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CHAPTER 2
REVIEW OF PREVIOUS WORK
2.1 INTRODUCTION
A wide range of alternative fuels can be used in diesel engines
using different methods. Fuels like vegetable oils, which have a high cetane
number can be used directly in conventional diesel engines. Alcohols can be
used in neat form if the compression ratio is raised significantly. They can
also be used with ignition improvers like dimethyl ether (DME), diethyl ether
(DEE) (Nagarajan 1997) or by employing hot surfaces for ignition. Using
gaseous fuels in a diesel engine directly is difficult as they have a high self-
ignition temperature and pose problems of injection. A simple way to burn
gaseous fuels in a normal diesel engine is in the dual fuel mode. Dual fuel
engines are modified diesel engines, which combine the features of SI and CI
engine versions. Dual fuel engines can use a wide range of fuels as the
primary source. They can operate at higher thermal efficiency as compared to
their CI engine counterpart’s atleast at high outputs with certain primary fuels
(Karim 1987, Karim 1989, Poonia 1999). Most of the advantages of CI
engines can be obtained in the dual fuel mode also.
Natural gas, producer gas, liquefied petroleum gas (LPG), hydrogen
and acetylene are some of the gaseous fuels that can be used. Dual fuel
engines are suitable for stationary and mobile applications. Apart from
permitting the use of alternative fuels, the dual fuel engines can reduce smoke
emissions of a diesel engine significantly (Karim 1987). This is because the
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amount of diesel injected is reduced. Further the injected gaseous fuel forms
a homogeneous mixture with air and then burns leading to smoke free
combustion. Many of the methods used to improve the performance of diesel
engines can also be applied to dual fuel engines.
In this chapter, a detailed literature review done on the following is
presented:
Performance and emission characteristics of dual fuel engines.
Gaseous fuel injection system.
Dual fuel engine combustion.
Effect of exhaust gas recirculation, water injection and ignition
improvers, on the performance and emission characteristics of
diesel/dual fuel engines.
2.2 DUAL FUEL ENGINES
Dual fuel engines work at normal diesel engine compression ratio.
The primary fuel is inducted with air using a carburetor or it can be directly
injected. The pilot fuel in a dual fuel engine is auto-ignited and its combustion
has all the features of diesel burning in a CI engine. The combustion of the
inducted primary fuel carries all the qualities of homogeneous charge burning
by flame propagation, which occurs in SI engines. Since the injected pilot
fuel is generally small, the performance and emission characteristics are
largely affected by the primary fuel. However, the pilot fuel injection timing,
quantity and quality play a significant role. The amount of primary fuel has
to be controlled depending on the output and the nature of the primary fuel.
Primary fuels like biogas, which have poor combustion qualities, will need a
relatively large quantity of pilot fuel to produce a strong ignition source. On
the other hand, fast burning fuels like LPG and hydrogen may lead to knock
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and rough engine operation with high levels of NOx, if the pilot fuel is not
controlled.
Too high or low pilot fuel quantity will yield poor performance of
the engine. At low loads, a small pilot fuel quantity will result in high HC
emission levels and poor thermal efficiency due to incomplete and slow
combustion. The brake thermal efficiency was found to be better with larger
pilot fuel at light loads. This is because of larger pilot fuel leading to stronger
ignition source and hence a complete and rapid combustion of gaseous fuel
takes place. At higher loads, more volume of gaseous fuel admission results in
uncontrolled reaction rates near the pilot fuel spray and leads to very high
combustion rates and hence very high rate of pressure rise leading to knock.
The ignition of the pilot fuel depends on the nature of the primary
gaseous fuel and pilot fuel. The gaseous fuel undergoes pre-ignition chemical
reactions during the compression stroke. This will lead to the formation of
active radicals, believed to interfere with the ignition of pilot fuel and the
subsequent combustion process. Advancing the injection timing by a few
degrees when compared to diesel operation will compensate for the increase
in the ignition delay of the pilot fuel (Karim 1983, Karim 1989, Poonia 1999,
Razavi 1998). Thus, the pilot fuel quantity is one of the most important
variables controlling the performance of dual fuel engines.
Karim and Burn (1980) conducted experiments on a single cylinder
four stroke, direct injection laboratory type diesel engine to study the effect of
lowering the intake temperature on the performance and combustion of an
engine in diesel and dual fuel modes. It was observed that lowering the intake
temperature improved air induction. However, there was an increase in
ignition delay, which resulted in poor part load performance. There was a
substantial reduction in NOx emission levels with dual fuel operation at lower
intake temperatures.
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Haragopal et al (1983) performed an experimental study on a single
cylinder, water cooled CI engine using hydrogen as a fuel in dual fuel mode.
It was stated that, with the introduction of hydrogen, the thermal efficiency
was observed to increase at high load. This is attributed to high diffusion rates
of hydrogen and faster energy release due to increased flame propagation
velocities. It was reported that at partial loads, with a small quantity of
injected diesel fuel, the flame propagating from the ignition centers do not
extend to all regions of the combustion chamber and leave some of the
homogeneously dispersed hydrogen unburnt, thus causing low thermal
efficiency. Consequently, the observed increase in maximum cycle pressure
with hydrogen introduction was low at part loads. The authors have also
stated that it was possible to supply 30 % of energy input through hydrogen at
full load. Further increase in hydrogen proportion caused violent knocking.
Charge dilution methods, such as intake manifold water introduction and
exhaust gas recirculation (EGR) are likely to increase the proportion of
hydrogen.
Varde and Frame (1983) conducted an experimental study to
investigate the possibility of reducing diesel particulates in the exhaust by
aspirating small quantities of gaseous hydrogen in the intake of a diesel
engine. A single cylinder direct injection diesel engine was used for the
experimental study. It was reported that at hydrogen flow rates equivalent to
about 10 % of the total energy, there was substantially reduced smoke
emissions at part loads. At full load, the reduction in smoke level was limited
due to lower amount of excess air in the cylinder. There was no significant
change in the HC emissions but oxides of nitrogen in the exhaust increased
with increased hydrogen flow rate. An increase in brake thermal efficiency at
high loads was also observed. The increase in brake thermal efficiency was
due to the high diffusion rate of hydrogen and faster energy release due to
higher flame propagation velocities once the ignition started at various
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locations. Very low hydrogen flow rates had adverse effects on the engine
thermal efficiency. It was concluded that the optimum hydrogen percentage
for smoke reduction was found to be between 10 and 15 % of the total energy.
