METHOD FOR PREVENTING A RISK OF FREEZING IN A REDUCING-AGENT FEEDING DEVICE OF A SELECTIVE CATALYTIC REDUCTION SYSTEM

Abstract

Disclosed is a method for preventing a risk of freezing in a device for supplying reducing agent to a selective catalytic reduction system in an exhaust line, a freezing temperature specific to the agent being stored in memory, the system including a controller operating the system and emitting pulses to an injector, the controller being inactive when the engine is switched off. With the combustion engine switched off, the controller is woken up at predetermined intervals to initiate an emission of a specific electric pulse to the injector with measurements of a current-strength and voltage of the electric pulse providing a value of the electrical resistance of the injector. A temperature of the reducing agent at the injector is estimated as a function of the measured resistance and, when at least the temperature thus estimated is below the freezing temperature, a purge of the device is initiated.

Claims

1. A method for preventing a risk of freezing in a device for supplying reducing agent to a selective catalytic reduction system in an exhaust line (10) of a motor vehicle combustion engine (1), a threshold temperature below which there is a risk of the reducing agent freezing being stored in memory, the catalytic reduction system comprising a controller in charge of the operation of the system and emitting pulses to an injector (6) of reducing agent into the system, the controller being inactive when the engine (1) is switched off, wherein, with the combustion engine (1) switched off, the controller is woken up (18) at predetermined and calibratable intervals to initiate an emission of a specific electric pulse to the reducing-agent injector (6) with measurements (19) of a current-strength and voltage of the electric pulse providing a value of the electrical resistance of the injector (6), and a temperature of the reducing agent at the injector (6) is estimated as a function of the measured resistance and, when at least the temperature estimated at the injector (6) is below the stored reducing-agent freezing temperature, a purge of reducing agent from the device for supplying the system with reducing agent is initiated (21).

2. The method as claimed in claim 1, wherein the specific pulse to the injector (6) is an injector (6) control pulse that is weak enough that the injector control pulse does not cause the injector (6) to open.

3. The method as claimed in claim 1, wherein at least one parameter additional to the estimated temperature in the injector (6) is taken into consideration to confirm the initiation of the purge, said at least one parameter being considered individually or in combination from among the following parameters: an outside temperature below a predetermined outside temperature, a weather forecast regarding an outside temperature that may be reached in a determined forthcoming time, a temperature in a reducing-agent tank (7) upstream of the injector (6) and, in the case of an injector that is cooled by the engine cooling circuit, a combustion engine (1) coolant temperature.

4. The method as claimed in claim 1, wherein a model of the cooling of the system, indicative of the thermal inertia of the system and that takes account of a currently-prevailing outside temperature, is formulated and, when the temperature measured at the injector (6) is higher than the freezing temperature of the reducing agent but the model anticipates that a temperature measured at the injector (6) will become lower than the freezing temperature of the reducing agent in a calibratable predetermined forthcoming period of time, a preventative purge of the device supplying reducing agent to the system is initiated.

5. The method as claimed in claim 4, wherein the predetermined forthcoming period of time is calibrated to be shorter than a predetermined interval between two awakenings (18) of the controller.

6. The method of claimed in claim 1, wherein after a purge, the prevention method is suspended (22) and awakenings (18) of the controller at predetermined intervals are no longer performed.

7. The method as claimed in claim 1, wherein the intervals can be calibrated as a function of the outside temperature, the intervals being shorter, the lower the outside temperature.

8. The method as claimed in claim 1, wherein, after the combustion engine (1) has been stopped, the device for supplying the system with reducing agent is depressurized (13).

