SE535889C2 - Procedures and systems for monitoring the quality deterioration of oil - Google Patents

Abstract

Method and system for monitoring deterioration of the quality of oil which accumulates in a gathering space (2) in a motor vehicle.In a situation where the oil in the gathering space is allowed to cool to its gelling point, a first temperature value and a second temperature value which represent the temperature of the oil in respective first and second regions of the gathering space are generated. Gelling of the oil is detected on the basis of the difference between these temperature values. When gelling is detected, a gelling point value which represent the oil's gelling point is determined on the basis of one or more measured values for the temperature of the oil in the gathering space at the time when the gelling is detected. The gelling point value is then used to determine whether or not the quality of the oil has deteriorated to such an extent that it needs changing.

Description

535 885 facing a corresponding annular recess of the stator to form a toroidal space between the rotor and the stator.

A number of vanes are arranged in the recesses of the rotor and stator. When the retarder is to exert a braking action on the driving shaft, retarder oil is pumped into the toroidal space from a collecting container. During the rotation of the rotor relative to the stator, the retarder oil will flow between the vanes of the stator and the rotor while generating a braking action on the rotor, whereby the driving shaft is braked. When the retarder's braking effect is to cease, the retarder oil is returned to the collection container.

In order to function satisfactorily, the oil used as a lubricant in an internal combustion engine, gearbox or rear axle and the oil used as a brake medium in a retarder must have a suitable viscosity. For example, oil used as a lubricant must not have too low a viscosity. If the viscosity becomes too low, the oil becomes so liquid that it no longer gives a satisfactory lubricating effect, which entails a risk of mechanical oatmeal. The oil must also not have too high a viscosity.

If the viscosity becomes too high, the oil becomes so viscous that it becomes difficult or in the worst case impossible to pump the oil through the lubricant channels in an internal combustion engine or gearbox or to and from the toroidal space in a retarder. This can lead to a lack of lubrication and mechanical breakdown in an internal combustion engine or gearbox and to a lack of braking action in a retarder.

The viscosity of the oil changes for various reasons. This change in viscosity can occur gradually, for example due to oxidation. In some cases, the oil can also be subjected to a sharp and sudden change in viscosity, for example if water or fuel comes into contact with the oil and causes its dilution.

An oil is continuously oxidized. The oxidation increases with increasing temperature of the oil and constitutes a significant problem with regard to engine oil and gearbox oil. Engine oil also risks being exposed to soot that penetrates the oil and causes increased viscosity of the oil, and to fuel that leaks into the oil and causes a reduced viscosity of the oil.

Rear axle oil is gradually degraded due to the shear of the oil between the gears of the rear axle differential gear, which leads to gradually increasing viscosity of the oil.

Retarder oil is gradually degraded due to the very high temperatures that the oil is subjected to during retarder braking, which leads to a gradually increasing viscosity of the oil.

When an oil in a motor vehicle deteriorates too much and risks not being able to fulfill its task, it must be replaced with new oil. Such an oil change can, for example, take place at certain time intervals which are of a predetermined length or which are calculated on the basis of relevant parameters, such as, for example, mileage and oil temperature. Since the time interval must be set with a certain safety margin in order not to risk accidents and other problems, the oil will in many cases be replaced while it is still fully functional, which leads to unnecessarily large oil consumption. In order to be able to determine with better precision when engine oil needs to be changed, various types of systems for monitoring the quality deterioration of engine oil have been developed. Through, for example, DE 100 13 590 A1 and EP 2 159 387 A1, it is previously known to measure the viscosity of engine oil with the aid of a viscosity meter, whereby the need for oil change is determined by analysis of measured viscosity values. By US 2005/0039521 A1 it is previously known to calculate the viscosity of engine oil on the basis of measured values regarding oil pressure, oil temperature and engine speed, whereby the need for oil change is determined by analysis of calculated viscosity values.

OBJECT OF THE INVENTION The object of the present invention is to provide a new and advantageous way of monitoring the deterioration in the quality of oil which is collected in a collection space in a motor vehicle. SUMMARY OF THE INVENTION According to the present invention, said object is achieved by means of a method having the features defined in claim 1 and a system having the features defined in claim 9.

