Fig. 62 shows the effect of swirl ratio on BTE and net and gross ITE for the 77% gasoline case. It was found that thermal efficiency is strongly affected by changing the swirl ratio. The thermal effi- ciency was found to increase with a more open swirl valve, indi- cating that better in-cylinder mixing is needed to realize the maximum thermal efficiency with the current multi-cylinder configuration (i.e., single diesel injection, injector technology, compression ratio).
Fig. 63 shows cylinder pressure and heat release rates for the diesel fuel SOI timing sweep from -30o to -60o ATDC at a swirl valve angle of 65.7o. When the diesel SOI timing was advanced to -70o ATDC, combustion was found to be unstable. The trends of pressure and heat release from the multi-cylinder engine experi- ment agreed well with the modeling predictions, as shown in Fig. 63. The two peaks for the case with a diesel SOI timing of -30o seem to indicate two-mode combustion. These trends also held for different swirl ratios and for the 85% gasoline case, as explained in the dual-fuel RCCI work of Hanson et al. [34]. The HC and CO emissions trends for the diesel SOI sweeps are shown in Fig. 64, and the NOx trends for 81% and 85% gasoline are shown in Fig. 65.
This work successfully demonstrated the application of dual-fuel RCCI operation on a multi-cylinder LD engine and the model results were valuable in directing the experiments. The work also showed the relationship between intake and temperature and gasoline-to-diesel fuel ratio during dual-fuel operation. The need for increased understanding of the performance of turbo- machinery for low temperature combustion also became apparent due to the observed importance of boost pressure.