Method of monitoring a vehicle tank level, storage medium, control unit and scr system

Abstract

The present application relates to a method for monitoring a liquid level in a tank of a vehicle, comprising the steps of: causing a first ultrasonic sensor to emit ultrasonic waves and to receive a first echo signal for detecting the liquid level in the tank; causing a second ultrasonic sensor to emit ultrasonic waves and to receive a second echo signal when the first ultrasonic sensor does not receive the first echo signal for a period of time; determining whether the tank is empty based on the second echo signal. The present application also relates to a machine-readable non-volatile storage medium, a control unit and a vehicle SCR system. According to the present application, it is possible to reliably determine whether the tank is emptied when the first ultrasonic sensor does not detect the liquid level in the tank.

Description

Technical Field

[0001] This application relates to a technique for monitoring the liquid level in the tanks of vehicles, especially diesel vehicles, and particularly in the exhaust gas treatment tanks. Background Technology

[0002] Vehicle engines produce exhaust gases with high levels of nitrogen oxides during operation. These gases cannot be directly released into the atmosphere but must be treated by a selective catalytic reduction (SCR) system before being released. For diesel vehicles, an effective technology for treating engine exhaust gases is the SCR method to reduce the nitrogen oxide content. The SCR method uses an exhaust gas treatment fluid (usually a urea solution) in the SCR system to treat the exhaust gases, converting harmful nitrogen oxides into harmless nitrogen and water vapor, thereby reducing harmful engine emissions.

[0003] SCR systems typically include a tank for holding exhaust gas treatment fluid, an injection device for injecting the fluid into the exhaust pipe, and a delivery device for drawing the fluid from the tank and pressurizing it for supplying it to the injection device. An ultrasonic sensor is installed inside the tank to detect the fluid level. This ultrasonic sensor measures the level using the interface between the exhaust gas treatment fluid and the air above it. However, when the tank is overfilled with exhaust gas treatment fluid, all the air is expelled, eliminating the interface between the fluid and air. No ultrasonic waves are reflected back to the ultrasonic sensor, so it cannot detect the level. Furthermore, when the fluid in the tank is excessively consumed or emptied, the level drops below the ultrasonic sensor's ultrasonic emission point, also making it undetectable. Therefore, using only this level sensor is insufficient to determine if the tank is empty. Summary of the Invention

[0004] In view of the problems described above, this application aims to provide a method for monitoring the liquid level in the liquid tanks of vehicles, especially diesel vehicles, particularly in the exhaust gas treatment liquid tanks, so as to reliably determine whether the liquid tanks have been emptied.

[0005] According to one aspect of this application, a method for monitoring the liquid level in a vehicle's fluid tank is provided, the fluid tank having a first ultrasonic sensor and a second ultrasonic sensor mounted at the bottom of the fluid tank, the first ultrasonic sensor being used to detect the liquid level in the fluid tank by emitting ultrasonic waves toward the top of the fluid tank and receiving echo signals, and the second ultrasonic sensor being used to determine whether liquid is present below the ultrasonic wave emitting portion of the first ultrasonic sensor by emitting ultrasonic waves along the bottom and receiving echo signals, the method comprising the following steps:

[0006] The first ultrasonic sensor emits ultrasonic waves and receives a first echo signal to detect the liquid level in the tank.

[0007] When the first ultrasonic sensor does not receive the first echo signal for a period of time, the second ultrasonic sensor is made to emit ultrasonic waves and receive the second echo signal.

[0008] The second echo signal is used to determine whether the liquid tank is empty.

[0009] According to another aspect of this application, a machine-readable non-volatile storage medium is provided, on which program instructions for implementing the aforementioned method are stored.

[0010] According to another aspect of this application, a control unit is provided, the control unit having a memory and a processor, the memory storing executable program instructions that, when executed, cause the processor to perform the aforementioned method.

[0011] According to another aspect of this application, a vehicle SCR system is provided, including a tank for containing exhaust gas treatment fluid, an injection device for injecting exhaust gas treatment fluid into an exhaust pipe, a delivery device for drawing exhaust gas treatment fluid from the tank and pressurizing and supplying it to the injection device, and the aforementioned control unit.

