Method of operating a reductant injection system

Abstract

The invention provides a method of operating a reductant injection system, the reductant injection system (10) comprising a reductant tank (4) storing a reductant, an injector (9), a pump (5), a supply line (7) for supplying pressurized reductant from the reductant tank (4) to the injector (9), and a pressure sensor (6) arranged to measure a reductant pressure in the supply pipe (7) and / or the injector (9). The method implements a diagnostic routine which comprises: a) monitoring the pressure within the supply line (7) and / or the injector (9) over a time window including a sequence of reductant injection events, thereby generating pressure data; b) computing at least one estimate reductant quantity from the pressure data, preferably by integrating pressure data over time during at least part of the time window; c) based on the at least one estimate reductant quantity, controlling the injector (9), notifying the operator of an injector fault and / or registering an injector fault.

Description

[0001] METHOD OF OPERATING A REDUCTANT INJECTION SYSTEM

[0002] Technical field

[0003] The present invention generally relates to emission control systems for the automotive industry, in particular to a reductant injection system for selective catalytic reduction.

[0004] Background Art

[0005] Reductant injection systems are widely used in the automotive industry to reduce emissions of polluting and harmful gases. In diesel engines for example, Diesel Exhaust Fluid (DEF), also known as AdBlue (RTM), is dispersed in the exhaust gas and consumed in a selective catalytic reduction (SCR), thereby decreasing the quantity of nitrogen oxides (NOx) released in the atmosphere.

[0006] Reductants injection systems typically comprise a reductant tank to store liquid reductant, a reductant injector for dispersing reductant in the exhaust gas, a supply pipe therebetween defining a passage for the reductant between the reductant injector to the reductant tank, and a pump to control flow of reductant within the supply pipe.

[0007] Unfortunately, most reductants have a freezing temperature comprised between -15°C and 0°C. Frozen reductant in injection systems is known to lower their performance. In particular, frozen reductant in the supply pipe or in the injector may block passage of liquid reductant coming from the tank, thereby preventing the injector from functioning altogether.

[0008] When the engine of the vehicle is running, heat from the engine may be used to keep the reductant above its freezing temperature and prevent it from freezing. However, when the engine is turned off, the reductant loses heat and may freeze before the next engine start. Simply using heat from the engine to melt already frozen reductant can take a significant amount of time, during which the reductant injection system is inoperable and toxic NOx gas is released.

[0009] One solution to this problem consists in arranging a separate heating unit in the reductant tank to improve the melting speed of the reductant, thereby slightly decreasing the downtime of the reductant injection system. An alternative or complementary solution consists in purging the supply pipe and the reductant injector upon engine shut down by reversing the flow of reductant, thereby introducing air and exhaust gases in the supply pipe and / or injector. With this solution, solidification of reductant in the supply pipe and injector is prevented and the passage of liquid reductant will be unobstructed.

[0010] Whilst this solution drastically reduces the downtime of the reductant injection system, it also leads to erratic injector behavior, especially during the first few injections following engine start up. Indeed, at engine start up, reductant must be reintroduced in the supply pipe and injector, thereby mixing with the air / exhaust gases already present and forming gas pockets within the system. These gas pockets make it difficult to determine the effective quantity of reductant injected in the exhaust gases and are therefore to be avoided.

[0011] Technical problem

[0012] It is an object of the present invention to provide to improve the control of reductant injection at start-up.

[0013] This object is achieved by a method for controlling a reductant injection system during engine start-up as claimed in claim 1.

[0014] General Description of the Invention

[0015] The present invention relates to operation ofa reductant injection system of a combustion engine, the reductant injection system comprising a reductant tank storing a reductant, an injector, a pump, a supply pipe for supplying pressurized reductant from the reductant tank to the injector, and a pressure sensor arranged to measure a reductant pressure in the supply pipe and / or the injector.

[0016] According to the invention, a method of operating a reductant injection system as claimed in claim 1 is provided, whereby a diagnostic routine is performed. The diagnostic routine comprises the following steps: a) monitoring the pressure within the supply line and / or the injector over a time window including a sequence of reductant injection events, thereby generating pressure data; b) computing at least one estimate reductant quantity from the pressure data, preferably by integrating pressure data over time during at least part of the time window; c) based on the at least one estimate reductant quantity, controlling the injector, notifying the operator of an injector fault and / or registering an injector fault.