According to the invention: - is generated continuously, in a situation where the oil in the collection space is allowed to cool from a temperature above the gelation temperature of the oil to a temperature corresponding to or lower than the gelation temperature of the oil and by means of measured values from temperature sensors, a first temperature value representing the temperature of the oil in a first region of the collecting space and a second temperature value representing the temperature of the oil in a second region of the collecting space, - a difference value representing the prevailing difference between the first the temperature value and the second temperature value, this difference value being compared with a limit value for detecting gel formation in the oil, - when gel formation is detected, a gel formation temperature value representing the gel formation temperature of the oil is determined, this gel formation temperature value being determined from n one or more measured values regarding the temperature of the oil in the collection space at the time of the detected gel formation, and - the gel formation temperature value is used to determine whether the quality of the oil has deteriorated to such an extent that there is a need for oil change.

When the oil in the collecting space cools, heat will be emitted from the oil via the side walls and bottom of the collecting space and via the oil surface. The gradual cooling at the edges of the oil volume in the collection space leads to density differences in the oil, which in turn leads to circulation of the oil in the collection space and heat transport by natural convection. As long as the temperature of the oil in the collection space is above the gelation temperature of the oil, the natural convection will lead to an equalization of the temperature of the oil in the collection space. The natural convection thus counteracts temperature stratification of the oil in the collection space, which means that there will be no significant temperature differences between the different areas of the oil in the collection space that have not yet been subjected to gel formation. However, when the oil in the collection space has reached such a low temperature that the oil is subjected to gel formation, the natural convection and the associated temperature equalization cease, which leads to more noticeable temperature differences in the oil in different parts of the collection space. By registering such temperature differences, it thus becomes possible to detect gel formation of the oil in the collection space. The solution according to the invention is furthermore based on the fact that a change in the quality of an oil leads to a changed gel formation temperature of the oil.

With the solution according to the invention, it becomes possible in a simple manner to monitor the deterioration of the quality of an oil in a motor vehicle and determine when oil change needs to take place, without the need for any other types of sensors than relatively simple and inexpensive temperature sensors. This monitoring can be performed under cold conditions when the ambient temperature is low enough to cool the oil to a temperature corresponding to or lower than the gel formation temperature of the oil, ie in a cold climate. Under conditions of warmer ambient temperature, the oil change intervals must be determined based on other criteria.

According to an embodiment of the invention, the gel formation temperature value is compared with an upper gel formation temperature limit value and / or a lower gel formation temperature limit value, wherein a warning signal or a message indicating the need for oil change is generated if said comparison shows that the gel formation temperature value is greater than or equal to the upper gel formation temperature limit or is less than or equal to the lower gel formation temperature limit. In this way, the determined gelation temperature value can be easily analyzed to determine whether the oil has acquired an inappropriately high or inappropriately low viscosity.

According to an alternative embodiment of the invention, gelation temperature values are determined at different times to obtain a series of gelation temperature values, these gelation temperature values being analyzed to determine whether the quality of the oil has deteriorated to such an extent that there is a need for oil change.

According to a further embodiment of the invention, preferably by means of linear regression or moving average value, a trend line is generated which shows how the gelation temperature value has changed with the operating time of the motor vehicle. Each new gel-forming temperature value determined is compared with the trend line of the previously determined gel-forming temperature values to determine whether the new gel-forming temperature value follows or deviates from the trend line, generating a warning signal or message indicating the need for oil change when it is determined to gelation temperature value or a given number of consecutive gelation temperature values deviate from said trend line. In this way, the last determined gel formation temperature value can be easily analyzed to find out if the oil has suddenly been subjected to a sharp increase or decrease in viscosity, for example due to fuel leakage or some other sudden cause of error, without having to resort to predetermined limit values. With the latter analysis method, however, it is not possible to detect that the oil has acquired an inappropriately high or inappropriately low viscosity as a result of a gradual change in the viscosity.

Other advantageous features of the method and system according to the invention appear from the dependent claims and the following description. The invention also relates to a computer program having the features defined in claim 13, a computer program product having the features defined in claim 14 and an electronic control unit having the features defined in claim 15.

BRIEF DESCRIPTION OF THE DRAWINGS The invention will be described in more detail below with the aid of exemplary embodiments, with reference to the accompanying drawings.