[0012] The method of this application for monitoring the fluid level in the fluid tanks of vehicles, especially diesel vehicles, and particularly in the exhaust gas treatment fluid tank, can determine whether the fluid tank has been emptied when the ultrasonic level sensor cannot detect the fluid level. Furthermore, the method can reliably determine the following filling conditions of the fluid tank: 1) the tank is empty; 2) the tank is overfilled; 3) the fluid level in the tank has dropped below the ultrasonic emission point of the ultrasonic level sensor, thereby responding differently to different filling conditions to ensure the normal operation of the vehicle system. Attached Figure Description

[0013] The above and other aspects of this application will now be understood and appreciated more thoroughly in conjunction with the accompanying drawings. It should be noted that the drawings are schematic only and not drawn to scale. In the drawings:

[0014] Figure 1 This is a schematic diagram of an SCR system according to a preferred embodiment of this application; and

[0015] Figure 2 This is a flowchart of a method for monitoring the liquid level in a liquid tank according to a preferred embodiment of this application. Detailed Implementation

[0016] Preferred embodiments of this application are described in detail below with reference to examples. Those skilled in the art should understand that these exemplary embodiments are not intended to limit the scope of this application.

[0017] Figure 1 A preferred embodiment of the exhaust gas SCR system for a vehicle, particularly a diesel vehicle, according to this application, is schematically illustrated. Associated with the exhaust pipe (not shown) of the vehicle engine, the system is used to inject an exhaust gas treatment fluid (typically an aqueous urea solution) into the exhaust pipe to reduce the nitrogen oxide content in the exhaust gas. The SCR system may include a tank 1 for containing the exhaust gas treatment fluid, an injection device 3 for injecting the exhaust gas treatment fluid into the exhaust pipe, and a delivery device 5 for drawing the exhaust gas treatment fluid from the tank 1 and pressurizing it for supplying it to the injection device 3. The delivery device 5 may include a supply pump 7 and a backflow pump 9. When the SCR system is operating, the supply pump 7 in the delivery device 5 draws the exhaust gas treatment fluid from the tank 1 through a suction line 11, pressurizes it, and supplies it to the injection device 3 through a supply line 13. The injection device 3 then atomizes the exhaust gas treatment fluid and injects it meteredly into the exhaust pipe. When the vehicle is stopped, the back pump 9 in the conveying device 5 back pumps the exhaust gas treatment liquid back to the liquid tank 1 through the return pipe 15 to prevent the exhaust gas treatment liquid from depositing or crystallizing in the injection device 3 and the conveying device 5.

[0018] Continue to refer to Figure 1 The SCR system may also include a first ultrasonic sensor 17, a heater 19, a second ultrasonic sensor 25, and a reflector 27 arranged within the tank 1. The first ultrasonic sensor 17 can be used to detect the exhaust gas treatment fluid level in the tank 1. The heater 19 can be used to heat the exhaust gas treatment fluid in the tank 1. The SCR system may also include a control unit 20, which is associated with the tank 1, the first ultrasonic sensor 17, the heater 19, the second ultrasonic sensor 25, the injection device 3, the delivery device 5, and other functional elements of the SCR system for controlling their operation and monitoring their status. This control unit 20 may, for example, be the vehicle's ECU.

[0019] A first ultrasonic sensor 17 is installed at the bottom 21 of the tank 1 and can be used to detect the level of the exhaust gas treatment fluid in the tank 1 by emitting ultrasonic waves toward the top 23 of the tank 1 and receiving the echo signal. Specifically, the first ultrasonic sensor 17 detects the level by emitting ultrasonic waves toward the interface between the exhaust gas treatment fluid in the tank 1 and the air above and receiving the echo signal reflected from the interface. The control unit 20 can determine the level of the exhaust gas treatment fluid in the tank 1 by comparing the echo signal with a predetermined reference value. However, when the tank 1 is overfilled with exhaust gas treatment fluid, all the air in the tank 1 is squeezed out, there is no interface between the exhaust gas treatment fluid and the air, no ultrasonic waves are reflected back to the first ultrasonic sensor 17, and therefore the first ultrasonic sensor 17 cannot detect the level. Furthermore, when the exhaust fluid in tank 1 is excessively consumed or emptied (e.g., manually emptied or leaked due to damage), the fluid level will drop below the ultrasonic emission point of the first ultrasonic sensor 17, and no ultrasonic waves will be reflected back to the first ultrasonic sensor 17, thus the first ultrasonic sensor 17 cannot detect the fluid level. Using only the first ultrasonic sensor 17 is insufficient to determine whether tank 1 has been emptied. In other words, when the first ultrasonic sensor 17 cannot detect the fluid level, it is impossible to determine whether tank 1 is empty. For example, when it is necessary to determine the fluid level in tank 1 during vehicle startup, if the first ultrasonic sensor 17 cannot detect the fluid level, it is impossible to determine whether tank 1 is empty. Therefore, determining whether tank 1 is empty when the first ultrasonic sensor 17 cannot detect the fluid level is crucial. As mentioned earlier, the cause of emptying may be that tank 1 has been manually emptied or leaked due to damage. In this case, it is necessary to determine whether tank 1 is empty, and if it is determined that tank 1 is empty, the user should be reminded to check the integrity of the tank.

