Lim, J., Walker, N., Kokjohn, S., and Reitz, R., "High Speed Dual-Fuel RCCI Combustion for High Power Output," SAE Technical Paper 2014-01-1320, 2014, doi:10.4271/2014-01-1320.
In recent years society's demand and interest in clean and efficient internal combustion engines has grown significantly. Several ideas have been proposed and tested to meet this demand. In particular, dual-fuel Reactivity Controlled Compression Ignition (RCCI) combustion has demonstrated high thermal efficiency, and low engine-out NOx, and soot emissions. Unlike homogeneous charge compression ignition (HCCI) combustion, which solely relies on the chemical kinetics of the fuel for ignition control, RCCI combustion has proven to provide superior combustion controllability while retaining the known benefits of low emissions and high thermal efficiency of HCCI combustion. However, in order for RCCI combustion to be adopted as a high efficiency and low engine-out emission solution, it is important to achieve high-power operation that is comparable to conventional diesel combustion (CDC).The present study includes experimental results that show that load increase at mid-speed operation is limited by increasing peak pressure rise rates (PPRR). Accordingly, as a high power output strategy, high speed engine operation was examined. Using CFD simulations, high speed engine operation using iso-octane and n-heptane as surrogate fuels was tested in a light-duty diesel engine. Compared to mid-speed (1900 rev/min) operation, high-speed (3000 rev/min) operation was shown to allow increased combustion controllability. The increased engine speed also reduced NOx formation residence times, resulting in reduced NOx emissions. In one particular case examined the PPRR was reduced by 56% and NOx emission decreased by 24% with high-speed operation. The improved combustion controllability also enabled the use of early injection strategies, which gave more time for the direct-injected fuel to mix, thus providing low soot and CO emissions. One potential disadvantage of high-speed operation is increased frictional losses. However, the Chen-Flynn model was used to estimate friction mean effective pressure (FMEP), which increased by only 0.5 bar as the speed was changed from 1900 to 3000 rev/min. As a result, considering the corresponding dramatic increase in power output and the accompanying low emissions and combustion controllability advantages of RCCI combustion, the present study suggests that high speed operation is a very promising path to high power density operation with RCCI combustion.