Why is diesel no longer “green”?


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

Diesel and gasoline are both products of fractional distillation of crude oil. Crude oil is a huge mishmash of different compounds formed when underground when algae and plankton were buried underground and pressurized. All these different compounds behave slightly differently. One of these different behaviors is their boiling points. Fractional distillation takes advantage of this by heating up the oil to different temperatures to capture what boils at each temperature. A distillation column can do this continuously, by having “stages” at varying heights that are heated to different temperatures in a gradient. So at each “stage,” you may have a different product.

At the lowest stages (the ones that don’t boil or boil only at very high temperatures), you have tar- and asphalt-type compounds. Higher up, you get bunker fuel, which is fuel oil for ships and heaters. Higher than that, you get diesel and mineral oils. Higher than that is kerosene, jet fuel, and gasoline, and so on. So they come from the same material, but they are different (although pretty similar) parts of that original material.

Our definition of “green” changed.

It used to be purely about fuel consumption…how many gallons do you burn. And Diesel engines are inherently more efficient that gasoline (technically Otto cycle) engines. So they burned less gas, and that was greener.

But because of diesel engines’ different combustion behavior, you also get differences in the exhaust. Diesel tends to have more particulates, more NOx (contributes to smog), and more unburned hydrocarbons. You can manage that to some extent with better engine designs and emission controls but those are also pollutants and we take a lot more notice of them now than we used to.

Diesel engines and gasoline engines operate using diffirent principles – you can almost think of it like different operating parameters. The diesel cycle is inherently more thermodynamically efficient than the Otto cycle (how gasoline engines operate). However, they burn less clean, essentially less of the fuel is spent during the process, and I believe the fuel is also less purified – what this translates to is there are more pollutants spit out in the exhaust. The Holy grail has always been to filter the exhaust enough that diesel cars were as clean burning as their gasoline counterparts. Car companies lead us to believe they achieved this. It turns out – they actually just managed trick the emissions tests, and they’re inherently as dirty as ever. If you clean them up – they end up being less fuel efficient, essentially the juice isn’t worth the squeeze. The result – the diesel boom deflated.

Years of study eventually showed that some of the waste products of diesel burning were just as bad as those from petrol (gasoline) – it’s *never* been greener, we just used to think that it was. As we learn, the science changes. And it’s an oil derivative, so it’s never going to be exactly “green” anyway.

We also discovered that the NOx emissions were killing people and just basic “smoke/dust” (“particulate matter”) from burning it was far worse.

That meant more stringent controls. The EU EN590 standards, etc. really cracked down on what was allowed, which meant that diesels essentially became the same as or more expensive than other ICE vehicles. Diesel now has to give out 200 times less sulphur than in the 90’s, for example. Things like AdBlue came on the market to try to reduce emissions from burning diesel in vehicles.

And then we found that EVEN THOUGH we’d done all that, things like NOx weren’t dropping as much as we’d expect. It was at that point that we discovered that many car companies were literally just cheating the emissions tests and actually pumping out as much as they used to, but hiding it behind software trickery designed to detect if the vehicle was being tested for emissions or driven normally. They claimed that they basically couldn’t make a car with the performance they wanted if they didn’t give out those emissions.

That led to BILLIONS of dollars of lawsuits and recalls for many/most diesel manufacturers. That cost almost certainly is being recouped across the industry, not just diesels, but it also means diesels are under far more scrutiny now and thus costs are rising.

Diesel being greener was always a con, basically. We just didn’t know it for sure and people still bought them for decades thinking they were cleaner.

I can remember calling bullshit on this back in the early 90’s – my dad was a fleet mechanic for lorries, etc. and claimed (as he’d been told) that diesels were cleaner. It was always bollocks. By the 2000’s we knew it was a lie. Nowadays, people thinking that diesel is cleaner because of what they grew up with is still rife (as your question shows).

After a bit of scrolling, I’m surprised not to have seen a mention of diesel exhaust fluid (DEF). Diesel engines made after 2010 are required by law to operate with DEF in accordance with the EPA. I’m not an expert on the science of it all, but I know my entire work fleet needs it. It helps turn exhaust into nitrogen and hydrogen, but again, I’m no expert.

You’d need a chemist to back me up on this (or correct me if I’m misinformed) but I was told years ago that the refining process for gasoline/petrol is more expensive than the process for diesel, so diesel was cheaper.

More recently, though, apparently chemists found some profitable uses for by-products leftover after gasoline/petrol production. You don’t get all these useful by-products from producing diesel, so refiners started charging more for diesel, since selling it meant they were missing out on the extra sales they would get for other products when refining gasoline.

Supposedly (in the USA, at least) taxes had nothing to do with it.

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)


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”.


Because youre only green in the wrestling business for so long. Once Nash picked up some experience he started to a learn a thing or two. Tbh Diesel now is a veteran and I think he would be offended that you still thought he was green.