JPH11148338A - Method to regenerate trap for nitrogen oxides of exhaust system of internal combustion engine - Google Patents

Abstract

PROBLEM TO BE SOLVED: To conduct lean burn running in an engine speed/load area large to the utmost by avoiding an exhaust peak in a period transient from lean burn to a theoretical air fuel ratio. SOLUTION: This method is related to a method to regenerate a trap for the nitrogen oxides in the exhaust system of an internal combustion engine provided with an electronic engine control unit. In this method, which air/fuel mixture of a lean or nearly theoretical ratio is supplied to an internal combustion engine according to various engine running states is judged by using this electronic engine control unit, and based on the result of the judgment, the basic regeneration cycle of the trap for nitrogen dioxides is started under a specified first starting condition. When transitions 52 and 54 from a lean burn 42 to a theoretical air fuel ratio 48 occur and a specified second starting condition is obtained, the additional regeneration cycle of the trap for nitrogen oxides is started. Through this operation, free liberation of the nitrogen oxides stored can be avoided.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a method for regenerating a trap for nitrogen oxides in an exhaust system of an internal combustion engine having an electronic engine control unit. Here, using this electronic engine control unit, depending on many engine operating conditions, the air / fuel ratio of lean or substantially stoichiometric air-fuel
It is determined which of the fuel mixture is being supplied and, using this, under predetermined first starting conditions,
A basic regeneration cycle of the nitrogen oxide trap is started.

[0002]

2. Description of the Prior Art In order to reduce the emission of nitrogen oxides, which occur particularly during lean burn operation, in motor vehicles equipped with an internal combustion engine designed for lean burn operation (lean burn engine), the above-mentioned problems are encountered. It is preferable to use a nitrogen oxide trap in combination with a general three-way catalyst. Nitrogen oxide molecules settle out of the trap coating and are removed from the exhaust. To allow long-term use of the nitrogen oxide trap, a regeneration cycle is required at some point of saturation. For this purpose, the engine is equipped with a rich air / fuel air-fuel ratio (eg, lambda

[Outside 1] = 0.75) for short-time operation. In such a state, the precipitated nitrogen oxides are separated into nitrogen and oxygen under the influence of the catalyst, and the oxygen is combusted with excess hydrogen or carbon monoxide to form water or carbon dioxide.

The problem with known nitrogen oxide traps is that
Under certain operating conditions, the nitrogen oxides already fixed
It is released again from the nitrogen oxide trap without conversion. The above-mentioned problem occurs particularly when the internal combustion engine shifts from the lean burn operation to the stoichiometric air-fuel ratio operation in a high speed / high load region. At this transition, if the nitrogen oxide trap has already stored a considerable amount of nitrogen oxides, liberation of unconverted nitrogen oxides is likely to occur. Such free liberation of nitrogen oxides may fail rigorous emissions tests, even if the emissions at steady state operation are satisfactory.

In order to avoid such emission peaks, the speed / load window in which the internal combustion engine is operated with a lean mixture is reduced so that at low loads and speeds such that the aforementioned problems do not occur, the engine speed is reduced. There is known a proposal to make the size such that a transition from the burn to the stoichiometric air-fuel ratio occurs. On the other hand, however, it is desirable to operate the engine with a lean air / fuel ratio at the highest possible rotational speed / load in order to maximize the potential for fuel savings.

[0005]

SUMMARY OF THE INVENTION It is an object of the present invention to avoid a peak emission at the transition from lean burn to the stoichiometric air-fuel ratio, and to perform lean burn operation at a rotational speed / load region as large as possible. It is to provide a method of the kind described above, which is assured that

[0006]

In order to achieve this object, according to the present invention, a transition from lean burn to a stoichiometric air-fuel ratio occurs, and when a predetermined second starting condition is obtained, nitrogen oxides are produced. An additional regeneration cycle of the trap is started.
With this additional regeneration cycle, the nitrogen oxide trap is regenerated before the transition to the stoichiometric air-fuel ratio and free liberation of the stored nitrogen oxide no longer occurs.

