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CHAPTER 9
COMBINED EFFECTS OF INJECTION TIMING AND
COMBUSTION CHAMBER GEOMETRY
9.1
INTRODUCTION
The combustion in a diesel engine is a complex process. It depends
on many factors such as engine design particularly combustion chamber
design, fuel properties, injection pressure and injection timing of fuel
(Kouremenos et al 1999). There are extensive research results showing that
fuel injection and air motion have great effects on mixture preparation,
combustion and combined improvement of smoke and NOx emission. The
injection parameters employed should match with the combustion chamber
design to attain improved performance and emissions from the DI diesel
engine. Fuel injection characteristics have significant effects on diesel engine
performance and emissions. For example, a further injection delay with a
higher injection rate, decrease NOx and soot emission simultaneously. Based
on the analysis of diesel combustion heat release rate, if the premixed
combustion is controlled within a reasonable limit, the combustion
temperature will be lower to some extent and the combustion generated NOx
emission will thus be lowered; if the diffusion combustion process keeps fast
and ends sharply, the soot emission will be reduced.
It was reported and noticed from the previous studies that blending
biodiesel leads to particulate emission reductions by interfering with the soot
formation process. However, in the case of biodiesel fuelling there was a
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well-documented increase of 5-20% in NOx emissions. As shown by Monyem
et al (2001), the NOx increase with biodiesel fueling was attributed to an
advance of fuel injection timing. The advance in injection timing was due to
the higher bulk modulus of compressibility, or speed of sound, in the fuel
blend, which leads to a more rapid transfer of the pressure wave from the fuel
pump to the injector needle and an earlier needle lift. It was established that
the injection timing influences all engine characteristics notably. The reason
was that the injection timing influences the mixing quality of the air fuel
mixture and hence the whole combustion process. The fuel property changes
between biodiesel and PBDF require a change in the engine operating
parameters such as injection timing, injection pressure etc. These operating
parameters can cause different performance and exhaust emissions than the
optimized settings chosen by the engine manufacturer. Hence it is necessary
to determine the improved optimum values of these parameters. In this
experimental phase, the attention was focused on finding the optimal injection
timing for TRCC with POME20 in terms of the performance parameters.
9.2
AIM AND OBJECTIVES
This phase of experimental work was planned to optimize the
combination of injection timing and combustion chamber geometry to achieve
higher performance with lower emissions in a DI POME20 fuelled diesel
engine having TRCC and to compare the results with that of standard engine
operated with POME20 and PBDF to find the optimal injection timing for the
POME20 in terms of the performance and emission characteristics. To
achieve this aim, the following objectives were set:
To investigate the combined effects of injection timing and
combustion chamber geometry on the performance, emission
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and combustion characteristics of POME20 fuelled DI diesel
engine having TRCC at different loads of operation.
To compare the engine performance, emission and combustion
characteristics of modified injection timing settings with that
of standard injection timing setting recommended by the
engine manufacturer.
9.3
EXPERIMENTAL PROCEDURE
The injection timing of the MICO jerk type pump was varied by
changing the number of shims under the pump body. The standard engine was
fitted with three shims to give standard injection timing of 23o before Top
Dead Center (bTDC). By changing the number of shims, the injection timings
were varied to 20o, 21o, 22o and 24o bTDC. To start with, the performance,
emission and combustion tests were carried out using PBDF and POME20 at
various loads for standard engine having HCC with fuel injection pressure of
200 bar and standard fuel injection timing of 23º bTDC, following the
experimental procedure explained in Chapter-7. Then the performance,
emission and combustion tests were carried out using POME20 at various
loads for the modified engine having optimized combustion chamber i.e.
TRCC with fuel injection pressure of 200 bar and standard fuel injection
timing of 23º bTDC. These values were also considered as baseline values
throughout the experimentation for comparison with the results obtained from
the optimized engine in terms of combustion chamber with POME20 and
PBDF at different injection timings. Hereafter in this chapter the term
‘optimized engine’ represents engine piston having TRCC operated with fuel
injection pressure of 200 bar and standard fuel injection timing of 23º bTDC.
Then the performance, emission and combustion tests were performed for
optimized engine with PBDF and POME20 at different injection timings viz.
20o bTDC, 21o bTDC, 22o bTDC, 24o bTDC.
