TECHNICAL FIELD

The present invention relates to a method for verifying the integrity of a SCR system in automotive vehicle. In particular it relates to an on-board diagnostic method for detecting a presence or absence of a leak in a SCR system.

BACKGROUND OF THE INVENTION

Known emission control technologies include systems for reducing the nitrogen oxides of exhaust gas produced by a vehicle engine. Such technologies include Selective Catalytic Reduction (SCR) systems.

SCR systems enable the reduction of nitrogen oxides by injection of a reducing agent, generally ammonia, into the exhaust line. This ammonia may derive from the pyrolytic decomposition of an ammonia precursor solution, whose concentration may be the eutectic concentration. Such an ammonia precursor is generally a urea solution.

Generally, a SCR system comprises a urea tank containing a urea solution. The SCR system comprises a feed line for conveying the urea solution from the urea tank to an injector located in the exhaust line of the vehicle, upstream of a SCR catalyst. The urea solution is conveyed by the action of a pump.

It is known to use a pump capable of rotating in two opposite directions. Such pump can be controlled to rotate either in a forward direction for feeding the injector with urea solution or in a reverse direction for purging the feed line. Generally, the injector is maintained open during purging operations. The purging of the feed line takes place by sucking air through the injector by operating the pump in the reverse direction.

As the environmental requirements and safety requirements with respect to emission of chemicals by vehicles are increasingly stringent, it is of the utmost important that the SCR system be kept leak-free. It is therefore practically required to equip such system with means to detect and preferably report any leaks that may occur. More particularly it is required to detect leaks that may occur in the feed line (or in the pressurized hydraulic circuit which comprises the elements/components/lines arranged between the pump and the injector). It is desirable that such leak detection occurs as soon as possible. DE102009029408A1 discloses a method for detecting a leak in a feed line of a SCR system for a vehicle. This known method consists in monitoring the building of a negative pressure in the feed line when the SCR system operates in a rear suction mode. DEI 02009029408 A 1 teaches operating the SCR system in the rear suction mode such that urea solution is sucked off from the feed line in order to avoid freezing in the feed line when the engine of the vehicle is turned off. A limitation of this known method is that it does not allow to detect a leak when the engine of the vehicle is running. This can lead to the spraying of urea solution in the atmosphere (through the leak orifice in the feed line) until the leak is effectively detected at the time when the engine of the vehicle is switched off.

It is therefore needed an improved method for detecting rapidly a leak in a SCR system.

SUMMARY OF THE INVENTION

It is, therefore, one aspect of the present invention to provide for a method for detecting a leak in a feed line of a SCR system of a vehicle, the system comprising a tank for the storage of an ammonia precursor solution, an injector, said feed line being arranged between the tank and the injector, and a pressure sensor arranged to measure a pressure in the feed line.

The method comprises a preliminary test comprising the following steps:

- closing the injector or opening the injector to generate a controlled flow rate;

- operating a pump comprised in said system, for allowing ammonia precursor solution to flow in the feed line from the tank to the injector;

- monitoring the rotational speed of the pump;

- detecting an abnormal operating condition when the monitored rotational speed of the pump exceeds a predetermined threshold speed value, for a given mass flow of ammonia precursor solution (and for a given pressure of the pressurized circuit),

Advantageously, upon detection of the abnormal operating condition, the method further comprises an intrusive leak test comprising the steps of:

a) closing the injector;

b) depressurizing the feed line;

c) monitoring pressure measured by said pressure sensor subsequent to the depressurizing; d) determining the presence of a leak in the feed line by detecting whether said monitored pressure deviates from a predetermined pressure evolution based on one or more measurements.

Thus, it is proposed to combine a non-intrusive preliminary test and an intrusive leak test. More precisely, it is proposed to detect a suspicious leak (i.e. abnormal operating condition) by using the non-intrusive preliminary test, and then to confirm the presence of a leak in the feed line by using the intrusive leak test. It is proposed to start the intrusive leak test of the present invention (i.e. running steps a) to d) consecutively) subsequent to the detection of an abnormal operating condition. In other words, the execution of the intrusive leak test of the present invention is conditioned to the detection of a suspicious leak (i.e.

abnormal operating condition). The preliminary test of the present invention is executed when the engine of the vehicle is running and when the SCR system operates in an injection mode (i.e. operating the pump for allowing ammonia precursor solution to flow in the feed line from the tank to the injector). It is an advantage of the combination of the non-intrusive preliminary test and the intrusive leak test to allow detecting leak while the engine of the vehicle is running, so as to quickly apply safety measures, for example deactivating the pump (i.e. stopping the flow of ammonia precursor solution in the feed line from the tank to the injector).

The preliminary test of the present invention is non-intrusive, in the sense that the monitoring of the rotational speed of the pump is carried out without disrupting the normal operation of the SCR system.

