Three way catalytic converter

The three-way catalytic converter ( TWC from English three way catalytic converter ) is a vehicle catalytic converter with lambda control for exhaust gas aftertreatment in vehicles with gasoline engines , converting carbon monoxide (CO), nitrogen oxides (NO _x ) and unburned hydrocarbons (HC) into carbon dioxide (CO ₂ ), nitrogen (N ₂ ) and water (H ₂ O). The name results from the simultaneous conversion of the three air pollutants .

In order to ensure a high degree of conversion of the pollutants, the combustion air ratio (air-fuel ratio or the “air ratio” lambda ) must be kept in a narrow range around the so-called lambda window with the help of a control loop with a lambda probe . The regulated three-way catalytic converter converts over 90 percent of the pollutants into harmless components and thus significantly reduces the pollutant emissions of an internal combustion engine . ${\displaystyle \lambda }$ ${\displaystyle \lambda =1}$

history

Eugene Houdry developed the first oxidation catalyst and received a patent for it in 1956. However, the catalyst was not used as it quickly became ineffective due to the octane booster tetraethyl lead . The introduction of the three-way catalytic converter is closely linked to the requirements of emissions legislation. In addition to the technical challenge of developing a catalytic converter that simultaneously removes all harmful components from the exhaust gas, infrastructural measures such as the elimination of tetraethyl lead from gasoline were necessary because it acts as a catalytic converter poison. In addition, a reduction in the content of sulfur compounds in the fuel was desirable.

In order to meet the stricter emissions regulations of the US American Environmental Protection Agency , vehicles were initially equipped with two-way oxidation catalytic converters, which convert carbon monoxide and unburned hydrocarbons into carbon dioxide and water, but do not remove nitrogen oxides from the exhaust gas. The first nationwide introduction took place in 1975 on the US market. The breakthrough came with John J. Mooney and Carl D. Keith in 1981 with the development of the three-way catalyst.

effect

The pollutants carbon monoxide, unburned hydrocarbons and nitrogen oxides are removed from the exhaust gas according to the following equations:

${\displaystyle \mathrm {2\ CO\ +\ O_{2}\longrightarrow \ 2\ CO_{2}} }$

${\displaystyle \mathrm {C_{m}H_{n}\ +\ (m+n/4)\ O_{2}\longrightarrow \ m\ CO_{2}\ +\ n/2\ H_{2}O} }$

${\displaystyle \mathrm {2\ NO\ +\ 2\ CO\longrightarrow \ N_{2}\ +\ 2\ CO_{2}} }$

If there is a deviation from the stoichiometric air-fuel ratio ( ) towards a "lean" mixture (excess air ), not all nitrogen oxides are broken down, as the required reducing agents are oxidized beforehand. If the mixture is “rich” (lack of air ), not all hydrocarbons and carbon monoxide are broken down. A short-term deviation can be absorbed by the oxygen reservoir. ${\displaystyle \lambda =1}$ ${\displaystyle \lambda >1}$ ${\displaystyle \lambda <1}$

Because of the exhaust gas aftertreatment, it is necessary to forego driving with a lean mixture to improve fuel consumption in certain areas of engine operation.

In older models, sulphate adsorbed on the catalyst was also reduced to hydrogen sulphide in the rich area and caused an unpleasant odor of rotten eggs.

Another side reaction can form ammonia in the catalyst . Vehicles with catalytic converters emit between 20 and 50 mg of ammonia per kilometer driven. Nitrogen indicators such as yellow lichen ( Xanthoria perietina ) have been found on busy roads . In Germany around 2% of ammonia emissions are due to traffic.

${\displaystyle \mathrm {2\ NO\ +\ 5\ H_{2}\ \longrightarrow \ 2\ NH_{3}\ +\ 2\ H_{2}O} }$

Cer (IV) oxide is used as an oxygen storage component and provides oxygen in the rich range according to the following equation:

${\displaystyle \mathrm {4\ CeO_{2}\ +\ 2\ CO\ {\xrightarrow {Pt,Pd}}\ 2\ Ce_{2}O_{3}\ +\ 2\ CO_{2}} }$

The starting component is formed again in the excess of oxygen:

${\displaystyle \mathrm {2\ Ce_{2}O_{3}\ +\ O_{2}\longrightarrow 4\ CeO_{2}} }$

Four way catalytic converter

In some applications, the three-way catalytic converter is combined with a gasoline particle filter . The combination is called a four-way catalytic converter.

Manufacturing

The catalyst consists of a ceramic cordierite honeycomb body or a metal carrier on which a so-called washcoat is applied. The washcoat consists of a mixture of different metal oxides. The metal oxides γ- aluminum oxide and zirconium dioxide represent the carrier substances for the catalytically active noble metals such as platinum , palladium and rhodium , ceria serves as an oxygen storage component . The cordierite honeycomb body is welded into a sheet metal housing for installation in the vehicle, which is lined with a mineral fiber mat to seal and mechanically hold the monolith. Metal supports can be welded directly into the exhaust system.

Substrate

Metal substrate catalyst

A ceramic monolith with a honeycomb structure is usually used as the substrate. The cordierite ceramic substrate used in most catalysts was invented by Rodney Bagley, Irwin Lachman and Ronald Lewis at Corning Glass. For this invention, they were inducted into the US National Inventors Hall of Fame in 2002.

