Combustion in SI and CI engines

You can find the detailed working of both these in any IC Engines reference book very easily.

But to answer your question briefly, here it goes :


SI engines :
They are the Spark Ignition engines. Meaning that the fuel in the engine is ignited with the help of a spark generated by the Spark plug in the cylinder.
The fuel - air mixture is introduced in the cylinder, compressed and then ignited with the help of the spark created by the spark plug and then the normal combustion process begins where the mixture burns and expand producing power.

They are usually the petrolor gasoline run engines which are using this technique for ignition.



CI Engines :
They are the Compression Ignition Engines.

In their case the fuel used is diesel generally. Here the main difference between the method of ignition is as follows.
The Air is drawn in the cylinder during the suction stroke and then compressed during the compression stroke, the compresion ratio being very high. Due to such high compression of the air it gets extremely presuried and the tempreature and pressure rise to the extent of the ignition point of the diesel.
Here the diesel fuel is introduced in the cylinder at the end of the compession stroke, and due to the very high pressure and tempreature of the air it immediately ignites and starts the power stroke.



Main Diffference between SI and CI :
The SI system uses a premixed fuel- air mixture introduces it to the engine at once.
Where as the CI systems introduce he air and the fuel seperately.
Secondly, SI uses a spark genreated by the spark plug to burn the a/f mix.
Whereas the CI compresses the air itself to a degree so high that the fuel automatically ignites due to its high tempreature and prssure and it DOES NOT need an external source like a spark plug to burn the mixture.


Effect of the A/F mixture ratio on engine performance:

In the SI engine the A/F ratio is predetermined and remains the same throughout the running if the vehicle. The speed is varied simply by altering the amount of mixture entering the cylinder.

Where as in the CI Engines the amount of ir in the cylinder always remains the same but the amount of fuel injected in the cylinder is increased or reduced as per speed requirement. The A/F ratio changes with speed fluctuations.

This change in A/F ratio in one case and being constant in the other is the main difference related to A/F ratios in the SI and CI engines.

The mixtuere must be as near as possible to the stochiometric ratio of 14:1 as the pefromance is the most efficient at this ratio.

A leaner mixture would produce lesser power and be less efficient during one cycle. Where as the richer mixture would lead to incomplete combustion of the fuel leading to wastage of fuel and higher CO and NO2 and hydrocarbon emissions causing both inefficiency and pollution.

The type of engine is classified based on the type of fuel they use. In SI engines a spark plug is used to ignite the fuel after it is being compressed. Where as in CI engine self ignition takes place after the fuel is compressed.

This difference is due to the difference in fire points and volatility of the fuels.

SI engines use petrol or gasoline as fuel, where as CI engines use Diesel, Kerosene as fuel.

Both these come under category of Internal Combustion engine formally known as IC engines. Since the combustion takes place inside the cylinder or combustion chamber.

Any IC will have strokes through which the fuel is processed. There are 4-stroke engines and 2-stroke engines.

In a 4-stroke engine we get power or thrust force for every 4 strokes of the piston inside the cylinder. In a 2-stroke engine we get power for every 2 strokes of the piston.

Again we have both SI & CI type in 4-stroke and as well as in 2-stroke.

SI Engine::

At first the fuel (Gasoline)+air is sucked in to the combustion chamber and at the end of the suction , the piston begins the compression, at the end of the compression the temperature of the fuel is increased and to start the combustion we have to supply additional thermal energy , this is done by spark plug. The spark produced by the spark plug ignites the compressed fuel and initiates the combustion. As a result of combustion, thrust force acts on the piston head and this force tries to push the piston down. This downward movement of piston is converted to mechanical energy ( some part of it stored in flywheel )

CI Engine::

At first the fuel (Diesel)+air is sucked in to the combustion chamber and at the end of suction, the piston begins the compression process. Due to compression the temperature of fuel increases and this temperature is sufficient to ignite the fuel. So at the end of compression, combustion starts due to self ignition of fuel. Later process is same as in Si engine.

1.)Can the size of these SI & CI engines be made same?

Ans.)No, because in CI engine there is no spark plug to initiate combustion, the combustion takes place due to compression process alone. So to have efficient compression the size of CI engine is a bit more and its Compression ratio is also high. Where as in SI engine we supply external thermal energy so we can reduce the size of its combustion chamber by providing higher energized sparks.

2.)Can we use Diesel as fuel in SI engine? And petrol in CI engine?

Ans.)The engine wouldn't even start.
Diesel fuel is like a thin oil. It is lacking the aromatic and easily ignited components in a petrol. These have already been separated out to produce petrol. Petrol is ignited under relatively little pressure by a spark.
Diesel fuel is sprayed into the hot, highly compressed air inside the engine's combustion chambers in a fine mist and under extremely high pressure, where the heat ignites it, but the fuel continues to be sprayed in as the power stroke progresses. The burn is pretty slow, but goes on for some time.
Petrol is a "one- shot- its- all- in" process, one big bang when ignited by the spark.

Diesel fuel is no more compatible with petrol engines than petrol is with diesel engines.

Having said all that, kerosene is and has been used in some "petrol" engines, but kerosene is a lighter fuel than diesel oil. These engines running kerosene might start on petrol, then change, or have a "hot bulb" for ignition.
The heat for the bulb is often initially provided by a blowlamp.

3.)Which engine is efficient?

Ans.)In the case of CI engines the value of compression ratio is higher; hence these engines have the potential to achieve higher thermal efficiency. In the case of SI engines the lower compression ratio reduces their potential to achieve higher thermal efficiency.

4.)Which one is heavier in weight ?

Ans.) In CI engines the compression ratio is higher, which produces high pressures inside the engine. Hence CI engines are heavier than SI engines.

5.)Comment on speed achieved by CI & SI engines.

Petrol or SI engines are lightweight, and the fuel is homogeneously burned, hence achieving very high speeds. CI engines are heavier and the fuel is burned heterogeneously, hence producing lower speeds.

6.)Compression ratio details in SI & CI Engines.

In the case of SI engines, the compression ratio of the fuel is in the range of 6 to 10 depending on the size of the engine and the power to be produced. In CI engines, the compression ratio for air is 16 to 20. The high compression ratio of air creates high temperatures, which ensures the diesel fuel can self-ignite.


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