Investigation of Low-Temperature Combustion in an Optical Engine Fueled with Low Cetane Sasol JP-8 Fuel Using OH-PLIF and HCHO Chemiluminescence Imaging

Low cetane JP-8 fuels have been identified as being difficult to use under conventional diesel operation. However, recent focus on low-temperature combustion (LTC) modes has led to an interest in distillate hydrocarbon fuels having high volatility and low autoignition tendency. An experimental study is performed to evaluate low-temperature combustion processes in a small-bore optically-accessible diesel engine operated in a partially-premixed combustion mode using low-cetane Sasol JP-8 fuel. This particular fuel has a cetane number of 25. Both single and dual injection strategies are tested. Since long ignition delay is a consequence of strong autoignition resistance, under the conditions examined, low cetane Sasol JP-8 combustion can only take place with a double injection strategy: one pilot injection event in the vicinity of exhaust TDC and one main injection event near firing TDC. In this work, the effects of autoignition properties are examined by comparing the behavior of a high cetane number JP-8 fuel with that of a low CN Sasol JP-8. The double injection strategy also served to reduce pressure rise rates during operation at light load (2 bar IMEP) conditions. Dual injection timing is optimized for peak IMEP, at which point simultaneous OH Planar Laser-Induced Fluorescence (OH-PLIF) and high-speed crank-angle-resolved HCHO chemiluminescence imaging are performed to analyze the partially-premixed combustion process. Fuel efficiency and engine-out emissions performance are also presented. In terms of IMEP, low CN Sasol JP-8 fuel is shown to be a satisfactory fuel for low-temperature combustion under light-load condition using the proper dual injection strategy. However, partially-premixed combustion operation in this work results in higher UHC emissions and lower fuel efficiency when compared with the high cetane JP-8 fuel.