摘要:
Nitrogen oxide storage catalysts are used to remove the nitrogen oxides present in the lean exhaust gas of lean-burn engines. Storage catalysts are thermally aged by high temperatures. Ageing is due to sintering of the catalytically active noble metal components of the catalyst and to formation of compounds of the storage components with the support materials. According to the invention, the formation of compounds of the storage materials can be largely reversed by treatment of the storage material with a gas mixture containing carbon dioxide, optionally water vapor and optionally nitrogen oxides at temperatures in the range from 200° C. to 950° C., preferably from 300° C. to 700° C. The reactivation can be carried out under emission-neutral conditions directly in the vehicle during driving operation by setting of suitable exhaust gas conditions and regulating the air/fuel ratio.
摘要:
A nitrogen oxide storage catalytic converter which is operated for a relatively long time at low exhaust-gas temperatures in the range between 120 and 250° C. exhibits a decreasing storage capacity as a result of incomplete regeneration at said temperatures. In order to re-establish the original storage capacity of the catalytic converter which is operated in this way, two-stage regeneration is proposed, wherein the storage catalytic converter is initially partially regenerated at the low exhaust gas temperature by means of a switch from the lean mode to the rich mode, and wherein subsequently, with rich exhaust gas again, the exhaust-gas temperature of the engine is raised into a range of between 300 and 400° C. for complete regeneration.
摘要:
Nitrogen oxide storage catalysts are used for removing the nitrogen oxides present in the lean-burn exhaust gas of lean-burn engines. Here, the purifying action is based on the nitrogen oxides being stored in the form of nitrates by the storage material of the storage catalyst during a lean-burn operating phase of the engine and the previously formed nitrates being decomposed in a subsequent rich-burn operating phase of the engine and the nitrogen oxides which are being liberated again being reacted with the reducing exhaust gas constituents over the storage catalyst to form nitrogen, carbon dioxide and water. Storage catalysts are thermally aged by high temperatures. The aging is due to sintering of the catalytically active noble metal components of the catalyst and to formation of compounds of the storage components with the support materials. According to the invention, the formation of compounds of the storage materials can be largely reversed by treatment of the storage material with a gas mixture comprising nitrogen dioxide and/or carbon dioxide and water vapor at temperatures of from 300 to 500° C. The reactivation can be carried out either directly on the vehicle by setting appropriate exhaust gas conditions or during a routine service by removal of the catalyst from the vehicle and treatment with a gas mixture in an appropriate apparatus.
摘要:
The fuel qualities for vehicles differ, for example, in Europe on a regional basis. This applies in particular to the sulphur content of the fuel. If vehicles with modern lean engines or with diesel engines which are equipped with nitrogen oxide storage-type catalytic converters in order to reduce the nitrogen oxide content in their exhaust gas pass through regions with a sulphur content in the fuel which is above the sulphur content which is specified for the vehicle, the engine controller will correspondingly frequently initiate desulphurization of the storage-type catalytic converter after the vehicle has been refuelled with this fuel. Any desulphurization entails an increased level of fuel consumption and greater ageing of the catalytic converter. It is proposed to avoid these adverse effects in that in lean motors the engine controller switches to stoichiometric operation in such a case, and in a diesel engine said engine controller prevents the desulphurization. The information about the region in which the vehicle is moving at a particular time can be transmitted to the engine controller by a navigation system.
摘要:
When a nitrogen oxide storage catalyst is being regenerated, the regeneration may be terminated for example as a result of a premature load change in the engine, which can lead to incomplete emptying of the storage catalyst. The residual filling level which remains in the catalyst following an incomplete regeneration of this nature is used as the starting value for calculation of the filling level during the next storage phase. After incomplete regeneration, the nitrogen oxide conversion rate is initially greater than would be expected, on account of the residual filling level. By taking this increased conversion rate into account when calculating the filling level during the storage phase, it is possible to further improve the accuracy of the calculation.
