Method for estimating a thawed volume present in liquid form in a tank

Abstract

A method for estimating a volume of thawed liquid in a motor vehicle tank, wherein the following operations are executed at regular time intervals: obtaining a temperature of the ambient air outside the tank using a thermometer; determining, according to said temperature outside the tank and by a first pre-established relation, a thermal energy transfer between the contents of the tank and the outside environment; determining, as a function of the power produced by a heating element, and by a second pre-established relation, a thermal energy transfer between the heating element and the contents of the tank; determining, as a function of the energy transfers, and by a third pre-established relation, an amount of thawed or refrozen liquid, during said time interval; the amounts of thawed and refrozen liquid are added during the preceding consecutive time intervals to estimate a volume of thawed liquid present in the tank.

Claims

1. A method for estimating a volume of thawed liquid forming part of a content located inside a motor vehicle tank including at least one heating element arranged inside the motor vehicle tank, an inside thermometer positioned in the motor vehicle tank to measure a temperature θ.sub.int0 of a liquid thawed inside the tank, an outside thermometer positioned out of the motor vehicle tank to measure a motor vehicle temperature θ.sub.exti of an ambient air outside the motor vehicle, at least one of submerged level sensors, a motor vehicle speed measuring means, a computer processing unit for calculating estimated values of the volume of thawed liquid and acquiring actual values of an inside temperature of the content of the motor vehicle tank, an outside temperature of the ambient air outside the motor vehicle, a motor vehicle speed, a liquid level, via data interchange means connected to the inside thermometer, the outside thermometer, the at least one submerged sensors, the motor vehicle speed measuring means, and including a means for storing a first pre-established relationship, a second pre-established relationship, a third pre-established relationship, pre-established values and variable values, the method comprising: determining, the motor vehicle having started, during a regular time intervals (ΔT.sub.i); detecting the actual temperature (θ.sub.exti) of the ambient aft outside the motor vehicle via the outside thermometer; estimating a heat-energy transfer (δE.sub.S) between the content of the motor vehicle tank and external surroundings by using the first pre-established relationship based on the actual temperature (θ.sub.exti) of the ambient aft outside the motor vehicle; estimating a heat-energy transfer (δE.sub.E) between the at least one heating element and the contents of the motor vehicle tank using the second preestablished relationship (R.sub.2(W)) based on a power (W) produced by the at least one heating element; estimating a quantity (δV.sub.i) of at least one of a thawed liquid and a refrozen liquid during the regular time interval (ΔT.sub.i) using a third preestablished relationship (R.sub.3(δE.sub.i)), based on the energy transfers (δE.sub.i=δE.sub.S+δE.sub.E) having been determined; and estimating a volume of the thawed liquid present in the motor vehicle tank (V.sub.wc=ΣδV.sub.i), based on a sum of the quantity of the thawed liquid and the quantity of the refrozen liquid during preceding successive time intervals.

2. The method as claimed in claim 1, further comprising: detecting an initial temperature (θ.sub.int0) inside the motor vehicle tank via the inside thermometer; comparing said initial temperature (θ.sub.int0) to pre-established temperature thresholds (θ.sub.S1, θ.sub.S2, θ.sub.S3); and determining an activation of the at least one heating element, and estimating an initial volume (V.sub.0) in liquid form present in the motor vehicle tank and a dead time (T.sub.DT) at the end of which an actual temperature (θ.sub.exti) of the ambient air outside the motor vehicle via the outside thermometer is determined for a first time interval (ΔT.sub.1), based on a function of a time (T.sub.P) for which the motor vehicle spent parked, and of a volume of liquid (V.sub.wcp) being present in the motor vehicle tank when the motor vehicle has been stopped before being parked.