It is shown in: Fig. 1 a principle sketch of a motor vehicle with an internal combustion engine and a system according to the present invention for monitoring the quality deterioration of the internal combustion engine engine oil, Fig. 2 a trend line showing how the gelation temperature value of an oil changes with the operating time of a motor vehicle, Fig. 3 is a schematic diagram of an electronic control unit for implementing a method according to the invention, Fig. 4 is a flow diagram illustrating a method according to a first embodiment of the invention, and Fig. 5 is a flow diagram illustrating a method according to a second embodiment. of the invention.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION Fig. 1 schematically shows a system 10 according to the present invention for monitoring the deterioration of oil 1 which is collected in a collecting space 2 in a motor vehicle 3. In the example illustrated. the collection space 2 of an oil sump in an internal combustion engine 4 of the vehicle. In this case, the oil thus consists of engine oil which is used as a lubricant for moving parts of the internal combustion engine. The oil 1 is pumped by means of an oil pump 5 up from the collecting space 2 and further out through lubricant channels 6 in the internal combustion engine. From the lubricant channels 6, the oil flows back down into the collection space 2.

The system 10 according to the invention could also be arranged to monitor the deterioration of the quality of oil collected in a collecting space in some other part of the vehicle, such as for instance of gearbox oil which is collected in an oil sump in a gearbox of the vehicle and / or of rear axle oil that is collected in a space at the bottom of a rear axle of the vehicle and / or with retarder oil that is collected in a collection container adjacent to a hydrodynamic retarder of the vehicle.

When the internal combustion engine 4 is running, the oil 1 is heated by the heat generated by the internal combustion engine. When the internal combustion engine is then switched off, the oil 1 will gradually cool to a temperature corresponding to the ambient temperature. If the ambient temperature is sufficiently low, the temperature of the oil 1 during cooling will drop to a temperature at which the oil is subjected to gel formation. The temperature at which gel formation occurs in the oil, ie the gel formation temperature of the oil, depends, among other things, on the quality of the oil.

One or more first temperature sensors 7a are arranged to measure the temperature of the oil in a first region 2a in the collecting space 2 and one or more second temperature sensors 7b are arranged to measure the temperature of the oil in a second region 2b in the collecting space. In the embodiment illustrated in Fig. 1, said first and second areas 2a, 2b in the collecting space are located at a distance from each other in the horizontal direction.

However, they could alternatively be located at a distance from each other in the vertical direction or in both the horizontal and vertical directions.

The system 10 according to the invention comprises an electronic data processing device 11 which is connected to said temperature sensor 7a, 7b for receiving from them measured values regarding the temperature of the oil 1 in said first and second areas 2a, 2b in the collecting space.

The electronic data processing device 11 is arranged that, in a situation where the oil 1 in the collecting space 2 is allowed to cool from a temperature above the gel formation temperature of the oil to a temperature corresponding to or lower than the gel formation temperature of the oil and by means of measured values from said temperature sensor 7a , 7b, generate a first temperature value VT1 representing the temperature of the oil in said first region 2a in the collection space and a second temperature value VTZ representing the temperature of the oil in said second region 2b in the collection space. The data processing device 11 is further arranged to continuously during the cooling of the oil determine a difference value Vdiff representing the prevailing difference between the first temperature value VT1 and the second temperature value Vw, and to compare this difference value Vdiff with a limit value VG for detecting gel formation of the oil.

The difference value Vdjff is suitably calculated as the difference Vdiff = VT1-VT2 or the difference Vdiff = VTZ-Vn, where gel formation of the oil is detected when it is determined that the difference value Vdiff has an absolute amount greater than or equal to the limit value VG.

The limit value VG is chosen so that it is slightly greater than the small oil temperature differences that can normally occur between the two areas 2a, 2b in the collection space 2 during the cooling of the oil before the oil temperature has dropped so low that gel formation has occurred in the oil.

When the data processing device 11 has detected gel formation, i.e. when the above-mentioned comparison between the difference value Vdjff and the limit value V6 shows that the oil 1 in the collection space 2 has been subjected to gel formation, the data processing device 11 is arranged to, based on one or more measured values regarding the temperature of the oil in the collection space 2 at the time of the detected gel formation, determine a gel formation temperature value VTQQ. which represents the gel formation temperature of the oil.

The data processing device 11 is further arranged to use this gel formation temperature value VTQG. to determine whether the quality of the oil has deteriorated to such an extent that there is a need for oil change or not.

The gel formation temperature value Vfge, is suitably determined on the basis of the temperature measured of the oil in said first region 2a and / or second region 2b in the collection space 2 at the time of the detected gel formation. For example, said first temperature value V11 or said second temperature value V12 at the time of the detected gel formation, or an average of these two temperature values, may be selected as the current gel formation temperature value Vwe.

