DE19511548A1 - Nitrous oxide reduction system in vehicle engine exhaust - Google Patents

Abstract

The engine (1) is operated alternately in lean and stoichiometric or enrichment conditions. In lean conditions, the nitrous oxide (NOx) is received by an absorber (3), which is regenerated under stoichiometric or enrichment conditions.The hydrocarbons (HC), carbon monoxide (CO) or nitrous oxide (NOx) contained in the exhaust gas are measured downstream of the absorber. The engine is changed over to lean operating conditions as soon as the hydrocarbon or carbon monoxide content of the gas exceeds a predetermined value or the nitrous oxide content falls below such a value. Where there is a sensor (4) for the gas contents in the exhaust pipe downstream of the absorber, the signal from this can regulate the proportions of fuel and air supplied to the engine.

Description

The invention relates to a method for nitrogen oxide reduction Decoration in the exhaust gas of an internal combustion engine according to the generic term of claim 1 and one for performing such Method suitable device according to the preamble of the An Proverbs 3

Such methods and devices come in particular Motor vehicles for use. In addition to the diesel engine here with the gasoline engine also a clear potential for lowering its resulting CO₂ emission if it is in lean operation will drive.

With this engine concept, combustion takes place in the same way as with a diesel engine with excess air, ie with a lambda value greater than one. Although this has the advantage of lower fuel consumption, the post-engine nitrogen oxide reduction is more difficult because the nitrogen oxides can no longer be reduced so easily in an overall oxidizing exhaust gas atmosphere. During the operation of the gasoline engine with the stoichiometric lambda value of one pollutant, ie the un unburned hydrocarbons (HC), carbon monoxide (CO) and nitrogen oxides (NO _x) effectively can be reduced with a three-way catalyst, this catalyst technology in ma ger operated gasoline engine not enough for NO _x reduction alone.

As a remedy, it is already known to drive the internal combustion engine alternately in lean operation and in stoichiometric operation, that is, with lambda equal to one, or enrichment operation, ie with lambda less than one, and to provide a NO _x adsorber in the exhaust gas line, e.g. B. as a separate unit in front of or behind a three-way catalyst. In lean operating phases, the nitrogen oxides are absorbed by the NO _x adsorber, and in stoichiometric or enrichment operating phases, the NO _x adsorber is generated by reversing the adsorption reactions and desorbing the nitrogen oxides and reducing them in a downstream catalyst part. Such an adsorber-catalyst system with lean-mix operation is described in the published patent application EP 0 560 991 A1. The switchover between lean phase operation and stoichiometric or enrichment operation phases takes place there at comparatively roughly estimated times, whereby, of course, the operation in lean operation is as long as possible and a stoichiometric or enrichment operation phase is then switched back to lean operation if, based on a corresponding estimate, it is assumed that the NO _x adsorber has regenerated again.

The invention is based on the technical problem of providing a method and a device of the type mentioned in the introduction, in which a switch is made as reliably as possible from a stoichiometric or enrichment operating phase to a lean operating phase when the NO _x adsorber has regenerated.

This problem is solved by a method with the characteristics of Claim 1 and by a device with the features of Claim 3 solved. The direct measurement of the content of un burned hydrocarbons, carbon monoxide or nitrogen oxides in the Exhaust gas downstream of the nitrogen oxide adsorber enables ge exact determination of the time at which the nitrogen oxide adsor About during a stoichiometric or enrichment operation sphase has largely regenerated again. When using egg This is immediately understandable with a direct nitrogen oxide measurement, because the nitrogen oxide content during and after regeneration of the nitrogen oxide adsorber steadily drops to almost zero. The inventor dung is also based on the knowledge that if the nitrogen oxide content during a stoichiometric or  an enrichment operation phase below a predetermined level has decreased, the content of carbon monoxide and also that of unburned hydrocarbons in the exhaust gas show significant increase. As an alternative to nitrogen oxide measurement can therefore be provided, the carbon monoxide or Koh Hydrogen content of the exhaust gas downstream of the nitrogen oxide dabsorber to measure and then always from the lean operation to switch to lean operation if the measured carbon monoxide or increased hydrocarbon content over a predetermined level gen is. The device according to the invention is suitable for through carry out such a procedure by providing an appropriate one Sensor in the exhaust pipe downstream of the nitrogen oxide adsor About whose measurement signal information to a device which is the air / fuel ratio for the Controls the internal combustion engine and therefore the presence of an ent speaking measurement signal information the internal combustion engine from stoichiometric or enrichment operation to lean operation switches.

