Diesel FuelsDr. Fatma Ashour

The term ‘diesel fuels’ comprises • Gas Oil’ (Solar) o Diesel Oil

Both fractions are heavy distillates obtained from crude oil.

The main difference between gas oil and diesel oil is that the former, is completely a distillate fraction while the latter may include some residual oils.

Diesel fuels

The usual boiling range of gas oil varies from 210 to 350C.

In the case of diesel oil, the initial boiling point is higher than that, and the final boiling point cannot be reached under atmospheric pressure.

In general, gas oil has a lighter color and a lower specific gravity (below 0.85) than diesel oil.

Boiling Range

Both gas oil and diesel oil are used to operate ‘diesel engines’ which are I.C.E. without spark ignition.

Diesel Engines

Diesel EngineThe diesel engine cycle consists of the following strokes:

Air Suction stroke: in which air is sucked inside the cylinder.

Air Compression stroke: in which the air is compressed adiabatically and its temperature therefore rises & the fuel is then injected. After the fuel has burned, the expansion proceeds adiabatically, driving the piston back.

It is important to note that the fuel is injected at a high pressure (after the compression stroke) in the form of fine droplets by means of an ‘atomizer’ (or injector).

 The compression ratio in diesel engines is much higher than that in gasoline engines. Since the fuel is not compressed with the air but injected after the compression, this could be done and gives a higher efficiency than lower compression ratios.

Air is compressed with a compression ratio typically between 15:1 and 22:1 resulting in 40-bar (4.0 MPa; 580 psi) pressure compared to 8 to 14 bars (0.80 to 1.4 MPa) (about 200 psi) in the petrol engine.

Compression Ratios of Diesel Engines

The performance of diesel fuels obviously differs from that of gasoline.

In the case of diesel fuels, it is essential that the fuel ignites easily-from the heat of compression of air-without using any spark.

Good diesel fuel will be composed mainly of

Paraffins, with preferably little or no branched hydrocarbons and aromatics.

 

Performance of Diesel Fuels

The performance or ignition quality of a diesel fuel is measured by one of the following methods:

A- Measuring the Angle of Delay

It is defined as ‘the angle travelled by the crank during the period between the fuel injection to the start of combustion’ of the engine.

Low grade diesel fuel will require a longer period of time between the injection of the fuel and the starting of ignition (i.e.: a big Angle of Delay).

 

Measurement of Ignition Quality

B- Diesel knock

In diesel engines, the injection of the fuel and its ignition as well as the expansion of the hot combustion gases, all take place in one stroke - the power stroke.

If the period between the injection of the fuel and its ignition is long (low grade diesel fuel) this leaves a relatively short period for the ignition and the expansion.

Consequently, the power is generated over this small period of time. The impulsive force acting on the piston will be so high resulting in what is known as the ‘diesel knock’

(i.e.: explosive ignition resulting in non uniform pressure distribution on the piston).

The performance or ignition quality of a diesel fuel is measured by the ‘Cetane Number’. Two reference fuels compounds are used for the determination of the octane number, these are;

n-cetane (n-hexadecane, n-C16 H34) which is normal paraffin with a long chain, and therefore expected to ignite easily. It is given an arbitrary cetane number of 100.

The -methyl naphthalene which is a ring structure having a side-chain and therefore expected to resist spontaneous ignition (i.e. poor diesel fuel constituent). It is therefore given an arbitrary cetane number of Zero.

Measurement of Cetane Number

A standard C.F.R. engine is used for that test

The engine is first operated using the fuel under test, then with mixtures of n-cetane and -methyl naphthalene. The composition of the latter mixtures (n-cetane and -methyl naphthalene) is varied until a performance similar to that obtained by the fuel tested obtained, as measured by the ‘angle of delay’ on the ‘card diagram’.

The cetane number may be defined as “the volume percent of n-cetane in a mixture of n-cetane and -methyl naphthalene which gives the same performance (or ignition quality) as the diesel fuel sample under test”.

