FIELD OF THE INVENTION AND PRIOR ART The present invention relates to a method for fault detection in an exhaust system which comprises

- an exhaust line intended to receive exhaust gases from a combustion engine,

- a fuel oxidation device situated in the exhaust line,

- a fuel injection device for injection of fuel into the exhaust line at an injection point situated upstream of the fuel oxidation device, which injection device is caused, e.g. at the time of regeneration of a particle filter situated downstream of the fuel oxidation device, to inject fuel into the exhaust gases with a mabs flow so regulated that the fuel injected generates a desired increase in the temperature of the exhaust gases during their passage through the fuel oxidation device,

- an SCR catalyst situated in the exhaust line, and

- a reducing agent injection device for injection of reducing agent into the exhaust line at an injection point situated upstream of the SCR catalyst, which injection device is caused to inject reducing agent into the exhaust gases with a mass flow so regulated that the reducing agent injected generates a desired conversion of NO _x in the exhaust gases during their passage through the SCR catalyst.

The invention relates also to a computer programme and a computer programme product comprising computer programme code for implementing a method according to the invention, and to an electronic control unit. To meet prevailing exhaust cleaning requirements, today's motor vehicles are often provided with a catalyst in the exhaust line to effect catalytic conversion of environmentally hazardous constituents of the exhaust gases to environmentally less hazardous substances. A method which has been employed for achieving effective catalytic conversion is based on injection of a reducing agent into the exhaust gases upstream of the catalyst. A reducing substance which forms part of, or is formed by, the reducing agent is carried by the exhaust gases into the catalyst and is adsorbed on active seats therein, resulting in accumulation of the reducing substance in the catalyst. The accumulated reducing substance may either be desorbed, i.e. become detached from the active seats, or react with an exhaust substance and thereby convert the latter to a substance with less environmental impact. Such a reduction catalyst may for example be of the SCR (selective catalytic reduction) type. This type of catalyst is hereinafter called an SCR catalyst. An SCR catalyst reduces NO _x in the exhaust gases. In the case of an SCR catalyst a reducing agent in the form of urea is usually injected into the exhaust gases upstream of the catalyst. The injection of urea into the exhaust gases results in the formation of ammonia which then serves as the reducing substance to assist the catalytic conversion in the SCR catalyst. The ammonia accumulates in the SCR catalyst by being adsorbed on active seats therein, and NO _x present in the exhaust gases is converted to nitrogen and water when it is brought into contact in the catalyst with ammonia accumulated on the catalyst's active seats. When an SCR catalyst is used in combination with dosing of reducing agent, it is important to control the injection of the reducing agent so that desired conversion of the respective exhaust substance is achieved without too large amounts of unconsumed reducing substance leaving the catalyst along with the exhaust gases and thereby being released to the surroundings or having to be dealt with in a slip catalyst. The consumption and hence the cost of the reducing agent may thereby be minimised while at the same time avoiding or at least minimising any unintended release of the reducing substance to the surroundings. Ammonia for example is a toxic and malodorous reducing substance, so it is desirable as far as possible to minimise its release to the surroundings while maintaining sufficient injection of reducing agent, e.g. in the form of urea, to achieve required NO _x conversion.

The injection of reducing agent into the exhaust line upstream of an SCR catalyst is usually regulated by means of a regulating valve in response to control signals from an electronic control device. The amount of reducing agent injected into the exhaust line, i.e. the dosage of the reducing agent, is regulated to achieve a desired conversion of NO _x in the exhaust gases during their passage through the SCR catalyst. A particle filter is often also provided in the exhaust line upstream or downstream of the SCR catalyst. An oxidation catalyst may be situated in the exhaust line upstream of the particle filter. When the particle filter needs to be regenerated, i.e. to be freed of particles deposited in it, fuel is injected into the exhaust gases upstream of the oxidation catalyst. This fuel accompanies the exhaust gases into the oxidation catalyst and is oxidised therein, generating such an increase in the temperature of the exhaust gases that combustion of the particles deposited in the filter takes place, thereby regenerating the filter.