According to a preferred embodiment of the present invention, a rich air / fuel mixture is supplied to the engine during a regeneration cycle.

In accordance with a further advantageous embodiment of the invention, the nitrogen oxide trapping rate of the nitrogen oxide trap corresponds to the amount of nitrogen oxide trapped by the nitrogen oxide trap. The nitrogen oxide amount value is approximately determined by the engine control unit using time differentiation,
The basic regeneration cycle is started in the first state in which the nitrogen oxide amount exceeds a predetermined first threshold, and the additional regeneration cycle is started in the second state in which the nitrogen oxide amount exceeds a second threshold lower than the first threshold. Beginning and after each execution of a basic or additional regeneration cycle, the nitrogen oxide amount value is initialized. In this way, additional regeneration of nitrogen oxide traps
It does not occur at all transitions between burn and stoichiometric, but only when a certain minimum amount of nitrogen oxides is additionally stored in the nitrogen oxide trap. Unnecessary regeneration cycles that increase fuel consumption are thus avoided. In a simpler alternative embodiment, the basic regeneration cycle is performed at regular intervals and an additional regeneration cycle is initiated at each transition from lean burn to stoichiometric. Further, it is possible to enable an additional reproduction cycle only in an additional start state in which a certain minimum period has elapsed since the last reproduction.

Since it is almost impossible to measure the actual trapping rate of nitrogen oxides within the cost allowable range, the engine speed, the engine load, the air-fuel ratio, and the exhaust gas near the trap for nitrogen oxides are measured. It is more effective to approximately determine the current trapping rate of nitrogen oxides using a functional relationship according to the temperature and the mass flow rate of the nitrogen oxides. Such functional relationships can be incorporated into memory as functional forms or look-up tables, and are preferably determined using data on a test stand.

In yet another embodiment, the free liberation of nitrogen oxides occurs essentially only at the transition of lean burn / stoichiometric from a certain speed / load range.
When the internal combustion engine is operating in the predetermined rotational speed / load region, the transition from the predetermined small region of the lean burn rotational speed / load region to the stoichiometric air-fuel ratio operation region is performed to start the additional regeneration cycle. Condition. The small region of the lean burn operation region is preferably in a high rotation or high load region. As a result of this additional (second) start condition, unnecessary regeneration cycles are avoided.

Further, the rich regeneration air-fuel ratio required for regeneration of the nitrogen oxide trap can be determined using a functional relationship according to the exhaust gas temperature and the exhaust mass flow rate in the region of the nitrogen oxide trap. The thus determined regeneration air-fuel ratio is preferably used in both the basic and additional regeneration cycles.

The basic regeneration time required for the basic regeneration cycle using the regeneration air-fuel ratio is preferably determined using a functional relationship according to the exhaust gas temperature near the nitrogen oxide trap and the exhaust gas mass flow rate. .

The additional regeneration time required to perform an additional generation cycle for regeneration using the regeneration air-fuel ratio is multiplied by the basic regeneration time by the ratio of the current nitrogen oxide amount value to the first threshold. It is preferably calculated by When the additional regeneration cycle is performed, the amount of nitrogen oxide stored in the nitrogen oxide trap is generally smaller than that in the basic regeneration cycle. The regeneration time can be shortened in order to minimize fuel consumption. In yet another embodiment of the present invention, a fixed offset value can be added to the playback time determined by the above method. This takes into account the time required for the rich peak from the internal combustion engine to reach the nitrogen oxide trap through the three-way catalyst.

The invention will be described in more detail hereinafter with reference to the drawings and by way of example.

