Difference between SI engine & CI engine with PDF

The Mechanical post welcomes you back! Today we are going to see about the difference between the SI engine and CI engine.

You can get your copy of PDF on Differences between SI and CI engine at the end of the article.

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 the 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? 


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

As the name suggests spark-ignition engine uses a 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? 


Commonly, diesel engines are known as Compression ignition (CI) engines. 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 are at their 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

SI Engine
CI Engine

A SI engine stands for Spark Ignition engine. 


 A CI engine stands for Compression Ignition engine.

SI engines are also commonly known as Petrol engines.


CI engines are also commonly known as Diesel engines. 

An SI engine works on the "Otto cycle".


A CI engine works on the "Diesel cycle".

In a SI engine, a mixture of air & fuel enters during the suction/intake stroke.
 

In a CI engine, only air enters during the suction/intake stroke.

In a SI engine, the air-fuel mixture is ignited with the help of a spark plug.

In a CI engine, the air-fuel mixture is ignited by the compression of the air-fuel mixture.


Fuel (petrol) is provided to the engine by the carburetor.

Fuel (Diesel) is directly injected into the cylinder with the help of a fuel injector.


SI engine has a compression ratio in the range of 10:1 to 14:1

CI engine has a compression ratio in the range of 14:1 to 22:1
 

Since, SI engines have a low compression ratio, their thermal efficiency is less.


Since, CI engines have a high compression ratio, their thermal efficiency is more.

It is easy to start a SI engine.

CI engine are comparatively difficult to start.


Spark Ignition engines produce less noise.


Compression Ignition engines produce comparatively loud noise.



Cost of maintenance of a SI engine is low.


Cost of maintenance of a CI engine is high.



SI engines are comparatively lighter in weight.


CI engines are on the heavier side.

SI engines are comparatively cheaper.


CI engines are expensive.

SI engine is used in small automobiles like bikes, scooter, cars etc.

CI engine is used for larger applications like ships, heavy duty trucks, buses etc.


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