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FUELS & COMBUSTION

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Improvement of Antiknock Characteristics of a Fuel

Dr.A.Geetha

Associate Professor

Department of Chemistry

Kongu Engineering College

KNOCKING

Knocking - Detonation- Pinking

An unwanted Sound

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What are Engine fuels

Petrol and Diesel are used as engine fuels.

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PISTON ENGINE

• Piston Engine can be divided into
• Spark Ignition (SI) Engine
• Compression Ignition Engine (CI) Engine

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• SI Engine consumes Petrol Octane Rating
• CI Engine consumes Diesel Cetane Rating

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Historical View

• There was a demand for more powerful engines. So they added more and larger pistons to the engines. Some were even built with 16 cylinders that means we will increase the size of the engines to increase their power and this is not a practical method.

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Practical Method

• The obvious method to get more power from an engine of a given size was to increase its compression ratio (a measure of the extent to which the gasoline–air mixture is compressed in the cylinder of an engine) The more the mixture is compressed before ignition, the more power the engine can deliver.

• The auto ignition of fuels causes a “knocking”.

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• It ignites twice; once due to the high pressure and again when the spark ignites the gasoline. This causes the car engine to be less efficient and it can also damage the engine.

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• Hence quality of an automobile gasoline is determined by its resistance to knock ”detonation during service”.

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• The antiknock quality of the fuel limits the power and economy that an engine using that fuel can produce.
• The higher the antiknock quality of the fuel, the more the power and efficiency of the engine.

SI Engine

In a four stroke engine,

Petrol + air gets mixed in the carburettor.

• Mixture sucked into the cylinder during suction stroke.
• Mixture gets compressed during compression stroke.
• The compressed mixture gets ignited by an electric spark

during power or ignition stroke.

• The product of combustion increases pressure and pushes

the piston out and expels the exhaust gases from the

cylinder.

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Knocking in SI Engine

In Petrol Engine,

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Sometimes, in some cases,

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• The fuel – air mixture may get heated to a temperature greater than its ignition temperature and spontaneous combustion occurs even before sparking. This is called pre-ignition.

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• Further, the spark also occurs and again combustion take place with the rest of the mixture faster and causes explosive.
• This is called knocking.

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• Knocking lowers the efficiency of engine which results in loss of energy.

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Chemical Structure and Knocking

The knocking tendency decreases as follows

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n-alkanes isoparaffins olefins naphthenes aromatics

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(aromatics have highest antiknock value whereas n-alkanes have lowest antiknock value)

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So, the presence of maximum quantity of aromatics and minimum quantity of n-alkanes is desirable in petrol.

Octane Number

• Octane number is used for gasoline(petrol).

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Definition: �It is the % of iso-octane in a mixture of iso-octane and n-heptane.�Octane Number is rated from 0 to 100.

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• Comparisons are made with blends of two pure hydrocarbons,

n-heptane and iso-octane (2,2,4-trimethylpentane).

Octane Number

• Higher octane number fuel will have lesser chance of knocking. Hence such fuel can be compressed to higher extent without observing any knocking.

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• Lesser octane number fuel will have higher chance of knocking. Hence such fuel can't be compressed to higher extent.

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• Octane number actually measures the resistance to auto-ignition. Higher the Octane Number, Higher will be the resistance to auto-ignition.

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For example,

Gasoline with a knocking ability that matches that of a blend of

90% isooctane and 10% n-heptane

has an octane number of 90 •

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How octane number of a fuel can be increased ?

1. By putting special additives into the fuel which discourage auto ignition.
2. By blending high-octane fuels in with the ordinary petrol.

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Anti-knocking additives

• Anti-knocking additives are substances which reduce the tendency of a fuel to auto-ignite, and so increase the octane number.

• Small amounts of lead compounds have been used as economical and effective anti-knock additives. tetraethyl lead

• But it damage the environment.

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• Other Additives
• Toluene , Xylene, tertiary-butyl-ether (MTBE) , Methanol or Ethanol Alcohol and Tertiary Butyl Alcohol

CI Engine

In a CI engine,

• Air alone gets compressed.
• Diesel is sprayed which must ignite spontaneously
• The product of combustion increases pressure and pushes

the piston out and expels the exhaust gases from the

cylinder.

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Knocking in CI Engine

In Diesel Engine,

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Sometimes, in some cases,

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• Even after the compression stroke is over and even after the diesel oil is sprayed, burning may not start.

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• So more and more fuel is injected automatically and sudden ignition may occur . This is called delayed-ignition.

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• This delayed ignition causes explosive.

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• This is called knocking.

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Chemical Structure and Knocking

The knocking tendency increases as follows

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n-alkanes isoparaffins olefins naphthenes aromatics

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(n-alkanes have highest antiknock value whereas aromatics have lowest antiknock value)

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So, the presence of maximum quantity of n-alkanes and minimum quantity of aromatics is desirable in diesel.

Cetane Number

• Cetane number is used for diesel.

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Definition: �It is the % of n-hexadecane (n-cetane) in a mixture of n-hexadecane and 1-methyl naphthalene.

�Cetane Number is rated from 0 to 100.

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Comparisons are made with blends of n-hexadecane and 1-methyl naphthalene.

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Additives (Improvement of cetane number)

• Ethyl nitrate and iso amyl nitrate

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CONCLUSION

OCTANE NUMBER AND CETANE NUMBER

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• Octane Number and Cetane Number are the standards to measure the tendency of fuel to ignite spontaneously.

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• Octane number measures the performance of gasoline while Cetane number measure the performance of the diesel.

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• The fuel having high octane number has the low cetane number and high cetane number fuel has low octane number.

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• This is one reason why we can’t use petrol in a diesel engine and diesel in petrol engine.

THANK YOU