High pressure at the end of compression stroke
 

I am simulating the compression stroke of a Diesel Engine in ANSYS ICE workbench.As, the piston move upward in the compression stroke, just before the injection the pressure calculated by the simulation is more than the actual pressure corresponding to the experiment.The actual value is about 20bar but from the simulation the pressure is calculated about 80bar.I am not getting, why the pressure is rising so much ??
How to solve this problem ??