9. An assembly comprising a motor vehicle exhaust line (10) and a selective catalytic reduction system, with a reducing-agent supply device comprising an injector (6) of reducing agent into the exhaust line (10) from a tank (7) of reducing agent, the assembly implementing the method of claim 1, the selective catalytic reduction system comprising a controller in charge of the operation of the system and comprising an injection driver emitting pulses to the injector (6) of reducing agent into the system, the controller having means for activating a purge of the reduction system supply device and being inactive when the engine (1) is switched off, wherein the assembly comprises means for implementing the method, the assembly comprising a wake-up device equipped with a time counter (17) and with means (18) for waking up the controller at predetermined intervals, the controller having means for controlling the injection driver so that upon awakening (18) of the controller, the controller sends a pulse to the injector (6) and comprising means for measuring the voltage and the current-strength of the pulse, means for calculating the resistance of the injector (6) from the measured voltage and measured current-strength, means for storing in memory a map that maps the resistance of the injector (6) as a function of temperature in order to estimate the temperature of the injector (6), and means for comparing the estimated temperature of the injector (6) against a stored reducing-agent freezing temperature, the means for activating a purge being operational when the estimated temperature at the injector (6) is below the threshold temperature below which there is a risk of freezing.

10. The assembly as claimed in claim 9, wherein the injection driver commands the opening and closing of the injector (6) according to a duty factor associated with an injection control frequency, the duty factor varying from 0 to 100%, and a validation command switching directly from 0 to 100%, the injection driver, upon the awakening (18) of the controller, sending to the injector (6) a pulse the duty factor of which is not sufficient to open the injector (6).

11. The method as claimed in claim 2, wherein at least one parameter additional to the estimated temperature in the injector (6) is taken into consideration to confirm the initiation of the purge, said at least one parameter being considered individually or in combination from among the following parameters: an outside temperature below a predetermined outside temperature, a weather forecast regarding an outside temperature that may be reached in a determined forthcoming time, a temperature in a reducing-agent tank (7) upstream of the injector (6) and, in the case of an injector that is cooled by the engine cooling circuit, a combustion engine (1) coolant temperature.

12. The method as claimed in claim 2, wherein a model of the cooling of the system, indicative of the thermal inertia of the system and that takes account of a currently-prevailing outside temperature, is formulated and, when the temperature measured at the injector (6) is higher than the freezing temperature of the reducing agent but the model anticipates that a temperature measured at the injector (6) will become lower than the freezing temperature of the reducing agent in a calibratable predetermined forthcoming period of time, a preventative purge of the device supplying reducing agent to the system is initiated.

13. The method as claimed in claim 3, wherein a model of the cooling of the system, indicative of the thermal inertia of the system and that takes account of a currently-prevailing outside temperature, is formulated and, when the temperature measured at the injector (6) is higher than the freezing temperature of the reducing agent but the model anticipates that a temperature measured at the injector (6) will become lower than the freezing temperature of the reducing agent in a calibratable predetermined forthcoming period of time, a preventative purge of the device supplying reducing agent to the system is initiated.

14. The method of claimed in claim 2, wherein after a purge, the prevention method is suspended (22) and awakenings (18) of the controller at predetermined intervals are no longer performed.

15. The method of claimed in claim 3, wherein after a purge, the prevention method is suspended (22) and awakenings (18) of the controller at predetermined intervals are no longer performed.

16. The method of claimed in claim 4, wherein after a purge, the prevention method is suspended (22) and awakenings (18) of the controller at predetermined intervals are no longer performed.

17. The method of claimed in claim 5, wherein after a purge, the prevention method is suspended (22) and awakenings (18) of the controller at predetermined intervals are no longer performed.

18. The method as claimed in claim 2, wherein the intervals can be calibrated as a function of the outside temperature, the intervals being shorter, the lower the outside temperature.

19. The method as claimed in claim 3, wherein the intervals can be calibrated as a function of the outside temperature, the intervals being shorter, the lower the outside temperature.

20. The method as claimed in claim 4, wherein the intervals can be calibrated as a function of the outside temperature, the intervals being shorter, the lower the outside temperature.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Other features, aims and advantages of the present invention will become apparent upon reading the detailed description that will follow and upon examining the appended drawings, given by way of non-limiting examples and in which:

[0031] FIG. 1 shows an assembly comprising a combustion engine and an exhaust line comprising a selective catalytic reduction system with a device for injecting reducing agent into the exhaust line, it being possible for the prevention of freezing of reducing agent in the injection device to be controlled using a method according to the present invention,

[0032] FIG. 2 illustrates a logic diagram according to one embodiment of the method for preventing the risk of freezing in a device supplying reducing agent to a selective catalytic reduction system in an exhaust line of a motor vehicle combustion engine according to the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0033] In what follows, upstream and downstream are to be considered in the sense of a flow of exhaust gases along an exhaust line.

