**2.2 Combustion experimental apparatus**

The specifications of the test engine are shown in Table 2. Its operating conditions are set at 1400 r/min, 0.575MPa and 1000 r/min, 0.279MPa. The test engine is operated on the commercially available diesel fuel with the cetane number of 51 in all the test cases. The coolant temperature is set to 80±3°C. Figure 3 gives the combustion experimental apparatus. The EGR cooler and the intercooler are water-cooled, with water circulation volume and water temperature that are adjustable. The inlet air temperature after the intercooler is maintained at 40±3°C during the whole experiment. The fuel injection is performed by a high-pressure common-rail electric-controlled system on the engine. The exhaust gas emissions are measured using a HORIBA MEXA-7100 gas analyzer, smoke density (soot) measured using an AVL 415s smoke meter, and particle matter (PM) measured using an AVL 472 partial-flow particulate sampler which allows double particulate filters to be exposed. In-cylinder pressure is acquired using a KISTLER cylinder pressure sensor.


Experimental Investigation on Premixed Combustion in a Diesel Engine with Ultra-Multihole Nozzle 57

(EGR rates from 0 to 80%) (referred to as case B). Experiments are carried out using the UMH nozzle and the original nozzle respectively. The cyclic fuel can't be completely injected into the combustion chamber before ignition because of smaller flow rate of the original nozzle, so experiment with the original nozzle can only achieve the conventional combustion. It means that this combustion can't eliminate the trade-off relationship between reduction of NOx and soot emissions, and accordingly the engine with the original nozzle is only tested in original condition. Experiments with the UMH nozzle, however, are carried out by adjusted EGR rates, injection pressures and injection timings to achieve the lowtemperature premixed combustion. These optimum parameters (include EGR rate, injection pressure and injection timing) are different for case A and B to achieve the minimum values of NOx and soot emissions while keep the break specific fuel consumption (BSFC) not to be

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

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

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

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

retarding to -1°ATDC causes continuing reduction of NOx but increase of soot.

between the premixed combustion and the conventional combustion.

significantly deteriorated because their excess air ratios are different.

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

varying the injection timing. The results are shown in Figure 4.

injection timing is achieved.


\*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.

$$\text{EGR(\%)} = \frac{\text{CO}\_{2,1} - \text{CO}\_{2,A}}{\text{CO}\_{2,E} - \text{CO}\_{2,I}}$$

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

#### **3. Experimental results and discussions**

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 (EGR rates from 0 to 80%) (referred to as case B). Experiments are carried out using the UMH nozzle and the original nozzle respectively. The cyclic fuel can't be completely injected into the combustion chamber before ignition because of smaller flow rate of the original nozzle, so experiment with the original nozzle can only achieve the conventional combustion. It means that this combustion can't eliminate the trade-off relationship between reduction of NOx and soot emissions, and accordingly the engine with the original nozzle is only tested in original condition. Experiments with the UMH nozzle, however, are carried out by adjusted EGR rates, injection pressures and injection timings to achieve the lowtemperature premixed combustion. These optimum parameters (include EGR rate, injection pressure and injection timing) are different for case A and B to achieve the minimum values of NOx and soot emissions while keep the break specific fuel consumption (BSFC) not to be significantly deteriorated because their excess air ratios are different.
