Vísperas

Higher intake manifold temperature led to advanced and rapid heat release processes, as shown in Figure 5.5. The peak heat release rate for the main combustion event was increased with the higher intake temperature. However, the peak of the cool flame heat release rate was reduced for the higher intake temperature cases. The increase in intake manifold temperature by 20°C led to an increase in in-cylinder temperature by about 50°C at the end of the compression stroke^*. The higher charge temperature enhanced the radicals’ primary initiation and propagation processes in the LTHR. It also enhanced the evaporation of the fuel and increased the homogeneity of premixed fuel-air mixture to support the early stage of the main combustion, leading to a higher peak HRR. However, due to the higher charge temperature, the cylinder temperature may have reached the NTC region earlier during the LTHR process. Hence, the NTC terminated the low temperature oxidation processes and led to reduced LTHR rates for these cases.

* The calculated end of compression temperature was based on assuming adiabatic, polytropic compression, with polytropic exponent of 1.35, from IVC 134°CA ATDC) to SoI (-21°CA ATDC).

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Figure 5.5 Effects of intake manifold temperature on LTC heat release (Top: 1500 rpm, 8 mg/cycle, fuel injection pressure 800 bar, SoI -21°CA ATDC, EGR rate 65%; Bottom: 1500 rpm, 16 mg/cycle, fuel injection pressure 1000 bar, SoI -12°CA ATDC, EGR rate 54%)

For the 50°C intake temperature cases, the LTHR rates were significantly higher than that for the higher temperature cases. As the temperature is the decisive factor for the occurrence of the NTC region, the terminating effects of the NTC on the low temperature oxidation processes would be less significant for the lower mixture temperature cases. The wide temperature difference between the start of reaction (decided by the intake charge temperature) and the temperature for NTC to occur (at constant temperature) enabled a higher intensity of LTHR rate.

Increased intake charge temperature led to advanced combustion events for the whole combustion process. The combustion phasing and duration shown in Figure 5.6 suggest that the higher intake temperature reduced the ignition delay for the LTHR (IDL). The higher charge temperature led to an earlier occurrence of primary radical initiation and propagation. While the NTC region is a function of the temperature, the cylinder temperature reached the NTC region and terminated the radical formation chains sooner for the higher intake temperature cases. With less

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increased slowly for the high intake temperature cases. However, the in-cylinder temperature is a combined result of the intake charge temperature and LTHR.

Hence, even with lower LTHR, the higher intake temperature cases may still have had a higher charge temperature and passed through the NTC region more rapidly.

This led to shorter ignition delay for the main combustion event.

Figure 5.6 Effects of intake manifold temperature 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, EGR rate 65%; Bottom: 1500 rpm, 16 mg/cycle, fuel injection pressure 1000 bar, SoI -12°CA ATDC, EGR rate 54%)

The duration of the first half of the main combustion was reduced significantly by the higher intake temperature, while the duration of the second half was not significantly affected. The rapid first half heat release of the main combustion event associated with the higher charge temperature which could lead to enhanced fuel oxidation. The reduction in low temperature oxidation rates during the early stage of the cycle by increased intake temperature could be another reason for the increases in the main combustion heat release rates. The reduction in the LTHR for the fuel may have led to a different charge composition for the main combustion event. The later part of the main combustion event, however, was mainly governed by the availability of oxygen in the mixture and would therefore be less sensitive to the intake temperature, as the results shown in Figure 5.6 demonstrate.

The combustion stability was improved by the higher intake temperatures, with the earlier and higher temperature combustion generally leading to reduced combustion variability as represented by variability in both heat release rate and combustion

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phasing, as shown in Figure 5.7. However, the standard deviation of SoCL was increased for the low load condition, which is consistent with a lower intensity of LTHR caused by the higher intake charge temperature.

Figure 5.7 Effects of intake manifold temperature on LTC combustion stability and emissions (Top: 1500 rpm, 8 mg/cycle, fuel injection pressure 800 bar, SoI -21°CA ATDC, EGR rate 65%; Bottom: 1500 rpm, 16 mg/cycle, fuel injection pressure 1000 bar, SoI -12°CA ATDC, EGR rate 54%)

The smoke emissions for the intermediate load condition were increased significantly with higher intake temperature despite a reduction in the intake oxygen content. The reduction in ignition delay due to the higher charge temperature and shorter NTC region in the LTC regime led to increased combustion temperature and subsequently increased soot emissions. This is reasonable, as for the LTC operating condition, the soot formation is suppressed by the low combustion temperature, and an increase in combustion temperature could lead to increased soot formation rates. However, the soot oxidation rates were not significantly increased under this mode due to low intake oxygen concentration, which is supported by the relatively small change in later-phase combustion duration as shown in Figure 5.6.

The THC and CO emissions showed different trends for the two fuelling cases with increased intake charge temperature, as shown in Figure 5.7. For the 8 mg/cycle condition, the THC was hardly affected by the higher intake charge temperature, and the CO was reduced by it. For the 16 mg/cycle condition, the increase in charge temperature led to increased THC and CO. More advanced main combustion events

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

50°C Intake O2 (%); Nox(ppm); Smoke(FSN); THC(*1k ppm); CO(%)

50°C CoV(IMEP), CoV(MaxHRR) (%); Standard Deviations (*0.1 °CA)

50°C 70°C 90°C

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normally tend to emit lower THC and CO emissions due to the higher combustion temperature and longer oxidation time before exhaust valve opening. However, the thermal ‘throttling’ effect of the higher intake charge temperature reduced the intake oxygen content; for hot EGR this has been shown to increase THC and CO emissions (Ogawa et al., 2007). The competitive effect of a higher combustion temperature and a reduction in oxygen content resulted in unchanged THC and reduced CO emissions for the low load operating condition, indicating that the higher temperature was the more dominant effect. However, at the 16 mg/cycle condition, the equivalence ratio was substantially higher and hence the reduction in oxygen content was a more dominant factor in increasing the THC and CO emissions. This resulted in a net increase in THC and CO emissions under this condition.

Higher intake charge temperature enhanced the LTC combustion process with improved combustion stability and advanced combustion phasing. It increased the peak of the main heat release rate but reduced the LTHR heat release rate. The NTC contributed to the reduced LTHR rate due to its earlier occurrence with higher intake charge temperatures. The first half of the main combustion event exhibited a higher heat release rate when the intake charge temperature was high. The reduced LTHR rate may have left more reactive reactants to burn during this stage. The smoke number was increased presumably due to the higher local combustion temperature derived from the faster main combustion event resulting from higher intake temperatures.

Other engine parameters are also able to encourage more rapid main combustion;

for example higher fuel injection pressures, which promote more rapid mixing, have been shown in Chapter 4 to enhance LTC reaction rates.