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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

23

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

24

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.

25

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

26

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

28

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

30

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

34

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.

35

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

36

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

37

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,

38

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.

39

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

42

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

48

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

50

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.

52

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

53

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.