In in cylinder charge temperature and this will

In
the dual fuel mode, a high-octane number fuel (primary fuel) like alcohols,
biogas, natural gas etc. is inducted along with air and then compressed like in
a CI engine (Fig.2.1). This compressed charge is then ignited by a pilot injection
of high cetane number fuel like diesel. In this method there is full
flexibility in changing the energy share of the inducted fuel. This requires an
additional injection system like Port fuel injector, carburettor or vaporizer
along with a control system and separate fuel lines and fuel tank Chauhan,
Bhupendra, 2011.

Advantages
of dual fuel engine –

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Ø  A wide range of Primary fuels
can be easily use

o   High octane number

o   Biogas, Alcohols, LPG, CNG,
Gasoline etc

Ø  Possibility of low NOx and
smoke

o   Primary fuels like alcohols
have high latent heat of evaporation this will cool in cylinder charge
temperature and this will result in lower levels of NOx emission.

o   In dual fuel engine we are reducing secondary fuel (i.e.
diesel) by inducting primary fuel which is lower number of carbon atom in its
molecule so this will help in reduction in smoke Papagiannakis, R. G, 2004. In addition to this is primary fuel is alcohols it has oxygen in its
molecule this will also help in reduction in smoke.

Ø  Can revert to single fuel
(CI) mode when needed

o   With proper electronic
control it is easy to stop primary fuel supply and it is easy to switch between
conventional and dual fuel mode of operation.

Ø  No need of additional
additive or co-solvent as fuel is inducted along with air in premixed form.

