Define unsteady BC
 

Hi everybody,
I am working on a gas flow between cylinder and piston of CI engine.
I have assumed Combustion chamber pressure as a pressure inlet.
I want to write a UDF to define a pressure inlet.
Equation between combustion chamber pressure and crankshaft angle is:
y=combustion chamber pressure
x=crankshaft angle
y(x)= -1.16e-06*x^3-8.24e-04*x^2+0.042*x+3.66

Now, my questions about writing a UDF for this case are:

1. How can I the number of steps?
2. How can I define start and end point?

I would be pleased if someone explain this for me with a written UDF.