C.F.R. Engine

The performance characteristics of diesel fuels may be improved by the addition of certain additives, such as alkyl nitrites, nitrates, nitro-and nitroso-compounds as well as peroxides.

It is important to note that, owing to the difficulty of measuring the cetane number-through the angle of delay-several substitute characteristics have been suggested, the most important of which are:

1. Aniline Point 2. Diesel Index

This is defined as the “minimum temperature at which two equal volumes of the sample and aniline are completely miscible with one another, this simplified method gives an indication about the extent of “aromaticity of the fuel”.

Accordingly, a highly paraffinic fuel will have a high aniline point, since there is a great difference between the molecular structure of aniline (an aromatic) and a paraffin. The opposite is true for high aromatic contents, since an aromatic is more easily soluble in another aromatic than is a paraffin.

 

Aniline Point

It has been observed that hydrocarbons of low specific gravities have lower aniline points than hydrocarbons with higher specific gravities.Thus the specific gravity as well as “the paraffinicity of the fuel” affect the aniline point.

when the aniline point of a diesel fuel (in F) is multiplied by its (A.P.I) gravity/l00), which is inversely proportional to the specific gravity, a value representing the real paraffinicity of the fuel is obtained.

Diesel Index

D.I. = Aniline Point(˚F )x ˚API 100

It has been found that, the diesel index of a diesel fuel is approximately equal to its cetane number.

The “diesel index” is sometimes used in the specifications of diesel fuels instead of the cetane number.

Diesel engines may be classified into the following 3 groups according to their speeds:

High speed engine: (speeds above 800 r.p.m). A high cetane number diesel fuel is necessary to run such an engine. The fuel is usually specified to have a minimum cetane number of 52 which could only be obtained by using gas oils.

  Intermediate speed engine: (speeds between 300 and 800

r.p.m.) The minimum cetane number required for such engines is about 40. Such fuels used may be gas oil or diesel oil.

Slow speed engines: (speeds below 300 r.p.m). No limit is specified for the cetane number of the fuels for such engines (usually diesel oil is used).

N.B. The lowest cetane number of diesel fuels is in the range of 20-25.

Classification of Diesel Engines

Carbon Residue

Sulphur Content, a maximum of 1.5- 2 %

Pour Point, a maximum of –10 C

Viscosity, a minimum of 30 secs Red. I at 100 F is specified for gas oil and a maximum of 45 secs. for gas oil and 60 secs for diesel oil.

Flash point, above 150 F - using the “Pensky-Martens” closed cup tester for safety purposes. However, this limit may be reduced to 130 F (at least in countries like Egypt).

Specifications of Diesel Fuels

Biodiesel

Feedstock (Raw Material)

Fats

Animal Fat

SoybeanRapeseed

Jatropha

WVO

Mixing

Reaction Separator Washi

ng

Production

Biofuels vs. Fossil Fuels

Biofuels: Strengths & Opportunities

1. Fossil fuels contribute to global warming by increasing GHG emissions

Biofuels reduce GHG emissions (closed carbon dioxide cycle)

BiofuelsFossils

Biofuels: Strengths & Opportunities

2. Energy Security: Decrease dependency on crude oil Decrease imports of crude oil Energy diversification

3. Renewable4. Less toxic5. Sustainable use of otherwise problematic

waste products

Biofuels: Strengths & Opportunities

6. Economic Benefits

Economic use of wasteland Useful co-products Carbon Credit Opportunities Additional distribution channel for agricultural

products Decentralized production and thus strengthen rural

economies

Special Advantages of Biodiesel

The properties of biodiesel (viscosity, ignition properties)

are similar to those of fossil diesel.

Cetane number and lubricating effect of biodiesel are

significantly higher than fossil diesel.

The toxicity of biodiesel is lower compared to fossil diesel.

Biodiesel can be blended with petrodiesel at any ratio.

International Tendencies (Brazil)

International Tendencies (Africa)

Bioethanol Production Africa 2006: 606,000 liters = 1.2% global production (US + Brazil: 75% global production)