Investigations of Crank Offset and It's Influence on Piston and Piston Ring Friction Behavior Based on Simulation and Testing 2007-01-1248
Due to the rapidly increasing raw oil price the reduction of fuel consumption has become one of the most important targets for the development of modern passenger car engines. After large progress has been achieved in the combustion process development - CAE has been one of the keys to success - nowadays further potential is being investigated.
The mechanical friction is very much in the focus of the engine development engineers. While in the Valve Train the potential of roller contacts and surface treatment is the main development direction, in the cranktrain the reduction of bearing diameters is being investigated. Due to increasing specific loads on the crankshaft there are clear limits. At the piston group the potential is almost untouched.
While optimizations of the piston skirt contour or the ring pack bring up the risk of negative influences on blow by and oil consumption, the application of a crank offset is an easy design measure having almost no risks.
This paper discusses the possibilities of friction reduction of pistons and piston rings by applying a crank offset. Testing and simulation are combined such that both disciplines play their best possible role:
Measurements of piston friction at a fired engine are used to validate the simulation model at some chosen engine working conditions with low measurement risks. The crank offset influence is determined for these working points
Multibody Simulation (MBS) is used to analyze a wide range of different engine working conditions and different crank offset values. The result is a database, characterizing the friction behavior and sensitivity regarding crank offset.
Finally the engine's rpm & load time history, as result of a vehicle cycle simulation, is used to “drive through” the previously generated database, to quantify the saved energy, average friction power decrease and reduction of fuel consumption for a standardized vehicle driving cycle.
With this new approach it is now possible, to evaluate not only the benefit under specific engine working conditions, but also to give a clear recommendation about crank offset magnitude under consideration of realistic vehicle driving conditions.