The injection of fuel into the exhaust line upstream of an oxidation catalyst is regulated, for example, by means of a regulating valve in response to control signals from an electronic control device. The amount of fuel injected into the exhaust line, i.e. the dosage of the fuel, is regulated so as to achieve a desired increase in the temperature of the exhaust gases during their passage through the oxidation catalyst.

To ensure that a reducing agent injection device and a fuel injection device in an exhaust system function as intended and inject correct amounts of reducing agent and fuel respectively, their function is monitored in various ways. When a malfunction is detected, however, it may in certain cases be difficult to establish whether it is caused by a fault in any of the injection devices or by something else, which means that locating and rectifying the problem may be a complicated and time-consuming operation.

OBJECT OF THE INVENTION

The object of the present invention is to propose a novel and advantageous way of detecting a malfunction in an exhaust system of the type indicated in the introduction. SUMMARY OF THE INVENTION

According to the present invention, said object is achieved by a method having the features defined in claim 1 . This method is characterised by

- determining an exhaust mass flow value which represents the mass flow of the exhaust gases released from the combustion engine,

- comparing the mass flow of the fuel injected by the fuel injection device, herein called the actual fuel mass flow, with an expected fuel mass flow which corresponds to that which, taking the exhaust mass flow value into account, is expected to be injected by the fuel injection device in order to generate the desired temperature increase; or comparing the desired temperature increase generated with an expected temperature increase which corresponds to that which, taking the exhaust mass flow value into account, is expected to be generated with the prevailing mass flow of the fuel injected by the fuel injection device,

- comparing the mass flow of the reducing agent injected by the reducing agent injection device, herein called the actual reducing agent mass flow, with an expected reducing agent mass flow which corresponds to that which, taking the exhaust mass flow value into account, is expected to be injected by the reducing agent injection device in order to generate the desired NO _x conversion; or comparing the desired NO _x conversion generated with an expected NO _x conversion which corresponds to that which, taking the exhaust mass flow value into account, is expected to be generated with the prevailing mass flow of the reducing agent injected by the reducing agent injection device, and

- finding that there is in the exhaust system an exhaust mass flow error due to exhaust leakage in the exhaust line upstream of said injection points or to incorrectly determined exhaust mass flow value if said comparisons show that the following conditions are fulfilled:

a) if the actual fuel mass flow is smaller than the expected fuel mass flow, or the temperature increase generated is greater than the expected temperature increase, and

b) if the actual reducing agent mass flow is smaller than the expected reducing agent mass flow, or the NO _x conversion generated is greater than the expected NO _x conversion.

The method according to the invention is based on the insight that an exhaust mass flow error due to exhaust leakage in the exhaust line upstream of the fuel and reducing agent injection points or to consistently incorrect measurement or calculation of the mass flow of the exhaust gases released from the combustion engine results in similarly incorrect behaviour of the SCR catalyst and the fuel oxidation device. Exhaust leakage upstream of the fuel and reducing agent injection points means that the exhaust mass flow through the SCR catalyst and the fuel oxidation device will be smaller than the exhaust mass flow from the engine determined by means of a mass flowmeter or by calculation. A similar incorrect state occurs if said mass flowmeter or calculation provides an incorrect exhaust mass flow value which is constantly greater than the actual exhaust mass flow from the engine. When the actual exhaust mass flow through the SCR catalyst is thus smaller than that which corresponds to the exhaust mass flow value determined, a smaller amount of NO _x than expected will reach the SCR catalyst per unit time, resulting in a smaller amount of reducing agent than expected being consumed per unit time to achieve a given NO _x conversion in the SCR catalyst. At the same time, the NO _x conversion generated by means of a certain amount of reducing agent injected per unit time at a certain exhaust mass flow determined from the engine will in this case be greater than expected.