[0034] Reference is made to FIG. 1 which shows a combustion engine 1 and an exhaust line 10 for removing the exhaust gases originating from combustion in the combustion engine 1. The exhaust line 10 may comprise, in the vicinity of an exhaust manifold of the engine 1, an oxidation catalytic converter 2 and a particulate filter 3, for a compression-ignition engine 1, notably a diesel engine 1 or engine running on gas oil.

[0035] In the case of a controlled-ignition combustion engine, notably an engine running on gasoline or on a mixture containing gasoline, the line 10 may comprise a three-way catalytic converter and a gasoline particulate filter.

[0036] An upstream nitrogen oxides probe 4a, also referred to as an upstream NOx probe is positioned downstream of the particulate filter 3. All the features relating to the reduction catalytic converter 2, to the particulate filter 3 and to the upstream NOx probe 4a are not essential for the implementation of the present invention.

[0037] Shown next is a device for supplying reducing agent by injecting reducing agent into the line, this supply device forming part of the selective catalytic reduction system. The injection device comprises a tank 7 of reducing agent, a pipe 5 leaving the tank 7 and opening into the exhaust line 10 via an injector 6. A pump, not illustrated in FIG. 1, may draw reducing agent from the tank 7 toward the injector 6.

[0038] As an accessory, at least one heating element may be provided in the tank 7. Likewise, a temperature sensor may be provided in the tank 7 to measure the temperature of the liquid agent that the tank 7 contains. A liquid reducing agent gauge may also advantageously be provided in the tank 7.

[0039] Optionally, and non-compulsorily, an accumulator of liquid agent for temporarily storing a dose of reducing agent, not shown in FIG. 1, may be connected to the pipe 5 by being positioned between the pump and the injector 6. The injection system may also possess a pressure sensor which measures the pressure prevailing in the pipe.

[0040] The reducing-agent supply device, notably the pipe 5 and the injector 6, may become blocked and even damaged if the reducing agent freezes. As a result, it is appropriate to purge the pipe 5 and the injector 6 of the supply device regularly. A systematic purge of the supply device is therefore performed when the vehicle stops. It is a systematic purge such as this that the present invention seeks to avoid in order to carry out a purge only when the risk of the reducing agent freezing is proven.

[0041] Downstream of the injector 6 in the exhaust line 10 is mounted a mixer 11 that mixes reducing agent with the exhaust gases, the reducing agent decomposing into ammonia.

[0042] An ammonia probe 8 is provided, this being positioned downstream of the selective catalytic reduction or SCR catalytic converter 9, remaining part of the catalytic reduction system, supplementing the device supplying reducing agent. Downstream of the SCR catalytic converter 9 there is a downstream NOx probe 4b that makes it possible to check whether all the nitrogen oxides have been reduced. The mixer 11, the downstream NOx probe 4b and the ammonia probe 8 are not essential to the implementation of the present invention, whereas the selective catalytic reduction catalytic converter 9 is, as are the tank 7, the pipe, the injector 6 and the controller of the system which is not depicted in FIG. 1.

[0043] With reference to FIG. 2, while considering FIG. 1 for the references not present in FIG. 2, the present invention relates to a method for preventing the risk of freezing in a device supplying reducing agent to a selective catalytic reduction system in an exhaust line 10 of a motor vehicle combustion engine 1. The catalytic reduction system comprises a controller in charge of the operation of the system and emitting pulses to an injector 6 of reducing agent of the system supply device, the controller being inactive when the engine 1 is switched off. The controller in charge of the operation of the system stores in memory beforehand a threshold temperature below which there is a risk of the reducing agent freezing.

[0044] In what follows, the method for preventing a risk of the reducing agent freezing will be described with reference to FIG. 2. However, FIG. 2 illustrates certain steps in the method which are purely optional and are not essential to the implementation of the method, these steps being mentioned hereinafter as being optional.

