chapter 3 conventional fuels and alternative fuels...

Chapter 3

CONVENTIONAL FUELS AND ALTERNATIVE FUELS

Fossil fuels are formed by natural resources such as anaerobic decomposition of

buried dead organisms. The age of the organisms and their resulting fossil fuels is

typically millions of years, and sometimes exceeds 650 million years. The fossil fuels

include coal, petroleum, and natural gas which contain high percentages of carbon.

Fossil fuels range from volatile materials with low carbon:hydrogen ratio like

methane, liquid petroleum and nonvolatile materials composed of almost pure carbon like

anthracite coal. Methane can be found in hydrocarbon fields as alone, associated with oil,

or in the form of methane clathrates. It is generally accepted that they formed from the

fossilized remains of dead plants and animals by exposure to heat and pressure in the

Earth's crust over millions of years.

3.1 Fuel & Oxidizer

Chemically, the fuel can be defined as one, which donates electrons. In contrast,

an oxidizer can be defined as one, which accepts electrons. This property of elements to

accept or donate electrons is known as electronegativity, which dictates whether an

element can be classified as fuel or an oxidizer. This word electronegativity is defined as

the power of an atom in a molecule to attract electrons towards it. The electronegativity

values for various elements are given in Table 3.1 below.

Table 3.1 Electronegativity of various elements [25].

Elements Electronegativity

O 3.5

N, Cl 3.0

C, S, I 2.5

H, P 2.1

B 2.0

Be, Al 1.5

Mg 1.2

Chapter 3 - CONVENTIONAL FUELS AND ALTERNATIVE FUELS

28

Oxygen has the second highest electronegativity. Although, it is available

abundantly in nature, it is mostly accompanied with nitrogen which, being an inert gas,

reduces the actual capability of oxygen. The fuels such as carbon, hydrogen, aluminium,

magnesium, etc. have lower electronegativity in comparison to oxygen. In combustion

studies, air is a very common oxidizer used most of the times in which the fuel elements

such as carbon, hydrogen, aluminium, boron, etc oxidized easily by oxygen. The most

common fuels that we use in our day-to-day life are basically hydrocarbons. Of course,

hydrogen is emerging as the next generation fuel being the least pollutant fuel. Metals are

used as additives in some special fuels particularly in rocket engine.

3.1.1. Gaseous Fuel and Oxidizer

Recently, gaseous fuels are preferred most over liquid and solid fuels as it is easier

to control emissions from gaseous fuel operated combustion devices as they do not

contain any mineral impurities and are easier to burn for achieving higher efficiency.

Besides this, a gas handling system is least expensive to establish and operate among all

others forms of fuels. The most common gaseous fuels that we use profusely are natural

gas and liquefied petroleum gas. Apart from these two fuels, other gases such as biogas,

producer gas, coke oven gas, acetylene, methane, propane and hydrogen are being used as

fuels. Table 3.2 summarizes several commonly used gaseous fuels along with their

applications.

Natural gas is generally found in porous rocks, which is believed to be formed by

anaerobic (bacteria-assisted) decomposition of organic matter under heat and pressure,

million years ago. It contains mainly methane (CH4) ranging from 75 to 99% by volume

and other gases such as CO, CO2, N2, C2H6 etc as shown in Table 3.3. It must be kept in

mind that natural gases like coal and coke oven gas has regional variations. Recently,

liquefied natural gas is being produced by condensing dry natural gas at -102ºC causing

cryogenic refrigeration systems.


29

Table 3.2 Types of gaseous fuel and oxidizer.

Fuel Oxidizer Application

1. Liquefied Petroleum Gas (LPG) Air/O2 Domestic, burner, furnace.

2. Natural Gas (NG) Air/O2 IC engines, furnace

3. Producer Gas Air/O2 IC engine

4. CH4, C3H8, H2 Air/O2 IC engine

5. Biogas Air/O2 Engine, burner

6. Acetylene Air/O2 Gas welding, cutting

Liquidfied Petroleum Gas mainly consists of propane and butane. The typical

composition of LPG is shown in Table 3.3. Propane and butane are generally produced by

atmospheric fractional distillation of crude oil. Interestingly, LPG can be stored as liquid

in tank at around pressure of 0.8 MPa at normal atmospheric temperature (around 35ºC).

But it becomes gas when it is released to ambient pressure (0.1MPa), because butane has

a boiling point of -0.5ºC while propane has boiling point of -42.1ºC at ambient pressure.

Table 3.3 Typical compositions of gaseous fuels [26].

Type CO2

% O2

%

N2

%

CO

%

H2

%

CH4

%

C2H6

%

C3H8

%

C4H10

%

SG Heating value

(kJ/m3)

Gross Net

Natural

Gas

… … 5.0 … … 90.0 5.0 … … 0.60 37300 33680

Biogas 33 … 1 … 1 65 … … … 0.80 28890 25700

Propane … … … … … … 2.2 97.3 0.5 1.55 95290 87840

Butane … … … … … … … 6.0 94.0 2.04 119580 110300

Coal Gas 2.1 0.4 4.4 13.5 51.9 24.3 … … … 0.42 19370 17360

Coke oven

Gas

2.2 0.8 8.1 6.3 46.5 32.1 … … … 0.44 21200 18960

Producer

Gas

8.0 0.1 50 23.2 17.7 1.0 … … … 0.86 5330 4950

Blast

furnace

11.5 … 60 27.5 1.0 … … … … 1.02 3430 3430

LPG … … … … … … … 70.0 30.0 1697 102577 64878


30

Biogas is gaining popularity as a renewable source of energy in Third World

countries, which can be easily produced by anaerobic digestion of biomass. In India, the

cattle dung is the main constituent through which biogas is produced in a digester. It

mainly contains methane and carbon dioxide. Typical compositions of biogas can be

found in Table 3.3. It finds application both in direct burning (cooking stove) and power

producing devices mainly in IC engines.

Producer gas can be generated by gasifying biomass, coal, etc. It finds

applications both in direct heating and power producing devices. A typical producer gas is

given in Table 3.3. It can be noted that primary fuels are carbon monoxide and hydrogen.

The other constituents of this gas are nitrogen (around 40%) and carbon dioxide which

make it a low calorific value fuel. However, using oxygen instead of air for gasifying the

solid fuel enhances the heating content of this gas.

In recent times, hydrogen is contemplated as the future green fuel as it produces

least emission. Hydrogen can be easily produced by reforming natural gas, partial

oxidation of liquid fuels and gasification of coal or biomass. Acetylene can be produced

by hydration of calcium carbide, which is being very much used in gas welding shops in

India.

Gaseous fuels can be characterized by their composition, density and heating

value. The compositions of various gaseous fuels are already shown in Table 3.3. The

heating value of a fuel can be defined as the amount of heat released per unit volume

when it undergoes oxidation at normal pressure and temperature (0.1 MPa and 298 K).

The Higher Heating Value (HHV) corresponds to the heating value of fuel when water is

condensed. In contrast, Lower Heating Value (LHV) of a fuel is referred as amount of

heat released by burning of 1 kg of fuel assuming that latent heat of vaporization in the

reaction products is not recovered. The following expression can relate both these two

heating values:

vfuelOH HmmHHVLHV )/(2

(3.1) Where ∆Hv is the latent heat of vaporization of water at 298.15 K.


31

3.1.2 Liquid Fuel and Oxidizer

Liquid fuel is one of the major energy sources, particularly in transport sector.

Some of the common and special liquid fuels and oxidizers are listed in Table 3.4, along

with their respective applications.

Liquid fuels are mainly obtained from the crude oil. Besides this, liquid fuels can

also be obtained from biomass, coal tar, sand and oil shale, etc. Basically, typical crude

oil is a mixture of alkanes (propane, butane etc.), alkenes, aromatics like benzene,

toluene, etc.) and cycloalkanes (napthene) of organic compounds containing C, H, O, N

and S elements.

Table 3.4 Types of liquid fuels and oxidizers .

Fuel Oxidizer Application

1. Gasoline Air SI engine, aircraft

piston engine

2. HSD Air CI engine

3. Furnace oil Air Furnaces

4. Kerosene Air Aircraft, gas turbine,

ramjet, domestic

5. Alcohols Air IC engine

6. Hydrazine, Unsymmetric Di-Methyl

Hydrazine (UDMH), Mono-Methyl

Hydrazine (MMH), Liquid hydrogen,

triethyl amine

Liquid O2, Red

Fuming Nitric

Acid (RFNA),

N2O4

Ramjet/scramjet,

liquid propellant

rocket

The crude oil from Assam, India contains on an average, 40% paraffin, higher

alkanes, 25% napthene and cycloalkanes and 35% aromatics. The ultimate analysis of a

crude oil indicates that, on an average, it contains 84% of carbon, up to 0.5% of nitrogen

and 0.5% of sulphur.


32

Table 3.5 Fuel properties of certain common liquid fuels [27,28].

Fuel type Automotive

Gasoline

Diesel

Fuel

Methanol Kerosene Aviation

Turbine

Fuel

Specific gravity 0.72-0.78 0.85 0.796 0.82 0.71

Kinematics viscosity

At 293 K (m2/s)

0.810-6 2.510-6 0.7510-6 3.62610-6

Boiling point range

(K) at STP

303-576 483-508 338 423-473 442

Flash point (K) 230 325 284 311 325

Auto-ignition

Temperature (K)

643 527 737 483

Stoichiometric air/fuel

by weight

14.7 14.7 6.45 15 15.1

Heat of vaporization

(kJ/kg)

380 375 1185 298.5 --

Lower heating value

(MJ/kg)

43.5 45 20.1 45.2 43.3

3.1.3 Solid Fuel and Oxidizer

Wood, coal, charcoal, soft coke, agricultural waste (biomass), animal dung are

some of the widely used solid fuels, which are derived basically from fuels available

naturally. Biomass contains plant products such as wood, leaves, bark, branches,

agricultural residues like rice husk, rice straw, wheat straw, groundnut shell, etc. Biomass

as a fuel has been in use since time immemorial. It is well known that coal has been

produced from biomass millions years ago due to natural calamities while entrapped

under high pressure and temperature conditions. Coals can be mined either from open or

deep underground mines. Municipal waste and industrial refuse can also be used as solid

fuels. Besides these, several special fuels using polymers are being devised in last fifty

years to meet the demand of solid fuels in the chemical rocket engines. Several

applications of certain solid fuels are given in Table 3.6.


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Table 3.6 Types of solid fuels and oxidizers.

Solid fuel Oxidizer Applications

1. Biomass (wood, sawdust,

etc.)