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The engine tests were carried out at 0%, 25%, 50%, 75% and 100%
load and their results were compared and analyzed with the standard injection
timing of 23o bTDC. In order to have a meaningful comparison of emissions
and engine performance, the investigation was carried out at same operating
conditions i.e. ambient temperature, atmospheric pressure, engine speed and
torque were maintained for all injection timing settings. The combined effect
of injection timing and combustion chamber geometries on the performance,
combustion and emission characteristics of the DI diesel engine operated with
POME20 and PBDF at different loads of operation was investigated. The
results were analyzed and compared with standard engine fuelled with
POME20 and PBDF. The results and discussion are presented in the next
section of this chapter.
9.4
RESULTS AND DISCUSSION
The performance, emission and combustion characteristics of the
standard engine with HCC at standard injection timing and optimized engine
with TRCC at different injection timings were determined, compared and
analysed for BSFC, BTE, UBHC, CO, NOx, smoke emissions and combustion
parameters such as ignition delay, cylinder peak pressure, exhaust gas
temperature and heat release rate.
9.4.1
Combustion Characteristics
One of the most important parameters in the combustion
phenomenon is the ignition delay. Figure 9.1 shows the variation of ignition
delay for the optimized and standard engine with PBDF and POME20 at
different injection timings. It was observed that the ignition delay period of
POME20 was significantly lower than that of PBDF when tested in the
standard engine at standard injection timing. This was due to higher cetane
number of POME20 compared with PBDF. It was also observed that for
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optimized engine the ignition delay periods were lower compared to standard
engine at all loads of operation. Further, it was found that at retarded injection
timings, the ignition delay decreased due to high in-cylinder temperature,
improved air motion and availability of Oxygen in POME. The ignition
delays for the retarded injection timings of 20o, 21 o and 22o bTDC were
measured as 5.8o CA, 5.9o CA and 6.1o CA respectively at full load compared
to 6.4o CA at standard injection timing of 23o bTDC for optimized engine
fuelled with POME20. However for advanced injection timing of 24o bTDC
the ignition delay was increased to 6.9o CA due to lower in-cylinder
temperature.
The comparison of the HRR curves for the standard engine and the
optimized engine with PBDF and POME20 at different injection timings is
shown in Figure 9.2. The maximum heat release rate of POME20 was lower
than that of PBDF in the standard engine at the standard injection timing.
This was because of shorter ignition delay for POME20 compared with that of
PBDF. In addition the poor spray atomization characteristics due to higher
viscosity was responsible for the lower heat release rate. The heat release rate
curve for optimized engine fuelled with POME20 and operated at standard
injection timing demonstrated similar but better than standard engine fuelled
with PBDF. This was caused by improved air fuel mixing and better
combustion. However the maximum heat release rate for retarded injection
timings were slightly lower compared to the standard and advanced injection
timings due to shorter ignition delay. As a result, the heat release rate during
diffusion combustion phase was increased. The maximum heat release rate for
retarded injection timing of 20o, 21o and 22o bTDC were recorded as 86.2
J/oCA, 85.2 J/oCA and 78.6 J/oCA respectively at full load compared to 87.6
J/oCA at standard injection timing of 23o bTDC for optimized engine fuelled
with POME20. However for advanced injection timing of 24 o bTDC the
maximum heat release rate was increased to 88.6 J/oCA.
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Figure 9.1 Variation of ignition delay at different injection timings
Figure 9.2 Comparison of HRR at full load at different injection timings
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The comparison of the cumulative heat release rate curves for the
standard engine and the optimized engine with PBDF and POME20 at
different injection timings is shown in Figure 9.3. The cumulative heat release
rate curve for optimized engine fuelled with POME20 and operated at
standard injection timing demonstrated similar, but better than standard
engine fuelled with PBDF. This was mainly attributed to improved
combustion due to increased air-fuel mixing as a result of better air motion.
However the cumulative heat release rate for retarded injection timings were
slightly lower compared to the standard and advanced injection timings
during early combustion zone due to shorter ignition delay. The cumulative
heat release rate for retarded injection timing of 20o, 21o and 22o bTDC were
recorded as 641 J, 653 J and 668 J respectively at 10o aTDC compared to 671
J at standard injection timing of 23 o bTDC for optimized engine fuelled with
POME20. However for advanced injection timing of 24o bTDC the
cumulative heat release rate was to 668 J at 10oCA aTDC at full load.