In a further development of the invention according to claim 2, the switch from a lean operating phase to a stoichiometric or an enrichment operating phase is carried out automatically when the nitrogen oxide content in the exhaust gas measured by a NO _x sensor used rises above a predetermined level downstream of the nitrogen oxide adsorber, since this indicates that the nitrogen oxide adsorber is saturated and that a regeneration phase is necessary.

A preferred embodiment of the invention is in the drawing shown and is described below. Here demonstrate

Fig. 1 is a schematic block diagram of a motor vehicle engine with associated control and catalyst system and

Fig. 2 diagrams of the time-dependent course of the content of pollutants in the exhaust gas in the system of Fig. 1 before and after a catalytic converter stage during a controlled lean-mix operation.

In Fig. 1, a motor vehicle internal combustion engine ( 1 ) is shown, from which an exhaust line ( 2 ) leads away, in which there is a three-way catalyst ( 3 ). A nitrogen oxide adsorber is integrated in the three-way catalytic converter ( 3 ). Such catalysts are known per se and therefore do not require a detailed description here. Alternatively, a separate NO _x adsorber unit can also be arranged in front of or behind a catalytic converter unit.

Downstream from the catalyst ( 3 ) and thus from the nitrogen oxide adsorber, a hydrocarbon sensor ( 4 ) is inserted into the exhaust pipe, the sensor output signal of which is fed to a signal processing stage ( 5 ). The from the signal processing stage ( 5 ) processed measurement signal information is then fed to a motor control unit ( 6 ) which controls the fuel / air ratio, ie the lambda value, for the internal combustion engine ( 1 ). It goes without saying that the system still has further components and connecting lines which are not of further interest in the present case and are therefore not shown in FIG. 1.

In operation, the engine control unit ( 6 ) controls the engine ( 1 ) either in lean operation, ie with a target lambda value greater than one, or in stoichiometric or in enrichment mode, ie with a lambda value equal to or less than one. Lean operation is always carried out for as long as possible, since this mode of operation has a significantly lower CO₂ emission. In the lean operating phases, the reduction reaction for the nitrogen oxides in the exhaust gas is made more difficult due to the higher oxygen content. The nitrogen oxides are absorbed by the NO _x adsorber in the oxygen-rich exhaust gas. After a certain period of lean operation, the NO _x adsorber becomes saturated and must be regenerated. This is done by switching to stoichiometric or enrichment operation at a suitable point in time, which is determined, for example, using an estimate of the amount of nitrogen oxide stored in the NO _x adsorber. In such an operating phase without excess oxygen in the combustion mixture, the intercalation reaction of the NO _x adsorber can be reversed so that it releases the adsorbed nitrogen oxides again. In addition, the nitrogen oxides in the catalytic converter ( 3 ) can now be reduced due to the absence of an oxidizing atmosphere in the exhaust gas. Accordingly, the nitrogen oxide content in the exhaust gas behind the catalytic converter ( 3 ) drops relatively steeply from the start of a stoichiometric or enrichment operating phase, in particular if the NO _x adsorber was previously close to its saturation and therefore the nitrogen oxide content in the exhaust gas behind the catalytic converter ( 3 ) had increased noticeably before the start of the stoichiometric or enrichment phase.

