03-IC Engine Fuels

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IC Engine Fuels

Their Properties and Tests

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Fuels for IC Engines

The main fuel for IC engines is derived from crude petroleum which is a mixture of many hydrocarbon compounds

Crude oil is made up almost entirely of carbon and hydrogen with some traces of other species

It varies from 83% to 87% carbon and 11% to 14% hydrogen by weight

The crude oil mixture which is taken from the ground is separated into component products by cracking and/or distillation using thermal or catalytic methods

It is entirely possible, but not economically feasible, to convert almost completely the crude petroleum into gasoline and diesel fuel

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A: No. 2D diesel fuel (good grade)

B: No. 1D light distillate (high-

speed diesel engine and JP-5 jet

fuel)

C: Kerosene

D: JP-4 jet fuel

E: Automotive gasoline

F: Aviation gasoline

Families of IC Engine Fuels

The Paraffin family (Alkanes) open-chain structure

(aliphatic hydrocarbons) with general formula CnH2n+2 The critical compression ratio (CR) for audible knock in an

SI engine decreases rapidly as the length of the chain of

the normal members is increased

Thus the normal paraffins in the volatility range of gasoline

are poor SI fuels

The reference scale to measure SI knock has been

established by arbitrarily selecting two primary reference

fuels iso-octane (2,2,4 trimethyl pentane) has been

arbitrarily assigned an "octane rating" of 100 while n-

heptane has been assigned an "octane rating" of zero


The knock ratings of the fuels are in rough proportion to

the self-ignition temperatures

In progressing downward with higher carbons, the

suitability of the fuels for SIE progressively decreases,

and for CI engines, progressively increases thus,

hexadecane (cetane) has a low self-ignition temperature

and therefore it is a good fuel to prevent knock in a CIE

The reference scale for measuring CI knock is based

upon hexadecane and heptamethylnonane as primary

fuels with assigned values of 100 cetane and 15 cetane

respectively


Interestingly, the air-fuel ratio (AFR) for the chemically

correct mixture is essentially constant even though the

fuel structure and phase change

Because of this constancy, various fuels can be supplied

to SIE without changing the carburettor adjustment

Also that the energy content per unit volume of mixture is

essentially constant and therefore the power output of

the engine is not affected by a change in fuels (unless

knock is present)

The fuels of this family are stable, clean burning and do

not attack gaskets or metals
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The Olefin family (Alkenes) the mono-olefins have

open-chain structures (aliphatic olefins) with genral

formula CnH2n

The physical properties correspond closely to similar

compounds in the paraffin family and are also clean

burning and a higher octane rating

Because of the free bond, the olefins are chemically

active and unite readily with hydrogen to form the

corresponding paraffin or naphthene

When they unite with oxygen, they form an undesirable

residue, gum which can cause smog
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The Di-olefin family diolefins have two double bonds

with a general formula CnH2n-2

These are undesirable fuel components because, upon

storage, reactions take place that lead to coloring of the

fuel and, also, to the formation of a cloudy gum

The Naphthene or Cycloparaffin family (Cyclanes)

have the same general formula as the olefins but are

saturated, ring-structure compounds

These are desirable components of motor gasoline

The Aromatic family (Benzene derivatives) ring-

structured hydrocarbons with general formula CnH2n-6
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These are excellent gasoline fuels and can be selectively

produced by catalytic cracking or by thermal cracking at

high temperatures (1200 F)

Benzene or benzol is an excellent blending agent to

raise the octane ratings of low-grade fuels

The aromatics have the highest densities of the HC and

therefore have the highest heating values per unit

volume

These fuels are stable in storage, smoky in burning with

high solvency powers


The Alcohols are a partial oxidation product of petroleum with a chain structure of the general formula ROH, where R is the paraffin group

They have good antiknock qualities with octane ratings in excess of 100

Alcohols absorb water from atmosphere

They also require a different AFR than gasoline

Gas or natural gas can be associated or unassociated

The composition of gas varies widely with methane usually predominating from 60 to 98% and with percentages of ethane and other paraffins along with carbon dioxide, helium, and nitrogen


A sour gas is one which contains hydrogen sulphide,

otherwise, it is called sweet gas

Before the gas enters the pipeline it must be sweetened,

dehydrated, and liquid HC removed

Synthetic or substitute natural gas (SNG) are produced

from solid wastes, petroleum crudes or fractions, or coal

Liquefied petroleum gas (LPG) may be propane, butane,

or a mixture of the two

Natural gas and LPG are excellent fuels for the SIE,

home, and gas-diesel with low emission pollutants

LNG and CNG are the two other versions of natural gas

Characteristics of Modern Gasoline

Modern gasoline may be made up of straight-run

gasoline (from fractional distillation), cracked gasoline

(from catalytic cracking), reformate (from catalytic

reforming), alkylate and polymerized gasolines

(produced from gases), with some butane or propane to

achieve the desired Reid vapour pressure

The two most important characteristics of gasoline are its

volatility and octane number

The gasoline sold on the market is a blend of a number

of products produced in several processes

The gasoline, irrespective of its origin, should have the

properties listed as follows:

