Investigating Air Handling Requirements of High Load Low Speed Reactivity Controlled Compression Ignition (RCCI) Combustion

Past research has shown that reactivity controlled compression ignition (RCCI) combustion offers efficiency and NOx and soot advantages over conventional diesel combustion at mid load conditions. However, at high load and low speed conditions, the chemistry timescale of the fuel shortens and the engine timescale lengthens. This mismatch in timescales makes operation at high load and low speed conditions difficult. High levels of exhaust gas recirculation (EGR) can be used to extend the chemistry timescales; however, this comes at the penalty of increased pumping losses. In the present study, targeting the high load - low speed regime, computational optimizations of RCCI combustion were performed at 20 bar gross indicated mean effective pressure (IMEP) and 1300 rev/min. The two fuels used for the study were gasoline (low reactivity) and diesel (high reactivity). The effects of intake pressure and EGR on combustion and emissions were studied using a full factorial design of experiments of genetic algorithm optimizations. The optimizations were setup for three values of EGR (30%, 45% and 55%) and equivalence ratios (0.8, 0.9 and 1.0). The results showed that gross indicated efficiency (GIE) increases with boost and EGR. A high gasoline percent (> 93.5%) has to be maintained across the range of boost and EGR. Approximately 50% of the gasoline was premixed at the lower EGR’s and this percentage increases with increasing EGR to maximize the efficiency. Direct injected gasoline, is injected post TDC at low EGR’s and high boost pressures to control the peak pressure rise rates and is advanced as the EGR increases. The start of injection (SOI) of diesel is early at low EGR’s to avoid stoichiometric, high temperature combustion and control the NOx emissions, and is brought closer to TDC as EGR increases. The pumping loop work for each case was estimated using a thermodynamic model prepared in CANTERA and the net indicated efficiency (NIE) for each case was calculated. The results showed that NIE had similar trends as GIE, but increasing the EGR beyond 55%, caused the NIE to decrease due to a high pumping loop penalty, indicating an optimum at this EGR and boost pressure.