4. RCCI For improved efficiency and low emissions on a multi- cylinder LD diesel engine
Curran et al. [90] investigated in-cylinder blending of gasoline with diesel fuel on a multi-cylinder LD diesel engine as a strategy to control in-cylinder fuel reactivity for improved efficiency and the lowest possible emissions. The objective of their study was to develop better understanding of the potential and challenges of RCCI on a multi-cylinder engine. More specifically, the effect of cylinder-to-cylinder imbalances and in-cylinder charge motion as well as the potential limitations imposed by real world turbo- chargers were investigated on a 1.9 L four-cylinder engine that is similar to the ERC LD engine. The investigation focused on one engine operating condition 2300 rev/min, 5.5 bar net mean effec- tive pressure (NMEP).
Parameter sweeps in the investigation included variations of the gasoline-to-diesel fuel ratio, intake charge mixture temperature, in-cylinder swirl level and diesel start-of-injection timings. In addition, engine parameters were trimmed for each cylinder to balance the combustion process for maximum efficiency and lowest emissions. An important observation was the strong influ- ence of the intake charge temperature on the cylinder pressure rise rate. The work was guided by the dual fuel modeling and experi- ments of Refs. [8,34]. The operating point mentioned was chosen such that no EGR would have to be used to allow simplified cylinder-to-cylinder balancing and to avoid thermal management of the EGR cooler. To further simplify the multi-cylinder engine experiments, a single direct injection of diesel fuel was used instead of a split injection strategy. The engine operating parameters were based on the multi-dimensional CFD modeling and single cylinder experiments performed by Kokjohn and Reitz [88]. The test engine specifications are given in Table 2, and a schematic diagram of the test engine was shown in Fig. 2.
The goal of the work was to provide a roadmap of how to ach- ieve improved efficiency with the lowest possible emissions with dual-fuel RCCI in a multi-cylinder LD diesel engine. To this end, the dual-fuel RCCI performance and emissions were compared against conventional diesel combustion at the 2300 rev/min, 5.5 bar NMEP modal point. Diesel SOI timing sweeps were performed to compare multi-cylinder dual-fuel performance against the model pre- dictions. No manual cylinder balancing was done for the conven- tional diesel combustion mode and the natural cylinder-to-cylinder imbalances are obvious in the graphs of NMEP and MFB50 for each cylinder in Figs. 54 and 55. These imbalances clearly show the need for control over each cylinder in the RCCI combustion mode. The performance and emission data for the CDC condition are shown in Table 36.