CHAPTER 2

LITERATURE REVIEW

CHAPTER 2

LITERATURE REVIEW

2.1 Introduction

The performance of a Spark ignition engine is mainly dependenf on the

air-fuel mixture preparation and distribution. Carburetors were designed to

deliver the varying air-fuel mixture requirements during idling, part load and

full load operating conditions of the spark ignition engine, and the main

emphasis was on the maximum brake power and driveability. Under transient,

operating conditions such as rapid acceleration, the carburetors were unable

to deliver the required air-fuel mixture. Hence electronically controlled

carburetors were developed to meet the transient air-fuel mixture

requirements. The major disadvantage of the carburetor is the improper

distribution of the air-fuel mixture to the cylinders. To overcome this

disadvantage, gaseous fuel injection system was developed.

The alarming environmental pollution, need for efficient utilization of

fossil fuels and the inventions in solid-state electionics have led to the

development of electronic control of fuel injection systems for commercial

applications. The various types of multi-point fuel injection systenrrs are

reviewed below.

2.2 Review of Multi-Point Fuel Injection Systems

2.2.1. K-Jetronic Fuel Injection System[5]

This system has an electronic fuel punnp, which develops a maximum

fuel pressure of 4 bar. The fuel injectors are of mechanical type, which open

when the fuel pressure exceeds 2 bar. The air-fuel mixture was controlled by

the mixture control unit and, also it has a fuel distributor and an accumulator.

The airflow in the intake manifold was sensed by mechanical flap type airflow

sensor and the mixture control unit is actuated through mechanical linkages.

Though the mixture distribution and control have improved considerably, the

system was an open loop system without any feed back of the exhaust

emissions. This was overcome in the KE-Jetronic system.

2.2.2 KE- Jetronic Fuel Injection System[6]

In this system solenoid injectors controlled by electronic control unit

(ECU) is used instead of mechanical fuel injectors. The oxygen sensor fitted

in the exhaust senses the exhaust oxygen content, which indicates whether

the air-fuel ratio is rich or lean. Based on this information the ECU optimizes

the fuel injection for operating the engine in the stoichiometric range. The

airflow in the intake manifold was sensed by an electromechanical airflow

sensor and communicated to the ECU. The control unit controls the opening

of the injectors and optimizes the fuel injection for the best performance and

emissions. The engine performance depends on precise quantity of airflow.

Thus the accurate measurement of airflow is necessary. Hence a precise

airflow sensor was incorporated in the L-Jetronic systems.

2.2.3 L- Jetronic Fuel Injection Systems[7]

In this system, an electromechanical airflow sensor was replaced with

the Hot-wire anemometer sensor for better accuracy. The ECU and the

associated hardware remain the same as in the KE-Jetronic system. All the

above injection system discussed does not have electronic ignition control.

This was incorporated in the Motronic fuel injection system.

2.2.4 Motronic Fuel Injection System

In this system the ignition and injection are controlled by the ECU for

the better engine performance and low emissions. The various other features

such as electronic throttle valve timing control, knock control are also

incorporated in the system.

2.3 Two-Stroke Fuel Injection Systems

The fuel injection systems discussed above are the multi-point fuel

injection systems used in four stroke spark ignition engines. These systems

are designed for particular engine applications and they cannot be directly

used in a two-stroke engine because of variation in speed of operation and

the varied quantity of fuel required in a two-stroke engine. The two stroke

engine requires fuel injection for every rotation of the crank while in the four-

stroke engine the injection is only once in two revolutions of crank. Moreover

the fuel quantity required by the typical two-stroke engines is only about one-

third of the fuel injected in a four-stroke engine. Hence the injection duration

has to be reduced drastically. The response of the injector is very poor for

very low injection timings particularly for idling conditions. A detail review of

the various two-stroke fuel injection systems developed is presented below.

2.3.1 Review of Two-Stroke Fuel Injection System

The main objective of the development of the two-stroke fuel injection

system was the elimination of short-circuiting losses, reduction of specific fuel

consumption, HC, CO emissions and reducing irregular combustion and cyclic

fluctuations.

Giichi Yamagishi, Tadanorisato, and Hiroyoshi lwasa(1972)[2] have

developed a mechanical fuel injection system. The performance of the engine

was studied with the injector located in the cylinder head, in the scavenging

port, and in the lower portion of the cylinder bore. In case of scavenging port

injection, it is required to set the end of injection before scavenging port

closing, which leads to a large amount of fuel being short circuited together

with the scavenging air as in the case of a carburetor. In the case of lower

portion of the cylinder bore injection, the range of injection timing is limited

and fuel short-circuiting is also inevitable. The cylinder head injection had the

best performance and lowest emissions. The main disadvantage of the

mechanical injection system is the injection delay, which occurs due to the

mechanical movement of the injector needle. This prevents the operation of

the engine at high speeds.

Edmond Vieilledent(1978)[3] has developed a low-pressure electronic fuel

injection for a 155 cc two-stroke SI engine. The injector was located in the

intake manifold and in the cylinder bore and the performance was studied.

