**3.3 Effects of injection pressure (rail pressure) on combustion characteristics**

The injection timing of case A and B are set at 2°ATDC and -1.5°ATDC respectively, EGR rates are set at 28% and 80% respectively. NOx, soot, HC, CO, BSFC and cylinder pressure are measured by varying the injection pressure. The results are shown in Figure 8.

It can be seen that soot, HC, CO and BSFC have some certain reduction except NOx with the injection pressure increase in case A. For case B, these changes are almost the same as case A when the injection pressure is less than 110MPa. But these performances are deteriorated

Experimental Investigation on Premixed Combustion in a Diesel Engine with Ultra-Multihole Nozzle 63

injection pressure increase from 90MPa to 110MPa, but the higher injection pressure can improve mixing of fuel and air, so soot can be significantly reduced. Over spray penetration,

however, leads to bad performance when the injection pressure reaches 120MPa.

Injection end θ2

fuel injection rate(mg/ms)

heat release rate(kJ/°CA)

Table 5. Premixed degree duration τpmix at different injection pressure

°ATDC MPa °ATDC °ATDC °CA °CA


**3.4 Combustion characteristics comparison between UMH nozzle and original nozzle**  Figure 10 shows that the relationship of NOx and soot emissions is compared between the UMH nozzle and the original nozzle (original engine). Because engine with the original nozzle can't achieve the complete premixed combustion and still belongs to the conventional combustion, which trade-off relationship between reduction of NOx and soot emissions can't be overcome. So the engine with the original nozzle is only tested in the original condition, this result is used to compare with that of the UMH nozzle. The engine with the UMH nozzle, however, can achieve the premixed combustion to obtain simultaneous reduction of NOx and soot emissions. Therefore it is tested by adjusted the EGR rate, injection pressure and injection timing to obtain a series of values of NOx and soot emissions. It can be seen that NOx is high but soot very low without EGR, and then NOx is swiftly decreased with the EGR rate increase. In sum, for case A, NOx and soot emissions can simultaneously achieve minimum values when EGR rate is 28%, and combing suitable injection pressure and injection timing. For case B, NOx is very low when EGR rate is 80%, and soot is also simultaneously reduced to very low value combing suitable injection pressure and injection timing. Table 6 shows combustion characteristics comparison

Combustion start θ3

fuel injection rate

90 75

110

110 12 12.1 10.1 0.1 90 12.5 12.8 10.8 0.3 75 13.4 12.9 10.9 -0.5

120 4 7.72 9.22 3.72 110 4.1 8.2 9.7 4.1 90 4.2 8.92 9.22 4.72 75 4.7 9.5 11 4.8

Ignition delay =θ3-θ1


case B

120

<sup>110</sup> <sup>90</sup>

heat release rate

75MPa

τpmix =θ3-θ2

fuel injection rate(mg/ms)

Fig. 9. Heat release rates at different injection pressure

case A

heat release rate 110 75MPa 90

1 6 11 16 21 26 31 crank angle(°)

75

Injection pressure

case


heat release rate(kJ/°CA)

A 2

B -1.5

Injection start θ1

fuel injection rate

<sup>110</sup> <sup>90</sup>

with the injection pressure further increase. This is maybe due to the spray wallimpingement with the injection pressure further increase.

a) Effects of injection pressure on NOx and soot emissions

b) Effects of injection pressure on HC and CO emissions

Fig. 8. Effects of injection pressure on engine performance

Figure 9 shows the heat release rates and Table 5 shows the premixed degree duration τpmix at different injection pressures. It can be seen that combustion advance and heat release peak increase with the injection pressure increase, and thus NOx is increased. But the advance of injection end point leads to longerτpmix with the injection pressure increase. In case A, τpmix increases from -0.5°CA to 0.1°CA with the injection pressure increase from 75MPa to 110MPa, so soot is decreased. Additionally, BSFC can be improved due to the combustion advance with the injection pressure increase. Higher injection pressure, of course, can improve mixing of fuel and air, finally improve combustion. Therefore HC and CO emissions are also decreased. In case B, althoughτpmix keeps almost unchanged with the injection pressure increase from 90MPa to 110MPa, but the higher injection pressure can improve mixing of fuel and air, so soot can be significantly reduced. Over spray penetration, however, leads to bad performance when the injection pressure reaches 120MPa.

