The basic difference of BS2 and BS3 engines is the presence of catalytic convertor. Catalytic convertor is present in BS3 engine which helps in reducing the formation of HC and CO. While in BS2 engine, no catalytic convertor is present which forms HC and CO. Thus in BS3 engine there is no emission of harmful CO and HC. The full form of BS is bharat stage, which is a standard of emission in India.