Karim (1987) has also reported that in dual fuel engines operating
on methane, ethane, hydrogen and natural gas at part load conditions, when
the gaseous fuel concentration is low, some of the homogeneously dispersed
gaseous fuel remains unburned and this leads to poor performance. It was
also reported that at high compression ratios, high intake temperatures and
high outputs, pre-ignition and knocking could cause engine damage.
Prabhukumar et al (1987) investigated the performance of a
hydrogen diesel dual fuel engine and noticed the onset of knock, as the
percentage of heat input derived from hydrogen increases beyond a certain
limit. A single cylinder direct injection diesel engine was used for the
experiments. At a higher rate of hydrogen induction, the richer hydrogen air
mixture is more prone to knocking. It was reported that induction of water
into the intake manifold along with hydrogen increases knock limited power
output (KLPO), as it serves as a powerful internal coolant in decreasing the
unburned mixture temperature. Brake thermal efficiency as well as the power
output decreased with the induction of water because of flame quenching. It
was also reported that water injection causes deterioration of the lubricating
oil quality. However, it improves the KLPO.
Karim and Moore (1990) investigated the performance of a single
cylinder, direct injection dual fuel engine fueled with methane enriched intake
charge. Knock was observed and reported that the dual fuel engine knocking
is of auto-ignition nature. Further, knocking has been characterized with high
rates of pressure rise, increase in heat transfer and consequent loss of brake
thermal efficiency.
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Karim (1991) has done extensive research to establish the nature of
the combustion process in dual fuel engines. A variety of gases like methane,
ethane, propane, butane, hydrogen, ethylene and acetylene as the primary fuel
were used. It is generally accepted that the performance of the dual fuel
engines irrespective of the type of gaseous fuel employed is better at medium
and high loads. However, it has been reported that at low outputs, efficiency
is inferior to diesel engines. Researchers have stressed the need to control the
quantity of both pilot and gaseous fuels depending on the load conditions for
better performance.
Liu and Karim (1995) reported that with methane induction, the
ignition delay period of the pilot fuel initially increases. By increasing the
amount of methane, ignition delay period falls due to improved pre-ignition
rate. Liu and Karim also examined the effects of admission of hydrogen and
its blends with methane on the knock characteristics and operation of a dual
fuel engine through modeling the chemical reaction activity of the pre-
ignition and subsequent combustion processes. It was reported that when
hydrogen flow was increased, the start of pre-ignition reactions advanced and
the reaction time to achieve maximum energy release became much shorter
and more energy was released rapidly, that resulted in knocking. Methane
addition also resulted in longer ignition delay than that observed with
hydrogen addition. It was noticed that lower compression temperature could
result in longer ignition delay with methane in dual fuel operation. Blending
small amounts of hydrogen with methane produces even longer ignition delay
than that observed with methane fuel.
Toshio Shudo (1999) carried out a research on a four stroke four
cylinder SI engine modified from an automobile gasoline engine (Nissan CA
20S). Hydrogen or methane was continuously supplied into the intake
manifold at 1500 rpm. In this research, the contributor to thermal efficiency in
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hydrogen premixed methane combustion was evaluated from the indicator
diagram. It was observed that in premixed combustion engine, there is a trade
off relationship between a degree of constant volume combustion and cooling
loss. These factors mainly dominate thermal efficiency. Compared to methane
combustion, hydrogen undergoes rapid combustion due to higher combustion
velocity. In both the fuels, advanced ignition timing tends to increase
combustion chamber wall temperature. Hydrogen combustion has a higher
amount of cooling loss at any ignition timing, when compared to methane
combustion. This was thought to be due to thinner temperature boundary layer
because of shorter quenching distance and higher combustion velocity.
Increasing excess air ratio reduces cooling loss thereby improving the thermal
efficiency.
Saravanan et al (2008) used hydrogen as air enrichment medium
with diesel as an ignition source in a stationary diesel engine system to
improve the engine performance and reduce emissions. Hydrogen air enriched
system in diesel engine enabled the realization of higher brake thermal
efficiency resulting in lower specific energy consumption. The results show
that the brake thermal efficiency increases to 29 % with 90 % hydrogen
enrichment, but results in knocking. Best results were obtained with 30 %
hydrogen with an efficiency of 28 % achieved without knocking over the
entire load range. The specific energy consumption decreases with increase in
hydrogen percentage over the entire range of operation. NOx concentration
decreases with lean mixtures of hydrogen. A low NOx level of 579 ppm was
noticed at 70 % load with 90 % enrichment. Particulate matter decreased
significantly from 4 to 1 g/kWh with 90 % hydrogen enrichment. A
significant reduction in smoke intensity was observed with an increase in
hydrogen enrichment with the lowest smoke level of 6 BSN with 90 %
enrichment.
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2.3 FUEL INJECTION SYSTEMS
Gaseous fuel can be admitted into the engine at two different
points, one on the intake manifold/port and another inside the cylinder (in-
cylinder injection), each having its own merits and demerits. Intake manifold
injection system uses a low pressure injector, which operates between the
pressure range of 2–5 bar. The gas flow rate can be varied by varying the
injection duration. Use of injectors can completely avoid the problem of
preignition and backfire in the intake manifold and the power output is similar
or greater than that of the conventional mode of operation.
Direct injection system uses injector in which the fuel is injected
directly inside the combustion chamber at a higher pressure. It is observed
that as the injection is closer to TDC, a heterogeneous mixture will be formed
inside the cylinder. Due to limited time available for mixing at the end of the
compression stroke, direct injection has a definite disadvantage.
Literature survey made on fuel injection techniques are discussed
below:
Maclarley and Worst (1980) carried out an engine test using timed
port and direct injection system configuration to circumvent backfire in
hydrogen fueled SI engine. Comparative performance evaluation was done in
the TX-650 gasoline test engine. Electronic control of fuel injection provided
control flexibility necessary for optimum overall engine performance. It was
observed that direct cylinder injection was susceptible to incomplete
combustion. Improvement in volumetric efficiency was affected by thermal
efficiency loss due to incomplete combustion. Thermal efficiency was 27 % at
3500 rpm with a comparison figure of 21 % for gasoline. However, port
injection required less sophistication. Problems associated with incomplete
mixing in direct injection was avoided, achieving highest thermal efficiency
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of 40 % at lower speeds. Port injection rather than direct injection system was
suggested for a better performance.