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Desirable Properties of Gasoline

Knock characteristics the octane rating the fuel

should have an octane rating to fit the engine

requirements

The tests for determining octane number are performed

using the ASTM-CFR (Cooperative Fuel Research)

engine, a variable compression ratio engine

Two octane numbers are: research octane number (RON)

and motor octane number (MON), the former is generally

higher

High octane fuels enable high compression ratios to be used

give increased power output and improved economy

Typically, a CR of 7.5 requires 85 octane fuel, while a CR of

10.0 would require 100 octane fuel

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Volatility expressed in terms of the volume percentage

that is distilled at or below fixed temperatures

If the fuel is too volatile

It will start the engine readily

When it is used at high ambient temperatures, the fuel is

liable to vaporise in the fuel lines and form vapour locks

If the fuel is not suffieciently volatile

The engine will be difficult to start, especially at low ambient

temperatures

In general, the fuel with the lowest distillation temperatures

is the best

Dilution of the lube oil may occur when the fuel condenses

of fails to vaporise in the engine

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The volatility also influences the cold start fuel economy

SIE are started on very rich mixtures, and continue to

operate on rich mixtures until they reach their normal

operating temperature increasing the volatility of the

gasoline at low temperatures will evidently improve the fuel

economy during and after starting

Gasoline stored for a long time in vented tanks is said to go

stale, the loss of more volatile components that are necessary

for easy engine starting

Gum and Varnish Deposits the fuel should not deposit either gum

or varnish in the engine

Corrosion the fuel and the products of combustion should be non-

corrosive

Cost the fuel should be inexpensive

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Additives Used in Gasoline

In order to achieve the required properties of gasoline,

as well as for other purposes, various additives are used:

Antiknock to reduce or eliminate the knock in SIE

Previously TEL or TML were used along with some

scavengers; now-a-days, MTBE or TBA are used

Deposit modifiers to alter the chemical character of

combustion chamber deposits and so reduce surface

ignition and spark plug fouling usually, phosphorous and

boron compounds are used

Antioxidants are used to prevent gum formation

amines of amount 0.5 to 6.5 kg per 1000 bbl

Detergents are used to prevent deposits in carburettor

and manifold alkyl amine phosphates, of amount, 5 kg

per 1000 bbl17
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Additives Used in Gasoline

Lubricants to lubricate valve giudes and upper cylinder regions 0.1 to 0.5 percent light mineral oils

Metal deactivators to destroy the catalytic activity of traces of copper amine derivatives, of amount, 0.5 kg per 1000 bbl

Anti-rust agents to prevent rust and corrosion arising from water (and air) fatty-acid amines, sulfonates or alkyl phosphates, of amount 0.5 to 6.5 kg per 1000 bbl

Anti-icing agents to prevent "gasoline freeze" from water in fuel and throttle-plate icing from water in air methyl alcohol and isopropyl alcohols of about 1% each are used for the purpose; sometimes, a surface-action agent, such as ammonia salts or phosphates (about 0.005%) is also used
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Characteristics of CI Engine Fuels

The most important characteristic of diesel fuel is the

cetane number which indicates how readily the fuel self-

ignites usually, the self-ignition temperatures of the

normal paraffins decrease as the length of chain

increases

Cetane (hexadecane, C16H34) is the primary standard of

the cetane scale, with an arbitrary rating of 100 (isocetane,

heptamethylnonane (HMN) is at the bottom of the scale

with a cetane number 15) while other paraffins have

cetane ratings which vary almost linearly with the length of

the chain thus the straight-run gasoline fuels which might

be poor SI fuels, are desirable for CIE
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The other important characteristic is the viscosity,

especially for lower-grade fuels used in the larger

engines, may need to have heated fuel lines

Another problem with diesel fuels is that, at low

temperatures, the high molecular weight components

can precipitate to form waxy deposit (cold filter plugging

point)

The requirements for a good CI fuel cannot be as simply

stated as those for gasoline

Because of the added complexity of the CIE from its

heterogeneous combustion process, which is strongly

affected by injection characteristics

Some of the desirable characteristics are listed below:20


Knock characteristics if an engine runs on a fuel with too

low cetane number, there will be diesel knock

Diesel knock is caused by too rapid combustion and is the

result of a long ignition delay period, since during this period

fuel is injected and mixes with air to form a combustible

mixture

Ignition occurs only after the pressure and temperature have

been above certain limits for sufficient time, and fuels with

high cetane numbers are those that self-ignite readily

The minimum cetane number of the good grade diesel fuel is

50 and for the other is 45

Starting characteristics the fuel should start the engine

easily requires high volatility to form readily combustible

mixture, a high cetane rating so that the self-ignition

temperature is low21


Smoke and odour the fuel should not produce smoke or

odour after combustion

Corrosion and wear should not cause corrosion before

combustion, or corrosion and wear after combustion

directly related to sulphur, ash, and residue contents of the

fuel

Handling ease should be liquid that will readily flow

under all conditions measured by the pour point and

viscosity of fuel; the fuel should also have a high flash

point since an advantage of the CIE is its use of fuels with

low fire hazards

Flash point of diesel fuel is at least 55C, while that for petrol

and kerosene are about 40C and 30C respectively

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Additives in diesel fuel to improve the cetane number are