[0045] When the engine 1 is switched off, which is what is illustrated by the reference 12, and after the system has been at least partially depressurized, something which is optional and illustrated by the reference 13, a first question step referenced 14 may optionally be carried out.

[0046] This first question step 14 relates to the following three conditions with, if at least one of the three conditions is present, a response yes O to the first question 14. If an ambient outside temperature is below a minimum ambient outside temperature, if a temperature of the tank 7 of reducing agent is below a minimum tank 7 temperature or if a battery voltage is below a minimum voltage, the response to the first question 14 is yes O and a preventative purge, referenced 15, of the SCR system supply device is carried out and then the controller in charge, having been kept awake until just after the vehicle stops in order to carry out this optional first question step 14, is put to sleep. Following the purge, the controller is returned to standby 16 and the method according to the present invention is suspended.

[0047] For this optional first question step 14, the controller in charge of the system comprises means for storing in memory a minimum outside temperature and a minimum tank 7 temperature, at least one of which temperatures could more or less rapidly lead to freezing of the reducing agent. The decision is then taken to purge the SCR system supply device, this purge being almost certain to be necessary while the engine 1 is stopped and the temperature of the reducing agent is gradually dropping.

[0048] There is then no further need to implement the rest of the prevention method according to the invention. The values of the outside temperature and of the temperature of the reducing-agent tank 7 may be communicated directly or indirectly by sensors to the controller in charge of the system. Indirectly means that these values are processed by another control unit other than the controller and then transmitted to the controller, it being possible for the other control unit to be a monitoring-control unit of the vehicle.

[0049] The condition regarding the battery voltage serves to avoid a potential purge, that is to occur while the vehicle is stopped, being prevented by too low a battery voltage. In this case, it is preferable for the purge to be performed just after the vehicle stops, so that this purge is achieved.

[0050] If the response to the optional first question 14 is no N, no purge is carried out just after this first question 14 and the method for preventing a risk of the reducing agent freezing is implemented.

[0051] At reference 17, the controller in charge of the system is placed on standby. However, as referenced by 18, the controller is woken up at predetermined and calibratable intervals. This serves to initiate an emission of a specific electrical pulse to the injector 6 of reducing agent with measurements 19 of a current-strength and voltage of the electrical pulse, providing a value for the electrical resistance of the injector 6. This taking of measurements of the current-strength, voltage and value of the electrical resistance of the injector 6 is referenced 19.

[0052] This value of electrical resistance is used to determine the temperature of the injector 6, the electrical resistance of the injector 6 being a function of the temperature and the temperature of the reducing agent at the injector 6 being able to be deduced from a temperature of the injector 6 estimated from the electrical resistance of the injector 6. An estimate of a temperature of reducing agent at the injector 6 as a function of the measured resistance is then made.

[0053] A second question step 20 is then performed. The essential question in this second question step 20 is: is the estimated temperature at the injector 6 below or equal to a threshold temperature stored in the controller below which there is a risk of the reducing agent freezing?.

[0054] If the response to this second question 20 is yes O, namely when at least the estimated temperature at the injector 6 is below the stored freezing temperature of the reducing agent, a purge of the SCR system supply device is initiated, this being illustrated by the reference 21. The controller is then placed on standby and is no longer woken up at determined intervals, this being referenced 22 in FIG. 2. The prevention method is then suspended.

[0055] Other optional conditions may form the subject of this second question step 20. For example, if the temperature of the reducing-agent tank 7 is below a minimum tank 7 temperature, if a battery voltage is below a minimum voltage or if an interval between two awakenings 18 is longer than a maximum interval, an optional response to the second question step 20 may be yes O and the SCR system supply device is purged, this being illustrated by the reference 21. The controller is then placed on standby and no longer woken up 18 at determined intervals, this being referenced 22 in FIG. 2.

[0056] The interval between two awakenings 18 being longer than a maximum interval is indicative of a malfunctioning of the way in which the controller is woken up 18. As a preventive measure, a purge may be performed, because the validity of the freezing diagnosis of the method may be compromised.

