## 2 The Heavy-Duty RLE Prototype
### Face seal Design Challenge: Experimental Confirmation  
As discussed by [PERSON] et al. (2012) [5], the RLE seal needs to have negligible gas leakage, negligible wear, and very low added friction. [PERSON] et al. (2012) described in detail the design and the philosophy behind the RLE face seal. A relatively low closing force compared to the bearing area is used so that very high cylinder pressures can be contained without metal-to-metal contact. Significant oil flow is established throughout the primary sealing ring, in order to keep it cool and also in order to ensure that sufficient oil is present to block the gas flow between the gap (film thickness ranges from up to 5 microns in the unloaded part of the cycle to under 1 micron under full load peak pressure). The abundance of lubricant also allows for a very low surface roughness to be used, which allows a much lower minimum film thickness prior to metal-to-metal contact compared to the piston rings that ride on a relatively high surface roughness. However, the fairly high spring pre-load and very tight flatness tolerance also ensure that oil leakage into the combustion chamber is negligible. As described by [PERSON] et al. (2012) [5], negligible means that in the worst case scenario, it would be a tiny fraction of the lubricant deposited on the liner by the piston rings during each stroke. However, based on the calculation assumptions presented by [PERSON] (2012), there may be almost zero net flow, given that the gas pressure will push much of this oil back from the sealing zone.  
Based on the preliminary testing, it appears that these requirements have been met. The RLE and BSL (Baseline) seem to have an approximately identical starting performance. There is no indication of blowby from the RLE from exhaust port #1, which is directly open to the atmosphere and isolated from the remaining crank case. However, the best indication of zero blowby from the RLE face seal is the trace of carbon. Given the poor fuel atomization of our fuel injection system, a significant amount of carbon is deposited on all surfaces that are in contact with combustion products within a few minutes of engine operation. However, the surfaces of the rotating liner flange very close to the sealing gap, and head inserts are completely clean from any traces of combustion products. Therefore, no gas flow of any significance has been established while the engine was operating, which is to be expected, given that the RLE face seal has no end-gap. Furthermore, even though we have not measured the liner driving torque for this prototype, it appears to be fairly low, based on the fact that the drive belt (Figure 8) is not tight, and yet there is no observed belt slippage (note, this indicates that the pinned piston rings also likely ride hydrodynamically). The relatively soft copper alloy primary sealing ring does not show any signs of wear. This is of course premature, given that the peak pressure so far has not exceeded approximately 100 bar. However, the engine has now operated for about 50 h, has been subjected to about 10 bar BMEP, and we have still seen no malfunction or wear of the sealing system.