piston motion_not completing the cycle after a number of cycles
 

Dear all,

I am running a 3D simulation of a solar stirling engine using dynamic mesh and in cylinder tool plus a UDF to govern the piston motion.

Recently I have noticed that the pistons do not reach their initial location at the end of one cycle.

This has started happening after a number of cycles during which the model run normally and without me changing anything in the UDF or the in-cylinder settings.
e.g. during the 50th cycle the model is ok but then, at the end of the 51st cycle the pistons do not fully return to their initial locations.

This is visible in the pressure vs time step plots and P-V diagrams.

I would appreciate if anyone had any ideas they could share.

Best regards,
Thanos