Crankshaft Offset and it’s Impact on Piston and Piston Ring Friction Behavior

Increasing fuel economy in modern passenger car engines has become one of the primary development targets, due to swiftly increasing raw oil prices. Initially, vast progress was achieved in the development of the combustion process. Computer Aided Engineering (CAE) has been one of the keys to success; however, further potential is now being investigated. Currently, the primary focus of engine development engineers is mechanical friction. The greatest potential for reducing friction in the valvetrain lies with the roller contacts and surface treatment. The primary focus of the development direction, for the cranktrain is in reducing bearing diameters. Due to increasing specific loads on the crankshaft there are clear limits that have been identified. Meanwhile, the potential for the development of the piston group is almost untouched. Optimization of the piston skirt contour and/or the ring pack introduces the negative risks blow-by and oil consumption. However, modifications to the crankshaft offset are relatively easy design measures that have almost no risk. The potential for friction reduction of the pistons and piston rings is outlined here through the application of a crankshaft offset. Testing and simulation are combined for the following disciplines: • Measurements of piston friction in a fired engine are used to validate the simulation model under some select engine working conditions that have low measurement risks. The influence of crankshaft offset is determined for these working points • Multibody Simulation (MBS) is used to analyze a wide variety of engine working conditions and different crankshaft offset values. The resulting database, characterizes friction behavior and sensitivity regarding crankshaft offset. The engine’s rpm and load time history, as a result of the vehicle cycle simulation, is used to “drive through” the previously generated database. The information gathered from that data is used to quantify the energy saved, decrease in the average friction power and the reduction of fuel consumption for a standardized vehicle driving cycle. Utilization of this new approach enables the evaluation of not only the benefits of operating the engine under specific working conditions, but also provides a clear recommendation concerning the magnitude of crankshaft offset under consideration for realistic vehicle driving conditions.