Ø  Good control over combustion

o   In dual fuel mode of
operation generally combustion starts after injection of secondary fuel so in
this mode there is good control over combustion. Challenges
in dual fuel engine – Ø  Primary fuel quantity,
secondary fuel quantity and its timing have to carefully control for avoiding
misfire, knocking and smoke.Following
parameters have impact on the level of power production potential and the
associated levels of exhaust emissions in a dual fuel engine Karim, Ghazi A.,
2003 –Ø  The type of fuel used the
physical and chemical properties of the fuel, its composition and heating
value, their variations with temperature and pressure and the physical and
chemical properties of the fuel. In addition to this at the end of compression
and at about to start of combustion the temperature and pressure are high at
this point values of the effective flammability limits and burning rates are
important.Ø  The equivalence ratio values
used, the autoignition characteristics, ignition energy and intake and exhaust
temperature and pressures. In addition to this presence of any diluents (i.e. EGR)
and operational knock limits were important.Ø  The type of secondary fuel
used, injection characteristics, timing, volumetric efficiency, turbocharging.Ø  Engine diesel related
parameters like engine compression ratio, bore, and stroke, combustion cham­ber
shape, engine speed range, and any surface activity Karim, Ghazi A., 2015.Use of
alcohols in Dual fuel mode Ajav
et al. (1998) studied effect of vaporized ethanol as a supplementary fuel in a
modified single cylinder engine. Experiments were conducted with two different
conditions in first mode air temperature was kept at 20 °C and in second mode
air was preheated at 50 °C. It was reported that heating of air did not had any
significant effect on BSFC. However, BSFC was found to decreases with increase
in load, this is because as load increase brake power also increases. Induction
of ethanol along with unheated air results in increased peak power and BTE.
However, CO emission was found to increase and NOx emission unchanged. Ethanol
fumigation was studied by Janousek 2010 on a 4-cylinder diesel engine using
atomization technique. The study was conducted speed varying from 1000 to 2400
rpm in the step of 200 rpm. With ethanol fumigation NOx emission was found to
be decrease. However, there was increase in HC and CO emissions as compared to
conventional diesel engine. In addition to this BTE with ethanol fumigation was
not compromised.  Comparison
of effect of ethanol-diesel blends and ethanol fumigation on BTE and emission
was studied on a single cylinder DI diesel engine by Abu-Qudais et al. 2000.
Fumigation method was found to better than blend and it resulted in increase in
BTE. Optimum percentage of ethanol was 20% and 15% for ethanol fumigation and
ethanol-diesel blending operation. 20% ethanol fumigation resulted in 7.5%
increase in BTE, 36% in HC emission and 55% CO emission. However, it resulted
in 48% reduction in engine out smoke. Cheng
et al. 2008 studied effect of methanol fumigation used in 10%, 20% and 30%
along with diesel DI on engine performance and emissions on 4-cylinder diesel
engine. Experiments were conducted at five different loads while keep engine
speed constant (i.e. 1800 rpm). They observed that at low load methanol
fumigation deteriorate BTE up to 13% in some cases. However opposite trend was
observed at high load. This is because with increase in level of methanol
fumigation combustion efficiency deteriorates at low load which could improve
at high load. About gaseous emissions, as amount of
fumigated methanol increases NOx emission was found to decrease. This is
because lower combustion temperatures were attained with methanol as it has
high latent of evaporation which absorbs more heat than diesel. The maximum
reduction was 20% observed at medium load with a 30% methanol. However, higher
HC and CO emission were observed as amount of methanol fumigation increase.
This is because of lower temperature were attained with methanol fumigation as
compare to conventional diesel case. In addition to this, in methanol
fumigation mode methanol is trapped in deposit, cervices and quench in the
engine. Particulate mass (PM) and smoke opacity was found to decrease with
increase in methanol fumigation, this was significant at medium and high load.
This is because of more amount of diesel is replaced with methanol, due to this
smaller amount of diesel is taking part in diffusion combustion. In addition to
this methanol has oxygen content in its molecule and lower C/H ratio as compare
to diesel fuel. Zhang
et al. 2011 studied and compared effect of fumigation of ethanol and methanol
on particle number concentration, size distribution and also other gaseous
emission from a compressed ignition engine. They observed that with both
ethanol and methanol BTE was fond to decrease at low loads and increases at
high load as compared to conventional diesel engine. In addition to this it was
noted that ethanol was performing better at lower
load and methanol was at high load. The total number concentration and
particulate mass concentration decrease with increase in fumigation level of
ethanol or methanol. Methanol was found to be more effective in reducing PM and
particulate number concentration; this because it has higher amount of oxygen
in its molecule, in addition to that it has
simpler molecular structure. However, both do not have any effect of GMD of the
particle. HC and CO emissions were found to increase with increase in
fumigation level because of cooling effect. Emissions with ethanol were lower as
compared to methanol. NOx emission was found to decrease with increase in
amount of fuel used in fumigation; this is because of charge cooling effect.
However, reduction did not depend on type of alcohol used in fumigation. Hebbar
et al. 2013 studied effect of alcohol fumigation on emission and performance
along with hot EGR and compared it with conventional diesel engine. It was
observed that with alcohol fumigation it is possible to reduce NOx and
accommodate more EGR %. In addition to this loss in BTE due to use of EGR (up
to 30%) can be minimize with the use of alcohol fumigation. Use of ethanol
along with EGR helps in reduction in HC and smoke emission as compared to
without ethanol.  This is because of
oxygen bought by ethanol. An
