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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, 21o 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 24o 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 23o 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 in-

cylinder 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 24o 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, 21o 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 20o 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 24o 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

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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, 21o 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 24o

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 NOx 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 in-

cylinder 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 in-

cylinder 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.