# Mechanism of Combustion Noise Influenced by Pilot Injection in PPCI Diesel Engines
## 1 Introduction  
Recently, 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. That is, these advanced combustion concepts cannot be applied for high load operating cases because the fast fuel-burning and heat release of premixed combustion will deteriorate combustion noise. However, 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]. In this way, the reduction of combustion noise by pilot injection has the potential to extend the PPCI mode to the full operating condition. Additionally, higher combustion efficiency, and lower CO\({}_{2}\) and NOx emissions can be obtained using pilot injection in the PPC mode compared with conventional diesel combustion [6; 7; 8]. Moreover, the advantages of pilot injection have gained increasing attention, particularly of researchers, regarding the detailed exploration of the effects of pilot injection on combustion noise. It has already been shown that increasing the fuel quantity of the pilot injection or advancing pilot injection timing will result in a progressive decrease of combustion noise [9; 10]. However, as reported by [PERSON] et al. [11], the combustion noise can be minimized in the conditions of the moderate pilot injection quantity and dwell. In addition, [PERSON] et al. [12] determined that the minimum combustion noise can be obtained when the energizing dwell between the main and pilot injections decreases below 200 \(\upmu\)s. Therefore, throughout these previous combustion noise investigations, it is observed that the conclusions drawn with respect to the pilot injection parameters required to suppress combustion noise are inconsistent. This is probably due to differences of the experimental conditions, but what is foremost is that the influences of pilot injection on combustion noise cannot be interpreted clearly. Meanwhile, the evaluation of combustion noise for situations of varying pilot injection strategies is particularly challenging. In order to determine the detailed relationship between pilot injection and combustion noise, a robust method is required. Over the last 20 years, numerous methods have been investigated for combustion noise evaluation and analysis. Among these methods, the maximum of the in-cylinder pressure rise rate (MPRR) has been the most intuitive tool for combustion noise assessment [13; 14; 15; 16; 17]. In addition, researchers have also demonstrated the existence of an overall relationship between the combustion noise and peak of heat release rate (HRR) [18; 19; 20]. It is worth noting that pressure spectrum analysis was of utmost importance in combustion noise assessment. [PERSON] et al. proposed a novel combustion noise assessment method on the basis of in-cylinder pressure spectrum decomposition [14; 21; 22]. Besides, cylinder pressure levels (CPLs) have been widely used in combustion noise analysis [13; 19; 23; 24], and in our previous work, CPLs was synthesized in several frequency segments to investigate the effects of pilot injection on combustion noise [25]. Unfortunately, the CPLs or synthesized CPLs can reveal the overall noise level only in the frequency domain, and the lack of time-domain information limits combustion noise analysis. However, with the application of signal processing techniques to in-cylinder pressure signals, wavelet transforms (WT) and time-frequency analysis have been proposed to further study the relationship between combustion noise and in-cylinder pressure [23; 26; 27; 28].