The present experimental study explored methods to obtain the
maximum practical cycle efficiency with Reactivity Controlled
Compression Ignition (RCCI). The study used both zero-dimensional
computational cycle simulations and engine experiments. The
experiments were conducted using a single-cylinder heavy-duty
research diesel engine adapted for dual fuel operation, with and
without piston oil gallery cooling. In previous studies, RCCI
combustion with in-cylinder fuel blending using port-fuel-injection
of a low reactivity fuel and optimized direct-injections of higher
reactivity fuels was demonstrated to permit near-zero levels of NOx
and PM emissions in-cylinder, while simultaneously realizing gross
indicated thermal efficiencies in excess of 56%.
The present study considered RCCI operation at a fixed load
condition of 6.5 bar IMEP an engine speed of 1,300 [r/min]. The
experiments used a piston with a flat profile with 18.7:1
compression ratio. The results demonstrated that the indicated
gross thermal efficiency could be increased by not cooling the
piston, by using high dilution, and by optimizing in-cylinder fuel
stratification with two fuels of large reactivity differences. The
best results achieved gross indicated thermal efficiencies near
60%. By further analyzing the results with zero-dimensional engine
cycle simulations, the limits of cycle efficiency were
investigated. The simulations demonstrated that the RCCI operation
without piston oil cooling rejected less heat, and that ~94% of the
maximum cycle efficiency could be achieved while simultaneously
obtaining ultra-low NOx and PM emissions.