[0020] The control unit 20 of this application can determine whether the liquid tank 1 is empty (referred to as the first filling condition) when the first ultrasonic sensor 17 does not detect the liquid level. The control unit 20 uses a second ultrasonic sensor 25 to determine whether the liquid tank 1 is in the first filling condition. The second ultrasonic sensor 25 is installed inside the liquid tank 1 at the bottom 21 and can be used to determine whether there is exhaust gas treatment liquid below the ultrasonic emission point of the first ultrasonic sensor 17 by emitting ultrasonic waves along the bottom 21 of the liquid tank 1 and receiving the echo signal, thereby determining whether the liquid tank 1 is empty. The second ultrasonic sensor 25 can be arranged below the ultrasonic emission point of the first ultrasonic sensor 17. Preferably, the second ultrasonic sensor 25 can be arranged at the lowest point of the bottom 21 of the liquid tank 1. The reflector 27 can be arranged facing the second ultrasonic sensor 25, and the second ultrasonic sensor 25 can be used to determine the type of medium (e.g., gas and / or liquid) between the second ultrasonic sensor 25 and the reflector 27 by emitting ultrasonic waves toward the reflector 27 and receiving the echo signal reflected by the reflector 27. Specifically, the presence of exhaust gas treatment fluid between the second ultrasonic sensor 25 and the reflector 27 can be determined by comparing the echo signal with a predetermined reference value. This allows it to determine whether exhaust gas treatment fluid is present below the ultrasonic emission portion of the first ultrasonic sensor 17, thereby determining whether the tank 1 is in a first filling state, i.e., whether the tank 1 is empty. In one example, the reflector 27 is a plate extending from the bottom 21 of the tank 1. In another example, the reflector 27 may be a plate extending from the side wall of the tank 1.

[0021] When the control unit 20 detects that the first ultrasonic sensor 17 cannot detect the liquid level within a certain time period (e.g., approximately 300 seconds, but may be more or less than 300 seconds), it activates the second ultrasonic sensor 25, causing it to emit ultrasonic waves toward the reflector 27 and receive echo signals to determine whether the liquid tank 1 is in the first filling state. Specifically, the presence of exhaust gas treatment fluid below the ultrasonic emission point of the first ultrasonic sensor 17 can be determined by comparing the echo signal of the second ultrasonic sensor 25 with a predetermined reference value. When it is determined that there is no exhaust gas treatment fluid below the ultrasonic emission point of the first ultrasonic sensor 17, the liquid tank 1 is in the first filling state, i.e., the liquid tank 1 is empty. However, if it is determined that there is exhaust gas treatment fluid below the ultrasonic emission point of the first ultrasonic sensor 17, there are two possible situations: 1) the liquid tank 1 is overfilled (which can be called the second filling state); or 2) the liquid level in the liquid tank 1 has dropped below the ultrasonic emission point of the first ultrasonic sensor 17 but has not been emptied (which can be called the third filling state).

[0022] In this situation, the control unit 20 needs to use the operation of the heater 19 to further determine whether the liquid tank 1 is in the second or third filling state. Specifically, the heater 19 operates differently when there is exhaust gas treatment liquid in the surrounding area and when there is no exhaust gas treatment liquid, especially in terms of different temperature rise rates and operating currents. For example, when there is exhaust gas treatment liquid around the heater 19, the temperature rise rate of the heater 19 is slower (stable heating), while when there is no exhaust gas treatment liquid around the heater 19, the temperature rise rate of the heater 19 increases sharply (unstable heating).

[0023] Heater 19 can be, for example, a PTC (Positive Temperature Coefficient) heater, whose heating element is made of PTC material, whose resistance increases with temperature. When using this material, as the exhaust gas treatment fluid rises to a certain temperature, the resistance of the heating element increases and the current decreases, thus reducing the power of heater 19 and preventing overheating of the exhaust gas treatment fluid. Therefore, heater 19 has a self-limiting temperature function. When there is no exhaust gas treatment fluid around heater 19, the temperature rise rate of PTC heater 19 is very rapid, and the operating current drops rapidly.

