Method for adjusting a fuel/air ratio of an internal combustion engine

Abstract

The invention relates to a method for adjusting a fuel/air ratio of an internal combustion engine (10), comprising a catalyst volume (26) with a first catalyst partial volume (26.1) and a second catalyst partial volume (26.2). The second catalyst partial volume (26.2) is arranged downstream from the first catalyst partial volume (26.1). An actual filling level of an exhaust gas constituent in the catalyst volume (26) is calculated from operating parameters of the internal combustion engine (10) and the exhaust system (14) using a computing model, and is adjusted to a nominal value by modifying the fuel/air ratio. The adjustment is carried out first for the second catalyst partial volume (26.2) and only later for the first catalyst partial volume (26.1).

Claims

1. A method for adjusting a fuel/air ratio of an internal combustion engine (10) which has an exhaust gas system (14) with a single catalyst having a catalyst volume (26) which is capable of storing an exhaust gas component, the method comprising: determining a first actual fill level of the exhaust gas component for a first catalyst partial volume (26.1) of the catalyst volume (26) from operating parameters of the internal combustion engine (10) and the exhaust gas system (14) with a calculation model, wherein the first catalyst partial volume (26.1) extends in a direction of an exhaust gas flow across a front region of the catalyst volume (26); determining a second actual fill level of the exhaust gas component for a second catalyst partial volume (26.2) with the calculation model, wherein the second catalyst partial volume (26.2) extends downstream of the first catalyst partial volume (26.1) across a rear region of the catalyst volume (26); adjusting the fuel/air ratio based on a deviation of the second actual fill level from a second setpoint value; and adjusting the fuel/air ratio based on a deviation of the first actual fill level from a first setpoint value after adjusting the fuel/air ratio based on the deviation of the second actual fill level from the second setpoint value.

2. The method as claimed in claim 1, wherein the second setpoint value is predetermined so that emissions are minimized.

3. The method as claimed in claim 1, wherein the first setpoint value is predetermined so that a fill level reserve against changes in the fuel/air ratio which occur upstream in front of the catalyst volume assumes a predetermined value.

4. The method as claimed in claim 3, wherein the first setpoint value is half as large as a maximum fill level of the first catalyst partial volume (26.1) and that the second setpoint value is between 25% and 35% of the maximum oxygen storage capacity or of the maximum fill level of the second catalyst partial volume (26.2).

5. The method as claimed in claim 1, wherein an exhaust gas temperature and an exhaust gas mass flow are taken into account in the determination of the actual values of the fill levels.

6. The method as claimed in claim 1, wherein the single catalyst is a three-way catalyst.

7. The method as claimed in claim 1, wherein the exhaust gas component is oxygen.

8. A control device (16) for adjusting a fuel/air ratio of an internal combustion engine (10) which has an exhaust gas system (14) with a single catalyst having a catalyst volume (26) which is capable of storing an exhaust gas component, wherein the control device (16) is configured to determine a first actual fill level of the exhaust gas component for a first catalyst partial volume (26.1) of the catalyst volume (26) from operating parameters of the internal combustion engine (10) and the exhaust gas system (14) with a calculation model, wherein the first catalyst partial volume (26.1) extends in a direction of an exhaust gas flow across a front region of the catalyst volume (26); determine a second actual fill level of the exhaust gas component for a second catalyst partial volume (26.2) with the calculation model, wherein the second catalyst partial volume (26.2) extends downstream of the first catalyst partial volume (26.1) across a rear region of the catalyst volume (26); adjust the fuel/air ratio based on a deviation of the second actual fill level from a second setpoint value; and adjust the fuel/air ratio based on a deviation of the first actual fill level from a first setpoint value after adjusting the fuel/air ratio based on the deviation of the second actual fill level from the second setpoint value.

9. The control device (16) as claimed in claim 8, wherein the second setpoint value is predetermined so that emissions are minimized.

10. The control device (16) as claimed in claim 8, wherein the first setpoint value is predetermined so that a fill level reserve against changes in the fuel/air ratio which occur upstream in front of the catalyst volume assumes a predetermined value.