According to a first alternative, the data processing device 11 is arranged to compare the gel formation temperature value Vfge. with an upper gel formation temperature limit value V61 and / or a lower gel formation temperature limit value V62, and to generate a warning signal, for example in the form of an acoustic or visual signal, or a message indicating the need for oil change if said comparison shows that the gel formation temperature value VTQQ . is greater than or equal to the upper gelation temperature limit value V61 or that the gelation temperature value Vrgei is less than or equal to the lower gelation temperature limit value V62. In this way, for example, a driver in the vehicle 3 can be made aware that the oil 1 is in such a condition that it should be replaced.

The upper gel formation temperature limit value V61 can be determined, for example, by allowing a reference oil with elevated gel formation temperature to be received in the collection space 2 or in a collection space with a corresponding design and then, in the manner described above, determine the gelation temperature of this reference oil. The upper gel-forming temperature limit value V61 can be set to correspond to the value of the gel-forming temperature thus determined. The upper gel-forming temperature-limit value V01 could of course also be determined in another way.

The lower gel formation temperature limit value V62 can be determined, for example, by allowing a reference oil with reduced gel formation temperature to be received in the collection space 2 or in a collection space with a corresponding design and then, in the manner described above, determine the gel formation temperature for this reference oil. The lower gel formation temperature limit value V62 can be set to correspond to the value of the gel formation temperature thus determined. The lower gel formation temperature limit value VGZ could of course also be determined in another way.

According to a second alternative, the data processing device 11 is arranged to determine gel formation temperature values Vïge. at different times and in the manner described above to obtain a series of gel formation temperature values, and to determine by analysis of these gel formation temperature values whether the quality of the oil 1 has deteriorated to such an extent that there is a need for oil change or not. In the latter case, the data processing device 11 may, for example, be arranged to use said series of gel-forming temperature values to generate a trend line TL (see Fig. 2) which shows how the gel-forming temperature value V-ge. changed with the operating time peak of the motor vehicle, and then compare each new gelation temperature value vTge. with the trend line TL for the previously determined gel formation temperature values to determine whether the new gel formation temperature value follows the trend line or deviates from it. Operating time here refers to the accumulated value of the time during which the motor vehicle has been in operation since the last oil change. When it is determined that a gel-forming temperature value V-ge, or a given number of consecutive gel-forming temperature values deviates from trend line TL, the data processing device 11 is arranged to generate a warning signal, for example in in the form of an acoustic or visual signal, or a message indicating the need for an oil change. In this way, for example, a driver in the vehicle 3 can be made aware that the oil 1 is in such a condition that it should be replaced.

The above-mentioned trend line TL can, for example, be generated by means of linear regression or moving average. Trend line is continuously updated when determining a new gel-forming temperature value V-ge. The assessment of whether a new gel-forming temperature value V-ge. deviates from trend line TL or not based on predetermined criteria. One such criterion may be, for example, that a gel formation temperature value v-give. is judged to deviate from the trend line TL if the gel formation temperature value is not within the prevailing standard deviation from the trend line, or is not within a given multiple of the standard deviation, such as for example four times the standard deviation, from the trend line.

The above-mentioned data processing device 11 may be implemented by means of a single electronic control unit of the motor vehicle 3, as illustrated in Fig. 1. However, the data processing device 11 could alternatively be implemented by means of two or more mutually cooperating electronic control units of motor vehicle 3. Fig. 4 shows a flow chart illustrating a first embodiment of a method according to the present invention for monitoring the deterioration of the quality of oil 1 collected in a collecting space 2 in a motor vehicle 3. This method comprises the following steps: A1) in a situation where the oil in the collecting space 2 is allowed to cool from a temperature above the gel formation temperature of the oil to a temperature corresponding to or lower than the gel formation temperature of the oil is generated continuously, by means of measured values from temperature sensors 7a, 7b, a first temperature value V11 20 25 30 35 535 889 13 represents the temperature of the oil in a first region 2a in the collecting space 2 and a second temperature value VU representing the temperature of the oil in a second region 2b in the collecting space, B1) a difference value Vdiff representing the prevailing difference between the first temperature value VN and the second temperature value V12 is determined continuously during the cooling of the oil and is compared with a limit value V6 for detecting gel formation in the oil, C1) when gel formation has been detected, a gel formation temperature value Vfge is determined. representing the gel formation temperature of the oil, this gel formation temperature value being determined on the basis of one or more measured values regarding the temperature of the oil in the collection space 2 at the time of the detected gel formation, D1) the gel formation temperature value V-gei is compared with an upper gel formation value V a lower gelling temperature limit value V62, and E1) a warning signal or message indicating the need for oil change is generated if the comparison in step D1 shows that the gelling temperature value VTQG is greater than or equal to the upper gelling temperature limit value V61 or is less than or equal to the lower gelation temperature limit value V62. Fig. 5 is a flow chart illustrating a second embodiment of a method according to the present invention for monitoring the deterioration in the quality of oil 1 collected in a collecting space 2 in a motor vehicle 3. This method comprises steps A2-C2 corresponding to the above steps A1- C1 and in addition the following steps: D2) gel formation temperature value V-ge. is compared with a trend line TL for previously determined gel formation temperature values to determine whether the new gel formation temperature value follows the deviation line or deviates from it, E2) a warning signal or a message indicating the need for oil change is generated if the comparison in step D2 shows that the new 35 535 B89 14 gelation temperature value Vfge. deviates from the said trend line TL, F2) if the comparison in step D2 instead shows that the new gel formation temperature value VTge. If the mentioned trend line TL is updated, the trend line is updated based on the new gel formation temperature value.

