Optical spray investigation and numerical spray model calibration for the RCCI combustion mode with ethanol/CNG and diesel fuel

The simultaneous reduction of fuel consumption and pollutant emissions such as Nitrogen oxides (NOX) and soot remains the primary goal of modern engine development. In this context, Low Temperature Combustion (LTC) concepts are considered promising approaches. The so-called Reactivity Controlled Compression Ignition (RCCI) combustion process is one of these LTC concepts. RCCI combustion with two differently reactive fuels enables highly efficient combustion while reducing NOX and soot raw emissions compared to conventional combustion processes. RCCI combustion is significantly affected by the thermodynamical conditions in the combustion chamber (i.e., pressure and temperature), by the injection parameters (i.e., injection duration, timing, and offset), and by the reactivity of the fuels. It is very important to understand how these parameters affect the mixture formation in RCCI combustion and consequently the engine efficiency and emissions. This paper therefore focuses on the analysis of RCCI mixture formation and combustion. To this end, optical fuel injection investigations were performed in a High Pressure Chamber (HPC) under steady-state conditions to gain in-depth insights into the fuel spray propagation and mixture formation of the differently reactive fuels. Ethanol (low reactivity fuel) and diesel (high reactivity fuel) were used in the study. The results from the optical investigations showed that RCCI combustion is very sensitive to the chamber temperature and that a small increase in temperature change leads to a significant reduction in the Ignition Delay (ID) and results in early combustion. The reactivity gradients and concentrations are governed by the injection timing and duration of the low and high reactivity fuels, and also the offset between them. It was found that with increased chamber pressure, the Liquid Penetration Length (LPL) and the ID decrease. However, there is no impact on the Gaseous Penetration Length (GPL). The results from the optical HPC investigations were used to calibrate the spray model for the numerical simulations of RCCI combustion. The calibrated spray models agree well with the experimental data regarding LPL and GPL for ethanol and diesel fuel, indicating that they are well calibrated.