[0057] For measuring the electrical resistance of the injector 6, the specific pulse to the injector 6 may be an injector 6 control pulse that is weak enough that it does not cause the injector 6 to open, such opening of the injector presenting no advantage and giving rise to disadvantages such as a needless flow of reducing agent, and noise.

[0058] Additional parameters for confirming purge, in addition to the estimated temperature in the injector 6, estimated by measuring the electrical resistance of the injector 6 during a pulse of current, were mentioned in the first question step 14. In addition to the outside temperature being below a predetermined outside temperature or to the temperature in a reducing-agent tank 7 upstream of the injector 6, consideration may be given to a weather forecast regarding an outside temperature that may be reached in a determined forthcoming time, and a temperature of a combustion engine 1 coolant, which is also indicative of the temperature of the injector 6, this being in the case of an injector that is cooled by the engine cooling circuit.

[0059] Graphs may estimate a drop in temperature in the injector 6 as a function of these parameters. Thus, a model of the cooling of the SCR system supply device, indicative of the thermal inertia of the device and of the surrounding elements, may be formulated.

[0060] This model may take account of a currently-prevailing outside temperature and, when the temperature measured at the injector 6 is higher than the freezing temperature of the reducing agent but the model anticipates that a temperature measured at the injector 6 will become lower than the freezing temperature of the reducing agent in a calibratable predetermined forthcoming period of time, a preventative purge of the device supplying reducing agent to the system is initiated.

[0061] For example, in the case of a reducing-agent temperature, estimated from a measurement of the electrical resistance of the injector 6, that is higher than the freezing temperature, when the model nevertheless detects a risk of freezing before a further forthcoming standby interval has ended and the awakening of the controller has come into effect, a preventative purge may be initiated during the current standby of the controller. The predetermined forthcoming period of time can therefore be calibrated to be shorter than a predetermined interval between two awakenings 18 of the controller.

[0062] The intervals may be able to be calibrated as a function of an outside temperature or of the model. The intervals may be shorter the lower the outside temperature, so as to guarantee the best protection of the reducing agent against freezing that may occur between the controller being placed on standby at the start of a standby interval and the awakening 18 of the controller at the end of the interval.

[0063] With reference mainly to FIG. 1, the present invention also relates to an assembly comprising a motor vehicle exhaust line 10 and a selective catalytic reduction system with a reducing-agent supply device comprising an injector 6 of reducing agent into the exhaust line 10 from a tank 7 of reducing agent. The selective catalytic reduction system comprises a controller in charge of the operation of the system and an injection driver emitting pulses to the injector 6 of reducing agent into the system. The controller is equipped with means for activating a purge of the reduction system supply device and is inactive when the engine 1 is switched off.

[0064] The assembly comprises a wake-up device equipped with a time counter and with means 18 for awakening the controller at predetermined intervals. The controller has means for controlling the injection driver so as upon its awakening 18 to send a pulse to the injector 6 and comprises or is connected to means for measuring the voltage and the current-strength of the pulse.

[0065] The controller comprises means for calculating the resistance of the injector 6 from the measured voltage and the measured current strength, means for storing in memory a map mapping the resistance of the injector 6 as a function of its temperature for an estimation of the temperature of the injector 6 and means for comparing the estimated temperature of the injector 6 against a threshold temperature stored in memory and below which there is a risk of the reducing agent freezing.

[0066] The means for activating a purge of the SCR system supply device are operational when the estimated temperature at the injector 6 is below the threshold temperature.

[0067] The injection driver, for controlling the injection of reducing agent into the exhaust line 10, provides the commands to open and close the injector 6 by means of a duty factor associated with an injection control frequency, the duty factor varying from 0 to 100% and a validation command switching directly from 0 to 100%. This is a conventional approach for injecting reducing agent.

[0068] By using this command but not injecting reducing agent into the line 10 from the injector 6, the injection driver can, on the awakening 18 of the controller, send to the injector 6 a pulse of which the duty factor is not sufficient to open the injector 6 but is sufficient to make a reliable measurement of the electrical resistance of the injector 6 as a function of the current and of the voltage that are detected at the terminals of the injector 6.