Air, O2 Domestic, IC engine

with producer gas

2. Coal, coke Air, O2 Domestic, IC engine

3. Special fuels Nitro Cellulose

(NC), Hydroxyl Terminated

Poly-Butadiene (HTPB),

Carboxyl Terminated Poly-

Butadiene (CTPB)

Nitroglycerine,

Ammonium

percolate,

Ammonium nitrate,

Nitrogen tetra oxide

Solid propellant rocket,

hybrid rocket

3.2 Conventional Fuels

Most of the road vehicles today operate on Petroleum derived Gasoline and Diesel

fuels. Between 30 to 70% of the Petroleum crude throughout the oil refineries is

converted to automotive fuels thus controlling the refinery economics. Since the first

internal combustion engine and the automobile were developed more than 100 years ago,

the properties of engine fuel have been changing. A number of factors have been

responsible for evolution of modern fuels. Crude oil prices, progress in refinery

processing technology, developments in engine and vehicle technology, vehicle

performance and durability requirements, and more recently the environmental

regulations besides the geo-political considerations are the important factors that have

brought changes in fuel quality.

In early days, the main objective of engine designers was to improve power out

put and reliability. The one obvious route to improve performance of the spark ignition

engine was to increase engine compression ratio. With increase in engine compression

ratio however, knocking combustion was encountered in SI engines demanding high

octane number fuels to suppress engine knock. In the pursuit to increase knock resistance

of Gasoline, tetra-ethyl lead (TEL), an anti-knock additive was discovered in 1921 by

Thomas Midgley of General Motors Research Laboratory and was introduced for use in

Gasoline on February 1, 1923 [29]. The principal requirements of automotive fuels are

given in Table 3.7.


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Table 3.7 Principal Quality Requirements of Automotive fuels.

Fuel Quality Relationship with engine and vehicle performance

Combustion

Quality

Better ignition and combustion qualities result in better fuel economy

and reduction in emissions of pollutants. High octane number for SI

engines and cetane number for CI engines are necessary for good

combustion.

High heat of

combustion

A smaller mass of fuel is to be carried on board of vehicle for the

same operation range.

High volumetric

energy content

A smaller fuel tank and lower vehicle space is necessary, improving

vehicle packaging. Liquid fuels being sold on volume basis, it results

in better economics for the operators.

Low temperature

performance

A significant fraction of fuel should vaporize at low engine

temperatures for a better engine cold start and warm-up, good low-

temperature drivability, fuel economy and emissions.

High temperature

performance

For ease of hot starting, reduced vapour lock and evaporative

emissions, fuels are blended appropriately to meet the needs of

seasonal and geographical variations in ambient temperature.

Oxidation

Stability

Good low temperature oxidation stability reduces fuel deterioration

during storage and deposit formation in the fuel system.

Deposit formation

control

Helps in maintaining the engine performance, fuel economy and

emissions close to the designed level by keeping the fuel and

combustion systems clean. Deposit control additives are low cost

products and now widely used for minimizing deposit formation.

Material

compatibility

Material compatibility is essential to prevent corrosion of metallic

and deterioration of rubber and elastomeric components of the fuel

system.

Flow

characteristics

Fuel has to be in fluid condition at low temperature and is important

particularly for diesel fuels. Also appropriate diesel fuel viscosity is

essential for flow as well as good injection characteristics.


35

High octane number fuels could be produced at a low cost with the use of TEL. It

led to increase in engine compression ratio to 10.5-11.0: 1 during late 1950s and 1960s in

the USA and Europe. However, since 1970 reduction of engine emissions has become an

over-riding requirement for the engine designers and fuel engineers. TEL was widely

used in Gasoline until 1975 when the Gasoline vehicles for the first time employed

catalytic converters for emission control. Since then, TEL has been gradually phased-out

from Gasoline and today Gasoline is almost totally lead-free all over the world. Besides

the environmental considerations, there are several other requirements that are to be met

by the engine fuels.

3.2.1 Motor Gasoline

Gasoline is a mixture of nearly 400 different types of hydrocarbons [30]. The types

of hydrocarbons present in Gasoline are normal-paraffins, iso-paraffins, olefins, aromatics

and to a smaller extent cyclo-paraffins. It has hydrogen to carbon ratio varying from 1.7

to 2.0 and is typically characterized by the molecular formula C8H16. The Gasoline is

liquid at room temperature with boiling range approximately of 35 – 315º C. The

principal properties of Gasoline that are specified in the fuel standards are:

Distillation, ºC

Reid vapour pressure

Specific gravity

Research Octane Number (RON)

Motor Octane Number (MON)

Oxidation stability

Gum content, mg/100 ml

Lead content, g/l

Sulfur, wt %

Benzene, vol. %


36

Octane Quality

High antiknock quality of Gasoline is needed to prevent or minimize knocking

combustion in high compression ratio SI engines. Knocking combustion in SI engines can

cause engine overheating, loss in efficiency and increase in emissions. Persistent

knocking can lead to mechanical damage to engine under high load operation.

Earlier lead antiknocks, tetra-ethyl lead and tetra-methyl lead were used to boost

octane number of Gasoline. Now however, due to catalyst poisoning and lead being a

health hazard by itself, Gasoline world over has become almost lead-free. The lead-free

Gasoline is blended with high-octane fuel components like aromatics, iso-parrafins,

alcohols and methyl tertiary butyl ether (MTBE) to improve anti-knock quality. The

regular grade Gasoline in Europe has a minimum of 91 RON and 82.5 MON. Due to

refinery economic reasons, the octane quality of premium unleaded Gasoline however, is

now kept at 95 RON and 85 MON as compared to 98 RON and 87 MON for the leaded

premium Gasoline earlier [31].

Volatility

The volatility of Gasoline is experimentally evaluated by its distillation range and

Reid vapour pressure (RVP). RVP measured by American Society for testing and

Materials (ASTM) is an important parameter of Gasoline front-end volatility. It has good

correlation with the evaporative losses during refueling, from tank vent and carburetor

when vehicle is running or during heat soaking after vehicle is stopped. Some other

parameter derived from the distillation characteristics and RVP are also used to evaluate

drivability performance of Gasoline. For hot weather drivability performance, vapour

lock index (VLI) is widely used in Europe and other countries.

Oxidation Stability

The fuel stays in the storage and transportation systems for several weeks after

production before it is consumed in vehicles or engines. During this period, the fuel

undergoes slow oxidation under the prevailing ambient conditions. Oxidation stability of

Gasoline is a measure of its suitability for long-term storage and its tendency to form

deposits in the engine especially the fuel system. The most commonly used methods are

induction period and existent gum test.


37

To improve oxidation stability of Gasoline anti-oxidants are usually added. The

type and amount of antioxidants depends on the Gasoline composition and storage

demands. These additives are based on aromatic diamine, alkyl phenol and amino-phenol

compounds. Additives called as metal deactivators are also used to nullify the catalytic

oxidation effect of some metals like copper present in the Gasoline.

Oxygenate Content

Various oxygenates, mainly ethers and alcohols are blended in unleaded Gasoline

to boost octane number of Gasoline. After the use of lead antiknocks has been banned,

use of ethers and alcohols for improving octane quality provides a relatively low cost

option to the fuel refiners. The main types of alcohols and ethers that have been used are

given in Table 3.8.

Table 3.8 Oxygenates used in Gasoline.

Alcohols Ethers

Methanol

Ethanol

Isopropyl alcohol

Isobutyl alcohol

Tertiary butyl alcohol (TBA)

Methyl tertiary butyl ether (MTBE)

Tertiary amyl methyl ether (TAME)

Ethyl tertiary butyl ether (ETBE)

The amount of oxygenates added to Gasoline is limited for use in the vehicles

designed to operate on the conventional hydrocarbon fuels mainly due to two reasons.

One is mixture leaning effect due to presence of oxygen in the fuel and another reason is

their adverse effect on the fuel system materials. In addition, the use of oxygenates

particularly alcohols increases fuel volatility and may lead to hot weather drivability

problems caused by vapour lock. Also, increase in RVP would increase evaporative

emissions, resulting in further environmental pollution caused by unburned fuel

emissions. Increase in aldehydes emissions with the use of oxygenates containing

Gasoline, particularly in the non-catalyst equipped cars is another concern.


38

3.2.2 Diesel Fuels

Diesel fuels are mixtures of a large number of hydrocarbons, which generally boil

within the temperature range of 150 to 390º C. Earlier, diesel fuels were produced mostly

by blending a number of refinery streams from the atmospheric distillation unit. However,

to meet the increasing demand of the diesel fuels, products of secondary refinery

conversion processes like thermal and catalytic cracking, hydro-cracking etc are also

blended in the current diesel fuels. The quality of the final product depends on the

characteristics of the crude oil processed and the characteristics of the various streams

blended in the product. The key properties of the diesel fuel, which affect engine

performance and emissions, are:

Ignition quality

Distillation (volatility) characteristics

Density

Viscosity

Hydrocarbon composition

Sulphur content, and

Stability and injection system cleanliness

Other significant properties include cold flow characteristics at low ambient

temperatures, water and sediment content, carbon residue etc.

Ignition Quality

Ignition quality of diesel fuel is an expression of the ease of self-ignition of diesel

injection spray in the hot compressed air inside the engine cylinder. Cetane number is the

most widely used and accepted measure of ignition quality of the diesel fuels.

A higher cetane number is being specified now for the diesel fuels as it results in

improved cold starting, warm-up, reduced combustion noise, lower fuel consumption and

exhaust emissions. Cetane or ignition improvers are used to improve ignition quality of

the diesel fuels, which do not naturally meet the specification limits. In premium quality

diesel fuels, oil companies use cetane improvers to produce diesel fuels even above the

specification limits in certain countries. Most commonly used cetane improving additives


39

are nitrates and peroxides like iso-propyl nitrate, cycle-hexyl nitrates, ethyl-hexyl nitrate

(EHN) and di-tertiaryl-butyl peroxide. These compounds decompose readily at high

compression temperatures in the engine, produce free radicals that accelerate

precombustion reactions in the fuel-air mixture and thereby reduce ignition delay. The

response of cetane improvers depends on the cetane number and hydrocarbon

composition of the base fuel.

Distillation Range

Higher the volatility of the fuel more readily it vaporizes, mixes with air and burns

in the combustion chamber. The low volatility components boiling above 350º C may not

burn completely forming engine deposits and producing high exhaust smoke emissions.

The mid boiling point often is taken as an overall representation of the fuel volatility and

has been observed to affect smoke emissions [32,33]. The volatility is also correlated to the

other physio-chemical properties of the fuel like, density, viscosity and ignition quality.

Density

The specific gravity of diesel fuel varies generally in the range 0.81-0.88. It is

interrelated to volatility, cetane number, viscosity and heat of combustion etc. In general,

an increase of 10% in density decreases heat of combustion by about 4% and, thus the

volumetric energy density of the fuel increases by approximately 6% [32]. The fuel

injection pump calibration being on volume basis, the fuel density influences engine

power, fuel economy and smoke emissions.

Viscosity

The viscosity of diesel fuel affects injection characteristics, fuel atomization, drop

size distribution, spray cone angle and penetration. An increase in viscosity reduces spray

cone angle, increase droplet size and spray penetration. Viscosity of fuel affects fluidity

of fuel at low temperatures. High viscosity can reduce fuel flow rates to the injection

system resulting in an inadequate fuelling of the engine. Low viscosity on the other hand,

results in an increase in leakage of fuel past the pumping elements. If the fuel viscosity is

too low, at high temperatures it could result in total fuel leakage. Thus, the fuel viscosity

influences metering characteristics of the injection system. Viscosity of fuel is important

for lubrication and protection of the injection equipment from wear. Therefore, for a


40

given engine application the fuel viscosity range is specifies. Most specification limits

kinematic viscosity of diesel fuel in the range of 2 to 5 centistokes.

