## 3 Testing the RLE and BSL Single Cylinder Engines under Idle Conditions
### The PCP-PMP Comparison Method, and First Estimate of RLE Friction Reduction
Additionally, it confirms that the bulk of the combustion takes place in the premixed section. Figure 15 describes Case 4 for the RLE, which had a more advanced combustion. This calculation indicates that while the combustion started at about the same time, virtually all the heat was released in the premixed stage, and all prior to the TDC. Note also that the maximum heat release rate around crank angle \(-8.5\) is very high in Case 4, which is responsible for creating the pressure wave noise right at the end of the premixed phase. This pressure wave also causes a local calculated negative heat release rate at about crank angle \(-6.5\), but this noise does not affect the average calculation. The heat release history confirms that the comparisons of PCP-PMP between Cases 1 (BSL, high compression ratio) and 4 (RLE most advanced) were not entirely fair for the RLE because the combustion characteristics of Case 4 will generate a higher PCP for the same total heat released per cycle. The comparison of the total heat released per cycle of 487 J for the BSL to 328 J for the RLE indeed shows a larger margin for the RLE, a reduction in total heat input by 33%, while the simplified PCP-PMP method only yields 18%. Figure 16 shows the heat release rates for both RLE Case 5 and Case 6 (retarded yet another 4 degrees). The first lesson from Figure 16 is the effect that the combustion retardation has on PCP. As expected, as the injection timing is retarded, the PCP drops. This will obviously happen on both BSL and RLE. Therefore, in the PCP-PMP method, it is important to compare cases with similar injection timing as expressed by the location of the heat release with respect to the TDC. Figure 14: Instantaneous heat release rate for BSL Case 1 along with the corresponding cylinder pressure. The premixed combustion looks sharper than what is normally reported in the literature, partly due to the low-pressure fuel injection and partly due to the over-advanced timing. Now, we consider if, indeed, RLE Case 5 (retarded by about 4 degrees) is a more fair comparison with BSL Case 1. Further examining Figure 16, we note that in Case 5, we can see that the maximum rate of combustion is much reduced compared to Case 4 (Figure 15), causing a reduction in the pressure wave noise, which also reduced the negative heat release just after the premixed combustion, and that some combustion continues after the TDC, much like Case 1 of the BSL shown in Figure 14 or Figure 11. The phasing of the maximum heat release rate, the beginning and end of the premixed combustion (starts at \(-10\), ends at \(-5\)), and the small tail end of heat release that stretches to just after the TDC, all match between the two cases. Case 4, in contrast (Figure 15), has a much higher maximum rate of heat release, and the premixed event was just about over at \(-7.5\) crank angle degrees. This confirms that Cases 1 and 5 are better matched for the PCP-PMP method comparison. The simplified PCP-PMP method yields a 38% reduction in fueling, while the integrated heat release comparison (319 vs. 487 ) yields 34.5%. If we compare RLE Cases 4 (over-advanced) and 5, given that we have identical integrated heat release (heat per cycle), one would expect that the CO\({}_{2}\) emissions would also be about identical. However, Tables 1 and 2 show a large difference in CO\({}_{2}\) concentration. This discrepancy is caused by the fact that our heat release calculation is adiabatic and  
Figure 16: Instantaneous heat release rate for RLE Case 5 (blue/green) and Case 6 (red/purple). One interesting observation in Figure 9 is that the approximately 4 degrees of physical rotation of the mechanical injection pump generated about 6 degrees of delay in the premixed combustion event. Figure 15: Instantaneous heat release rate for RLE Case 4.  
does not consider the loss in indicated thermal efficiency caused by the excess heat loss of Case 4 (in Case 4, we know that the heat loss is greater due to the longer exposure of the combustion chamber walls to high temperature and pressure).