The cylinder pressure variations with crank angle for the optimized
engine and standard engine at different injection timings with POME20 and
PBDF are shown in Figure 9.4. The pressure variations of the optimized
engine operated with POME20 followed the similar pattern of pressure rise as
that of the standard engine operated with PBDF at all load conditions at
standard injection timing. However the values of pressure data were higher at
all operating conditions. This was as a result of better combustion due to
enhanced air fuel mixing in TRCC. However the cylinder pressure variations
and peak cylinder pressure decreased while retarding the fuel injection timing
and increased while advancing the fuel injection timing. The cylinder gas
temperature at the start of retarded fuel injection timing was increased and
this in turn decreased the ignition delay and reduced the amount of fuel
injected into the cylinder within the ignition delay period. As a consequence
the combustible mixture available within the ignition delay and the premixed
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Figure 9.3 Comparison of CHRR at full load at different injection timing
Figure 9.4
Comparison of cylinder pressure at full load at different
injection timings
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heat release in the subsequent combustion process, were decreased leading to
the decrease of in-cylinder pressure and peak cylinder pressure. However
advancing the injection timing increased the ignition delay due to lower incylinder temperature, which in turn increased the amount of air-fuel mixture
prepared in the cylinder during ignition delay period. As a consequence the
combustible mixture available within the ignition delay and the premixed heat
release in the subsequent combustion process were increased, leading to the
increase of in-cylinder pressure and peak cylinder pressure. The cylinder
pressure trend of the optimized engine with POME20 at retarded injection
timing of 21o bTDC lies in between 22o bTDC and 20o bTDC. However the
cylinder pressure variations of the optimized engine with POME20 at
advanced injection timing of 24 o bTDC lies above the standard injection
timing of 23o bTDC.
The variation of peak pressures with respect to brake power for
optimized engine and standard engine at different injection timings with
PBDF and POME20 is shown Figure 9.5. It can be seen from Figure 9.5 that
the peak pressure for the optimized engine was higher than that for standard
engine with the POME20 fuel operation at standard injection timing. This was
because of better combustion as a result of better air fuel mixing. The peak
pressure for optimized engine (75.8 bar) operated with the retarded injection
timing of 21o bTDC was slightly lower than the standard injection timing of
23o bTDC (77.1 bar) caused by the lesser amount of heat release in the
premixed combustion phase due to shorter ignition delay. A decrease in the
peak pressure was observed for retarding the injection timing from 21º to 20º
bTDC. Moreover, the peak pressure for POME20 (75.8 bar) was slightly
lower than that of PBDF (78.5 bar) with the optimized engine having TRCC
operated with the retarded injection timing of 21o bTDC at full load. This was
attributable to the lower calorific value of POME than that of conventional
PBDF.
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Figure 9.6 shows the variation of EGT for the standard engine and
optimized engine with PBDF and POME20, at different injection timings. It
was noticed that the EGT of the POME20 blend was higher than that of PBDF
Figure 9.5 Variation of peak pressures at different injection timings
Figure 9.6 Comparison of EGT at different injection timings
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and EGT for optimized engine was higher than the standard engine at
standard injection timing. This was attributable to more complete combustion
due to better air fuel mixing and the presence of Oxygen in the POME20.
Further, it was found that the EGT increased first with retarded injection
timings and then decreased with further retardation. This was because of
delayed combustion and more of the heat was released during the diffusion
phase compared to advanced injection timing. The EGT for retarded injection
timing of 22o, 21 o and 20o bTDC were measured as 551oC, 556oC and 541oC
respectively at full load compared to 548o C at the standard injection timing of
23o bTDC for the optimized engine fuelled with POME20. However for the
advanced injection timing of 24o bTDC the EGT was increased to 568oC.
9.4.2
Performance characteristics
BSFC is a comparative parameter that shows how efficiently an
engine is converting fuel into work. The BSFC variations for standard engine
and optimized engine with PBDF and POME20, for different injection
timings starting from 20 o to 24o bTDC is shown in Figure 9.7. The BSFC for
TRCC (0.252 kg/ kW-hr) was lower than baseline HCC (0.293 kg/ kW-hr)
with POME20 at the standard injection timing. Further, when the injection
timing was retarded, at first the BSFC decreased and then it increased. Lowest
BSFC was observed at an injection timing of 21º bTDC.
