**3.2 Effects of EGR rate on combustion characteristics**

The injection timing of case A and B are set at 2°ATDC and -1.5°ATDC respectively, injection pressure is all 110MPa. NOx, soot, HC, CO, BSFC and cylinder pressure are measured by varying EGR rates. The results are shown in Figure 6.

Experimental Investigation on Premixed Combustion in a Diesel Engine with Ultra-Multihole Nozzle 61

in case A due to lower excess air ratio (lower oxygen concentration). Longer τpmix, however, dominates combustion process compared to oxygen concentration decrease when the EGR rate is high in case B due to higher excess air ratio. There is much time for fuel and air to mix prior to ignition with the aid of EGR. This is beneficial to the formation of a homogeneous mixture.

HC, CO and BSFC have a slight change when the EGR rate is less than 22%, then begin to increase with further increase the EGR rate, especially worsen when the EGR rate reaches 33% in case A. For case B, however, HC, CO and BSFC have a slight change when EGR rate is less than 30%, then begin to worsen with further increase of the EGR rate. This is because

Therefore soot has been greatly decreased again when EGR rate is high in case B.

Injection end θ2

fuel injection rate(mg/ms)

heat release rate(kJ/°CA)

Table 4. Premixed degree duration τpmix at different EGR rates

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

are measured by varying the injection pressure. The results are shown in Figure 8.

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

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

Combustion start θ3

% °ATDC °ATDC °ATDC °CA °CA


0 2 12 9.7 7.7 -2.3 22 2 12 11.6 9.6 -0.4 28 2 12 12.1 10.1 0.1 33 2 12 12.6 10.6 0.6

0 -1.5 4.1 6.94 8.44 2.84 20 -1.5 4.1 6.95 8.45 2.85 40 -1.5 4.1 7.52 9.02 3.42 50 -1.5 4.1 7.73 9.23 3.63 60 -1.5 4.1 7.75 9.25 3.65

Ignition delay =θ3-θ1

case B

0% 20% 40% 50% 60%

heat release rate


80% fuel injection rate

τpmix =θ3-θ2

fuel injection rate(mg/ms)

of an incomplete combustion causing with the EGR rate increase.

33%

heat release rate

Fig. 7. Heat release rates at different EGR rates

0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 crank angle(°)

case A

0%

22%

28%

Injection start θ1

case


heat release rate(kJ/°CA)

A

B

EGR rate

fuel injection rate

b) Effects of EGR rate on HC and CO emissions

c) Effects of EGR rate on BSFC

Fig. 6. Effects of EGR rate on engine performance

It can be seen from Figure 6 that NOx is linearly decreased with the increase of the EGR rate. For case A, NOx is decreased by 71% and 89% respectively when the EGR rate is from zero to 28% and 33%. Soot is almost unchanged when the EGR rate is less than 28%, but further increasing the EGR rate causes swift increase of soot emission. For case B, NOx is decreased by 88% when the EGR rate is from zero to 80%, but soot has a complicated tendency. Firstly soot has a slight change when the EGR rate is low, then reaches the maximum value when the EGR rate is 20%, however soot begins to decrease swiftly with further increase of the EGR rate.

Figure 7 shows the heat release rates and Table 4 shows the premixed degree duration τpmix at different EGR rates. The reduction of oxygen concentration with EGR causes NOx decrease and soot increase, but on the other hand, longerτpmix due to EGR causes soot decrease. Therefore effects of EGR on soot emission are as follows. Lower EGR rates don't cause significant change of soot. But soot is deteriorated when the EGR rate is more than 28% in case A due to lower excess air ratio (lower oxygen concentration). Longer τpmix, however, dominates combustion process compared to oxygen concentration decrease when the EGR rate is high in case B due to higher excess air ratio. There is much time for fuel and air to mix prior to ignition with the aid of EGR. This is beneficial to the formation of a homogeneous mixture. Therefore soot has been greatly decreased again when EGR rate is high in case B.

HC, CO and BSFC have a slight change when the EGR rate is less than 22%, then begin to increase with further increase the EGR rate, especially worsen when the EGR rate reaches 33% in case A. For case B, however, HC, CO and BSFC have a slight change when EGR rate is less than 30%, then begin to worsen with further increase of the EGR rate. This is because of an incomplete combustion causing with the EGR rate increase.

Fig. 7. Heat release rates at different EGR rates

60 Fuel Injection in Automotive Engineering

caseB

0 10 20 30 40 50 60 70 80 90 EGR rate(%)

caseB

0 10 20 30 40 50 60 70 80 90 EGR rate(%)

caseB

0 20 40 60 80 100 EGR rate(%)

0 0.0005 0.001 0.0015 0.002 0.0025

CO(g/kW.h)

NOx soot

soot(g/kW.h)

a) Effects of EGR rate on NOx and soot emissions

> HC CO

soot(g/kW.h)

NOx(g/kW.h)

0 0.05 0.1 0.15 0.2 0.25 0.3

b) Effects of EGR rate on HC and CO emissions

HC(g/kW.h)

CO(g/kW.h)

c) Effects of EGR rate on BSFC

BSFC(g/kW.h)

It can be seen from Figure 6 that NOx is linearly decreased with the increase of the EGR rate. For case A, NOx is decreased by 71% and 89% respectively when the EGR rate is from zero to 28% and 33%. Soot is almost unchanged when the EGR rate is less than 28%, but further increasing the EGR rate causes swift increase of soot emission. For case B, NOx is decreased by 88% when the EGR rate is from zero to 80%, but soot has a complicated tendency. Firstly soot has a slight change when the EGR rate is low, then reaches the maximum value when the EGR rate is 20%, however soot begins to decrease swiftly with further increase of the EGR rate.

Figure 7 shows the heat release rates and Table 4 shows the premixed degree duration τpmix at different EGR rates. The reduction of oxygen concentration with EGR causes NOx decrease and soot increase, but on the other hand, longerτpmix due to EGR causes soot decrease. Therefore effects of EGR on soot emission are as follows. Lower EGR rates don't cause significant change of soot. But soot is deteriorated when the EGR rate is more than 28%

Fig. 6. Effects of EGR rate on engine performance

0 10 20 30 40 EGR rate(%)

case A

NOx soot

0 10 20 30 40 EGR rate(%)

case A

0 10 20 30 40 EGR rate(%)

case A

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6

BSFC(g/kW.h)

HC CO

HC(g/kW.h)

NOx(g/kW.h)


Table 4. Premixed degree duration τpmix at different EGR rates