The ceramic monoliths have a geometric surface area of ​​around one to five square meters per liter of catalyst volume, depending on the channel size.

Washcoat

For the production of the catalyst, an acidic metal oxide suspension is first produced. This can be applied to the cordierite carrier by various methods. In the simplest case, the honeycomb body is immersed in the suspension. The excess suspension is blown out, the coated honeycomb body is then dried and calcined . The fine channels of the honeycomb body then contain a thin layer of a metal oxide mixture. In the next step, this layer is impregnated with precious metal salt solutions. The precious metals are fixed on the carrier by further drying and calcining. Alternatively, the noble metals can be impregnated on the metal oxides before the cordierite carrier is coated. Finally, the catalyst is packed in a mineral fiber mat and welded into a metal sheet.

technical features

In addition to its chemical composition, important technical features of a three-way catalytic converter are its light-off behavior, pressure loss , thermal shock behavior and washcoat adhesion.

Problem of full load enrichment

In gasoline engines for passenger cars, it is common practice to maximize performance in the full load range by enriching the fuel-air mixture. One speaks of full load when maximum power is called up ("full throttle"). To do this, the fuel-air mixture must be made slightly enriched compared to part load, as the flame speed is higher in slightly rich mixtures (faster so-called burn-through of the mixture). How much the mixture is enriched depends on the programming of the respective engine control unit; Values ​​up to λ  = 0.8 are common (as of 2018). In addition, the full load enrichment improves the internal cooling of the combustion chambers: Since the evaporation of fuel requires energy, the combustion chambers are additionally cooled by the additional fuel.

The incomplete combustion causes the exhaust gas temperature to drop, which means that engine operating points can be reached at which the exhaust manifold, the turbocharger or the catalytic converter would be destroyed without enrichment.

By deviating from , the three-way catalytic converter loses part of its effectiveness, since hydrocarbons and carbon monoxide can no longer be completely converted. A public discussion about this problem is undesirable. Full load enrichment is a technique that was used decades ago in carburettor engines. ${\displaystyle \lambda =1}$

In the previous NEDC cycle, no high accelerations, such as those that are common when driving onto the autobahn, were called up, so that under these test conditions, unlike in practice, no full load enrichment took place and was accordingly not included in the measurement. In the WLTP cycle that has been in force since 2017 and the tests in accordance with the RDE regulations, the accelerations are significantly higher. In the RDE test, however, no limit value for carbon monoxide has yet to be complied with; currently (2019) only nitrogen oxides and particles are limited there.

literature

• Bosch, technical instruction: exhaust technology for gasoline engines . Robert Bosch GmbH KH / VDT, Stuttgart, Bosch no .: 1 987 722 020.
• Bosch, technical briefing: Motronic . Combined ignition and injection system for gasoline engines, 2nd edition, Robert Bosch GmbH KH / VDT, Stuttgart, September 1985, Bosch no .: 1 987 722 011.
• H. Bode: Materials Aspects in Automotive Catalytic Converters . Verlag Wiley-VCH (2002), ISBN 3527304916 .
• RM Heck: Catalytic Air Pollution Control: Commercial Technology . 544 pages, Verlag John Wiley & Sons (2009), ISBN 0470275030 .

Web links

Commons : three way catalyst  - collection of pictures, videos and audio files

• The exhaust gas catalytic converter - Otto and Diesel Kat on kfztech.de, accessed on November 25, 2014

Individual evidence

1. ^ Bosch: Motronic technical briefing . P. 41.
2. ^ JJ Mooney, CD Falk: Three-Way Conversion Catalysts: Effect of Closed-Loop Feed-Back Control and Other Parameters on Catalyst Efficiency . In: SAE Technical Papers , doi : 10.4271 / 800462 .
3. ↑ LUBW: Ammonia in the environment. Measurement programs and measurement results 2003–2007. ( Memento of the original from July 14, 2014 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. (PDF; 1.9 MB), December 2008. 
4. ↑ Christof Vieweg: Fine dust: petrol engines also need a filter . In: The time . February 25, 2017, ISSN  0044-2070 ( zeit.de [accessed February 26, 2020]).
5. ^ Robert Bosch GmbH (ed.): Otto engine management: systems and components . 2nd edition, Vieweg, Braunschweig / Wiesbaden 2003, ISBN 9783322939296 , p. 19.
6. ^ Robert Bosch GmbH (ed.): Otto engine management: systems and components . 2nd edition, Vieweg, Braunschweig / Wiesbaden 2003, ISBN 9783322939296 , p. 102.
7. ↑ Klaus Schreiner: Basic knowledge of the internal combustion engine: Questions - calculate - understand - exist . Springer, Wiesbaden 2014, ISBN 9783658061876 , p. 112.
8. ↑ Dieter Klemp, Djuro Mihelčić, Bernhard Mittermaier: Measurement and evaluation of traffic emissions . Volume 21 of publications by Forschungszentrum Jülich: Energie et Umwelt series , Forschungszentrum Jülich, 2012, ISBN 9783893365463 , p. 143 ff.
9. ↑ Emission Standards: Europe: Cars and Light Trucks. Retrieved April 15, 2019 .