Varde and Frame (1984) performed an experimental study using
electronic hydrogen fuel injection in the intake manifold of a single cylinder
SI engine. The injector was capable of injecting a predetermined quantity of
fuel with a small variation in fuel delivery from cycle to cycle. It was stated
that hydrogen injection allowed the engine to operate on a leaner equivalence
ratio, reduced cyclic pressure variation, increased brake thermal efficiency
and totally avoided the backfire when compared to carburetion technique.
Shoichi Furuhama (1985) suggested that to prevent the preignition
of hydrogen in the intake manifold, hydrogen is to be supplied into the intake
system only during the suction period or to be injected into the cylinder only
during the intake period with a relatively low pressure, which in turn can
avoid backfire. Verhelst and Sierens (2001) converted a V8, SI engine to use
hydrogen fuel on sequential timed multipoint injection system. Special
features such as ignition characteristics, injection pressures, lubricant oil and
excess oxygen were analyzed by the use of hydrogen in IC engines. It was
suggested to operate the engine in lean mode with equivalence ratio of 2 to
avoid backfire.
James Heffel (1998) evaluated a series of commercially available
natural gas fuel injector originally designed for heavy duty diesel application
for use with hydrogen fuel. Results show that sonic flow, pulse width
modulated electronic gaseous fuel injectors provide accurate and stable
metering of hydrogen at fuel pressures between 2 to 20 bar. A linear flow rate
of hydrogen was observed with low standard deviation error during pulse
width modulation. It was concluded that the performance of injectors
evaluated was within the necessary tolerance for hydrogen application with
internal combustion engine.
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Das et al (2000a) developed an electronically controlled gas
injection system for hydrogen fueled SI engine. A pulse width modulated
choked flow gas injection system was developed. Changing the pulse width of
the control pulse given to the injector regulates the fuel injected. The
important functional components of the injection system comprise of three
parts: optical encoder, control circuit, and solenoid injector. Hydrogen and
CNG were inducted using timed manifold injection (TMI) technique on the
same injector under similar operating conditions. Maximum brake thermal
efficiency obtained was 31 % at 2200 rpm for hydrogen.
Eiji tomita et al (2001) conducted an experimental study on a single
cylinder, four-stroke diesel engine operated in dual fuel mode. Hydrogen was
inducted into the intake port along with air and diesel oil was injected into the
cylinder. A wide range of injection timing was studied. When the injection
timing was advanced, the diesel oil was well mixed with hydrogen air mixture
and initial combustion became mild. NOx emissions decreased because of
lean premixed combustion without the region of high temperature burned gas.
Emissions such as CO, HC and CO2 decreased without emitting smoke, while
brake thermal efficiency was marginally lower than that in ordinary diesel
combustion.
Lee et al (1995) studied the performance of dual injection hydrogen
fueled engine by using solenoid in-cylinder injection and external fuel
injection technique. The external fuel mixture preparation has the advantage
that it is simple and gives higher efficiency but it produces low power output
due to the occurrence of backfire at high loads. In turn, direct in-cylinder
injection produces higher power output with the elimination of backfire but its
thermal efficiency becomes relatively lower due to poor hydrogen air mixing
rate. It was observed that at 50 % load the thermal efficiency of external
mixture was 27 % compared to direct in-cylinder injection of 23 %. The lower
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thermal efficiency of direct injection is due to shorter hydrogen air mixing
duration. To overcome the problems with external mixture injection and
direct cylinder injection the authors have tried dual injection by combining
both external and direct injection at a ratio of 30 % (mass of external fuel to
total fuel) and a spark timing of 10o
bTDC. The maximum pressure in dual
injection was found to be 48 bar compared to 45 bar in direct in-cylinder
injection. The increase in thermal efficiency for dual injection was about 22
% at low loads and 5 % at high loads compared to direct injection. Authors
suggested that by considering the dual injection, the stability and maximum
power of direct injection cylinder with maximum efficiency of external
mixture hydrogen engine could be obtained.
Das (2002) have tried various fuel induction methodologies such as
carburetion, continuous manifold injection (CMI), timed manifold injection
(TMI), low pressure direct cylinder injection (LPDI) and high pressure direct
cylinder injection (HPDI). From the test results, it was observed that
carburetion is not suitable for gas engines because of its uncontrolled
combustion. As far as CMI is concerned the engine did not show a
substantially different response from carburetion. The variation in indicated
thermal efficiency was found to be 40 % for TMI compared to 32 % for CMI
at an equivalence ratio of 0.35. In direct cylinder injection with LPDI, it was
very tough for the injector to survive on severe thermal environment of the
combustion chamber over a prolonged engine operation and time for mixing
hydrogen with air was less resulting in a drop in brake thermal efficiency.
Hence TMI was selected which gave a maximum brake thermal efficiency of
39 % at a compression ratio of 9:1 with an equivalence ratio of 0.575 at 1600
rpm, compared to 33 % for LPDI system and the NOx emission was found to
be 100 ppm. Further increase in equivalence ratio from 0.575 to 1.0 resulted
in NOx emission to increase upto 1400 ppm.
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Saravanan et al (2007a) conducted experiments to determine the
optimized injection timing, injection duration and quantity of injection of
hydrogen fuel in the manifold in dual fuel mode in diesel engine. The
optimised injection timing was at gas exchange TDC and an injection
duration of 30 °CA with hydrogen flow rate of 7.5 lpm. The brake thermal
efficiency was found to increase by 9 % compared to diesel operation. Smoke
and NOx emissions were found to be lower than diesel at all the loads. An
increase of 7 % in exhaust gas temperature than diesel was noticed.
Saravanan et al (2007b) tested diesel engine for its performance and
emissions characteristics of hydrogen diesel in dual fuel mode. Hydrogen was
injected into the intake port along with air, while diesel was injected directly
inside the cylinder. Hydrogen injection timing and injection duration was
varied for a wider range with constant injection timing of 23° bTDC for diesel
fuel. Emissions such as HC, CO and smoke decreased. The maximum
efficiency of 30 % was noticed and NOx emission reduced to a lower value of
888 ppm when compared to diesel operation at full load for optimized
injection timing of 5° after gas exchange TDC and injection duration of 90
°CA.