11. The control device (16) as claimed in claim 10, wherein the first setpoint value is half as large as a maximum fill level of the first catalyst partial volume (26.1) and that the second setpoint value is between 25% and 35% of the maximum oxygen storage capacity or of the maximum fill level of the second catalyst partial volume (26.2).

12. The control device (16) as claimed in claim 8, wherein an exhaust gas temperature and an exhaust gas mass flow are taken into account in the determination of the actual values of the fill levels.

13. The control device (16) as claimed in claim 8, wherein the single catalyst is a three-way catalyst.

14. The control device (16) as claimed in claim 8, wherein the exhaust gas component is oxygen.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Exemplary embodiments of the invention are represented in the drawings and explained in greater detail in the following description. The same reference numbers in different figures designate respectively identical elements or elements which are comparable at least in terms of their function. In each case in a schematic form, in the figures:

(2) FIG. 1 shows an internal combustion engine with an exhaust gas system as a technical field of the invention;

(3) FIG. 2 shows a flow chart as an exemplary embodiment of a method according to the invention; and

(4) FIGS. 3a-e show in each case the fill level of two catalyst partial volumes in percent in the case of a control strategy with which the fill level in the second catalyst partial volume is initially adjusted.

DETAILED DESCRIPTION

(5) The invention is described below on the basis of the example of a three-way catalyst, but can also be expediently applied to other types of catalysts. For the sake of simplicity, an exhaust gas system with a single catalyst is assumed below. The invention can, however, expediently also be applied to exhaust gas systems with a plurality of catalysts. The front and rear catalyst partial volumes described below can in this case extend over a plurality of catalysts or lie in different catalysts.

(6) In detail, FIG. 1 shows an internal combustion engine 10 with an air supply system 12, an exhaust gas system 14 and a control device 16. An air flow meter 18 and a throttle valve 19 arranged downstream of air flow meter 18 are located in air supply system 12. The air flowing via air supply system 12 into internal combustion engine 10 is mixed with petrol in combustion chambers 20 of internal combustion engine 10, which petrol is injected via injection valves 22 directly into combustion chambers 20. The resultant combustion chamber fillings are ignited and burned with ignition devices 24, for example, spark plugs. A rotational angle sensor 25 detects the rotational angle of a shaft of internal combustion engine 10 and allows control device 16 as a result to trigger ignitions at predetermined angle positions of the shaft. The exhaust gas resulting from the combustion processes is discharged by exhaust gas system 14.

(7) Exhaust gas system 14 has a catalyst volume 26. Catalyst volume 26 is, for example, a three-way catalyst which, in a familiar manner on three reaction paths, converts the three exhaust gas components nitrogen oxides, hydrocarbons and carbon monoxide and possesses an oxygen-storing action. Catalyst volume 26 has, in the represented example, a first catalyst partial volume 26.1 and a second catalyst partial volume 26.2. Exhaust gas 28 flows through both catalyst partial volumes. First, front catalyst partial volume 26.1 extends in the direction of flow across a front region of three-way catalyst 26. Second, rear catalyst partial volume 26.2 extends downstream of first catalyst partial volume 26.1 across a rear region of catalyst volume 26. Naturally, further catalyst partial volumes can be present in front of front catalyst partial volume 26.1 and behind rear catalyst partial volume 26.2 as well as between both catalyst partial volumes, for which further catalyst partial volumes, where applicable, the respective fill level is also modeled.

(8) A front exhaust gas sensor 32 exposed to exhaust gas 28 is arranged immediately in front of catalyst volume 26 upstream of catalyst volume 26. A rear exhaust gas sensor 34 also exposed to the exhaust gas is arranged immediately after catalyst volume 26 downstream of catalyst volume 26. Front exhaust gas sensor 32 is preferably a wide band lambda sensor which enables a measurement of air ratio over a wide air ratio range. Rear exhaust gas sensor 34 is preferably what is known as a switching-type lambda sensor with which air ratio =1 can be measured particularly accurately because the signal of this exhaust gas sensor changes abruptly there. Cf. Bosch, Kraftfahrtechnisches Taschenbuch, 23.sup.rd Edition, page 524.

(9) In the represented exemplary embodiment, a temperature sensor 36 exposed to the exhaust gas is arranged in thermal contact with the exhaust gas on three-way catalyst 26 which detects the temperature of catalyst volume 26.

