I believe DICV owned Bharatbenz is going to eat into a major chunk of Tata & AL sales. If Volvo or Man could replicate their strategy, monopoly of the Indians could be over very soon.
I guess that most of the TATA motors engines are based on Mercedez Benz truck engines used on first generation trucks and present engines are came from TATA Daewoo and TATA Hipsona
Neptune engines and U truck platforms are from AVIA Trucks that owned by AL
Tata heavy duty trucks employ Cummins engines (sourced from Tata-Cummins and Cummins India). They are the world leaders in Heavy duty diesel engine technology. This is common practice. Some of their engines in LCVs, however, are developed in-house, probably derived from old Mercedes designs.
Neptune engines are homegrown, developed with assistance from AVL.