2.4 COMBUSTION CHARACTERISTICS OF GASEOUS FUEL
IN DUAL FUEL MODE
Auto ignition temperature, minimum ignition energy, wider
flammability range and shorter quenching distance are some of the properties,
which determine the suitability of a fuel for engine application. Unless the
properties are appropriately exploited to an advantage for improved engine
characteristics, they might give rise to various unwanted combustion
problems. Low ignition energy enables the conventional ignition system to be
effective even with very low spark energy but it also results in surface
ignition. Surface ignition causes undesirable combustion phenomenon such as
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flashback, pre-ignition and rapid rate of pressure rise. The simplest method to
avoid backfire is to ensure the absence of combustible mixture in the intake
manifold. Provision of crank case ventilation is needed to avoid any abnormal
combustion in the crankcase. By optimizing the rate of pressure rise, the
knocking problem in gaseous engine can be eliminated.
The abnormal combustion in an engine is classified as:
a) Knocking
b) Pre-ignition and backfire
The literature survey carried out on the combustion characteristics
of gas operated dual fuel engines are discussed below:
Lee et al (1995) constructed an intake port hydrogen injector using
a solenoid driven gas valve and experiments were carried out with this system
to investigate the combustion characteristics of hydrogen fuel including
flashback phenomenon. It was concluded that by using solenoid driven gas
valve, the amount of hydrogen supplied could be controlled very easily by
changing the duration of the solenoid driving signal. The cylinder peak
pressure of hydrogen operation was above 50 bar and was higher than that of
gasoline operation by more than 10 bar. Owing to high cylinder pressure, the
amount of NOx emissions increases. NOx emission concentration of
hydrogen operation was 856 ppm/kW and that of gasoline operation was 371
ppm/kW. A stable engine operation was observed between equivalence ratios
of 0.32 to 0.8. Above 0.8 equivalence ratio, a decrease in BMEP due to
incomplete combustion of hydrogen was noticed. To operate the engine at a
higher speed without flashback, equivalence ratio and injection timing should
be controlled accurately considering the delay of the solenoid.
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Poonia et al (1998) conducted experiments on a single cylinder DI
water-cooled LPG diesel dual fuel engine at various intake temperatures and
pilot quantities. Pilot fuel quantity and intake temperatures are two important
parameters controlling the combustion process in a dual fuel engine. It was
observed that the ignition delay in the dual fuel mode was always greater than
that in the diesel mode. At a given intake temperature and pilot quantity the
ignition delay increases with an increase in power output. Thus, gas to air fuel
is a very important factor in controlling ignition delay. At low outputs, the
heat release in the first stage due to the combustion of the pilot fuel and
entrained gas is the dominant factor. The subsequent heat release, which was
mainly due to the combustion of the gas, was affected favorably by the
amount of pilot fuel injected. At high outputs, after the combustion of the
pilot fuel and entrained gas, the remaining gas burns in two stages. The first
of these was at a high rate, which was significantly affected by the pilot
quantity or the intake temperature.
The maximum rate of pressure rise increases with increase in pilot
diesel quantity. The peak pressure in the dual fuel mode was significantly
higher than diesel operation at high outputs, particularly when the intake
temperature is high due to rapid combustion of the gas air mixture. The
combustion duration in the dual fuel mode was higher than diesel values at
low outputs. However, it was lower than diesel values at high outputs. It was
suggested that, high pilot diesel quantities have to be used at low outputs to
ensure proper combustion of gaseous fuel. As the power output increases, the
pilot quantity has to be reduced to control rapid combustion and knock.
Karim and Moore (1999) studied the combustion phenomenon in
dual fuel engines with very rich mixtures and with oxygen enriched charge
using different pilot quantities. It was noticed that the heat release rate from a
dual fuel engine appeared to have two distinct phases. First with combustion
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of pilot fuel and small quantity of gaseous fuel and in second phase, the heat
release rate was mainly due to combustion of gaseous fuel. The second phase,
which tends to be slower, depends mainly on the pilot fuel quantity.
Nwafor (2000) investigated the combustion knock characteristics of
diesel engines running on natural gas using pilot injection of diesel as a means
of initiating combustion. The cylinder pressure crank angle and heat release
diagrams indicate that dual fuel operation exhibits a longer ignition delay and
slower burning rates. Maximum peak cylinder pressure was reduced and the
initial rate of pressure rise was lower compared to diesel fuel operation. The
power output of the dual fuel operation was inferior to diesel fuel. In dual fuel
engines, three types of knock were identified; they are diesel knock due to
combustion of premixed pilot fuel, knock due to auto ignition of end gas and
erratic knock due to secondary ignition of the alternative fuel. The main
factors that influence the occurrence of these knock is the pilot quantity, delay
period, load, speed, gas flow rate and time interval for secondary ignition.
Increasing the pilot fuel and reducing primary fuel reduces the knocking
phenomena in dual fuel engines.
Jehad at al (2000) studied analytically the aspect of combustion
duration affected by engine’s operating parameters like compression ratio,
equivalence ratio, spark plug location, spark timing and engine speed. In turn,
how combustion duration affects the engine performance parameters like
BSFC, BMEP, thermal efficiency as well as emissions characteristics were
analysed. It was found that any attempt to increase the combustion duration
either by reducing the compression ratio or locating the spark plug near the
periphery or operating at leaner mixtures would improve the engine fuel
economy with a sacrifice in power output. It was suggested that combustion
duration has to be between 4–6 ms and the engine should run on a mixture
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slightly leaner than stoichiometric. From the emissions point of view, NOx
emission is lower when combustion duration is high.
Nwafor (2001) investigated the combustion characteristics of dual
fuel combustion of natural gas in an unmodified diesel engine. Natural gas
was fumigated and a small quantity of pilot diesel fuel was injected for
initiating the combustion. The combustion process of dual fuel engine was
noted to lie between that of CI engine and SI engine leading to five stages of
the combustion process, unlike the four-stage combustion of neat diesel fuel
operation. It involves an evolution of two stages of ignition and combustion
processes, a longer ignition delay combined with low sudden pressure rise due
to combustion of pilot fuel and short delay period combined with higher
pressure rise due to combustion of primary fuel and finally the diffusion
combustion stage. The ignition delay of dual fuel engine increases with
decrease in engine speed, load, mixture composition and system temperature.
