Difference between BS3 and BS4 engines

Daimler India Commercial Vehicles (DICV) today said BS3 vehicles should not be allowed to sell in the country after April 1, the deadline for switch to BS4 norms, as it is already scaling down such vehicles. 

BS is basically the emission standards given by Government of India to regulate the air pollutants coming from internal combustion engine. BS stands for Bharat Stage and are set by the central pollution control board.

1. BS3 is compulsory for all india, while BS4 is compulsory for all metro cities. 
2. Most petrol powered passanger car in india have alrady converted to BS4 many year ago.

3. Difference Between BS3 &BS4:

   BS3 
   carbonmonoxide 2.30 g/km 
   HC 0.20 g/km 
   NOX 0.15 g/km
   BS4 
   carbonmonoxide 1.00 g/km 
   HC 0.10 g/km 
   NOX 0.08g/km
4. BS4 vehicle is costly than BS3