## 1 Introduction
Its chemical formula only has C-H and C-O bonds, but no C-C bond, thus it produces practically no soot emission and a significant reduction of CO, HC, \(\mathrm{NO_{x}}\) concentrations in the exhaust gas [28]. [PERSON] et al. [29] revealed the effects of adding DME on the gasoline engine and found that HC and \(\mathrm{NO_{x}}\) emissions were diminished. Similarly, [PERSON] et al. [30] found that combustion stability was improved, and CO emission was decreased when increasing the amount of DME in the mixture with gasoline. Furthermore, the experimental results showed that the laminar burning velocities of the traditional fuels in the air could be improved with the addition of DME [31; 32]. The presence of a small amount of DME in the traditional fuel could improve the ignition quality of SI engines [33]. A study by [PERSON] et al. [34] showed that the range of engine loads was widened with the addition of DME to LPG. They pointed out that 20% of DME-blended fuel could be used as an optimal alternative fuel for LPG engines [33]. Biogas is produced from organic wastes such as agricultural waste, cattle, and poultry manures, which are abundant in the rural areas of tropical regions. It contains mainly CH\({}_{4}\) and CO\({}_{2}\) with variable compositions depending on primary resources. Also, biogas has a high octane number and high auto-ignition temperature. Biogas is proved to be an excellent renewable fuel for stationary engines [6]. Due to the high octane number, biogas is a potential fuel for application on a high compression ratio SI engine converted from diesel engine [35; 36]. \(\mathrm{NO_{x}}\) and smoke emissions in this engine decreased significantly with the biogas-fueled engine, whereas HC and CO emissions increased marginally [37]. Although the unique properties of biogas are beneficial for enhancing engine performance, there are still some problems that arise when the biogas engines are applied in practice. The presence of CO\({}_{2}\) in biogas improves the anti-knocking property of the fuel but reduces burning velocity, which affects the thermal efficiency and pollutant emission of the engines [38]. The application of biogas in conventional engines shows some limitations, but biogas can be used more effectively in the new combustion strategy engine, such as HCCI engines [39]. As mentioned above, to improve combustion efficiency, different fuel blends should be fueled in engines instead of a single fuel [40]. As the engine operates under low load conditions, the increase of high cetane-rating fuels in the blend with biogas is needed to improve the ignition performance. By contrast, the addition of high cetane-rating fuel should be reduced as the engine operates at a high load regime [41]. In this concept, the biogas-DME hybrid fuel engines are desirable. The properties of low-temperature oxidation and the high cetane number of DME can lead to abnormal ignition, but it can serve as an addition to improve the combustion of the biogas engine [42; 43]. Due to the high cetane number of DME, its unique application in dual-fuel operation mode leads to the early start of the combustion phenomena, particularly at a relatively high load regime [27]. [PERSON] et al. [44] studied the DME-diesel dual-fuel combustion in a naturally aspirated engine and concluded that as the DME quantity increased, the start of combustion advanced. Hence, for practical usage of DME in dual combustion mode, additional fuel for the combustion inhibitor is needed. [PERSON] et al. [45] used LPG to the control combustion phase of the LPG-DME HCCI engine. [PERSON] et al. [46] used the exhaust gas recirculation (EGR) rate to control the combustion and emission characteristics of the DME-diesel dual-fuel PCCI engine. Generally, in practical application, if DME is the main fuel, the addition of a high octane-rating fuel for a combustion inhibitor is needed. By contrast, if a high octane-rating fuel is used as the main fuel, DME can be used as a combustion enhancer. It is the case of a biogas DME hybrid dual-fuel engine.