The poor part load performance of dual fuel engine improved through
enrichment of pilot fuel.
Shrinivasa et al (2005) carried out experimental investigations on a
single cylinder vertical water cooled 5.2 kW CI engine run in dual fuel mode
with diesel as injected primary fuel and LPG as inducted secondary gaseous
fuel. The combustion studies were carried out based on the heat release
patterns calculated thermodynamically in the dual fuel mode. From the
results, it was observed that the brake thermal efficiency improves with an
increase in LPG flow rate until onset of knock at higher loads. It was found to
increase from 30 % in diesel mode to 34 % in dual fuel mode at LPG flow
rate of 0.6 kg/h at full load. Exhaust smoke level reduced with increasing
LPG flow rate at all loads. It decreased significantly at full load from 29 HSU
to 14 HSU from diesel fuel to dual fuel mode. The peak cylinder pressure and
maximum rate of pressure rise increased with increase in load. At full load,
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the peak pressure was 73 bar and 84 bar in single and dual fuel mode
respectively. The corresponding maximum pressure rise rates were 6.8 bar
/°CA and 8 bar/°CA in diesel and dual fuel mode. The combustion
temperature increased with load. At full load, calculated peak temperatures
were 1940 K and 2020 K in single and dual fuel mode respectively. The
calculated maximum equilibrium concentration of NOx increased with
increase in LPG flow rate at full load. The equilibrium CO concentration was
negligibly small at all operating conditions because of overall lean mixture.
2.5 NOx REDUCTION TECHNIQUES
The major problem in gas operated dual fuel engine is the
production of oxides of nitrogen (NOx) which can be reduced by some of the
following techniques:
Exhaust gas recirculation.
Water injection.
Adding high cetane fuel like DME, DEE.
Retarding injection timing.
Adding charge diluents such as nitrogen, helium, etc,.
Increased coolant flow rate.
High conductivity materials to dissipate heat.
Catalytic reduction.
Multistage injection.
Intercooling of charge air in turbocharged engines.
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2.5.1 Exhaust Gas Recirculation
The literature survey done by using EGR in gas operated engine is
discussed below:
Daisho et al (1993) reported that about 50 % reduction in NOx
emissions could be obtained with 20 % EGR without deteriorating smoke and
unburned hydrocarbons. Exhaust gas recirculation (EGR) into the combustion
chamber has been employed to reduce NOx emissions in diesel engines.
Water vapor and carbon dioxide is the major constituent in exhaust gases.
These gases have high specific heats and thus enable the exhaust gas to be
used to reduce the temperature during the combustion process in the cylinder.
EGR does not influence the ignition delay period significantly, but suppress
the sharp increase in cylinder pressure. Hot EGR can help in improving the
light load performance of a dual fuel engine. It was observed that by
increasing the EGR, NOx decreases but hydrocarbons tend to increase.
Exhaust gas recirculation is therefore usually limited to 5–10 %. EGR can be
useful for normal and lean burn engines and for diesel engines.
Poonia et.al (1996) have done experiments on the intake throttling
and hot and cold EGR in LPG diesel dual fuel engine. It was reported that
EGR improves the thermal efficiency and reduces the HC emissions at low
and high loads. Cyclic fluctuations were found to be lower with hot EGR. It
was also found that throttling of the intake charge improves the combustion
rate, raises the brake thermal efficiency and reduces HC levels at low and
medium outputs. This is because throttling the inlet air tends to increase the
effective fuel air ratio of the charge by reducing the amount of air inducted
per stroke.
Ladommatos et al (1998) have stated that diluents CO2 and H2O are
the principal constituents of EGR, which causes an increase in ignition delay
40
and a shift in the start of combustion, which results in the products of
combustion spending shorter periods at high temperatures, which lower the
NOx formation rate. The shift in the combustion process towards the
expansion stroke resulted in earlier quenching of the combustion flame, which
yields higher levels of products of incomplete combustion in the exhaust. By
using hot EGR there will be an increase in inlet charge temperature, which
reduces the ignition delay period, which also enhances the evaporation of the
fuel that could result in fuel rich mixtures in regions of the combustion
chamber where air entrainment is restricted by the high viscosity of hot air.
As a result, high levels of soot may be produced due to increased rate of fuel
pyrolysis at high temperatures that prevail during combustion.
From the results, it was observed that for an increase of 7 % mass
of CO2 concentration in the intake air, which in turn replaced the oxygen,
causes the time taken for the first 10 % of the mass burnt to decrease
substantially and was found to be less than 0.5 °CA. Similarly, for 7 % mass
increase in CO2 concentration cause the delay period to increase from the
baseline of 7.9 °CA to 15.6 °CA. With oxygen concentration in the inlet
charge of 20.3 % volume of air, the premixed and diffusion burning period is
found to be 9 °CA while with 16.3 % volume of air, the duration of premixed
burning is 14 °CA and diffusion burning is 4.5 °CA. Therefore, increase in
CO2 concentration made the combustion to shift from premixed burning
towards diffusion combustion. It was also observed that the effect of charge
dilution strongly affect the combustion duration, therefore proper selection of
EGR percentage will determine the effective combustion.
Ming Zheng (2000) studied the impact of EGR on diesel engine.
Due to the vitality of EGR in NOx reduction, it is prudent to explore the
applicable limits of EGR. Notably higher EGR could degrade the energy
efficiency and mechanical durability. Excessive EGR also cause operational
41
instabilities that further aggravate the engine efficiency and durability. The
authors suggested that instability can be reduced by modifying the EGR
stream thermally or chemically through EGR treatments such as cooling the
EGR and oxidation of EGR. EGR cooling is more effective to reduce NOx
and extends the EGR applicable limits. Moreover, cooled EGR also has the
potential to stabilize the engine operation by holding the temperature of
recirculated exhaust gas constant. Although excessive EGR results in
dramatic NOx reduction, the engine operation also approaches zones with
higher cyclic variations. Such instabilities are largely associated with
prolonged ignition delay and incomplete combustion, which are caused by
increased CO2 and decreased O2 in the engine intake. Uncontrolled EGR
components such as combustibles are commonly introduced into the engine
combustion chamber. By applying oxidation with a catalyst in the EGR loop,
elimination of recycled combustibles is possible leading to stabilizing the
cyclic variations.
