Hello Mr Bhaswar Mukhopadyay,
CNG vehicles are different from the normal petrol and diesel vehivles. A normal vehicle can be installed with a CNG kit, as this is not factory fitted its not that much efficient but nowadays CNG engines are manufactured by the companies inly with alternative fuel as Petrol. These cars will run on both CNG and Petrol. There are CNG autorickshaws too in the market which is very much efficient. The CNG gas emits minimum amount of harmful gases and so it is less polluting and is the most preferred form of fuel nowadays. The installation takes a little bit of expense but its nothing infront of our Environmental cost as well as the petrol prices.