# Exactly what does Volkswagen's "defeat device" do and why? **URL:** \ **Category:** Factual Questions\ **Created:** [September 22, 2015, 7:59pm UTC](https://boards.straightdope.com/t/exactly-what-does-volkswagens-defeat-device-do-and-why/732231 "2015-09-22T19:59:40Z")\ **Posts on this page:** 20\ **Page:** 1
**Author:** ![Frankenstein\_Monster](https://avatars.discourse-cdn.com/v4/letter/f/ccd318/32.png) [@Frankenstein\_Monster](https://boards.straightdope.com/u/Frankenstein_Monster)\ **Post date:** [September 22, 2015, 7:59pm UTC](https://boards.straightdope.com/t/exactly-what-does-volkswagens-defeat-device-do-and-why/732231/1 "2015-09-22T19:59:40Z")
So Volkswagen got caught using a “defeat device” in their diesel engines. Perhaps simplifying, I understand that the defeat device would detect when the car is on a dyno for emission testing and then keep the exhaust clean. When on the road it would run the engine “dirty”. I am interested in the technical and engineering side of this. [Here is the EPA complaint (PDF link).](http://www3.epa.gov/otaq/cert/documents/vw-nov-caa-09-18-15.pdf) > [@EPA](#): > > During EPA emission testing, the vehicle’s ECM ran software which produced compliant emission results under an ECM calibration that VW referred to as the “dyno calibration” … At all other times during normal vehicle operation… the ECM software ran a separate “road calibration” which reduced the effectiveness of the emission control system (specifically the selective catalytic reduction or the lean NOx trap). What does it mean when the engine “reduces the effectiveness of the selective catalytic reduction or the lean NOx trap”? What did they gain from this? VW has said (AFAIK) they will recall the cars and fix them. Is this as simple as refraining from activating the defeat device? What would be the consequence of running clean on the road? (Poor performance? Engine wear?) Or is this going to be a more elaborate fix? Historical note: there was a similar scandal in 1998 when [DOJ, EPA announce[d] one billion dollar settlement with diesel engine industry for clean air violations](http://yosemite.epa.gov/opa/admpress.nsf/b1ab9f485b098972852562e7004dc686/93e9e651adeed6b7852566a60069ad2e?OpenDocument). What was the technical story in those older engines and their defeat devices? ---
**Author:** ![control-z](https://avatars.discourse-cdn.com/v4/letter/c/eada6e/32.png) [@control-z](https://boards.straightdope.com/u/control-z)\ **Post date:** [September 22, 2015, 8:44pm UTC](https://boards.straightdope.com/t/exactly-what-does-volkswagens-defeat-device-do-and-why/732231/2 "2015-09-22T20:44:50Z")
[This Wired article](http://www.wired.com/2015/09/vw-fool-epa-couldnt-trick-chemistry/) explains it: > [@](#): > > Michigan’s Stefanopolou says computer sensors monitored the steering column. Under normal driving conditions, the column oscillates as the driver negotiates turns. But during emissions testing, the wheels of the car move, but the steering wheel doesn’t. That seems to have have been the signal for the “defeat device” to turn the catalytic scrubber up to full power, allowing the car to pass the test. So basically if the car is running but you’re not steering, the emissions systems run full blast. ---
**Author:** ![Smapti](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/smapti/32/17938_2.png) [@Smapti](https://boards.straightdope.com/u/Smapti)\ **Post date:** [September 22, 2015, 8:47pm UTC](https://boards.straightdope.com/t/exactly-what-does-volkswagens-defeat-device-do-and-why/732231/3 "2015-09-22T20:47:16Z")
Seems to me that if the device can make the engine run clean when it’s being tested, then it’s completely capable of making the engine run clean _all the time_. Is there some advantage to letting it run dirty under normal operating conditions? ---
