Why is diesel no longer “green”?

Question

When I was growing up, diesel was always considered the greener option than petrol, and this was reflected in the prices, diesel always cost less because it was taxed at a lower rate.

Now they say it’s worse than petrol and prices are now higher for diesel.

So what changed, or what did we suddenly learn about diesel that we didn’t know before?

Bonus question, considering they’re both made from crude oil, what’s the difference in how they’re refined?

In: Chemistry

Answer 1

Everyone has answered most of this already. As someone else pointed out with modern diesel exhaust systems and engine control the exhaust is actually as good or better than most Otto cycle cars.

But I haven’t seen an answer yet in regards to price. I am not 100% sure and don’t have sources, but I believe the main reason for the swap in which fuel was cheaper is not because of tax incentives for “green” or “clean” fuels but because the federal requirements for the fuel became much more stringent (especially in California).
Throughout the years the amount of sulphur allowed in the fuel has plummeted remarkably. (Pre 1993 it was unregulated; post 1993 5,000 ppm, post 2006 way down to 15ppm)

https://www.epa.gov/diesel-fuel-standards/diesel-fuel-standards-and-rulemakings

To get fuel down to this low of sulphur levels you either need to make it from “sweet” crude oil. (Oil that naturally has less sulphur; as opposed to “sour”)
Or / and they have to do expensive processes to remove the sulphur which I know nothing about and won’t pretend to.
Both of which drive up the cost of the fuel noticeably.
I also believe it is this “sulphur removing process” that changed the fuel in a way that made seals shrink and crack and made old vehicles start leaking when the regulations began. Which angered many people and turned them off of the new cleaner fuel.

I believe there was also a minimum cetane increase with the 1993 fuel regulation rollout up to 40. (Again California further demands a minimum of ~50).
Cetane is a measurement of how quickly a fuel will burn after injected. And the quicker this happens the more “complete” it will burn. This generally makes the exhaust less toxic as well as there is more CO2 and less “other toxic things”.

https://en.m.wikipedia.org/wiki/Cetane_number