Mohammed (2003) conducted an experimental investigation to
study the effect of EGR on combustion pressure rise and thermal efficiency of
a dual fuel engine running on diesel and CNG. The effects of EGR ratio,
engine speed, temperature of recycled exhaust gases, intake charge pressure,
engine compression ratio, combustion noise and thermal efficiency were
examined for the dual fuel engine. The use of 5 % EGR ratio has the positive
effects on increasing the thermal efficiency, reduced combustion noise and
reduced NOx emission. When the dual fuel engine used hot EGR, the
maximum pressure rise rate was higher at all loads and at all EGR ratios than
cooled EGR. The choice of cooled EGR is reduced NOx emission and
whereas hot EGR is to improve thermal efficiency.
Taggart et al (2003) commissioned a heavy duty diesel engine for
single cylinder operation, fueled with pilot ignited natural gas injected
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directly into the cylinder. A study on the impact of cooled EGR on the engine
performance and gaseous emissions was carried out. Various engine speeds,
loads and injection timing were tested over a range of EGR fractions. The
results indicate that NOx emissions varied linearly with the intake oxygen
mass fraction until NOx emissions reached 20 % of their non-EGR levels.
Further increase in EGR resulted in a reduced rate of reduction in NOx
emissions. The NOx emissions were found to be independent of engine speed
and load. Overall activation energy for NOx formation was determined by
correlating the NOx reductions with flame temperature. The combustion by
products including CO and unburned hydrocarbons increased significantly at
higher EGR fractions. The engine performance was not significantly affected
except at very high EGR fractions.
James Heffel (2003) conducted experiments on a ford ZETEC 4
cylinder, 12.1 compression ratio engine, specially designed to run on pure
hydrogen using a lean burn fuel metering electronic port injector.
Experiments were conducted to ascertain the effect of exhaust gas
recirculation and a standard 3–way catalytic converter on NOx emissions and
engine performance. The air fuel ratio varied from 100:1 to 50:1, further
increase in lean burn condition was limited to the onset of knock. The
maximum torque obtained without EGR was 94 N-m compared to 88 N-m
with EGR. NOx emission without EGR was 2500 ppm and with EGR, it
reduced to 4 ppm with a drop in brake thermal efficiency from 38 % to 34 %
for the same operating conditions. From the results, it was observed that with
EGR and a standard 3-way catalytic converter system, the NOx emissions
from a hydrogen fueled engine could be reduced even to 1 ppm.
Saravanan et al (2007) investigated the effect of cooled EGR in
hydrogen enriched single cylinder diesel engine. It was concluded that the
brake thermal efficiency increased by 6 % without EGR, with cooled EGR it
43
was lower than dual fuel engine and higher than neat diesel at full load
operation. The NOx emissions decreased to a minimum of 464 ppm with 25 %
EGR. Smoke intensity decreased by 48 % in dual fuel mode and lower than
dual fuel mode with EGR.
2.5.2 Water Injection
Various methods were attempted to control the emissions of IC
engines. But, most of the methods that control NOx affect smoke and
particulate emissions adversely. Use of water diesel emulsion in diesel
engines is one of the methods for simultaneous reduction of both NOx and
smoke without any penalty in fuel consumption. Brake thermal efficiency was
improved by the use of emulsified fuels at certain operating conditions due to
the micro explosions of the water diesel emulsion (Subramanian 2001,
Tadashi Murayama 1978). Water has also been introduced in diesel engines
by injecting it directly into the cylinder or in the intake manifold.
The advantages of using water emulsified fuels in diesel engines
are:
Improvement in brake thermal efficiency.
Reduction in smoke and particulate levels.
Good reduction in NOx due to thermal, chemical and dilution
effects of water.
The disadvantages are (Subramanian et al 2001):
At low outputs, water present in the pilot fuel can adversely
affect the performance.
HC and CO emission increases.
44
Increase in ignition delay, peak pressure and maximum rate of
pressure rise.
An extensive research work was carried out by Miyauchi et al
(1981) to study the effect of steam addition on NO formation. Experiments
were conducted on laminar methane air premixed flames and NO species
were measured along the profile of the results. The NO concentration was
found to reduce by the addition of steam even though the maximum flame
temperature was kept constant. Not formed due to the chemical effects is
usually called prompt NOx. Reactions of hydrocarbon fragments like CH,
CH2 radicals with N2 are thought to be the major source for this prompt NOx
formation. The HCN radicals react with oxygen and form oxides of nitrogen
rapidly. Due to the effect of added water, OH radical concentration was
increased. These OH radicals react with HCN and prevent the formation of
NOx considerably.
Prabhukumar et al (1983) carried out an investigation on improving
the KLPO when water was inducted with the intake charge of a hydrogen
diesel dual fuel engine. Under normal hydrogen diesel dual fuel operation,
the KLPO occurred when percentage heat input derived from hydrogen was
about 60 %. The induction of water into the intake manifold along with the
hydrogen increased the KLPO, as it served as a powerful internal coolant in
decreasing the unburned mixture temperature. The percentage of heat input
derived from hydrogen at KLPO increased to 87 % for the water induction
rate of 0.7 lpm. The brake thermal efficiency decreased with the induction of
water due to escape of gaseous fuel during the combustion process because of
quenching. Ignition delay increased, maximum rate of pressure rise and peak
pressure decreased with water induction due to slow combustion rate.
Deterioration of lubricating oil was observed with more than 0.7 lpm of water
induction.
45
Prabhukumar et al (1987) investigated the performance of a
hydrogen diesel dual fuel engine and noticed the onset of knock as the
percentage of heat input derived from hydrogen increased beyond a certain
limit. A single cylinder direct injection diesel engine was used for
experiments. At higher rates of hydrogen induction, the richer hydrogen air
mixture was more prone to knocking. It was reported that induction of water
into the intake manifold along with hydrogen increased KLPO, as it served as
a powerful internal coolant in decreasing the unburned mixture temperature.