Computer program code for implementing a method according to the invention is suitably included in a computer program which can be read into the internal memory of a computer, such as the internal memory of an electronic control unit of a motor vehicle. Such a computer program is suitably provided via a computer program product comprising a computer readable data storage medium, which data storage medium has the computer program stored thereon.

Said data storage medium is, for example, an optical data storage medium in the form of a CD-ROM, a DVD disc, etc., a magnetic data storage medium in the form of a hard disk, a floppy disk, a cassette tape, etc., or a flash memory or a ROM type memory. , PROM, EPROM or EEPROM. Fig. 3 very schematically illustrates an electronic control unit 20 comprising an execution means 21, such as a central processing unit (CPU), for executing computer software. The execution means 21 communicates with a memory 22, for example of the RAM type, via a data bus 23. The control unit 20 also comprises data storage medium 24, for example in the form of a flash memory or a memory of the type ROM, PROM, EPROM or EEPROM. The execution means 21 communicates with the data storage medium 24 via the data bus 23. A computer program comprising computer program code for implementing a method according to the invention, for example in accordance with any of the embodiments illustrated in Figs. 3 and 4, is stored on the data storage medium 24.

The invention is of course in no way limited to the embodiments described above, but a number of possibilities for modifications thereof should be obvious to a person skilled in the art, without this for that reason deviating from the basic idea of the invention as defined in the appended claims. .

Claims (1)

