**3.1 Effects of injection timing on combustion characteristics**

56 Fuel Injection in Automotive Engineering

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The operating conditions are set at 1400 r/min, 0.575MPa, 1.18-2.29 of excess air ratio (λ) (EGR rates from 0 to 33%) (referred to as case A) and 1000 r/min, 0.29MPa, 1.68-2.92 of λ

where, E, I, and A denote exhaust gas, inlet gas and atmosphere respectively.

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\*BSFC is an acronym for 'brake specific fuel consumption'.

Table 2. Specifications of the test engine

Fig. 3. Combustion experimental apparatus

EGR rate is denoted by the follow formula.

**3. Experimental results and discussions** 

Model CA6DF2 Combustion chamber reentrant Compression ratio 16.5 Ricardo swirl ratio of inlet port 2.8

> The EGR rates of case A and B are set at 28% and 80% respectively, and the injection pressure is all 110MPa. NOx, soot, HC, CO, BSFC and cylinder pressure are measured by varying the injection timing. The results are shown in Figure 4.

> It can be seen that NOx and soot emissions are simultaneously decreased by 43% and 94% respectively with retarding the injection timing from -4°ATDC to 3°ATDC in case A. For case B, NOx and soot emissions are also simultaneously decreased by 42% and84% respectively with retarding the injection timing from -4°ATDC to -1.5°ATDC, further retarding to -1°ATDC causes continuing reduction of NOx but increase of soot.

> It is not difficult to understand NOx reduction with retarding the injection timing. The heat release rates at different injection timing are shown in Figure 5. The fuel injection rate curves are also plotted in Figure 5 and set at the same start point, accordingly the corresponding heat release rate curves must be shifted. In this way, it is convenient to compare the combustion characteristics, and to distinguish if the premixed combustion at different injection timing is achieved.

> The premixed combustion of this investigation means combustion that occurs after the cyclic fuel completely injected into the combustion chamber. Therefore it is important the duration between the injection end point and the combustion start point (Shimazaki,2003). This duration affects combustion characteristics especially emissions, because it represents the degree of the premixed combustion. Here it is defined as the premixed degree duration denoted by τpmix. The cyclic fuel has not been completely injected into the combustion chamber prior to ignition when τpmix is less than zero, it means the complete premixed combustion can't be achieved, still belongs to the conventional combustion. However, the cyclic fuel has just been completely injected into the combustion chamber prior to ignition when τpmix is equal to zero, but it is short for fuel and air to completely mix, which can not form homogeneous mixture. The homogeneity of mixture tends to improve with the increase of τpmix, and accordingly soot and NOx emissions tend to decrease simultaneously when high levels of EGR were used. Soτpmix is a very important parameter to help compare between the premixed combustion and the conventional combustion.

Experimental Investigation on Premixed Combustion in a Diesel Engine with Ultra-Multihole Nozzle 59

The premixed degree duration τpmix at different injection timings is shown in Table 3. It can be seen that the premixed combustion has not been achieved at -4°ATDC of the injection timing in case A. The premixed combustion has just been achieved at -1°ATDC of the injection timing while τpmix is equal to zero. Theτpmix is equal to 0.3°CA at 3°ATDC of the injection timing, which means that it is longer for fuel and air to mix prior to ignition. The longerτpmix is, the more homogeneous mixture is. In this way, it is possible to achieve the homogeneous charge combustion, eventually soot and NOx emissions are simultaneously reduced to very low when high levels of EGR were used. This is different with the conventional combustion. For case B, the premixed combustion has already been achieved at -4°ATDC of the injection timing whileτpmix is equal to 3.43°CA, so soot is low. The τpmix is equal to 3.9°CA with retarding the injection timing to -2°ATDC, therefore soot is already very low. But it can be seen from Figure 5 that the combustion rate is very low with further retarding the injection timing to -1°ATDC, which is not beneficial to complete combustion,

> Combustion start θ3

°ATDC °ATDC °ATDC °CA °CA



HC, CO and BSFC have a slight change until 1°ATDC of the injection timing, but the further retarding injection timing causes deterioration of these performances in case A. These performances are significantly worsened after 1°ATDC of the injection timing because the combustion period is far away from the top dead center (TDC). For case B due to higher EGR rate, HC and CO emissions are swiftly increased with retarding the injection timing to after -1.5°ATDC because of slowing combustion rate, and BSFC is also worsened due to far

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

Ignition delay =θ3-θ1

Τpmix =θ3-θ2

and accordingly causes increase of soot.

away from TDC of the combustion period.

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

measured by varying EGR rates. The results are shown in Figure 6.

Injection end θ2

Table 3. Premixed degree durationτpmix at different injection timing

Injection start θ1

case

A

B

b) Effects of injection timing on HC and CO emissions

Fig. 4. Effects of injection timing on engine performance

Fig. 5. Heat release rates at different injection timing

case B


case B


case B


case B




heat release rate


0 0.001 0.002 0.003 0.004 0.005 0.006 0.007 0.008


fuel injection rate(mg/ms)

HC CO CO(g/kW.h)

NOx soot soot(g/kW.h)

a) Effects of injection timing on NOx and soot emissions

0 0.5 1 1.5 2 2.5 3

0 0.1 0.2 0.3 0.4 0.5 0.6

NOx soot

soot(g/kW.h)

NOx(g/kW.h)

b) Effects of injection timing on HC and CO emissions

CO(g/kW.h)

HC(g/kW.h)

c) Effects of injection timing on BSFC

fuel injection rate(mg/ms)

heat release rate(kJ/°CA)


fuel injection rate

BSFC(g/kW.h)

Fig. 4. Effects of injection timing on engine performance

heat release rate

0° 1° 2°

3°ADTC

case A


case A


case A


case A


0 0.5 1 1.5 2 2.5 3 3.5

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6


heat release rate(kJ/.°CA)

BSFC(g/kW.h)

HC CO

HC(g/kW.h)

NOx(g/kW.h)

Fig. 5. Heat release rates at different injection timing



fuel injection rate

The premixed degree duration τpmix at different injection timings is shown in Table 3. It can be seen that the premixed combustion has not been achieved at -4°ATDC of the injection timing in case A. The premixed combustion has just been achieved at -1°ATDC of the injection timing while τpmix is equal to zero. Theτpmix is equal to 0.3°CA at 3°ATDC of the injection timing, which means that it is longer for fuel and air to mix prior to ignition. The longerτpmix is, the more homogeneous mixture is. In this way, it is possible to achieve the homogeneous charge combustion, eventually soot and NOx emissions are simultaneously reduced to very low when high levels of EGR were used. This is different with the conventional combustion. For case B, the premixed combustion has already been achieved at -4°ATDC of the injection timing whileτpmix is equal to 3.43°CA, so soot is low. The τpmix is equal to 3.9°CA with retarding the injection timing to -2°ATDC, therefore soot is already very low. But it can be seen from Figure 5 that the combustion rate is very low with further retarding the injection timing to -1°ATDC, which is not beneficial to complete combustion, and accordingly causes increase of soot.


Table 3. Premixed degree durationτpmix at different injection timing

HC, CO and BSFC have a slight change until 1°ATDC of the injection timing, but the further retarding injection timing causes deterioration of these performances in case A. These performances are significantly worsened after 1°ATDC of the injection timing because the combustion period is far away from the top dead center (TDC). For case B due to higher EGR rate, HC and CO emissions are swiftly increased with retarding the injection timing to after -1.5°ATDC because of slowing combustion rate, and BSFC is also worsened due to far away from TDC of the combustion period.
