# What is a Lean NOx Trap?



## Flyingman (Sep 13, 2009)

https://en.wikipedia.org/wiki/NOx_adsorber

Purpose and function of a NOx adsorber[edit]

A NOx adsorber is designed to reduce oxides of nitrogen emitted in the exhaust gas of a lean burn internal combustion engine. Lean burn engines, particularly diesels, present a special challenge to emission control system designers because of the relatively high levels of O2 (atmospheric oxygen) in the exhaust gas. The 3-way catalytic converter that has been successfully used since the 1980s on stoichiometric engines (such as fueled by petrol, LPG, CNG, or ethanol) will not function at O2 levels in excess of 1.0%, and does not function well at levels above 0.5%. Because of the increasing need to limit NOx emissions from diesel engines, technologies such as exhaust gas recirculation (EGR) and selective catalytic reduction (SCR) have been used, however EGR is of limited effectiveness and SCR requires a continuous supply of reductant to the exhaust.

The NOx adsorber was designed to avoid the problems that EGR and SCR experienced. An adsorbant such as zeolite traps the NO and NO2 molecules - acting as a molecular sponge. Once the trap is full (like a sponge full of water) no more NOx can be absorbed. Various schemes have been designed to "purge" or "regenerate" the trap. One of possible reactants used to this aim is diesel fuel. Injection of diesel fuel before the adsorber can purge it - the NOx is made to desorb and react with hydrocarbons under rich conditions to produce water and nitrogen. Also hydrogen is a good reductant, but is dangerous and difficult to store. Some experimental systems have used fuel reformers for on-board hydrogen generation.

Market use[edit]

A NOx trap is used on the Volkswagen Jetta Clean TDI and the Volkswagen Tiguan concepts. Both are projected to be introduced into the American market by 2008.[2] They were to be marketed as part of the BlueTec program from Audi, Daimler-Chrysler, and Volkswagen.
Technical details[edit]

The NOx adsorber is based on a monolithic catalyst support that has been coated with a NOx adsorbing washcoat such as one containing zeolites. Alkali/alkaline oxide (carbonate) can also be used as the adsorbant.

Trap's effective storage changes with temperature. They start to store NOx between 150 and 200 °C but at 500 °C the storage diminishes rapidly.[citation needed].

Traps are gradually poisoned by SOx which adsorbs more strongly than NOx. It necessitates a periodic high temperature regeneration that tends to reduce the adsorber's operating life.[3]


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## Michael47 (May 9, 2014)

So this is my guess as to what happened at VW. Engineering sold management on these traps, but were unable to build ones with enough total capacity for actual road conditions. They worked, but only well enough to pass the official test. Then the regeneration took too long to keep them going on the highway. Just guessing, you inderstand, as I have no connection to VW by which to know for sure.


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## Flyingman (Sep 13, 2009)

Is our DPF a LTN as well? Just asking.:dunno:


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## floydarogers (Oct 11, 2010)

Flyingman said:


> Is our DPF a LTN as well? Just asking.


No.


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## Doug Huffman (Apr 25, 2015)

Hmm, zeolites are exactly what is in a water conditioner/softener column, a Culligan water softener, or a reactor plant coolant water polisher/ion exchanger. Oh, and the supplemental oxygen system on F-22/F-35 fighters.

Good reliable technology. My greatest professional coup was directing servicing a radioactive ion exchanger.


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## mefferso (Feb 26, 2014)

I was wondering how that NOx trap worked. Thank for the post


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## dunderhi (Dec 10, 2006)

Per the attached ICCT report, BMW began transitioning to use both SCR and LNT for U.S. cars in 2013. Interestingly, in Europe BMW used either LNT or SCR, but not both.

Page 8:
View attachment ICCT_NOx-control-tech_revised 09152015.pdf


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## F32Fleet (Jul 14, 2010)

Michael47 said:


> So this is my guess as to what happened at VW. Engineering sold management on these traps, but were unable to build ones with enough total capacity for actual road conditions. They worked, but only well enough to pass the official test. Then the regeneration took too long to keep them going on the highway. Just guessing, you inderstand, as I have no connection to VW by which to know for sure.


Yep. I bet warranty costs were going to be too high.


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## floydarogers (Oct 11, 2010)

F32Fleet said:


> Yep. I bet warranty costs were going to be too high.


Now is the time for all BMW (and MB) diesel owners to call upon the EPA to make BMW (MB) extend their warranties on the DEF/SCR systems on our cars. And redesign them so that we won't be made poor by the maintenance costs.


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## bjbolduc (Dec 19, 2012)

Michael47 said:


> So this is my guess as to what happened at VW. Engineering sold management on these traps, but were unable to build ones with enough total capacity for actual road conditions. They worked, but only well enough to pass the official test. Then the regeneration took too long to keep them going on the highway. Just guessing, you inderstand, as I have no connection to VW by which to know for sure.


I agree with that and it makes sense
Except they put the emissions bypass logic in the VW with scr. That doesn't make sense to me.


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