[0015]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring first to FIG. 1, a multi-cylinder internal combustion engine 10 is controlled by an electronic engine control unit 12, which controls the current engine speed, the air mass flow in the intake passage. Sensor 3
A plurality of input signals 24 are received, such as a signal from zero or the current position of the driver's accelerator pedal. The engine control unit includes the electronic throttle valve 20, the ignition system 1
8 and the algorithm controlling the injection system 26 is executed. The electronic throttle valve 20 and the injection system 18 allow the air-fuel ratio lambda of the mixture supplied to the cylinder to be varied over a wide range, and in particular under certain operating conditions a lean air-fuel ratio can be set. Exhaust from the engine is supplied to an exhaust treatment system. It comprises a three-way catalytic converter 14 and a nitrogen oxide trap 16. Using the temperature sensor 22, the exhaust gas temperature in the spatial vicinity of the exhaust treatment system is measured.

FIG. 2 shows the amount X of nitrogen oxide trapped by the nitrogen oxide trap and the set air / fuel ratio lambda (

[Outside 2] ) And the time change of the value NOx representing the nitrogen oxide emission amount are qualitatively shown. At the start of the sequence shown in FIG. 2, the internal combustion engine is operating in lean burn mode with an air / fuel lambda of 1.5. At each time interval, the engine control unit calculates the current trapping rate of nitrogen oxides using a functional relationship according to engine speed, engine load, air-fuel ratio, exhaust temperature and exhaust mass flow rate, and differentiates this ratio. To give a nitrogen oxide amount value X. When this value exceeds the threshold value S1 (60), the basic regeneration cycle is executed at the regeneration air-fuel ratio of 0.75 during the period TR1, and the nitrogen oxide amount value X is initialized to zero.

In the example shown in FIG. 2, the transition from the lean burn to the stoichiometric air-fuel ratio operation mode occurs at time ts. At this time, since the nitrogen oxide amount value X is above the second threshold value S2, the period TR2 shorter than TR1 is used.
During this period, an additional regeneration cycle in which the regeneration air-fuel ratio is 0.75 is executed, and the value X is initialized to zero. Only after this additional regeneration cycle is the stoichiometric air-fuel ratio

[0018]

(Equation 1)

Is set to

In the prior art method without additional regeneration cycles, as shown by the break curve in FIG.
Undesired nitrogen oxide emission peaks 66 occur in contrast to the method of the present invention.

FIG. 3 shows a schematic diagram of the rotational speed / load of the engine. The maximum engine torque MD at each engine speed n is given by the full load curve 46. In region 42, lean burn operation of the internal combustion engine is permitted by the engine control unit, and above or to the right of this region the engine is operated at the stoichiometric air-fuel ratio in the region indicated by reference numeral 48. Only at the transition of the lean burn region 42 from the small region 50 (eg 52, 54) free liberation of unconverted nitrogen oxides takes place. Thus, only when the engine control unit detects the transition from the small area 50 to the area 48, the additional regeneration cycle is started.

As shown in FIG. 4, the execution of the monitoring loop, which is continuously performed during operation of the engine according to the method of the present invention, begins with the determination of the nitrogen oxide value X in step 82. In step 84, X is the first threshold value S1
Is compared to If this value is exceeded, a basic regeneration cycle is started. For this purpose, in step 86, the air-fuel ratio required for regeneration is determined according to the exhaust gas temperature and the mass flow rate near the nitrogen oxide trap.

[Outside 3] And the required basic reproduction time TR1 are calculated. With these parameters, a basic regeneration cycle is performed at step 88 and the nitrogen oxide amount value X is initialized to zero. If S1 is not exceeded in step 84, X is further compared in step 90 with a second lower threshold S2. When the engine control unit detects the transition from the small area 50 to the area 48 shown in FIG. 2 and X exceeds the threshold value S2, the engine control unit starts an additional regeneration cycle. Additional playback time TR2 is TR1
Is smaller by the ratio of the current nitrogen oxide amount value X to the threshold value S1, as shown in step 94. Then, in step 96, the additional regeneration cycle is started, and the nitrogen oxide amount value X is initialized to zero.

