Quote - https://theconversation.com/fact-check-are-diesel-cars-really-more-polluting-than-petrol-cars-76241
Diesel was promoted as a more environmentally friendly fuel as part of the EU’s response to the 1997 Kyoto Protocol to reduce greenhouse gas emissions, especially carbon dioxide (CO₂). Diesel engines are “lean-burn”, meaning they use less fuel and more air to get the same performance as a petrol engine.
So, while diesel fuel contains slightly more carbon (2.68kg CO₂/litre) than petrol (2.31kg CO₂/litre), overall CO₂ emissions of a diesel car tend to be lower. In use, on average, this equates to around 200g CO₂/km for petrol and 120g CO₂/km for diesel.
But even when governments were promoting diesel cars, we knew there were issues with toxic emissions (those immediately harmful to humans, not CO₂). Heating air in an engine produces nitrogen oxides (NOₓ) which include the toxic nitrogen dioxide (NO₂), greenhouse gas nitrous oxide (N₂O) and nitric oxide (NO), which reacts with oxygen to form NO₂. In a petrol car, these can be cleaned up by a three-way catalytic converter so that it emits on average around 30% less NOₓ than a diesel car, without after-treatment.
We know that long-term exposure to nitric oxide can significantly increase the risk of respiratory problems, and so these emissions have been regulated for some time. The fine particulate matter (PM) that diesel engines produce also causes cancer and can have acute respiratory effects.
Particulate filters in car exhausts can reduce PM emissions by more than 90%, but they require good operating conditions and regular maintenance. They can also produce more nitrogen dioxide, making diesel one of the main sources of this toxic gas.