3. The method as claimed in claim 2, wherein determining the activation of the at least one heating element, the value of the initial volume (V.sub.0) in liquid form present in the motor vehicle tank and the dead time (T.sub.DT) is executed in: i. activating said at least one heating element if the initial temperature (θ.sub.int0) inside the motor vehicle tank is above a first given threshold (θ.sub.S1), the initial volume (V.sub.0) is equal to an updated volume (V.sub.act) (V.sub.0=V.sub.act) and the dead time (T.sub.DT) is equal to zero (T.sub.DT=0), or if the initial temperature (θ.sub.int0) inside the motor vehicle tank is below the first given threshold (θ.sub.S1) greater than or equal to a second given threshold (θ.sub.S2) the at least one heating element is activated; and ii. estimating via the computer processing unit the dead time and the initial volume of the liquid if the initial temperature inside the motor vehicle tank (θ.sub.int0) is below a third given temperature threshold (θ.sub.S3) (θ.sub.Int0<θ.sub.S3), the initial volume (V.sub.0) is equal to zero (V.sub.0=0) and the dead time (T.sub.DT) is equal to a predetermined value (T.sub.DT=T.sub.0), or if the initial temperature inside the motor vehicle tank (θ.sub.int0) is comprised (θ.sub.S3<θ.sub.Int0<θ.sub.S2) between the third temperature threshold (θ.sub.S3) and a second given temperature threshold (θ.sub.S2) higher than the third threshold (θ.sub.S3), iii. estimating the quantity of the liquid at the moment of the previous stopping of the motor vehicle if an estimated volume of liquid present in the motor vehicle tank at the moment of the previous stopping of the vehicle (V.sub.WCP) is below a given threshold (V.sub.inf) (V.sub.WCP<V.sub.inf), the initial volume (V.sub.0) is equal to zero (V.sub.0=0) and the dead time (T.sub.DT) is equal to said predetermined value (T.sub.DT=T.sub.0), or if said estimated volume of liquid present in the tank at the moment of the previous stopping of the vehicle (V.sub.WCP) is above said given threshold (V.sub.inf) (V.sub.WCP<V.sub.inf), iv. estimating the quantity of the liquid after the vehicle having parked if the time for which the vehicle has been parked (T.sub.p) is above a given threshold (T.sub.Pmax), the value of the initial volume (V.sub.0) is equal to zero (V.sub.0=0), and the dead time (T.sub.DT) is equal to zero, or if the time for which the vehicle has been parked (T.sub.p) is below said given threshold (T.sub.Pmax), the initial volume (V.sub.0) is equal to the estimated volume (V.sub.WCP) of liquid present in the motor vehicle tank at the moment of the previous stopping of the vehicle decreased by a volume (V.sub.R) of liquid that has refrozen during the time for which the volume has been parked (V.sub.0=V.sub.WCP−V.sub.R), and the dead time (T.sub.DT) is equal to zero (T.sub.DT=0).

4. The method as claimed in claim 2, further comprising: increasing the estimated volume of thawed liquid present in the motor vehicle tank by the value of the initial volume (V.sub.WC=V.sub.0+ΣδV.sub.i).

5. The method as claimed in claim 1, further comprising: obtaining a signal (ζ) from at least one level sensor present in the motor vehicle tank during the regular time interval (ΔT.sub.i); and assessing a validity of the signal: if the signal (ζ) is valid, evaluating an updated value (V.sub.act) for the volume of the liquid present in the motor vehicle tank via said at least one level sensor, and replacing the value of the estimated volume of thawed liquid (V.sub.WCP) obtained at the end of the regular time interval (ΔT.sub.i) with said updated volume value (V.sub.act), or if the signal (ζ) is invalid retaining the value for the estimated volume of the thawed liquid (V.sub.WC) having been obtained at the end of the regular time interval (ΔT.sub.i); and repeating determining the actual temperature of the ambient air outside the motor vehicle tank via the outside thermometer for a subsequent regular time interval (ΔT.sub.i).

6. The method as claimed in claim 1, wherein the pre-established first relationship (R.sub.1(θ.sub.exti, S.sub.i)) is dependent on the external actual temperature (θ.sub.exti) and on the motor vehicle speed (S.sub.i) which is obtained during determining the heat energy transfer between contents of the motor vehicle tank and the external surroundings.