(10) Control device 16 processes the signals of air flow meter 18, rotational angle sensor 25, front exhaust gas sensor 32, rear exhaust gas sensor 34 and temperature sensor 36 and forms from them actuation signals for adjusting the angle position of throttle valve 18, for triggering ignitions by ignition device 20 and for injection of fuel through injection valves 22. Alternatively or additionally, control device 16 also processes signals of other or further sensors for actuation of the represented actuators and also further or other actuators, for example, the signal of a driver command transmitter 40 which detects an accelerator pedal position. Propulsion operation with switching off of the fuel supply is triggered, for example, by releasing the accelerator pedal.

(11) The flow chart of FIG. 2 shows an exemplary embodiment of a method according to the invention.

(12) A block 100 represents a main program for control of internal combustion engine 10 in which, for example, throttle valve 19, ignition device 20 and injection valves 22 are actuated so that a desired torque is produced.

(13) Since a fill level of a catalyst cannot be measured, the invention provides modeling of the fill levels, preferably oxygen fill levels, in at least two catalyst partial volumes 26.1, 26.2 of catalyst volume 26 with the aid of a calculation model 16.1. Calculation model 16.1 is a sub-program executed in control device 16.

(14) This modeling is carried out in step 102 both for first catalyst partial volume 26.1 and also for second catalyst partial volume 26.2. The mass flow of excess oxygen and shortage of oxygen at the inlet of catalyst volume 26 is produced from the product of the signal of front exhaust gas sensor 32 which detects an oxygen concentration with the exhaust gas mass flow which is known in control device 16 from the measured air mass supplied to the internal combustion engine and the metered fuel for this purpose.

(15) A predetermined fraction of this excess is stored in first catalyst partial volume 26.1 and a predetermined fraction of the shortage is compensated for by a reduction in the fill level of first catalyst partial volume 26.1. The predetermined fractions which can be different from one another are dependent on the temperature of catalyst volume 26, on the exhaust gas mass flow and on the fill level of first catalyst partial volume 26.1. The in each case complementary fractions represent input variables for the change in the fill level of second catalyst partial volume 26.2 and are thus also dependent on the exhaust gas mass flow and on the temperature of catalyst 26.

(16) The fill level of second catalyst partial volume 26.2 is in particular dependent on the fill level of first catalyst partial volume 26.1. The greater the fill level of the first catalyst partial volume, the smaller, for example, the percentage of a current oxygen excess which can still be stored by first catalyst partial volume 26.1 and the greater the complementary percentage of the current oxygen excess which forms an input variable for the calculation of the fill level of second catalyst partial volume 26.2. The relative oxygen fill level (in %) in first catalyst partial volume 26.1 lying in front of it, relative to the maximum oxygen storage capacity of this first catalyst partial volume 26.1 and the development of this relative fill level, is therefore equally decisive for the development of the oxygen fill level in second catalyst partial volume 26.2 at the outlet of catalyst 26.

(17) Values of the percentages and the maximum oxygen storage capacity are stored in control device 16 so that control device 16 can calculate both the fill level of first catalyst partial volume 26.1 and the fill level of second catalyst partial volume 26.2 from the exhaust gas mass flow known to said control device 16, a measured or modeled temperature of catalyst 26 and the predetermined and stored values for the percentages.

(18) How the catalyst volume is ultimately filled with stored oxygen or emptied of stored oxygen largely depends on the exhaust gas temperature and the exhaust gas mass flow. In the case of a high exhaust gas temperature and low exhaust gas mass flow, the catalyst volume is in extreme cases gradually emptied or filled from the front to the rear, which means that initially first catalyst partial volume 26.1 is completely filled or emptied before the second catalyst partial volume is filled or emptied. In the case of a low exhaust gas temperature and large exhaust gas mass flow, the catalyst is in extreme cases filled or emptied simultaneously at the front and rear.

(19) A check is performed in a step 104 as to whether the fill level of second catalyst partial volume 26.2 lies in a predefined interval with surrounds a setpoint value for the second fill level. If this is not the case, the program branches into step 106 in which an actuating variable for the lambda air ratio is determined as a function of a deviation of the actual fill level of the second catalyst partial volume from its setpoint value.

