The Rotating Liner Engine (RLE) is a design concept where the cylinder liner of a heavy-duty Diesel engine rotates at about 2–4 m/s surface speed to eliminate the piston ring and skirt boundary friction near the top and bottom dead center. Based on prior testing results from our single cylinder RLE prototype (a converted four-cylinder Cummins ISB 3.9 Diesel) compared to a similar baseline, under idle conditions, the friction reduction is approximately 50 kPa (0.5 bar) in FMEP (friction mean effective pressure), which translates to about 40% for a complete engine. In this new set of experiments, we compare the RLE performance under load of up to about 7 bar IMEP (indicated mean effective pressure). It has been proven that the elimination of metallic contact between the compression rings and cylinder wall persists for up to 75 bar peak cylinder pressure and 1.5–2.3 m/s liner surface speed (283–426 rpm) for the 850–1280 rpm crankshaft speed. Additionally, the RLE FMEP is substantially reduced under load, which is a trend opposite to standard engines. Presumably, at idle the RLE cylinder and piston are cooler than the standard engine because the substantial heat dissipation by the metallic contact of the rings and skirts with the liner does not take place. Under load, however, the increased heat transfer from the combustion raises a piston and rotating cylinder temperatures, which reduces local lubricant viscosity and reduces mid-stroke viscous losses as well as rotating liner parasitic losses. Furthermore, based on higher speed baseline engine data, it appears that there is also a reduction in the hydrodynamic piston terms. The reason is attributed to the increased lubricant film thickness at mid-stroke, due to the combined effects of the liner rotation and the traditional wedge effects of the piston skirt and rings. This effect seems to more than compensate for the increase of the hydrodynamic parasitic friction of the liner rotation with engine speed (we currently have a constant 3:1 ratio of the crankshaft to liner speed). The combined fuel efficiency benefit is shown to exceed 10% for medium loads and speeds.