Stability

Resistance of fuel to degradation during storage before it is consumed is an

important requirement. More cracked products are being blended in diesel fuels to

increase its yield from the same crude barrel. Heavy residues from the atmospheric

distillation units are catalytically cracked to produce distillate fuels. During long-term

storage of diesel fuels particularly those containing thermally and catalytically cracked

stocks, high amounts of sediments are formed due to slow oxidation occurring at

atmospheric temperature conditions. The distillate products from cracking processes

consist of higher amount of olefins and also nitrogen and sulphur containing compounds

than those from the atmospheric distillation units. The olefins are more prone to oxidation

and the oxidation process is further enhanced by nitrogen containing compounds, such as

pyrolles and indoles. The oxidation products in the fuel are polymerized finally to high

molecular weight compounds called ‘gums’. The gums are of two kinds, ‘soluble gum’

that remains dissolved in the fuel and the ‘insoluble gum’, which gets precipitated out in

the fuel. The insoluble gum is also referred as ‘sediments’, which causes a number of

problems in the engine. The gums cause plugging of fuel filters, the problem being more

severe for the paper element micro-filters.

Chemical Composition

Aromatic content is of great concern as it increases particulate and poly-aromatic

hydrocarbons (PAH’s) emissions. The diesel fuels specifications in most of the countries

now limit aromatic content to 10% maximum. Also limits on the PAH’s are being

specified. One side effect of reduction in aromatic content is reduction in lubricity

characteristics of the diesel fuels resulting in high wear rates of the injection pump

elements and injector needles. Detergent additives are used in fuel to solve injector

cleanliness problems. Small dosage of additives can keep injectors clean while high

dosages of these additives can partially clean the heavily clogged injector holes [34].

Sulphur Content

Sulphur containing compounds naturally occur in the fuel and diesel fuels have

significantly higher sulphur content than Gasoline. Sulphur on combustion produces


41

sulphur dioxide (SO2), most of which is exhausted into atmosphere. A small fraction of

SO2 about 1 to 3% is oxidized in the oxygen rich diesel exhaust to sulphur trioxide (SO3)

and to sulphates found in particulate emissions [35]. The sulphur trioxide on combining

with water forms sulphuric acid that causes wear of metallic components. It is well

known that high sulphur levels of diesel fuel increases wear of piston rings and cylinder

liners, the wear rates being higher at lower coolant temperatures. In addition, sulphur

increases deposit formation in the combustion chamber and the deposits become harder in

presence of sulphur. Sulphuric acid aerosols adsorbed on the foot particles and are

emitted as particulate emission. Depending on sulphur content, its contribution can be

significantly large to particulate emissions. This is why, sulphur content of diesel fuels is

being reduced to very low levels (< 500 ppm and even down to 50 ppm) as more and

more stringent emission standards are being implemented [31].

Lubricity

The diesel fuel itself provides lubrication of diesel pumping and injection

elements. The heavier, high viscosity hydrocarbons and polar compounds are believed to

be the lubricating compounds providing natural lubricity to the diesel fuel. The polar

compounds get adsorbed on the injection systems surfaces and act as anti-friction layer.

Hydro-treating refining processes are used to remove sulphur from the diesel fuels. In the

process, polar compounds are also removed. As the sulphur content of diesel fuel

decreases, the lubricity of diesel fuel goes down. Low lubricity can result in excessive

injection pump wear and in some cases in total mechanical failure.

3.3 Alternative Fuels

More and more stringent environmental regulations being enacted in the USA and

Europe have lead to research and development activities on clean alternative fuels.

Energy security has been another important consideration. A number of liquid and

gaseous fuels are among the potential fuel alternatives. Most important among them are,

alcohols, ethanol and methanol, natural gas, liquefied Petroleum gas (LPG), hydrogen,

ethers like di-methyl ether (DME), vegetable oil esters commonly called as ‘biodiesel’,

bio gas etc. The US Clean Air Act Amendments (CAAA) besides most of these

alternative fuels lists reformulated Gasoline, electricity and any other fuels that permit

attaining the legislated emission standards as the ‘clean alternative fuels’ [31].


42

Some of the important properties of different alternative fuels are compared in

Table 3.9 with those of typical Gasoline and diesel fuels.

Table 3.9 Properties of various fuels for vehicles [39,40].

Property Gasoline Diesel Methanol Ethanol Natural

gas

Propane DME RME Hydrogen

Mol. wt 110 170 32.04 46.07 18.7 44.10 46.1 2.015

Density 0.72-

0.78

0.82-

0.88

0.796 0.794 0.72 0.51

(liquefied)

0.67

(liquid)

0.882 0.090

Lower heating

value MJ/kg

44.0 42.5 19.9 26.8 50.0 46.3 28.4 37.7 120

Heat of

vaporization,

kJ/kg

305 250 1110 904 509 426 410 at

20º C

Boiling point,

ºC

30-215 180-

370

65 78 -160 -43 -24.9 330-

340

-253

Octane

number,

research

90-98 - 112 111 120-130 112 - - 106

Octane

number, motor

80-90 - 91 92 120-130 97 - - -

Cetane number - 45-55 - - - - >55 51-52 -

Stoichiometric

A/F ratio, mass

14.7 15.0 6.43 8.94 17.12 15.58 9.0 11.2 34.13

Vapour

flammability

limits, (vol. %)

0.6-8.0 0.6-

7.5

5.5-26 3.5-15 5-15 9-9.5 3.4-

18.6

- 4-75

Stoichiometric

CO2 emissions,

g/MJ fuel

71.9 69.0 71.2 54.9 64.5 69.0 0

Adiabatic

flame

temperature

(K)

2266 2151 2197 2227 2268 2383

High Petroleum crude prices generated a lot of interest in ethyl alcohol produced

from agricultural products during 1980s notably in Brazil. On the other end, during the

same period environmental considerations were foremost in the USA for vigorous

technological development activities related to methanol. At that time, methanol was

considered a more convenient and economically attractive carrier of natural gas across the


43

continents for import of cheaper energy available in the form of natural gas. Methanol

being liquid it was better suited than natural gas for storage on-board of vehicles. Interest

in methanol however, has almost more or less died down it being otherwise highly toxic

to living beings and its corrosive nature for many materials used in fuel handling and

engine fuel systems. Presently, natural gas and biodiesel have attracted an increasing

interest of the governments, vehicle manufacturers and fuel suppliers. Hydrogen is also

considered a long-term potential alternative.

According to an estimate, the reserves will last for 218 years for coal, 41 years for

oil, and 63 years for natural gas, under a business-as-usual scenario [36,37,38].

3.3.1 Alcohols

Alcohol is made from renewable resources like biomass from locally grown crops

and even waste products such as waste paper, grass and tree trimmings etc. Alcohol is an

alternative transportation fuel since it has properties, which would allow its use in

existing engines with minor hardware modifications. Alcohols have higher octane number

than Gasoline. A fuel with a higher octane number can endure higher compression ratios

before engine starts knocking, thus giving engine an ability to deliver more power

efficiently and economically. Alcohol burns cleaner than regular Gasoline and produces

lesser carbon monoxide, hydro carbons and oxides of nitrogen [41,42,43]. Alcohol has higher

heat of vaporization, therefore it reduces the peak temperature inside the combustion

chamber leading to lower NOx emissions and increased engine power. However, the

aldehyde emissions go up significantly. Aldehydes play an important role in formation of

photochemical smog.

Methanol (CH3OH) is a simple compound. It does not contain sulfur or complex

organic compounds. The organic emissions (ozone precursors) from methanol

combustion will have lower reactivity than Gasoline fuels hence lower ozone forming

potential. If pure methanol is used then the emission of benzene and PAH’s are very

low [42]. Methanol gives higher engine efficiency and is less flammable than Gasoline but

the range of the methanol-fuelled vehicle is as much as half less because of lower density

and calorific value, so larger fuel tank is required. It has invisible flames and it is

explosive in enclosed tanks. The cost of methanol is higher than Gasoline. Methanol is

toxic, and has corrosive characteristics, emits ozone creative formaldehyde. Methanol


44

poses an environmental hazard in case of spill, as it is totally soluble with water. Ethanol

is similar to methanol, but it is considerably cleaner, less toxic and less corrosive. It gives

greater engine efficiency. Ethanol is a grain alcohol, and can be produced from

agricultural crops e.g. sugar cane, corn etc. Ethanol is more expensive to produce, has

lower range, having cold starting problems and requires large harvest of these crops.

Higher energy input is required in ethanol production compared to other energy crops and

it leads to environmental degradation problems such as soil degradation [24].

Methanol and ethanol can be produced from renewable sources as well as from

fossil fuels. Methanol is mainly produced from natural gas. Coal and cellulose containing

biomass like wood etc. may also be used to produce methanol. Ethanol is produced

almost entirely from the renewable sources from fermentation of carbohydrate containing

biomass like sugar, grains, tapioca etc. Neat ethanol (95% ethanol + 5% water) and

anhydrous ethanol blended up to 20% in Gasoline have been widely used in Brazil during

1980s. In the USA, use of ethanol initially started in the agricultural surplus states like

Nebraska for blending in the reformulated Gasoline as oxygenate. Now, ethanol is the

preferred oxygenate replacing methyl tertiary butyl ether (MTBE). The 10% ethanol-

Gasoline blends used in the USA are commonly referred as ‘Gasohol’. Germany,

Sweden, New Zealand and California focused mainly on methanol as an automotive fuel

due to its potential near the natural gas field and it being liquid can be more easily

handled and transported compared to natural gas.

Physical and chemical characteristics of alcohols make alcohols as excellent fuels

for the SI engines. The ignition quality of alcohols being poor, these cannot replace diesel

fuels directly and a source of ignition is to be provided for their combustion in the diesel

engine cylinder. Gasoline at least in 15% v/v amount was added to alcohols to increases

visibility of flame from the fire safety considerations.

Alcohols are not suitable fuels for compression ignition as they have very poor

ignition quality. Cetane number of methanol and ethanol is 5 and 8, respectively. Further,

the alcohols are not easily soluble in the diesel fuels. To prepare alcohol-diesel blends

high amounts of additives (emulsifiers/solublizers) are required and ignition improvers

are to be used to compensate for loss in ignition quality [44,45]. The following three

approaches have been largely considered practically feasible for total or part replacement

of diesel fuels by alcohols:


45

(i) Improving ignition quality of alcohols by use of ignition improvers.

(ii) Use of glow or spark plugs as a positive source of ignition.

(iii) Dual-fuel operation, using pilot diesel injection as an ignition source for

alcohol-air mixtures [46,47].