Brake thermal efficiency as well as power output decreased with induction of
water, as a result of flame quenching. It was also reported that water injection
caused deterioration of the lubricating oil quality.
Patro (1993) studied the burning rate of fuel mass analytically from
experimental P–V diagrams. Using the above approach hydrogen enriched
dual fuel diesel engine combustion process was analysed. Hydrogen, in lower
volumetric supplementation rate of around 30 lpm burned predominantly in
the premixed mode. However, when the flow rate of hydrogen
supplementation is higher, in the order of 50 lpm diffusion combustion of
hydrogen fuel was quite noticeable. When charge diluents like helium,
nitrogen or water in appropriate proportion was used along with hydrogen
fuel, the engine knocking tendency is suppressed and burning efficiency is
improved. Nitrogen was very effective in reducing ignition delay and
shortening the flame length, so that the burning rate was not far too ahead of
the mixture preparation rate. Water caused the burning process to occur at low
temperature and pressure conditions, helping towards better mixture
formation rate and so, higher combustion efficiency. Water is diluent was
quite advantageous for fuel economy measures. The burning rate
characteristics of hydrogen in the presence of water diluent are quite similar
to the typical DI diesel burning rate diagram.
46
Mathur et al (1993) conducted experiments on a single cylinder,
four stroke water cooled portable diesel engine system of 4.4 kW rating. It
was modified to operate in dual fuel mode with hydrogen as the main fuel. In
order to improve the engine performance and KLPO, various diluents such as
helium, nitrogen and water with various proportions were used. Helium as a
diluent was found to control the engine knock but the thermal efficiency and
percentage hydrogen energy substitution exhibited no positive gains. Nitrogen
showed the best influence on engine performance and KLPO improvement.
Water induction in small concentration, demonstrated the highest full load
hydrogen substitution although the engine’s thermal efficiency and KLPO
were marginally affected.
Masahiro et al (1994) analysed the effect of increase in intake air
humidity by adding water into the intake air. Results were compared with
their model developed for NOx formation. It was indicated quantitatively that
the effect of absolute humidity on NO formation was significantly large. A 20
% reduction in NOx was observed with an increase of 0.01 kg of absolute
humidity. Since the specific heat and gas weight of the burned zone increased
by the added amount of water, combustion gas temperature was significantly
reduced. Msahiro et al (1997) investigated the effect of port injected water in
a DI diesel engine on NOx reduction. A 50 % reduction in NOx was observed
by injecting 0.03 kg of water per unit kg of air. The reduction in NOx was
observed to be due to decrease in temperature of the burned gas due to an
increase in the specific heat of the in–cylinder gas. A marginal increase in
smoke emissions was reported. Brake thermal efficiency decreased
marginally as the water content was increased.
Susumu et al (1996) have conducted experiments with in–cylinder
water injection by modifying the injector suitably. Water and fuel were
injected into the engine alternatively by using a split injector. A significant
47
reduction in NOx levels was observed. The reduction of NOx was primarily
due to lowered flame temperature and the water injected quantity. Smoke
level started increasing after a certain water to diesel ratio, in addition to
increasing the in HC emission.
Morse et al (2002) performed experimental and simulation study on
AVL single cylinder research engine to quantify the effects of fuel humidity
on the performance of an IC engine fueled by hydrogen. Initial expectations
were that with respect to fuel humidity in a hydrogen fueled engine: NOx
could be reduced caused by increased heat capacity of the charge resulting in
lower cylinder temperature and dissociation of water vapor at high
temperatures, which consequently influence the reaction mechanism for NOx
formation. The first effect was investigated by using a thermodynamic cycle
simulation and simple NO kinetics model known as extended Zeldovich
mechanism. The simulation predicted that fuel humidity and excess air ratio
where the most effective means of reducing the concentration of NOx.
Increasing the water mole fraction from 0 to 0.33 reduced the concentration of
NOx by more than 90 %. The dissociation of water vapour at high temperature
was investigated using CHEMKIM (chemical kinetics simulation code). The
results indicate that water dissociation at high temperature did not appear to
influence NOx formation. Therefore, the reduction in NOx was primarily due
to the increase in heat capacity of the cylinder charge resulting in lower gas
temperature.
Greeves et al (2004) carried out experimental work on an
automotive type diesel engine to determine the effect of water diesel
emulsion, water injection at inlet manifold and injection into the cylinder.
Water injection directly into the cylinder through a separate injection pump
and a three hole injector nozzle at a pressure of 165 bar with a fuel injection
timing of 20o bTDC were used. The results for water injection in the inlet
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manifold showed that NOx decreases progressively with increase in water/fuel
(W/F) ratio. When the W/F ratio was 0.5, the NOx reduction was 30 %.
Smoke, CO and ignition delay increased together with a marginal increase in
specific fuel consumption. The data for water injection into the cylinder
showed very similar reduction of NOx to those obtained with water injection
in the manifold but there was a greater increase of smoke with increase in
W/F ratio. In the case of water, emulsion for a given W/F ratio a greater
reduction of NOx was observed than the outer techniques. In addition,
reduction of smoke, CO and a marginal reduction in specific fuel
consumption were observed. Ignition delay increased more rapidly with
increase in W/F ratio, beyond W/F ratio of 0.6 the unburned HC increased.
Subramaniam et al (2006) conducted experiments to study the
effect on exhaust NOx by charge dilution by nitrogen, CO2 and water in a neat
hydrogen fueled SI engine. Hydrogen was supplied through an electronic fuel
injection system into the manifold. It was observed that NO level with
hydrogen fueling becomes significant after an equivalence ratio of 0.55;
highest levels were seen near to an equivalence ratio of 0.80. Charge diluents
like hydrogen, CO2 and water can lead to a considerable reduction in NOx
levels because of the thermal effect and due to the reduction in oxygen
concentration (dilution effects). From these experiments on the three diluents
evaluated, it was suggested that dilution effect to control NOx emission was
more effective than thermal effects.
2.5.3 Use of Ignition Improvers
Zhili Chen et al (2001) carried out experiments on a CI engine with
DME as an ignition source with LPG as fuel. The results showed 4–5 %
apparent improvement in indicated thermal efficiency of HCCI mixture of
LPG / DME. The NOx emission reduced from 16 ppm to 3 ppm. It was
observed that the UHC and CO emissions increased. The quantity of DME
49
required to cause the ignition is 13.2 % higher than LPG, but increase in LPG
rate also causes diesel knock.
