Fossil fuel engines produce the most emissions
- Diesel, gasoline, petrol and natural gas-powered buses all emit carbon dioxide, from the burning of fossil fuels (coal, oil and natural gas).
- They also emit nitrogen, which is harmless.
- Other emissions are nitrous and sulphur oxides which cause acid rain
- Water vapour
- Cyclic aromatic compounds, aldehydes and ketones from reforming processes in the combustion area
- Fuel losses vaporize to produce hydrocarbon emissions
- Alcohols from the windshield cleaner
Hydrogen (as a combusted fuel) powered vehicles emit pure water and heat.
Buses powered by fuel cells emit pure water, heat, and oxygen, but still require gaseous hydrocarbon gases or hydrogen to operate.
Most electric buses emit no gases at all except for alcohols from the windshield solutions, but some of the batteries used for storage may have emissions.
- Lead acid batteries can leak hydrogen sulfide gas which is toxic and hydrogen gas which is explosive.
- Lithium-ion batteries have zero emissions, but there are polluting elements in the production process, and the batteries do produce heat
- Some trolley buses are still in use which have no on board power source but draw power from overhead lines. These are essentially pollution free
Some cars are powered by compressed air or flywheel energy storage and these technologies may be available for buses in the future and be essentially clean
