Friday, September 25, 2015

A chemist’s thoughts on the Volkswagen scandal (and CJ's questions)

I have scant thoughts about the Volkswagen scandal, which I will cover down below. That said, I do not know the first thing about diesel engines or nitric oxide (or mama, or trains, or prison, for that matter*). But via Twitter, I saw Old Biddy had some opinions about it, and she is a lot more of a subject matter expert, so I asked her to write about it:
I worked on new zeolite catalysts for diesel deNOx for several years.  The revelation that Volkswagen designed the catalytic converters on their TDI diesel automobiles to cheat on smog tests was absolutely shocking and depressing to me.
A short primer on automotive emissions control: In the United States, gas-powered cars have what is known as a three-way catalytic converter and use a combination of Rh, Pt, and/or Pd loaded onto a porous ceramic monolith through which the exhaust passes.  Other additives, such as cerium oxide, may also be present to maximize catalytic performance.  Three-way refers to the three things the converter does – oxidize CO to CO2, reduce NOx to nitrogen, and oxidize unburnt fuel and soot to CO2.  Oxygen sensors and fuel injectors cycle the system back and forth between “lean” (excess O2) and “rich” (excess fuel) to make sure that NOx gets reduced and CO, soot and unburnt fuel get oxidized.  In addition to the catalyst chemistry and fuel injector systems, there is a great deal of engineering technology present to maximize the performance under sub par conditions such as cold starts, etc.

However, the three-way catalytic converter technology is not suitable for application on so-called ‘‘lean-burn’’ engines that operate at high air/fuel ratios, including diesel-powered cars.  The high air-fuel ratio makes it harder to reduce the NOx emissions.  Prompted in part by the shift to cleaner diesel feeds, which are less likely to poison catalysts, and more stringent environmental standards, there’s been lot of research done in the last 15 years on developing better catalysts for diesel deNOx.  

So, what are the options for diesel/lean DeNOx?  It would be advantageous to develop a diesel analog of the three-way converter, in which fuel injection strategies and a bit of extra fuel can be used to reduce NOx.  This is called hydrocarbon-selective catalytic reduction (HC-SCR).  We’re not really there yet.  Several workaround technologies are in use.

  1. NOx storage-reduction (NSR, also called Lean NOxTraps/LNT) uses alkali earth metals such as Ba to bind NOx as nitrates, which then get reduced when the fuel injection switches into a richer mode.  Pt/Ba/Al2O3 is a common formulation. Periodically, the catalyst gets poisoned by sulfur and must be regenerated, just like the catalyst bed in a drybox.
  2. Ammonia-SCR: In ammonia selective catalytic reduction (NH3-SCR), urea is sprayed into the exhaust stream at high temperature.  It decomposes to ammonia and helps reduce NOx to N2.  A separate tank in needed to store the urea and, of course, it will need to be refilled on a regular basis.

Both NSR and NH3-SCR also require the use of additional catalysts to actually do the DeNOx.  Currently, the state of the art for NH3-SCR is copper on zeolites,* but Pt has also been widely used, particularly in the NSR systems.  In addition to the catalytic converter, diesel engines contain a diesel particulate filter (DPF) to collect soot.  The soot can be burned off at high temperature.

What Volkswagen did: Volkswagen is sort of vague on exactly what types of deNOx technologies they use, but from what I’ve read, it sounds like the smaller cars had NSR and the larger ones had a combination of NSR and NH3-SCR using Cu-zeolites (Cu-CHA, if you’re a zeolite geek).  It is unclear whether the Cu-zeolites were used in all cars, or just the ones with SCR.  When the cars are being driven, the steering column vibrates and the deNOx parts of the catalytic converter were not turned on.  Urea was not sprayed into the exhaust pathway, and the feedback loop necessary for proper NSR was probably not turned on either.  When the car was being smog tested, the steering column was stationary and the catalytic converter was turned on.  I suspect the DPF technology was still being run properly, because soot is a much easier problem for a layperson to notice.

I’m still puzzled why they’d take the risk and cheat the system this way.  Fines of $37,500 per car are nothing to scoff at. I’m also sort of dumbfounded that TDI owners didn’t notice that they never needed to fill their SCR tank, but perhaps VW just told them that it wouldn’t need to be filled very often. Did the catalytic converter technology not hold up to everyday driving, or did it just decrease the performance and mileage and they thought they’d sell more cars and happier owners if they bypassed it without telling people?  At first, I cynically assumed the latter explanation, especially since only a few states do routine smog testing.  