When the actual exhaust mass flow through the fuel oxidation device in the case mentioned above is smaller than the exhaust mass flow which corresponds to the exhaust mass flow value determined, a smaller amount of exhaust gases than expected will reach the fuel oxidation device per unit time, resulting in a smaller amount of fuel than expected being consumed per unit time to achieve a given increase in the temperature of the exhaust gases during their passage through the fuel oxidation device. At the same time, the temperature increase generated by means of a certain amount of fuel injected per unit time at a certain exhaust mass flow determined from the engine will in this case be greater than expected. An actual exhaust mass flow through both the fuel oxidation device and the SCR catalyst which is smaller than that which corresponds to the exhaust mass flow value determined constitutes an exhaust mass flow error due either to exhaust leakage in the exhaust line upstream of the fuel oxidation device and the SCR catalyst or to incorrectly determined exhaust mass flow value, and such an error will result in the aforesaid conditions a) and b) being simultaneously fulfilled. Purely theoretically, simultaneous malfunction of the fuel injection device and the reducing agent injection device might also result in conditions a) and b) being simultaneously fulfilled, but the likelihood of this is so slight that it may in practice be disregarded as a plausible cause of error.

Other advantageous features of the method according to the invention are indicated by the dependent claims and the description set out below.

The invention relates also to a computer programme having the features defined in claim 4, a computer programme product having the features defined in claim 5 and an electronic control unit having the features defined in claim 6.

BRIEF DESCRIPTION OF THE DRAWINGS The invention is described below in more detail on the basis of embodiment examples with reference to the attached drawings, in which

Fig. 1 is a schematic diagram of a combustion engine with associated exhaust system,

Fig 2 is a schematic diagram of an electronic control unit for implementing a method according to the invention, Fig. 3 is a flowchart illustrating a method according to a first embodiment of the invention, and

Fig. 4 is a flowchart illustrating a method according to a second embodiment of the invention.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION The invention is described below as applied to a motor vehicle. The invention is however not restricted to that application but may be employed in all contexts where an SCR catalyst with associated reducing agent injection device and a fuel oxidation device with associated fuel injection device form part of an exhaust system for a combustion engine.

Fig. 1 depicts schematically a combustion engine 1 of a motor vehicle 2. The exhaust gases leaving the engine are received by an exhaust system 3 which comprises an exhaust line 4 intended to receive the exhaust gases generated by the engine. The exhaust gases from the engine flow through the exhaust line 4 and emerge into the surroundings via an exhaust outlet at the downstream end of the exhaust line. In the exhaust line 4, a fuel oxidation device 5 and a particle filter 6 are arranged in series with one another, the particle filter being situated downstream of the fuel oxidation device. In the exhaust line 4 an SCR catalyst 7 is also arranged in series with the fuel oxidation device 5. In the example illustrated, the SCR catalyst 7 is situated downstream of the fuel oxidation device 5 and the particle filter 6 but might alternatively be situated upstream of them. In the embodiment depicted in Fig. 1 and described below, the fuel oxidation device 5 takes the form of an oxidation catalyst. It might alternatively take the form of a set of two or more oxidation catalysts connected in series and/or in parallel or be composed of one or more fuel oxidising units of some other type.

An injection point 8 for fuel is provided in the exhaust line 4 upstream of the oxidation catalyst 5. The injection of fuel into the exhaust line 4 is by means of an injection device 20 comprising one or more injection means 21 situated in the exhaust line in the form of injection nozzles or the like. A fuel tank 9 is connected to said injection means 21. Fuel supply to said injection means 21 is regulated by means of a regulating valve 22 situated between the fuel tank 9 and said injection means 21 . The regulating valve 22 is controlled by an electronic control device 10 which determines how much fuel is at the time to be injected into the exhaust gases. The control device 10 uses the regulating valve 22 to regulate the amount of fuel injected into the exhaust gases. In a feed line 23 between the fuel tank 9 and the regulating valve 22 a pump 24 is provided to feed fuel from the fuel tank to the regulating valve so as to maintain a given pressure of the fuel fed to the regulating valve. When the particle filter 6 needs to be regenerated, i.e. to be freed of particles deposited in it, fuel is injected into the exhaust gases by means of the fuel injection device 20. This fuel accompanies the exhaust gases into the oxidation catalyst 5 and is oxidised therein, generating such an increase in the temperature of the exhaust gases that combustion of the particles deposited in the particle filter 6 takes place, thereby regenerating the filter. The control device 10 is arranged in a conventional way to regulate the mass flow of the fuel injected via the injection device 20 so that this fuel injected generates a desired increase in the temperature of the exhaust gases during their passage through the oxidation catalyst 5. The temperature increase generated by a certain amount of fuel injected depends inter alia on the mass flow of the exhaust gases which pass the injection point 8 before flowing into the oxidation catalyst 5.

