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.