In a particular embodiment, the rotational speed of the pump is measured by a Hall effect or other type of speed sensor.

In another particular embodiment, the rotational speed of the pump is estimated by using back Electro-Motive Force (EMF) method. The back EMF method is well known in the art and is not further described hereafter.

The intrusive leak test of the present invention consists in depressurizing the feed line while the injector is maintained closed. The present invention is based on the observation that, in case of the presence of a leak in the feed line, the underpressure (i.e. negative relative pressure) in the feed line will significantly reduce or disappear over time. More precisely, in case of the presence of a leak in the feed line, the depressurization will result in the entrainment (i.e. sucking) of air inside the feed line, through the leak orifice, such that the underpressure in the feed line will start to reduce. The intrusive leak test according to the invention can be used to detect leaks in the feed line, by comparing the underpressure to a threshold value or by monitoring the pressure evolution in the feed line, following a closing of the injector and a

depressurization of the feed line, and by comparing the monitored pressure evolution to a predetermined pressure evolution (i.e. reference pattern of pressure). The depressurization of the feed line is performed during a

predetermined period of time. The monitoring of the pressure evolution in the feed line is performed during this predetermined period of time.

The predetermined pressure evolution can be determined in advance in calibrated non-leak conditions. If a deviation is detected between the monitored pressure evolution and the predetermined pressure evolution, it may be concluded that a leak is present in the feed line.

The expression "SCR system" is understood to mean a system for the catalytic reduction of the NOx from the exhaust gases of an internal combustion engine, preferably of a vehicle, using for example urea as an ammonia precursor solution. The present invention is advantageously applied to diesel engines, and in particular to the diesel engines of passenger cars or heavy goods vehicles.

In a first particular embodiment, the system comprises a single pump. The pump is preferably a positive-displacement pump, driven by a motor and the operation of which is generally controlled by a controller. It is preferably a rotary pump (gear or gerotor pump type) and hence generally comprises a stator and a rotor and can preferably operate in two opposite rotational directions, one generally corresponding to supplying the feed line with liquid and the other generally corresponding to a purge of the feed line. The invention hence gives good results with a gear pump. Preferably the gear pump is pressure regulated when running in forward mode.

The reverse direction (of a pump capable of rotating in two opposite directions) has so far been used for purging feed lines in SCR systems, but never for detecting leaks in such feed lines. Thus, the method according to the invention allows a new operating mode where the pump is controlled to operate in the reverse direction while the injector is maintained closed. More precisely, at step b) the pump operates in the reverse direction such that it creates a depressurization in the feed line.

In a second particular embodiment, the system can comprise a 4-way valve that makes it possible to reverse the flow in the line without changing the direction of rotation of the pump. In a third particular embodiment, the system comprises two pumps. A pump allowing ammonia precursor solution to flow in the feed line from the tank to the injector and a second pump allowing to reverse the flow in the feed line. Advantageously, at step b) the pump allowing ammonia precursor solution to flow in the feed line from the tank to the injector, is stopped and the second pump is activated such that it creates a depressurization in the feed line. In a particular embodiment, the first pump is a linear pump, preferably having a piston.

In an advantageous embodiment, the injector, the pump(s) and the pressure sensor are operatively controlled by an electronic control unit.

An electronic control unit (ECU) is a programmable component (e.g., a processor) that can be used to control a variety of processes onboard the vehicle.

The ECU can be configured to carry out the preliminary test and the intrusive leak test as described above.

The ECU is preferably configured to drive the closing of the injector and the rotating of the pump at selected points in time, and to monitor the time evolution of the sensed pressure in the feed line subsequent to said closing.

In an advantageous embodiment, if the presence of a leak in the feed line is not confirmed at step d), then it is performed a step of calibrating the

predetermined threshold speed value with a new speed value corresponding to a zero injection (i.e. injector is closed).

A calibration of the threshold speed value improves the efficiency of the preliminary test. Such calibration permits to take into account the change over time of the ageing of the system (pump wear, filter plugging, motor...). This step of calibrating allows to increase the efficiency of the preliminary test (i.e. reduce number of false suspicious leaks).

In a particular embodiment, the ECU can store the predetermined pressure evolution (i.e. reference pattern of pressure), and can be configured to determine whether a deviation between the sensed pressure evolution and the

predetermined pressure evolution exceeds a certain maximum margin. If the margin is exceeded, the ECU can generate a suitable leak detection alarm signal.

In another particular embodiment, the ECU can be configured to detect the presence of a leak in the feed line when the monitored pressure exceeds a predetermined threshold pressure value and an absence of a leak in the feed line when the monitored pressure stays below the predetermined threshold pressure value over a predetermined time period. This embodiment is easy to implement. Advantageously, the ECU can be configured to relay a leak detection signal to an On-Board Diagnostics system.