[0017] This injection system diagnosis provides an assessment of the performance of the reductant injection system, or more precisely of the condition of the reductant injector, and allows adapting the operating conditions of the injector should a fault be detected, such as e.g. partial clogging of bores in an injection cap of the reductant injector.

[0018] The diagnostic routine can be implemented together in the context of the herein described purging sequence, i.e. where the reductant injection events are ‘purging injection events”. It may however be implemented later than at startup, during the normal engine operation, with respect to a sequence of reductant injection events with predetermined reductant quantity and injection frequency. For example, injection events to inject reductant quantities determined in accordance with the reductant injection / dosing strategy in function of measured or predicted emissions.

[0019] In embodiments, computing at least one estimate reductant quantity involves processing pressure data by: d) removing a direct current component from the pressure data, the direct current component being preferably derived from an average pressure over the reductant injection events or from a moving average; and e) applying an absolute function.

[0020] Additionally or alternatively, computing at least one estimate reductant quantity from the pressure data may comprise computing, for each reductant injection events of the sequence, a corresponding first estimate reductant quantity, and incrementing a first counter if the first estimate is above a respective first reductant quantity threshold, and / or incrementing a second counter if it is below a second reductant quantity threshold, wherein the first reductant quantity threshold is greater than or equal to the second reductant quantity threshold. Step c) then involves comparing the first and second counters to predetermined thresholds.

[0021] A control unit may conclude that the injector is over-injecting if the first counter is greater than a predetermined threshold and / or if the cumulated integrated value is greater than the first cumulated threshold. Additionally or alternatively, the control unit may conclude that the injector is under-injecting if the second counter is greater than a predetermined threshold and / or if the cumulated integrated value is lower than the second cumulated threshold. An injector fault may be stored, the injector may subsequently be controlled and / or the operator may be notified of an injector fault based on this conclusion.

[0022] The invention thus provides an injection system diagnosis which double checks the performance of the injection system by counting the number of times individual injections have exceeded a predetermined threshold (based on the integral of the pressure data of individual reductant injection events), and by comparing an estimate reductant quantity for all of the reductant injection events (based on the integral of the pressure data for all injection events). Such a double check diagnosis is particularly relevant when performed at reductant injector start-up due to the aforementioned erratic injector behavior prior / during purging injection events. But the diagnostic can also be applied for sequences of reductant injection events operated later than at startup, when the engine is hot and running for several minutes or dozens of minutes.

[0023] Preferably, processing pressure data involves data involves filtering to remove pumping and / or electrical noise, preferably by means of a band pass filter and / or a band stop filter calibrated at a purging injection frequency.

[0024] In embodiments, the injection system diagnosis is performed once an exhaust temperature has exceeded a predetermined exhaust temperature threshold, the temperature threshold being preferably 150°C.

[0025] In embodiments, at start-up of said reductant injection system, a gas purging sequence is performed, which comprises the steps of: - operating the reductant pump to maintain a predetermined pressure in the supply line, and

[0026] - performing a plurality of injection events, referred to as purging injection events, whereby the injector is opened for a predetermined duration, thereby purging gas from the supply line.

[0027] By preemptively using purging injection events to purge I flush gas in the supply line at start-up of a reductant injection system, i.e. before a demand injection is processed, it has been found that the initial erratic injection behavior of said reductant injection system could be prevented, leading to more consistent performances in terms of injected reductant quantities and timings of such injections.

[0028] In embodiments, the method further comprises the step of computing a second estimate reductant quantity for all of the purging injection events of the respective gas purging sequence, and step c) involves comparing the second estimate reductant quantity to predetermined threshold(s).

[0029] In embodiments, performing a plurality of injection events includes performing between 2 and 10 purging injection events, preferably between 2 and 4.

[0030] In embodiments, the purging injection events are performed at a predetermined injection frequency, in particular between 0.40 and 2 Hz.