In the example illustrated in Fig. 1 , a first temperature sensor is provided to measure the temperature of the exhaust gases upstream of the oxidation catalyst 5, and a second temperature sensor 12 to measure the temperature of the exhaust gases downstream of the oxidation catalyst 5. The control device 10 is connected to these temperature sensors 1 1 , 12 in order to receive from them measured values for the prevailing exhaust temperature at the respective measuring points. The control device 10 can use the measured values from the temperature sensors 1 1 , 12 as a basis for determining the temperature increase ΔΤ which the fuel accompanying the exhaust gases into and oxidising in the oxidation catalyst 5 imparts to the exhaust gases during their passage through the oxidation catalyst 5. As the control device 10 is intended to control the fuel injection so that the fuel injected generates a desired increase in the temperature of the exhaust gases, the temperature increase ΔΤ generated will correspond to this desired temperature increase.

An injection point 13 for reducing agent is situated in the exhaust line 4 upstream of the SCR catalyst 7. The injection of reducing agent into the exhaust line 4 is by means of an injection device 30 comprising one or more injection means 31 situated in the exhaust line in the form of injection nozzles or the like. A storage container 14 for reducing agent is connected to said injection means 31 . The supply of reducing agent to said injection means 31 is regulated by means of a regulating valve 32 situated between the storage container 14 and said injection means 31. The regulating valve 32 is controlled by the electronic control device 10 which determines how much reducing agent is at the time to be injected into the exhaust gases. The control device 10 uses the regulating valve 32 to regulate the amount of reducing agent injected into the exhaust gases. In a feed line 33 between the storage container 14 and the regulating valve 32 a pump 34 is provided to feed reducing agent from the storage container to the regulating valve so as to maintain a given pressure of the reducing agent fed to the regulating valve.

The control device 10 is arranged in a conventional way to regulate the mass flow of the reducing agent injected via the injection device 30 so that the reducing agent injected generates a desired conversion of NO _x in the exhaust gases during their passage through the SCR catalyst 7. The NO _x conversion generated by a certain amount of reducing agent injected depends inter alia on the mass flow of the exhaust gases which pass the injection point 13 before flowing into the SCR catalyst 7.

An NO _x sensor 15 is provided in the exhaust line 4 downstream of the SCR catalyst 7 to measure the NO _x concentration in the exhaust gases downstream of the SCR catalyst. The control device 10 is connected to this NO _x sensor 15 in order to receive from it measured values for the prevailing NO _x concentration in the exhaust gases downstream of the SCR catalyst. The control device 10 can use measured values from the NO _x sensor 15 as a basis for determining the NO _x conversion ΔΝΟ _χ which the reducing agent injected into the exhaust gases causes in them during their passage through the SCR catalyst 7. As the control device 10 is intended to control the reducing agent injection so that the reducing agent injected generates a desired NO _x conversion, the NO _x conversion generated ΔΝΟ _χ will correspond to this desired NO _x conversion.

The NO _x concentration in the exhaust gases upstream of the SCR catalyst 7 may be determined by means of an NO _x sensor in the exhaust line upstream of the SCR catalyst but might also be determined in any of the conventional ways for calculating it, e.g. on the basis of engine load, engine speed, injection angle and, where applicable, EGR (exhaust gas recirculation) content.

The control device 10 is arranged to determine or receive an exhaust mass flow value V _ex h which represents the mass flow of the exhaust gases released from the engine to the exhaust line 4. This exhaust mass flow value V _exh may be determined by means of a mass flowmeter situated in the exhaust line 4 but might also be determined in any of the conventional ways for calculating it, e.g. on the basis of engine load and engine speed.

As the injection of fuel via the fuel injection device 20 and the injection of reducing agent via the reducing agent injection device 30 take place with mass flows determined by the control device 10, this control device has access to information about the mass flow of the fuel injected by the injection device 20, herein called the actual fuel mass flow, and the mass flow of the reducing agent injected by the injection device 30, herein called the actual reducing agent mass flow.