The BSFC for TRCC (0.249 kg/ kW-hr) operated with the retarded
injection timing of 21o bTDC was slightly lower than standard injection
timing of 23o bTDC. Compared to the optimized engine operated at standard
injection timing of 23o bTDC, the retarded injection timing of 21o bTDC
provided a lower BSFC of about 1.22%. This was due to better combustion of
POME20 due to better air fuel mixing as a result of improved swirl and
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turbulent kinetic energy. However, an increase in the BSFC was observed at
retarded injection timing from 21º to 20º bTDC. The BSFC decreased with
increase in load for the retarded injection timings too. However BSFC for
POME20 (0.249 kg/ kW-hr) was slightly higher than that of PBDF (0.245 kg/
kW-hr) with the optimized engine having TRCC operated with the retarded
injection timing of 21o bTDC at full load. This was due to the lower calorific
value of POME than that of conventional PBDF.
Figure 9.8 shows the comparison of BTE of the standard engine
and optimized engine with PBDF and POME20 at different injection timings.
It shows that the BTE increased with the increase in brake power for all fuels
and all types of combustion chambers at different injection timings. The BTE
for TRCC (33.09% at full load) was higher, when compared to the HCC at all
loads, when operated with POME20 at standard injection timing. This was
due to better mixture formation of POME20 and air, as a result of better air
motion in TRCC. This led to better combustion of the biodiesel and thus
increased the BTE.
It was also observed that the brake thermal efficiency increased
marginally with retarding the injection timing from 23º bTDC to 21º bTDC.
This was mainly due to improved combustion as a result of better mixture
formation. However further retardation in injection timing was found not so
beneficial. In addition, advancement of injection timing was also found not
desirable as it led to drop in BTE of the engine. With POME20 as fuel, the
BTE at full load increased from 33.09% to 33.36% on retarding the injection
timing by 1o and to 33.5% on retarding by 2o from standard injection timing of
23o bTDC. On advancing the injection by 1o to 24o bTDC the thermal
efficiency declined to 32.41%.
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Figure 9.7 Variation of BSFC at different injection timings
Figure 9.8 Comparison of BTE at different injection timings
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Figure 9.9 shows the variation of BSEC of the standard engine and
optimized engine with PBDF and POME20 at different injection timings. The
BSEC was higher for standard engine compared to the optimized engine with
PBDF and POME20 at standard injection timing. This was due to better
mixture formation, as a result of better air motion in TRCC, which in turn led
to better combustion. However BSEC of the optimized engine with POME20
was higher than the PBDF at all loads of operation at standard injection
timing. This can be attributed to the lower calorific value of POME20. The
results also showed that the BSEC decreased with retarding the injection
timing initially from standard timing i.e. retarding the injection timing from
23º bTDC to 21º bTDC and then increased with further retardation. It was
also observed that the best results were associated with an injection timing of
21º bTDC for POME20 with the optimized engine. It was also noticed that the
BSEC increased marginally with advancing the injection timing from 23º
bTDC to 24º bTDC. This was due to shorter ignition delay for POME20. Due
to shorter ignition delay, combustion was initiated much before TDC was
reached, which increases compression work and hampers power output. With
optimized engine compared to standard injection timing, the BSEC for
retarded injection timings of 21o and 22o bTDC were lower by 1.2% and 0.8%
respectively. However, for retarded injection timing of 20o bTDC and
advanced injection timing of 24 o bTDC the BSEC were higher by 3.9% and
2.1% respectively.
9.4.3
Emission characteristics
The comparison of UBHC emissions for standard engine and
optimized engine with PBDF and POME20 at different injection timings is
shown in Figure 9.10. UBHC emissions were reduced over the entire range of
loads for both standard engine and optimized engine fuelled with POME20
when compared to PBDF operation. It was noticed that optimized engine emit
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Figure 9.9 Variation of BSEC at different injection timings
Figure 9.10 Variation of UBHC emissions at different injection timings
148
less level of UBHC compared to standard engine at standard injection timing.
