature by compression, such that when fuel is sprayed it  burns immediately.

What is the Spark Ignition (SI) Engine? 


Generally, Petrol engines are called as spark ignition engine. 2 stroke petrol engine and 4 stroke petrol engine both come under Spark Ignition engine banner.

As the name suggests spark ignition engine uses spark to ignite the fuel and air mixture,which then expands in the engine cylinder to give work output.

What is a Compression Ignition (CI) Engine? 


Generally, diesel engines are known as Compression ignition (CI) engine. In a CI engine, only air enters the cylinder through the inlet valve. 

This air is then compressed by the piston such that its temperature as well as the pressure increases. 

When the temperature and pressure of the air is at its maximum, diesel is sprayed. This causes combustion of the diesel due to the high temperature and work is obtained.

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Difference Between SI and CI Engine


  • The Spark ignition (SI) works on Otto cycle whereas Compression ignition (CI) engine is based on the Diesel cycle.

  • In a SI engine a mixture of petrol and air is admitted into the cylinder during the suction stroke with the help of carburetor.

  • While Diesel is injected directly into combustion chamber at high pressure at the end of compression stroke in a CI engine. Usage of Carburetor is eliminated but high pressure fuel pump and injector is required.

  • Throttle valve in carburetor controls the quantity of the mixture introduced in combustion chamber in a SI engine.

  • Quantity of fuel reaching the combustion chamber is regulated by fuel pump in a CI engine.

  • Petrol engine  SI engine requires an ignition system with the spark plug in combustion chamber.

  • No spark plug is required as combustion of fuel happens due to very high pressure and temperature in combustion chamber of CI engine.

  • Compression ratio of SI engine is about 6 to 10.5.

  • Compression ratio of a CI engine is about 14 to 22.

  • Higher r.p.m. is achieved in SI engine due to lighter weight.

  • Maximum r.p.m. is lower in a CI engine.

  • In a SI engine maximum efficiency is lower ( about 30% ) is achieved due to low compression ratio.

  • In a CI engine higher efficiency ( about 40% ) is achieved due to higher compression ratio.

  • Engine can be easily started due to spark plug in a SI engine.

  • Engine cannot be started easily due to high compression ratio in a CI engine.

  • Petrol engine is lighter in weight .

  • Diesel engine is heavier due to higher compression ratio and many more components.

  • Petrol engines are generally employed in light duty vehicles like scooters, cars etc.

  • Diesel engines are generally employed in heavy vehicles like bus, truck etc.

  • Light in weight due to low pressure developed.

  • Heavy in weight due to high pressure developed.

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