Abstract
Pilot injection combined with exhaust gas recirculation (EGR) is usually utilized to realize the partially premixed compression ignition (PPCI) mode in diesel engines, which enables the simultaneous decrease of nitrogen oxide and soot emissions to satisfy emission regulations. Moreover, the ignition delay of main injection combustion can also be shortened by pilot injection, and then combustion noise is reduced. Nevertheless, the mechanisms of pilot injection impacts on combustion noise are not completely understood. As such, it is hard to optimize pilot injection parameters to minimize combustion noise. Therefore, experiments were conducted on a four-stroke single-cylinder diesel engine with different pilot injection strategies and 20% EGR as part of an investigation into this relationship. Firstly, the combustion noise was analyzed by cylinder pressure levels (CPLs). Then, the stationary wavelet transforms (SWTs) and stationary wavelet packet transform (SWPT) were employed to decompose in-cylinder pressures at different scales, and thus the combustion noise generated by pilot and main combustion was investigated in both the time and frequency domain. The results show that pilot injection is dominant in the high frequency segment of combustion noise, and main injection has a major impact on combustion noise in the low and mid frequency segment. Finally, the effects of various pilot injection parameters on suppressing combustion noise were analyzed in detail.
Highlights
The requirement of continuous mitigation of particulate matter (PM) and nitrogen oxide (NOx) emissions has become a significant challenge regarding the improvement of combustion in diesel engines
The application of the pilot-main injection strategy to the premixed charge compression ignition (PCCI) combustion mode can reduce combustion noise by shortening the ignition delay (ID) of main injection, which is introduced as partially premixed compression ignition (PPCI) [4,5]
As discussed in the following paragraphs, the in-cylinder pressures of the PPCI diesel engine were processed by several methods
Summary
The requirement of continuous mitigation of particulate matter (PM) and nitrogen oxide (NOx) emissions has become a significant challenge regarding the improvement of combustion in diesel engines Advanced combustion concepts, such as low-temperature combustion (LTC) and premixed charge compression ignition (PCCI) [1,2,3], have arisen as a solution for the reduction of emissions, but some limitations must still be faced. The application of the pilot-main injection strategy to the PCCI combustion mode can reduce combustion noise by shortening the ignition delay (ID) of main injection, which is introduced as partially premixed compression ignition (PPCI) [4,5]. Higher combustion efficiency, and lower CO2 and NOx emissions can be obtained using pilot injection in the PPC mode compared with conventional diesel combustion [6,7,8]
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