Widespread use of alcohols as motor fuels so far has not established. Firstly

significant cost benefits did not exist and also emission benefits compared to Petroleum

fuels have not been attractive. In addition, there are some negative factors and undesirable

effects on engines relative to conventional fuels. The main advantages and disadvantages

of alcohols with respect to conventional Gasoline and Diesel fuels are summarized in

Table 3.10 & Table 3.11.

Table 3.10 Advantages of alcohol motor fuels compared to Gasoline and Diesel.

Property/Performance Parameter

Compared to Gasoline and Diesel fuels

Advantages

Octane number Higher

A higher engine compression ratio in

SI engines can be used resulting in

higher thermal efficiency

Latent heat of vaporization Higher Lower intake temperature may be

used to increase charge density and

higher volumetric efficiency

Adiabatic flame

temperature

Lower Potentially lower NOx emissions and

lower heat losses

Flame luminosity Lower Lower heat losses from combustion

PM emissions Lower Due to clean burning characteristics

PM emissions are even lower than the

Gasoline engines

Toxic emissions - Lower benzene and 1,3 butadiene

emissions

Nature of sources Renewable esp. of

ethanol

Sources more widespread around the

world, hence better energy security.

Lower net CO2 emissions


46

Table 3.11 Disadvantages of alcohol motor fuels compared to Gasoline and Diesel. Property/Performance

Parameter

Compared to

Gasoline and

Diesel fuels

Disadvantages

Volumetric energy

content

Much lower Higher volumetric fuel consumption

hence larger fuel storage space and

weight

Cetane number Much lower Cannot be directly used in compression

ignition engines. Needs a source of

ignition increasing complexity of

engine/fuel system

Vapour pressure Lower Poor cold starting and warm up

performance, higher unburned fuel

emissions during starting/warm up phase

CO and NOx Emissions Similar No definite trend is observed, So, no

advantage over Petroleum fuels have been

noted

Aldehyde emission Higher Formaldehyde and acetaldehyde emission

are higher

Material

corrosion/adverse effects

Higher Methanol and a lesser degree ethanol are

more corrosive to metals, elastomers and

plastic components. Needs selection of

suitable materials for the fuel system.

Engine wear Higher Washes away lubricants film during cold

starting, resulting in higher cylinder and

piston ring wear

Flame Luminosity Almost invisible Neat alcohols present fire safety hazards.

Addition of Gasoline or other material

required to increase flame luminosity


47

3.3.2 Natural Gas

Natural gas has been used now for more than 50 years as fuel for stationary

engines for power generation, gas compression and agricultural machinery. Presently, a

large number of natural gas vehicles (NGVs) are in operation throughout the world in

Argentina, Australia, Canada, Italy, India, New Zealand, countries of former Soviet

Union and a number of other Asian and European countries. Natural gas is finding favour

as an alternative fuel due to its large-scale availability and potentially high environmental

benefits. In the USA, stringent particulate emission standards for the urban buses

implemented in the year 1994 and later, provided impetus to the development of natural

gas fuelled urban buses. Table 3.12 gives the number of NGVs annual gas consumption

and number of natural gas filling station in operation in different regions worldwide for

the year 2010 and Table 3.13 represents natural gas vehicle growth since 2000.

Table 3.12 Worldwide Population of NGVs in 2010 [48].

Country Natural Gas

Vehicles

Monthly Sales Average

N/m3

Refuelling

Stations

Iran 2,070,930 330,000,000 1490

Argentina 1,901,116 207,305,000 1878

Brazil 1,646,955 165,812,800 1782

India 1,100,000 NA 596

Italy 676,850 62,030,000 770

China 500,000 NA 1652

Colombia 320,036 45,000,000 614

Thailand 211,402 95,600,000 423

Bangladesh 200,000 91,550,000 500

Egypt 139,804 38,000,000 129

USA 110,000 105,000,000 1100

Russia 100,052 27,710,000 249

Germany 85,000 14,600,000 863

Korea 28,324 81,680,000 166

Sweden 23,125 6,770,000 134

Switzerland 9,279 1,320,000 15


48

Table 3.13 Natural gas vehicle growth since 2000 [48].

Region Average % NGV growth since 2000

Asia 52.50%

Europe 15.40%

North America 0.40%

South America 25.90%

Africa 19.30%

Total 29.80%

The principal constituent of natural gas is methane (80 to 95% by volume). The

balance is composed of small and varying amounts of other hydrocarbons such as ethane,

propane, butane and heavier hydrocarbons and non – hydrocarbon gases carbon dioxide,

nitrogen, water, hydrogen sulphide and other trace gases. Typical composition is given in

Table 3.14. The natural gas before transportation or use is upgraded by removing water,

hydrogen sulphide and condensable higher hydrocarbons. It helps in prevention of

condensation of these compounds in pipeline and also valuable by-products are obtained.

Natural gas, once flared-off as an un-needed byproduct of petroleum production, is now

considered a very valuable resource [49].

Table 3.14 Composition of CNG.

Constituents % Volume Methane 93.20 Ethane 04.27 Propane 01.38 i-Butane 00.18 n-Butane 00.20 i-Pentane 00.04 n-pentane 00.03 Carbon dioxide 00.27 Nitrogen 00.43 Moisture content 2.0 ppm


49

In a nationwide survey across the USA reported in 1992, variations in

composition of natural gas could result in variations of 14% in density, 20% in Wobbe

index and 25% in stoichiometric air-fuel ratio [50]. The Wobbe index is defined as

W=H/√ρ, where H is volumetric heating value and ρ is specific gravity. It has an almost a

linear relation with air-fuel equivalence ratio. Reasonably small variations in Wobbe

index have little effect on emissions using modern engines with three way catalysts and

closed loop feedback control. However, large variations in gas composition can have

significant effects on engine performance and emissions, especially if the engine

performance and emissions are optimized on a fixed gas composition and engine is not

equipped with means of adjusting to other composition can also affect the composition

and reactivity of the exhaust HC emissions.

Oil wells are primary source and refineries are a secondary source of natural gas

where the dissolved gases in the Petroleum crude are released during the refining process,

but in lesser volumes. CNG is a safe fuel. Being lighter than air, it diffuses easily into the

atmosphere and does not form a sufficiently rich mixture for combustion to take place. In

this respect, CNG is superior to other fuel. Storage of propane on vehicles is not as

cumbersome as CNG, but the cost of propane is higher than that of CNG. CNG represents

a more cost effective emission reduction measure than quite a few other options that have

been the subject of serious debate in recent years.

International Standards Organization (ISO) 15403-2000 provides specifications

for the natural gas delivered to the vehicle and not to the pipeline gas [51]. These

specifications also include parameters like water content, sulphur content, condensate and

free oil that may come from the natural gas compressor.

Natural gas liquefies at -161ºC at atmospheric pressure. To use liquefied natural

gas (LNG) as automotive fuel cryogenic systems are required. There are other problems

too with the use of LNG. Liquid phase in the fuel tank should not become enriched with

other higher hydrocarbons during refilling cycles. Leakage of even small amounts of

LNG in an enclosed space may form explosive mixtures and risk of fire hazards increases

by manifold. Therefore, in most of the NGVs today, natural gas is stored on board in

high-pressure cylinders at a pressure of 200 to 300 bars as CNG, storage of natural gas at

high pressure on board provides an acceptable range of vehicle operation.


50

High antiknock quality of natural gas makes it a fuel that is better suited for SI

engines. The natural gas engine operation may be broadly classified in the following

types:

(i) Bi-fuel Operation: The conventional Gasoline vehicles are converted to

operate either on Gasoline or natural gas, as the operator prefers.

(ii) Dedicated or Mono-fuel Operation: Vehicle operating only on gas using a

positive source of ignition such as spark plug or hot surface ignition like glow

plug.

(iii) Dual-Fuel Operation: When the diesel injection constitutes 10% or less of

the total fuel, the diesel injection system particularly the injectors are replaced

by the new ones of a different calibration. These engines are known as ‘pilot-

injection’ engines and do not have the dual-fuel flexibility of operation on

diesel alone. In a new development of ‘pilot injection’ engine high-pressure

natural gas is directly injected in the combustion chamber and ignited by the

pilot diesel spray.

As natural gas has a very high antiknock quality, dedicated natural gas engines

can be built with much higher compression ratio than the Gasoline engines resulting in

significant improvements in fuel efficiency and lower carbon dioxide emissions. This is

particularly useful for heavy-duty vehicle application. Lean burn spark or low plug-

ignited, high compression ratio engines can be built to give very low particulate emissions

and high-energy efficiency. The stoichiometric SI engines can utilize the three-way

catalyst (TWC) emission technology and therefore, it provides the greatest emission

reduction potential. Problems with thermal stresses and low power density have favoured

the use of lean-burn combustion system over TWC in heavy-duty application.

3.3.2.1 Effect of Natural Gas on Emissions

With natural gas operation, large reductions in engine-out emissions compared to

either Gasoline or diesel fuel operation can be achieved. It could be mentioned that most

light-duty SI natural gas engines are stoichiometric similar to their Gasoline-fuelled

counterparts. With natural gas, mixture enrichment during cold starting is not required

unlike the Gasoline operation. Hence, use of natural gas results in lower unburned fuel

emissions during cold staring and warm-up phase.


51

CNG buses without after-treatment have high emissions of formaldehyde, which

is considered a possible human carcinogen. The formaldehyde emissions can be reduced

with an oxidation catalyst but not to the low level of a diesel bus equipped with catalytic

regeneration particulate trap (CRT). In addition to emissions benefits, NGV has other

differences from the vehicles operating on the conventional Petroleum fuels as below:

(i) The natural gas is stored in high-pressure cylinders. It results in weight penalty of

the vehicles and for heavy-duty vehicle it may increase weight of the vehicle by

600 to 1000 kg to provide an acceptable range of operation. Low weight cylinders

of composite material are available that reduce the cylinder weight by more than

half. These cylinders have a liner made of steel, aluminium or non-metal, which is

over-wrapped by hoop or fully wound carbon/fiberglass filament in a resin matrix.

These composite material cylinders however, are more expensive than the

conventional steel cylinders and the cost may be higher by a factor of 3 or 4.

(ii) Compared to Petroleum fuels, emissions of carbon dioxide, a green house gas are

lower in the dedicated natural gas engines as a higher engine compression ratio

can be used.

(iii)Low emissions of non-methane hydrocarbons from natural gas vehicles result in

low photochemical reactivity and ozone forming potential of the exhaust gases.

Emissions of air toxics such as benzene and 1-3, butadiene are very low.

3.3.3 Liquefied Petroleum Gas

LPG is commonly known as propane (C3H8), a combustible hydrocarbon based

fuel. It comes from the refining of crude oil and natural gas. There are currently three

grades of propane available, HD5 for ICEs, commercial propane and commercial

propane-butane mixture for other uses. The commercial grade of propane for automotive

use is known as HD5 and composition is shown in Fig. 3.1. The exact composition of

propane varies slightly between different parts of the country and different refineries.

Compared to Gasoline the energy content of LPG is 74%.


