Due to the well-known fact of higher thermal efficiency, diesel engines are more popular than any other fuelled prime movers. Because of this feature, the diesel engines are extensively used in almost all on road and off road applications. Power generation is another widespread application of the diesel engines. Over recent past years, stringent emission legislations have been imposed on reduction of emission parameters emitting from diesel engines. Central Pollution Control Board (CPCB) of India has proposed further reduction in emissions applicable from 2013, which are the most stringent limits in the world for this category of engines up to 75 kW.This paper deals with the strategies applied and experimentation details to meet the proposed CPCB Stage-II emission limits. The criticality increases exponentially for naturally aspirated versions. The experimental investigations are carried out on various capacities of 1.0 l to 3.25 l naturally aspirated diesel genset engines.In the upgradaion work, the naturally aspirated engines of different power levels are calibrated and optimized to meet the proposed CPCB Stage-II emission limits. Various combustion parameters such as intake swirl, air flow, combustion bowl and its geometry, compression ratio, nozzle configuration, hydraulic through flow, nozzle hole size, number of holes, injection pressures, start of injection, injection duration, high pressure pipe along with exhaust gas recirculation are optimized and results reported.