[0031] In embodiments, the purging injection sequence is timed so as not to overlap with a demand injection event.

[0032] In embodiments, the reductant is water or aqueous urea.

[0033] In embodiments, a quantity of reductant injected per injection is comprised between 150 and 400 mg, preferably 200 and 300 mg.

[0034] In embodiments, the predetermined pressure is between 5 and 6 bars.

[0035] In embodiments, the gas purging sequence is performed only in case a reductant purging sequence has been previously performed. Such reductant purging sequence is typically performed at shut down and registered in the Engine control unit. Hence, in embodiments, an enabler for the gas purging sequence may be the existence of a Flag (or similar information in the control unit) that a reductant purging sequence has previously been performed.

[0036] Brief Description of the Drawings

[0037] Preferred embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings in which:

[0038] Fig. 1 is a schematic view of a reductant injection system;

[0039] Fig. 2 is a flow chart of an embodiment of the present method for gas purging in the reductant injection system;

[0040] Figs. 3a-c are frequency spectra of a pressure signal as measured by a pressure sensor in the supply pipe for reductant injection systems with different injectors;

[0041] Figs. 4a-c are graphical representations of the pressure signals (Pressure vs. time) corresponding to Figs. 3a-c (with no direct current component);

[0042] Figs. 5a-c are graphical representations (pressure vs time) of the pressure signals of Figs. 4a-c after filtering;

[0043] Figs. 6a-c are graphical representations of the integrals of the pressure signals of Fig. 5a-c;

[0044] Fig.7: is a flow chart of an embodiment of the diagnostic method for reductant injection system.

[0045] Description of Preferred Embodiments

[0046] Figure 1 shows a schematic view of an embodiment of reductant injection system 10 for injection of reductant in an exhaust 2 of an internal combustion engine 1 . The reductant injection system 10 comprises a reductant tank 4 to store liquid reductant (e.g. AdBlue, RTM), a reductant injector 9 for dispersing reductant in the exhaust 2, a supply pipe 7 therebetween defining a passage for the reductant between the reductant injector 9 and the reductant tank 4, a pressure sensor 6 to monitor the pressure within the supply pipe 7, and a pump 5 to control flow of reductant within the supply pipe 7. The reductant injection system is part of a SCR system, which uses a SCR catalyst 11 downstream of the reductant injector 9, and a NOx sensor 12.

[0047] In its normal use, the pump 5 is operated (continuously or intermittently) to pump reductant from the tank and maintain a predetermined pressure in the supply pipe 7. When the injector is opened, a quantity of reductant is discharged, typically as a spray. As the reductant / exhaust gas mixture reaches the SCR catalyst, selective reduction of NOx occurs, whereby N2, H2O and CO2 are produced.

[0048] A control unit 3 is generally configured to control the reductant injector 9 by triggering demand injection events. A demand injection event is here defined as a reductant injection triggered by the control unit to prevent emissions of NOx or other harmful gases. More specifically, the control unit 3 may implement reductant injection strategies whereby a demand injection event is triggered when emissions of NOx or other harmful gases are predicted. Said reductant injection strategies (or dosing strategies) generally depend on several parameters such as e.g. injected fuel quantity, engine speed, engine temperature, number of operating hours, the measured quantity of NOx in the exhaust gas, etc.

[0049] Conventionally, the reductant injector comprises a valve group selectively controllable by an electromagnetic actuator (solenoid type). An injection event is thus typically obtained by energizing the actuator to influence an opening of the valve during a given duration. Hence, performing a demand injection event typically involves operating the reductant injector to open during a predetermined time period to discharge a target reductant quantity. The target reductant quantity typically depends on the reductant injection / dosing strategy. A mapping is typically used that relates the target reductant quantity and injector opening duration for at least one reductant pressure.