To evaluate the effect generated by the fuel injected by the injection device 20, the control device 10 is arranged to

- compare the actual fuel mass flow with an expected fuel mass flow which corresponds to that which, taking the exhaust mass flow value V _eX h into account, is expected to be injected by the injection device 20 in order to generate the desired temperature increase ΔΤ, or

- compare the temperature increase generated ΔΤ with an expected temperature increase which corresponds to that which, taking the exhaust mass flow value V _eX h into account, is expected to be generated with the prevailing mass flow of the fuel injected by the injection device 20.

The aforesaid expected fuel mass flow is determined in the control device 10 by means of a calculation model of conventional type on the basis inter alia of the exhaust mass flow value V _eX h and the temperature increase ΔΤ, and the aforesaid expected temperature increase is determined in the control device 10 by means of a calculation model of conventional type on the basis inter alia of the exhaust mass flow value V _eX h and the mass flow of the fuel injected by the injection device 20.

To evaluate the effect generated by the reducing agent injected by the injection device 30, the control device 10 is arranged to - compare the actual reducing agent mass flow with an expected reducing agent mass flow which corresponds to that which is expected, taking the exhaust mass flow value V _eX h into account, to be injected by the injection device 30 in order to generate the desired NO _x conversion ΔΝΟ _χ , or

- compare the NO _x conversion generated ΔΝΟ _χ with an expected NO _x conversion which corresponds to that which is expected, taking the exhaust mass flow value V _exh into account, to be generated with the prevailing mass flow of the reducing agent injected by the injection device 30.

The aforesaid expected reducing agent mass flow is determined in the control device 10 by means of a calculation model of conventional type on the basis inter alia of the exhaust mass flow value V _ex h and the NO _x conversion ΔΝΟ _χ , and the aforesaid expected NO _x conversion is determined in the control device 10 by means of a calculation model of conventional type on the basis inter alia of the exhaust mass flow value V _eX h and the mass flow of the reducing agent injected by the injection device 30.

The control device 10 is arranged to find that there is in the exhaust system 3 an exhaust mass flow error due to exhaust leakage in the exhaust line 4 upstream of said injection points 8, 13 or to incorrectly determined exhaust mass flow value V _eX h if the aforesaid comparisons show that conditions a) and b) set out below are fulfilled:

a) if the actual fuel mass flow is smaller than the expected fuel mass flow, or the temperature increase generated ΔΤ is greater than the expected temperature increase, and b) if the actual reducing agent mass flow is smaller than the expected reducing agent mass flow, or the NO _x conversion generated ΔΝΟ _χ is greater than the expected NO _x conversion. When there is found to be an exhaust mass flow error, action may be taken to correct the exhaust mass flow value so that the exhaust mass flow value V _exh determined or received by the control device is caused to correspond better to the actual exhaust mass flow through the oxidation catalyst 5 and the SCR catalyst 7. The actual exhaust mass flow through the oxidation catalyst 5 and the SCR catalyst 7 may for example be estimated by means of a calculation model on the basis of relationships well known to one skilled in the art between the aforesaid actual fuel mass flow and the temperature increase generated ΔΤ or on the basis of relationships well known to one skilled in the art between the aforesaid actual reducing agent mass flow and the NO _x conversion generated ΔΝΟ _χ . Supplementary investigations may also be done to locate and rectify any leaks in the exhaust line upstream of said injection points 8, 13.

The control device 10 may comprise a single electronic control unit as in Fig. 1 or two or more mutually cooperating electronic control units. Fig. 3 is a flowchart illustrating an embodiment of a method according to a first embodiment of the present invention for fault detection in an exhaust system 3 of the type indicated above. As a first step S1 , an exhaust mass flow value V _eX h is determined which represents the mass flow of the exhaust gases released from the engine to the exhaust line 4. As a second step S2, fuel is injected into the exhaust line 4 by means of the fuel injection device 20, and reducing agent by means of the reducing agent injection device 30, the fuel injection mass flow being regulated so that the fuel injected generates a desired temperature increase ΔΤ in the exhaust gases during their passage through the fuel oxidation device 5, and the reducing agent injection mass flow being regulated so that the reducing agent injected generates a desired NO _x conversion ΔΝΟ _χ in the exhaust gases during their passage through the SCR catalyst 7.

