WO1999035386A1

WO1999035386A1 - Method for regenerating a nitrogen oxide trap in the exhaust system of an internal combustion engine

Info

Publication number: WO1999035386A1
Application number: PCT/IB1999/000007
Authority: WO
Inventors: Guido Dickers; Klemens Grieser; Johann Himmelsbach; Patrick Phlips
Original assignee: Ford Global Technologies, Inc.
Priority date: 1998-01-09
Filing date: 1999-01-06
Publication date: 1999-07-15
Also published as: EP0940570B1; EP0937876B1; DE59800195D1; EP0937876A1; EP0940570A1

Abstract

The invention relates to a method for regenerating a nitrogen oxide trap in the exhaust gas system of an internal combustion engine having a device for influencing the amount of fuel injected, wherein the amount of fuel injected is varied for the purpose of initiating or ending a regeneration cycle carried out with a rich air/fuel ratio. The regeneration cycles are only carried out if the amount of the nitrogen oxide picked up by the nitrogen oxide trap has exceeded a predetermined value. This amount is determined by integration of a nitrogen oxide pick-up rate determined by a functional relationship. The threshold value is adapted to the ageing of the catalytic converter and of the nitrogen oxide trap. For further saving of fuel the nitrogen oxide trap is regenerated on transition from stoichiometric to lean mixture operation.

Description

METHOD FOR REGENERATING A NITROGEN OXIDE TRAP IN THE EXHAUST SYSTEM OF AN INTERNAL COMBUSTION ENGINE

The invention relates to a method for regenerating a nitrogen oxide trap in the exhaust system of an internal combustion engine having devices for influencing the mass flow of air supplied and the quantity of fuel injected, wherein for the purpose of initiating or ending a regeneration cycle carried out with a rich air/fuel ratio the amount of fuel injected is varied.

Such a nitrogen oxide trap (NOx trap) is preferably used in combination with a conventional three-way catalytic converter in motor vehicles of which the internal combustion engine is designed for lean mixture operation (a so-called lean burn engine) . The nitrogen oxide trap decreases the nitrogen oxide emissions which occur particularly in lean mixture operation: nitrogen oxide molecules are deposited on the coating of the trap and are thus removed from the exhaust gas. The deposited nitrogen oxides are split into nitrogen and oxygen under the influence of a catalyst, the oxygen being burned with excess hydrogen or CO to water or C02 respectively. However, under the exhaust gas conditions prevailing in lean mixture or stoichiometric operation of the engine these splitting reactions can only take place to a very small extent, if at all. Instead the nitrogen oxides are deposited on the trap until a degree of saturation is reached, so that the purification efficiency of the nitrogen oxide trap is greatly diminished.

It is therefore known to subject the nitrogen oxide trap at regular intervals to a regeneration cycle in which at the end of a predetermined period of operation with lean or stoichiometric air/fuel ratios splitting of the stored nitrogen oxide is effected by a marked over-enrichment of the air/fuel mixture for a short time. For example, for this purpose instead of operating the engine with lambda = 1.6 (lean mixture operation) , it may be operated every 60 s for a period of 0.5 to 1 s with lambda = 0.75 (highly over- enriched) , where lambda denotes the air/fuel ratio relative to the stoichiometric ratio.

This cyclical over-enrichment required for regeneration leads to an increased requirement of fuel. It is desirable to minimise the additional consumption of fuel due to these . regeneration cycles and thus to improve the overall efficiency of the nitrogen oxide trap system.

A further problem with known methods of regenerating nitrogen oxide traps is that in these methods the short-time over-enrichment is achieved by increasing the fuel supply. This typically leads to a 30% to 40% increase in the torque output of the engine, which despite its short duration can lead to a change in speed which is noticeable by the driver. Especially at low speeds (e.g. walking pace) such sudden changes in speed are not acceptable. Hence the increased torque output has hitherto been compensated by artificially impairing the engine efficiency by means of a change in the ignition angle. This, however, leads to a high consumption of fuel.

