La Misa

As previous studies have shown, fuel injection pressure significantly influences both conventional and low temperature diesel combustion (see Sections 2.3 and 4.4). An increase in the fuel injection pressure led to advanced cool flame reactions with higher LTHR rates; this is shown in detail in Figure 5.8. The higher fuel injection pressure led to improved fuel-charge mixing (Fang et al., 2010). During the cool flame reaction period, the more evenly distributed fuel in the charge had increased an opportunity to meet and react with oxygen molecules, leading to more rapid cool flame heat release. The LTHR heat release rate started to drop earlier for the high injection pressure cases. This suggests that the LTHR could have started to

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terminate when the total heat released from the cool flame reaction reached a threshold where the mixture temperature was in the NTC region.

Figure 5.8 Effects of fuel injection pressure on LTC heat release (Top: 1500 rpm, 8 mg/cycle, SoI -21°CA ATDC, EGR rate 65%, intake temperature 70°C; Bottom: 1500 rpm, 16 mg/cycle, SoI -12°CA ATDC, EGR rate 54%, intake temperature 80°C)

The main combustion was affected by the increase in fuel injection pressure with different trends for the two fuelling conditions. For the 8 mg/cycle condition, the main combustion heat release rate was increased and advanced when the fuel injection pressure was increased from 600 bar to 800 bar. A further increase in the fuel pressure led to retarded high temperature heat release and lower peak heat release rates. Conversely, the higher injection pressure led to advanced main combustion events with higher peak values for the higher fuelling conditions. The improved mixing could have led to over-mixing of the fuel, which delayed the combustion event for the low load LTC high fuel injection pressure (1000 bar) case. However, for the higher fuelling conditions, the air-fuel-ratio is lower; the increased mixing intensity led to earlier and more intense combustion process with higher peak heat release rates.

The combustion phasing parameters such as start of LTHR, CA5, CA50 and CA90

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fuelling condition, as shown in Figure 5.9. This is most likely a result of improved mixing enhancing the fuel burn rate, leading to a shorter duration of the first half of the main combustion event. For the low load condition, however, only the start of LTHR was advanced by the higher fuel injection pressure, and the phasing parameters of the main combustion showed no consistent trend. The possible over-mixing at the highest fuel injection pressure case made it more difficult for the ignition to occur. The duration of the LTHR was increased due to the later start of the main combustion (CA5) for this case. The duration of the first half of the main combustion was increased due to the slow combustion caused by the possible over-mixing.

Figure 5.9 Effects of fuel injection pressure on LTC combustion phasing (IDL: ignition delay of LTHR; DurL: SoCI-CA5; DurF: CA5-CA50; DurP: CA50-CA90) (Top: 1500 rpm, 8 mg/cycle, SoI -21°CA ATDC, EGR rate 65%, intake temperature 70°C; Bottom: 1500 rpm, 16 mg/cycle, SoI -12°CA ATDC, EGR rate 54%, intake temperature 80°C)

The combustion variability was unchanged or reduced by the increasing fuel injection pressure for the 16 mg/cycle condition, but variability was increased for the low fuelling conditions, as shown in Figure 5.10. At the higher fuelling condition, improved mixing due to the high fuel injection pressure may have improved the utilisation of the limited quantity of oxygen in the charge, leading to a more stable LTC combustion.

This may explain why, with increased injection pressure, the mid-load LTC combustion stability was improved. However, at the lower load where there was more excess oxygen, high fuel injection pressure may have led to over-mixing of fuel which in turn caused weak flame propagation and bulk quenching. This hypothesis is supported by the higher THC and CO emissions for the low load high fuel injection pressure case shown in Figure 5.10. For the low load LTC, the significant increase in

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Duration (°CA) ^{800 bar}_{1000 bar} 1200 bar

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the variability of the peak heat release rate and IMEP suggests the occurrence of over-mixing.

Results presented in Chapter 4 showed that fuel injection pressure can strongly influence the LTC emissions. The smoke, THC and CO emissions were reduced with increased fuel injection pressure for the 16 mg/cycle conditions, as shown in Figure 5.10. The improved homogeneity of the mixture led to low formation rates of soot even with increased combustion temperature. The higher combustion temperature and the increased turbulence resulting from the high injection pressure reduced the THC and CO emissions. However, for the low fuelling condition, the lowest THC and CO were for the intermediate 800 bar fuel injection pressure case. Increasing or reducing the fuel injection pressure from this point increased the THC and CO emissions. For the low fuel injection case, the poor mixing due to less kinetic energy in the injection event could lead to mixing of the fuel; partial oxidation of under-mixed fuel is a possible source of combustion by-products like CO and THC. On the other hand, the high injection pressure may have resulted in over-mixing and increased the possibility of quenching, which is a source of THC and CO emissions.

Figure 5.10 Effects of fuel injection pressure on LTC combustion stability and emissions (Top:

1500 rpm, 8 mg/cycle, SoI -21°CA ATDC, EGR rate 65%, intake temperature 70°C;

Bottom:1500 rpm, 16 mg/cycle, SoI -12°CA ATDC, EGR rate 54%, intake temperature 80°C) 0 CoV(IMEP), CoV(MaxHRR) (%); Standard Deviations (*0.1 °CA)

600 bar Intake O2 (%); Nox(ppm); Smoke(FSN); THC(*1k ppm); CO(%)

800 bar CoV(IMEP), CoV(MaxHRR) (%); Standard Deviations (*0.1 °CA)

800 bar 1000 bar 1200 bar

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