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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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