This was due to better combustion of POME as a result of improved swirl and
squish motion of air in re-entrant combustion chambers and the presence of
Oxygen in POME. However, marginal increase in UBHC emissions was
observed for retarded injection timings. Compared to standard injection
timing, the percentage increase of UBHC emissions for retarded injection
timing of 20o, 21 o and 22o bTDC were 8.26%, 4.34% and 2.82% respectively.
This was due to a slight decrease in premixed combustion phase and decrease
in cylinder wall temperature. However for advanced injection timing of 24 o
bTDC UBHC emissions were decreased by 5.21% compared to standard
injection timing.
Figure 9.11 shows the comparison of CO emissions with brake
power for the two types of combustion chambers at different injection
timings. CO emissions from all types of combustion chambers fuelled with
POME20 decreased significantly when compared with that of PBDF at all
loads at standard injection timing. This was because of presence of Oxygen in
POME20. It decreased with TRCC than the HCC. Higher air movement in
TRCC and the presence of Oxygen in POME, led to better combustion of fuel
resulting in the decrease in CO emissions. Secondly, increase in the
proportion of Oxygen in POME promotes further oxidation of CO during the
engine exhaust process. However marginal increase in CO emissions was
noticed with retarded injection timings due to poor premixed combustion
phase.
Figure 9.12 shows the variation of CO2 emissions for standard and
optimized engines with PBDF and POME20 at different injection timings.
The CO2 emissions increased with the addition of POME in PBDF and
reached a maximum value for optimized engine than the standard engine at
standard injection timing. The more amount of CO2 in the exhaust emission is
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Figure 9.11 Variation of CO emissions at different injection timings
Figure 9.12 Variation of CO2 emissions at different injection timings
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an indication of the complete combustion of fuel. Thus higher CO2 emission
for the optimized engine indicates effective combustion due to the Oxygen
content in the POME20, better air motion and air fuel mixture preparation. It
was noticed that CO2 emissions varied from 3.6% at low load to 8.4% at full
load for the optimized engine operated at standard injection timing fuelled
with POME20. Compared to standard injection timing, the percentage
decrease of CO2 emissions for retarded injection timing of 20o, 21o and 22o
bTDC were 7.1%, 3.5% and 2.4% respectively. This was due to a slight
decrease in premixed combustion phase and decrease in in-cylinder
temperature. However for advanced injection timing of 24o bTDC, CO2
emissions were increased by 2.38% compared to the standard injection
timing. This was due to enhanced premixed combustion phase and complete
combustion of fuel.
Figure 9.13 shows the variation of NOx emissions for the standard
engine and the optimized engine with PBDF and POME20 at different
injection timings. The NOx emissions were higher for TRCC than the HCC at
standard injection timing. The reason for the increase in NO x was due to
higher combustion temperatures arising from improved combustion as a result
of better mixture formation in TRCC and the availability of Oxygen in
POME. There was an increase of about 9.2% NOx for TRCC compared to the
HCC when fuelled with POME20 and 17.35% with standard PBDF at
standard injection timing. However retarding the injection timing decreased
NOx emission. Compared to standard injection timing, the percentage
decrease of NOx for retarded injection timings of 20o, 21o and 22o bTDC were
12.24%, 7.9% and 3.82% respectively. This was due to shorter ignition delay
owing to high cylinder pressure and temperature at retarded injection timing.
The shorter ignition delay shortens the mixing time which leads to slow
burning rate and slow rise in pressure and temperature. However for advanced
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injection timing of 24o bTDC, NOx emissions were increased by 2.8%
compared to standard injection timing. This was due to longer ignition delay.
The longer ignition delay increases the mixing time which enhances the
precombustion phase and increases incylinder temperature and NOx
formation.
The smoke intensity comparison for optimized engine and standard
engine at different injection timings with PBDF and POME20 is shown in
Figure 9.14. At standard injection timing, smoke emissions from both
optimized engine and standard engine for the POME20 blend decreased
significantly when compared with those of PBDF. It was also observed that
the smoke emissions were lower for TRCC than the HCC type of combustion
chamber. This was attributable to more complete combustion as a result of
better air fuel mixing and the presence of Oxygen in the POME. As can be
seen from the Figure 9.14, retarding the injection timing increased smoke
emission when compared with the standard injection timing.
