Powertrain Friction Reduction by Synergistic Optimization of Cylinder Bore Surface and Lubricant - Part 2: Engine Tribology Simulations and Tests

In the present work, a system approach to the tribological optimization of passenger car engines is demonstrated. Experimental data and simulation results are presented to demonstrate the role of surface specifications, ring pack, and lubricant on the piston/bore tribology. The importance of in-design “pairing” of low-viscosity motor oils with the ring pack and the cylinder bore characteristics in order to achieve maximum reduction in GHG emissions and improvement in fuel economy without sacrificing the endurance is elucidated.

Earlier motored friction data for two different gasoline engines - Ford Duratec and Mercedes Benz M133 - using motor oils of different viscosity grades are now rationalized using AVL EXCITE® piston/bore tribology simulations. The main difference between the engines was the cylinder bore surface: honed cast iron vs thermally sprayed, and the valve train type: direct-acting mechanical bucket (DAMB) vs roller finger follower (RFF). The simulation data show that mirror-like bores have very low asperity friction and therefore allow safe deployment of ultralow viscosity oil for improved fuel economy. However bearings may remain vulnerable to wear under high load conditions, calling for higher performance resource conserving lubricants.