[jabl]

AFAIU modern diesels have lower combustion temperatures in order to limit NOx formation. I imagine this comes at a cost in thermodynamic efficiency.

Similarly particle filters, catalytic converters and whatnot reduce efficiency via exhaust backpressure.

So all in all, reducing non-CO2 emissions do come at a cost in CO2 emissions (or fuel consumed, if you like). Is it as much as 15%? No idea.. And is it all worth it? I'd argue yes, old-school diesel exhaust is nasty stuff.

[bri3d]

Yes, this is the exact trade from Dieselgate, as well - higher combustion temperatures increase efficiency, reducing CO2 output, but at the expense of much higher NOx production. It’s easily 15% or more.

[TylerE]

Exhaust back pressure is not an issue at all on diesels because they all run big turbos.

As for your later point...I very much concur. I started masking (n95) during the pandemic, and haven't stopped. I have a... large number of health issues including several respiratory ones. Exhaust in general really is nasty stuff. I live in pretty quiet smallish (100k) town, and it can be bad enough around here with all the pollen, but I wasnt recently on a whirlwind trip through the north east that saw me visit the dense urban cores of DC, Philly, Manhattan, and Boston. The difference in odor on the occasions I'd take my mask off on the sidewalk were kinda shocking as someone not used to it.

[dexwiz]

Out of curiosity do you live in the Great Lakes basin? I moved from Indiana to the West Coast, and I am always shocked at how bad the air quality is when I go back, even in rural areas. During wild fire season I monitor AQI, and when I zoom out I always see that Indiana is just as bad as the smokiest days on the West Coast.

Something about the large, hardly noticeable depression traps bad air at a regional level. I wouldn’t move back for a myriad of reasons, but everyone is always surprised when I list air quality as one.

[selecsosi]

Similar migration (IN->CA), it has changed my life. I had the worst sinus/migraine issues growing up and my breathing has dramatically improved after moving out of Midwest, even compared on the worst wildfire weeks out here.

Coming from NWI, I know the mills had a lot to do with subpar air quality, but I had similar issues when I lived on the far Northside of Chicago so it seems to be more regionally affecting.

It's a similar top reason on my list to not living there long term again

[dexwiz]

Same with the sinus infections. Cold air + pollution = two weeks of sinus infections every winter. I haven’t had one since leaving.

[TylerE]

North Carolina. Locally it's not so much exhaust type stuff, but all the damn pollen, which I'm somewhat allergic too. Pine trees for miles. I'm about 50 miles inland from the coast so lots of wetlands so in the summer (Which means basically... late March through mid October - it's going to hit 82 here toomorrow) it's oppressively hot and humid.

[jabl]

> Exhaust back pressure is not an issue at all on diesels because they all run big turbos.

Not sure what you're arguing here. Isn't it quite obvious that resistance in the exhaust system means that the engine has to do more work to push the exhaust gases out; work that otherwise could be used to turn the crankshaft. Now of course a lot of that extra energy is wasted in any case, particularly if there's nothing like a turbocharger to make use of it.

[TylerE]

Practically all diesels have turbos. You have literally have to go back to tiny tractor engines form The 60s to Find ones that aren’t. Turbos already provide a ton of back pressure. What’s downstream of that is pretty irrelevant

[jcgrillo]

My tractor from the early 00s doesn't have a turbo. Plenty of VW and Mercedes Benz cars built in the 80s and 90s don't have turbos.

Backpressure aft of the turbo is something you really want to avoid because it makes the turbine much less effective. The whole point is to use a pressure and thermal gradient to do work.

[12thhandyman]

Ford and VW put a naturally aspirated diesel into their vehicles up until the early/mid 90s.

[TylerE]

Absolutely none of which have DPFs or EGRs.

[lupusreal]

Backpressure to the turbo reduces the efficiency of the turbo, no?

[throwaway48476]

I was under the impression the engine had to be run hotter in order to break down the NOx particulates in the exhaust gas recirculation scheme which led to higher fuel burn.

[themaninthedark]

No, it is a lower temp which results in less burn hence the need to recirculate the exhaust back through to completely burn it....

[m463]

hmmm interesting. In a gasoline engine with high temperatures, NOx is produced, but it is fixed by the catalytic converter. So I guess that's not possible with a diesel, so they have to lower temperatures. That must lower the efficiency of the engine because (I think) the temperature difference governs efficiency.

[themaninthedark]

https://www.natso.com/what-is-the-difference-between-exhaust...

Here is a quick overview of how the system works.

Currently only smaller engines are EGR+DPF only. It takes having an SCR(&DEF) system to reduce emissions to legal limits on most larger engines now and the technology has been combined to have an engine with EGR+DPF&SCR(&DEF).

There are issues with engines and the active regeneration that has to occur in order to clean the DPF(basically engine has to get very hot and burn out the buildup) and also issues with DEF, which is an ammonia solution, both with the electronic dosing units failing and solution purity.

[12thhandyman]

I’m curious how combustion temperatures are lowered like you say - is it a pv=nrt relationship hinging on changes in compression ratio or something like injector technology?

[jabl]

Yes, the latest generation diesel engines have lower compression ration than engines from a couple decades ago. Maybe something like 16:1 instead of 19:1. But also exhaust gas recirculation: https://en.wikipedia.org/wiki/Exhaust_gas_recirculation