摘要:
A support for sensor elements, especially temperature, throughflow or chemical sensors, comprises a supporting element composed of a ceramic material and having at least one throughgoing opening, a thin ceramic foil arranged on the supporting element so as to cover the opening to support a sensor element in the region of the opening.
摘要:
Nitrogen oxide storage catalysts are used to remove the nitrogen oxides present in the lean exhaust gas of lean-burn engines. As a result of the stress due to high temperatures in vehicle operation, they are subject to thermal aging processes which affect both the nitrogen oxide storage components and the noble metals present as catalytically active components. The present invention provides a process with which the catalytic activity of a nitrogen oxide storage catalyst which comprises, in addition to platinum as a catalytically active component, basic compounds of strontium and/or barium on a support material comprising cerium oxide, said catalytic activity being lost owing to the thermal aging process, can be at least partly restored. The two-stage process is based on the fact that strontium and/or barium compounds formed during the thermal aging with the support material, which also comprise platinum, are recycled to the catalytically active forms by controlled treatment with specific gas mixtures.
摘要:
An emission control system for the cleaning of the exhaust gases of a lean burn engine with two or more cylinders comprises a first exhaust leg for the exhaust gases of a first group of cylinders and a second exhaust leg for the exhaust gases of a second group of cylinders. A nitrogen oxide storage catalyst is arranged in each exhaust leg. The two exhaust legs are combined downstream of the storage catalysts at a confluence to form a common exhaust leg. The common exhaust leg contains an SCR catalyst. The first and second groups of cylinders are each supplied alternately in periodic intervals with lean and rich air/fuel mixtures. Lean or rich exhaust gases are thus obtained in the combustion in the cylinders and released into the corresponding exhaust legs. Lean and rich exhaust gases are adjusted with respect to one another so as to result in a lean exhaust gas after the combination of the exhaust gases in the common exhaust leg. The regeneration of the storage catalysts may result in the formation of ammonia, which is stored by the SCR catalyst and reacted with nitrogen oxides which pass through the storage catalysts in an unwanted manner during the storage phases.
摘要:
Modern exhaust-gas purification systems in motor vehicles with a lean-burn engine include a starting catalyst fitted close to the engine and a main catalyst arranged in the underbody region, with both the starting catalyst and the main catalyst being formed by nitrogen oxide storage catalysts. The nitrogen oxide storage catalysts are in each case regenerated by the engine being briefly switched from lean-burn mode to rich-burn mode when the nitrogen oxide concentration in the exhaust gas downstream of the storage catalysts rises above a predetermined value.The starting catalyst is exposed to particularly high temperatures and is therefore prone to faster ageing of its nitrogen oxide storage capacity than the main catalyst. To check the nitrogen oxide storage capacity of the starting catalyst, a regeneration which is due for the catalyst system is selected in such a way in terms of its duration and the extent to which the exhaust gas is enriched that substantially only the starting catalyst is regenerated, whereas the main catalyst is not. In this arrangement, the criterion for terminating this partial regeneration is the breakthrough of rich exhaust gas through the starting catalyst. After the engine has been switched back to lean-burn mode, the time which elapses until the concentration of nitrogen oxides in the exhaust gas downstream of the catalyst requires regeneration again is measured. The measured time is a measure of the remaining nitrogen oxide storage capacity of the starting catalyst.
摘要:
To remove the nitrogen oxides from the exhaust gas from lean-burn engines, these engines are equipped with a nitrogen oxide storage catalyst, which has to be regenerated frequently by the engine being briefly switched to rich-burn mode. The regeneration is usually initiated when the nitrogen oxide concentration downstream of the catalyst rises above a permissible value. In this context, there is a risk of the bed temperature of the catalyst during and after regeneration being pushed into a range with incipient thermal desorption of the nitrogen oxides on account of the heat which is released during the conversion of the nitrogen oxides by the reducing constituents of the exhaust gas. This can lead to increased nitrogen oxide emission both during the regeneration itself and after the engine has been switched back to lean-burn mode. To eliminate this problem, it is proposed to divide the rich-burn mode into two rich pulses which follow one another in time, the first rich pulse being of shorter duration than the second rich pulse.