Reducing friction is an important aspect to increase the efficiency of internal combustion engines (ICE). The majority of frictional losses in engines are related to both the piston skirt and piston ring–cylinder liner (PRCL) arrangement. We studied the enhancement of the conformation of the PRCL arrangement based on the assumption that a suitable conical liner in its cold state may deform into a liner with nearly straight parallel walls in the fired state due to the impact of mechanical and thermal stresses. Combining the initially conical shape with a noncircular cross section will bring the liner even closer to the perfect cylindrical shape in the fired state. Hence, a significant friction reduction can be expected. For the investigation, the numerical method was first developed to simulate the liner deformation with advanced finite element methods. This was validated with given experimental data of the deformation for a gasoline engine in its fired state. In the next step, initially conically and/or elliptically shaped liners were investigated for their deformation between the cold and fired state. It was found that, for liners being both conical and elliptical in their cold state, a significant increase of straightness, parallelism, and roundness was reached in the fired state. The combined elliptical-conical liner led to a reduced straightness error by more than 50% compared to the cylindrical liner. The parallelism error was reduced by 60% to 70% and the roundness error was reduced between 70% and 80% at different liner positions. These numerical results show interesting potential for the friction reduction in the piston-liner arrangement within internal combustion engines.

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<div class="section abstract"><div class="htmlview paragraph">The increasing demand for environmentally friendly and fuel-efficient transportation and power generation requires further optimization and minimization of friction power losses. With up to 50% of the overall friction, the piston cylinder unit (PCU) shows most potential within the internal combustion engine (ICE) to increase mechanical efficiency. Calculating friction of internal combustion engines, especially the friction contribution from piston rings and skirt, requires detailed knowledge of the dynamics and lubrication regime of the components being in contact. Part I of this research presents a successful match of simulated and measured piston inter-ring pressures at numerous operation points [<span class="xref">1</span>] and constitutes the starting point for the comparison of simulated and measured piston group friction forces as presented in this research.</div><div class="htmlview paragraph">The authors utilized a single-cylinder floating-liner engine (FLE), based on a heavy-duty diesel truck engine, to determine crank angle resolved friction of the piston cylinder unit. The temperatures of the PCU were measured, and surface temperature distribution and thermal deformation were calculated to ensure realistic oil viscosity and piston and liner deformation under operating condition within the friction simulation.</div><div class="htmlview paragraph">Friction measurements were conducted under motored and fired engine condition. To derive the friction contribution of each ring and the piston skirt separately, motored strip-down tests were conducted as well. Piston ring friction was simulated with a validated ring dynamic simulation tool in combination with flow simulations of the surfaces using a deterministic correlation approach. To consider friction properly within the simulation, high-quality surface representation is needed. Precise optical three-dimensional measurements of the cylinder liner surface and the artificially surface generation for accurate numerical surface representation without measurement errors, revealed to be key. With the simulation model the friction contribution of each ring was compared separately with the strip-down measurement. Moreover, the piston secondary motion and the friction behavior of the piston skirt was calculated and compared to the measurements as well. As a result, the overall friction of the PCU was compared for motored and fired condition. The friction mean effective pressure (FMEP) as well as the crank-angle resolved friction forces from measurement and simulation were analyzed in detail. The comparison between simulation and FLE measurement was done for engine speeds from 10 to 1500 rpm, at oil temperatures from 40° to 100° C and engine loads up to 15.5 bar IMEP (indicated mean effective pressure).</div><div class="htmlview paragraph">The friction contribution (FMEP) of the simulation and measurement matches very well. The detailed examination of the crank angle resolved friction forces shows very good correlation for the hydrodynamic, and boundary lubrication regions.</div><div class="htmlview paragraph">This research successfully proves the ability to predict the friction forces and power losses of the different components of the PCU in combination with honed cylinder liners. It also reveals the importance of the quality of the input parameters such as surface topographies, surface temperatures respectively thermal deformations, oil parameters and contact geometries. Reliable input in combination with experimental data for the validation of the simulation models, enables the utilization of the simulation tools for reliable predictive design approaches.</div></div>

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