I plotted a graph which shows the behavior of a working engine: RPM as $x$ against time as $y$, around 4000 $(x,y)$ points are imported from the experiment data.
I am trying to find an expression which fits the graph and can be used further with a multi-body simulation to simulate the process.
The real question is: which function would be best representing the experimental behavior of the graph?
I tried the curve fitting tool from Matlab and Lagrangian interpolation but not satisfying the graph. A brief idea about a choice of function to use and its procedure and how the graph can be analyzed would help me understand analyzing my data in future in a better way.
Note: The noise from 0-1.3 seconds can be assumed constant.