What is the difference between Engine of Tractor and Engine of Trucks?

Typically, displacement. A semi truck is technically a tractor.

The infamous Cummins 5.9 direct injection diesel of Dodge Ram pickup fame was originally developed for farm tractor applications. Displacement of tractor motors may vary, dependent on size, function, etc. For a road tractor, 11 - 16 litre displacement is typical (most in the 12.7 - 14.6 litre range), whereas a dump truck may be likely to have lower motor displacements, but could also have just as high a displacement as a road tractor, dependent on how the truck is specified.