Double Injection Strategies for Ethanol-Fuelled Gasoline Compression Ignition (GCI) Combustion in a Single-Cylinder Light-Duty Diesel Engine 2016-01-2303
Ethanol has been selected as a fuel for gasoline compression ignition (GCI) engines realising partially premixed charge combustion, considering its higher resistance to auto-ignition, higher evaporative cooling and oxygen contents than widely used gasoline, all of which could further improve already high efficiency and low smoke/NOx emissions of GCI engines. The in-cylinder phenomena and engine-out emissions were measured in a single-cylinder automotive-size common-rail diesel engine with a special emphasis on double injection strategies implementing early first injection near BDC and late second injection near TDC. Three key parameters are investigated to optimise the double injection strategy including the proportion of the first and second injection mass, the first injection timing, and the second injection timing, while other operating conditions were held constant at the engine speed of 2000 rpm, intake air temperature of 80°C, common-rail pressure of 50 MPa, and indicated mean effective pressure (IMEP) of about 950 kPa. From the experiments, it is found that the higher proportion of the first injection results in higher in-cylinder pressure, pressure rise rate, and apparent heat release rate, which contributes to the increased indicated MEP (IMEP), indicated engine efficiency and reduced indicated specific fuel consumption (ISFC). However, the increased noise was problematic. The engine-out emissions of smoke, uHC, and CO also show a decreasing trend with increasing first-injection fraction; however, this leads to the increased NOx emissions due to the advanced combustion phasing and increased peak aHRR. Regarding the first injection timing variations, more advanced first injection tends to show improved performance due primarily to the extended pre-combustion mixing time and hence increased mixture homogeneity. The advanced first injection, however, increases the NOx emissions as the increased mixture homogeneity causes more advanced combustion phasing and thereby increasing the combustion temperature. For the second injection timing variations, the results exhibit that the decreased IMEP and increased ISFC limit the late second injection while it is the high noise and NOx emission to limit the early second injection. Compared with the diesel baseline condition, the optimised engine operating conditions of the present study with 54% first injection at 170°CA bTDC and 46% second injection at 3°CA bTDC achieves 32% higher indicated engine efficiency and 23% lower NOx emissions while smoke emissions are kept low at about 2% opacity and the combustion efficiency estimated using uHC/CO emissions stays high at 96%. The estimated noise was also low at 83.7 dB. The ISFC is reduced by 1% despite 33% lower calorific value of ethanol than a conventional diesel fuel.