[0024] The control unit 20 determines whether the liquid tank 1 is in a second or third filling state based on the heating status of the heater 19. For this purpose, the heater 19 is positioned above the ultrasonic emission point of the first ultrasonic sensor 17. For example, the heater 19 is mounted on the side wall of the liquid tank 1.

[0025] When the control unit 20 needs to further determine whether the liquid tank 1 is in a second or third filling condition—that is, when the control unit 20 cannot detect the liquid level through the first ultrasonic sensor 17 and determines through the second ultrasonic sensor 25 that there is exhaust gas treatment liquid below the ultrasonic emission part of the first ultrasonic sensor 17—it activates the heater 19 or maintains the heating state of the heater 19 and monitors the heating state of the heater 19 to determine whether the liquid tank 1 is in a second or third filling condition. Specifically, when the control unit 20 detects that the heater 19 is heating stably, it can determine that the liquid tank 1 is in a second filling condition; when it is heating unstablely, it can determine that the liquid tank 1 is in a third filling condition. For example, when the heater 19 is heating stably, the temperature rise of the heater 19 is normal, thus it can be determined that there is also an exhaust gas treatment liquid level above the ultrasonic emission part of the first ultrasonic sensor 17, and therefore the liquid tank 1 is determined to be overfilled (second filling condition). When heater 19 is not heating stably, its temperature rises too quickly, indicating that the liquid level in tank 1 has dropped below the ultrasonic emission point of the first ultrasonic sensor 17 but has not been emptied (third filling condition). If heater 19 is a PTC heater, control unit 20 detects its operating current. If the rate of decrease in operating current is normal, it indicates that heater 19 is heating stably, and tank 1 is in the second filling condition. However, if the rate of decrease in operating current is too fast or falls below a predetermined minimum current value, it indicates that heater 19 is heating stably, and tank 1 is in the third filling condition.

[0026] When the control unit 20 determines that the liquid tank 1 is in the first liquid level condition, i.e., the liquid tank 1 is empty, the control unit 20 may issue an alarm to remind the user to check the liquid tank 1. For SCR systems with an automatic filling function, the control unit 20 may temporarily disable this function. If the user checks that the liquid tank 1 is intact, the automatic filling function can be reactivated or manual filling can be performed; if the user finds that the liquid tank 1 is damaged, the entire liquid tank 1 needs to be replaced. When the control unit 20 determines that the liquid tank 1 is in the second liquid level condition, i.e., the liquid tank 1 is overfilled, the control unit 20 may remind the user that there is sufficient exhaust gas treatment fluid available in the liquid tank 1 and maintain the heating state of the heater 19 (e.g., at low temperatures). When the control unit 20 determines that the liquid tank 1 is in the third liquid level condition, i.e., the liquid level in the liquid tank 1 has dropped below the ultrasonic emission part of the first ultrasonic sensor 17 but has not been emptied, the control unit 20 may turn off the heater 19 and issue an alarm to remind the user to add exhaust gas treatment fluid to the liquid tank 1. For SCR systems with an automatic filling function, the control unit 20 may initiate the filling operation of the liquid tank 1.

[0027] The SCR system of this application can reliably determine whether the liquid tank 1 has been emptied when the first ultrasonic sensor 17 cannot detect the liquid level in the liquid tank 1. In addition, the above technology can reliably determine which of the following filling conditions the liquid tank 1 is in: 1) the liquid tank 1 is empty; 2) the liquid tank 1 is overfilled; 3) the liquid level in the liquid tank 1 has dropped below the ultrasonic emission part of the first ultrasonic sensor 17, thereby making different responses to different filling conditions and ensuring the normal operation of the vehicle system.

[0028] It is understood that, within the scope of this application, those skilled in the art can make various modifications to the structure and function of the SCR system described above, particularly the control unit 20. Additionally, in other embodiments, when exhaust gas treatment fluid is present between the second ultrasonic sensor 25 and the reflector 27, the concentration of the exhaust gas treatment fluid can be further determined based on the echo signal from the second ultrasonic sensor 25.

[0029] This application also relates to a method for monitoring the liquid level in a liquid tank, optionally applicable to the SCR system described above and implemented using the control unit 20 described above. One feasible flow of the method is as follows: Figure 2 The illustration is shown in the middle.

[0030] like Figure 2 As shown, in step S1, the first ultrasonic sensor 17 emits ultrasonic waves toward the top 23 of the liquid tank 1 and receives the first echo signal to detect the liquid level in the liquid tank 1.

