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IssacZachary
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Re: ABG: German cities can ban older diesel cars immediately Hamburg will start enforcing new rules by the end of the mo

Tue May 29, 2018 7:55 am

DarthPuppy wrote:And I hear you about the durability. A long time ago before I became more aware of the environmental issues, I had a diesel. They are very efficient, durable and require little maintenance. They definitely are very appealing until you know the downsides for air quality.

Getting back on topic about diesels, I wonder how this would affect commercial vehicles in Germany. The title says "older diesel cars". What about diesel trucks. They also run off the lean, NOx producing diesel cycle. And commercial companies use big diesel vehicles because of their economic benefits, reliability and longevity.

On the other hand, I still don't see why technology can't fix this. Doing research, diesel engines used to produce less NOx than gasoline cars did back before EFI and catalytic converters. But like other any other lean burn technology, it is hard to get rid of NOx when there is lots of oxygen and nitrogen present in the exhaust.

Doing some math, I once figured out how much oxygen flow my engine would need. I also found an oxygen separator that could put out that much oxygen and used less than 1hp. The idea was to cool the exhaust, feed that into the intake, and add the oxygen necessary. No nitrogen, no NOx! The problem is that the separator is very big and would take up both the rear seat and trunk area. It was also expensive. And I'd probably need a bigger radiator to cool the exhaust properly.

If NOx can be eliminated, then things that could make major increases in efficiency could be implemented, like higher compresión ratios, advanced injection timing and no more EGR. The same technology could also be used on gasoline engines, allowing for highly efficient, lean burning, direct injection engines.
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Re: ABG: German cities can ban older diesel cars immediately Hamburg will start enforcing new rules by the end of the mo

Thu May 31, 2018 10:12 pm

Interesting read:

http://www.bbc.co.uk/news/resources/idt-sh/how_toxic_is_your_car_exhaust

The results weren't surprising. An older gasoline car will likely get way worse NOx emissions than newer diesels. But the sad truth is that diesels can get better NOx emissions, but manufacturers are cheating and don't care.
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Re: ABG: German cities can ban older diesel cars immediately Hamburg will start enforcing new rules by the end of the mo

Thu May 31, 2018 10:41 pm

I read the whole piece, and I think I can explain the NOx surprise. Keep in mind that this is based on personal automotive knowledge that is now largely out of date, but since it seems to apply, here goes:

The automotive catalytic converter was first adopted because of huge particulate emissions from...gasoline engines. At the time diesel cars were rare enough that they barely registered in the equation. The cat converters worked very well at removing the particulates from gas engine exhausts, with one small penalty: they produced more NOx in the conversion. This wasn't considered a huge issue, (in large part because NOx isn't black, and while it has an acrid smell it doesn't stink as badly as a rich-running gas engine exhaust) but regulators and manufacturers did try to deal with it, if fairly simply: by making the engines run leaner. This improved the results through two different-but-similar benefits: less fuel in the combustion chamber meant fewer particulates produced, and thus fewer NOx molecules replacing them out the tailpipe, and also because less air (because of less fuel) in the combustion chamber meant less nitrogen as well, and so still less NOx out the tailpipe. There was one minor drawback, of course: most of the engines ran like shit much of the time. Only the better designed fuel systems, like the Lambda Sonde and similar EFI + O2 sensor systems, ran fairly well. Anyway, back to the comparison in the piece: the VW gas engine, with many miles and many years on it, was producing considerably more particulates than it would have when new, due to things like exhaust blowback and burning lubricating oil. The Skoda diesel, designed when it was, was actually trying (and succeeding) in reducing NOx, albeit at the expense of more particulates. If you were to attach a catalytic converter to the rear half of the Skoda's exhaust, it would likely start to spew NOx like nobody's business.

So essentially you have two vehicles that focus on opposite sides of the particulate/NOx problem, with predictable inverse results. They are both dirty, but in different ways.
Last edited by LeftieBiker on Thu May 31, 2018 11:39 pm, edited 1 time in total.
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Re: ABG: German cities can ban older diesel cars immediately Hamburg will start enforcing new rules by the end of the mo

Thu May 31, 2018 11:15 pm

I'll have to research that one. I've never heard of catalytic converters causing more NOx, and it doesn't even compute as to how that would happen in my brain. I've only heard of NOx happening in high heat and high pressure environments. I can't see a catalytic converter causing anywhere near heat or pressure that is produced by the actual combustion event, the event that is, as far as I understand, the culprit of NOx production. Three-way cats even reduce NOx.

But what do you mean by leaner? Normally leaner means more air and less fuel, not less air. Air is dependent upon RPM and throttle position. Since the RPM is geared to the wheels, it's the throttle position that counts here. As you lean out an engine it loses power, meaning you have to open up the throttle more to get the same amount of power in order to do the same amount of work (for an example, maintain 65mph down a freeway.)

Also less air leads to more particulates, not less. That or a less homogenous air/fuel charge, which basically creates the same thing, rich pockets of fuel in the combustion chamber, which is why diesels create more particulates in spite of being leaner running engines.

I also was under the impression that catalytic converters were adopted to reduce HC and CO emissions, not particulate emissions.

A particulate/NOx balance does make some sense. Richer, more particulate and less NOx. Leaner, less particulate and more NOx. More EGR, more particulate and less NOx. Although here we have an example of the oposite. The car that runs richer is producing more NOx.

