Potential of Miller Cycle in Reactivity Controlled Compression Ignition Combustion

In order to overcome the “Diesel dilemma” of conventional diffusive combustion, highly-premixed combustion strategies such as Homogeneous Charge Compression Ignition (HCCI) have been investigated during last years. This approach is based on an optimized premixed combustion, allowing significant reductions of engine-out NOx and soot emissions. However, HCCI combustion operation was limited at mid and high loads by excessively high pressure gradients, leading to unacceptable noise levels and reliability issues. Thus, in this paper, the application of the Miller cycle to Reactivity Controlled Compression Ignition (RCCI) is analysed in terms of combustion process, pollutant emissions and efficiency, with the aim of complementing the existing knowledge about premixed combustion strategies.
An experimental investigation has been performed using a single-cylinder heavy-duty research engine, at mid load (14 bar IMEP). At this engine load, to keep controlled the ignition processes, in-cylinder thermodynamic conditions and global reactivity of the fuel blend have to be lowered. This research work has been done following parametrical studies. More in detail, an effective compression ratio sweep, performed by using the Miller cycle, is evaluated at three different in-cylinder fuel blending ratios, (increasing the amount of low reactivity fuel in the blend).
Results show that a simultaneous reduction of soot and NOx emissions up to EURO VI limits is attainable keeping fuel consumption reduced.