[0050] When the engine 1 is shut-down, a liquid purge is performed, whereby reductant within the supply pipe 7 and the reductant injector 9 is forced back in the tank by reversing the flow of reductant, thereby sucking air and exhaust gases in the supply pipe 7 and the injector 9. Freezing of reductant in the supply pipe 7 and injector 9 is therefore prevented. The flow of reductant may be reversed by operating the pump 5 in a direction opposite from its normal use or by using another, secondary pump (not represented). However, as previously mentioned, gas pockets present in the supply pipe 7 or the injector 9 may lead to erratic injector behavior and are to be avoided.

[0051] A - Gas purging sequence

[0052] To solve this issue, the present invention provides a method whereby a gas purging injection sequence is triggered at start-up. Advantageously, the gas purging injection sequence further includes an injector diagnosis.

[0053] The gas purging injection sequence is preferably performed before a first demand injection is triggered, with each gas purging injection being about four times as long as a nominal demand injection. As will be clear below, this method decreases the time required after engine 1 start-up for the injector 9 to be fully operable, i.e. with no gas pockets in the supply pipe 7 or the injector 9. The method further improves the reliability of the diagnostic.

[0054] In general, the present gas purging sequence involves performing a plurality of injection events, referred to as purging injection events, whereby the injector is opened for a predetermined duration, in order to flush any gas from the supply line. The gas purging sequence spans a certain time period, the gas purging period, that encompasses all of the purging injections. For example, the gas purging sequence may begin with the start of the first purging injection event and end after the end of the last purging injection event, possibly with a margin. During the gas purging sequence, respectively the gas purging period, the pump is operated to maintain a predetermined pressure in the supply line, which may be typically the standard operating pressure for reductant injection, e.g. around 5 or 6 bar.

[0055] The purging sequence is summarized in Fig.2.

[0056] The gas purging sequence is normally performed as soon as possible at the start of the reductant injection system (and hence typically at engine start-up - step 20), i.e. before demand injection events are performed, according to the standard control strategy programmed in the control unit, in function of measured or estimated emissions.

[0057] It may be noted that pump operation (step 22) is required to maintain the supply pressure at the predetermined level during the gas purging sequence, but the pump may be already in function before start of the gas purging sequence, since it is often started quite early at engine startup.

[0058] As indicated at step S24, the purging sequence includes N purging injector events, so that in theory a volume of reductant of N x VPinj is introduced into the supply line 7, where VPinj is the reductant volume to be injected during a purging injection event (i.e. for a given injector opening time). The purging reductant volume is calibrated based on the configuration of the reductant system (pipe length, pipe diameter, injector design), so that the volume allows flushing / evacuating gas pockets though the injector when open.

[0059] Once the series of purging injections is performed, the reductant injection / dosing strategy is enabled, such that demand reductant injection events can be performed based on the operation state.

[0060] B - Diagnostic routine

[0061] A diagnostic strategy will now be explained, which performs an injector diagnostic based on pressure data analysis acquired during a predetermined monitoring period with a plurality of injection events.

[0062] In brief, at least one estimated reductant quantity is computed based on pressure data measured in the supply pipe, typically by integrating over time, based on which corrective actions may be taken, injector fault can be deduced, and / or driver notifications can be triggered.

[0063] Advantageously, the diagnostic strategy can be applied during the purging sequence, i.e. over a tome window encompassing a plurality of purging injection events. However, the diagnostic strategy can also be implemented during normal operation of the reductant injection system, i.e. over a time period encompassing a plurality of nominal reductant injection events.

[0064] An embodiment of a method implementing the present diagnostic strategy will be described below based on figure 7. It is exemplified for three situations of the reductant injection system 10. In all three situations, the reductant injector comprises an injector cap with three bores (not shown) for spraying reductant into the exhaust piping. In the first case, noted “a”, the injector cap is in nominal condition, i.e. fully operational, none of the three bores are clogged. In situation ‘b’, the reductant injector has a partially faulty injector cap, with one of its three bores clogged. In situation “c”, the injector has a non-functional injector cap, wherein all of its three bores are clogged. Indexes a, b and c are used in Figs 3 to 6.