52

Composition of LPG

0

20

40

60

80

100

Constituents

% A

mou

nt

Series1 90 5 2 1.5 1.5

Propane Propylene Butane Iso-butane Methane

Fig 3.1 Composition of LPG.

In the USA, LPG contains more than 85% propane while in Europe and Asia,

propane constitutes just about half of LPG, the balance being largely the butane. It

remains in gaseous state at normal ambient temperatures and pressures (the boiling point

of propane and butane at atmospheric pressure is about -45º C and -2º C, respectively).

The pressure inside storage tank keeps LPG liquid, and it becomes gas when released

from the tank. The liquid form has an energy density 270 times greater than the gaseous

form, making it efficient for storage and transportation. The benefits of LPG as a clean

burning motor fuel results in practice largely from its ability to change between the liquid

and gaseous phase much more readily compared to natural gas.

In Europe, LPG motor fuel was first used in 1950s, especially in Italy and the

Netherlands who offered tax incentives making it economically more attractive. In 2003,

worldwide population of LPG vehicles stood at 9.5 millions consuming annually about

16.5 million tons of LPG [48]. Population of LPG vehicles in some countries in the year

2003 is given in Table 3.15.

South Korea had the largest LPG passenger car fleet that stood at 1.7 million

units, ahead of Italy (1.2 million), Poland (1.1 million) and Turkey (1 million). The use of

LPG as motor fuel would help diversify the transport energy supply while exploiting local

resources that may be present in abundance in several countries.


53

Table 3.15 LPG Vehicle Fleet Worldwide in 2003 [48].

Country Number of Motor

Vehicles (thousands)

Consumption of LPG

(thousand tons)

Australia

Czech Republic

France

Italy

Japan

Mexico

Netherlands

Poland

Russia

South Korea

Turkey

United States

Worldwide

492

145

180

1,220

290

700

290

1,100

550

1,723

1,000

190

9,416

1,213

68

166

1,202

1,528

1,200

435

1,700

780

3,740

1,260

730

16,445

Most LPG vehicles employ bi-fuel systems for operation either on Gasoline or

LPG. It provides flexibility to vehicle operation, which is important as the number of

LPG filling stations is usually small. One drawback with a bi-fuel system is that neither

fuel can achieve optimum performance. Optimization of engine for LPG operation is

possible only for the dedicated gas engines. However, variation in propane/butane ratio in

LPG possess a problem as the octane number of the two main constituents, propane

(RON is 112) and butane (RON is 94) is quite different. When more stringent emission

standards like Euro IV are to be met, the bi-fuel vehicles may require a major technology

upgradation. For bi-fuel vehicles like conventional motor fuels, specific technological

development will be necessary to ensure compliance with the stringent emission

standards.

The advantages and disadvantages of LPG as a motor fuel are similar to those for

natural gas. The main advantages and disadvantages of LPG compared to Gasoline are

given below:


54

(i) Good cold start and warm-up characteristics due to its gaseous state

(ii) Higher antiknock quality of LPG provides an opportunity for use of a higher

compression ratio and improvement of engine performance and thermal

efficiency

(iii) Emissions are substantially lower compared to Gasoline vehicles. LPG has

disadvantage compared to natural gas in respect of non-methane hydro carbon

(NMHC) emissions as these consists of higher amounts of reactive olefinic

hydrocarbons. The ozone forming potential of LPG with Gasoline, Diesel and

CNG is compared in Fig.3.2. LPG has significantly lower smog formation

potential compared to Gasoline and Diesel fuels.

(iv) LPG operation results in negligible PM emissions compared to Diesel.

(v) LPG is relatively a low sulphur fuel.

(vi) Lower energy content results in higher volumetric fuel consumption

(vii) As the fuel on board is at a higher pressure, additional safety regulations are to

be implemented. As LPG is heavier than air, restrictions on vehicle parking in

confined space are also to be applied.

0

50

100

150

200

250

300

350

400

CNG LPG Gasoline Diesel

Ozo

ne F

orm

ing

Pote

ntia

l, m

g/km

Fig 3.2 Comparison of ozone forming potential of different fuels for cars, during

summer [36].

Introduction of gaseous fuels in the intake manifold decreases the air partial

pressure notably compared to Gasoline. This reduction in power is inherent in the

structure of gaseous fuels.


55

One way to compensate this loss is to use supercharger or turbocharger in order to

increase air flow rate. Superchargers and turbochargers provide more power from

the engine by compressing the inducted air higher density than ambient.

Volumetric efficiency goes up with turbochargers and superchargers along with

better bsfc. Turbocharger has a turbine and compressor in a common shaft.

Turbine is driven by the exhaust gas. Using of exhaust gas provides the recovery

of waste energy which leads to increase in the overall efficiency. An intercooler or

aftercooler is applied in order to provide further increase in the combustion air

density. Supercharger is operated on the same principle with turbocharger. But the

driven of compressor is achieved by engine's crankshaft. Turbo lag which

indicates the delay between boost and throttle response. This can be a problem in

SI engines. However this is not noticeable in large diesel engines. The drawback

with supercharger appears in cruise conditions. Because supercharger can not

adjust itself to this condition due to direct connection to crankshaft as easily as

turbocharger can. An electric clutch that turn the supercharger on and off and a

by-pass application which takes air from the supercharger output and introduce it

in the intake are the current methods for the solution.

Another approach that has been considered is to enrich the oxygen content of the

intake air by using a membrane gas separator. The oxygen enrichment approach is

under research at the present time and is not available on purchased vehicle.

The second reason which causes power loss is related to the intake manifold air

density. The heat of vaporization of Gasoline helps to decrease the temperature of

mixture, producing the dense mixtures. Although propane and methane have higher heat

of vaporization value, they are already in gaseous state when inducted into the intake

manifold and they do not provide this cooling effect. Development of liquid fuel injection

systems for LPG engines should provide better performance and efficiency. Beside this

liquid fuel injection provides better A/F ratio control. Back-fire is almost eliminated due

to introducing less volume of explosive gases in the inlet system. Cooling effect of

endothermic expansion of the liquid increases the resistance to pre-ignition and knock.

This leads higher compression ratio which means higher power output.


56

Higher compression ratio improves thermal efficiency and provides more power

that can be produced by the engine. Higher octane rating of propane compared to

Gasoline allows higher compression ratio for the engine.

Natural gas and propane are generally considered to reduce engine maintenance

and wear in SI engines. The most commonly cited benefits are extended oil change

intervals, increased spark plug life, and extended engine life. Natural gas and propane

both exhibit reduced soot formation over Gasoline. Reduced soot concentration in the

engine oil is believed to reduce abrasiveness and chemical degradation of the oil.

Gasoline fuelled engines particularly carbureted engines require very rich operation

during cold starting and warm up. Some of the excess fuel collects on the cylinder walls,

"washing" lubricating oil off walls and contributing to accelerated wear during engine

warm up. Gaseous fuels do not interfere with cylinder lubrication.

Engines powered by gaseous fuels are generally considered easier to start than

Gasoline engines in cold weather. Because gaseous fuel are already vaporized before

inducted into engine. However, under very cold temperatures, cold-start difficulty occurs

for propane and natural gas. This is probably due to ignition failure caused by very

difficult ionization conditions, sluggishness of mechanical components.

Hot starting can cause difficulties for gaseous fuelled vehicles, especially in warm

weathers. After an engine is shut down, the engine coolant continues to draw heat from

the engine, raising its temperature. If the vehicle is restarted within a critical period after

shutdown, (long enough for the coolant temperature to rise, but before the entire system

cools), the elevated coolant temperature will heat the gas more than normal, lowering its

volumetric heating value and density. This would cause mixture enleanment.

Gasoline shows very little change over the normal temperature or pressure range.

Propane, however, is gas at ambient conditions. Its physical properties depend mainly on

the temperature and pressure at which they are being stored. There must be space left in a

propane fuel tank. As the temperature rises, the volume of liquid increases significantly.

Due to this, propane system has some kind of safety fill stop device to prevent tank fills to

not more than 80 to 85%. This provides room for liquid expansion if the temperature

increases after the tank is filled. Due to low viscosity of propane and its storage under


57

pressure, it may leak through small cracks, pumps, seals and gaskets more readily than

Gasoline.

3.3.4 Biodiesel

Use of vegetable oils as diesel engine fuel is almost as old as the diesel engine

itself. In a 1912 speech, Rudolf Diesel said, “the use of vegetable oils for engine fuels

may seem insignificant today, but such oils may become, in the course of time, as

important as Petroleum and the coal – tar products of the present time” [52]. However, due

to availability of cheaper Petroleum crude, interest in fuels derived from vegetable oils

diminished. The revival of biodiesel production started with farm co-operatives in the

1980s in Austria and in 1991, the first industrial-scale plant started biodiesel production

with a capacity in excess of 10,000 m3 per year. Through 1990s, plants were established

in many European countries, including the Czech Republic, France, Germany and

Sweden. In 1998, the Austrian Biofuels Institute identified 21 countries with commercial

biodiesel projects. In the 1990s, France launched the production of biodiesel obtained

from rapeseed oil. The European Directive 2003/30/EC proposed to promote the use of

biofuels or other renewable fuels for transport to reach 2% share of the total automotive

fuel market by December 31, 2005 and 5.75% by December 2010. Of this, biodiesel is

expected to constitute the major part [53].

Biodiesel is a renewable fuel that is produced from a variety of edible and non-

edible vegetable oils and animal fats. The term “biodiesel” is commonly used for methyl

or ethyl esters of the fatty acids in natural oils and fats that meet the specifications for

their use in the CI engines. Straight vegetable oils are not considered as biodiesel

although attempts have been made to use these as well in the CI engine. Biodiesel is

typically produced by a reaction of vegetable oils or animal fats with an alcohol such as

methanol or ethanol in the presence of a catalyst to yield mono-alkyl esters. Glycerin is

obtained as a by-product, which is removed. The straight mineral oils have very high

viscosity that makes flow of fuel difficult even at room temperatures and presence of

glycerin in the vegetable oil causes formation of heavy carbon deposits on the injector

nozzle holes.


58

A variety of vegetable oils such as those from soybean, rapeseed, sunflower,

jatropha – carcass, palm, and cottonseed etc. have been widely investigated for

production of biodiesel. Rapeseed oil and some other vegetable oils when transformed to

their methyl esters have many characteristics such as density, viscosity, energy content,

and cetane number close to that of diesel. The more widely used are Rapeseed Methyl

Ester (RME) in Europe and Soybean Methyl Esters (SME) in the US. They are

collectively known as Fatty Acid Methyl Esters (FAME). Recently non-edible oil

produced from jatropha-curcass seeds has gained interest as this plant can be easily grown

on wastelands.

The vegetable oil esters are practically free of sulphur and have a high cetane

number generally in the range 46 to 60 depending upon the feedstock. The cetane number

of methyl esters tends to be slightly lower than of ethyl or higher esters [54]. Biodiesel

from saturated feed stocks such as animal fat and recycled restaurant cooking fats will

generally have a higher cetane number than the esters of oils high in poly-unsaturates

such as soybean oil. Due to presence of oxygen, biodiesel have a lower calorific value

than the diesel fuels.