**Author:** ![friedo](https://avatars.discourse-cdn.com/v4/letter/f/8edcca/32.png) [@friedo](https://boards.straightdope.com/u/friedo)\ **Post date:** [September 22, 2015, 8:49pm UTC](https://boards.straightdope.com/t/exactly-what-does-volkswagens-defeat-device-do-and-why/732231/4 "2015-09-22T20:49:58Z")
> [@Smapti](#): > > Seems to me that if the device can make the engine run clean when it’s being tested, then it’s completely capable of making the engine run clean _all the time_. Is there some advantage to letting it run dirty under normal operating conditions? You get better performance without the scrubber running. ---
**Author:** ![Machine\_Elf](https://avatars.discourse-cdn.com/v4/letter/m/82dd89/32.png) [@Machine\_Elf](https://boards.straightdope.com/u/Machine_Elf)\ **Post date:** [September 22, 2015, 8:53pm UTC](https://boards.straightdope.com/t/exactly-what-does-volkswagens-defeat-device-do-and-why/732231/5 "2015-09-22T20:53:26Z")
Some diesel engines employ [selective catalytic reduction (SCR)](https://en.wikipedia.org/wiki/Selective_catalytic_reduction) technology to reduce NOx output. In addition to an SCR catalyst that is physically installed in the exhaust pipe, the system requires the injection of a water-based urea solution (called diesel exhaust fluid, DEF) into the exhaust upstream of the catalyst. The urea breaks down into ammonia, which helps facilitate the reaction that turns NOx back into N2 and O2. The DEF is a consumable; owners must buy the stuff and refill a tank somewhere on the vehicle from time to time (generally after several thousand miles). Presumably during dyno tests the car is programmed to use liberal quantities of DEF, keeping NOx below allowable limits, and during open-road driving the system is much more stingy with it, allowing drivers to travel an acceptably long distance between DEF refills. The fix may be as simple as programming a higher DEF flow rate, and the negative consequence of such a fix would be more frequent DEF refills (hassle and expense) for the owner. The other cheat I read about involved high rates of exhaust gas recirculation (EGR) during dyno tests. This limits oxygen availability and reduces NOx formation, but it also requires limiting the maximum amount of fuel that can be injected - in other words, the engines deliver limited power/torque. Out on the open road, EGR gets dialed back, the max allowable fuel quantity goes up (giving drivers a nice powerful engine), and NOx goes through the roof. The likely fix for this would be heavy EGR all the time; the negative consequence for owners would be an underpowered car. The heavy-duty diesel scandal in the 1990s was a little different. Engines had been programmed to recognize when they were being tested, and then they applied a delay to the fuel injection timing. Combustion happened later in the expansion stroke, reducing peak temperatures and limiting NOx formation; this also delivered less-than-awesome fuel efficiency. Out on the open road, the system advanced the fuel injection timing, giving higher peak combustion temps, higher efficiency, and higher-than-allowable NOx emissions. I don’t recall how they ultimately fixed it; it may have been some combination of changing the fuel injection timing and/or upgraded exhaust emissions control hardware. ---
**Author:** ![Frankenstein\_Monster](https://avatars.discourse-cdn.com/v4/letter/f/ccd318/32.png) [@Frankenstein\_Monster](https://boards.straightdope.com/u/Frankenstein_Monster)\ **Post date:** [September 22, 2015, 9:34pm UTC](https://boards.straightdope.com/t/exactly-what-does-volkswagens-defeat-device-do-and-why/732231/6 "2015-09-22T21:34:29Z")
Great info, **Machine Elf**! I had read something about urea injection but didn’t realize this is what “selective catalytic reduction” referred to or what it meant in practice. Now I found where to buy this stuff on Amazon and learn how to fill up via Youtube videos! The fix presumably involves increasing the car’s consumption of this stuff. Doesn’t seem so bad. If “lean NOx trap” in this complaint refers to EGR, then indeed it sounds like they will inevitably have to reduce the car’s performance (reduced top speed and 0-60 mph acceleration). Is the performance reduction going to be significant? Are there ways to compensate? Will be interesting to see how it plays out. ---
