Maitines

An increase in EGR rate by several percentage points showed a significant influence on LTC heat release rate, for both the low temperature heat release and the main combustion events, as shown in Figure 5.2. It retarded the main combustion event and reduced the peak heat release rate significantly. The peak of the low temperature heat release rate was about 10% that of the main combustion event.

However, the influence of the EGR rate on the heat release rate during the LTHR was still detectable. An assessment of the heat release rate curves for the LTHR period, shown in the right hand column of Figure 5.2, showed that higher EGR rates led to a later cool flame and a lower LTHR reaction rate at constant start of injection timing. As the intake manifold temperature was kept constant for the different EGR levels, the LTHR processes appeared to be sensitive to the charge composition. For the higher EGR rates, the reduction in oxygen concentration and increase in heat capacity due to the increased concentration of CO2 and H2O in the charge should lead to slower fuel reaction rates and lower charge temperatures during the LTHR period. This lower temperature and lower O2 concentration may have reduced the formation and propagation rates of the initial radicals, and the concentration of the intermediate compounds during the LTHR period.

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Figure 5.2 Effects of EGR rate on LTC heat release (Top: 1500 rpm, 8 mg/cycle, fuel injection pressure 800 bar, SoI -21°CA ATDC, intake temperature 70°C; Middle: 1500 rpm, 16 change in intake charge composition had a more significant influence on the phasing of the main combustion than it had on the LTHR. This indicated that the two

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this set of tests. The in-cylinder temperature would take a longer time to reach the temperature threshold where the NTC region terminates (i.e. when the high temperature reactions start), and hence the duration of the LTHR (DurL) was increased. In terms of the main combustion event, the different charge composition caused by the EGR and the LTHR processes led to a more significant influence on the combustion phasing parameters. The retarding of the start of the main combustion phase and of the CA50 by an increase in the EGR rate was longer than the delay in the start of the LTHR. The duration of the first half of the main combustion events, DurF, was increased by the increase in EGR rate, indicating a slower heat release process for the premixed combustion event. The duration of the second half of the main combustion (DurP) was increased by the higher EGR rates for the 8mg/cycle fuelling case while it was reduced for the 16mg/cycle fuelling cases. A lack of oxygen in the charge, a reduction in the concentrations of intermediate radicals and an increase in the ignition delay were all expected to contribute to the retarded and slower main combustion event.

For the 16 mg/cycle cases, the change in LTHR by the EGR rate is less significant than for the 8 mg/cycle cases. The higher fuel concentration could reduce the LTHR sensitivity to the intake charge dilution ratio. The doubled fuel injection quantity led to a higher equivalence ratio for these conditions, which will have increased the fuel concentration and lead to increased primary radicals formation and propagation processes even with reduced oxidant concentrations. Thus, the in-cylinder temperature increased more rapidly for this load condition and entered the NTC region sooner after the start of the cool flame reactions than for the low load case.

The rapid increase in temperature for the 16 mg/cycle fuelling condition also led to a shorter NTC region than for the 8 mg/cycle fuelling condition. For the 16mg/cycle conditions, the change in duration of the LTHR (which incorporated both the cool flame reactions and the NTC region) was only about a half of that from the low fuelling conditions.

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Figure 5.3 Effects of EGR rate on LTC combustion phasing (IDL: ignition delay of LTHR; DurL:

SoCI-CA5; DurF: CA5-CA50; DurP: CA50-CA90) (Top: 1500 rpm, 8 mg/cycle, fuel injection pressure 800 bar, SoI -21°CA ATDC, intake temperature 70°C; Middle: 1500 rpm, 16 mg/cycle, fuel injection pressure 1000 bar, SoI -12°CA ATDC, intake temperature 70°C;

Bottom: 2500 rpm, 16 mg/cycle, fuel injection pressure 900 bar, SoI -15°CA ATDC, intake temperature 80°C)

The EGR level significantly influences combustion stability and emissions, as discussed in detail in Chapter 4; the emissions and combustion stability for the specific points of interest in this chapter are shown in Figure 5.4. The combustion stability represented by CoV(IMEP) and CoV(MaxHRR) was degraded with higher EGR rates. The standard deviations of SoCL, CA5 and CA50 were also increased, indicating a more variable timing of the start of LTHR and main combustion by the increased intake charge dilution. The NOx and smoke emissions were both reduced with the increased EGR rate, while the THC and CO emissions were increased due to the reduction in the intake charge oxygen concentration, as shown in Figure 5.4.

All these results are consistent with the previous chapter’s view of the effect of EGR rate on LTC emissions. The longer NTC region increased the time available for

Duration (°CA) ^{52% EGR 54% EGR 56% EGR}

-5

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combined effect from reduced oxygen concentration, possible over-mixing and bulk quenching due to the longer ignition delay and reduced peak bulk temperature.

Figure 5.4 Effects of EGR rate on LTC stability and emissions (Top: 1500 rpm, 8 mg/cycle, LTHR caused by the higher dilution ratio, there was a longer period for the reaction to release enough chemical energy to push the charge temperature into the NTC CoV(IMEP), CoV(MaxHRR) (%); Standard Deviations (*0.1 °CA)

62% EGR Intake O2 (%); Nox(ppm); Smoke(FSN); THC(*1k ppm); CO(%)

52% EGR CoV(IMEP), CoV(MaxHRR) (%); Standard Deviations (*0.1 °CA)

52% EGR Intake O2 (%); Nox(ppm); Smoke(FSN); THC(*1k ppm); CO(%)

47% EGR CoV(IMEP), CoV(MaxHRR) (%); Standard Deviations (*0.1 °CA)

47% EGR 50% EGR 52% EGR

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At higher load, the increase in fuel injection quantity led to increased fuel concentration taking part in the low temperature oxidation processes, resulting in higher reaction rates. The enhanced cool flame reaction for the higher fuelling conditions led to a more rapid increase in the charge temperature. This higher temperature resulted in an earlier occurrence of the NTC regime, which terminated the cool flame reactions.

For the same fuel injection quantity, engine speed showed an insignificant influence on the LTC mechanism. The duration of the cool flame reaction and ignition delay on time basis (in ms) were almost the same for the 1500 rpm and 2500 rpm conditions with 16 mg/cycle fuelling. This indicates that the chemical kinetics dominated the low temperature oxidation processes. Thus, the effects of intake charge temperature, fuel injection pressure and timings were studied for 1500 rpm cases only. The effects of engine speed were evaluated further and the results are presented in Chapter 7.