But after thinking about it some more, I’m starting to wonder if the technology itself was not quite ready for prime time.  Some people have speculated that the temperatures required might be higher and cause more engine wear and tear.  I think it may depend on what catalyst is being used.  The Cu-zeolite catalysts work best above 300°C, but many NSR Pt catalysts perform quite well at 200-300°C.  But it may be that the Cu-zeolite technology just isn’t ready for extended use in automotive applications.  Back when I worked on HC-SCR, the performance of a fresh catalyst was very good, but it would get deactivated over time. The commercialized system and the use of NH3-SCR were supposed to solve a lot of the problems that we observed, but perhaps that isn’t the case.  If you only turn on the catalytic converter once in a while, this won’t be a problem.  I’ve been working in another area of catalysis since 2010, so I haven’t been keeping up to date on the literature, so it could be that VW is just getting greedy and the catalytic converters work just fine.

Anyway, at the moment there are a lot of questions and not a lot of answers.  Hopefully more details will come to light soon.
* For a recent review on Cu-zeolite deNOx, see Peden et al, Chem Soc. Rev., ASAP, DOI: 10.1039/c5cs00108k

Thanks to Old Biddy for her knowledgeable thoughts on the issue. Thoughts/questions that I have:
  • I had no idea that urea was used in diesel vehicles to cut down on NOx - that’s fascinating. 
  • How many people had to be in on this? (i.e. were involved in making the software to turn off the SCR systems?) I guess that it could be a very small group of people? 
  • Still, how in the hell did no one at Volkswagen notice this? 
  • How did anyone not notice this in the general car-buying public, too? 
Finally, I have a question related to the chemical manufacturing industry: both Volkswagen cars and chemistry produces pollutants. The software program that Volkswagen installed (the “defeat device”) is basically turning off the pollution prevention device in order to save money/get better performance. Surely this happens in industrial chemistry - how should chemists react when we see these sorts of schemes pop up? Anyone have any tales of successful (or unsuccessful) whistleblowing?

*But I do know about getting drunk.

24 comments:

  1. Thanks to old biddy and CJ for posting this. Awesome stuff to learn about.

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  2. From a Cummins PDF (http://www.wccressey.com/Cummins%20Filtration%20DEF%20FAQ.pdf), urea solution consumption is about 2% of diesel consumption, so at 40 mpg, you should consume about 1L every 400 miles.

    This sounds like a pain in the butt.

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  3. Yeah, it's an extra thing to worry about, although it's no more complicated than filling up your windshield wiper fluid. The VW tank was about 1.7 liters, so that sounds like once a fill-up, but they do a lot of handwaving about how it just supplements the NSR purifier, so I can see a typical consumer just assuming it would get filled up at an oil change rather than every time they fill up their diesel tank.
    I think some trucks stops have urea pumps alongside the diesel pump, so this mitigates the hassle somewhat, at least for truckers.

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    1. When diesel exhaust fluid-requiring diesels hit the pickup truck market, DEF started appearing everywhere. I think I've seen it at all the local auto parts places, as well as many of the local gas stations here in central NJ. Availability probably isn't that big of a hassle.

      The cost is likely more of a hassle. I thought I'd seen estimates of a little more than one gallon of DEF per 1000 gallons of diesel in routine pickup truck service. With Home Depot listing an online price of $12 for 2 1/2 gallons, that's not too horrible. But if the actual use is closer to Hap's number, it will start to add up (especially at the 5-7 MPG typical for a long-haul trucker...).

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    2. I think the pdf gives an estimate of a fillup about every 300 miles for a truck getting 6 mpg.

      It sounds more often, but not more complicated than refilling windshield wiper fluid - I don't fill up my windshield wiper fluid once every five weeks (about how long it would take to go through 1.7 L of urea DEF). I guess it would depend on the other factors (reliability, gas mileage, etc.) but it probably wouldn't be a deal-breaker. (At $12 for about 10 L, that adds about $0.12/gallon - effectively, since you can't add DEF to fuel - to gas costs).

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    3. Actually, it's probably easier for a truck driver to add DEF than windshield washer fluid: the newest trucks seem to have DEF tanks adjacent to their fuel tanks. For a pickup truck, the reservoir is under the hood... but I think most have been engineered to accept a commonly-sized container by the time they signal for replenishment.

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    4. Which would make sense. The PDF noted that the DEF and fuel spigots are different sizes, so you can't confuse them, which is good.

      If I bought a diesel, I would have had to figure this out - I don't have any understanding of how a diesel and a gas car are operationally different.

