## 3 Testing the RLE and BSL Single Cylinder Engines under Idle Conditions
### The PCP-PMP Comparison Method, and First Estimate of RLE Friction Reduction  
The above cases set the stage for meaningful comparisons of PCP minus PMP of comparable RLE and BSL cases in order to establish the total heat release needed to sustain the engine. It can be assumed that the indicated thermal efficiency of both engines is about identical, and since the only load on the engine at idle is its own internal friction, the fMEP is directly proportional to the heat needed to sustain the engines. This total heat release per cycle is proportional to the PCP-PMP, as long as the comparison is made for cases where about the same percentage of the total heat release is complete at the TDC, and/or as long as the peak cycle pressure is around the TDC (in other words, as long as the combustion phasing with respect to the TDC is identical). The only case where peak pressure is substantially after the TDC is Case 3 (see Figure A5 in the Appendix A). We collected enough information to justify meaningful comparisons. For the cases shown in Tables 1 and 2, we can compare Cases 1 and 4 (favorable for the BSL as discussed), 1 and 5, 2 and 5, and 3 and 6. The CO\({}_{2}\) measurements are not complete, but appear consistent with the PCP-PMP comparison method. The most important corroboration is actually via the integrated adiabatic heat release calculations, which will be discussed in Section 3.7; they show where in the cycle the heat release is taking place. Overall, the comparison in Tables 1 and 2 is consistent with the trends. The lowest margin of RLE improvement in fuel economy is when comparing Cases 1 and 4, which is 18%. RLE Case 4 is considerably over-advanced as shown by its P-V diagram (Figure 11,red curve). This over-advancing was also apparently causing excessive heat loss, and the actual indicated thermal efficiency was low, as indicated by the higher (compared to RLE Case 5) exhaust CO2 concentration. However, when the timing was retarded one step (Case 5), and the combustion of the RLE was a lot closer to BSL Case 1, we obtained a 38% reduction in PCP-PMP compared to Case 1. However, we are comparing the RLE with a higher compression ratio BSL, where it can be argued that the pressure loading on the BSL piston rings creates an excessive fMEP loss for the BSL (the RLE piston rings and skirt are operating hydrodynamically, so their fMEP contribution is not sensitive to pressure loading). However, when we compare RLE Case 5 with BSL Case 2, where the compression ratio is actually lower than the RLE, we are still obtaining a 32% reduction in fuel consumption. When the timing is considerably retarded on both engines, the margin is reduced to 23%. This is reasonable, based on the fact that the retarded timing reduces the pressure loading on the BSL piston rings, but also, it needs to be reiterated that this comparison favors the BSL where the peak pressure is considerably past the TDC (Figure 13a for the RLE and Figure A5 for the BSL). Some readers may wonder about the validity of the PCP-PMP method, by suggesting that the leakage from the face seal reduces the peak pressure on the RLE. This was expanded upon by [PERSON] et al. 2019 [PERSON] et al. 2019 [PERSON] et al. The equation used to calculate the instantaneous heat release rate was borrowed from [PERSON] and Platts [8] and is shown here as Equation (1). \[dQ_{hr}=\frac{\gamma}{\gamma-1}\cdot p\cdot dV+\frac{1}{\gamma-1}\cdot V\cdot dp \tag{1}\]  
where \(Q_{hr}\) is the heat input by combustion, \(\gamma\) is the ratio of specific heats (function of temperature), \(p\) is the pressure, and \(V\) is the volume. Note that we ignored heat loss; we are only considering the adiabatic heat release in order to be consistent with the prior PCP-PMP method. For all the graphs shown in this section, the dQ plotted is for 1/20 th of a crank angle, which is our encoder resolution. Figure 14 shows the heat release rate for BSL Case 1. This confirms the prior statements that most of the heat release was prior to the TDC, but some combustion continued after the TDC.