The smoke opacity for retarded injection timing of 20o, 21o and 22o
bTDC were measured as 56.8%, 54.2% and 52.6% respectively at full load
compared to 52% at standard injection timing of 23o bTDC for optimized
engine fuelled with POME20. Compared to standard injection timing, the
percentage increase of smoke emissions for retarded injection timing of 20o,
21o and 22o bTDC were 9.23%, 4.23% and 0.11% respectively. This was
caused by lower in cylinder temperature due to reduction in premixed
combustion phase as a result of the shorter ignition delay. The lower incylinder temperature inhibits the oxidation of soot particles that results in
higher smoke emissions. However for advanced injection timing of 24o bTDC
smoke opacity was decreased by 6.9% and compared to standard injection
timing. This was due to enhanced premixed combustion that results in higher
in-cylinder temperature which promotes the oxidation of soot particles.
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Figure 9.13 Comparison of NOx emissions at different injection timings
Figure 9.14 Comparison of smoke emissions at different injection
timings
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9.5
SUMMARY
In this experimental phase, experiments were carried out to study
the combined effects of injection timing and combustion chamber geometry
on the performance, emission and combustion characteristics of POME20
fuelled DI diesel engine. The tests were performed at standard injection
pressure of 200 bar for optimized engine having TRCC with PBDF and
POME20 at different injection timings viz. 20o bTDC, 21o bTDC, 22o bTDC,
23o bTDC and 24o bTDC. The results were compared with that of POME20
and PBDF operated standard engine to decide on the proper injection timing
for biodiesel fuelled optimized engine and PBDF operated optimized engine
for validation. From the experimental results the following conclusions can be
drawn.
1.
It was observed that the performance of the engine initially
improved and then decreased with retarded injection timings
i.e. BTE increased marginally and BSEC and BSFC were
slightly decreased with retarding the injection timing from 23º
to 21º bTDC. Further retardation of injection timing to 20º
bTDC was found not so beneficial, moreover advancement of
injection timing to 24º bTDC was not desirable as it led to
drop in thermal efficiency and increase in BSFC and BSEC of
the engine.
2.
It was observed that there was a significant reduction in CO,
UBHC and smoke intensity for the engine having TRCC due
to better mixture formation and combustion at the standard
injection timing. However CO, UBHC and smoke intensity for
the engine having TRCC marginally increased with retarded
injection timing due to poor initial phase of combustion.
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3.
The increased swirl and squish of optimized engine improved
charge mixing which resulted in better combustion and
increased
the
combustion
chamber
temperature.
This
increased NOx emission of optimized engine having TRCC at
the standard injection timing. However retarding the injection
timing significantly decreased NOx emission due to lower incylinder temperature as a result of poor premixed combustion
phase caused by shorter ignition delay.
4.
The improved air motion and better mixing in the optimized
engine having TRCC decreased the ignition delay and
increased the EGT when compared to standard engine having
HCC at the standard injection timing. The optimized engine
having TRCC also developed maximum peak pressure and
maximum heat release rate for POME20 compared to the
standard engine at the standard injection timing. However, it
was found that retarding the injection timing further lowered
ignition delay, maximum peak in-cylinder pressure, maximum
heat release rate and peak in-cylinder temperature and thereby
reduced NOx emission.
The present phase of investigation showed that the performance,
combustion and emission characteristics of biodiesel fuelled previously
optimized engine in terms of combustion chamber can be improved by
suitably varying the injection timing. On the whole, the engine having TRCC
operated with the retarded injection timing of 21o bTDC was found to be
superior in terms of performance, combustion and exhaust emissions
improvement over the standard, other retarded and advanced injection
timings. Compared to the optimized engine operated at standard injection
timing of 23o bTDC, the retarded injection timing of 21o bTDC provided a
better performance of 1.24%, 1.22% and 1.19% in terms of BTE, BSFC and
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BSEC respectively and NOx emission level improvement of 7.9%. However
CO, UBHC and smoke emission levels were slightly deteriorated. Based on
the above results, the optimized engine having TRCC operated at retarded
injection timing of 21o bTDC was chosen for further studies, to evaluate the
effect of varying the injection pressure to further enhance the existing
performance and emissions characteristics of a biodiesel fuelled DI diesel
fuelled engine.