(20) The actuating variable is, for example, a correction value for an actuating signal for the injection valves with which fuel is metered into the combustion chambers of the internal combustion engine. In order to increase the oxygen fill level, a lean fuel/air mixture (lambda>1) is set. In order to reduce the oxygen fill level, a rich fuel/air mixture (lambda<1) is set.

(21) The method subsequently returns to main program 100 in which, among other things, the corrected actuation signals are formed and output. The loop of steps 100 to 106 is passed through repeatedly until the actual fill level of second catalyst partial volume 26.2 reaches its setpoint value. This is ascertained in step 104. In this case, the method branches from step 104 into a step 108. A check is performed in step 108 as to whether the fill level of first catalyst partial volume 26.1 lies in a predefined interval which surrounds a setpoint value for this first fill level. If this is not the case, the program branches into step 110 in which an actuating variable for the lambda air ratio is determined as a function of a deviation of the actual fill level of first catalyst partial volume 26.1 from its setpoint value.

(22) The actuating variable is also, here, for example, a correction value for an actuation signal for the injection valves with which fuel is metered into the combustion chambers of the internal combustion engine. In order to increase the oxygen fill level, a lean fuel/air mixture (lambda>1) is set. In order to reduce the oxygen fill level, a rich fuel/air mixture (lambda<1) is set.

(23) The method subsequently returns to main program 100 in which, among other things, the corrected actuation signals are formed and output. The loop of steps 100 to 104, 108 and 110 is passed through repeatedly until the actual fill level of the first catalyst partial volume reaches its setpoint value. This is ascertained in step 108. In this case, the method branches from step 108 back into main program 100 without forming a correction value.

(24) FIG. 3 illustrates a control strategy which corresponds to the flow chart of FIG. 2 for the situation which is to be produced after a propulsion operation phase. In detail, FIG. 3 shows in its partial figures a) to e) in each case the fill level of both catalyst partial volumes 26.1, 26.2 in percent in the case of a control strategy with which initially the fill level in second catalyst partial volume 26.2 is adjusted so that the current emissions are minimized. To this end, the fill level of the second catalyst partial volume is, in the example under consideration, initially reduced from 100% according to FIG. 3 to 50% according to FIG. 3d. The initial fill level of 100% is produced, for example, in the case of a preceding propulsion operation phase with the fuel supply switched off.

(25) The fill level is reduced by adjusting a rich exhaust gas atmosphere which has a lack of oxygen at the inlet of three-way catalyst 26. The control deviation formed as a difference between the actual value (100%) and setpoint value (50%) of second catalyst partial volume 26.2 is initially 50%. The rich exhaust gas atmosphere is generated by the injection of sufficiently large quantities of fuel into the combustion chambers of the internal combustion engine. In this case, the fill level of first catalyst partial volume 26.1 is reduced, in the example under consideration of FIG. 3a to FIG. 3d, gradually to zero, which is undesirable on the grounds mentioned above.

(26) Later, preferably when the actual level of the fill level of second catalyst partial volume 26.2 has reached its setpoint value, in a further method step, the metering of fuel to combustion chambers 20 of internal combustion engine 10 is therefore reduced to such an extent that at the inlet of three-way catalyst 26 has a lean exhaust gas atmosphere having excess oxygen. Control is carried out to this end on the basis of a control deviation which is produced as the difference between the actual value of the fill level in first catalyst partial volume 26.1 and its setpoint value as long as the fill level of first catalyst partial volume 26.1 lies, for example, at half of the maximum fill level, i.e. at 50%. As a result of this, a maximum fill level reserve in first catalyst partial volume 26.1 is produced with which dynamic lambda disturbances, which can occur upstream of the catalyst volume, can be balanced out by discharging or absorbing oxygen.

(27) This control strategy is based on the following observations: the oxygen fill level in a comparatively small rear catalyst partial volume at the outlet of the catalyst volume is decisive for the current exhaust gas composition behind a catalyst volume. As long as this catalyst partial volume can both store oxygen and release stored oxygen, both a reduction and an oxidation of exhaust gas components to be converted are possible and no breakthroughs of oxygen and other exhaust gas components arise.