Nagarajan et al (1997) used ethanol as a fuel for C.I. engine. The
problem regarding the use of ethanol was its low cetane number (8). Hence,
diethyl ether (DEE) was introduced along with ethanol through the intake
port, which undergoes earlier combustion during the compression stroke itself
that in turn create a hotter environment for ethanol combustion. The DEE
requirement for starting was higher (57 % by mass basis) compared to the
entire range of operation from no load (3.0 %) to full load (2.5 %). This was
found to be due to lower charge temperature, dilution of ethanol, which may
be introduced in smaller quantities or may be due to the residual gases present
from the previous cycle. The improvement in brake thermal efficiency was
around 19–48 % at full load and between 20–30 % at low loads. The increase
in thermal efficiency was attributed to better vaporization, mixing and
combustion characteristics of injected ethanol in the hotter environment
created by the early combustion of DEE. The increase in pressure rise was
found to be 3.2 bar/ CA at no load to 5.6 bar / CA at full load for ethanol-
operated engine compared to diesel fuel operation of 3 bar/ CA at no load to
5.2 bar/ CA at full load. The increase in rate of pressure rise was attributed to
longer ignition delay of ethanol. The CO emissions were more in ethanol-
DEE (0.8–1.4 %) than diesel (0.10–0.28 %).
Miller et al (2007) used LPG as a primary fuel with DEE as an
ignition enhancer in a direct injection diesel engine. DEE is reported as a
renewable fuel and to be a low emission high quality diesel fuel replacement.
A single cylinder, four-stroke air cooled naturally aspirated DI diesel engine
having rated power output of 3.7 kW at 1500 rpm was used for the
experiments. It was reported that the brake thermal efficiency was lower by
about 23 % to full load with NOx reduction of about 65 % than diesel
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operation because of the temperature drop in the cylinder. The maximum
reduction in smoke and particulate was observed to be 85 % and 89 %
respectively when compared to diesel operation. However, an increase in CO
and HC emission was observed.
Saravanan et al (2008) conducted experiments on a single cylinder
diesel engine using hydrogen in dual fuel mode with DEE as an ignition
source. The optimized conditions were found to be 5oCA aTDC for injection
for hydrogen, 30oCA for hydrogen injection duration in the dual fuel mode
and 40oCA aTDC for DEE. Hydrogen in dual fuel and with DEE operation
showed an increase in brake thermal efficiency by about 22 % and 35 %
respectively compared to diesel. Hydrogen diesel and DEE operation
exhibited a significant reduction in NOx and smoke emissions compared to
diesel fuel. A severe knocking was observed beyond 75 % load due to the
instantaneous combustion of hydrogen.
2.6 OBSERVATIONS BASED ON LITERATURE SURVEY
The observations based on the literature survey conducted are:
The dual fuel engine can utilize a wide range of alternative fuels
effectively.
It can work with higher thermal efficiency and very low smoke
level as compared to neat diesel engine at medium and high
outputs.
The performance, emissions and combustion characteristics of a
dual fuel engine is significantly affected by the nature of the
primary gaseous fuel and the pilot fuel.
51
The quantity of the pilot and the primary fuel plays a significant
role.
The combustion process in dual fuel engine is a complex
combination of both SI and CI engine version.
The dual fuel engine leads to rise in HC and CO emissions
particularly at part loads.
It will also lead to higher NOx levels at full load, when the
combustion rates are high.
The peak pressure of hydrogen-operated engine is higher which
leads to NOx emission, noise and vibration.
Higher cooling loss in hydrogen combustion is due to the effect
of higher burning velocity and shorter quenching distance. The
thermal efficiency is affected due to the effect of high cooling
loss.
Backfire and pre-ignition problems are severe in carburetion
system.
Timed injection is very effective in the reduction of backfire.
Optimum injection timing and injection duration is necessary
for gas injection system in order to get proper mixing of fuel
with air.
Electronically controlled injectors are more versatile compared
to hydraulically operated or mechanically operated injectors in
terms of performance, response and flexibility in timings.
EGR and water injections are effective methods to decrease the
tendency of backfire.
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Charge diluents, such as intake manifold water injection and
EGR can increase the gas substitution rate.
By reducing the peak pressure and rate of pressure rise,
knocking problem can be eliminated.
EGR can help in improving the light load performance of a
dual fuel engine.
EGR cause an increase in ignition delay and a shift in the
location of the start of combustion. This makes the products of
combustion spending shorter period at high temperatures, which
lowered the NOx formation rate.
The shift of combustion towards expansion stroke quenches the
flame leading to shorter combustion duration.
Higher levels of soot can be produced due to increased rates of
fuel pyrolysis at high temperatures prevailing during diffusion
combustion.
The heat losses to the walls increase with increase in EGR rate.
The choice of a cooled EGR is reduced NOx emission and
combustion noise, whereas hot EGR is to improve thermal
efficiency.
The addition of diluents (nitrogen, helium, water) improves the
knock limited engine operation.
Emulsified fuels can be used to control smoke and NOx
emissions in diesel engines in addition to improvement in brake
thermal efficiency. HC and CO emission increases. At low
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outputs, water present in the pilot fuel can adversely affect the
performance.
Water injection into the manifold, decreases NOx emission and
thermal efficiency, increases smoke, HC, and CO.
DEE has a high cetane number of 125 and high energy density
than diesel fuel.
DEE results in shorter ignition delay, which lowers the
maximum cylinder pressure and decreases the rate of increase in
pressure rise.
DEE operation exhibited a significant reduction in NOx and
smoke emissions, increases HC and CO emissions in the
exhaust at part loads compared to diesel fuel.
2.7 OBJECTIVES OF THE PRESENT RESEARCH WORK
The objectives of the present research work are:
To study the performance, emission and combustion characteristics
of acetylene in a diesel engine by adopting the following techniques in dual
fuel mode:
1. Carburetion technique.
2. Carburetion technique with port injection of water.
3. Carburetion technique with port injection of DEE as an
ignition improver.
4. Timed manifold injection technique.
5. Timed port injection technique.
6. Timed manifold injection technique with cooled EGR.