Is it necessary to specify ignition timing for diesel combustion simulation.

Please elaborate, what exactly do you mean?

No. You specify injection timing.

Copy, you specify injection timing and second you can "influence" the injection timing...but it depends on the combustion model that you use:
- if you use a model associated with the Shell ingnition model you specify some ignition characteristic such as characteristic temperature, cetane number, etc.
- if you use comb. model from the ECFM family you can define only the cetane number and by means of that you can vary the time of combustion

I am using shell auto ignition model and default value used is 1100K.Is it correct?

Yes it is correct, you can try to vary the start of injection by means of the costant AF04 of the model. Try to vary it (by means of posdat.f for example)...a value of 2.5e6 may be a good fist attempt to match the correct ignition time