## 1 Introduction  
In the foreseeable future, internal combustion engine vehicles will still play an important role in long-distance transportation, but internal combustion engine technology faces more stringent emissions regulations [1; 2]. Some investigations have proved that adding some small-molecule oxygenated fuels into diesel can significantly reduce the CO, HC, and soot emissions, and even avoid the soot-NOx trade-off in the diesel engine combustion process [3]. In order to reduce the harmful emissions of diesel engines, alternative fuels, such as methanol, ethanol, biodiesel, dimethyl ether (DME), and polyoxymethylene dimethyl ether (PODE), have attracted more and more attention [4; 5; 6; 7]. Polyoxymethylene dimethyl ether (PODE) is one of the most promising small molecule oxygenated fuels that can be made from methanol [8; 9], which is abundant and inexpensive. Its molecular formula is CH3O-(CH2O)n-CH3, and the composition distribution of PODE obtained by different production technologies is slightly different, with n values typically ranging from 3 to 8 and 3 to 5 predominating. Table 1 shows the physical properties of several components of PODE [10], with their common characteristics being high cetane number and oxygen content, no sulfur, no aromatic hydrocarbons, and the ability to be miscible with diesel fuel [11; 12], which are physically suitable for use in diesel engines alone or blended with diesel without major changes to the diesel engine [10]. After extensive experimental studies, the potential of PODE to improve engine performance and emissions has been well documented. [PERSON] et al. [13; 14] conducted combustion visualization tests based on different PODE/diesel blends in a single-cylinder optical engine and found that particulate matter (PM) emissions were reduced by 40% when 12.5% PODE\({}_{3.5}\)/diesel was blended directly in an unmodified engine. The experimental results revealed that PODE can improve the combustion quality and reduce the soot emission due to its oxygen content. [PERSON] et al. [15] also found from engine tests that the absence of C-C bonds in PODE leads to a significant reduction in particle emissions from PODE/diesel blends compared with diesel. In addition, blended fuels can shorten the combustion duration, increase the maximum in-cylinder pressure, and improve the brake thermal efficiency (BTE) [16; 17]. Although PODE can improve engine combustion, this does not necessarily imply that a larger blending ratio is preferable. The power performance of a diesel engine will be affected by blends with a high PODE content [17], and as the blending ratio rises, so does the break-specific fuel consumption [18]. Furthermore, numerous studies have demonstrated that the addition of PODE dramatically enhanced the NO\({}_{\text{x}}\) emissions of diesel engines [19; 20]. [PERSON] et al. [3; 21] studied the combustion behavior of a pure PODE composed of about 80% PODE\({}_{3}\) and 20% PODE\({}_{4}\) on a heavy-duty single-cylinder engine and showed that the CO and HC emissions exhibited extreme peaks. The NO\({}_{\text{x}}\) emissions showed a strong dependence on the applied exhaust gas recirculation (EGR) rate, but when the stoichiometric conditions were approached by increasing the EGR rate, the engine thermal efficiency showed a decrease. Therefore, the combustion of pure PODE or a high ratio of blended fuel is of little significance to improve the fuel economy and emissions of the engine, and PODE has a better application prospect when blended with diesel at a low ratio. EGR is a good way to achieve highly efficient and clean combustion [12]. Due to the absence of C-to-C bonds, PODE considerably reduces soot formation, allowing, in turn, significantly higher EGR rates to reduce NO\({}_{\text{x}}\) emissions [10], [PERSON] et al. [22] proved that blending PODE\({}_{3}\) fuels with diesel is an effective technique to reduce soot emissions with minimal effect on NO\({}_{\text{x}}\) emissions.