Predictive Piston Cylinder Unit Simulation - Part II: Novel Methodology of Friction Simulation Validation Utilizing Floating-Liner Measurements 2023-01-0415
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 [1] and constitutes the starting point for the comparison of simulated and measured piston group friction forces as presented in this research.
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.
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).
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.
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.