An On-Board Diagnostics (OBD) system is a dedicated vehicular system that inter alia keeps track of malfunctions, in particular malfunctions in systems with a potential environmental impact. Thus, the OBD system may play an important role in verifying whether a vehicle meets applicable emission standards. As leakage of liquids carried on board the vehicle can have important environmental impact, it is advantageous to import leak detection information in the OBD system.

According to an aspect of the present invention, there is provided a computer program product comprising code means configured to cause a processor to carry out the steps of a method as described above.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is illustrated, in a non limitative way, by the accompanying Figures 1 to 5.

Figure 1 is a schematic view of a particular embodiment of a SCR system to which the present invention may be applied;

Figure 2 illustrates a flowchart of operations depicting logical operational steps for detecting a leak in the feed line, in accordance with a particular embodiment of the invention;

Figure 3 depicts the result of measurements illustrating the correlation between pump speed and feed line pressure;

Figure 4 illustrates schematically a particular embodiment of an ECU; and Figure 5 illustrates schematically a particular embodiment of a leak detection functional block of the ECU of Figure 4.

DETAILED DESCRIPTION

The same reference numerals are used to indicate the same elements (or functionally-similar elements) throughout the separate Figures 1 to 5.

Figure 1 illustrates a particular embodiment of a SCR system to which the present invention may be applied.

The SCR system comprises a urea tank (1) containing a urea solution. The urea tank (1) is equipped with the following components:

a gauge (2) (i.e. level sensor);

a heating element (3);

- a filter (4);

a temperature sensor (5); and a current sensor for the heating element (6).

The urea solution is conveyed by the action of a pump (7) towards an injector (12) located in an exhaust line (11) for discharging the exhaust gases of the engine of the vehicle, upstream of a SCR catalyst (17). The urea solution circulates between the tank and the injector via a feed line (18). In this example, the pump (7) is a gear pump. The pump (7) is capable of rotating in a forward direction for feeding the injector with urea solution and in a reverse direction for purging the feed line when required. In the example of Figure 1, the pump (7) and the injector (12) are controlled by an electronic control unit (ECU). For example, the pump (7) is driven by a BLDC motor (15) and which is controlled by the ECU. The ECU can receive a signal (relative to the outlet pressure of the pump) measured by a pressure sensor (10) and a signal (relative to the rotational speed of the pump) measured by a speed sensor (8). For example, the control of the rotational speed of the motor (15) is achieved by sending, to the motor (15), a given voltage which may be in the form of a PWM voltage so that the outlet pressure of the pump follows a given pressure setpoint value. The SCR system also comprises a heating filament (9) for the feed line and pump. The SCR system further comprises a return (or bypass) line equipped with:

a non-return valve (13) that prevents the liquid from going round in circles (in the loop created by the feed line and the one for return to the tank) during the purge (when the pump rotates in reverse); and

a calibrated orifice (restriction) used to set the flow rate and to add resistance in order to increase the pressure (by increasing pressure drop in the return line).

The ECU includes a series of computer-executable instructions, as described below in relation to Figure 2, which allow the ECU to determine the presence or absence of a leak in the feed line based on pressure measurements. These instructions may reside, for example, in a RAM of the ECU. Alternatively, the instructions may be contained on a data storage device with a computer readable medium (for example, USB key or CD-ROM).

Figure 2 illustrates an intrusive leak test according to a particular embodiment of the invention. More precisely, Figure 2 illustrates a flowchart of instructions depicting logical operational steps for detecting a leak in the feed line, in accordance with a particular embodiment of the invention. Beginning at step 21, the ECU closes the injector (12). At step 22, the ECU controls the pump for rotating in the reverse direction, while maintaining the injector (12) closed. This creates a depressurization in the feed line. At step 23, the ECU measures the pressure in the feed line by means of the pressure sensor (10) and compares the measured pressure to a threshold pressure value, for example -200mbar. In this example, the ECU is configured to detect a presence of a leak in the feed line when the measured pressure exceeds -200mbar and to detect an absence of a leak in the feed line when the measured pressure stays below -200mbar over a predetermined time period, for example 20s. Advantageously, this time period is determined on the basis of the volume of the line and the dimensions of the smallest leak to be detected.

Figure 3 depicts an example of the result of speed/pressure measurements recorded during a test campaign on a system similar to that illustrated in Figure 1. The graph of Figure 3 illustrates the evolution of the pressure in the feed line in case of the presence of a leak in the feed line. The test campaign has been performed with a leak orifice of 0.0635mm diameter.