It can now be seen that during the regeneration, stoichiometric or enrichment operating phase of the NO _x adsorber, a significant increase in the content of unburned hydrocarbons in the exhaust gas occurs after the catalytic converter ( 3 ) as soon as the NO _x content falls below a certain level. Since this only low NO _x content in the exhaust gas indicates at the same time that the NO _x adsorber is completely regenerated again, this observation can be observed of the increase in the hydrocarbon content correlated with the achievement of a low NO _x content value for the self-timed switching from exploit stoichiometric or enrichment operation in the lean operation. As soon as the hydrocarbon sensor ( 4 ) detects a significant increase in the hydrocarbon content, this information is used by the engine control unit ( 6 ) to switch the engine ( 1 ) back to lean operation by changing the air / fuel ratio regulates a target lambda value above the stoichiometric value one. It is thus ensured in this system that the stoichiometric or enrichment operation is ended again exactly when the NO _x adsor is regenerated again. In this way, the less favorable operating phases with lambda values less than or equal to one can be kept as short as possible. As an alternative to the hydrocarbon sensor ( 4 ), a carbon monoxide sensor can be used. Because it is shown that a significant increase in the carbon monoxide content in the exhaust gas also starts again when the NO _x content has dropped below a certain level. Therefore, when using such a CO sensor, in the same way as when using an HC sensor, the ability of the system for targeted, timely switching from stoichiometric or enrichment operation for NO _x adsorber regeneration to low-emission engine operation results.

Instead of the carbon monoxide sensor ( 4 ), a NO _x sensor can alternatively be used. The switchover from the NO _x adsorber ( 3 ) regenerating, stoichiometric or enrichment operating phase to lean operation is carried out in this case when the measured NO _x content in the exhaust gas behind the adsorber catalyst ( 3 ) falls by a predetermined amount . In this application, a targeted switchover from lean operation to stoichiometric or enrichment operation can also take place, in that the NO _x sensor tracks the gradual increase in the NO _x content in the exhaust gas during lean operation and the motor control unit ( 6 ) becomes such let veran switching when it detects the associated with the saturation of the _NOx adsorber continuous steep rise of the NO _x content.

Referring to Fig. 2, the insight underlying the invention is hereinafter referred to the relationship between decreasing NO _x -Ge halt and nä explained increase in the hydrocarbon content of the coals and monoxidgehalts in the exhaust gas downstream of the adsorbent-catalyst (3) ago. Fig. 2 shows two test series diagrams, which were obtained with a system according to FIG. 1, but with fixed switching times between lean operating phases and stoichiometric operating phases, in order to make the above-mentioned connection clear. The upper diagram shows the time course of the NO _x content, the CO content and the HC content in the exhaust gas upstream of the catalytic converter, while the lower diagram shows the NO _x , CO and HC contents obtained in parallel during the same measurement time in the exhaust gas downstream of the catalytic converter, the contents being shown in arbitrary units, since only the qualitative curve is important.

As can be clearly seen in the upper diagram, the NO _x content and the CO content in the untreated exhaust gas during lean operating phases (M) lasting approx. 60 s are at significantly lower values than during intermediate stoichiometric operating phases (S), the duration of which is in each case 30s was set. At the same time, the HC content in the untreated exhaust gas during the lean operating phases (M) is somewhat higher than during the stoichiometric operating phases (S).

