## 1 Introduction
### Lubricant Formulation in RLE Commercial Units  
In addition to generating experimental proof of the potential contribution of the boundary and mixed lubrication in diesel engines, the authors also expect that, given the very high predicted reduction in FMEP compared to conventional methods, and given the relatively moderate increase in engine cost, the RLE will find its way in Heavy-Duty commercial engines. If that is to take place, the question that arises is what type of lubricant will be ideal for the RLE? Would the standard 15w40 lubricant be ideal, or should RLE units follow the trend of reduced viscosity? For the testing of the first light-duty RLE prototype ([PERSON] et al. 2005) [6], the standard 5w30 low-viscosity light-duty lubricant was used. The reason for choosing the above grade was that it was expected that the low viscosity would amplify the boundary terms for the low-speed un-throttled motoring conditions. Additionally, since the peak pressures for motoring were relatively low (just under 20 bar), the low viscosity oil would not provide any challenges for lubricating the RLE face seal. During the initial testing of the RLE diesel prototype, the 15w40 grade has been consistently used. [PERSON] et al. (2012) [5] presented calculations that showed that if a high-temperature, high-shear rate lubricant of about 0.0035 Pa*s is used (lower than the one exhibited by 15w40 and about identical to the modern low-viscosity oils currently proposed for Heavy-Duty use), then the seal film thickness will be adequate to avoid contact. Even though the low load cases that we have been running so far in the prototype would not exceed the limit of the seal calculated by [PERSON] (2012) even with a 5w30 oil, and even though we expect that these calculations were conservative for the above reasons and computational complications discussed by [PERSON] et al. (2012), the security that the high-viscosity oil provides is considered critical for the prototype's preservation. Of course, once more data are accumulated, and the prototype is no longer as precious to us as it is now, lower viscosity can be tried as well so that more can be learned about RLE operation under reduced viscosity. When the conversion is redesigned for larger engines, the bearing surfaces of the RLE face seal can be redesigned larger to accommodate for the lower viscosity with a higher confidence level. However, it is not clear yet if the lower viscosity may require higher liner speeds to achieve the same results in minimizing the boundary terms of the piston rings, especially when operating under the high pressures of the future engines. If that proves to be the case, the 15w40 grade may prove more desirable. Furthermore, most of the benefits of the low-viscosity oil can be gained by the 15w40 grade, if one simply increases the clearance in the crankshaft bearings and piston skirts. This would allow exploitation of the high viscosity for the top-end cylinder and RLE seal lubrication, while the rest of the hydrodynamic terms can still benefit from the reduced shear rate. It needs to be noted that in any hydrodynamic bearing design, the compromise between reduced viscous losses and reduced maximum load capacity work in an identical manner regardless whether the designer reduces the viscosity or increases clearance. Due to the recent regulations in fuel economy for Heavy-Duty truck engines in North America, a new trend of reduced lubricant viscosities has emerged. As discussed above, and supported by [PERSON] et al., the reduced viscosity will reduce the friction in the intake and exhaust stroke, while having almost no effect in the high-pressure part of the cycle, which seems to dominate the mechanical losses. Therefore, as expected, the benefit is marginal in terms of fuel economy, while introducing a risk of potential increased wear. In order to control the wear impact on the piston rings and cylinder liner, more aggressive and expensive additive packages have been developed.