There is an old expression that says, “Bad data in, bad data out”, or words to that effect. A key skill an accomplished CAE engineer must have is to be able to verify the quality of input data but also judge the quality and physical validity of simulation results. It is by means of these skills, combined with sophisticated physical modelling packages that CAE can truly aid the faster and more cost-effective development of systems and their componentry.
Assuming the accuracy and validity of the physics within the models we are using has been verified, checking the quality of the data we have been provided or that we measure that is destined to model parameterisation, is of paramount importance to ensure that the simulation results will not be misleading.
In Dymola (Dynamic Modelling Laboratory) we represent physical systems by describing the physical relationships between physical interfaces and the variables and parameters contained within a given model. These acausal physical relationships, unlike most other simulation tools, can be multi-domain; i.e. integrate multiple physical domains into one model in an acausal manner. In this example, the effect of cam and valve lift data quality on the simulation results is analysed. A crank angle resolved engine model from the VeSyMA-Engines library is used to analyse the fluid and multibody mechanics physical domain effects of using noisy valve and cam lift data to parameterise the camshaft models. This excerercise comes from a recent project where the customer wanted to expore the interaction of the valve 1-d fluid dynamics with the multibody mechanics of the valvetrain system.
The data presented in figures 1a and 1b show the original and corrected valve and cam lift data directly extracted from the parameterisation of an equivalent engine in a 1-d engine fluids package:
Figure 1a: Noisy valve and cam lift data derived from a manual test rig. X axis is cam rotation angle in degrees.
Figure 1b: Corrected valve and cam lift data. X axis is cam rotation angle in degrees.
Figure 2 shows that whilst the fluid dynamics of the mass flow of air flowing through the intake valve is not significantly affected by the noisy data, the valve acceleration is significantly affected, causing excessive and unrealistic excitation to the multibody mechanics of the valve train. This issue was not apparent in the 1-d engine fluid dynamics simulation tool as the multibody mechanics could not be represented. The cam and valve lift data was found to have been measured on a manual test rig and not taken from CAD. CAD data availability allowed new correct data to be used within the VeSyMA-Engines valvetrain components and the correct multibody dynamics to be simulated.
Figure 2: Fluid dynamics and valvetrain multibody mechanics simulation results using corrected (blue) and non-corrected (red) cam and valve lift data. X axes show time in seconds.
Written by: Alessandro Picarelli – Chief Engineer
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