# SI and CI Engine

Why we use Petrol in SI Engine & Diesel in CI Engine

hosam_ozo

• Member Since Apr-2010 | Aug 8th, 2010

First thing you have to know is what is called cetane and octane number.

The octane number is the percentage of octane in the mixture of fuel
The centane number is the percentage of cetane
The octane number expresses the self ignition temperature, as this no. increases the self ignition temperature increases too and this make the knock to be avoidable and the cetane no. is vice verse

**Octane no. increase self ignition increase and cetane no decrease"" SIE needs high octane no"
**Cetane no increase the octane no decrease and the self ignition temperature decrease" CIE needs this"

Case 1: SI engine we dont want the Self Ignition to be reached we use a fuel have a high octane to avoid the knock "which damage the engine"

Case 2: CI the compression ratio is too high "12:22" and this kind of engine depends essentially on the self ignition temperature so we use a fuel with high cetane no "low octane" as the high cetane "low octane" gives low self ignition temperature and that is we need in the CIE..

#### sargkuna Profile Answers by sargkuna Questions by sargkuna

• Aug 7th, 2010

The engine wouldn't even start. Diesel fuel is like a thin oil. ...

#### hosam_ozo Profile Answers by hosam_ozo

• Aug 8th, 2010

First thing you have to know is what is called cetane and octane number.

The octane number is the percentage of octane in the mixture of fuel
The centane number is the percentage of cetane
The octane number expresses the self ignition temperature, as this no. increases
the self ignition temperature increases too and this make the knock to be
avoidable and the cetane no. is vice verse

**Octane no. increase self ignition increase and cetane no decrease"" SIE
needs high octane no"
**Cetane no increase the octane no decrease and the self ignition temperature
decrease" CIE needs this"

Case 1: SI engine we dont want the Self Ignition to be reached we use a fuel
have a high octane to avoid the knock "which damage the engine"

Case 2: CI the compression ratio is too high "12:22" and this kind of engine
depends essentially on the self ignition temperature so we use a fuel with high
cetane no "low octane" as the high cetane "low octane" gives low self ignition
temperature and that is we need in the CIE..

#### santhosh319 Profile Answers by santhosh319

• Oct 10th, 2010

The theroy behind it is
petrol explodes and diesel burns
a spark can ignite petrol but not diesel,so compressed air.

#### puneetrawat Profile Answers by puneetrawat

• Jul 14th, 2011

self ignition of petrol is not that high...u need an external source to ignite it......spark plug...
diesel as is its characteristic to be ignited at high pressure ratios is used in ci engines

• Jul 15th, 2011

because the petrol is more volatile then diesel and therefore even simple spark does help to ignite petrol thus we use si engine but diesel being less volatile it need to be compressed to attain the required ignition temperature thus we use ci engine for diesel

#### Tarun

• Aug 8th, 2011

because petrol has high self ignition temperature so it need spark to ignite but diesel has low self ignition temperature so it can be ignited using hot compressed air..

• Aug 9th, 2011

Si engines means spark ignition engines. With the help of spark plug, spark is produced in engine cylinder. Before this process occurs, mixture of air and fuel entered in engine cylinder during suction stroke and it compresses during compression stroke. Just before end of compression stroke, spark is ignited so that combustion of that air and fuel mixture takes place....

Ci engines mean compression ignition engines. Same process takes place as that of si engines only the difference is that instead of spark plug, fuel injector is used. Only compressed air is entered in cylinder and it get compress during compression stroke. Fuel is injected just before end of compression stroke. Temperature of that air is sufficient to burn thru fuel....

#### ABHIJAY

• Jul 10th, 2014

Si engines means spark ignition
engines. With the help of spark
plug, spark is produced in engine
cylinder. Before this process occurs,
mixture of air and fuel entered in
engine cylinder during suction
stroke and it compresses during
compression stroke. Just before end
of compression stroke, spark is
ignited so that combustion of that
air and fuel mixture takes place....