negative densities.

Mihir

HI, I'm performing a full cycle IC Engine simulation under motoring conditions. I start with the exhaust stroke and then move on to intake,compression and expansion strokes.During the run,just at the beginning of the compression stroke i get the foll. error:

Error#076 Negative densities at more than 100 cells.Run stopped.

Does anyone know the cause of this error and how to resolve the same? thanx in advance

Mihir.

PS: I'm using ideal f(T,P) for density.