LM Fuels Power Point

8/6/2019 LM Fuels Power Point

1/14

By Luke Moores


2/14

Winter and Summer Petroly On average Between 30-40% of each barrel of crude oil

is used for petrol

y Its not as simple as distilling off the right bit at therefinery:

y You must blend it to get the right properties

y It is designed to have the correct volatility


3/14

Amixture of petrol vapour an

d air is ign

ited in

the cylin

derof a cars engine.

y The mixture of petrol and air is made in the carburettor

When

the weather is very cold the petrol is difficultto vaporise, the car becomes difficult to start.

So as to overcome this problem the petrol companies

make different blends for different times of the year.During the winter they add more volatile components

(smaller chained hydrocarbons) to the petrol so

that it vaporises more easily.


4/14

Equally, in hotter weather you dont want the more volatile

components, as the petrol will vaporise too easily whichwould be dangerous.

All commercial petrols are a precise blend of hydrocarbons

of high, medium and low volatility.As well as altering the petrol for the different seasons in acountry, the blend will be different in different countriesdepending on their climate.


5/14

The Problem of KnockingAnother important consideration that blenders must take

into account is the octane numberof the petrol.

This is a measure of the tendency of the petrol to cause theengine to knock.

In both petrol and diesel engines, the fuel-air mixture

has to ignite in each cylinder at the correct time which isusually just before the piston reaches the top of the cylinder.


6/14

As the piston compresses the fuel-air mixture it heats up, the more it is

compressed the hotter it gets.

Modern cars achieve greater efficiency than older ones by using highercompression ratios, often compressing the gases in the cylinder by afactor of 10.

Most hydrocarbons auto-ignite under these conditions as the fuel-airMixture combusts when compressed. When this happens two explosionsoccur:

One due to the compression and another when the spark occurs. Thisdisturbs the cycle of the engine and produces a knocking sound in the

engine.

The force from the expanding gases is no longer occurring at the propertime, so engine performance is significantly lowered and the inside of thecylinder can be damaged and the engine vibrates violently.


7/14

Octane NumbersThe tendency of a petrol-based fuel to auto-ignite is

Measured by its octane number.

2,2,4-trimethylpentane is a branched alkane with a lowtendency to auto-ignite. It is given an octane number of 100.

Heptane however, a straight chained alkane, therefore auto

Ignites easily and is given an octane number of 0.


8/14

The octane number of any fuel is tworked out by the

percentage of 2,2,4-trimethylpentane to heptane it contains

which knocks at the same compression rate as the fuel.

For example, four-star has a high octane number of 97

an

d kn

ocks at the same compression

ratio as a mixture of97% 2,2,4-trimethylpentane and 3% of heptane.


9/14

Name of alkane Octane Number

Heptane 0

Hexane 25

Pentane 62

3-methylhexane 65

2-methylpentane 73

3-methylpentane 75

2,3-dimethylpentane 91

2-methylbutane 93

utane 9

2-methylpropane >100


10/14

Making Petrol to Get the Right

Octane NumberDifferent cars have different compression ratios:

High performance petrol engines such as in sports cars usually

have a high compression ratio and therefore need a high octane

fuel otherwise there would be knocking and the engine

performance would be reduced.

There are two ways of dealing with the knocking problem:

1) put special additives in the petrol which discourage auto

Ignition.

2) blend high-octane compounds with ordinary petrol.


11/14

Unleaded fuelAnti-knock additives are substances which reduce auto ignition.

Therefore increasing the octane number of the petrol.

From the 20s till 1992, petrol engines were designed to run on

petrol with small quantities of lead.

However, public concern over environmental effects led to gradual

phasing-out of leaded petrol. They poison the metal catalysts in

catalytic converters which reduce the levels of other pollutants in

the exhaust.


12/14

Refining and BlendingRather than develop other lead-free additives,

the petrol companies looked to using refining and blending

techniques to get high octane numbers without the need for

any additives.

The hydrocarbons which give the best performance in a

Petrol engine are not the ones which are the most plentiful in

crude oil.

Its the job of the refinery to provide the hydrocarbons to suit

our needs.


13/14

DieselContains far larger hydrocarbonmolecules than those found in petrol.

Diesel from crude oil contains about 75% saturated hydrocarbons and

25% aromatic hydrocarbons.

While a petrol engine takes in a mixture of fuel vapour and air and

Ignites it with a spark, a diesel engine takes in just air, compresses it and

Then injects pure fuel into the compressed air. The heat of the

Compressed air ignites the fuel spontaneously. Thus a diesel

engine works entirely by auto ignition.


14/14

Which Alkane to use?

The structure of an alkane has a very important role on its

tendency to auto-ignite and its octane number. generally, the

shorter the alkane chain the higher the octane number.

Short chain alkanes are therefore more volatile, so they canbe used to increase the octane number so as to improve cold

starting. Even gaseous alkanes such as butane can be used

For this once dissolved in the petrol.

LM Fuels Power Point

Documents

Transcript of LM Fuels Power Point