7. The method as claimed in claim 1, wherein: first pre-established relationship (R.sub.1(θ.sub.exti)) to determine a heat-energy transfer (δE.sub.S) between the contents of the motor vehicle tank and the external surroundings, the second preestablished relationship (R.sub.2(W)) to determine a heat-energy transfer (δE.sub.E) between the at least one heating element and the contents of the motor vehicle tank, and the third preestablished relationship (R.sub.3(δEi)) to determine the quantity (δVi) of the thawed liquid and the refrozen liquid, are established experimentally.

8. The method as claimed in claim 1, wherein the liquid contained in the motor vehicle tank is at least one of water, urea dissolved in water, a ternary mixture made up of water, urea, and an alcohol.

9. The method as claimed in claim 8, wherein the alcohol forming the ternary mixture is selected from at least one of methanol, ethanol, ethylene glycol, and isopropanol.

10. A motor vehicle comprising a motor vehicle tank for storing a liquid comprising: at least one heating element; an inside thermometer positioned inside the motor vehicle tank to measure a temperature (θ.sub.int0) of the liquid inside the motor vehicle tank; an outside thermometer positioned out of the motor vehicle tank to measure a temperature (θ.sub.exti) of an ambient air outside the motor vehicle; at least one of submerged sensors; a motor vehicle speed measuring means; a computer processing unit connected by data interchange means to the inside thermometer, the outside thermometer the at least one of the submerged sensors the motor vehicle speed measuring means and comprising a means for storing a first preestablished relationship, a second preestablished relationship, a third preestablished relationship, pre-established values and variable values and coded instructions loaded into the computer processing unit to: determine, the motor vehicle having started, during a regular time interval (Δ.sub.ti) an actual temperature (θexti) of the ambient air outside the motor vehicle tank via the outside thermometer; estimate a heat-energy transfer (δE.sub.S) between a content of the motor vehicle tank of and external surroundings by using the first pre-established relationship based on the actual temperature (θ.sub.exti) of the ambient air outside the motor vehicle; estimate a heat-energy transfer (δE.sub.E) between the at least one heating element and the content of the motor vehicle tank using the second preestablished relationship (R.sub.2(W)) based on a power (W) produced by the at least one heating element; estimate a quantity (δV.sub.i) at least one of a thawed liquid and a refrozen liquid during the regular time interval (ΔTi) using the third preestablished relationship (R.sub.3(δE.sub.i)), based on the energy transfers (δE.sub.i=δE.sub.S+δE.sub.E) having been determined; and estimate a volume of the thawed liquid present in the motor vehicle tank (V.sub.WC=ΣδV.sub.i) based on a sum of the quantity of the thawed liquid and the quantity of the refrozen liquid during preceding successive time intervals.

Description

(1) The invention will be better understood from studying the attached figures, which are provided by way of examples intended to support the present description and which are entirely unlimiting, in which:

(2) FIG. 1 is a schematic view of a tank.

(3) FIG. 2 is a flow diagram indicating the various steps in the implementation of the invention.

(4) By way of an example on which to base the description which follows, the invention will more specifically concern itself with a method for calculating the volume present in liquid form in a tank containing urea dissolved in water or in an alcohol. However, it should be emphasized that the elements of the method apply mutatis mutandis to any other tank containing a liquid liable to pass from a solid phase to a liquid phase under the temperature conditions observed during ordinary use of the vehicle in which the tank is installed.

(5) FIG. 1 depicts a device for storing urea comprising a tank 1, inside which is arranged a plurality of heating elements 2 fixed, by way of example, to the lateral walls or to the bottom of the tank or else to the middle of the interior volume of the tank.

(6) The tank may also contain submerged sensors such as a level sensor 4 or a quality sensor 5. These sensors have the particular feature of emitting a valid signal when immersed in a pocket of liquid contained in the tank and of emitting an incoherent signal or of not emitting any signal at all when trapped within frozen liquid.

(7) The level sensor may be a capacitive effect sensor comprising measurement cells the electrical capacitance of which changes according to the liquid or solid state of the urea. The sensor may also be of the ultrasound type comprising a transducer positioned in such a way that the ultrasound produced by the transducer reflects off the interface separating the liquid from the gaseous (or solid) part sitting on top of the liquid. The reflected waves are analyzed by a reception means. When the sensor is trapped in urea in solid form, the echo generated by the reflection of the signal does not reach the reception means within a predetermined time interval, and the signal received is therefore considered to be invalid.

