A diesel generator is the set of diesel engine and 
alternator. Disel engine is prime mover which rotates the 
alternator and alternator generates the electricity. 
Further diesel engine works on principle of carnot cycle, 
air is compressed in a cylender, then it is compressed up 
to high pressure, disel is injected to this compressed air 
thus cumbustion takes place,this energy force to move the 
piston in the cylendrical passage, piston is further 
connected to crank shaft. on the other hand generator 
wotrks on principle of electo magnetic induction,  when a 
DC excited rotor is rotated then at synchonous speed EMF 
induced in static armature.