The emission studies [55] show that the use of biodiesel results in reduction of CO,

HC and PM, but slight increase in NOx emissions. Reduction in CO emission could

probably be attributed to presence of oxygen in the fuel molecule. Decomposition of

biodiesel produces a variety of oxygenated hydrocarbons in addition to hydrocarbons.

Response of the standard HC measurement technique, the heated flame ionization

detector is different for the methyl esters than HC emission [56] and this could be partly

responsible for the difference in HC emissions between the normal diesel fuels and

biodiesel. The methyl esters have a lower compressibility, which results in advance of

dynamic injection timing with biodiesel compared to diesel. Change in injection timing

and differences in cetane number and combustion characteristics and particulate

emissions are observed to be significantly lower with biodiesel compare to diesel fuels.

Volumetric fuel consumption with biodiesel is higher than diesel due to its lower

heating value. An increase of 10-11% in fuel consumption compared to diesel may be

expected when comparing their heating values.


59

As biodiesel is produced from vegetable oils or animal fats, its use has been

promoted as a means for reducing greenhouse gas CO2 emissions that would otherwise be

produced from the combustion of Petroleum-based fuels. The total impact that biodiesel

could have on global warming would be a function not just of its combustion products but

also of the emissions associated with the full biodiesel production and consumption

lifecycle. On an average the carbon content on mass basis of plant-based biodiesel is

77.8% and of animal fat based biodiesel is 76.1% compared to 86.7% for the Petroleum

based diesel.

Since biodiesel is free from sulfur hence less sulfate emissions and particulate

reduction is reported in the exhaust. Due to near absence of sulfur in biodiesel, it helps

reduce the problem of acid rain due to transportation fuels [57].

Higher thermal efficiency, lower BSFC and higher exhaust temperatures are

reported for all blends of biodiesel compared to mineral diesel [58].

Biodiesel is oxygenated fuel (hence more complete combustion) and causes lesser

particulate formation and emission. This is also due to oxygenated nature of biodiesel

where more oxygen is available for burning and reducing hydrocarbon emissions in the

exhaust [59,60,61].

The biodiesel have a slightly higher carbon content per unit energy (2.068

kg/100MJ) than the conventional diesel (2.042 kg/100MJ) and thus may be expected to

give higher CO2 emissions on combustion. The measured data however, suggest that the

combustion generated CO2 from biodiesel and conventional diesel are substantially

similar [62].

The cost of producing methyl or ethyl esters from edible oils is currently much

more expensive than hydrocarbon based diesel fuel. Due to the relatively high costs of

vegetable oils (about 1.5 to 2 times the cost of diesel), methyl esters produced from it

cannot compete economically with hydrocarbon-based diesel fuels unless granted

protection from considerable tax levies applied to the latter. In absence of tax relief, there

is a need to explore alternate feedstock for production of biodiesel.

The cost of biodiesel can be reduced if we consider non-edible oils and used-

frying oils instead of edible oils. Non-edible oils such as mahua, karanja, babassu,


60

jatropha, neem etc., are easily available in many parts of the world, and are cheaper

compared to edible oils. Most of these non edible oils are not used to their potential and in

fact produced in surplus quantities. Several countries including Netherlands, Germany,

Belgium, Austria, USA, Japan and India discard used frying oils. With the mushrooming

of fast food centers and restaurants in the world, it is expected that considerable amounts

of used-frying oils will be discarded. This oil can be used for making biodiesel, thus

helping to reduce the cost of water treatment in the sewerage system and in the recycling

of resources [24].

3.3.5 GAS-TO-LIQUID (GTL) FUELS:

GTL conversion is a broad term for a group of technologies that are used to

produce synthetic liquid hydrocarbon fuels from a variety of feed stocks. These fuels have

characteristics similar to those of Petroleum fuels and would form a more convenient

substitute for them. The synthetic gas, a mixture of carbon monoxide and hydrogen is

produced from a variety of feed stocks like coal, natural gas and biomass, and is

converted to a mixture of hydrocarbons of different molecular weights and structures. The

chemical conversion process was first developed by Petroleum deficient but coal rich

Germany during 1920s and is known as Fischer-Tropsch (F-T) process after the name of

its inventors. Therefore, GTL diesel is also known as F-T diesel. The basic process

consists of two steps.

1. Production of synthesis gas, and

2. F-T synthesis.

Synthesis gas is produced by steam reforming of natural gas, coal or biomass or

by partial oxidation of hydrocarbons like natural gas. Steam reforming reactions are:

COHyOHCH y 22 )5.01( (3.2) The value of n depends on the type of feedstock. For example, for typical

hydrocarbon feed stocks y = 2.2 to 4 as they have high content of hydrogen and for coal

y<<1. The partial oxidation reaction for natural gas to generate synthesis gas proceeds as

below;

COHOCH 242 224 (3.3)


61

The steam reforming and partial oxidation reactions are endothermic in nature and

the energy needed is supplied by the combustion of the feedstock itself with oxygen.

Fischer-Tropsch synthesis in generic form is described by the reaction.

OnHCHnnHnCO 222 )(2 (3.4)

(-CH2-) is the basic building block of paraffin hydrocarbons. The product is

primarily straight chain hydrocarbons with small quantities of isoparaffins and olefins.

Therefore, the F-T fuel has a high cetane number and is best suited as fuel for the diesel

engines. The F-T synthesis takes place over cobalt based catalyst at temperatures between

180° to 250° C and pressures ranging from 20 to 40 bar. As the catalyst gets poisoned by

sulphur the synthesis gas is made sulphur free before F-T synthesis. Commercial plants

are in operation in South Africa (Sasol) that uses coal and natural gas, and in Malaysia

and Qatar based on natural gas.

The properties of GTL fuels depend on the pressure, temperature and the catalyst

used for synthesis. The Table 3.16 gives the properties of GTL fuel and the range in

which these are generally obtained. The GTL fuel when compared to conventional diesel

has,

High hydrogen content

Similar heat of combustion

Lower density and hence, lower energy content per unit volume

Higher cetane number

Ultra-low sulphur

Near zero or very low aromatic content

The GTL diesel is composed of hydrocarbons like Petroleum derived diesel fuels.

Hence its effect on engine performance would so the trends similar to those obtained with

change in properties of the conventional diesel fuels. Use of GTL diesel alone as well in

blends with conventional diesel has been investigated on light and heavy duty Euro III

and Euro IV diesel engines. Reduction in emissions with 100% GTL fuel are significant

particularly the particulate, unburned HC and CO emissions. The soot emissions are low

with GTL fuels as these have negligible aromatic content. Combustion with GTL fuels


62

results in reduced HC and CO emissions due to higher cetane numbers and lower

densities. The NOx emissions are found to reduce slightly or similar to conventional

diesel fuels. The specific fuel consumption is also similar although, some studies have

reported 2 to 3% improvements in fuel efficiency. However, due to lower densities, the

volumetric fuel consumption is nearly 5% higher.

Table 3.16 Properties of GTL Diesel Fuels [63,64,65].

3.3.6 Di-methyl Ether (DME)

During 1990s interest began to be focused in di-methyl ether (DME) as a potential

diesel engine fuel. DME can be produced from dehydration of methanol. Haldor Topsoe

developed a process for direct production of DME from synthesis gas [66]. The synthesis

gas (CO+H2) can be produced from a variety of raw materials e.g., natural gas, coal,

biomass etc. The DME produced from biomass can be categorized as a renewable fuel,

while DME produced from natural gas can act as an energy carrier in liquid form which is

much easier to transport across continents than the natural gas.

DME is the simplest ether and has chemical formula CH3-O-CH3. It has vapour

pressure of 5.1 bar at 20º C and can be stored, transported and dispensed like LPG. It is

environmentally benign, is not harmful to ozone layer and it readily degrades in

troposphere to carbon dioxide and water. DME is considered non-toxic and is not

classified as a carcinogen, teratogen or mutagen. It is non-corrosive and burns with

Properties Range

Density @ 20° C 0.765-0.800

Kinematic viscosity at 40° C, mm2/s 1.97-2.50

Cetane number 64-75

Distillation -

Initial boiling point, ºC 187-210

95% evaporation point 320-363

Sulphur, ppm by mass <1

Total aromatics,% mass 0.14-0.15

H/C atomic ratio 2.10-2.14

Lower heat of combustion, MJ/kg 43.49-43.84


63

visible blue flame. Important properties of DME are listed in Table 3.9. Its calorific value

is 33% lower than the conventional diesel but it has a high cetane number making it a

suitable fuel for CI engines. DME has no carbon-carbon bonds and oxygen constitutes

35% of its weight. These factors contribute to an almost smoke free combustion.

Density of DME is about 80% of diesel fuel and calorific value is just about two-

third of diesel. Therefore, compared to diesel twice the volume of DME should be

injected to get the same engine power. Moreover, it has a high compressibility and, low

viscosity and lubricity compared to diesel. Thus, the fuel injection system designed for

diesel fuel cannot be used for DME [67].

Gray and Webster studied [68] emissions of a 5.9 liter Cummins engine equipped

with oxidation catalyst with DME and diesel fuel. Table 3.17 summarizes the overall

average regulated exhaust emissions with the engine operating on DME and diesel fuel.

Table 3.17 Emissions Results with DME and Diesel Fuel on a 5.9 liter Cummins engine with Oxidation Catalyst, g/hp-h [69].

Fuel CO CO2 NOx HC PM

DME 0.253 544.7 3.33 0.427 0.02

Diesel 0.443 588.5 3.54 0.180 0.08

Emission operation on DME reduced CO emissions by 43% and PM emissions by

75% compared to diesel. NOx emissions were only slightly lower. However, the HC

emissions more than doubled, but most HC emissions were unburned DME that is

environmentally benign. Use of exhaust catalysts may be beneficial in reducing unburned

DME and methane emissions.

DME provides good engine cold starting. DME although is non corrosive to

metals but some rubber and elastomer components may not be compatible with it.

Therefore, material of seals has to be carefully selected. DME has a poor lubricity

requiring use of additives to protect injection equipment against excessive wear. It burns

with visible blue flame and the flame luminosity is quite good. This is important from fire

safety angle. It being gas at room temperature and atmospheric pressure, precautions to

prevent its leakage need to be taken as it could form explosive mixtures with air.


64

3.3.7 Hydrogen

Interest in hydrogen as a potential alternative automotive fuel has grown due to

need of reducing dependence on fossil fuels and to minimize air pollution. Hydrogen can

be produced from a variety of fossil and non-fossil sources. Presently the most economic

process to manufacture hydrogen is from hydrocarbon like natural gas or naphtha by

steam reforming. Coal gasification is another method. In these processes however, carbon

dioxide is also produced. Production of hydrogen by electrolysis of water is used in some

industrial plants but it is very expensive due to high consumption of electricity. Use of the

solar energy to produce hydrogen by photo-electrolysis is another potential route.

