OBRAS A REALIZAR .1 CALZADAS EXPRESAS 78.1 CALZADAS EXPRESAS78

The three-way catalytic converter is installed in the exhaust-emission control systems of manifold-injection engines and gasoline direct-injection engines.

Assignment

Three toxic components are generated dur- ing the combustion of the A/F mixture: HC (hydrocarbons), CO (carbon monoxide), and oxides of nitrogen (NOx). It is the job of

the three-way catalytic converter to convert these into harmless components. The prod- ucts which result from this converion are H2O (water vapor), CO2(carbon dioxide),

and N2(nitrogen).

Operating concept

The toxic components are converted in two phases: Firstly, the carbon monoxide and the hydrocarbons are converted by oxidation (Fig. G, Equations 1 and 2). The oxygen needed for the oxidation process is available in the exhaust gas in the form of the residual oxygen resulting from incomplete combus- tion, or it is taken from the oxides of nitro- gen whereby these reduce as a result (Fig. G, Equations 3 and 4).

The concentration of the toxic substances in the untreated exhaust gas is a function of the excess-air factor λ (Fig. 2a). For carbon monoxide and hydrocarbons (HC), the con- version level increases steadily along with in- creasing excess-air factor (Fig. 2b). At λ = 1, there is only a very low level of toxic compo- nents in the untreated exhaust gas. With high excess-air factors (λ > 1), the concen- tration of these toxic components remains at this low level.

Conversion of the oxides of nitrogen (NOx) is good in the rich range (λ < 1) . The

lowest levels of NOxare present during stoi-

chiometric operation (λ = 1). Even a small increase in the exhaust-gas oxygen content as caused by operation at λ > 1 impedes the nitrogen reduction and causes a sharp in- crease in its concentration.

In order to maintain the three-way catalytic converter’s conversion level for all three toxic substances at as high a level as possible, these must be present in a chemical balance in the exhaust gas. This means that the A/F mixture composition must have a stoichio- metric ratio of λ = 1, so that the “window” for the A/F mixture ratio l is necessarily very restricted. A/F mixture formation must be controlled by a Lambda closed-loop control circuit.

Catalytic emissions control Three-way catalytic converter 77

Figure 2

a Before catalytic aftertreatment (untreated exhaust gas)

b After catalytic after- treatment c Voltage characteris-

tic of the two-step Lambda sensor Lambda-Regelbereich (Katalysatorfenster) Excess-air factor λ Rich Lean 0.975 1.0 NO_X NOX CO CO

Lambda control range (catalytic-converter window) a b c HC HC 1.025 1.05 U_λ

Toxic components in the exhaust gas

2

æ

Reaction equations in the three-way catalytic converter G (1) 2 CO + O2 ➞ 2 CO2 (2) 2 C2H6 + 7 O2 ➞ 4 CO2 + 6 H2O (3) 2 NO + 2 CO ➞ N2 + 2 CO2 (4) 2 NO2 + 2 CO ➞ N2 + 2 CO2+ O2

Design and construction

The catalytic converter (Fig. 3) comprises a steel casing (6), a substrate (5), and the ac- tive catalytic noble-metal coating (4). Substrates

Two substrate systems have come to the forefront

Ceramic monoliths

These ceramic monoliths are ceramic bodies containing thousands of narrow passages through which the exhaust gas flows. The ceramic is a high-temperature-resistant magnesium-aluminum silicate. The mono- lith, which is highly sensitive to mechanical tension, is fastened inside a sheet-steel hous- ing by means of mineral swell matting (2) which expands the first time it is heated up and firmly fixes the monolith in position. At the same time the matting also ensures a 100 % gas seal.

Ceramic monoliths are at present the most commonly used catalyst substrates.

Metallic monoliths

The metallic monolith (metal catalytic con- verter) is an alternative to the ceramic monolith. It is made of finely corrugated, 0.05 mm thin metal foil which is wound and soldered in a high-temperature process. Thanks to its thin walls, more passages can be accomodated inside the same area, which means less resistance to exhaust-gas flow, a fact which is important in the case of high- performance engines.

