Question

Which fuels are less polluting and why?

Answer 1

I think so CNG is a less polluting fuel as it takes very less energy and doesn’t polluted

Answer 2

Less pollution and more efficiency: CNG emits significantly less pollution directly than gasoline or oil when combusted [ e.g., carbon dioxide (CO2), unburned hydrocarbons (UHC), carbon monoxide (CO), nitrogen oxides (NOx), sulfur oxides (SOx) and PM (particulate matter) ]

They are least polluting. Actually pollution caused by burning them is zero because when they burns with oxygen the products of combusion is water/water vapour.