New Method for Evaluating and Optimizing Transient Piston Friction and Cooling Using a High-Power Laser in Motored Operation

The input of combustion heat in engines has a major impact on the piston friction and the resulting wear of the piston skirt. The new methodology presented here enables the simulation of combustion heat input during motored operation, and thus a detailed investigation of the piston friction under realistic piston temperature profiles of real engine operation is possible.

For this purpose a standardized engine test bench for motored friction evaluations was expanded to include, among other things, a movable high-power diode laser with special defocusing optics. The setup of the test engine is based on the FEV teardown step methodology [1] and has open access to the engine piston from above due to a cylinder head dummy. Thus, the heat input by means of a high-power diode laser into the piston crown can be made. The reduced engine structure also enables a precise and highly accurate evaluation of the piston friction. A previously conducted validation process of the methodology ensures the most accurate possible replication of fired piston temperature profiles. The comparison between the piston temperatures measured in fired operation and those simulated in motored operation for a partial load operating point shows a maximum variance deviation of only 15°C depending on the measuring point.

The new methodology is also used in particular for the evaluation and detection of critical piston friction conditions. Experiments in this context are presented and discussed exemplary by using three measurement series at different operating temperatures and engine speeds.

There is a gradual increase in the laser power for each series of measurements and thus in the heat input into the piston. The increase in heat input leads to a significant increase in friction at all operating points due to thermal expansion and the associated decrease reduction in piston clearance. Depending on the operating temperature and the engine speed, a critical piston friction condition is achieved and detected by the level of friction increase. The additional use of ultrasonic sensors and the knock sensor installed as standard makes a simultaneous measurement of the structure-borne sound signals possible. The increase in the acceleration levels of all sensors correlates here with the increase in piston friction. An evaluation of the noise, vibration, and harshness (NVH) measurement in both the frequency range and the crank angle (CA) range shows conspicuous high-frequency excitation levels that occur in the top dead center area. This correlation can be proven for all three measurement series.

The results obtained here may open a path to an improved piston cooling strategy in the future.