10

The performance of the cylinder bore injection had minimum emissions and

specific fuel consumption, while the manifold injection system simulated the

carburetor conditions. A capacitor discharge injection system was developed

to improve the high-speed response of the injector. The operating voltage of

the injector was increased to increase-the response of injector.

R Douglass and G.P. Blair (1982)[4] have developed a low-pressure

electronic fuel injection system for a 100 cc two-stroke SI engine. The injector

control was completely manual. The injector timing was set by means of a

timing disc and inductive pick-up affixed to the crankcases. Triggering was

from a steel pointer on the end of the crankshaft. The perforniance of the

engine was studied with the injector located in the inlet manifold, into rear

transfer duct, direct into cylinder, and into swirl cylinder. In transfer port

injection the start of injection was delayed to minimize short-circuiting losses.

It has been reported that the cylinder bore injection reduces fuel consumption

by 30% and exhaust emission by 50-60%. However there is a reduction of

10% of power.

Grasas-Alsina.C, Freixa.E, Esteban.P, and masso.J(1986)[8] have

implemented a low -pressure discontinuous fuel injection system for a 350cc

two stroke SI engine . The system is electronically controlled to supply

different quantities of fuel required in relatively short duration of time. They

employed two electro valve-type fuel injectors operating simultaneously at a

line pressure of 300 kPa . Two locations of the injectors have been tested into

c the inlet duct and into transfer ducts. The results obtained in this work are

11

compared with the carburetor fuel supply system and the following

conclusions were drawn.

1. The results did not depend upon injection timing

2. The same maximum power was obtained with both the systems at any

given working speed.

3. A considerable reduction in BSFC was obtainable whenever the BMEP

was not too close to the maximum attainable at the tested speed.

4. In case of transfer injection, injection timing is a significant factor.

5. In case of transfer injection, the power at wide-open throttle decreased

slightly at low engine speeds.

John beck.N., Johnson.w.p, Barkhimer.R.L, and Pattee3on(1986)[9] have

developed an accumulator type electronic unit injector for achieving high

injection rates and spray characteristics, which are independent of engine

speed. The injection pressure was 100 bar and the injector was located in the

cylinder head. Performance, as compared to a carbureted engine, shows 20

to 30% reduction in the fuel consumption and a 5 to 10 folds reduction in

unburned hydrocarbons in the exhaust.

Bizian Francisek and Pauletic Radislar(1986)[10] have analyzed the

performance of 59cc two-stroke SI engine with the injector located at the

intake manifold, transfer port and cylinder bore. The fuel injection system was

c an L- Jetronic system developed for four stroke engine. In cylinder bore

injection, the injection spray was made to impinge on the piston crown.

12

towards the head and towards the scavenging port by changing the injector

location and the performance was analyzed. Maxinnum reduction in specific

fuel consumption and emissions with the injector facing the scavenging port

were obsen/ed. However minimum reduction in specific fuel consumption and

emissions were observed when the injector was located in the manifold.

Tadanori Sato and Mitsushige Nakayama(1987)[11] have developed a

mechanical fuel injection system with various nozzle configurations. The

performance of the engine with injector located in the cylinder bore, transfer

port and in the cylinder head were presented. The specific fuel consumption

of the injection sngine at the full load is 25 to 45% lower than that of the

carburetor engine. The minimum specific fuel consumption of the injection

engine is 300 g/kWh.

Diethard plohberger et al.(1988)[12J have developed a semi-direct injection

system for a 250 cc two-stroke SI engine. The performance of the engine with

the injector located in the transfer port, in the cylinder bore, cylinder head and

in the intake manifold was analyzed. The injector was located in the transfer

port such that the spray directly enters the cylinder. In the cylinder bore ,*

injection the injector was located such that the injection spray is towards the

piston crown to enhance evaporation of the fuel spray. The injection spray

towards the piston crown gave the maximum reduction in specific fuel

consumption and emissions. ^

Duret, P. et al.(1989)[13] developed a single cylinder compressed air assisted

fuel injection (lAPAC) engine of 246 cc capacity. Fuel injector is in the cylinder

head. A conventional low-pressure automotive style electronic fuel injector is

13

used as a metering unit to deliver the fuel. A large poppet valve activated by a

camshaft controls the delivery of the compressed air and fuel to the engine

cylinder. The specific fuel consumption of the injection engine is 260 g/kWh.

The exhaust emission is below 10 g HC / kWh.

Blair et al.(1991)[14] describe the application of direct air assisted fuel

injection for the reduction of emissions and fuel consumption from a single

cylinder, crankcase scavenged engine of 270cc swept volume. The engine

makes use of a piston controlled induction system, a fixed exhaust timing and

an untuned exhaust system. A modest target BMEP in the range of 550-600

kPa was set for this design. A multi-cylinder version of such an engine with

favourable exhaust timing would allow an increase in BMEP of about 25%.