[0024]

The trap for nitrogen oxides is regenerated before the transition to the stoichiometric air-fuel ratio and free liberation of the stored nitrogen oxides does not occur.

[Brief description of the drawings]

FIG. 1 is a schematic diagram of an internal combustion engine and an engine control system for performing the method according to the invention.

FIG. 2 is a schematic diagram showing a temporal change in engine characteristics.

FIG. 3 shows the rotational speed / value for explaining the method according to the invention.
4 is a schematic map of a load characteristic.

FIG. 4 is a schematic flowchart of a method according to the present invention.

[Explanation of symbols]

 Reference Signs List 10 internal combustion engine 12 engine control unit 16 trap for nitrogen oxide

 ──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Patrick, Philps Cologne, Germany, Vogelzangar Beck, 12 (72) Inventor Roland, Erdman Pulheim, Germany, Delhofen ner Straße

Claims (9)

[Claims] A trap for nitrogen oxide in an exhaust system of an internal combustion engine having an engine control unit.
6) The method of claim 6, wherein the engine control unit is used to supply either a lean or substantially stoichiometric air / fuel mixture to the internal combustion engine depending on a number of engine operating parameters. The basic regeneration cycle of the nitrogen oxide trap is started under the predetermined first start condition using the engine control unit, and the lean air condition mode is switched from the lean burn operation mode to the stoichiometric air / fuel ratio operation mode. The additional regeneration cycle of the trap for nitrogen oxides is started when the predetermined second start condition is obtained after the transition of (i). 2. The method according to claim 1, wherein a rich air / fuel mixture is supplied to the internal combustion engine during both regeneration cycles. 3. The nitrogen oxide trap rate of the nitrogen oxide trap (16) at that time and a nitrogen oxide amount value (X) corresponding to the amount of nitrogen oxide trapped by the nitrogen oxide trap. The basic regeneration cycle is started in a state where the nitrogen oxide amount exceeds a predetermined first threshold value (S1), and the basic regeneration cycle is started by the engine control unit. The additional regeneration cycle is started in a state where the second threshold value lower than the first threshold value is exceeded, and the nitrogen oxide amount value (X) is initialized after each execution of the basic or additional regeneration cycle. The method according to claim 1 or 2, wherein 4. The approximate determination of the current nitrogen oxide trap rate includes the current engine speed, engine load, air-fuel ratio, and the temperature and mass of the exhaust near the nitrogen oxide trap (16). 4. The method according to claim 3, wherein the method is performed using a functional relationship depending on the flow rate. 5. When the internal combustion engine is operated at a predetermined rotational speed / load region (42) with a lean mixture,
The additional regeneration cycle is started in a transition state from a predetermined small area (50) of the lean burn rotation speed / load area (42) to an operation area (48) at the stoichiometric air-fuel ratio. 5. The method according to any one of 1 to 4. 6. The method according to claim 5, wherein the sub-region (50) of the lean-burn operation region (42) is in a region where the rotational speed and / or load is high. 7. A rich regeneration air-fuel ratio required for regeneration of the nitrogen oxide trap is determined using a functional relationship according to an exhaust gas temperature and an exhaust mass flow rate in the vicinity of the nitrogen oxide trap. The method according to any one of claims 3 to 6, wherein 8. A basic regeneration time (TR1) required for a basic regeneration cycle using the regeneration air-fuel ratio is determined by using a functional relationship based on an exhaust gas temperature and an exhaust mass flow rate in the vicinity of the nitrogen oxide trap. 8. The method according to claim 7, wherein the method is determined by: 9. The additional regeneration time required to execute an additional regeneration cycle using the regeneration air-fuel ratio is determined by the ratio of the current nitrogen oxide amount value (X) to the first threshold value (S1). Method according to claim 8, characterized in that it is determined by multiplying the basic playback time (TR2) and adding a predetermined offset time to the product.