referred to as ignition accelerators their

concentrations are greater than those of antiknock

additives used in petrol

By adding 1% v/v of amyl nitrate, C5H11ONO2 increases

the cetane number by about 6; ethyl nitrate, C2H5ONO2and ethyl nitrite, C2H5ONO are other effective substances

Ignition delay is most pronounced at slow speeds

because of the reduced temperature and pressure

during compression

Cold-starting can be a problem, and under severe

conditions, heaters may be needed; also excess fuel may

be injected

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Sometimes, volatile fuels with high cetane numbers, such

as ether, can be added to the intake air

Sometimes a cetane index is used, as the only

information needed is fuel viscosity and density with no

need for engine tests

The cetane index can be used only for straight petroleum

distillates without additives

Other fuels that are suitable for diesel engines are derived

from coal and vegetable oils

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Tests for Diesel

Viscosity is exactly defined as the ratio of shearing

stress in a fluid to the rate of shear, and is a measure of

the resistance of the fluid to flow

Viscosity changes rapidly with temperature, thus the test

temperature must be specified

The viscosity of the fuel exerts a strong influence on the

shape of the fuel spray high viscosity causes low

atomization and high penetration of the spray jet

In small combustion chambers, the effect of viscosity may

be critical

The SU viscosity required for most high-speed engines

ranges between 35 and 70 sec (100F)

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Tests for Diesel

Gravity is an indication of the density or mass per unit

volume of the fuel defined as the mass of a unit

volume of fluid (at 60F) to that of the same volume of

water (at 60F)

The API gravity is defined in terms of the specific gravity

API Gravity = [(141.5/sp. Gr. At 60F/60F) 131.5]

In general, high API gravities imply high cetane fuels

Sulphur Corrosion and increased wear is caused in

the engine along with carbon deposits on the piston and

rings and deterioration of lub oil due to the presence of

sulphur in fuels

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Tests for Diesel

Sulphur trioxide is formed during diesel combustion which

may attack lub oil on the cylinder walls to form resinous

materials that harden to form varnish and carbon

SO3 may also form sulphuric acid

Sulphur content over 1.0% is detrimental, while amounts of

0.5% are economically feasible

Carbon Residue is the result of the fuel burned with a

limited amount of oxygen

In the Conradson carbon test, a sample of the fuel in a

crucible is heated to a high temperature for a relatively

long period of time; the percentage by mass of residue to

the original sample is the carbon residue

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Tests for Diesel

High carbon residues contribute to deposits in the

combustion chamber and around nozzle tips, thus

interfering with the spray shape

Ash In testing for ash, the fuel is heated until the

vapours can be ignited when the flame dies away, any

carbonaceous material is oxidized by heating in a flame

or muffle furnace the unburned residue is called ash

The ash content is a measure of the abrasiveness of the

products of combustion that could cause wear in the

engine

Water and Sediment of all specifications for diesel, the cleanliness

factor is probably the most important because of the precisely fitted

parts in the fuel pump and nozzle

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Tests for Diesel

Flash Point is the lowest temperature of the fuel that

allows inflammable vapours to be formed

It is found by heating the fuel slowly and then sweeping a

flame across the surface of the liquid, a distinct flash is

obtained at the flash point

It is important for safety purposes and serves as a

measure of the fire hazard

Distillation range should be as low as possible without

unduly affecting the flash point, the burning quality, or

the viscosity of the fuel

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Tests for Diesel

Ignition Quality The ease of igniting the fuel in the

engine by autoignition is called the ignition quality of the

fuel; the diesel index number (DI) has been empirically

found to correlate, approximately, the cetane number of

most commercial fuels which is defined as:

DI =[aniline point (F) API Gravity (60F)]/100

The DI and the cetane rating of the fuel for most high-

speed diesels should be of the order of 50-60

Cetane ratings below 40 may cause exhaust smoke, with

increased fuel consumption and loss of power

Pour Point is determined by cooling a sample of oil in a test jar

until , when the jar is displaced from the vertical to horizontal

position, no perceptible movement of the fuel will occur (within 5

secs)30
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Relation between Cetane Rating and DI

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Tests for Diesel

Heating Value or the heat of combustion is detrmined

by burnign the fuel with oxygen in a bomb and noting the

temperature rise of a cooling bath

If all of the water vapour can be condensed, the HHV is

obtained, if none of the water vapour is condensed, the

LHV is obtained

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Tests for Diesel

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03-IC Engine Fuels