In parallel, the lower diagram shows that the NO _x content in the catalyst-treated exhaust gas during a respective lean operating phase (M) initially increases relatively slowly from zero. Towards the end of the defined lean operating phases (M), the NO _x content then rises at a certain point in time due to the saturation of the NO _x adsorber. At the time (t 1) from the lean operation (M) in the stoichiometric operation (S) is switched. Thereupon the NO _x content in the catalyst-treated exhaust gas initially rises very steeply and then falls steeply and then again over a shoulder ( 7 ) to zero. At the time (t₂) of switching to a new lean operating phase (M), the NO _x content behind the catalyst begins to rise again. At the same time, the lower diagram shows that the CO and HC content in the catalyst-treated exhaust gas remain at a low value during the entire period of a lean operating phase (M). From the time (t₁) of switching to a stoichiometric operating phase (S), the HC content initially increases only very slightly. However, as soon as the NO _x content has dropped below a certain value, which lies between zero and the value on the shoulder ( 7 ) of the NO _x curve drop, both the CO and the HC content start from this point in time (t 3). to climb steeply. Only after switching to a new lean operating phase (M) will the CO and HC content then fall back to their low values in lean operation from this point in time (t₂). It follows that the steep increases in the CO and HC content during a stoichiometric or enrichment phase are time correlated with the reduction in the NO _x content, which is given by the fact that the gradient of the time course of the NO _x content falls below a predetermined value. This is the reason that not only the detection of the NO _x content, but alternatively that of the CO or HC content in the exhaust gas downstream of the catalyst is suitable for realizing a targeted switchover from stoichiometric or enrichment operation to lean operation.

In the system of FIG. 1 feasible practical operation is therefore precisely at these times (t₃) the beginning of a steep HC rise in the catalyst-treated exhaust gas again switched from stoichiometric or enrichment operation to lean operation, so that the former operating phases are shorter than are in the test series shown in Fig. 2, which among other things increases the fuel savings to be achieved and thereby reduces the CO₂ emissions. The longer stoichiometric operating phases (S) of FIG. 2 were only chosen to be able to clearly illustrate the steep HC increase in the catalyst-treated exhaust gas. Similar results as for the HC content result for the CO content in the exhaust gas, which is why, analogously, as mentioned, a CO sensor instead of the HC sensor ( 4 ) is suitable for the system of FIG. 1. If the alternative NO _x sensor is used, the switch from lean operation to stoichiometric or enrichment operation can also be controlled by this NO _x sensor when it detects the beginning of a steep increase in the NO _x content in the catalyst-treated exhaust gas, as it does can be seen in the lower diagram of FIG. 2 at the end of a respective lean operating phase (M).

Claims (5)

1. Process for nitrogen oxide reduction in the exhaust gas of an internal combustion engine, in which

• - The internal combustion engine ( 1 ) is operated alternately in lean operation and in stoichiometric or enrichment operation, wherein
• - The nitrogen oxides (NO _x ) are adsorbed in a lean operating phase by a stick oxide adsorber ( 3 ) and
• - The nitrogen oxide adsorber is regenerated in stoichiometric or enrichment phases,

characterized in that

• - The content of the exhaust gas of hydrocarbons (HC), carbon monoxide (CO) or nitrogen oxides (NO _x ) is measured downstream of the nitrogen oxide adsorber and
• - Each is switched from a stoichiometric or enrichment phase to a lean operating phase as soon as the hydrocarbon or carbon monoxide content rises above a predetermined level or as soon as the nitrogen oxide content falls below a predetermined level.

2. The method according to claim 1, in which the nitrogen oxide content of the gas is measured downstream of the nitrogen oxide adsorber ( 3 ), further characterized in that each switch from a lean operating phase to a stoichiometric or enrichment operating phase as soon as the measured nitrogen oxide content rises above a predetermined level. 3. Device for nitrogen oxide reduction in the exhaust gas of an internal combustion engine ( 1 ), with

• - In the exhaust pipe ( 2 ) of the internal combustion engine ( 1 ) arranged nitrogen oxide adsorber ( 3 ) and
• - A device ( 6 ) for regulating the fuel / air ratio for the internal combustion engine,

marked by

• - A hydrocarbon, carbon monoxide or nitrogen oxide sensor ( 4 ) in the exhaust line ( 2 ) downstream of the nitrogen oxide adsorber ( 3 ), the measurement signal information of the device ( 6 ) for re regulation of the fuel / air ratio for the purpose of switching from lean operation control to stoichiometric or enrichment control can be fed.