experimental study was conducted on a DI diesel engine along with PFI of
n-butanol with EGR on a single cylinder research engine by (Chen et al., 2013).
Effects of EGR rate and butanol amount on efficiency, combustion and emissions
were evaluated and compared with blends of butanol-diesel which is injected
directly. They observed that as amount of butanol increases peak of heat
release rate (HRR) increase, shorter combustion duration and there was no
significant effect on start of ignition under low EGR rates. However opposite
of this was observed at high EGR rates peak of HRR drops and combustion
duration elongates as butanol amount was increased. This is because lower
oxygen concentration at high EGR rates. Smoke was found to decrease with
increase in amount of butanol at both low and high EGR rates. This because of
butanol has oxygen in its molecule and port injected butanol is increasing
homogeneity of mixture. However, NOx was found to increase with increase in
amount of butanol at low EGR rates. This is because effect of lower cetane
number and increase in oxygen concentration were found to be dominating as
compared to charge cooling effect bought by higher latent heat of evaporation
by butanol. HC and CO were found to increase with both increases in amount of
butanol and EGR rates. This is because small fraction of butanol can escape
easily in crevices and quenching near combustion chamber wall. With increase in
EGR rates and butanol concentration ISFC found to increase and ITE decreasing.
This is because of incomplete combustion of butanol and high EGR also
deteriorate combustion. Under similar concentration of butanol, port injection
of butanol resulted in higher levels of HC, CO emission and lower ITE as
compared to butanol-diesel blends.In
another study Zhu et al. (2014) studied influence of diesel injection timing,
EGR rates and amount of butanol used in PFI on combustion, emission and fuel
economy. Experiments were performed at constant load and speed of 1.0 MPa
(IMEP) and 1800 rpm respectively, 20% and 40% (by Vol.) fraction of butanol
used. In addition to this, 15% and 45% (by Vol.) EGR rates were used. It was
observed that at low EGR i.e. 15% diesel injection timing has control over
combustion and peak of HRR found to increase and advance with advancing diesel
injection timing. In addition to this combustion finish in shorter crank angle
duration. However, amount of butanol did not have any effect on start of
combustion. Peak of HRR was found to increase with increase in amount of
butanol. At 45% EGR, with increase in amount of butanol peak cylinder pressure
was found to decrease, however HRR peak was increase. This is because here
ignition delay was found to increase with increase in amount of butanol and
combustion was got delayed at 40%. It was observed that ignition delay,
pressure rise rate and peak mean temperature were found to increase with
advancing diesel injection timing, amount of butanol and EGR rate. However
opposite tread observed for combustion duration. This was because more
fuel was burning in premixed phase. HC and CO were found to increase with EGR
rate and butanol amount, however smoke was found to
decrease with advancing diesel injection timing. This is because with advancing
diesel timing peak mean temperature was found to increase. In addition to this
more time available for mixing and better mixing. NOx emission was found to
increase with increase in amount of butanol and advancing diesel injection
timing. This was because both of these factors results in increase in ignition
delay due to that more fuel is burning in premixed phase. However, with
increase in EGR rates NOx was found to decrease. For smoke opposite to NOx
emission trend was observed. Smoke was found to decrease with increase in
butanol and advancing diesel injection timing, this is because better mixing
and butanol has oxygen in its molecule. With increase in EGR rate smoke was
found to increase because EGR decrease oxygen availability. ITE was found to
increase with advancing diesel injection timing this is because better
combustion phasing and shorter combustion duration. Soloiu
et al. (2013) studied effect of n-butanol (PFI) and direct injection of diesel
or bio-diesel on efficiency, combustion and emissions at idling speed and lower
load 1-3 bar IMEP (indicated mean effective pressure). Low loads selected
because higher loads are typically not required at idling speed. It was
observed that increasing amount of butanol results in drop and delaying the
peak pressure both at 1 bar and 3 bar IMEP along with direct injection of
diesel. It is observed that with as amount of butanol increase the ignition
delay of diesel reduces. This result in lower peak for premixed combustion and
most of fuel is burning in diffusion combustion. In addition to lower peak
temperature were observed with butanol diesel dual fuel case than conventional
diesel case. There was drop in soot-NOx trade off with addition of butanol this
is because of lower temperature were attained and fuel air mixture was more
homogenous. However, HC and CO emissions were found to increase with increase
in butanol.Sahin
et al. (2015) studied and compared effect of n-butanol in dual fuel mode of
operation and in the form of blends (butanol-diesel blends) on engine emission,
engine performance in turbocharged diesel engine at different speed (2000 rpm
and 4000 rpm) and different loads. In both blend and dual fuel mode of
experiments same amount of butanol was used that is 2%, 4% and 6% by volume
basis. Smoke was found to decrease with both n-butanol diesel dual fuel mode
and n-butanol-diesel blending; however, n-butanol diesel dual fuel mode was
more effective in reducing smoke. This is because ignition delay increases so
better mixing. For n-butanol diesel dual fuel mode NOx was found decrease at a
speed of 2000 rpm for all operating conditions. However there was no effect
on NOx emission at a speed of 4000 rpm. In case of n-butanol-diesel blending
case, NOx was found to increase for 4% and 6% n-butanol concentration. This was because
with n-butanol diesel blends oxygen concentration increased. HC
emission increased slightly for
n-butanol-diesel blend; however, it increased
significantly for n-butanol diesel dual fuel case. For both n-butanol-diesel
blend and n-butanol diesel case BTE decreased and BSFC
increased for selected speed and