(28) The advantages of this procedure become clear as a result of the following comparison with other processes in the case of which control only of an oxygen fill level is carried out in a rear catalyst partial volume of the catalyst volume or in the case of which control to an average oxygen fill level of both catalyst partial volumes is carried out jointly.

(29) A state is considered which is produced after a propulsion operation phase with the fuel supply switched off: in the propulsion operation phase, the internal combustion engine has pumped air into the exhaust gas system. The oxygen fill level of the catalyst volume then initially lies at 100%. A storage, required for conversion of nitrogen oxides, of further oxygen is then not possible. The stored oxygen should be cleared out to such an extent that the catalyst volume once again reaches its full conversion capacity as quickly as possible. To this end, a rich lambda is set before the catalyst volume. For the sake of simplicity, it is assumed that the exhaust gas temperature is high and the exhaust gas mass flow is low so that the oxygen store of the catalyst volume is gradually emptied from the front to the rear, that both partial volumes possess the same maximum oxygen storage capacity and that the setpoint fill level is in each case 50% of the maximum oxygen storage capacity of the catalyst.

(30) In the case of a control which is only based on the actual value of the oxygen fill level in the rear catalyst partial volume, the control setpoint value of 50% is only reached when the catalyst volume has already been largely emptied of oxygen because the fill level in the rear catalyst partial volume only drops when the catalyst partial volume has been completely emptied before this. There is therefore the risk of a rich breakthrough as a result of the emptying of oxygen out of the catalyst volume, in particular if, under these conditions, a dynamic deviation of the lambda towards rich comes about before the catalyst volume.

(31) In the case of control of the average oxygen fill level of the entire catalyst volume, the control setpoint value of 50% is reached as soon as the front half of the catalyst volume has been completely emptied and the rear half of the catalyst volume is still completely filled. In this case, the oxygen would not have been sufficiently emptied out of the catalyst volume, which would result in increased NO.sub.X emissions.

(32) The adjustment according to the invention of at least two fill levels into a front and a rear catalyst partial volume makes it possible in this exemplary case to adjust both the fill level of the rear catalyst partial volume so that it lies in the conversion window and also adjust the fill level of the front catalyst partial volume so that the largest possible fill level reserve for dynamic deviations of the lambda in front of the catalyst volume towards rich or lean is achieved.

(33) Due to the fact that the invention, in this situation which occurs after propulsion operation, initially empties the rear catalyst partial volume as quickly as possible to such an extent that, in this catalyst partial volume, oxygen can be both stored and released, the NO.sub.X emissions can be minimized. The front catalyst partial volume is nevertheless completely emptied. There is therefore the risk of rich breakthrough. According to the invention, the front catalyst partial volume is therefore subsequently filled again with oxygen to such an extent that a fill level reserve against dynamic deviations of the lambda before the catalyst is present both towards rich and towards lean.

(34) In this case, it must of course be taken into account that the fill levels of both catalyst partial volumes are coupled and cannot be adjusted independently of one another. This example, which relates to a situation which occurs after a propulsion operation, represents an extreme case with the largest possible initial control deviation in the direction of an excessive fill level. It can, however, expediently be applied to situations with smaller control deviation or control deviation in the direction of an excessively low fill level. It can also be applied to situations in which only the fill level of individual zones deviates from the setpoint fill level (e.g. after a short propulsion operation phase).

(35) In its most general form, the adjustment of a fill level profile of the catalyst volume is thus provided with the following steps: the fill level in a rear catalyst partial volume is set so that the current emissions are minimized. The fill level of one or more catalyst partial volumes lying in front of it is subsequently adjusted so that the fill level reserve against lambda disturbances in front of the catalyst volume is maximized. The coupling of the fill levels of the different catalyst partial volumes can be taken into account in the adjustment of the fill level profile. In particular, the dependency of this coupling on the catalyst temperature and on the exhaust gas mass flow can be taken into account. The setpoint fill levels of the catalyst partial volumes can be adapted dynamically to the current fill level distribution in the catalyst volume so that the two aims of the minimum current emissions and the maximum fill level reserve against dynamic disturbances are simultaneously satisfied as optimally as possible. In a situation in which currently no fill level reserve against dynamic disturbances is present, it can, for example, be expedient to accept, for a short time, higher emissions as a result of the increase in this fill level reserve in order as a result to ensure lower emissions in the long term.