(8) The level sensor may also be of mechanical type and comprise a float, the rise of which indicates the volume contained in the tank. When the float is blocked in the ice, it emits a constant signal considered to be invalid.

(9) The quality sensor 5 operates in a similar way to the level sensor using ultrasound. The ultrasound emitted by the piezoelectric transducer reflects off a reflector 51, and makes a given number of outward-return trips between the reflector and the transducer, passing through the liquid on each pass. When the sensor is trapped in urea in solid form, the journey time of the sound wave does not reach the receiver in a predetermined time interval and the signal delivered is considered to be invalid.

(10) Conversely, when one of these sensors provides a measurement considered to be valid, it is possible to deduce therefrom that the reservoir contains a minimum volume of liquid V.sub.act. This updated volume V.sub.act may be the result of an estimate based on volume values stored in memory and acquired experimentally when the sensor is awakened, or indicative of the actual volume of liquid present in the tank when the entire contents of the tank are in phase liquid.

(11) When the tank contains at least one of these level or quality sensors, referred to in the broadest sense as submerged sensors, it will be possible to make use of the signal delivered as will be explained in detail later on in this description.

(12) The tank also comprises an injection pump 3 associated with a means of producing a signal indicative of the quantity of liquid injected in a given time interval, and a thermometer 6 making it possible to evaluate the temperature θ.sub.int0 of the liquid inside the tank. Once again it will be noted here that this temperature measurement is reliable when the entire contents of the tank are in liquid form.

(13) The submerged sensors 4 and 5, the inside thermometer 6 and the injection pump 3 are connected to a computer processing unit 9.

(14) The device also comprises a thermometer 7 external to the tank, positioned in the vehicle outside the passenger compartment and engine compartment proper, in order to measure the temperature θ.sub.exti of the ambient air outside the vehicle and outside the tank, and a means 8 for measuring vehicle speed. As a general rule, these instruments are connected to the central processor 10 of the vehicle. The computer processing unit 9 therefore comprises a link to the central processor 10 in order to acquire the values of the external temperature and of the speed of the vehicle.

(15) The computer processing unit contains a memory in which are stored coded instructions which, when executed by the processing unit, make it possible to execute the steps of the method according to the invention.

(16) This software comprising said coded instructions may also be stored on a readable medium which is then loaded into said processing unit.

(17) FIG. 2 is flow diagram detailing the various steps in implementing the method.

(18) Implementation of the method involves two distinct phases. A first, reset, phase, during which the initial data at vehicle startup or after a short-term stoppage are determined, and formed of steps 101 to 107. This setup phase is succeeded by a phase of actually scrutinizing or evaluating the quantity of liquid present in the tank, and formed of steps A to E (201 to 206). This evaluation phase is executed in a loop at regular time intervals ΔT.sub.i.

(19) Upon vehicle startup, when the ignition key is inserted and the units of the vehicle are activated, a value for the temperature θ.sub.int0 prevailing inside the tank is obtained using the thermometer 6. This initial internal temperature θ.sub.int0, even though its value can be considered to be very much compromised with errors when the tank is frozen or partially frozen, is, as a general rule, measured at a point close to the injection pump pickup point, and remains indicative of the temperature prevailing around this point and of the possible presence of liquid at this level.

(20) When (101) the temperature inside the tank θ.sub.int0 is above a first given temperature threshold θ.sub.S1, the tank is considered to contain only liquid and the volume present in the tank is then measured directly using the submerged level sensor 4. It will be seen here that the measurement of this temperature is thus relatively reliable.

(21) By way of example, for a tank containing urea, this first temperature threshold θ.sub.S1 may usefully be fixed at 5° C. Otherwise, the volume V.sub.0 comprised in the tank is considered to be entirely in liquid form and equal to the value V.sub.act given by the level sensor 4. The method then passes on directly to the scrutinizing phase.