loads. This was because with the addition of
n-butanol heating value of fuel decreased.  Han
et al. studied different fuel i.e. n-butanol, ethanol, gasoline and diesel in
low temperature mode. They explored either single fuel or dual fuel strategies
at a load range of 0.8-1.2 MPa of IMEP. Effect of EGR, intake boost and fuel
dispatching ratio along with start of injection sweep was studied. They
observed that for attaining LTC with the diesel single-shot injection extremely
high EGR rates (60-70 %) required, this is not feasible in production engine.
In addition to this for soot emission reduction high injection pressure and
intake boost required. Indicated brake thermal efficiency decreases as LTC mode
is attained. One more problem with diesel fuel is its lower volatility and high
reactivity make difficulties in mixture formation which is required for
attaining LTC mode. For gasoline-diesel experiment a different engine was used
which was having a compression ratio of 16:1 (all other experiments were done
at a CR of 18:1). It was possible be get low NOx and smoke emissions at a load
range of 0.8 to 1.0 MPa IMEP. However, it is difficult increase load range even
with a lower compression engine. Gasoline was found to auto-ignited; however,
with the help EGR it was possible to postpone auto-ignition and even with high
EGR rates it was possible to that diesel fuel burns first. With gasoline port
fuel injection HCCI type of combustion was possible but it requires complex
control on EGR and intake boost. Ultra-low NOx and smoke emission were
observed. In addition to this IMEP of 1.16 MPa was attained with gasoline HCCI.
In case of butanol diesel dual fuel, butanol was found to auto-ignite at low
concentration of butanol before diesel injection. However, with the help of
high EGR it is possible to hinder auto-ignition of butanol. Advancing diesel
injection timing of diesel makes auto-ignition more aggressive which was
already a problem. Diesel injection timing has a reasonable control over CA5
and CA50. However, it is not having any impact of start of combustion (i.e.
auto-ignition of butanol). Increasing amount of butanol leads to more advance
and strong auto-ignition of butanol and this require even higher rates of EGR.
Two peaks in HRR were observed and indicated efficiency was found to decrease
as amount of butanol increase. This is because more fuel was burning is first
peak of HRR (auto-ignition of butanol) as amount of butanol increases and this
was located near TDC. Butanol HCCI shows near zero soot emission this is
because of longer ignition and fuel has oxygen molecule. With moderate EGR and
boost low NOx and soot emission can be attained and load range can be extended
to 1.0-1.3 MPa. In case of ethanol-diesel dual fuel auto-ignition of butanol was
not observed because of higher octane number and latent heat of evaporation as
compared to butanol and gasoline. With lower NOx and soot emission load a load
of 1.65 MPa can be achieved however high amount of EGR is required.The
effect of n-butanol for enabling HCCI, PCCI, and RCCI combustion mode was
studied in single cylinder research engine equipped with common rail fuel
injection system. Effect of each combustion mode on exhaust emission, engine
efficiency, engine safety and control over combustion phasing was evaluated. It
was observed that for enabling PCCI and HCCI combustion mode butanol was found
to be more suitable as compared to diesel. For controlling combustion phasing
in n-butanol HCCI combustion mode, EGR was the primary factor to be control.
Low NOx, soot emissions and comparable engine efficiency were attained with
both n-butanol PCCI and n-butanol HCCI. However, in case of RCCI combustion
mode auto-ignition of butanol was observed in compression stroke which was
injected in port. Thus, for proper ignition timing and combustion phasing high
amount of EGR and injection timing of diesel needed to be retardated. Here also
similar levels of NOx emission were observed which were observed in n-butanol
PCCI and n-butanol HCCI. However, soot emission was high with RCCI combustion
mode as compared to other two combustion mode evaluated 2015-01-1816.On
the whole use of alcohols in dual fuel mode of
operation is an efficient way to use
renewable alternative fuel. It results simultaneous
reduction in NOx and smoke emissions with better performance as
compared to neat diesel operation. However, HC and CO
emissions were usually found to increase. As dual fuel mode
of operation gives flexibility to change energy share online can be
studied. However, only limited studies have been
reported at limited operating points. Several parameters like
effect of fuel ratio and injection parameters for direct injection are needed
to study in detail for exploring full potential of a dual
fuel engine using alcohols as a primary fuel.     2.4.
SUMMARY OF LITERATUREAlcohols
are viable fuel for CI engines due to their moderate cetane number, higher volatility,
lower adiabatic flame temperature and higher flame speed as compared to neat
diesel. These properties led to similar performance and
efficiency. High latent heat of vaporization, fuel bound oxygen and
lower carbon to hydrogen ratio of alcohols
helps in reduction in engine out emissions. Even though alcohols have several
benefits as compared to diesel fuel, however use of neat
alcohols is not feasible because of poor performance during cold start
and high load range is limited by knocking. Use of
alcohols is also limited by another important
factor that is its availability. From above
it can be conclude that use of alcohols
along with diesel is a good option.Use of
alcohols except butanol along with
diesel in blended form is limited by its solubility. However,
some additives need to be added in order to safety of fuel injection system. Physical
and chemical properties fuel changes when we add
alcohols in diesel and because of this based on operating conditions like load,
speed affects performance and emission characteristics
of the engine.   

Based on
above, a system is required which can change amounts of alcohols
and diesel online based on operating condition of engine in order
to get best performance out engine
using these fuels.  Since the
common rail injection system can enable diesel to be injected in pulses its
effect on combustion and performance in the dual fuel mode can be studied to
ascertain the benefits.