However, for the duration of a regeneration cycle it is not only the amount of NOx which is stored in the nitrogen oxide trap that is of importance. Both the catalytic converter and the nitrogen oxide trap undergo ageing. As a result the optimum duration of the regeneration cycle in the case of a "young" vehicle is different from that in the case of an "old" vehicle.

The object of the present invention is to improve a method of the aforementioned kind so as to minimise the excess fuel consumption due to the regeneration cycles. A further object is also to adapt the duration of the regeneration cycle to the ageing of the catalytic converter and the nitrogen oxide trap.

According to the present invention, at least in one regeneration cycle the nitrogen trap is regenerated immediately before a reduction in the amount of fuel injected, for lean mixture operation.

When from a situation in which a relatively high torque is required (stoichiometric operation) the torque requirement is decreased, an obvious way of minimising the fuel consumption would be to change over directly to lean mixture operation. In contrast to this, the invention contemplates, immediately before making a reduction in the amount of fuel injected so as to start lean mixture operation, increasing the amount of fuel so as to perform a regeneration of the nitrogen oxide trap.

This has the advantage that less fuel is consumed for the regeneration.

It is preferred that at the same time as the amount of fuel injected is varied for the purpose of initiating or ending the regeneration cycle the mass flow of air supplied to the engine is also varied in such a way that the torque output of the internal combustion engine during the regeneration cycle remains substantially unchanged compared with the torque output immediately before or after the regeneration cycle, and that the duration of the regeneration cycle is changed as a function of the storage capacity of the nitrogen oxide trap and of the oxygen storage capacity of the catalytic converter. The duration of the regeneration cycle is thereby adapted to the decreasing oxygen storage capacity of the preceding 3-way converter and the additional use of fuel thereby minimised. The change in the mass flow of air supplied to the internal combustion engine is effected by means of the device for influencing the mass air flow, e.g. an electronically controllable throttle valve or an electronically controllable by-pass valve connected in parallel with the air flow passage of a conventional mechanical throttle valve. During the regeneration cycle the mass flow of air is decreased in such a way that despite the greater supply of fuel the torque output is not changed at all, or at least not to an extent detectable by the driver. Consequently no artificial impairment of efficiency is necessary: instead the engine can still be operated during the regeneration with optimum efficiency for the regeneration air/fuel ratio.

In an advantageous embodiment of the invention the air/fuel ratio required for the regeneration of the nitrogen oxide trap can be determined by means of a functional relationship in dependence on the exhaust gas temperature in the region of the nitrogen oxide trap and the mass flow of exhaust gas. The regeneration air/fuel ratio is determined such that a complete regeneration can be effected with the least possible over-enrichment. With a higher exhaust gas temperature and greater mass flow of exhaust gas the necessary over-enrichment tends to be less. The said functional relationship and also the further functional relationships mentioned below are preferably obtained by series of experimental tests and implemented in the form of a mathematical function adapted to the test results or by means of a multidimensional table store. The temperature and the mass flow of the exhaust gas in the region of the nitrogen oxide trap are determined in known manner by measurements or calculations (for example on the basis of the amount of fuel injected and of the mass flow of air.)

In accordance with the invention it is preferred to adapt the storage capacity of the nitrogen oxide trap and/or the oxygen storage capacity of the catalytic converter according to a model which takes into account: duration of operation of the engine mileage of the vehicle (total distance and variation over time) catalytic converter operating conditions (e.g. temperature, mass flow of exhaust gas, lambda) nitrogen oxide trap operating conditions (e.g. temperature, mass flow of exhaust gas, lambda) or a desired combination thereof.

