Originally Posted by NayBreak
A cut and paste from Tog's post above:
Two VW models failed the ICCT test for two completely different reasons. In the VW Passat, the nitrogen oxide is filtered out of the emissions with a system that injects a urea-based solution. The ICCT suspects that VW had reduced the amount of urea injected to spare its customers having to refill the urea tanks themselves.
With the Jetta, the second VW model in the ICCT test, VW uses a nitrogen oxide reservoir catalytic converter that stores the oxide particles before they are released. But this system has to regenerate itself. The bigger it is, the faster it can do so, but the more fuel it consumes. The ICCT believes that in the Jetta, the reservoir system fitted was kept small to keep fuel consumption low. As a result, especially on long journeys at high speeds, the cars emitted nitrogen oxide unchecked. That kind of construction flaw is hard to fix in a recall.
So that bolded part doesn't show up in an emissions test, because it takes time for the smaller reservoir to lose its functionality. Combined with "test" mode being common to bypass all of the modern electronic controls during testing, why would the Jetta need a "cheat device"? It would seem that using a smaller reservoir would lead to passing emissions (reservoir is functioning), but failing testing at high speed over long distances (reservoir capacity exceeded).
The reservoir storing the NOx needs to be regenerated. This is done by squirting extra fuel into the cylinders. I guess if the resevoir -LNT is large, then it will take more fuel to regen. Somehow they maximize the conditions for the test by using a small LNT and regen just enough. (Edit: it's the test so minimizing emissions is goal. So they regen more often or use a little more fuel. They give up a bit of mpg). I don't think you could go to zero because then at some point no trapping of NOx occurs. When off test they use even less regen and the small LNT does little at hghwy speeds. That's a blind guess.
Well worth a read:Volkswagen’s Little Engine That Couldn’t
By Tim De Chant on Tue, 22 Sep 2015
For years, engineers across the industry had struggled to find a way to keep the best parts of diesel engines—torque and efficiency—while ditching the bad—soot, nitrogen oxides (NOx), and sulfur dioxide emissions. To reduce NOx emissions, there are two main approaches. One is selective catalytic reduction, or SCR, which involves injecting urea into the exhaust stream to react with NOx and turn it into harmless nitrogen and oxygen. The approach is incredibly effective, but it requires an additional tank to store the urea, a heater to keep it fluid, a pump, a valve, a mixer, and a catalyst to speed the reaction. Despite that, most automakers ended up settling on SCR, in part because it worked so well.
For larger, more expensive vehicles, hiding the extra kit involved in SCR is easier, both in terms of packaging and price. But on smaller cars like the Volkswagen Golf and Jetta, SCR is less than ideal. There’s not a lot of room to put the urea tank, pump, and other equipment, and price-conscious consumers may balk at the added expense, which costs the manufacturer about $50 more for a 2.0-liter engine than the alternative, according to a study by ICCT. That equates to a cost savings of over $500 million over 11 million cars.
Luckily for Volkswagen, the company’s engineers were hard at work on the alternative, one based on research Toyota had done in the mid-1990s on an idea known alternately as a NOx storage catalytic converter or a lean NOx trap, or LNT. LNT uses a catalyst to absorb and store NOx so it doesn’t escape into the atmosphere. When the catalytic converter is full, the system burns off the stored NOx by pumping an extra burst of fuel into the cylinders, most of which passes through to the converter where it burns the NOx into harmless nitrogen and oxygen.
When working properly, LNT systems tend to run a fuel-rich mixture for a second or two “every minute or so, sometimes more often,” says German, the ICCT senior fellow. “You’re injecting a lot of extra fuel.” Running rich that often can depress fuel economy, one of the main selling points of diesel cars.
The professor in the other quote said " it was easy" to swich engine maps for the test and dyno mode seems standard for an ecu.
Maps= values for fuel,ignition, at rpm and load. Plus egr, Scr events.
You start to see how hopeless the carburetor is for emissions on a gasoline car. Just too sloppy. Largely why they've dissapeared for cars in the 80's. Even though it's an amazingly simple precision device with some black magic. Bit like a mechanical vs quartz watch.Edited by Tog - 9/23/15 at 12:01am