Hydrogen is a colorless, odourless and nontoxic gas. It burns with an invisible and

smokeless flame. The combustion products of hydrogen consist of mainly water and some

nitrogen oxides. The major hurdles in the use of hydrogen as a fuel are lack of production,

distribution and storage infrastructure. On board storage of hydrogen is a major challenge.

Hydrogen has very low boiling point (-253ºC) and a very low volumetric energy density.

The following methods of on-board storage of hydrogen are under consideration and

some of them are being used in demonstration vehicles:

(i) Compressed H2 in high-pressure cylinders at 20-70 MPa: It results in high

weight penalty and safety risks.

(ii) As a metal hydride: Hydrogen can be stored as a metal hydride like iron-

titanium metal hydride (FeTiH2), magnesium hydride, and magnesium-nickel

hydride or adsorbed on carbon. Metal hydrides release hydrogen on heating by

a heat source like vehicle exhaust gas. The main problems of hydride storage

system are limited storage capacity, contamination of storage materials by the

impurities in hydrogen, and high cost.

(iii) Storage of liquid hydrogen in cryogenic tanks: Liquefaction of hydrogen is

highly energy intensive. Energy spent in liquefaction of hydrogen to 20 K is

nearly equal to the energy content of the liquid hydrogen. Thermal insulation

of the cryogenic tanks at 20 K is also very challenging.

(iv) Chemical hydrogen carriers: Hydrogen can be stored as a constituent of a

chemical compound like methyl-cyclohexanol, sodium boro-hydride (NaBH4)


65

etc. A catalyst is required to dehydrogenate the chemical compound at high

temperature e.g. 500º C for hydrogenous methyl-cyclohexanol [70].

Volumetric energy density of compressed hydrogen is just one-third energy

density of natural gas. Liquid hydrogen also has a very low volumetric energy density,

which is about one-fourth of Gasoline. The liquid, hydride and compressed hydrogen

storage methods are compared in Table 3.18 for storing 5-gallon (19 liter) Gasoline

equivalent of energy storage. Hydrogen storage space required is at least 10 to 12 times

that for Gasoline. Storage and fuel weight for hydrides is 27 times and for compressed H2

is 4 to 5 times of Gasoline.

Table 3.18 Comparison of hydrogen storage methods [71].

Gasoline Liquid H2 Hydride Fe-Ti (1.2%)

Compressed H2 (70MPa)

Energy stored, MJ Fuel mass, kg Tank mass, kg Total Fuel System mass, kg Volume, l

6.64 102 14 6.5 20.5 19

6.64 102 5 19 24 178

6.64 102 5 550 555 190

6.64 102 5 85 90 227

Hydrogen fuel-cell vehicles are expected to have more commercial potential in

long run. Though it is believed that significant production volumes for customers will not

be available until 2010-2020 time frame, automotive manufacturers world over like

Toyota, Honda, General Motors, Ford, Chrysler, BMW are going ahead with limited

production and field trials of fuel cell powered cars and buses. Hydrogen fuelled ICE

vehicles are however, regarded as transition or ‘bridging’ strategy to stimulate building of

hydrogen infrastructure and related hydrogen infrastructure and related technologies.

Hydrogen has significantly different combustion characteristics than Gasoline.

Octane rating of hydrogen is 106 RON, making it more suitable for SI engines. The

laminar flame speed of hydrogen is 3 m/s, about 10 times that of Gasoline and methane.

Hydrogen has very wide flammability limits ranging from 5-75% by volume (φ= 0.07-9),

which may lead to pre-ignition and backfiring problems. Its adiabatic flame temperature

is higher by about 110º C than for Gasoline (Table 3.9). If inducted along with intake air,

the volume of hydrogen is nearly 30% of the stoichiometric mixture, decreasing the


66

volumetric efficiency and engine power considerable. Another option is direct injection of

liquid hydrogen into the engine cylinder that provides some advantages like cooling of

charge, higher volumetric efficiency and no danger of backfiring.

Hydrogen on combustion produces water and there are no emissions of carbon

containing pollutants such as HC, CO and CO2 and air toxics benzene, PAH, 1-3

butadiene and aldehydes. Trace amounts of HC, CO, and CO2 originating from burning of

lubricating oil however, may be emitted. NOx is the only pollutant of concern from

hydrogen engines. Very low NOx emissions are obtained with extremely lean engine

operation (φ<0.05) [70]. Injection of water into intake manifold or exhaust gas

recirculation which in this case consists primarily water vapour, can further suppress

formation of nitrogen oxides. In addition, water injection provides charge cooling and

control of pre-ignition and backfiring in the engines using external mixture preparation.

The direct fuel injection in the cylinder mitigates some of the problems faced by the

engines with external mixture preparation.

Hydrogen fuelled engines produces almost no CO2 and its global warming

potential is insignificant. Considering the total well-to-wheel energy analysis however,

when hydrogen is produced from fossil resources hydrogen fuelled vehicles provide no

overall reduction in greenhouse gas emissions and in some cases even worse than the

vehicles fuelled by the conventional Gasoline and diesel fuels.

Also the addition of H2 to other traditional slower burning fuels with narrow

operational mixture range such as those of methane and bio-gases, can accelerate

significantly the flame propagation rates, extend greatly the lean operational mixture

range while reducing the emissions of CO2 [72,73].

3.4 Vehicle Emissions and Air Pollution

Since, 1970 more and more stringent vehicle emission regulations have been

implemented in the developed countries like USA, Europe and Japan, but still vehicles

contribute significantly to the urban air pollution [74].

Combustion of various fossil fuels leads to emission of several pollutants, which

are categorized as regulated and unregulated pollutants. Regulated pollutants are ones,

whose limits have been prescribed by environmental legislations (such as USEPA, EURO

and Bharat norms) whereas there are some pollutants for which no legislative limits have


67

been prescribed. These are categorized as unregulated pollutants. Regulated pollutants

include NOx, CO, HC, particulate matter (PM) and unregulated pollutants include

formaldehyde, benzene, toluene, xylene (BTX), aldehydes, SO2, CO2, methane

etc.,[75,76,77].

These regulated as well as unregulated pollutants contribute to several harmful

effects on human health, which are further categorized as short-term and long-term health

effects. The short-term health effects are caused by CO, nitrogen oxides, PM,

formaldehyde (primarily regulated pollutants) etc., while long-term health effects are

caused mainly PAHs, BTX, formaldehyde (primarily unregulated pollutants) etc. CO is

fatal in large dosage, aggravates heart disorders, affects central nervous system, and

impairs oxygen-carrying capacity of blood by forming carboxy-hemoglobin. Nitrogen

oxides cause irritation in respiratory tract. HC cause drowsiness, eye irritation, and

coughing [78,79,80].

These pollutants also contribute towards several regional and global

environmental effects. Regional environmental effects such as summer smog are because

of aldehydes, carbon monoxides, nitrogen oxides etc. Winter smog is because of

particulate. Acidification is caused by nitrogen oxides, sulphuric oxides etc. Several

global effects like ozone layer depletion, global warming etc. are caused by CO2, CO,

methane, non-methane hydrocarbons, nitrogen oxides etc [81,82].

According to a report of Central Pollution Control Board (CPCB), out of the total

pollution load, 65% of CO, 22% of HC and 12% of Nitrogen oxides are due to Gasoline

and diesel vehicles [83].

Since CNG fuel is in gaseous form, it does not need to be vaporized. Therefore, no

fuel enrichment process is needed during cold starting or in transient conditions. This

contributes in the reduction of CO emission. In addition, due to higher H/C ratio, CNG

combustion produces 25% less carbon dioxide than Gasoline or diesel at the same engine

efficiency [84].

The main problem that all researchers and manufacturers are facing now is the

low power output of CNG engine due to loses in volumetric efficiency, low flame speed

and absence of fuel evaporation [85].


68

One major step towards emission control was introduction of exhaust oxidation

catalysts on passenger cars in the US and Japan in 1975. since then, high level of

advancements in engine technology including ‘death of carburettor’ in the US in 1990

and its replacement by multi-port fuel injection (MPFI), multi-valves per cylinder,

electronically controlled variable valve lift and timing, Gasoline direct injection (GDI)

engine have taken place [74].

Exhaust catalytic conversion being one of the mainstay of emission control, it has

seen many improvements like three-way catalytic control of HC, CO and NOx

simultaneously, electrically heated catalyst for emission reduction during engine

reduction during engine start-up, NOx storage catalysts to function under lean engine

operation such as in GDI engines etc [74].

The vehicles primarily emit the harmful gases CO, unburned fuel/hydrocarbons

also called as volatile organic compounds (VOC) and NOx. Among the mobile source,

diesel vehicles are the main contributors to smoke and particulate matter (PM) emissions.

Vehicles also emit sulphur di and tri-oxides (SOx), their amount depending upon the

sulphur content of the engine fuel. Some of the vehicle emissions in the atmosphere

produce other harmful chemicals, the secondary pollutants. The main secondary

pollutants are: oxidants like ozone, nitrogen dioxide (NO2) and total suspended

particulates (TSP) including host of other organic compounds like peroxy-acetyl nitrate

(PAN) etc. Carbon dioxide is not a pollutant for local environment but it being a green

house gas its contribution to global warming is causing an increasing concern. It is

estimated that CO2 is responsible for about 50% of the global greenhouse effect.

The adverse morbidity and mortality effect of air pollutants like suspended

particulate matter (SPM), respirable suspended particulate matter (RSPM or PM10), sulfur

dioxide (SO2), carbon monoxide (CO) and Ozone (O3) are well documented. In the case

of Delhi, the situation deteriorated in the 1990s as vehicles growth outpaced population

growth and economic development, vehicle had risen to nearly 3.6 million by 2001.

During the period, Delhi’s population increased from 9.5 to 13.8 million and road-length

from 22,000 to 25,000 km. The World Bank estimated that a person was dying every 70

minute in Delhi in 1995 from air pollution [86]. The Supreme Court (SC) of India ruled in

1998 that all the public transport should move away from diesel to CNG by 31st March,

2001 and by 1st December, 2002 all the buses were converted to CNG.


69

The United States, with less than 5% of the world’s total population, consumes

25% of the world’s current energy production and generates about 25% of the world’s

carbon emissions [87].

The emission norms and year of implementation for India are shown in Table 3.19

and Table 3.20.

Table 3.19 Emission norms for India – for heavy duty vehicles > 3.5 Tones (g/kWh) [88].

1992 1996 2000 2001 2005 2010

PRE EURO - 0 EURO - 1 EURO - 2 EURO - 3 EURO - 4

NOx 18.00 14.40 8.00 7.00 < 5.00 < 3.00

CO 14.00 11.20 4.50 4.00 2.50 1.00

HC 3.50 2.40 1.10 1.10 0.66 0.50

PM N.A N.A 0.36 0.15 < 0.15 < 0.10

Table 3.20 Emission norms and year of implementation.

