As indicated in Fig. 66, efficient combustion strategies were achieved over the entire load/speed range. NOx and soot emissions regulations were met across the entire range with the exception of soot at high load. However, high EGR levels at the 23 bar IMEP condition reduced combustion efficiency, and at the 4 bar IMEP condition, the ultra-low reactivity of methane caused the com- bustion efficiency to be relatively poor. However, the low load and high load case employed a later second injection, which occurred at approximately -20o aTDC and acted as a strong ignition source. The
13.5 bar IMEP case required the least amount of diesel fuel (on percent basis) and it was noticed that the diesel fraction decreased as load was increased, because the background equivalence ratio of methane was increased, thus increasing its reactivity and less diesel fuel was needed to ignite the mixture. This trend continued until 16 bar IMEP.
A comparison between the optimum 9 bar IMEP gasoline/diesel HD RCCI strategy developed by Kokjohn et al. [8,26,34,33] and the 9 bar IMEP natural gas/diesel strategy described in Fig. 66 is seen in
Fig. 67. The gasoline/diesel strategy was optimized at a relatively high intake pressure of 1.75 bar absolute, while the natural gas/ diesel strategy utilized 1.45 bar intake pressure. The combustion event of the gasoline/diesel strategy was retarded with lower intake pressure, whereas the natural gas/diesel strategy was un- affected by the changed intake pressure.
The use of natural gas as the low-reactivity fuel in the RCCI combustion strategy is seen to be able to yield clean, quiet and efficient combustion throughout the entire tested load/speed range. Additional work by Nieman [92] has shown that the use of triple injections is effective to further reduce NOx and PM at high and low loads, as shown in Fig. 68 and Table 39.