**Author:** ![Quartz](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/quartz/32/267_2.png) [@Quartz](https://boards.straightdope.com/u/Quartz)\ **Post date:** [September 22, 2015, 11:30pm UTC](https://boards.straightdope.com/t/exactly-what-does-volkswagens-defeat-device-do-and-why/732231/7 "2015-09-22T23:30:06Z")
> [@Machine\_Elf](#): > > Some diesel engines employ [selective catalytic reduction (SCR)](https://en.wikipedia.org/wiki/Selective_catalytic_reduction) technology to reduce NOx output. In addition to an SCR catalyst that is physically installed in the exhaust pipe, the system requires the injection of a water-based urea solution (called diesel exhaust fluid, DEF) into the exhaust upstream of the catalyst. So they were literally taking this piss? ---
**Author:** ![Habeed](https://avatars.discourse-cdn.com/v4/letter/h/a587f6/32.png) [@Habeed](https://boards.straightdope.com/u/Habeed)\ **Post date:** [September 22, 2015, 11:53pm UTC](https://boards.straightdope.com/t/exactly-what-does-volkswagens-defeat-device-do-and-why/732231/8 "2015-09-22T23:53:07Z")
Let’s see…$15 for a gallon of DEF every 1000 miles. Right now, diesel is about 2 bucks a gallon, and you can go 40 miles per gallon. Essentially, this raises your consumable costs by 30%, defeating any money saved by driving a diesel at all. (you can drive a gas car that gets 30-35 mpg instead and pay less for the car and less for the fuel) ---
**Author:** ![Conspicuous\_Conspiracist](https://avatars.discourse-cdn.com/v4/letter/c/5fc32e/32.png) [@Conspicuous\_Conspiracist](https://boards.straightdope.com/u/Conspicuous_Conspiracist)\ **Post date:** [September 23, 2015, 12:46am UTC](https://boards.straightdope.com/t/exactly-what-does-volkswagens-defeat-device-do-and-why/732231/9 "2015-09-23T00:46:39Z")
Current price for DEF is approximately $2.79 / gallon at the pump or $12.99 for 2.5 gallons off the shelf. At the current dosing rate, a Passat would use 2.5 gallons every 10,000 miles. I’m sure the ‘fix’ will be to increase the DEF dosing rate from the current 1% to somewhere in the 3% to 5% range. What I’m not sure of is what the ‘fix’ will be for non-SCR equipped cars (all US VW models were equipped with SCR starting in 2015, Passat has had SCR for a few years), as you can’t just crank up the DEF. ---
**Author:** ![Tom\_Tildrum](https://avatars.discourse-cdn.com/v4/letter/t/e95f7d/32.png) [@Tom\_Tildrum](https://boards.straightdope.com/u/Tom_Tildrum)\ **Post date:** [September 23, 2015, 4:05am UTC](https://boards.straightdope.com/t/exactly-what-does-volkswagens-defeat-device-do-and-why/732231/10 "2015-09-23T04:05:31Z")
Popular Mechanics [suggests](http://www.popularmechanics.com/cars/a17430/ezra-dyer-volkswagen-diesel-controversy/) that the problem may be unfixable for models that don’t have a urea injection system. ---
**Author:** ![Frankenstein\_Monster](https://avatars.discourse-cdn.com/v4/letter/f/ccd318/32.png) [@Frankenstein\_Monster](https://boards.straightdope.com/u/Frankenstein_Monster)\ **Post date:** [September 23, 2015, 10:17am UTC](https://boards.straightdope.com/t/exactly-what-does-volkswagens-defeat-device-do-and-why/732231/11 "2015-09-23T10:17:49Z")
> [@Tom\_Tildrum](#): > > Popular Mechanics [suggests](http://www.popularmechanics.com/cars/a17430/ezra-dyer-volkswagen-diesel-controversy/) that the problem may be unfixable for models that don’t have a urea injection system. Oh I’m sure it’s fixable. Since all the cars did pass the emission tests. They just have to run in the same mode they ran while in that test. I don’t think there’s a definitive answer yet about what the performance or mileage loss will be (if any). ---
