Abstract
Recently, interests for the gaseous fuel have been grown since the supply of the shale gas in United States have increased. Also for the concern of environmental issues, a number of studies about advanced diesel combustion has been widely introduced such as homogeneous compression charge ignition (HCCI), premixed charge compression ignition (PCCI), and reactivity controlled compression ignition (RCCI) for the reduction of emissions, especially nitrogen oxides (NOx) and particulate matters (PM) while maintaining or enhancing indicated thermal efficiency as compared to that of conventional diesel combustion (CDC).
Dual-fuel combustion has the potential to achieve near-zero NOx and PM emissions with higher indicated thermal efficiency under mid-load conditions. However, it showed low combustion efficiency due to THC slip by crevice effect. Also, under the high load condition, the maximum pressure rise rate(MPRR) drastically increase (over 10 bar/deg) due to the simultaneous auto-ignition of gaseous fuels, so that the noise vibration harness (NVH) problem occurs. There is a limitation to substitute high ratio of volatility fuels such as gaseous fuels since high volatility fuels have poor reactivity for the auto-ignition. However, introducing higher ratio of gaseous fuel is needed to lower PM emissions and save diesel fuel. Therefore, it is important to investigate the potential of using gaseous fuel as dual fuel combustion for widening high load conditions with stabilized combustion, and with low level of MPRR and emissions.
In this study, the test was performed on a 0.4 L light-duty single cylinder diesel engine. The engine was operated at 1,500 rpm and gIMEP 14.5 bar with varying the ratio of low reactivity gaseous fuel, in this experiment propane, to diesel fuel (0 / 80 /87 / 95 %) and diesel injection timings ( 0 / 2.5 / 5 / 7.5 °CA bTDC) while maintaining the overall equivalence ratio at 0.55. The intake pressure was fixed at 1.6 bar, which is normally the maximum value at 1,500 rpm for conventional diesel engine. Exhaust gas recirculation (EGR) was also added for some cases to reduce NOx emission.
The results showed that coefficient of variation (CoV) of IMEP for all cases were under 5 %, which can be said as a stable combustion. The heat release rate (HRR) showed double peak after the injection, which can be regarded as propane auto-ignition after the diesel auto-ignition. EGR added dual-fuel combustion cases showed a promising MPRR (under 10 bar/deg) and 90 % reduction of PM emissions compared to CDC. The results emphasize that suitable operation strategies with minimum diesel ratio (5 %) can be used in high load conditions in diesel engines.
Key Words : Engine System, New CI Engines and Components