Lieghton .S et al.(1994)[15] have developed the orbital combustion process,

small engine Fuel Injection system for outboard two-stroke marine engine

applications. In this system the fuel is injected into a mixing chamber at a

high pressure and mixed with the air. This air-fuel mixture is then injected into

the cylinder at the start of compression. A highly stratified fuel mixture is

formed at the end of the compression stroke, which reduces the specific fuel • , ; . . , .

consumption by 40% and the exhaust emissions by 60%.

Kum-jung et al.(1995)[16] have developed a low-pressure air assisted fuel

injection system for a 400 cc two-stroke SI engine. The injectors were

modified to have various cone angles. A spray cone angle of 70° and a droplet

size of 6 microns were found to give the best performance. The injector was

located in the cylinder head and the spray was directed towards the piston

crown for good evaporation and mixing. In this system the fuel is injected into

14

a mixing chamber at 7.5 bar where compressed air at 5 bar is being

continuously supplied by an external pump. The air-fuel timing were controlled

by the electronic control unit to achieve optimum air fuel mixture ratios. The

premixed air fuel mixture was injected into the cylinder by a poppet valve fixed

in the cylinder head. High-pressure injection of 70 bar was also tried for

improving the mixture characteristics. The reduction in specific fuel

consumption and HC emissions for the low-pressure injection was better than

for the high-pressure injection system.

Marc L Syverten et al.(1996)[17] have analyzed the injection and the ignition

effects on the two-stroke direct injection engine emission and efficiency. The

in-cylinder air motion and the fuel injection spray were varied and the mixture

formation and emission were analyzed. The factors that affect the emissions

are injection spray type, spark plug location, injection timing, fuel air mixing

and combustion. A wide spray produces a well-mixed fuel cloud in the vicinity

of spark plug, which improves combustion. A narrow spray produces stratified

air fuel mixture near the spark plug, which is unpredictable.

Marco Nuti and Roberto Pardini(1998)[18] have reviewed the various types of

direct injection system. The direct injection system of the two-stroke engine

maintains the advantages of the two-stroke engine while improving the

combustion and emission with the four stroke engines.

Cornel Stan and Jean-louis Lefebvre(1999)[19] have developed a direct

injection concept for two wheelers equipped with two-stroke engines. The

electronically controlled fuel injection system was developed for two-stroke

engines with swept volumes of 50 cc and 25 cc. The engine results show that

15

the engine torque remains in ail the speed ranges at least at the same level

as for the base engine equipped with carburetors, while the BSFC decreases

to 35- 45%. But the most important result is the reduction of pollution with 80-

94% for the HC emissions and 90% for the CO emissions.

William P. Johnson et al(1999)[20] have developed electronic direct fuel

injection for a 46 cc handheld utility engine and a 50 cc two-wheeler engine.

The system is based on the accumulator fuel injection operating pnnciple,

which involves pressurizing fuel with in an injection nozzle and subsequently

releasing the pressurized fuel into the combustion chamber. This concept

provides very short injection duration throughout the dynamic operating range

of the engine as well as high injection frequency capability.

2.4 Alternative Fuels

In view of the possible depletion of fossil reserves research is being

done on various alternative fuels including renewable and nonrenewable

resources. These include biogas, producer gas, methanol, ethanol, LPG,

CNG etc. in this regard the CNG provides the better option out of the

alternative and relative, cleaner fuels.

An attempt has been made here to critically review and asses the

research efforts carried out on CNG engine system and identify the gaps,

controversies and limitations that need further study.

16

2.4.1 Historical Review

The use of natural gas has been known since earliest historical times.

Today world's proven reserves of natural gas exceed of those of crude oil[21].

Although different estimates are put forward with regard to the ratio of natural

gas crude reserve, this factor is constantly increasing as quite often and in the

search of crude reserves, dry fields were found and the resulting gas field is

capped off [22]. In view of the present lower consumption rate of gas, the ratio

of reserve to end-use is also much greater for natural gas than for the crude

oil[23]. Apart from the existing vast reserves of natural gas, it can also be

produced from coal and biomass conversion thus making it a wider available

base crude oil[24].

2.4.2 Earlier Research on CNG Engines

According to B.Bonnetl et al.(1972) the use of natural gas has been known

since very early phase of oil industries development. Oil industry was

established in 1859, and only within a few years the natural gas industry came

into existence. Gas was being produced and piped for supply [21].

Ken Deffeyes(1990) reviewed geological estimates of methane availability

[22]. It has been estimated that world's proven reserves of natural gas exceed

those of crude oil. Different estimates are put forward with regard to the ratio

of natural gas to crude reserve.

Enoch J. Durbin[23], Ralph D. Fleming et al.[24], have estimated the present c

lower consumption of gas, and observed that as the ratio of reserve to end

use is also much greater for natural gas than for the crude oil.

17

T.W. Ryam[25], studied the methane number, its heat effects on the engines

and the engine knock rating.

M.Leiker et al.[26] evaluated the engine anti-knocking property with the

methane number. They emphasized it with the practical application to gas

engines.

John kubesh et al. [27] correlated methane number and octane number.

