FUELS

DEPARTMENT OF AUTOMOBILE ENGINEERING RENGI KURIAN VARGHESE

S6 AUE ROLL NO:46

INTRODUCTION

• The rating of fuel is necessary because it gives the following characteristics of fuel :

• Self ignition or pre ignition• Ignition delay• Detonation or knocking• Behaviour towards various engine opreating

conditions.

Petrol fuel is rated by octane number rating

And diesel fuel is rated by cetane number rating

RATING OF FUELS

Octane Number Rating

The property of a fuel which describes whether fuel will or will not self-ignite is called octane number rating.

• Shows the anti-knocking quality of fuel.• Measures resistance to instantaneous

combustion.• Higher the octane number ,less likely the

fuel will self ignite.

• Unknown fuel whose octane number is to be found is compared with a reference fuel consisting of mixtures of iso-octane and normal heptane.

• Iso octane have octane no of 100.• Normal heptane have 0 octane number.

OCTANE TEST

• Octane test is conducted in CFR engine.• Procedure• First CFR engine is operated with the fuel

whose octane number is to be calculated.• Go on increasing the compression ratio and

conduct the test till the engine detonates.• Now conduct the test with various

proportions of iso-octane and normal heptane .

• A particular proportion of volume of above mixture shows the same detonating effect.

• Note down the proportions of volume.• Octane number of unknown fuel is the

percentage volume of iso octane in the mixture of iso-octane and normal heptane.

• Example, a blend of 10% n-heptane and 90% iso-octane by volume has an octane number of 90.

• A high octane number fuel is used in high compression engine.

• Whereas, engines with low compression ratios can use fuels with lower octane number.

TYPES OF OCTANE RATING

• Mainly two types• Research octane number (RON)• Motor octane number (MON)

RESEARCH OCTANE NUMBER METHOD

• This method measures anti –knock performance under relatively mild operating conditions.

• The engine is ideally run at mild load and low speed and octane test is carried out.

MOTOR OCTANE NUMBER METHOD

• This method measures anti-knock performance under relatively severe operating conditions.

• The engine is ideally run at severe load and high speed and octane test is carried out.

FUEL SENSITIVITY

• The difference in octane number between the research method and the Motor method octane numbers is known as the fuel sensitivity; thus

• fuel sensitivity (FS) = RON-MON• Generally FS ranges from 0 to 10.• It measures knock characteristics of a fuel

when condition of operation changes.

• Fuel sensitivity shows how a fuel behaves when engine working condition changes

• A low FS no usually means that knock characteristics of that fuel are insensitive to engine geometry.

ANTIKNOCK INDEX

• The average of two octane no rating methods , RON and MON , is known as antiknock index ; thus

• Antiknock index = (RON+MON)/2

CETANE NUMBER

• Cetane number is a measure of ignition quality of diesel fuel.

• It shows how fast fuel ignites.• Octane and cetane nos of the fuel is the

inverse of the same property (ignition).• A fuel which has high octane number has

low cetane number.

• Cetane no of any fuel is the percentage of cetane by volume in a mixture of cetane and x-methyl naphthalene which has the same diesel knock characteristics as the fuel under test.

• It is similar to the octane number for gasoline.

• Higher the cetane rating of the fuel lesser is the propensity for diesel knock.

• The cetane number is a numerical measure of determining the ignition delay.

• Low cetane fuels increases ignition delay so that start of combustion is near to top dead center.

CETANE TEST

• Cetane test is conducted in CFR engine.• Procedure• First CFR engine is operated with the fuel

whose cetane number is to be calculated.• Go on increasing the compression ratio and

conduct the test till the engine detonates.• Now conduct the test with various

proportions of cetane and x-methyl naphthalene.

• A particular proportion of volume of above mixture shows the same knocking effect.

• Note down that proportions of volume.• Cetane number of unknown fuel is the

percentage volume of cetane in the mixture of cetane and x-methyl naphthalene.

• For higher speed engines the cetane number required is 50 , for medium speed engine about 40 , and for slow speed engines about 30 .

DIESEL INDEX

• It is a cheap method of predicting ignition quality.

• It gives an indication of ignition quality obtained from certain physical characteristics of fuel as opposed to an actual determination in a test engine.

Diesel Index(DI)=(Aniline point(°F)*API gravity(°))/100

The aniline point of fuel is temperature at which equal parts of fuel (diesel) and pure aniline dissolve each other.

• Aniline point therefore gives an indication of chemical composition of fuel.

• Thus a higher API gravity reflects a low specific gravity and indicates a high paraffinic content which corresponds to a good ignition quality.