A method according to claim 1, 535 889 Method for monitoring the deterioration of the oil collected in a collection space (2) in a motor vehicle (3), characterized in that the method comprises the following steps: A) in a situation where the oil in the collection space (2) is allowed to cool from a temperature above the gel formation temperature of the oil to a temperature corresponding to or lower than the gel formation temperature of the oil is continuously generated, using measured values from temperature sensors (7a, 7b), a first temperature value (VH) representing the temperature of the oil in a first range ( 2a) in the collection space and a second temperature value (VTZ) representing the temperature of the oil in a second area (2b) in the collection space, B) a difference value (Vdiff) representing the prevailing difference between the first temperature value (VH) and the second the temperature value (VTZ) is determined continuously during the cooling of the oil and is compared with a limit value (VG) for detecting gel formation in the oil , C) when gel formation has been detected, a gel formation temperature value (vfgel) is determined which represents the gel formation temperature of the oil, this gel formation temperature value (V-ge.) Being determined on the basis of one or more measured values regarding the temperature of the oil in the collection space (2) at the time of the detected gel formation, and D) the gel formation temperature value (VTgg) is used to determine whether the quality of the oil has deteriorated to such an extent that there is a need for oil change or not. characterized in that the gel formation temperature value (v-ge.) is determined on the basis of the temperature measured of the oil in said first and / or second region (2a, 2b) in the collection space at the time of the detected gel formation,. Method according to claim 1 or 2, characterized in that gel formation of the oil is detected when it is determined that the difference value (Vdiff) has an absolute amount greater than or equal to the limit value (VG) . . Method according to one of Claims 1 to 3, characterized in that: - the gel formation temperature value (v-ge.) Is compared with an upper gel formation temperature limit value (V61) and / or a lower gel formation temperature limit value (V62), and - that a warning signal or a message indicating the need for oil change is generated if said comparison shows that the gel formation temperature value (v-ge.) is greater than or equal to the upper gel formation temperature limit value (VG1) or is less than or equal to the lower gel formation temperature limit value (V92) . Method according to claim 4, characterized in that said upper gel formation temperature limit value (V61) is determined by performing steps A-C with a reference oil with elevated gel formation temperature in the collection space (2) or in a collection space with a corresponding design. Method according to claim 4 or 5, characterized in that said lower gel formation temperature limit value (V92) is determined by performing steps A-C with a reference oil with reduced gel formation temperature in the collection space (2) or in a collection space with a corresponding design. A method according to any one of claims 1-3, characterized in that: - individual test cycles each comprising the steps AC are performed at different times to obtain a series of gel formation temperature values (vTgej), and - that by analyzing said gel formation temperature values (V-ge.) Determines whether the quality of the oil has deteriorated to such an extent that there is a need for an oil change or not. Method according to claim 7, characterized in that: - a trend line (TL) showing how the gelation temperature value (Vïgej) has changed with the operating time of the motor vehicle is generated, preferably by means of linear regression, or moving average, - that each new gel formation temperature value (VTW) is compared with the trend line (TL) of the previously determined gel formation temperature values to determine whether the new gel formation temperature value (V-ge.) follows the trend line or deviates from it, and - that a warning signal or a message indicating the need for an oil change is generated when it is determined that a gel formation temperature value (V-ge.) or a given number of consecutive gel formation temperature values deviates from said trend line (TL). . System for monitoring the deterioration of the oil collected in a collection space of a motor vehicle, characterized in that: - the system (10) comprises an electronic data processing device (11) which is arranged to, in a situation where the oil in the collection space is allowed to cool from a temperature above the gel formation temperature of the oil to a temperature corresponding to or lower than the gel formation temperature of the oil and by means of measured values from temperature sensors, generate a first temperature value (VM) representing the temperature of the oil in a first region of the collection space and a second temperature value (V12 ) representing the temperature of the oil in a second area of the collecting space, - that the data processing device (11) is arranged to continuously during the cooling of the oil determine a difference value (Vdjff) which represents the prevailing difference between the first temperature value (VH) and the second temperature value (VU), and to compare this ski value (VM) with a limit value (VG) for detecting gel formation of the oil, - that the data processing device (11) is arranged that, when detected gel formation and based on one or more measured values regarding the temperature of the oil in the collection space at the time of the detected the gel formation, determining a gel formation temperature value (Vfgej) representing the gel formation temperature of the oil, and - that the data processing device (11) is arranged to use the gel formation temperature value (VTgg) to determine whether the quality of the oil has deteriorated to such an extent that there is a need for oil change or not. System according to claim 9, characterized in that the data processing device (11) is arranged: - comparing the gel formation temperature value (V-ge.) With an upper gel formation temperature limit value (V61) and / or a lower gel formation temperature limit value (V62), and to generate a warning signal or a message indicating the need for oil change if said comparison shows that the gel formation temperature value (V-ge.) is greater than or equal to the upper gel formation temperature limit value (V61) or is less than or equal to the lower gelation temperature limit (V62). System according to claim 9, characterized in that the data processing device (11) is arranged: - to determine gel formation temperature values (VTW) at different times to obtain a series of gel formation temperature values, and - to determine by analysis of said gel formation temperature values (vTgeQ whether the quality of the oil has deteriorated to such an extent that there is a need for an oil change System according to claim 11, characterized in that the data processing device (11) is arranged: - to generate a trend line (TL) which shows how the gel formation temperature value (VTgg) changed with the operating time of the motor vehicle, - comparing each new gelling temperature value (Vfgei) with the trend line (TL) of the previously determined gelling temperature values to determine whether the new gelling temperature value (VTW) follows the trend line or deviates from this, and - to generate a warning signal or a message indicating the need for o ljebyte when it is determined that a gel-forming temperature value (VTW) or a given number of consecutive gel-forming temperature values deviates from the said trend line (TL). Computer programs comprising computer program code for causing a computer to implement a method according to any one of claims 1-8 when the computer program is executed on the computer. A computer program product comprising a computer storage medium readable by a computer, the computer program code of a computer program according to claim 13 being stored on the data storage medium. Electronic control unit of a motor vehicle comprising an execution means (21), a memory (22) connected to the execution means and a data storage medium (24) connected to the execution means, the computer program code of a computer program according to claim 13 being stored on said data storage medium (24) .