Stanky L. Genslak. John S. Heenam et al. K.Johnes et al.[28,29,30.] studied

about the engine performance. The significant result of their studies shows

that (i) a stoichiometric mixture of natural gas and air occupies about 10%

more volume than a stoichiometric gasoline/air mixture with the same energy

content, (ii) for a fixed engine displacement thus, the amount of air-fuel

mixture that can be inducted and burned in each stroke is about 10% less

(natural gas), resulting in a comparable penalty in engine power output, (iii) a

gasoline engine converted to natural gas engine will thus produce about 10%

full throttle than on gasoline, (iv) due to all these factors the volumetric

efficiency is also low and (v) to overcome this, it is necessary to increase the

compression or turbo charging.

World gas industry[32] has defined a parameter known as wobbe-number to

account for the effect of gas composition on energy delivery. The number is

an index of the energy flow rate through an orifice or valve in response to a

given pressure drop. Higher the wobbe number means greater heating value

of gas.

18

Glean B. 0'nail[33] has emphasized that wobbe number is an important index

because fuel composition and properties like heating value and molecular

weight can affect the maximum power output of an engine.

K. Johon et al.[34] studied the effect of gas composition on engine

performance.

Ralph D. Fleming et al.[35] gathered data on the efficiency, performance and

emission of a single cylinder engine at different compression ratios and with

air fuel ratio varied from rich to lean limit. They found that HC and NOx

emission increased with high compression ratio.

Mark A. Deluchi et al.[36] studied, the comparison of methanol Vs natural gas

vehicles with respect to resources supply, performance, emission, fuel

storage, safety, costs and transitions.

R.W. McJones et al.[37] studied the natural gas fueled engine and reported

that these engines have lower exhaust emissions with respect to the other

conventional engines.

Willamson E.I. has discussed the necessity of alternative fueis, promotion

policies of the government, availability of conversion kits and fuel storage on

the vehicle[38].

Harrison, John B., made an attempt to study the CNG performance datum to

provide a basis for comparative assessment, optimum performance which

could be achieved from CNG, in an engine of a given type without any outside

influences such as petrol, carburetor, pressure regulator or petrol ignition

19

advance characteristics. Performance problems resulting from poor design of

mixer are noted, and their solution through redesign of the mixer was

inducted[39].

Gettel L.E., perry G.C., Smith M.C., studied the performance test results on

different CNG kits and their degradation. The reasons for this were discussed

and indication was given for the best probable and best operating strategy for

kit design[40].

Elder Stephen T et al. in their work determined the effects of varying fuel

compositions on vehicle fuel consumption, power output, and emissions and

tuning[41].

Rosen, Jerome from their survey found that CNG as a fuel had less knock,

longer spark plug life and relatively less oil changes and instant winter

startups and lower maintenance cost than gasoline vehicles[42].

Ghandhi Dasan. P, Ertas. A, Anderson.E., have discussed the properties of

CNG source and potential fuel supply, safety, toxicity and health hazards,

engine performance, fuel storage and fuel tank and refilling[43].

Scott.C, Sayen et al. modified the vehicle for a dedicated CNG operation with

emphasis on lower emissions, fuel economy, engine efficiency, and

driveability without sacrificing performance. They also studied about lowering

of compression ratio, reduced peak cylinder temperature and inhibited NOx

formation, and the loss in BMEP due to lowered compression ratio[44].

20

Jim, Phillips, Scott, Vaughan, Holly changed engine piston and head to obtain

compression ratio of 13:1 in order to regain power (due to the gaseous fuel).

General motors electronic control module was reprogrammed for optimal

spark advance for natural gas operation[45].

Karim G.A., Wierzaba I., reviewed the safety operation of conventional

engines using natural gas, and they stated that CNG is safer than the

gasoline and other alternative fuels such as propane or hydrogen. The safety

procedures adopted in the design and operation of a conventional laboratory

engine using rich mixture of methane was also studied[46].

Bell, Stuart R. et al.[47] studied conversion of gasoline engine to natural gas

engine with commercially available kits. Performance and emissions

characteristics of the installed kits are discussed.

Sturman O. Eddie, Pena James A, Petersen[48], designed an injection

system especially for low energy density gaseous fuels. The injector

incorporated design features that is necessary to optimize the performance for

CNG fuel and the background of magnetic latching technology are discussed.

The application of the technology to an advanced, pressure balanced,

gaseous fuel injector is also described.

Single cylinder engines have been reported to lose 15 to 20 % of their power

at a given speed and wide open throttle, when fuelled with natural gas instead

of gasoline (Oearce[49], Morore and Roy[50], Karim and Ali[51], Beats[52]

perry et al.[53] ).About 10% of the power loss is due to displacement of air by

21

gaseous fuel rather than liquid fuel. The remainder appears to be due to the

high ignition threshold and the slow burning rate of methane air mixtures.