(22) When the temperature inside the tank θ.sub.int0 is below this first temperature threshold θ.sub.S1, the heating elements 2 are activated (102).

(23) The method therefore envisages a series of setup steps the purpose of which is to determine an initial volume of liquid V.sub.0 present in the tank, and a dead time T.sub.DT used for delaying the start of the scrutinizing phase.

(24) The value of the dead time T.sub.DT during which the starting of the scrutinizing phase during which an estimated volume of liquid is suspended is equal to a zero value or a non-zero value preestablished experimentally. When this value is non-zero it corresponds to the heating time T.sub.1 needed for a first quantity of liquid to appear. By way of example, considering a temperature of −40° C., the value T.sub.1 of the dead time is of the order of 5 to 6 minutes, depending on the power of the heating elements installed in the tank.

(25) When (103) the initial temperature θ.sub.int0 inside the tank is below a third temperature threshold θ.sub.S3 corresponding, for example, to the thawing temperature, it will be considered that a significant proportion of liquid is in solid form and that the initial volume V.sub.0 is equal to zero and that the dead time is equal to T.sub.1. For the case of urea, this third threshold θ.sub.S3 corresponds to a temperature of −9° C.

(26) When (104) the initial internal temperature θ.sub.int0 is comprised between a second threshold θ.sub.S2 and the third temperature threshold θ.sub.S3, it will be considered that the initial volume of liquid present in the tank is dependent on the estimated or actual volume V.sub.WCP the last time the vehicle was stopped. The value of the second temperature threshold θ.sub.S2 is generally equal to the value of the first threshold θ.sub.S1. For the case of urea, this second threshold θ.sub.S2 may usefully be fixed at 5° C.

(27) If this volume V.sub.WCP is below a given threshold V.sub.inf, then a prudent evaluation of the volume V.sub.0 will be adopted, this volume then being considered to be zero. The dead time T.sub.DT is then equal to T.sub.1 (105).

(28) When the volume V.sub.WCP is above V.sub.int, a value is obtained for the time T.sub.P spent parked that has elapsed between the previous stopping and the restarting of the vehicle.

(29) If (106) this time spent parked T.sub.P is above a given threshold T.sub.Pmax the value of the initial volume V.sub.0 is then considered to be zero and the dead time T.sub.DT is equal to zero. This is because although the temperature θ.sub.int0 inside the tank is above the freezing point θ.sub.S3, the variations in temperature during the time spent parked are uncontrolled and do not make it possible to determine a value for the initial volume. By way of example, the threshold for the time spent parked T.sub.Pmax may beneficially be of the order of two hours.

(30) If (107), the time spent parked T.sub.P is below the preestablished threshold T.sub.Pmax, then it will be considered that, throughout the duration of the stop, the heating elements are deactivated and the contents of the tank receive no incoming energy (E.sub.E=ΣδE.sub.E=0).

(31) In order to calculate the value of the outgoing energy transferred between the contents of the tank and the external surroundings during the time spent stopped, an external temperature value that corresponds to the harshest conditions is adopted. In the specific case of urea, this temperature is equal, by way of example, to −40° C. Using a first relationship R.sub.i(θ.sub.exti) that for a given time interval links an external temperature θ.sub.exti and the heat-energy transfer δE.sub.S between the contents of the tank and the external surroundings, a total quantity of energy lost by the contents of the tank and which is negative, E.sub.S=ΣδE.sub.S with δE.sub.S=R.sub.1(−40° C.) is determined for the duration spent parked. The energy balance E=E.sub.E+E.sub.S is a negative balance.

(32) Use is then made of a third relationship R.sub.3(E) that links, for a given time interval, a balance of the energy exchanged between the contents of the tank and a quantity of liquid that has frozen or thawed. Because the energy balance is negative, the value obtained corresponds to a quantity of liquid V.sub.R that has refrozen during the time spent parked. This value V.sub.R then needs to be deducted from the value of liquid V.sub.WCP present in the tank at the time of the previous stop in order to form the value of the initial volume V.sub.0=V.sub.WCP−V.sub.R.