As a third step S3, calculation models are used to determine

- an expected fuel mass flow which corresponds to that which is expected, taking the exhaust mass flow value V _exh into account, to be injected by the injection device 20 in order to generate the desired temperature increase ΔΤ, and

- an expected reducing agent mass flow which corresponds to that which is expected, taking the exhaust mass flow value V _ex h into account, to be injected by the injection device 30 in order to generate the desired NO _x conversion ΔΝΟ _χ .

As a fourth step S4, the actual fuel mass flow injected by the injection device 20 is compared with said expected fuel mass flow, and the actual reducing agent mass flow injected by the injection device 30 is compared with said expected reducing agent mass flow.

If the comparisons at step S4 show that the actual fuel mass flow is smaller than the expected fuel mass flow and that the actual reducing agent mass flow is smaller than the expected reducing agent mass flow, it is then found as a fifth step S5 that there is in the exhaust system 3 an exhaust mass flow error due to exhaust leakage in the exhaust line 4 upstream of the aforesaid injection points 8, 13 or to incorrectly determined exhaust mass flow value V _eX h - If the comparisons at step S4 show that such is not the case, steps S1 -S4 are repeated at a later time.

Fig. 4 is a flowchart illustrating an embodiment of a method according to a second embodiment of the present invention for fault detection in an exhaust system 3 of the type indicated above.

The first and second steps S1 ' and S2' of the method illustrated in Fig. 4 correspond to steps S1 and S2 indicated above of the method illustrated in Fig. 3.

As a third step S3' calculation models are used to determine

- an expected temperature increase which corresponds to that which is expected, taking the exhaust mass flow value V _exh into account, to be generated with the prevailing mass flow of the fuel injected by the injection device 20, and

- an expected NO _x conversion which corresponds to that which is expected, taking the exhaust mass flow value V _ex h into account, to be generated with the prevailing mass flow of the reducing agent injected by the injection device 30.

As a fourth step S4', the temperature increase generated ΔΤ is compared with said expected temperature increase, and the NO _x conversion generated ΔΝΟ _χ is compared with said expected NO _x conversion.

If the comparisons at step S4' show that the temperature increase generated ΔΤ is greater than the expected temperature increase and that the NO _x conversion generated ΔΝΟ _χ is greater than the expected NO _x conversion, it is then found as a fifth step S5' that there is in the exhaust system 3 an exhaust mass flow error due to exhaust leakage in the exhaust line 4 upstream of the aforesaid injection points 8, 13 or to incorrectly determined exhaust mass flow value V _eX h - If the comparisons at step S4' show that such is not the case, steps S1 '-S4' are repeated at a later time.

Computer programme code for implementing a method according to the invention is with advantage included in a computer programme which can be read into the internal memory of a computer, e.g. the internal memory of an electronic control unit of a motor vehicle. Such a computer programme is with advantage provided via a computer programme product comprising a data storage medium which can be read by an electronic control unit and which has the computer programme stored on it. Said data storage medium is for example an optical data storage medium in the form of a CD ROM disc, a DVD disc etc., a magnetic data storage medium in the form of a hard disc, a diskette, a cassette tape etc., or a flash memory or a memory of the ROM, PROM, EPROM or EEPROM type.

Fig. 2 illustrates very schematically an electronic control unit 40 comprising an execution means 41 , e.g. a central processor unit (CPU), for execution of computer software. The execution means 41 communicates with a memory 42, e.g. of the RAM type, via a data bus 43. The control unit 40 comprises also a data storage medium 44, e.g. in the form of a flash memory or a memory of the ROM, PROM, EPROM or EEPROM type. The execution means 41 communicates with the data storage medium 44 via the data bus 43. A computer programme comprising computer programme code for implementing a method according to the invention, e.g. in accordance with either of the embodiments illustrated in Figs. 3 and 4, is stored on the data storage medium 44.

The invention is of course in no way restricted to the embodiments described above, since many possibilities for modifications thereof are likely to be obvious to one skilled in the art without therein having to deviate from the invention's basic concept such as defined in the attached claims.