Tests on multi-cylinder engines have been reported to yield similar results,

though quantitative results differ considerably because of differing test

conditions. Pearce[49] operated a lOOOcc 4-cylinder engine on both natural

gas and gasoline. On natural gas he obtained 86% of the break power

measured with gasoline fuelling. Genslak[54] did similar tests with a 5700 cc

V8 engine and showed that it produced 85% of the power of the same engine

operating on gasoline, at optimum air-fuel ratio and ignition timing. -

Further test bed work with multi cylinder engines and natural gas, was done

by Affleck, Harrow and Mills[55] as part of a program to convert a small car to

natural gas. Their tests were done on a 2-litre displacement four-cylinder

engine. The engine was tested as delivered on gasoline, and then modified

for best economy when operating on natural gas. The modifications included

raising the compression ratio from 8.2:1 to 11.2:1, and fitting a natural gas

carburetor calibrated to produce lean mixtures for low fuel consumption. In

this form the peak power produced was 94% of that of gasoline at low speed,

decreasing to 67% of that of gasoline at high speed. The authors state that

this decrease in power is caused by lean carburetion for minimum fuel

consumption.

Under optimum conditions of air-fuel ratio and ignition timing, the power

available from a spark ignition natural gas engine is 89 to 90% of that from

gasoline. This loss can be explained mainly by the displacement of air in the

engine cylinder by the gaseous fuel. As Flemming and Allsup[56] show, for a

22

natural gas of specific composition, tiie energy available per unit mass of

mixture in a chemically correct mixture with air is 97% of that available from a

chemically correct gasoline -air mixture. The gaseous fuel will displace about

10% of the air inducted into the cylinder, so a given volume of fuel-air mixture

will have less than 90% of heat available from the same volume of gasoline -

air mixture. The maximum power output on natural gas will of course depend

on the composition of the natural gas but for the same engine will generally be

less than 90% of that for gasoline fuelling.

23

2.5 Compressed Natural Gas: Properties and Combustion

Characteristics

The exact composition of natural gas depends on whether the gas is

sourced fronn an oil or condensate field i.e. whether it is the associated gas or

It exists by itself, which is referred to as non-associated gas. Associated gas

may contain significant amounts of heavier hydrocarbon such as ethane,

propane and butane together with lighter liquids such as pentane, hexane etc.

In this category methane can be as low as 50%. Non-associated gas contains

a much higher percentage of methane. Additionally both these varieties

contain a much higher percentage of methane and varying amounts of carbon

dioxide, nitrogen and other contaminants. The gas composition may vary

substantially between different wells.

2.5.1 Natural Gas Characteristics

The characteristics of methane are as shown in table 2.1 and can be

taken as a close approximation to those of natural gas.

24

Table 2.1 Characteristics of Methane [57.58,59,60,61,62]

Fuel Property Natural Gas (Methane)

Molecular weight, kg/mole 16

Specific gravity at NTP relative to air 0.55

Density at NTP, kg/m^ 0.651

Limits of flammability. Vol % 5-15

Stoichiometric composition in air, Vol% 9.48

Minimum energy required for ignition in air MJ 0.29

Heat of combustion (high), MJ/kg 55.53

Heat of combustion (low), MJ/kg 50.02

Auto ignition temperature, K 813

Flame temperature in air, K 2148

Burning velocity in air, (m/sec) 0.37-0.45

Methane number 100

Wobbe number 1363

Heat of evaporation, (MJ/kg) 0.51

Vapour pressure at 311 K GAS

Research outane rating (RON) 130

Ignition temperature, K 922-994

Highest useful compression ratio • 15.6

Freezing point, K 91

Boiling point, K 111

Critical temperature, K 190.5

Critical pressure, atm 45.8

Latent heat of vaporization, kJ/kg 510

25

Table 2.2 Thermodynamic properties of Methane and Gasoline[63]

Fuel Property Natural Gas (Methane)

Gasoline

Formula CH4 C4 to C12

Molecular weight 16 100-105

Composition, %

Carbon 75 85-87

Hydrogen 25 12-15

Oxygen (oxygenated or reformulated gasoline's 0 0-4

only)

Freezing Point, K 91 233

Boiling point, K 111 300-498

Vapour pressure, kPa @ 311 K Not Applicable 48-103

Viscosity, mPa-s @ 293 K 0.01 0.37-.44

Latent Heat of Vaporization, kJ/kg 510 349

Flash point, K 85 230

Autoignition Temperature, K 823 530

Fiammability Limits, Vol%

Lower 5 1.4

Higher 15 7.6

Stoichiometric Air-Fuel Ratio, 17.2 14.7

Octane Number

Research 130 88-100

Motor 120 80-90

26

2.5.2 Methane Number

For quantifyir.g the knocking tendency of gaseous fuels, the parameter

methane number is used, which is similar to the octane number of petrol. It

gives the methane /volume ratio of a methane/hydrogen or methane/carbon

dioxide mixture(%). In this scale pure methane is assigned a methane number

of 100, indicating extreme knock resistance and hydrogen is assigned a

methane number of 0. Natural gas being predominantly methane based has a

high methane number[25]. On the octane scale the value corresponds to

approximately 130 RON, there by making natural gas a highly knock

resistance fuel.

