The Effect of Methane Addition on the Low-Temperature Oxidation Preparation and the Thermal Ignition Preparation of Dimethyl Ether Under Representative Engine In-Cylinder Thermal Conditions

Dimethyl ether (DME) is a highly reactive diesel substitute that can be used as a pilot fuel to ignite low- reactivity methane (CH4) in heavy-duty engines. To optimize the efficiency and emissions of CH4/DME dual-fuel engines, it is crucial to study the fundamental combustion characteristics of DME mixed with methane. This study focuses on the influence of CH4 addition on the low-temperature oxidation (LTO) preparation stage and the thermal ignition (TI) preparation stage of DME in the two-stage ignition process, as these two stages respectively control the ignition delay of the first and second stages. The comparison is made between pure DME and a 50% CH4 and 50% DME blended fuel, operating under thermodynamic conditions representing the engine in- cylinder environment at 30 atm pressure, 650K temperature, and a stoichiometric equivalence ratio. The results show that the addition of methane hardly affects the control mechanism of the two-stage ignition of DME. Specifically, the LTO preparation stage is still promoted by the increase in OH radicals in the DME’s low-temperature oxygenation pathways to form KET, and the second stage is still controlled by the H₂O₂ loop mechanism. The kinetic analysis also reveals that methane addition can compete for some of the OH radicals in the LTO preparation stage, which has a suppressing effect on ignition. However, in the TI preparation stage, methane can promote the loop reaction of OH→HO₂→H₂O₂→OH and promote ignition. For the operating conditions studied here, although methane consumes a total of 7.24% prior to thermal ignition, it only consumes 0.63% of the total amount in the LTO preparation stage and 1.81% of the total amount in the TI preparation stage. This indicates that methane is mainly consumed in the LTO stage, accounting for 65% of the total methane consumption amount. It can be concluded that the kinetic effect of methane has a relatively small impact on the ignition delay of the DME, at least for the conditions investigated here. In other words, the dilution and thermal effects caused by adding methane are the main reasons for the prolonged time of LTO/TI preparation. Overall, more fundamental research is warranted to understand the role of methane in the two-stage ignition process of DME, which could facilitate the development of CH4/DME dual-fuel engines.