[0065] At step S1 , a plurality of reductant injection events are performed. For example, in the context of the purging sequence, a plurality of purging injection events is performed periodically (e.g. with 250mg of aqueous urea) at a frequency of 2Hz. The pressure within the supply pipe 7 is continuously monitored (and recorded) at step S1 over a time period T 1 , encompassing a predetermined number of injection events by means of pressure sensor 6. The pressure sensor 6 is here arranged in the supply pipe 7, but could also be arranged within the injector 9. The pressure sensor 6 outputs an analog pressure signal which is sent to and processed by the control unit 3. The measured pressure over time window T1 is represented by the plots in Fig. 4 for the 3 situations a, b and c. The pressure oscillations correspond to about 10 injections over a period of 5 seconds, with each injection lasting approximatively 200 ms. The corresponding frequency spectrum of this signal (minus the direct current component) is represented in Fig. 3, i.e. in Fig. 3a for reductant injection system ’a’, on figure 3b for reductant injection system ‘b’ and on figure 3c for reductant injection system ’c’.

[0066] At step S1 , the control unit 3 removes the direct current component of the pressure signal by e.g. subtracting its average value or its moving average from the pressure signal. The control unit 3 further filters the signal to remove pumping and / or electrical noise, e.g. by means of a band pass filter and / or a band stop filter. If the injections of the predetermined number of injections is periodic, the band pass filter can be calibrated at the injection frequency (e.g. 2 Hz).

[0067] Figures 5a, 5b and 5c respectively represents the output corrected and filtered pressure signal of step S3 for input signals corresponding to those of figure 4a, 4b and 4c.

[0068] An absolute value function is applied to the signal of step S2 and subsequently integrated at step S3 over two consecutive injections, and the value of the integral is evaluated at C2. Figures 6a, 6b and 6c respectively represents integrals of the input signals corresponding to figure 5a, 5b and 5c over the entire window T 1 . As can be seen, where the holes are all clogged (situation ‘c’), the filter signal (Fig. 5c) is around zero and the resulting integral is null. In the intermediate situation, with one hole clogged (situation ‘b’), the pressure amplitude variation is smaller tan in the nominal case (‘a’) and the resulting integral (in dashed lines in Fig.6b) is less than the integral for the nominal case (solid line in Fig.6b).

[0069] So at first, integrals are computed for two consecutive injection events (i.e. two by two). If the integrated value is above a first threshold, a first counter is incremented at step S5. Conversely, if the integrated value is below a second threshold, a second counter is incremented at step S6. Steps S4-C2-S5 or S4-C2-S6 are repeated for each pair of consecutive injections of the predetermined number of injections.

[0070] In addition to the two-by-two integration approach, a cumulated integrated value is also calculated for the N injections events, i.e. over time period T1. This cumulated integrated value is then evaluated at C2’. If the cumulated integrated value is above a first cumulated threshold, a first flag is raised at step S5’. Conversely, if the cumulated integrated value is below a second cumulated threshold, a second flag is raised at step S6’.

[0071] The control unit 3 then checks at C3 if the first counter is greater than a predetermined threshold and if the first flag of step S5’ is raised. If so, the control unit 3 concludes that the injector 9 is over-injecting. The control unit 3 also checks at C4 if the second counter is greater than a predetermined threshold and if the second flag of step S6’ is raised. If so, the control unit 3 concludes that the injector 9 is under-injecting.

[0072] Finally, the control unit 3 controls the injector 9 and / or notifying the operator of an injector 9 fault based on the comparisons of the previous step. Alternatively, the diagnosis may first be repeated to confirm over- or under- injection behaviors.

[0073] The present diagnostic strategy has just been disclosed in combination with the above described purging sequence methodology, namely at engine start-up.

[0074] However, the diagnostic strategy can be implemented at later timings, during engine operation, for a plurality of reductant injection events that are operated by the control unit for predetermined reductant quantities and with a given frequency.

[0075] It may be noted that when the diagnostic strategy is implemented at engine startup, step S1 is advantageously preceded by a pressure and temperature check. For example, the control unit 3 first checks if the exhaust temperature in the exhaust 2 has reached a predetermined temperature threshold, and if the pressure in the supply pipe 7 has reached a predetermined pressure threshold (or level), e.g. 6 bar. If the thresholds have been exceeded, the method may then proceed to step S1 , otherwise, the control unit 3 continues monitoring the exhaust temperature and the supply pipe pressure. The exhaust temperature is typically monitored by a temperature sensor 8 which may be arranged downstream of the reductant injector 9 in the exhaust gas flow direction.