Natural gas has high ignition temperature and resistance to self-

ignition. These give it excellent anti-knock properties[26]. Methane has an

equivalent research octane number of 130 RON, which :s the highest for any

commonly used fuel that in turn improves thermal efficiency[28]. Large

quantities of propane, butane etc. in the gas increase the tendency to knock

some what, while the inert constituents such as CO2 and nitrogen lower down

such tendencies. For normal gas these effects tend to balance and, her.ce the

antiknock properties are similar to those of methane. Because of its antiknock

•Droperties, natural gas can safely be used with engine at compression ratios

as high as 15;1. Natural gas engines using these higher compression ratios

:an reach significantly higher efficiencies than are possible with gasoline.

27

2.5.3 Density

Due to the low density of natural gas, a stoichiometric mixture of

natural gas and air occupies about 10% more volume than a stoichiometric

gasoline/air mixture with the same energy content. For a fixed engine

displacement, therefore, the amount of air-fuel mixture that can be inducted

and burned in each stroke is about 10% less resulting in a comparable

penalty in engine power output. A gasoline engine converted to natural gas

will thus produce about 10% less power at full throttle than on gasoline. In

dedicated natural gas engines, this can be overcome by increasing the

compression ratio. The higher efficiency due to the increased compression

ratio results in more work output for each unit of mixture inducted, thus

offsetting the reduced maximum induction rate.

2.5.4 Flame Speed

Because of high activation energy [28], the laminar -flame speed of

natural gas mixtures is lower than that of other hydrocarbons. This effect is

most significant under lean conditions. The low flame speed of natural gas

results in a longer duration of combustion, impairing efficiency unless the

spark timing is advanced to compensate for it. The need for advanced timing

can be offset to a considerable degree by the use of high compression ratios

and compact turbulent combustion chambers. These increase flame speed

and decrease the distance the flame must travel.

28

2.5.5 Ignition Energy

The methane molecule is stable and compact. Therefore it has high

activation energy [28]. The minimum energy required for ignition is therefore

higher compared to liquid fuels. This may require high-energy ignition source

for combustion.

2.5.6 Wobbe Number

The composition of natural gas varies considerably from source to

source. Changes in the balance of methane, other hydrocarbons and inert

gases aflect both the density and the volumetric energy content of the

mixture. Increased amounts of higher hydrocarbons increase in the volumetric

energy content. While increased amounts of inert gases reduce it. Too great

a concentration of higher hydrocarbons will enrich the mixture and reduce the

octane number, leading to excessive emissions and knock. Too great a

concentration of inert gases will result in an excessively lean mixture,

reducing power output and possibly rough operation, if the mixture is already

lean.

To account for the effects of gas composition on energy delivery, the

gas industry has defined a parameter known as Wobbe number. The Wobbe

number W is defined as the higher heating value H (on a volumetric basis) of

the gas divided by the square root of its specific gravity with respect to air i.e.

W= H / (specific gravity )'"̂

29

As this equation indicates, the Wobbe number has units of kJ/kg. The

Wobbe number is an index of the energy flow rate through an orifice or valve

in response of the gas that will flow through a given orifice in a given time.

Virtually all natural gas appliances (including gas carburetors on engines)

meter the fuel by means of valves or orifices. Thus if two gas mixtures have

the same Wobbe number the heat output of a burner or the air-fuel mixture of

an engine will be the same with either mixture[33,34].

Wobbe number is important because fuel composition and properties

like heating value and molecular weight can affect the maximum power output

of an engine. Two gases having the methane contents of 74% and 80% were

tested and it has been observed that tuning the engine for optimum power

condition for one gas, resulted in power loss of up to 11% with the other. The

study revealed that with optimized engine settings, the full throttle power

output, which is proportional to the energy content of unit volume of

stoichiometric air-fuel mixture, is a direct function of gas composition.

2.5.7 Quenching Distance

At a given pressure, combustion may be suppressed by confining the

gases to vicinity of the surface. Two parallel plates may be brought together

until combustion can no longer be sustained. The maximum distance between

the plates when the combustion is suppressed is called the quenching

distance. Quenching distance for CNG is almost same as that of the gasoline,

but it needs high spark to filiate the combustion.

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2.6 Natural Gas Combustion

The normal combustion process in a spark ignited CNG fuelled engine

takes place as described below.

After the initial spark there is an ignition delay while the flame kernel

created by the spark grows to significant size. Following that, the flame front

spreads through the combustion chamber. The rate of spread is determined

by the flame speed, which is a function of air-fuel ratio, temperature and

turbulence level. The flame front increases in volume of the hot burned charge

outward. Overall cylinder pressure increases through compression heating.

Elements of unburned mixture burn out as the piston descends.

The higher the compression ratio higher the theoretical efficiency, but

the rate of improvement becomes small for compression ratios about 12:1. In

addition, frictional losses tend to increase with the increasing compression

ratio, so that most practical engines have an optimum compression ratio

between 12 and 15. The ratio of specific heats(k) is a function of the air-fuel

ratio. The ratio is typically 1.4 for diatomic gases such as O2 and H2 and about

1.3 for triatomic gases such as CO2 and H2O, which are produced by

combustion. Engines using lean mixture tend to have better efficiency for this

reason. The lower temperature of the burned gases results in less heat

transfer to the cylinder walls, which tends to improve efficiency, since a lean

mixture contains less fuel for same cylinder volume, the work output per

stroke is less.

