applied sciences Article The Rotating Liner Engine (RLE) Diesel Prototype: Reducing Internal Engine Friction by about 40% under Idle Conditions Dimitrios Dardalis 1,*, Amiyo Basu 2,*, Matt J. Hall 2,* and Ronald D. Mattthews 2,* 1 RSET Inc., University of Texas at Austin, Austin, TX 78712, USA 2 University of Texas at Austin, Austin, TX 78712, USA * Correspondence: [email protected] (D.D.); [email protected] (A.B.); [email protected] (M.J.H.); [email protected] (R.D.M.) Abstract: The Rotating Liner Engine (RLE) concept is a design concept for internal combustion engines, where the cylinder liner rotates at a surface speed of 2–4 m/s in order to assist piston ring lubrication. Specifically, we have evidence from prior art and from our own research that the above rotation has the potential to eliminate the metal-to-metal contact/boundary friction that exists close to the piston reversal areas. This frictional source becomes a significant energy loss, especially in the compression/expansion part of the cycle, when the gas pressure that loads the piston rings and skirts is high. This paper describes the Diesel RLE prototype constructed from a Cummins 4BT and the preliminary observations from initial low load testing. The critical technical challenge, namely the rotating liner face seal, appears to be operating with negligible gas leakage and within the hydrodynamic lubrication regime for the loads tested (peak cylinder pressures of the order of 100 bar) and up to about 10 bar BMEP (brake mean effective pressure). Preliminary testing has proven that the metal-to-metal contact in the piston assembly mostly vanished, and a friction reduction at idle conditions of about 40% as extrapolated to a complete engine has taken place. It is expected that as the speed increases, the friction reduction percentage will diminish, but as the load increases, the friction reduction will increase. The fuel economy benefit over the US Heavy-Duty driving cycle will likely be of the order of 10% compared to a standard engine. Citation: Dardalis, D.; Basu, A.; Hall, M.J.; Mattthews, R.D. The Rotating Keywords: engine efficiency; engine friction; engine lubrication; carbon emissions reduction Liner Engine (RLE) Diesel Prototype: Reducing Internal Engine Friction by about 40% under Idle Conditions. Appl. Sci. 2021, 11, 779. 1. Introduction https://doi.org/10.3390/app11020779 The Rotating Liner Engine (RLE, www.rotatingliner.com) is a unique lubrication Received: 30 November 2020 concept aimed at Diesel engines, where the cylinder liner rotates to eliminate the boundary Accepted: 12 January 2021 friction environment for the piston rings and skirt around the TDC (top dead center) of Published: 15 January 2021 compression-expansion. It is well understood from the literature that this specific form of friction loss is Publisher’s Note: MDPI stays neu- responsible for a substantial portion of the mechanical losses in Diesel engines (especially tral with regard to jurisdictional clai- at high loads). In fact, the observed considerable increase in FMEP (friction mean effective ms in published maps and institutio- pressure) as a function of engine load can be traced mostly to this particular factor and it is nal affiliations. also responsible for the principal wear mechanism that limits engine life. 1.1. Methods for Measuring Instantaneous Piston Assembly Friction Part of the challenge of evaluating the contribution of the piston ring and skirt bound- Copyright: © 2021 by the authors. Li- censee MDPI, Basel, Switzerland. ary friction is the fact that it is almost impossible to measure in a practical firing engine. The This article is an open access article floating liner method is one of the methods that has shown promise in isolating this term, distributed under the terms and con- but unfortunately, the increasing pressures also affect the sealing mechanism of the floating ditions of the Creative Commons At- liner while piston side loads also interfere with the axial force balance via deflections and tribution (CC BY) license (https:// friction. As a result, the measured piston assembly liner force does not necessarily represent creativecommons.org/licenses/by/ the actual friction. Another factor that limits the accuracy of floating liner testing is the fact 4.0/). that the state of wear of the cylinder liners and piston rings is generally in the close to new Appl. Sci. 2021, 11, 779. https://doi.org/10.3390/app11020779 https://www.mdpi.com/journal/applsci Appl. Sci. 2021, 11, x FOR PEER REVIEW 2 of 32 necessarily represent the actual friction. Another factor that limits the accuracy of floating Appl. Sci. 2021, 11, 779 2 of 31 liner testing is the fact that the state of wear of the cylinder liners and piston rings is gen- erally in the close to new condition with the honing marks still fresh, and the piston ring profile is still mostly intact as it was optimized by its designers. However, a real heavy- conditionduty engine with as the it operates honing marks in the still field