A adsorber or trap (also called Lean trap, abbr. LNT) is a device that is used to reduce oxides of nitrogen (NO and NO₂) emissions from a

lean burn Lean-burn refers to the burning of fuel with an excess of air in an internal combustion engine. In lean-burn engines the air–fuel ratio may be as lean as 65:1 (by mass). The air:fuel ratio needed to stoichiometry, stoichiometrically combust gas ...

internal combustion engine An internal combustion engine (ICE or IC engine) is a heat engine in which the combustion of a fuel occurs with an oxidizer (usually air) in a combustion chamber that is an integral part of the working fluid flow circuit. In an internal comb ...

by means of

adsorption Adsorption is the adhesion of atoms, ions or molecules from a gas, liquid or dissolved solid to a surface. This process creates a film of the ''adsorbate'' on the surface of the ''adsorbent''. This process differs from absorption, in which a ...

Purpose and function of a adsorber

A adsorber is designed to reduce oxides of nitrogen emitted in the exhaust gas of a lean burn

. Lean burn engines, particularly diesels, present a special challenge to emission control system designers because of the relatively high levels of O₂ (atmospheric

oxygen Oxygen is a chemical element; it has chemical symbol, symbol O and atomic number 8. It is a member of the chalcogen group (periodic table), group in the periodic table, a highly reactivity (chemistry), reactive nonmetal (chemistry), non ...

) in the exhaust gas. The three-way

catalytic converter A catalytic converter part is an vehicle emissions control, exhaust emission control device which converts toxic gases and pollutants in exhaust gas from an internal combustion engine into less-toxic pollutants by catalysis, catalyzing a redox ...

that has been successfully used since the 1980s on

stoichiometric Stoichiometry () is the relationships between the masses of reactants and products before, during, and following chemical reactions. Stoichiometry is based on the law of conservation of mass; the total mass of reactants must equal the total m ...

engines (such as fueled by

petrol Gasoline (North American English) or petrol ( Commonwealth English) is a petrochemical product characterized as a transparent, yellowish, and flammable liquid normally used as a fuel for spark-ignited internal combustion engines. When formul ...

, LPG,

CNG Compressed natural gas (CNG) is a fuel gas mainly composed of methane (CH4), compressed to less than 1% of the volume it occupies at standard atmospheric pressure. It is stored and distributed in hard containers at a pressure of , usually in c ...

, or

ethanol Ethanol (also called ethyl alcohol, grain alcohol, drinking alcohol, or simply alcohol) is an organic compound with the chemical formula . It is an Alcohol (chemistry), alcohol, with its formula also written as , or EtOH, where Et is the ps ...

) 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 emissions from diesel engines, technologies such as

exhaust gas recirculation In internal combustion engines, exhaust gas recirculation (EGR) is a nitrogen oxide () emissions reduction technique used in petrol engine, petrol/gasoline, diesel engines and some hydrogen internal combustion engine vehicle, hydrogen engines. ...

(EGR) and

selective catalytic reduction Selective catalytic reduction (SCR) means converting nitrogen oxides, also referred to as with the aid of a catalyst into diatomic nitrogen (), and water (). A reductant, typically anhydrous ammonia (), aqueous ammonia (), or a urea () soluti ...

(SCR) have been used, however EGR is of limited effectiveness and SCR requires a continuous supply of reductant to the exhaust. The adsorber was designed to avoid the problems that EGR and SCR experienced. An adsorbent such as

zeolite Zeolites are a group of several microporous, crystalline aluminosilicate minerals commonly used as commercial adsorbents and catalysts. They mainly consist of silicon, aluminium, oxygen, and have the general formula ･y where is either a meta ...

traps the NO and NO₂ molecules—acting as a molecular sponge. Once the trap is full (like a sponge full of water) no more can be absorbed. Various schemes have been designed to "purge" or "regenerate" the trap. Normally the engine is operated for a short time in rich mode and produces CO as reactant. Another possible reactants used to this aim is diesel fuel. Injection of diesel fuel before the adsorber can purge it—the is made to desorb and react with

hydrocarbon In organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. Hydrocarbons are examples of group 14 hydrides. Hydrocarbons are generally colourless and Hydrophobe, hydrophobic; their odor is usually fain ...

s under rich conditions to produce

water Water is an inorganic compound with the chemical formula . It is a transparent, tasteless, odorless, and Color of water, nearly colorless chemical substance. It is the main constituent of Earth's hydrosphere and the fluids of all known liv ...

and

nitrogen Nitrogen is a chemical element; it has Symbol (chemistry), symbol N and atomic number 7. Nitrogen is a Nonmetal (chemistry), nonmetal and the lightest member of pnictogen, group 15 of the periodic table, often called the Pnictogen, pnictogens. ...

. Also

hydrogen Hydrogen is a chemical element; it has chemical symbol, symbol H and atomic number 1. It is the lightest and abundance of the chemical elements, most abundant chemical element in the universe, constituting about 75% of all baryon, normal matter ...

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

A trap is used on the

Volkswagen Jetta The Volkswagen Jetta () is a compact car/small family car manufactured and marketed by Volkswagen since 1979. Positioned to fill a sedan (automobile), sedan niche slightly above the firm's Volkswagen Golf, Golf hatchback, it has been marketed ov ...

TDI and the

Volkswagen Tiguan The Volkswagen Tiguan () is a sport utility vehicle produced by German manufacturer Volkswagen since 2007, sitting between the smaller T-Roc and the larger Touareg in the company's crossover SUV range. The first generation was based on the ...

concepts. Both are projected to be introduced into the American market by 2008. They were to be marketed as part of the BlueTec program from Audi, Daimler-Chrysler, and Volkswagen. In 2015 an

ADAC The ADAC, officially the Allgemeiner Deutscher Automobil-Club (), is Europe's largest automobile association. The ADAC is the largest wikt:Verein, ''verein'' (club) in Germany, with around 21 million members. Its headquarters are located i ...

study (ordered by ICCT) of 32 Euro6 cars showed that few complied with on-road emission limits, and LNT cars had the highest emissions.NOx control technologies for Euro 6 diesel passenger cars
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Technical details

The adsorber is based on a monolithic catalyst support that has been coated with a adsorbing washcoat such as one containing

zeolites Zeolites are a group of several Microporous material, microporous, crystalline aluminosilicate minerals commonly used as commercial adsorption, adsorbents and Catalysis, catalysts. They mainly consist of silicon, aluminium, oxygen, and have the ge ...

. Alkali/alkaline oxide (carbonate) can also be used as the adsorbant. Traps are gradually poisoned by SO_''x'' which adsorbs more strongly than . It necessitates a periodic high temperature regeneration that tends to reduce the adsorber's operating life.{{cite web, last1=Hu, first1=Horan, title=Eaton Aftertreatment System (EAS) For On-Highway Vehicles, url=http://energy.gov/sites/prod/files/2014/03/f9/2006_deer_hu.pdf, publisher=12th Diesel Engine-Efficiency and Emissions, accessdate=24 September 2015

References

Engine technology Gas technologies