This results in the regeneration cycle which is provided on transition from stoichiometric operation to lean burn operation only lasting as long as is absolutely necessary. A further result is that a subsequent regeneration cycle is only initiated when this, too, is necessary, and that the ageing can also be taken into account. In contrast to this, in the case of prior art solutions the regeneration cycle is carried out at fixed intervals, e.g. every 60 s. Since, however, the current degree of filling of the nitrogen oxide trap varies in dependence on the engine operating conditions, the known solutions can result either in unnecessarily frequent regeneration - associated with increased fuel consumption - or in too infrequent regeneration - associated with poorer exhaust gas purification. In contrast to this, with the invention the current rate of pick-up of nitrogen oxides by the nitrogen oxide trap is determined approximately. This rate is determined at fixed intervals and integrated. The integral is thus a measure of the degree of filling of the nitrogen oxide trap. This integral is compared with a threshold value. If the threshold value is exceeded the trap is saturated, and a regeneration cycle is then initiated. The threshold value can be adapted to the ageing of the nitrogen oxide trap. At the end of the regeneration cycle the integral is returned to a starting value (e.g. zero) corresponding to the now regenerated nitrogen oxide trap. Thus regeneration takes place when and only when it is really necessary.

The accompanying diagram shows schematically the course of a method of regeneration according to the invention.

A device (not shown) for putting the method into practice includes an internal combustion engine having a nitrogen oxide trap in its exhaust gas system. The internal combustion engine obtains fuel from a fuel injection system and also air through an electronically controllable throttle valve. An electronic engine control unit controls the injection system, the throttle valve, the ignition system etc. and receives a plurality of input signals, e.g. the engine speed, the accelerator pedal position, the exhaust gas temperature, etc. From these input signals engine characteristics, e.g. the desired torque, the air/fuel ratio or the like are determined.

In the example shown in the Figure the internal combustion engine is operating stoichiometrically with an air/fuel ratio of lambda = 1. For regeneration of the nitrogen oxide trap regeneration cycles 1,2 are provided in which a rich air/fuel ratio of lambda = 0.75 is set. The times at which regenerations are necessary are determined by the engine control unit determining, by means of a functional relationship - dependent on the current speed and the torque of the internal combustion engine, on the air/fuel ratio and on the exhaust gas temperature and exhaust gas mass flow in the region of the nitrogen oxide trap - the approximate current nitrogen oxide pick-up rate of the nitrogen oxide trap. This is integrated over time. The oxygen stored in the catalytic converter is likewise calculated. When there is a lower torque requirement, before changing over to lean mixture operation the regeneration cycle 1 is set for optimum duration from the nitrogen oxide pick-up and oxygen pick-up. Afterwards these values are returned to zero. The integration then recommences. If the integral should then have exceeded a predetermined threshold value, this indicates that the pick-up capacity of the trap is exhausted, and the regeneration cycle 2 is therefore started. After the regeneration cycle 2 the integral is set to the value zero. When the engine has been operating at full load for a predetermined time, the integral is likewise reset to zero. The duration of the regeneration cycle and the air/fuel ratio set for it are determined in dependence on the exhaust gas temperature in the region of the nitrogen oxide trap and on the exhaust gas mass flow.

During the regeneration cycles more fuel is injected by the engine control unit in order to obtain a richer air/fuel ratio. At a constant throttle valve angle this results in a short-time higher engine torque MD. In order to avoid these variations in torque, the throttle valve angle and hence the mass flow of air are reduced during the regeneration cycles in such a way that the torque remains substantially constant.

Claims

1. Method for regenerating a nitrogen oxide trap fitted after a 3-way catalytic converter in the exhaust system of an internal combustion engine having devices for influencing the amount of fuel injected, wherein for the purpose of initiating or ending a regeneration cycle the amount of fuel injected is varied, characterised in that at least in one regeneration cycle the nitrogen oxide trap is regenerated immediately before a reduction in the amount of fuel injected, for lean mixture operation.

2. Method according to claim 1, characterised in that the duration of the regeneration cycle is varied as a function of the oxygen storage capacity of the catalytic converter and/or of the storage capacity of the nitrogen oxide trap.

3. Method according to claim 2, characterised in that the air/fuel ratio required for the regeneration of the nitrogen oxide trap is determined as a function of the exhaust gas temperature in the region of the nitrogen oxide trap and of the mass flow of exhaust gas.

4. Method according to claim 2 or claim 3, characterised in that the storage capacity of the nitrogen oxide trap and/or the oxygen storage capacity of the catalytic converter is adapted according to a model which is arranged for - duration of engine operation mileage of the vehicle catalytic converter operating conditions nitrogen oxide trap operating conditions or for a desired combination thereof.