(33) The strategy for determining the volume V.sub.0 and the dead time T.sub.DT as detailed hereinabove comes from an experimental approach and may undergo numerous arrangements in which the number of significant temperature thresholds, in this instance equal to three thresholds, is increased or decreased. Likewise, the number and value of the thresholds adopted for the times spent parked may be adapted at will.

(34) After having determined the initial volume V.sub.0 present in liquid form in the tank and a dead time T.sub.DT, and after having waited for a duration equal to said dead time (108), the method moves on to the scrutinizing phase during which the estimated volume of thawed liquid V.sub.WC present in the tank is estimated dynamically.

(35) Under the action of the heating elements, the urea progressively changes from solid form to liquid form. Further, under certain circumstances, it is also possible to see urea reappear in solid form.

(36) The evaluation phase may then begin to be executed in a loop.

(37) Beginning with the first time interval ΔT.sub.1, and then at successive regular and constant time intervals ΔT.sub.i, during a step A (201), the value of the temperature θ.sub.exti of the ambient air prevailing outside the tank and indicative of the temperature outside the vehicle is acquired using the thermometer 7 arranged outside the tank.

(38) During a step B (202), a value of the outgoing energy δE.sub.S exchanged during this time interval between the contents of the tank and the external surroundings is determined using the first relationship R.sub.1(θ.sub.exti). Optionally, it is advantageous also to take account of the vehicle speed S.sub.i in order to improve the precision with which this exchange is evaluated. A value for the loss δE.sub.S=R.sub.1(θ.sub.exti, S.sub.i) is obtained. When this speed is not known it is considered by default that the vehicle is running at high speed, for example at 140 km/h, so that the energy loss is increased by default. This energy transfer δE.sub.S is a loss, and therefore has a negative value.

(39) The relationship R.sub.i(θ.sub.exti) is a relationship preestablished experimentally following campaigns of measurements, linking the external temperature θ.sub.exti with the quantity of energy δE.sub.S. Optionally, it may be advantageous to take the vehicle speed S.sub.i during the time interval ΔT.sub.i considered into consideration. The relationship R.sub.1 then becomes a relationship of the type R.sub.1(θ.sub.exti, S.sub.i). The results obtained are specific to the shape of the tank and to where it is sited within the vehicle, and to the nature of the liquid contained in the tank. This relationship R.sub.1(θ.sub.exti) may take the form of a table of results, of curves or else of mathematical formulae stored in the memory of the computer processing unit 9.

(40) The energy value adopted is the value that corresponds to the most severe winter conditions so that the corresponding energy loss is the maximum loss likely to be observed.

(41) Next, in step C (203), the value of the energy transferred to the contents of the tank by the heating elements 2 is determined using a second relationship R.sub.2(W). This value is dependent on the power W delivered by the heating elements. δE.sub.E=R.sub.2(W) during the time interval ΔT.sub.i. This incoming energy transfer has a positive sign. It will be noted here that the power delivered by the heating elements may vary according to the power available in the battery. Likewise, when the vehicle is fitted with the “Stop and Start” function, certain manufacturers deactivate the heating elements in order to avoid excessive electricity consumption.

(42) The relationship R.sub.2(W) is also a relationship that is preestablished experimentally following measurement campaigns carried out according to the heating power incorporated into the tank. The results obtained are specific to a given shape of tank, and to the nature of the liquid contained in the tank. This relationship R.sub.2 may take the form of a table of results, curves or even of mathematical formulae stored in the memory of the computer processing unit 9.

(43) In step D (204), the balance of total energy transferred to the contents of the tank during the time interval ΔT.sub.i considered, and δE.sub.i=δE.sub.E+δE.sub.S, is calculated.

(44) This balance is positive as a general rule, which means that the quantity of energy transferred to the liquid accelerates the thawing. However, when the power of the heating elements is deliberately limited in order to save battery, or else under certain cold conditions not representative of realistic conditions, it is possible to see negative balances leading to a quantity of liquid freezing.