**Author:** ![Frankenstein\_Monster](https://avatars.discourse-cdn.com/v4/letter/f/ccd318/32.png) [@Frankenstein\_Monster](https://boards.straightdope.com/u/Frankenstein_Monster)\ **Post date:** [September 23, 2015, 10:34am UTC](https://boards.straightdope.com/t/exactly-what-does-volkswagens-defeat-device-do-and-why/732231/12 "2015-09-23T10:34:06Z")
From **Tom Tildrum** ’s link > [@Popular Mechanics](#): > > The second question concerned fuel economy. It’s been widely noted that four-cylinder TDIs tend to smash their EPA fuel economy estimates in real-world driving. The last TDI Jetta SportWagen I drove was rated at 42 mpg highway, but on 60-mph two-lane roads I averaged more like 50 mpg. That’s a huge difference. Did running non-compliant emissions improve fuel economy? Interesting, this sounds just like that older 1998 cheat described in **Machine Elf** ’s post. Wonder if this was part of the VW cheat, too? How well is that mpg difference established? Since that depends so much on driving style and the EPA cycle is quite conservative. Thing is, I don’t see this particular cheat mentioned in the EPA complaint, the way I read it. So it’s speculation. ---
**Author:** ![ralph124c](https://avatars.discourse-cdn.com/v4/letter/r/8797f3/32.png) [@ralph124c](https://boards.straightdope.com/u/ralph124c)\ **Post date:** [September 23, 2015, 12:27pm UTC](https://boards.straightdope.com/t/exactly-what-does-volkswagens-defeat-device-do-and-why/732231/13 "2015-09-23T12:27:58Z")
Diesel cards deliver higher MPG, because the energy content of diesel oil is higher than gasoline. In terms of gallons per barrel of crude oil, the efficiency is about the same. Plus, diesels require more frequent oil changes, so no net benefit. ---
**Author:** ![Quartz](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/quartz/32/267_2.png) [@Quartz](https://boards.straightdope.com/u/Quartz)\ **Post date:** [September 23, 2015, 12:40pm UTC](https://boards.straightdope.com/t/exactly-what-does-volkswagens-defeat-device-do-and-why/732231/14 "2015-09-23T12:40:11Z")
I’ve seen a claim elsewhere that the US emissions specifications for diesel cars were specifically designed to prevent the use of diesel. What’s the straight dope? ---
**Author:** ![filmore](https://avatars.discourse-cdn.com/v4/letter/f/7993a0/32.png) [@filmore](https://boards.straightdope.com/u/filmore)\ **Post date:** [September 23, 2015, 2:20pm UTC](https://boards.straightdope.com/t/exactly-what-does-volkswagens-defeat-device-do-and-why/732231/15 "2015-09-23T14:20:12Z")
Has anyone been able to switch on the EPA mode and tested what the performance change is with normal driving? ---
**Author:** ![Francis\_Vaughan](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/francis_vaughan/32/3093_2.png) [@Francis\_Vaughan](https://boards.straightdope.com/u/Francis_Vaughan)\ **Post date:** [September 23, 2015, 2:24pm UTC](https://boards.straightdope.com/t/exactly-what-does-volkswagens-defeat-device-do-and-why/732231/16 "2015-09-23T14:24:23Z")
> [@ralph124c](#): > > Diesel cards deliver higher MPG, because the energy content of diesel oil is higher than gasoline. In terms of gallons per barrel of crude oil, the efficiency is about the same. Plus, diesels require more frequent oil changes, so no net benefit. It is more complex than this. Pure unit volume diesel is about 10% higher energy, but per unit mass it is close enough to the same not to matter. But we buy it by volume, so that 10% does matter. What you get in crude oil varies well to well. Some wells are better for gasoline, some better for diesel. This follows regional trends. West Texas Intermediate is lighter than the two main competitors, Brent and Dubai, and commands a small but significant price premium because of this. WTI will yield slightly more gasoline than the others versus diesel. The global economics of this difference may be enough to skew the value of diesel versus gasoline cars to an oil producing country. But the energy efficiency of an ICU does also change between spark and compression ignition. In general diesel is more efficient than just the volumetric energy density of the fuel would suggest. The advent of common rail injection has made an important difference to cars. Differences in price of the two fuels varies in different markets, so it is hard to make solid comparisons. As diesel cars have become more popular diesel fuel has increased in price in some markets. But in many countries the cost of fuel is dominated by taxes and tariffs, so comparisons are skewed. The upshot is however that when you take the premium price that diesel cars take into consideration, the value equation can become a lot closer than might be hoped. ---