Fig. 9. Heat release rates at different injection pressure

62 Fuel Injection in Automotive Engineering

with the injection pressure further increase. This is maybe due to the spray wall-

case B

NOx soot

70 80 90 100 110 120 injection pressure(MPa)

case B

HC CO

70 80 90 100 110 120 injection pressure(MPa)

case A

70 80 90 100 110 injection pressure(MPa)

0 0.005 0.01 0.015 0.02

CO(g/kW.h)

soot(g/kW.h)

a) Effects of injection pressure on NOx and soot emissions

0 0.5 1 1.5 2

0 0.02 0.04 0.06 0.08 0.1

soot(g/kW.h)

NOx(g/kW.h)

b) Effects of injection pressure on HC and CO emissions

BSFC(g/kW.h)

HC(g/kW.h)

CO(g/kW.h)

Figure 9 shows the heat release rates and Table 5 shows the premixed degree duration τpmix at different injection pressures. It can be seen that combustion advance and heat release peak increase with the injection pressure increase, and thus NOx is increased. But the advance of injection end point leads to longerτpmix with the injection pressure increase. In case A, τpmix increases from -0.5°CA to 0.1°CA with the injection pressure increase from 75MPa to 110MPa, so soot is decreased. Additionally, BSFC can be improved due to the combustion advance with the injection pressure increase. Higher injection pressure, of course, can improve mixing of fuel and air, finally improve combustion. Therefore HC and CO emissions are also decreased. In case B, althoughτpmix keeps almost unchanged with the

c) Effects of injection pressure on BSFC

impingement with the injection pressure further increase.

NOx soot

> HC CO

case A

70 80 90 100 110 120 injection pressure(MPa)

case A

70 80 90 100 110 injection pressure(MPa)

case B

70 80 90 100 110 120 injection pressure(MPa)

0 0.5 1 1.5 2 2.5

0 0.5 1 1.5 2 2.5 3

BSFC(g/kW.h)

HC(g/kW.h)

NOx(g/kW.h)

Fig. 8. Effects of injection pressure on engine performance


Table 5. Premixed degree duration τpmix at different injection pressure

### **3.4 Combustion characteristics comparison between UMH nozzle and original nozzle**

Figure 10 shows that the relationship of NOx and soot emissions is compared between the UMH nozzle and the original nozzle (original engine). Because engine with the original nozzle can't achieve the complete premixed combustion and still belongs to the conventional combustion, which trade-off relationship between reduction of NOx and soot emissions can't be overcome. So the engine with the original nozzle is only tested in the original condition, this result is used to compare with that of the UMH nozzle. The engine with the UMH nozzle, however, can achieve the premixed combustion to obtain simultaneous reduction of NOx and soot emissions. Therefore it is tested by adjusted the EGR rate, injection pressure and injection timing to obtain a series of values of NOx and soot emissions. It can be seen that NOx is high but soot very low without EGR, and then NOx is swiftly decreased with the EGR rate increase. In sum, for case A, NOx and soot emissions can simultaneously achieve minimum values when EGR rate is 28%, and combing suitable injection pressure and injection timing. For case B, NOx is very low when EGR rate is 80%, and soot is also simultaneously reduced to very low value combing suitable injection pressure and injection timing. Table 6 shows combustion characteristics comparison

Experimental Investigation on Premixed Combustion in a Diesel Engine with Ultra-Multihole Nozzle 65

3. Case B can use higher EGR rate to achieve much more lean-homogeneous charge premixed combustion because the excess air ratio of case B is higher than that of case A. Therefore it can obtain remarkably simultaneous reduction of NOx and soot emissions

This study was financially supported by Wuxi Fuel Injection Equipment Research Institute. The authors wish to express their gratitude to the Executive of Wuxi Fuel Injection

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and slight increase of BSFC by 8.7% compared with the original engine.

the injection timing is -1.5°ATDC.

**5. Acknowledgements** 

Equipment Research Institute.

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**6. References** 

0745.

0200.

01-0742.

1744.

28% (excess air ratio is 1.49), the injection pressure is 110MPa and the injection timing is 2°ATDC. For case B, however, NOx and soot emissions of the UMH nozzle compared with the original nozzle are simultaneously reduced by 78.1% and 76.2% respectively when the EGR rate is 80% (excess air ratio is 1.68), the injection pressure is 110MPa and

between the UMH nozzle and the original nozzle. NOx and soot emissions of the UMH nozzle are reduced by 68.2% and 20% respectively than that of the original nozzle in case A, but BSFC increases by 10.4%. For case B, NOx and soot emissions of the UMH nozzle are reduced by 78.1% and 76.2% respectively than that of the original nozzle, but BSFC only increases by 8.7%. This is because the excess air ratio of case B is higher than that of case A. Therefore case B can use higher EGR rate than case A, which leads to the achievement of much more lean-homogeneous charge combustion.

Fig. 10. NOx—soot emissions comparison between UMH and original nozzle


Table 6. Combustion characteristics comparison between UMH and original nozzle

### **4. Conclusions**


28% (excess air ratio is 1.49), the injection pressure is 110MPa and the injection timing is 2°ATDC. For case B, however, NOx and soot emissions of the UMH nozzle compared with the original nozzle are simultaneously reduced by 78.1% and 76.2% respectively when the EGR rate is 80% (excess air ratio is 1.68), the injection pressure is 110MPa and the injection timing is -1.5°ATDC.

3. Case B can use higher EGR rate to achieve much more lean-homogeneous charge premixed combustion because the excess air ratio of case B is higher than that of case A. Therefore it can obtain remarkably simultaneous reduction of NOx and soot emissions and slight increase of BSFC by 8.7% compared with the original engine.