Coating

The ceramic and metallic monoliths require an aluminum oxide (Al2O3) substrate coat-

ing, the so-called “Washcoat” (4). This coat- ing serves to increase the converter’s effec- tive surface area by a factor of around 7000. On the oxidation catalytic converter, the ef- fective catalytic coating applied to the sub- strate contains the noble metals platinum and/or palladium. On the three-way con- verter, rhodium is also applied. Platinum and palladium accelerate the oxidation of the hydrocarbons (HC) and of the carbon monoxide. Rhodium accelerates the reduc- tion of the oxides of nitrogen (NOx).

Depending upon the engine’s displace- ment, a catalytic converter contains about 1...3 g of noble metal.

78 Catalytic emissions control Three-way catalytic converter

Figure 3 1 Lambda oxygen sensor 2 Swell matting 3 Thermally insulated double shell 4 Washcoat (Al2O3 substrate coating) with noble-metal coating 5 Substrate (monolith) 6 Housing 4 5 6 1 2 3 HC + CO + NO2

Three-way catalytic converter with Lambda oxygen sensor

3

æ

Operating conditions

Operating temperature

The catalytic converter’s temperature plays a decisive role in emission-control efficiency. Considering a three-way catalytic converter, no worthwhile conversion of toxic sub- stances takes place until temperature exceeds 300 °C. Operation within a temperature range of 400...800 °C is ideal with regard to high conversion levels and a long service life.

At temperatures between 800...1000 °C, thermal aging is accelerated due to the sin- tering of the noble metals and of the Al2O3

substrate layer, and this leads to a reduction of the effective surface. The time spent at 800...1000 °C is of vital importance, and above 1000 °C thermal aging increases dras- tically and leads to the catalytic converter becoming practically 100 % ineffective. Engine malfunction (ignition misfire) can cause the temperature inside the catalytic converter to exceed 1400 °C. Since such tem- peratures melt the substrate and completely destroy the catalyst, it is imperative that the ignition system is highly reliable and main- tenance-free. Modern engine-management systems are able to detect ignition and com- bustion miss, and in such cases interrupt the fuel injection to the cylinder concerned so that unburned A/F mixture cannot enter the exhaust-gas tract.

Unleaded fuel

Another prerequisite for long-term opera- tion is the use of unleaded fuel. Otherwise, lead compounds are deposited in the pores of the active surface and reduce their num- ber. Residues from the engine oil can also “poison” the catalyst and damage it so far that it becomes ineffective.

Installation point

Strict emissions-control legislation demands special concepts for heating the catalytic converter when the engine is started. The catalytic converter’s installation point is de- termined by such concepts (for instance, secondary-air injection, shift of the timing

in the “retard” direction). The three-way cat- alytic converter’s sensitivity regarding oper- ating temperature limits the choice of instal- lation point. The temperature conditions needed for a high conversion level make it absolutely imperative that the three-way converter is installed close to the engine. In the case of the three-way catalytic con- verter, a configuration featuring a “pre-cat” near the engine followed by a second (main) underfloor catalytic converter has come to the forefront. Catalytic converters near the engine demand that their coating techniques be optimized to provide for high-tempera- ture stability. Underfloor converters on the other hand, require optimisation in the so- called “low light-off ” direction (low start-up temperature) and good NOxconversion

characteristics.

An alternative is available with just one “overall” catalytic converter which is then installed close to the engine.

Effectiveness

For a spark-ignition engine with homoge- neous mixture distribution operating at

λ= 1, catalytic treatment of the exhaust gas using a three-way catalytic converter is at present the most effective emission-control method. Included in this system is the Lambda closed-loop control which monitors the composition of the A/F mixture. Using the three-way catalytic converter, the pollu- tant emissions of carbon monoxide, hydro- carbons, and oxides of nitrogen can be prac- tically eliminated provided the engine oper- ates with homogeneous A/F-mixture distribution and at stoichiometric A/F ratio. Notwithstanding the fact that it is not always possible to comply fully with these operating requirements, one can still presume an aver- age pollutants reduction of more than 98 %.

NO

accumulator-type