fresh, will and reach the a piston state relatively ring profile early is still in mostlyits useful intact life aswhere it was the optimized honing marks by its designers.have disappeared However, from a real much heavy-duty of the top engine pistonas ring it operates stroke, and in thethe field piston will rings reach have a state worn relatively out and reached early in thei its usefulr steady-state life where profile. the honing It would marks be imprac- have disappearedtical to run the from floating much liner of the for top sufficient piston ring hours stroke, (possibly and theover piston 1000) ringsin order have to wornreachout this andstate. reached Therefore, their this steady-state method has profile. value It in would isolating be impractical changes in tothe run frictional the floating characteristics liner for sufficientwith different hours operating (possibly overor design 1000) inparamete order tors. reach However, this state. these Therefore, results may this methodnot be suffi- has valueciently in accurate isolating to changes draw conclusions in the frictional on the characteristics absolute energy with differentcontribution operating of different or design fric- parameters.tion terms. However, these results may not be sufficiently accurate to draw conclusions on theAnother absolute method energy that contribution has been ofproposed different in friction the past terms. to isolate the individual compo- nentsAnother in a firing method engine, that the has P-w been method, proposed was in demonstrated the past to isolate by Marek the individual et al. in 1991. compo- The nentsprimary in a value firing of engine, this approach the P-w is method, that it gives was demonstratedthe overall instantaneous by Marek et friction al. in 1991. torque The of primarya single valuecylinder of this engine approach without is thatany itmodifica gives thetions overall to the instantaneous engine itself. friction One of torque the down- of a singlesides cylinderof this method engine is without that it anyonly modifications worked for low to the load engine cases itself. and Onefor single of the cylinder downsides en- ofgines this methodwith a low is that moment it only workedof inertia. for While low load the cases overall and instantaneous for single cylinder torque engines cannot with by aitself low identify moment and of inertia. separate While individual the overall compon instantaneousents, since we torque know, cannot when by in itselfthe cycle, identify high andinstantaneous separate individual friction torque components, takes place, since we can know, deduce when the in thelikely cycle, source. high Figure instantaneous 1 shows frictiona sample torque of the takes instantaneous place, we friction can deduce torque. the The likely conclusion source. Figurefrom this1 shows work ais sample that the offriction the instantaneous torque is dominated friction torque.by the high-press The conclusionure part from of the this cycle, work where is that the the piston friction as- torquesembly is boundary dominated terms by theare high-pressureactive (note, accord part ofing the to Miura cycle, whereet al [1]., the the piston frictional assembly losses boundaryof the crank terms bearings are active are almost (note, independent according to of Miura instantaneous et al. [1], thebearing frictional load, lossesand are of more the crank bearings are almost independent of instantaneous bearing load, and are more or or less constant throughout the cycle). This trend remains even when the engine load is less constant throughout the cycle). This trend remains even when the engine load is zero, zero, but not as pronounced as when the engine produces torque. This was the case, de- but not as pronounced as when the engine produces torque. This was the case, despite spite the fact that the oil sump temperatures for those tests were fairly low, amplifying the fact that the oil sump temperatures for those tests were fairly low, amplifying the the hydrodynamic terms. At increasing loads, the dominating spike increases, increasing hydrodynamic terms. At increasing loads, the dominating spike increases, increasing the the overall FMEP by 20% (despite the rather low brake mean effective pressure (BMEP) overall FMEP by 20% (despite the rather low brake mean effective pressure (BMEP) that that this air-cooled, naturally aspirated Diesel can generate). It can be safely deduced that this air-cooled, naturally aspirated Diesel can generate). It can be safely deduced that the the piston assembly boundary friction (plus the injector pump, but in these type of en- piston assembly boundary friction (plus the injector pump, but in these type of engines, the gines, the injection pressure was fairly low and, therefore, this contribution was small) is injection pressure was fairly low and, therefore, this contribution was small) is the primary the primary contributor for this 20% increase in total FMEP. It can also be deduced that contributor for this 20% increase in total FMEP.