Optical investigation of propane-dimethyl ether (DME) fuel blends under compression-ignition engine conditions

Propane, DME, and propane-dimethyl ether (DME) blends were investigated in an optically accessible engine under compression ignition (CI) engine conditions. Schlieren imaging was used to provide information on the physical development of fuel jets under non-reacting conditions. OH* chemiluminescence imaging and in-cylinder pressure data were used to assess the combustion characteristics of propane, DME, and propane-DME blends varying from 10% to 30% DME by mass. First, near-top dead center (TDC) single-injection experiments were carried out to examine the impact of injection timing, intake temperature, and injected fuel energy on the ignition and combustion process. Ideal start of injection (SOI) timing was determined based on maximizing indicated efficiency and varied with DME concentration from -13.25 CAD for propane and -8.25 CAD for the 70% propane-30% DME blend. Below an intake temperature of 180 ◦C, combustion efficiency significantly decreased for propane and the 90% propane-10% DME blend. The lack of mixing-controlled compression ignition (MCCI) combustion using a single-injection strategy necessitated a dual-injection strategy. For the dual-injection experiments, pilot SOI timing was swept from -50 CAD to -20 CAD, and main injection timing was held constant at -5 CAD. Propane only exhibited MCCI behavior at the most delayed pilot SOI, while the 70% propane-30% DME blend exhibited it at all pilot SOI timings. Lastly, using a similar dual-injection strategy, a continuously operated glow plug was used to assist ignition at a reduced intake temperature and achieve reliable mixing controlled combustion. Under low load and compressed gas temperatures representative of cold start conditions, the combustion of the fuel jets nearest the glow plug was reliable, but the ignition of the fuel jets far from the glow plug was improbable, and combustion efficiency was low. Increasing fueling led to better azimuthal propagation of the pilot combustion and increased momentum of the pilot combustion products, positioning them closer to the injector before the main injection. This facilitated the autoignition of more fuel jets compared to the lower load case and an appreciable increase in combustion efficiency.