On this graph, shown on the x-axis is the time (in second) and on the y-axis are the pump rotational speed (in rpm) and the pressure in the feed line (in mbar). On this graph, the curve CI corresponds to the pump rotational speed and the curve C2 corresponds to the pressure in the feed line. The portion 31 of the graph illustrates a first period of time during which the pump rotates in the forward direction at approximately 1500rpm. During this first period of time the pressure in the feed line is maintained at approximately 5000mbar. The portion 32 of the graph illustrates a second period of time during which the pump changes its rotational direction. The pump switches from the forward direction operating mode to a reverse direction operating mode. In the reverse direction operating mode the pump rotates in the reverse direction at approximately -4000rpm. This creates a depressurization in the feed line. During this second period of time the pressure in the feed line drops from approximately 5000mbar to approximately - 380mbar. The portion 33 of the graph illustrates a third period of time during which the rotational speed of the pump is maintained at approximately - 4000rpm. As illustrated in Figure 3, during this third period of time the underpressure (i.e. negative pressure) in the feed line cannot be maintained. This rise of pressure indicates the presence of a leak in the feed line. Indeed, in case of the presence of a leak in the feed line, the depressurization results in the entrainment (i.e. sucking) of air inside the feed line, through the leak orifice. This air is then sucked by the pump and the underpressure in the feed line starts to reduce. For example, when the pressure in the feed line exceeds -200mbar the ECU can generate a suitable leak detection alarm signal.

Figure 4 illustrates schematically a particular embodiment of an ECU capable to carry out the method in accordance with a particular embodiment of the invention.

Advantageously, the pump (7) and other active components (gauge (2), heating element (3), temperature sensor (5), quality sensor, etc.) are

integrated/assembled in a module, commonly referred as AdBlue ® Delivery Module (ADM). Such module is well known in the art and is not further described hereafter.

In the example of Figure 4, the ECU 40 comprises a functional block 401 which executes an algorithm for estimating an effective consumption of urea, on the basis of information provided by the ADM, for example information on the pressure, the temperature, the concentration of the urea solution and the motor velocity.

The ECU 40 further comprises a functional block 402 which executes an algorithm for detecting whether the injector (12) is (partially) clogged and for estimating the clogging rate of the injector, on the basis of the estimated effective consumption (calculated by functional block 401) and an information ("dosing setpoint") relative to the quantity of urea solution which is supposed to be injected into the exhaust line by the injector.

The ECU 40 also comprises a functional block 403 which executes an algorithm for detecting a presence or absence of a leak in the SCR system, on the basis of the estimated effective consumption (calculated by functional block 401), the information ("dosing setpoint") relative to the quantity of urea solution which is supposed to be injected into the exhaust line by the injector, and (optionally) the estimated clogging rate of the injector (calculated by functional block 402).

Figure 5 illustrates schematically a particular embodiment of the functional block 403 of Figure 4.

In the illustrated example, the functional block 403 comprises a first sub- functional block 4031 which executes an algorithm for detecting an abnormal operating condition in the SCR system. Advantageously, the first sub-functional block 4031 executes a non-intrusive test which consists in:

- closing the injector or opening the injector to generate a controlled flow rate; - operating the pump such that urea solution flows in the feed line from the tank to the injector;

- monitoring the rotational speed of the pump;

- detecting an abnormal operating condition when the monitored rotational speed of the pump exceeds a predetermined threshold speed value, for a given mass flow of ammonia precursor solution.

For example, the rotational speed of the pump is approximately 1500rpm for a "0" mass flow at a regulated pressure of 5000mbar. For example, if the monitored rotational speed of the pump exceeds 1750rpm when there is no injection (dosing setpoint = 0), then it can be concluded that there is a potential leak in the feed line. For example, if the expected rotational speed of this pump is 1600rpm for a certain dosing setpoint (a delta speed of lOOrpm), whilst the monitored speed for this dosing setpoint is only 1550rpm (a delta speed of only 50rpm), then it can be concluded that there is a partial clogging of the injector.

If it is concluded that there is no potential leak in the feed line, then the functional block 403 generates a "Leak flag" signal comprising the information "No leak".

On the contrary, if it is concluded that there is a potential leak in the feed line, then the functional block 403 generates a "Leak flag" signal comprising the information "Leak suspected". In addition, the functional block 403 runs a second sub-functional block 4032 which executes an algorithm for confirming or invalidating the presence of a leak in the feed line. In a preferred embodiment, the sub-functional block 4032 executes the intrusive leak test as described above in relation to Figure 2.

If it is concluded that there is a leak, then the functional block 403 generates a "Leak flag" signal comprising the information "Leak confirmed".

On the contrary, if it is concluded that there is no leak, then the functional block 403 generates a "Leak flag" signal comprising the information "No leak". In addition, the functional block 403 runs a third sub-functional block 4033 which executes an algorithm for calibrating the predetermined threshold speed value that has been used by the first sub-functional block 4031. Advantageously, this calibrated threshold speed value will be used by the first sub-functional block 4031 at a next non-intrusive leak test. The calibrating may consist in closing the injector and recording at steady state the pump velocity at a given pressure.