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    5. something something glow plugs something something

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    6. Hey, CJ, I'm just happy that the hours I've spent poring over pickup truck brochures might have been useful for something besides picking out my next truck.

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    7. Currently Chevy, thinking about Ram.

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    8. I have always fantasized about owning an F-250. I think that's the same fantasy where I run a steakhouse and own a ranch in Wyoming.

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    9. Heh... in my dream world, there's a shiny new 28' Regulator center console behind what would just about have to be a 3/4 ton truck. (In reality, there's sometimes a kayak in the back of the Silverado.)

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  4. Great post, but I never would have picked you for a D.A.Coe fan.

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    1. I'll hang around as long as you will let me.

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  5. Off-topic, but related to catalysts: you may enjoy this:

    http://retractionwatch.com/2015/09/25/five-decades-later-is-it-time-to-retract-a-nobelists-retraction/#more-32695

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  6. As an American, I moved to Germany for my doctoral thesis, and lived within biking distance of Wolfsburg. Considering the "Küngelei" (= corruption) that I witnessed there from on top, the current nonsense with VW is really not too surprising.

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  7. I looked it up, they use 1:2 w/w of lab-grade urea in DI water, and the retail price of the solution was around 3 USD/gallon. This does not seem too excessive. They could have just designed a bigger urea solution tank, people would accept that they have to add more.

    My impression from reading about it last night was that VW originally bought the the NOx removal technology from Mercedes Benz, but later switched to their own simpler variant that never quite worked as advertised. So after they painted themselves to the corner and saw they would never be able to pass in California, so they started tweaking the software a little, and this eventually developed into a full-fledged cheating mode... Please notice that other more expensive brands like BMW or Mercedes did not have these issues.

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  8. Actually $3/gallon for the urea solution- 2/3 is water, is a bit pricey. I don't own a diesel (my 2006 Fusion just turned over 200,000 miles; but I am still grumpy that it doesn't run on a controlled D2 to He nuclear reaction). Diesels operate at a compression ratio of ~ 18:1 (15-22) vs ~ 8:1 for a gasoline internal combustion engine. At that high compression, and consequently higher temperature, the diesel fuel self ignites, hence no spark plug is needed. The glow plug warms things up when starting. Higher temps mean higher efficiency, if i am remembering my Carnot cycle properly. Diesel fuel is higher molecular weight -typically C-12, (vs C-5 to C-8 for gasoline) and is essentially the same as home heating oil, for those of us unfortunately not on a gas main.

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  9. Let me chime in, not from the chemistry or environmental side, but from a more political side. From what I have read, VW followed the rules - which read that the emissions system must be in line with standards AT THE TIME OF TESTING. It's my understanding, which may be wrong because I've not been able to find the reference, that when the testing connector was attached, the software system sensed the emissions system was being tested and kicked in the controls necessary to meet the standards Therefore, the cars were, indeed, in line with the standards at the time of testing. While that may not have been the intent of the rule, that was the consequence of the rule, and we're all familiar with the unintended consequences of poorly written legislation; they didn't "cheat" as some folks are hollering, they simply followed the rules - all of them - to the letter. If VW is fined (which they probably will be) they should viciously fight the fine in open court and point out the poorly written regulation.

    They gamed the system, but the system is there to be gamed. They simply used ALL of the regulation to their advantage, just a business should do. We don't fault a business for using the tax regulations to their advantage; fundamentally, in our over-regulated society, why should this be any different?

    Here's a link to a Computer World article on it: http://goo.gl/breJ7E

    Harry

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    1. Regulatory AmmendmentSeptember 28, 2015 at 8:52 PM

      I always find this to be a specious argument. Is what Shkreli is doing illegal by the word of the law? No, but his actions are mobilizing the public to make sure that it will be. Businessmen should remember that regulations are a living document and that, ultimately, their business exists at the pleasure of the public.

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    2. RA - It won't be specious in court - or in appeals. Courts, and primarily appeal courts will judge the action on the way the law was written. Business exists at the pleasure of the public in so much that public purchases or does not purchase the product or service. The VW TDIs performed (road performance, not emissions performance) far better than any other diesel and that's why they are/were popular with the consumer. They are zippy as hell and the Jetta ride is just heavenly. After this, I wish I had the money to purchase 10 of them and put them in storage because I'll bet the next version won't be as fun to drive.

      I am a business owner in a regulation-driven environment. Yes, they are living, but do not exist in a vacuum. Bad and poorly written regulations are changed and business adapt, but businesses will tend to run their companies based on the way a regulation is written.

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