Claims

Claims1. A method of operating a reductant injection system, the reductant injection system (10) comprising a reductant tank (4) storing a reductant, an injector (9), a pump (5), a supply line (7) for supplying pressurized reductant from the reductant tank (4) to the injector (9), and a pressure sensor (6) arranged to measure a reductant pressure in the supply pipe (7) and / or the injector (9), wherein a diagnostic routine comprises: a) monitoring the pressure within the supply line (7) and / or the injector (9) over a time window including a sequence of reductant injection events, thereby generating pressure data; b) computing at least one estimate reductant quantity from the pressure data, preferably by integrating pressure data over time during at least part of the time window; c) based on the at least one estimate reductant quantity, controlling the injector (9), notifying the operator of an injector fault and / or registering an injector fault.

2. The method according to any claim 1 , wherein computing at least one estimate reductant quantity involves processing pressure data by: d) removing a direct current component from the pressure data, the direct current component being preferably derived from an average pressure over the reductant injection events or from a moving average; and e) applying an absolute function.

3. The method according to any of the preceding claims, wherein computing at least one estimate reductant quantity involves processing pressure data by: computing, for each reductant injection event of the sequence, a corresponding first estimate reductant quantity, and incrementing a first counter if the first estimate is above a respective first reductant quantity threshold, and / orincrementing a second counter if it is below a second reductant quantity threshold, wherein the first reductant quantity threshold is greater than or equal to the second reductant quantity threshold; and step c) involves comparing the first and second counters to predetermined thresholds.

4. The method according to claims 3, wherein it is concluded that the injector (9) is over-injecting if the first counter is greater than a predetermined threshold and / or if a cumulated integrated value is greater than a first cumulated threshold; and / or wherein a control unit (3) concludes that the injector (9) is under-injecting if the second counter is greater than a predetermined threshold and / or if a cumulated integrated value is lower than a second cumulated threshold; and the injector (9) is controlled and / or an injector fault is signaled or stored based on said conclusion.

5. The method according to any of claims 2 to 4, wherein processing pressure data involves data filtering to remove pumping and / or electrical noise, preferably by means of a band pass filter and / or a band stop filter calibrated to allow the injection frequency to pass.

6. The method according to any of the preceding claims, wherein the injection system diagnosis is performed once an exhaust temperature has exceeded a predetermined exhaust temperature threshold, the temperature threshold being preferably 150°C.

7. The method according to any of the preceding claims, wherein at start-up of the reductant injection system (10), a gas purging sequence is performed, which comprises the steps of:- operating the reductant pump (5) to maintain a predetermined pressure in the supply line (7);performing a plurality of reductant injection events, referred to as purging injection events, whereby the injector (9) is opened for a predetermined duration, thereby purging gas from the supply line (7).

8. The method according to claim 7, comprising computing a second estimate reductant quantity for all of the purging injection events of the respective gas purging sequence; and step c) involves comparing the second estimate reductant quantity to predetermined threshold(s).

9. The method according to claim 7 or 8, wherein performing a plurality of reductant injection events includes performing between 2 and 10 purging injection events, preferably between 2 and 4.

10. The method according to any of claims 7 to 9, wherein the purging injection events are performed at a predetermined injection frequency, in particular between 0.40 and 2 Hz.

11. The method according to any of claims 7 to 10, wherein the purging injection sequence is timed so as not to overlap with a demand injection event.

12. The method according to any of claims 7 to 11 , wherein the reductant is water or aqueous urea.

13. The method according to any of claims 7 to 12, wherein a quantity of reductant injected per injection is comprised between 150 and 400 mg, preferably 200 and 300 mg.

14. The method according to any of claims 7 to 13, wherein the predetermined pressure is between 5 and 6 bars.

15. The method according to any of claims 7 to 14, wherein the gas purging sequence is performed only in case a reductant purging sequence has been previously performed.

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