Edit
My take on it is this. NOx is produced close to, but leaner than stoichiometric. In a modern gasoline engine the goal is to make everything as homogenous as possible and as close to stoichiometric as possible. Then the resulting NOx, CO and HC emissions are eliminated in the catalytic converter. Particulates are near zero in a modern gasoline engine, with or without the cat.

In a diesel you have one area that is very rich and another that is very lean in the same combustion chamber. Now you have it so rich it causes particulates. The overall lean mix does reduce CO and HC to near nothing, but ashes are harder to burn so you get particulates. There is an area where things are a bit leaner than stoichiometric, causing that nasty NOx. Then the rest is too lean to cause NOx, but still burns up HC and CO nicely.

In an older gasoline engine you don't get as good of a homogenous air fuel mix. You get cylinders and even pockets in each cylinder that are leaner and richer, which in turn produce larger amounts of NOx and HC and CO emissions respectively. The overall combustion isn't lean enough to eliminate the HC and CO like in a diesel either. And there really isn't any lean zone lean enough to stop NOx production like in a diesel. So you get way more NOx, HC and CO. This was the way it was back when diesels came out. They produced way less NOx, HC and CO than gasoline engines, with only particulates being higher.

But the NOx emissions are hard to reduce in a diesel. In a modern gasoline engine they are still being produced, but they are easily reduced with the HC emissions in a three way cat. The HC is used to strip the oxygen off the NOx and so you basically kill two birds with one stone. And whatever HC and CO emissions are left after reducing NOx these are burnt up in the second part of the catalytic converter with either the little oxygen left in the exhaust or with added air from a pump.

But a diesel always runs lean. There is no way to make a diesel run well at stoichiometric. So you get a lot of oxygen in the exhaust. With the extra oxygen any catalytic converter will not be able to use the HC emissions effectively to strip oxygen off the NOx molecules because it's too easy for them to just strip normal oxygen molecules from the exhaust. Also, a diesel produces less HC emissions from the engine in the first place, so again you have less to work with. So now you have NOx and nothing to lower them. Unless you squirt something like extra fuel into the exhaust. That's how VW got good emissions but poor fuel mileage at the lab under testing conditions. Or DEF.

But regardless, if you have to chose between a newer diesel and an older gasoline, the newer diesel will likely get the better emissions. So it's not just a diesel vs. gasoline. It's a newer diesel vs a newer gasoline.

Which ironically brings up a valid point. If older diesels are banned for NOx reasons, so should older gasoline vehicles that are also getting much worse NOx than modern vehicles.
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Re: ABG: German cities can ban older diesel cars immediately Hamburg will start enforcing new rules by the end of the mo

Thu May 31, 2018 11:46 pm

The drawback of writing late at night while tired: I made the same typo/transposition twice. I meant to write less fuel in the combustion chamber, not less air. I corrected it. As for why cat converters were originally adopted, it's my understanding that it was directly because of smog in California, and the need to reduce the particulates in the exhaust in the LA Basin especially. That was the origin of the stricter California emissions rules, now CARB state emissions rules, and the catalytic converter was found to be effective at that.

As for the chemistry of NOx formation, I was taught that the tradeoff in using the catalytic converters (aside from having to remove that lovely valve-lubricating lead from gas) was that they produced more NOx as they removed particulates. I never learned the mechanism behind that. It may have been that what I was taught was just plain wrong. I'll look into that.
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Re: ABG: German cities can ban older diesel cars immediately Hamburg will start enforcing new rules by the end of the mo

Thu May 31, 2018 11:52 pm

Last post before my nap. This somewhat contradictory explanation fits. I (and I suspect some of my sources, back in the day) was confusing N2O with NOx. From

https://www.explainthatstuff.com/catalyticconverters.html :


But when it comes to climate change, auto engineers and environmentalists have long pointed out another serious issue. Although cats turn most nitrogen oxides into nitrogen and oxygen, they also produce small amounts of nitrous oxide (N2O) in the process, a greenhouse gas that's over 300 times more potent than carbon dioxide. The trouble is that with so many vehicles on the road, even small amounts of nitrous oxide add up to a major problem. Back in 2000, the Intergovernmental Panel on Climate Change noted: "The introduction of catalytic converters as a pollution control measure in the majority of industrialized countries is resulting in a substantial increase in N2O emissions from gasoline vehicles." Fortunately, newer catalytic converters produce dramatically less nitrous oxide than older ones. Even so, while catalytic converters have certainly helped us to tackle short-term air pollution, there are concerns that, when it comes to long-term climate change, they could be making matters worse.
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Re: ABG: German cities can ban older diesel cars immediately Hamburg will start enforcing new rules by the end of the mo

Fri Jun 01, 2018 12:25 am

Ok. Typos aside, then I'm not saying you're wrong, just something I've never heard before.
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Re: ABG: German cities can ban older diesel cars immediately Hamburg will start enforcing new rules by the end of the mo

Fri Jun 01, 2018 12:29 am

It's N2O that the converters produce, not NOx. That's something I learned incorrectly, for whatever reason. There is still a tradeoff, but with a different molecule on one side.
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Re: ABG: German cities can ban older diesel cars immediately Hamburg will start enforcing new rules by the end of the mo

Fri Jun 01, 2018 2:36 am

LeftieBiker wrote:It's N2O that the converters produce, not NOx. That's something I learned incorrectly, for whatever reason. There is still a tradeoff, but with a different molecule on one side.

Ok. That makes sense.

Usually whenever I think I disagree with someone, I find out later we were actually thinking alike.

P.S. Too bad you can't just put an older catalytic converter on your intake.
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