Norms Year of Implementation

1996 (Euro 0) 1996

1998 ( Catalytic Converter Norms) 1998

Bharat Stage I (Euro I) 1999

Bharat Stage II (Euro II) 2000/2001

Bharat Stage III (Euro III) April, 2005

Bharat Stage IV (Euro IV) April, 2010

3.4.1 Carbon Monoxide (CO)

Carbon monoxide is formed due to deficiency of oxygen during combustion. It is

an odorless gas but is highly toxic. On inhalation it is rapidly absorbed by lungs and

combines with hemoglobin in the blood forming carboxy-hemoglobin. CO has 200 to 240

times greater affinity than oxygen to combine with hemoglobin [89]. The CO-hemoglobin

complex is far more stable than oxy-hemoglobin. This exposure to CO reduces oxygen

carrying capacity of the blood to body tissues. The decrease in release of oxygen due to


70

CO intoxication damages tissue and cells and adverse effects are higher and more rapid to

the brain and nervous system as these have a higher oxygen demand. The toxic effects of

CO depend both on the exposure time and concentration as shown in Fig. 3.3.

The early signs of CO poisoning are shortness of breath, rapid breathing,

headache, dizziness, impaired judgment i.e., confusion and lack of motor coordination.

These signs and symptoms results due to reduced supply of oxygen to brain tissues, a

condition called hypoxia. Nausea, vomiting and diarrhea may appear later. Exposure to

high CO concentrations or for a longer period may lead to cardiac arrest, pulmonary

edema, loss of consciousness and eventually to death. If the concentration of CO in the

inhaled air is high enough, loss of consciousness and death may occur within a short time.

Treatment of CO intoxication includes remove of affected person from exposure to the air

having high CO and administration of 100% oxygen to accelerate dissociation of

carboxy-hemoglobin to hemoglobin. Hemoglobin then can combine with oxygen and

correct the tissue hypoxia [89].

Fig. 3.3 Toxicity of carbon monoxide [90].

3.4.2 Nitrogen Oxides

Oxides of nitrogen NO and NO2 are formed during combustion at high

temperatures. During combustion in IC engines, the principal oxide of nitrogen formed is


71

NO. The concentration of NO2 is small in SI engine emissions (less than 2% of total

nitrogen oxides), but is high in diesel engines ranging from about 10 to 20%. NO2 is a

strong oxidant and in the presence of ultraviolet radiations leads to formation of ozone via

NO2 - photolysis as below:

UV

(3.6)(3.5)

32

2

MOMOOONOhvNO

Where M is a third body needed to absorb the energy of reaction.

NO2 is a reddish brown gas. It has an irritating odour and low solubility. Hence, it

reaches deep in lungs causing irritation. In low concentrations, NO2 may produce mild

irritation in upper respiratory tract. In asthmatic persons, such exposure may cause

asthmatic attacks. When nitrogen oxides get dissolved in aqueous medium of mucous in

the nose and throat lining, they produce nitrous and nitric acids. In acute exposure, these

acids cause immediate irritation to the mucous of the respiratory tract. Similar symptoms

may appear as eye irritation. In the lungs, reactions include cough and bronchospasm. If a

sufficient quantity of gas reaches lungs, it may cause acute bronchitis, pulmonary edema

and even death. Several weeks later of the acute pulmonary reaction, a chronic

inflammation of lungs may result leading to lung fibrosis. Such a progressive process may

lead to respiratory failure.

Nitrogen oxides too like CO react with blood hemoglobin [89]. A variety of

nitrogen containing substances reacts with hemoglobin oxidizing iron to its ferric state

(Fe+3). This chemical state of iron may form methemoglobin, a black pigment like

substance. Methemoglobin does not combine reversibly with oxygen thus, reducing

oxygen carrying capacity of the blood to tissues. It also results in destruction of red blood

cells. When the concentration of methemoglobin exceeds 10% of the total hemoglobin

level, bluish coloration of skin (cyanosis) is caused in the affected person due to lack of

oxygen. At higher concentrations, symptoms like shortness of breath, weakness and

headache may occur. The symptoms on exposure to nitrogen oxides develop in a slow but

harmful way. The major signs of exposure include development of bluish skin

particularly of lips, fingers and toes.


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3.4.3 Photochemical Smog

Photochemical smog, initially known, as ‘Los Angeles Smog’ is a brownish-grey

haze caused by reactions between hydrocarbons and nitrogen oxide in atmosphere in

presence of solar ultraviolet radiations. It consists of a variety organic compounds, ozone

and nitrogen oxides trapped above the ground level due to temperature inversion. The

photochemical smog is characterized by an unpleasant odour due to some gaseous

pollutants.

The VOCs other than methane, which include hydrocarbons and their derivatives,

undergo several chemical reactions with nitrogen oxides in presence of sunlight to form

photochemical smog. A simplified model of photochemical smog formation includes NO2

photolytic cycle.

The harmful constituents of photochemical smog are NO2, O3, PAN and

aldehydes. PAN and aldehydes in high enough concentration can cause eye irritation and

plant damage. Ozone concentrations close to roads are generally small. Movements in

atmospheric air carry the gaseous emissions away and as the air takes along the reacting

species, reactions continue to take place for several hours. The impact of photochemical

smog may therefore be felt several kilometers away from the source of emissions.

3.4.4 Ozone

Ozone is a colourless gas with pungent odour like chlorine. It is soluble in cold

water and alkalis. Ozone is a strong oxidant used in bleaching and sterilizing water. In the

stratosphere, it forms a life saving ozone layer, which acts as a shield for ultraviolet rays.

However, at ground level ozone due to its highly oxidizing properties is harmful. The

odour threshold of ozone depending upon individuals is about 0.01 to 0.05 ppm. It causes

irritation of mucous membranes of airways and lungs. Ozone like other irritants such as

sulphur dioxide, chlorine etc., causes marked reduction in airflow to lungs and ventilation

(bronchoconsrtiction). Exposure to low level of ozone (0.3 to 0.9 ppm) produces

symptoms like cough, dryness of throat and chest discomfort [89]. These effects will

subside when exposure is discontinued. Eye irritation may also be caused by exposure to

ozone. Exposure to 0.2 ppm of ozone for three hours produces vision disorders like visual

acuity, decreases night vision and disturbs balance of muscles controlling position of


73

eye [91]. The adverse effects of ozone increase substantially in presence sulphur dioxide

and are much more than the effect of either pollutant separately.

3.4.5 Particulates

Particulates are fine solid or liquid particles suspended in air. Particulate matter

emitted by the vehicles also, may be in solid or liquid phase. Solid particles emitted by

vehicles are largely made of carbonaceous matter (soot) consisting a small fraction of

inorganic substance. Different type of liquid phase substances and other materials are also

either adsorbed or absorbed on these particles. During engine start up and warming

particularly in cold weather, the heavy hydrocarbons in the exhaust gas may condense to

form ‘white smoke’. Among ICEs, diesel engines operating at light loads and small two

stroke SI engines employing total loss lubrication system are the main sources of liquid

phase particulate emissions. When lead antiknocks were used in fuel, the Gasoline-fuelled

engines emitted solid lead containing particles. The SI engines may also emit the carbon

particles when operating on over-rich mixtures. However, lead antiknocks have been

phased out of Gasoline in most countries due to toxicity of lead and poisoning of the

catalytic converters being employed for emission control on Gasoline vehicles. Use of

unleaded Gasoline results in very low particulate emission just about 20 mg/km in the

vehicles without catalytic converters [92].

One of the principal sources of solid carbon particles (soot) emitted in air is the

diesel engine. The particulate emission from diesel engines consists of unburned soot,

soluble organics from fuel and lubricating oil, ash particles coming form oil additives and

wear particles. The major source of particulate emission in diesel engines is

heterogeneous mode of combustion where fine soot particles are generated. These fine

soot particles grow and agglomerate as the combustion progresses. Later, during

expansion and in the exhaust system heavy hydrocarbons from fuel and oil, sulphates

produced form combustion of fuel sulphur and water is absorbed on the soot core of

particles.

Particles smaller than 2.5 µm constitute more than 90% of total mass of

particulates emitted by diesel engines. According to Stoke’ law, settling time is inversely

proportional to the square of particle diameter. The rate at which the fine particles settle

down is very low and these remain airborne for several days. The particles of less than 2.5


74

µm are of main concern as they take long time to settle, remain airborne for days and

reach the respiratory system of the human beings. Particles particularly smaller than 1

µm, are too small to be trapped in the upper portion of lungs and penetrate deep into

lungs. The particles larger than 2.5 µm settle down fast and are also filtered by the

respiratory system, and hence are not a serous health hazard. Adverse effects of various

pollutants originating from vehicles are summarized in Table 3.21.

Table 3.21 Adverse Effects of Principal Pollutants [91,93].

Pollutants Short-term health effects Long-term health effects

Oxidants Difficulty in breathing, chest

tightness, eye irritation

Impaired lung function, increased

susceptibility to respiratory function

Ozone Soreness, coughing , chest

discomfort, eye irritation

Development of emphysema,

pulmonary edema

Total

suspended

particulate

/Respirable

suspended

particulate

Increased susceptibility to

other pollutants

Many constituents especially poly-

organic matter are toxic and

carcinogenic, contribute to silicosis,

brown lung

Sulphates Increased asthma attacks Reduced lung function when oxidants

are present

Nitrogen

Dioxide

Similar to those of ozone but

at a higher concentration

Development of cyanosis especially at

lips, fingers and tows, adverse changes

in cell structure of lung wall

Carbon

monoxide

Headache, shortness of

breath, dizziness, impaired

judgment, lack of motor

coordination

Effects on brain and central nervous

system, nausea, vomiting, cardiac and

pulmonary functional changes, loss of

consciousness and death.


75

Particulates pose health hazard due to two reasons:

(i) They can penetrate deep into lungs and deposit there affecting performance of

lungs adversely, and

(ii) These can have synergistic effect with other pollutants present in air. Sulphur

oxides and water combining together produce sulphuric acid that may be

adsorbed on soot particles and carried to lungs. Carcinogens like PAHs when

adsorbed on soot particles can be carried to lungs and cause cancer.

3.4.6 Noise

Noise is a well known environmental problem associated with major cities world

wide. A great deal of effort has been devoted to extensive noise surveys and modeling

studies in various cities of the world. In most of the surveys, vehicular traffic has been

identified as a major factor contributing to urban noise. The impact of traffic noise

exposure on human beings in terms of annoyance has been examined by using various

statistical noise indices in a number of studies. Most of the studies pertain to outdoor

noise measurements. A limited number of studies have been carried out on the noise

levels and its spectral distribution inside vehicles [94].

CNG run vehicles also have quieter operation, less vibrations and less odour than

equivalent diesel engines. However, high vehicle cost, shorter driving range, heavy fuel

tank, expensive distribution and storage network and potential performance and

operational problems compared to liquid fuels are some of the drawbacks of CNG [95].

The fact that CNG driven vehicles have new body structure would have also

certainly contributed to a reduction in interior noise. It is found that the decision to run

public transport on CNG fuel in Delhi has a positive impact on the noise environment in

buses, auto-rickshaws and taxis [96].

chapter 3 conventional fuels and alternative fuels...

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