Concerns over rising pollution levels, global warming and rapid depletion of conventional petroleum fuel resources have led to focused attention towards cleaner fuels and advanced combustion concepts in engines. Reducing combustion noise in engines is also critical. Gaseous fuels like natural gas, biogas, ammonia, hydrogen, etc. are being studied for developing relatively clean and efficient fuels. The economic and environmental benefits of renewable natural gas have made it one of the promising fuels and hence it is being used in dual-fuel engines along with diesel pilot as an ignition source. More recently, reactivity-controlled compression ignition (RCCI) is found to be a promising low-temperature combustion (LTC) method using fuels of varied reactivities, which has been explored more using liquid fuels as both low reactivity and high reactivity fuels. However, the RCCI with gaseous fuels needs to be explored more over the entire load range by studying the performance, combustion, emissions, and noise characteristics.
The present study investigates strategies for using gaseous fuels in dual-fuel and RCCI combustion concepts at different loads and their efficacy to achieve lesser emissions with simultaneous reduction of NOx and soot, without compromising on performance and combustion noise. In the first set of experiments, combustion, emissions and noise characteristics of the conventional dual-fuel engine operated with methane and diesel pilot were studied using a flexible dual-fuel engine setup developed to enable operation under desired gaseous (primary) fuel composition. Comparative results on combustion and performance characteristics with different methane energy shares at various engine loads revealed that there is a better utilisation of gaseous fuel at full load conditions, resulting in improved performance. Cylinder pressure spectra derived from recorded in-cylinder pressure were used for carrying out the quantitative analysis of sound levels. The results suggested that higher methane share helps in reducing the sound pressure levels at all loading conditions.
Further experiments were conducted with methane and diesel to achieve RCCI combustion regime at lower load conditions. The diesel injection timing was advanced and it was found that low-temperature combustion regime was achieved beyond 30 CAD BTDC with characteristic low-temperature heat release and simultaneous reduction of NOx and soot. The injection timing of 40 CAD BTDC was optimal in terms of performance and emissions and is suitable for further studies. Combustion stability quantified in terms of the coefficient of variation (COV) of IMEP limited the further advancement of injection timing. The developed CFD model revealed the presence of lesser locally-rich regions and high-temperature regions, which resulted in the trends observed in experiments. The study is further extended to explore RCCI combustion with gaseous fuels over the wide load range for understanding the emissions, combustion performance and noise characteristics.