Measuring and Simulating Friction between Piston Pin and Connecting Rod on a Tribometer Test Bench to Define Locally Resolved Friction Coefficients

Measuring and simulating the contact between piston pin and connecting rod (conrod) is very complex. The pin can rotate freely in the conrod as well as in the piston. Further, there is no defined oil supply with a constant pressure as it is for example in main bearings. A tribometer test bench was adapted to measure friction between pin and conrod. The system is loaded with a constant force and oil supply is realized as defined deficient lubrication. During one part of the schedule, the rotational speed is defined as ramp to measure friction coefficient over speed, in another part the speed was pivoted from positive to negative speed within less than 500 milliseconds. With this measurement method, the different friction coefficients between radial slider and pivot bearings could be quantified. The measurements were conducted for four different pin-coatings. The surface structure of each pin was scanned with a confocal microscope before and after measurement, which is an important input to define the contact pressure in simulation. Within the elastohydrodynamic simulation the oil supply approximates the same deficient lubrication as on the test bench. In a first step, the simulation with a constant friction coefficient was compared to measured values. These results demonstrated a big mismatch between the friction moments at lower rotational speed. The measurement is integrated over the whole contact surface, whereas in simulation it is separated in more than 2000 EHD elements. To improve simulation in a second step, an asperity contact pressure dependent friction coefficient was defined for each element. This lead to a very good accordance between simulation and measurement. The definition of a local friction coefficient can be transferred by similarity relation from test bench to higher loads and speeds, like they appear in internal combustion engines. With this methodology, friction in mixed lubrication regions of complex systems can be simulated accurately.