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Three basic combustion methods are adopted for natural gas engines.

For a homogeneous charge engine, the mixture surrounding the spark plug is

nominally of the same composition as the bulk of the mixture in the cylinder.

Such engines are subjected to the ignition limitations as discussed above. In a

stratified charge engine used with very lean mixtures, the mixture in the

immediate vicinity of the spark is made richer than the rest of the charge, so

that ignition and early flame growth occur more quickly and reliably. The flame

can then spread quickly to the leaner remainder of the charge.

An extreme form of the stratified-charge engine is one having a

separate prechamber where ignition occurs. The expansion due to

combustion in the prechamber causes the burning gases to shoot into the

main chamber through the prechamber orifices in one or more turbulent jets,

providing excellent mixing and rapid combustion through out main chamber.

2.7 Natural Gas Production

Natural Gas is present in the earth and is often produced in association

with production of crude oil. However, wells are also drilled for the express

purpose of producing natural gas.

The main constituent of natural gas is methane, the lightest and

simplest hydrocarbon, composed of one carbon and four hydrogen atoms.

Ethane is typically the only other hydrocarbon found in significant amounts in

natural gas, though often less than 10 volume percent. Natural gas may also

include carbon dioxide, nitrogen and very small amounts of hydrogen and

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helium. The composition of natural gas is important because its heating value

and physical properties may change which can affect combustion.

The properties of natural gas are dominated by methane. Methane is

widely acknowledged to be formed from four sources: 1) organic matter that is

decomposed in the presence of heat; 2) organic matter that is converted

through the actions of microorganisms; 3) oil and other heavy hydrocarbons

that produce methane in the presence of heat; and 4) coal which releases

methane over time. There is a theory that methane is present in large

quantities deep within the earth, from which it migrates upward via cracks and

fissures. This theory, known as the abiogenic theory, is not proven but if found

true would suggest that very large reserves of methane exist in the earth.

Very large reserves of natural gas are believed to lie at depths of 4600-

B200 meters, called "deep gas". Since methane remains stable up to its

autoignition temperature of 550°C, it is found at depths where oil is not found,

presumably because oil will be transformed in part to methane at lower

temperatures. Deep gas is expensive to drill for, but the quantities are

estimated to be very large. Technology has been developed, to enhance

recovery of deep gas when it is found.

Very little processing needs to be done to natural gas to make it

suitable for use as a fuel. Water vapor, sulfur, and heavy hydrocarbons are

removed from natural gas before it is sent to its destination, usually via

pipeline.

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2.8 Combustion-Generated Pollutants from CNG Engine

The use of natural gas in automotive fuel results in reduced

concentration of major harmful species in the engine exhaust [36]. The effect

of natural gas fueling on some of the exhaust pollutants is discussed below.

2.8.1 Carbon Monoxide (CO)

Carbon monoxide is the result of incomplete combustion and is a

function of overall mixture strength, the efficiency with which the fuel and air

are mixed and the length of time available for combustion [36]. The CO

emission with natural gas is lower because it easily forms a more

homogeneous mixture with air and can run leaner than gasoline vehicles [37].

Since natural gas engines do not require cold enrichment, the contribution to

reducing CO levels under cold conditions is substantial.

2.8.2 Hydrocarbons (HC)

Total hydrocarbon emission from natural gas vehicles tend to be

higher, since methane is slower to react than their hydrocarbons in very lean

mixtures. The flame speed may too low for combustion to be completed in the

power stroke. However, the non-methane hydrocarbon (NMHC) or reactive

HC emissions, which are of real environmental concern, are considerably

lower. The NMHC emissions are in direct proportion to the methane content of

gas and can vary between 15-20% of the total HC emission. The contribution

of HC's towards smog formation is measured by their rate of reaction with c

hydroxyl radical. Methane is practically non-smog producing HC as it has a

very low photochemical reactivity as can be seen from table 2.8.2.1.

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Table 2.8.2.1 Photochemical reactivity of some organic compounds

Compound Rate constant for reaction with Hydroxyl

Radical, Kx10^ (ppm"̂ min"̂ )

Trans-2- Butane 10.5

1,2,4 trinnethyl Benzene 4.9

Formaldehyde 2.1

Ethane 0.045

Methane 1

0.0012

Conventional methods of measuring NMHC in natural gas vehicles,

which determines reactive hydrocarbons by subtracting methane content of

the gas measured separately from the measured total HC's, can give large

inaccuracies because the methane component in a natural gas vehicle

emission is quite substantial. There is a need to develop some direct

measurement technique for NMHC's.

2.8.3 Oxides of Nitrogen

The rate of formation of NOx is exponentially dependent on

temperature. In SI engines, due to lean air-fuel ratio and lower flame

temperature of natural gas, the levels of NOx emissions are lower compared

to gasoline. However, in dedicated CNG vehicles, where the ignition timing

and compression ratio are optimized, the NOx levels are higher due to more

heat release and also NOx emission are increased with increase in the

manifold pressure [25] and spark advance [36].

