Compression ignition engines used in heavy-duty applications are typically powered by diesel fuel. The high energy density and feedstock abundance provide a continuing source for the immense energy demand. However, the heavy-duty transportation sector is challenged with lowering greenhouse gas and combustion by-product emissions, including carbon dioxide, nitrogen oxides, and particulate matter. The continuing development of engine management and combustion strategies has proven the ability to meet current regulations, particularly with higher fuel injection pressure. Nonetheless, a transition from diesel to a renewable alternative fuel source will play a significant role in reducing greenhouse gases while maintaining the convenience and energy density inherent in liquid fuels. Dimethyl ether is a versatile fuel that possesses combustion properties suitable for compression ignition engines and physical properties helpful for clean combustion. The higher volatility of DME may permit lower injection pressure than diesel fueling systems to achieve adequate atomization and mixing. In this work, an empirical study of the DME fuel injection pressure was conducted based on combustion and emission characteristics.
The DME injection pressure was tested from 200bar to 770bar. A plunger-type injection system with an enlarged high-pressure reservoir was adopted to manage steady fuel injection pressures at 1200rpm. Each injection pressure condition was subject to a full-range exhaust gas dilution sweep into low-temperature combustion. The low sooting propensity of DME was apparent as engine-out soot emissions were below 2mg/kWh under all conditions. A higher injection pressure showed higher NOx emissions up to 70% EGR, thereafter NOx emissions persisted similarly low. A prominent improvement in combustion efficiency was observed from 325bar to 500bar followed by a minor improvement to 660bar. Correspondingly, exhaust gas speciation showed a similar trend in hydrogen, methane, and unburned DME emissions.
