Energy Conversion Efficacy of Neat Dimethyl Ether Combustion with Heat Release Characterization and Emission Analysis

Dimethyl ether (DME) is widely regarded as a suitable energy source for compression ignition power systems because of its high reactivity. It has been widely reported that DME possesses a significantly low propensity to form soot, hindering the innate NOx-soot trade-off encountered with diesel fuel operation. Beyond the fuel-borne oxygen content of DME, its unique physical properties present a contrasting combustion behavior which may be advantageous to direct injection systems, especially concerning the mixing-controlled combustion mode. This work aims to detail the energy conversion efficacy of DME through heat release characterization and exhaust emission speciation. The tests were controlled within a single-cylinder research engine with an off-board high-pressure injection system to handle liquified DME up to 1000bar. To mitigate interference in fuel additives over the combustion behavior, the high-pressure fuel system specifically managed neat DME. The in-cylinder pressure was the indicator for combustion behavior whereas exhaust emissions were sampled with Fourier transform infrared spectroscopy (FTIR) for chemical speciation. The in-cylinder combustion profile was aligned with the actual rate of injection to designate the characteristics between the injection and combustion events. Overall, the fuel-to-heat conversion efficiency is comparable, while the combustion efficiency is slightly greater owing to lower carbon monoxide emissions. Without injection-combustion overlap, i.e. low load and primarily premixed combustion, the heat release pattern of DME was like diesel. Under extended injection-combustion overlap, diesel finished injection near 30% to 40% of cumulative heat release whereas DME injection finished near 50 to 75%. The end of the injection was followed by a sharp drop in heat release, e.g. the end of combustion. The application of exhaust gas dilution, however, altered the end of the combustion pattern and promote carbon monoxide emissions.