(45) Next (205), using the third relationship R.sub.3(δE.sub.i).sub.1 the quantity of liquid δV.sub.i that has frozen or thawed during the time interval ΔT.sub.i is calculated according to the energy balance δE.sub.i obtained.

(46) The relationship R.sub.3(δE.sub.i) is also obtained experimentally by cross comparison of the results of the previous two experimental campaigns.

(47) These experimental measurements include variable heating powers. The quantity of liquid obtained is measured in order to determine the energy actually transferred to the contents of the tank, by considering that the energy exchanged with the external surroundings corresponds to the relationship R.sub.1(θ.sub.exit) and that the relationship R.sub.2(W) corresponds to the energy supplied by the heating elements. In a similar way to the relationships R.sub.1(θ.sub.exit) and R.sub.2(W), the relationship R.sub.3(δE.sub.i) may take the form of tables, curves or mathematical relationships stored in the memory of the computer processing unit. The relationship R.sub.3(δE.sub.i) adopted is the one that corresponds to the most severe conditions (endogenic and environment conditions), so that the true thawing efficiency will always be higher than the assumed efficiency.

(48) This elemental volume is either positive, if the quantity of energy supplied is sufficient to thaw the urea, or negative if, during the time period considered, some ice has re-formed.

(49) In step E (206), the estimated total volume V.sub.WC of thawed liquid present in the tank V.sub.WC=V.sub.0+ΣδV.sub.i, is calculated.

(50) As has been indicated hereinabove, the experimental relationships R.sub.1(θ.sub.exti), R.sub.2(W) and R.sub.3(δE.sub.i) have been determined so that the estimated quantity of liquid is a quantity deliberately estimated by default and lower than the quantity of liquid actually present in the tank.

(51) Hence, it may seem advantageous to make use of the volume indications provided by the submerged sensors.

(52) During a step F, after having calculated the value V.sub.WC, a signal ζ is obtained (301) from one or more submerged sensors. According to the arrangements explained above, the validity of this signal is estimated (302). What is understood here as being valid is that this signal is characteristic of the behavior of the submerged sensor in a pocket of liquid. Contrastingly, a signal that is invalid means that the signal is characteristic of the behavior of the sensor when the sensor is trapped in ice.

(53) When this signal coming from the level sensor 4 is considered to be valid (303), then an estimated or actual value V.sub.act of the volume of liquid present in the reservoir is determined and this value is substituted for the value V.sub.WC previously calculated. This value V.sub.act may therefore represent the actual value of liquid present in the tank and measured by the submerged level sensor 4 or alternatively may represent a value estimated from values, determined by tests and stored in memory, correlating the signal from the sensor with the default quantity of liquid present in the tank.

(54) As has already been mentioned, the relationship R.sub.1, (θ.sub.exti), the relationship R.sub.2(W) and the relationship R.sub.3(δE.sub.i) are relationships obtained experimentally which make it possible to determine the value V.sub.WC and which are established in such a way as to give a liquid volume value estimated by default. This value V.sub.WC is therefore, in practically all instances, lower than the value V.sub.act which is itself lower than or equal to the value of the volume of liquid actually contained in the tank.

(55) When the signal ζ is not considered to be valid (304), the value V.sub.WC obtained is retained.

(56) The method continues by resuming acquisition of an external temperature θ.sub.exti for a subsequent time interval ΔT.sub.i+i.

(57) It will be noted that, when the tank contains a sufficient quantity of liquid for the level sensor to provide a valid indication corresponding to the value of the volume of urea in liquid form actually present in the tank and equal to the updated value V.sub.act, the estimated value V.sub.WC is replaced, in each time interval, by the updated value V.sub.act, and therefore remains limited to that value. The method can therefore be applied without interruption.

(58) It may also be considered that it is no longer necessary to continue to calculate the estimated value of the volume of liquid V.sub.WC when the temperature inside the tank θ.sub.int is above the first temperature threshold θ.sub.S1. In that case, it becomes necessary to acquire a value θ.sub.int for the temperature inside the tank when, for example, the estimated value V.sub.WC is replaced in several successive time intervals by the updated value V.sub.act.