[0031] Next, in step S2, it is determined that the first ultrasonic sensor 17 did not receive the first echo signal for a period of time (e.g., about 300 seconds).

[0032] Next, in step S3, the second ultrasonic sensor 25 emits ultrasonic waves along the bottom 21 of the liquid tank 1 and receives the second echo signal.

[0033] Next, in step S4, based on the second echo signal, it is determined whether there is exhaust gas treatment liquid below the ultrasonic emission part of the first ultrasonic sensor 17, so as to determine whether the liquid tank 1 is empty.

[0034] If it is determined in step S4 that there is no exhaust gas treatment liquid below the ultrasonic emission part of the first ultrasonic sensor 17, it can be determined that the liquid tank 1 is empty, and then proceed to step S5.

[0035] In step S5, an alarm is issued to remind the user to check liquid tank 1.

[0036] On the other hand, if it is determined in step S4 that there is exhaust gas treatment liquid below the ultrasonic emission part of the first ultrasonic sensor 17, it can be determined that the liquid tank 1 has not been emptied, and then proceed to step S6.

[0037] In step S6, the heater 19 in the liquid tank 1 is started or the heating state of the heater 19 is maintained, and the heating state of the heater 19 is monitored.

[0038] Next, in step S7, it is determined whether the heating state of the heater 19 is stable heating. For example, this determination is made by whether the temperature rise rate of the heater 19 exceeds a predetermined temperature rise rate, whether the operating current decrease rate of the heater 19 exceeds a predetermined current decrease rate, or whether the operating current value of the heater 19 is lower than a predetermined minimum current value.

[0039] If it is determined in step S7 that the heater 19 is in a stable heating state (e.g., the temperature rise rate does not exceed the predetermined temperature rise rate, the operating current decrease rate does not exceed the predetermined current decrease rate, or the operating current value of the heater is not lower than the predetermined minimum current value), it can be determined that the liquid tank 1 is overfilled, and then proceed to step S8.

[0040] In step S8, the user is reminded that there is sufficient exhaust gas treatment fluid available in the liquid tank 1 and that the heater 19 can maintain its heating state (e.g., at low temperatures).

[0041] On the other hand, if it is determined in step S7 that the heater 19 is in an unstable heating state (for example, the temperature rise rate exceeds the predetermined temperature rise rate, the operating current decrease rate exceeds the predetermined current decrease rate, or the operating current value of the heater 19 is lower than the predetermined minimum current value), it can be determined that the liquid level in the liquid tank 1 has dropped below the ultrasonic emission part of the first ultrasonic sensor 17 but has not been emptied, and then proceed to step S9.

[0042] In step S9, heater 19 is turned off, and an alarm is issued to remind the user to replenish the exhaust gas treatment fluid in tank 1. For SCR systems with automatic filling function, the filling operation of tank 1 can be initiated.

[0043] In addition, to avoid false readings, other steps can be performed before executing steps S1 to S9. For example, before executing steps S1 to S9, check whether the first ultrasonic sensor 17, the second ultrasonic sensor 25, and the heater 19 are powered on and / or whether any errors are reported. Furthermore, when the liquid in tank 1 solidifies due to low temperature (e.g., the freezing point of urea solution is -11°C), the first ultrasonic sensor 17 will also be unable to detect the liquid level. Therefore, the temperature in tank 1 can be detected before executing steps S1 to S9. If the temperature in tank 1 is below the freezing point, the heater 19 can be activated first, followed by executing steps S1 to S9.

[0044] It should be further understood that, such as Figure 2The method shown can be executable program instructions stored on a machine-readable non-volatile storage medium. It should also be understood that the control unit 20 may have a memory and a processor, wherein the memory may store executable program instructions that, when executed, cause the processor to perform actions such as... Figure 2 The method shown.

[0045] It should be noted that the SCR system for exhaust gases, particularly for diesel vehicles, its control unit, and related methods have been described above. These systems can determine whether the reservoir 1 is empty when the ultrasonic level sensor fails to detect the reservoir level. Furthermore, the aforementioned technology can reliably determine the following filling conditions of the reservoir 1: 1) the reservoir 1 is empty; 2) the reservoir 1 is overfilled; 3) the liquid level in the reservoir 1 has dropped below the ultrasonic wave emission point of the ultrasonic level sensor. This allows for different responses to different filling conditions, ensuring the normal operation of the vehicle system. Therefore, the scope of this application covers various scenarios where the filling condition is determined based on an ultrasonic sensor in the vehicle's reservoir and, possibly, a heater.