**Author:** ![jz78817](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/jz78817/32/1028_2.png) [@jz78817](https://boards.straightdope.com/u/jz78817)\ **Post date:** [September 23, 2015, 3:07pm UTC](https://boards.straightdope.com/t/exactly-what-does-volkswagens-defeat-device-do-and-why/732231/17 "2015-09-23T15:07:56Z")
> [@Frankenstein\_Monster](#): > > If “lean NOx trap” in this complaint refers to EGR, then indeed it sounds like they will inevitably have to reduce the car’s performance (reduced top speed and 0-60 mph acceleration). Is the performance reduction going to be significant? Are there ways to compensate? Will be interesting to see how it plays out. no, the “lean NOx trap” is in the exhaust system. It’s basically an NOx “adsorber” and I think VW integrated it into the particulate filter. EGR is something separate which helps the engine develop less NOx in the first place. as far as I can tell, when VW cars are _not_ running in the “test mode” calibration, they dial back EGR aspiration and the engine simply produces too much NOx for the trap to handle. > [@Quartz](#): > > I’ve seen a claim elsewhere that the US emissions specifications for diesel cars were specifically designed to prevent the use of diesel. What’s the straight dope? no. Starting in 2008 the EPA said they’d have to meet the same standards as gas engines, no exemptions. ---
**Author:** ![Doughbag](https://avatars.discourse-cdn.com/v4/letter/d/aeb1de/32.png) [@Doughbag](https://boards.straightdope.com/u/Doughbag)\ **Post date:** [September 23, 2015, 3:10pm UTC](https://boards.straightdope.com/t/exactly-what-does-volkswagens-defeat-device-do-and-why/732231/18 "2015-09-23T15:10:43Z")
Apparently VW may not the only ones using this trick, others are also under suspicion (Volvo, Renault & Hyundai) [Cite](http://www.thefiscaltimes.com/2015/09/22/Volkswagen-May-Not-Be-Only-Car-Company-Cheating-Emissions) ---
**Author:** ![Francis\_Vaughan](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/francis_vaughan/32/3093_2.png) [@Francis\_Vaughan](https://boards.straightdope.com/u/Francis_Vaughan)\ **Post date:** [September 23, 2015, 3:23pm UTC](https://boards.straightdope.com/t/exactly-what-does-volkswagens-defeat-device-do-and-why/732231/19 "2015-09-23T15:23:26Z")
> [@Quartz](#): > > I’ve seen a claim elsewhere that the US emissions specifications for diesel cars were specifically designed to prevent the use of diesel. What’s the straight dope? You could argue that tight NOx rules make life harder for diesel than gasoline because of the generally higher burn temperature. But to claim that the rules were set tighter than need be with the intent of disadvantaging diesel is another matter. As we see, diesels do need to make extra efforts to meet the NOx rules, but they have met them. ---
**Author:** ![Frankenstein\_Monster](https://avatars.discourse-cdn.com/v4/letter/f/ccd318/32.png) [@Frankenstein\_Monster](https://boards.straightdope.com/u/Frankenstein_Monster)\ **Post date:** [September 23, 2015, 4:49pm UTC](https://boards.straightdope.com/t/exactly-what-does-volkswagens-defeat-device-do-and-why/732231/20 "2015-09-23T16:49:11Z")
> [@filmore](#): > > Has anyone been able to switch on the EPA mode and tested what the performance change is with normal driving? Now somebody should try this! Apparently the clean mode kicked in when the car seemed to be otherwise running but the steering wheel remained motionless. So could you activate clean mode on the road simply by disconnecting/loosening/fooling the steering column sensor somehow? [Next page](https://boards.straightdope.com/t/exactly-what-does-volkswagens-defeat-device-do-and-why/732231.md?page=2)