(59) As has already been mentioned, how the laws R1, R2, R3 are determined is dependent on the shape and siting of the tank and also on the nature of the liquid contained therein. Although the foregoing description discloses a tank containing urea, it is entirely possible to adapt the method to a tank containing water, urea dissolved in water, or what is commonly referred to as a ternary mixture, containing urea, water and an alcohol.

(60) The alcohol that forms the ternary mixture may beneficially be selected from alcohols such as methanol, ethanol, ethylene glycol or isopropanol.

(61) In the case of a tank containing urea used to treat exhaust gases, knowing the estimated volume V.sub.WC of liquid present in the tank allows the injection pump connected to the gas treatment device to be activated as early on as possible or, conversely, makes it possible to limit this throughput in the event of there being insufficient liquid in the tank. When the pump is activated, care is taken to determine the quantity of liquid V.sub.Inj injected, so as to deduct this volume from the volume of thawed liquid V.sub.WC calculated according to the above method. The new value for the volume of liquid V.sub.WC′ is then equal to V.sub.WC-V.sub.Inj. So, the method according to the invention makes it possible as early on as possible and as effectively as possible to manage an exhaust gas treatment strategy when the vehicle is experiencing extremely low temperature conditions.

(62) The method described hereinabove therefore makes it possible to provide a dynamic and evolving model for the management of a tank containing a liquid liable to freeze under normal conditions of use while at the same time circumventing uncertainties regarding the measuring of the temperature inside the tank.

KEY TO PARTS

(63) 1 Tank. 2 Heating element. 3 Injection pump. 4 Submerged level sensor. 5 Submerged quality sensor. 51 Reflector. 6 Thermometer arranged inside the tank. 7 Thermometer arranged outside the tank. 8 Means for measuring vehicle speed. 9 Computer processing unit. 10 Vehicle central computer. 101 to 108 Setup: determining the volume V.sub.0 and the dead time T.sub.DT. 201 to 206 Determining the estimated quantity of liquid present in the tank. 301 to 303 Updating the estimated volume of liquid. δE.sub.S Transfer of heat energy between the contents of the tank and the external surroundings during the time interval ΔT.sub.i. δE.sub.E Transfer of heat energy between the heating elements and the contents of the tank during the time interval ΔT.sub.i. δE.sub.i Balance of energy exchanged with the contents of the tank during the time interval ΔT.sub.i. R.sub.1 First preestablished relationship linking the external temperature (θ.sub.exti) and the heat-energy transfer (δE.sub.S) between the contents of the tank and the external surroundings. R.sub.2 Second preestablished relationship linking the power (W) produced by the heating elements and a heat energy transfer (δE.sub.E) between the heating elements and the contents of the tank. R.sub.3 Third preestablished relationship linking the energy balance (ΔE) and the quantity (δV.sub.i) of liquid thawed or refrozen. S.sub.i Vehicle speed during the time interval ΔT.sub.i considered. T.sub.DT Dead time. T.sub.1 Preestablished non-zero duration. ΔT.sub.i Time intervals. ΔT.sub.1 First time interval during which step A is executed. T.sub.P Time spent parked. T.sub.Pmax Preestablished threshold of time spent parked. θ.sub.int0 Initial temperature inside the tank. θ.sub.S1 First preestablished temperature threshold. θ.sub.S2 Second preestablished temperature threshold. θ.sub.S3 Third preestablished temperature threshold. θ.sub.exti Temperature of the ambient air outside the tank for a time interval ΔT.sub.i considered. V.sub.0 Initial volume in liquid form. V.sub.R Value for liquid refrozen during the time spent parked. δV.sub.i Quantity of liquid frozen or thawed during the time interval ΔT.sub.i considered. V.sub.WC Estimated volume of thawed liquid present in the tank. V.sub.WCP Estimated volume of liquid present in the tank at the moment of the previous stopping of the vehicle. V.sub.inf Preestablished volume threshold. V.sub.act Updated volume of liquid present in the tank as estimated using a submerged sensor. V.sub.Inj Volume of liquid injected by the injection pump during the time interval ΔT.sub.i considered. ζ Signal from a submerged sensor.