## 1 Introduction
In order to optimize the operational range and improve the performance of biogas-diesel dual-fuel engines, DME is mixed with biogas due to its high cetane number and low boiling point. Based on the above properties of DME, the biogas-DME blend can be easily ignited and burnt. As biogas is a high octane number, and DME is a high cetane rating, the biogas-DME blend with the various mixing ratios has different octane ratings. Thus, under a specific operating condition, the most suitable octane-rating mixture could be obtained by adjusting the blending ratio of the biogas-DME mixture properly. This helps to achieve high efficiency and low emissions of the dual-fuel engine under different operating conditions. Experimental results showed that in a biogas-diesel dual-fuel engine, \(\mathrm{NO_{x}}\) and smoke emissions decreased significantly [37]. [PERSON] et al. [47] studied the combustion and emission characteristics of biogas-diesel dual-fuel combustion engine and found that \(\mathrm{NO_{x}}\) and smoke emissions were lower by about 39% and 49%, compared to that of the diesel engine. [PERSON] et al. [48] carried out the performance research in a compressed natural gas (CNG)-diesel dual-fuel engine and showed that a simultaneous reduction of \(\mathrm{NO_{x}}\) and soot could be realized. The research of [PERSON] et al. [49; 50] on dual-fuel engine fuel with hydrogen-enriched biogas showed that the soot emission could be significantly reduced with an increase in hydrogen content in the fuel mixture. [PERSON] and [PERSON] [51] indicated that soot and \(\mathrm{NO_{x}}\) could be reduced by biogas dual-fuel combustion by 70% and 37%, respectively, using natural gas or biogas-diesel dual-fuel engine systems. The low burning velocity of biogas causes the incomplete combustion. This in turn increases HC and CO emissions, which leads to the lack of fitness in the biogas dual-fuel engine. With the addition of DME in the biogas DME hybrid dual-fuel engine, this problem can be overcome. In previous works, we have studied biogas dual-fuel engines [49; 50; 52]. We found that the dual combustion mode is an efficient solution for biogas applications on internal combustion engines. However, the variation of the fuel ratio between biogas and diesel could not improve the flame speed of biogas, which affects the engine performance and emissions in certain operating conditions. Research into technology to improve the flame speed of biogas is, thus, important for an efficient application of the fuel in the internal combustion engine. Due to the high cetane number, DME is a potential substitution of diesel fuel in the CI engine, but it is not an appropriate fuel for the SI engine. However, DME can be served as a combustion enhancer of biogas in dual fuel combustion mode because biogas has a high octane number. This is the new concept in a biogas-DME hybrid dual-fuel engine, which seems to be rarely found in the literature. The purpose of this study is a contribution to develop the new concept of a biogas-DME hybrid dual-fuel engine in which DME is a combustion enhancer. The study will be carried out by simulation on a retrofitted mono-cylinder diesel engine. The effects of DME content, biogas compositions and diesel injection on performance and emissions, particularly soot emission, of the engine will be examined in detail.