## 3 Testing the RLE and BSL Single Cylinder Engines under Idle Conditions
### Comparison of High Compression Ratio BSL with the RLE  
In this subsection, we show some P-V diagrams that help us analyze the combustion characteristics of the various cases that we are presenting. If the pressure rise due to combustion has the slope shown by the black and red curves of Figure 11, it means that the volume was still decreasing when combustion occurred; therefore, the combustion took place before TDC. In the case of the blue curve, the pressure rise is almost vertical, meaning that there was no appreciable volume change during combustion, so the premixed combustion took place at the TDC. If the slope was reversed, then the pressure rise would have been taking place while the volume was increasing, and it would have taken place after TDC (this is the typical way diesel engines are set up to run). Additionally, a portion of the top of the compression and expansion curves, as in the red graph of Figure 11, overlaps. This means that the combustion had ended before the minimum volume was reached, so the pressure followed the corresponding adiabatic curve from the end of combustion, to the minimum volume, and back over the early expansion. In the black curve of Figure 11, some degree of combustion continued until the minimum volume or slightly after; the expansion followed a slightly higher adiabatic curve than the late compression. As discussed above, the RLE has been upgraded with a new gearbox and also with a proper TDC location. The data of Figure 11 (red line) were taken before ever modifying the injection pump timing, and with the same injector. Additionally, we are still using the exact same injection pump as we had in the original BSL experiments. It is obvious from Figure 11 that the injection timing in the RLE was substantially over-advanced and all combustion was complete prior to the TDC, whereas for the BSL, even though the premixed combustion was clearly taking place prior to the TDC, some combustion and increase in pressure continued after the TDC. The RLE red line graph also shows a pressure oscillation that was not present in the data presented by [PERSON] [7]. It seems that we are having some abnormal combustion along with the pressure wave in the channel that connects the combustion chamber to the pressure transducer (a channel of approximately 20 mm is connecting our transducer to the combustion chamber, see Figure A2; under certain conditions, a pressure oscillation is seen in the signal). In the blue case in Figure 11, the injection timing was retarded (still on the RLE). As expected, the peak pressure was reduced, making it a more fair comparison; however, some pressure noise towards the very top is still visible (this is even more apparent on the Pressure vs. Crank Angle graph). Figure 11: Top of the PV diagram of the original BSL case (black), RLE with original injection timing (red), and RLE with retarded timing (blue). The blue graph in Figure 11 appears to be a much better comparison to the old BSL data (black). However, the combustion still appears to be faster. We are not exactly sure as to why, other than the fact that we were forced to service the injector between the two tests due to contamination, and the cracking pressure may have increased. As we said above, the obsolete mechanical fuel system cannot be controlled directly; instead, we tried different operating parameters in a trial and error mode and observed the results in the pressure signal. We had a new injector prepared with a lower cracking pressure and tested it in the RLE. The result was a slower combustion event (the combustion duration and location with respect to the TDC is presented later with the instantaneous heat release rate calculations, see Section 3.7). Again, we are focusing on the conditions of 750 rpm and 71 \({}^{\circ}\)C coolant and oil temperature. The results are shown in Figures 12a and 13b. Figure 12a shows the pressure vs. crank angle graph, where the slower combustion also minimized the pressure wave that we obtain on the signal right at the end of the premixed combustion.