Compensation for the differences in LHV of diesel-OME blends by using injector nozzles with different number of holes: Emissions and combustion

The reduction of particulate matter (PM) emissions has become a major topic in the development of compression ignition (CI) engines. The aftertreatment of exhaust gases by means of particulate traps (DPF) has given promising results in terms of effectiveness. However, the backpressure and necessity to regenerate causes an increase in fuel consumption and operational cost. A possible strategy to reduce this cost is the abatement of PM levels of engine out exhaust gases by employing oxygenated fuels. This solution reduces the backpressure in the exhaust as well as fuel expensive regeneration events. In the present work the performances, in terms of combustion and emissions, of two different blends composed of diesel and a mixture of polyoxymethylene dimethyl ether (OME, POMDME or PODE) were investigated in a single-cylinder, direct injection, 4-stroke, heavy duty diesel engine. In order to minimize the influence on combustion characteristics (i.e. ignition delay, premixed and diffusion combustion portion, etc.), the compensation for the smaller caloric value of the blends in comparison to diesel was performed with an increased number of injector nozzles holes. This strategy was chosen to avoid effects on emissions caused by a prolonged duration of injection, increased fuel pressure or increased nozzle diameter. In total three different number of nozzle holes were tested with a reference diesel case (7-hole) and two with according blends (8-hole, ca. 22% OME; 9-hole, ca. 42% OME). The results show an acceleration of the diffusion combustion rate and the resulting improvements in the indicated specific fuel consumption (- 5.7%) with increasing OME portion. In addition, mainly due to a reduction of up to 90% of the soot emissions, the NOx-soot trade-off improved as well. The NOx emissions showed decreased values for the less oxygenated, and increased values for the stronger oxygenated blend. This behaviour is attributed to the faster combustion, which, on the one hand reduces the residence time to form NOx and on the other hand increases the temperature, which enhances NOx formation again. The investigation of the trade-off between shorter residence time and higher temperature can only be performed with constant injection parameters (injected energy, fuel pressure and duration of injection) among the different fuels.