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2.8.4 SOx and Particulate

As the sulphur percentage in natural gas is lower than that observed in

gasoline, SOx emission Is negligible in natural gas vehicles. The particulate

matter emission in converted gas engines is practically non-existent whereas

dual fuelling reduces the particulate emission by 50-70%.

2.8.5 Polynuclear Aromatics

Natural gas does not contain higher aromatic hydrocarbons and hence

is better with regard to emission of certain polynuclear aromatics (PAN),

which are known human carcinogens.

2.8.6 Lead Emission

High antiknock quality of natural gas eliminates the use of antiknock

agents and consequently the lead emission is avoided. One indirect benefit is

that the exhaust catalysts can be used safely in natural gas vehicles without

the fear of catalyst poisoning.

2.8.7 Noise

Due to smoother combustion process, noise emission of a gas engine

is considerably lower than that of a gasoline engine.

2.8.8 Greenhouse Effect

Since natural gas powered vehicles will have substantial amounts of

methane in the exhaust, one major concern is the potential threat of

accelerating the global warming, as methane is a strong greenhouse gas.

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Two factors working against this are the fact that natural gas vehicles produce

much lower CO2, because per unit of energy it contains less carbon than other

fossil fuels and the residence time of methane in the atmosphere is lower

compared to CO2. These factors are estimated to offset the additional

methane emitted. However, the total emission of greenhouse gas from a fuel

should not be viewed at all point of the use only, but must include production

and distribution also.

2.8.9 Ozone Formation

There are no studies on the effect on ozone formation of replacing

gasoline vehicles with natural gas vehicles. However, there are reasons to

believe that the use of natural gas vehicles would reduce ozone more than

would the use of other liquid vehicles. Methane, the primary constituent of HC

exhaust from natural gas vehicles is 100 times less reactive than other liquid

fuels [36].

Hydrocarbons from natural gas vehicles appear to be less reactive than

HC from other liquid fuel vehicles, and thus should result in less ozone

formation.

2.8.10 Evaporative Emissions

Methane that escapes from the fuel system is of no concern from a

health standpoiht. Methane is completely non-toxic, non-carcinogenic, and

virtually non-smog producing [36].

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2.9 Compressed Natural Gas as an S.I. Engine Fuel

Compressed Natural Gas is currently one of the most widely used

alternative fuels in the on going development of internal combustion engines

for lower emissions and better efficiency. The basic noticeable difference

between gasoline and CNG is that, the former is liquid at room temperature

and CNG remains in gaseous state even at much lower temperature (-161°C).

CNG has lower density than gasoline, but it has high octane number (120-

130) compared to gasoline (83-93). This makes it suitable for S.I. engine.

CNG operating engine can also be operated at high compression ratio,

without any detonation problem, thereby increasing cyclic efficiency. Higher

self-ignition temperature (SIT) of CNG (732 °C) compared to petroleum results

in much lesser risk of inflammation or explosion in case of leakage. High auto

ignition temperature makes the use of CNG fuelled diesel engine very difficult.

On the other hand the same properties permit a CNG fuelled spark ignition

engine to operate in the higher ranges of compression ratios than a usual

gasoline fuelled engine.

2-- • • Main constituent of CNG is methane. Methane has high hydrocarbon

ratio among all hydrocarbons and therefore results in lower CO2 production

from SI engine compared to fuels like gasoline or methanol. CNG is stored in

a robust cylinder and is lighter than air, so in case of leakage it escapes to the

atmosphere. Gasoline when spilled spreads on the ground endangering a

large area surrounding the spill. In the event of an accident for the gasoline-c

operated vehicle, splashed gasoline will cause a fire hazard for hours where

as CNG leaking would disappear with in moments. Due to CNG's high self-

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ignition tennperature, an additional safety point is the reduced probability of

fire of explosion in the event of a fuel leak.

CNG is insoluble in engine oil (lubricating oil); it neither contuses nor

dilutes the lubricating oil. Thus the lubricating oil may retain its quality for

longer period, reducing the maintenance cost. CNG does not form deposits on

spark plug because of its clean burning characteristics and thus the life of

spark plug increases. The nature of pollutants and their levels of emissions

are much less in CNG-fuelled engine as compared to gasoline engine. CNG

normally contains no sulfur and lead. So, CNG combustion does not produce

exhaust SOx or lead emissions thereby eliminating particulate matters [64-65].

However, in spite of excellent combustion characteristics the use of

CNG as a fuel in SI engine poses some combustion problems, which are

greatly influenced by the technique-employed in formation of fuel-air mixture.

It has been found that carburetion and continuous manifold injection are not

suitable techniques for long-term operation. Another possible technique for

CNG air mixture formation is that it could be tried on SI engines employing

direct injection systems, which appears to be the most promising technique

and needs considerable amount of R&D effort for adoption and

standardization.

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