## 3 Testing the RLE and BSL Single Cylinder Engines under Idle Conditions
### The PCP-PMP Comparison Method, and First Estimate of RLE Friction Reduction
In other words, our analysis only considers the heat release that was directly used to raise the cylinder pressure and perform mechanical work, and not the heat lost to the combustion chamber walls. The indicated thermal efficiency with respect to expansion ratio, however, is identical, which is consistent with the similar values to the total adiabatic heat release. We can differentiate, however, between the friction reduction and fuel consumption reduction. The friction reduction is likely directly proportional to the adiabatic total heat release. However, the reduction in fuel consumption at idle will be proportional to the total heat release (which includes the heat lost to the combustion chamber walls). [PERSON] and [PERSON] (2018) suggest that the actual heat release is about 15% higher than the adiabatic heat release. Therefore, the percentage reduction in fuel consumption may prove to be 15% higher (i.e., a 30% value would be 34.5%). We are planning on performing fuel flow measurements in the future. Given that the PCP-PMP method is corroborated by the more sophisticated heat release rate calculation, we can also compare Case 5 with Case 2 (28% benefit for the RLE), as well as Case 3 with Case 6 (20% benefit for the RLE even though Case 6 is not entirely warmed up). It is only reasonable for the RLE margin to be reduced when the timing is retarded, as the pressure loading on the BSL piston assembly is relieved (but also, the RLE was not fully warmed up). However, even with a reduced compression ratio for the BSL compared to the RLE, the substantial friction reduction persists. The trend of greater fMEP reduction at increasing pressure loading (i.e., higher loads) will likely persist (based also on the SVE experience), but unfortunately, the testing precision will have to be improved, since a smaller fraction of the fuel energy will be expended towards overcoming internal friction. A sensitivity analysis of this heat release calculation was conducted to compare the estimated magnitude of the friction reduction of the RLE at idle with the potential error. As discussed above, it was calculated that BSL Case 1 had approximately 4 cc lower clearance volume (the relatively large difference in peak pressure was partly due to valve timing and excessive valve lash that caused an early intake valve closing, plus slightly different camshaft specs). Additionally, there is approximately +/ -0.25 crank angle error on the TDC of the BSL for Case 1, plus some errors in the real piston location as opposed to the encoder position, due to the torsional vibrations mentioned earlier (these are more pronounced in the RLE, where the liner driving mechanism adds a torsional spring and flywheel upstream of the encoder, see Figure 10). Therefore, we did a test calculation of the BSL total heat release, assuming extreme errors of 1 full degree TDC shift (towards the direction where the heat release would decrease), plus a reduction in clearance volume of 10 cc instead of 4 cc, which would also reduce the total heat released per cycle (if the clearance volume was that much lower in the BSL, we would not have had reduced the motoring pressure via the minor changes performed prior to resurrecting the BSL unit). Even at this extreme range of parameters, which is far outside the confidence level, the BSL heat release is still 396 J per cycle, or 24% higher than RLE Case 5. The main reason why the heat release calculation is insensitive to the TDC location is that even in Case 1, most of the heat release occurred substantially before TDC. If the premixed combustion took place near TDC, then it is very likely that the calculation would show more sensitivity to TDC location and clearance volume. In other words, given the nature of combustion for both engines for most cases, the integrated heat release calculation is equivalent to the PCP-PMP method. The margin of error on the simpler method is obviously very low as the experimental error is very low compared to the difference.