The major pollutant/parts in petrol based vehicles is/are

The major pollutants/parts in petrol-based vehicles are Fuel tank, Crankcase and Carburetor.
The crankcase is formed by the portion of the cylinder block below the cylinder bores and the stamped or cast metal oil pan that forms the lower enclosure of the engine and also serves as a lubricating oil reservoir or sump.
Carburetor, also spelled carburettor, device for supplying a spark-ignition engine with a mixture of fuel and air. Components of carburetors usually include a storage chamber for liquid fuel, a choke, an idling jet, a main jet, a venturi-shaped air-flow restriction, and an accelerator pump.