[0046] It should also be understood that the first ultrasonic sensor 17 and the second ultrasonic sensor 25 can be the same sensor. In this case, an ultrasonic diverter can be used to enable the sensor to simultaneously perform the functions of the first ultrasonic sensor 17 and the second ultrasonic sensor 25.

[0047] The present application has been described in detail above with reference to specific embodiments. Obviously, the above description and the embodiments shown in the accompanying drawings should be understood as exemplary and not as limiting the present application. Those skilled in the art can make various modifications or alterations without departing from the spirit of the present application, and such modifications or alterations do not depart from the scope of the present application.

Claims

1. A method for monitoring the liquid level in a vehicle liquid tank (1), the liquid tank (1) having a first ultrasonic sensor (17) and a second ultrasonic sensor (25) mounted on the bottom (21) of the liquid tank (1), the first ultrasonic sensor (17) being used to detect the liquid level in the liquid tank (1) by emitting ultrasonic waves toward the top (23) of the liquid tank (1) and receiving echo signals, the second ultrasonic sensor (25) being used to determine whether there is liquid below the ultrasonic emission portion of the first ultrasonic sensor (17) by emitting ultrasonic waves along the bottom (21) and receiving echo signals, the liquid tank (1) further comprising a heater (19) for heating the liquid in the liquid tank (1), the heater (19) being arranged at a position higher than the ultrasonic emission portion of the first ultrasonic sensor (17), the method comprising the steps of: The first ultrasonic sensor (17) is made to emit ultrasonic waves and receive the first echo signal to detect the liquid level in the liquid tank (1); When the first ultrasonic sensor (17) does not receive the first echo signal for a period of time, the second ultrasonic sensor (25) is made to emit ultrasonic waves and receive the second echo signal. Based on the second echo signal, determine whether the liquid tank (1) is empty; When it is determined based on the second echo signal that there is no liquid below the ultrasonic emission part of the first ultrasonic sensor (17), the liquid tank (1) is determined to be an empty tank. When it is determined based on the second echo signal that there is liquid below the ultrasonic emission part of the first ultrasonic sensor (17), the heater (19) is started or maintained to heat the liquid in the liquid tank (1), and the heating status of the heater (19) is monitored. When it is determined that the heating state of the heater (19) is stable heating, it is determined that the liquid tank (1) is overfilled so that there is no interface between the liquid and the air in the liquid tank; When it is determined that the heating state of the heater (19) is unstable heating, it is determined that the liquid level in the liquid tank (1) has dropped below the ultrasonic emission part of the first ultrasonic sensor (17) but has not been emptied.

2. The method as described in claim 1, characterized in that, The method further includes monitoring the heating state of the heater (19) based on the heating temperature of the heater (19), determining that the heater (19) is in an unstable heating state when the rate of increase of the heating temperature exceeds a predetermined temperature rise rate value, and / or when the heating temperature exceeds a predetermined temperature value, and determining that the heater (19) is in an unstable heating state when the rate of increase of the heating temperature does not exceed a predetermined temperature rise rate value, and / or when the heating temperature does not exceed a predetermined temperature value.

3. The method as described in claim 1, characterized in that, The method further includes monitoring the heating state of the heater (19) based on the change of the operating current of the heater (19). When the rate of change of the operating current exceeds a predetermined rate of change of current value, or when the operating current reaches a predetermined limit current value, the heater (19) is determined to be in an unstable heating state. When the rate of change of the operating current does not exceed a predetermined rate of change of current value, or when the operating current does not reach a predetermined limit current value, the heater (19) is determined to be in a stable heating state.

4. The method as described in claim 1, characterized in that, The liquid tank (1) is the exhaust gas treatment liquid tank in the vehicle SCR system, and the liquid in the liquid tank (1) is exhaust gas treatment liquid.

5. A machine-readable non-volatile storage medium having stored on it program instructions for implementing the method as described in any one of claims 1 to 4.

6. A control unit (20) having a memory and a processor, the memory storing executable program instructions that, when executed, cause the processor to perform the method as claimed in any one of claims 1 to 4.

7. A vehicle SCR system, comprising a tank (1) for containing exhaust gas treatment fluid, an injection device (3) for injecting exhaust gas treatment fluid into an exhaust pipe, a delivery device (5) for drawing exhaust gas treatment fluid from the tank (1) and pressurizing and supplying it to the injection device (3), and a control unit (20) as claimed in claim 6.

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