The Rotating Liner Engine (RLE) is an engine design concept where the cylinder liner rotates in order to reduce piston assembly friction and liner/ring wear. The reduction is achieved by the elimination of the mixed and boundary lubrication regimes that occur near TDC. Prior engines for aircraft developed during WW2 with partly rotating liners (Sleeve Valve Engines or SVE) have exhibited reduction of bore wear by factor of 10 for high BMEP operation, which supports the elimination of mixed lubrication near the TDC area via liner rotation. Our prior research on rotating liner engines experimentally proved that the boundary/mixed components near TDC are indeed eliminated, and a high friction reduction was quantified compared to a baseline engine. The added friction required to rotate the liner is hydrodynamic via a modest sliding speed, and is thus much smaller than the mixed and boundary friction that is eliminated. The magnitude of the friction reduction, especially for cases of high BMEP and/or low piston speeds, can be expected to be very high compared to conventional optimization approaches.The RLE applications are to conventional truck engines and downsized engines. For the case of a conventional engine, the fuel economy gain at full load is about 3.5%, but the overall fuel consumption benefit through the load cycle is estimated in the 7-10% range (with the larger benefit for high EGR engines). The efficiency benefit for downsized engines is expected to be lower, due to their expected increased load factor, and the consequent reduced importance of friction. However, downsized engines will likely suffer from accelerated wear, and the RLE characteristic of reduced bore and ring wear can be of great value.In this paper, we present the design details of a Rotating Liner Engine (RLE) conversion of a 3.9L Cummins B-Series engine. The design is such that the original bore size is maintained, and all cylinders can be converted, while the part count is reduced compared to a prior design proposal. A semi-external driving mechanism is proposed, such that existing engines can be readily converted without need to modify castings. The main technological challenge of the RLE is the face seal, which needs to contain the combustion gas at very high pressure, yet exhibit very low friction and wear. In this paper, we present the modeling, which indicates that a 172 bar (2,500 psi) peak cylinder pressure can be contained without metallic contact, and with 8-22 Watts viscous friction for a range of liner speeds of 300-600 rpm. Our model includes factors such as gas loading, mechanical and thermal distortions, hydrodynamic/squeeze film pressure, and hydrostatic pressure.Furthermore, a review of recent literature on engine friction is presented, which supports the developer's expectation that the RLE will give very favorable and long-lasting friction reductions compared to current techniques such as plateau honing and optimized piston ring profiles.