EP1922474A1

EP1922474A1 - System for determining rate of dilution of a motor vehicle heat engine lubricating oil by the supply fuel thereof

Info

Publication number: EP1922474A1
Application number: EP06808275A
Authority: EP
Inventors: Pascal Folliot; Piet Ameloot; Mehdi El Fassi
Original assignee: Peugeot Citroen Automobiles SA
Current assignee: PSA Automobiles SA
Priority date: 2005-09-05
Filing date: 2006-09-01
Publication date: 2008-05-21
Also published as: FR2890411B1; WO2007028919A1; FR2890411A1

Abstract

The invention concerns a system for determining rate of dilution of a motor vehicle heat engine lubricating oil by the supply fuel thereof during the operation of the engine using a delayed fuel injection in its cylinders, said system comprising means (12) for estimating the rate of dilution dependent on the current operating mode of the engine based on a dilution rate variation model. Said system includes means (18) for determining the current running mode of the vehicle and means (16) for selecting at least one parameter of the dilution rate variation model based on the determined running mode.

Description

System for determining the dilution ratio of the lubricating oil of a motor vehicle engine engine with feed fuel thereof.

The present invention relates to a system for determining the dilution ratio of the lubricating oil of a motor vehicle engine.

Motor vehicle exhaust lines now include pollution control means, such as NOx traps and particulate filters, which need to be regularly purged or regenerated.

For this purpose, the engine is then associated with means for controlling its operation to switch it between a standard lean mode operation and a rich regeneration mode operation.

In the standard lean mode operating mode, a NOx trap stores the nitrogen oxides contained in the exhaust gas and when this trap is saturated, its regeneration is triggered by switching the engine to the rich operating mode of destocking. NOx, or mode deNox, in which the engine produces and in the exhaust line of the reducers, such as for example CO and HC.

Similarly, to regenerate a NOx trap poisoned by sulfur oxides commonly contained in the fuel additives, or to regenerate a particulate filter by burning the soot particles it stores, an engine is also switched to a operating in rich mode to release in the exhaust line unburned hydrocarbons, such as HC, so that they are catalyzed, thereby causing a rise in temperature necessary for the regeneration of the NOx trap or particulate filter.

The tilting of the engine in a rich mode operation is conventionally performed by modifying at least one engine operation control parameter, that is to say at least one parameter relating to the injection of fuel into the cylinders of the engine. ci (quantity, phasage, pressure, etc.) and / or a parameter of the air loop of the engine (air flow, recycling, turbo compressor pressure, etc.). In particular, during this operation in rich regeneration mode, a late injection of fuel into the cylinders after the top dead point (TDC) of their cycle is used. For example, post-injections are applied to one or more cylinders after their TDC, or the main fuel injection in each of these cylinders is out of phase so that a portion thereof occurs after its TDC.

However, these late fuel injections in the cylinders after their top dead point induce a dilution of the engine lubricating oil with the fuel thus injected late.

Indeed a tiny part of it will then go into the crankcase via piston rings segments.

However, such dilution of the lubricating oil causes a drop in its viscosity and therefore its pressure, its premature aging or a dilution of the additives present therein.

Known in the state of the art systems for determining the dilution ratio of the engine lubricating oil by fuel late injected into it.

Conventionally, such systems estimate this rate as a function of the characteristics of the applied rich mode operations and of the time spent by the engine in each of its different types of operation.

However, such systems do not take into account the type of taxiing of the vehicle (at a stop, in the city, in a severe city, on the road, on a motorway, etc.) to calculate the dilution rate.

However, the type of rolling is an important criterion that has a direct influence on the operating state of the engine and therefore on the dilution ratio of its lubricating oil.

The object of the present invention is therefore to solve this problem by proposing a system for determining the dilution ratio of the oil which estimates it according to the type of running of the vehicle.

For this purpose, the subject of the present invention is a system for determining the dilution ratio of the lubricating oil of a motor vehicle engine engine with feed fuel thereof. during at least one operation of the engine implementing a late injection of fuel into its cylinders, the system comprising means for estimating the dilution ratio according to the current operating mode of the engine from a variation model the dilution ratio, characterized in that it comprises means for determining the type of running current of the vehicle and means for selecting at least one parameter of the variation model of the dilution ratio according to the type of rolling determined.

According to particular embodiments of the invention, the system comprises one or more of the following features:

the means for determining the current running type of the vehicle comprise means for calculating a criterion representative of the type of rolling as a function of the speed of the vehicle and the torque of the engine and means for estimating the type of rolling current in the vehicle according to the calculated rolling criterion and a predetermined mapping of rolling types;

the engine is associated with an exhaust line comprising exhaust gas depollution means, the engine being adapted to switch in the at least one operation implementing a late injection of fuel for the regeneration of these, the means for estimating the dilution ratio comprise means of calculating an increase in this rate when the engine is switched in the at least one operation from a law of variation according to the relation: t

Δτ (t) = J AREF (I) XS (I) x where Δτ is the increase in the dilution ratio, t is the time, A ^^ is a coefficient of proportionality representative of the average speed of increase of the dilution rate, taking into account the evaporation of the fuel contained in the lubricating oil between two successive applications of the at least one operation and depending on at least one operation and the type of running current of the vehicle, and S (t) is a temporal function representative of the instantaneous overconsumption of fuel by the engine in the at least one operation, and the value of the coefficient of proportionality AREP is selected by the selection means according to the current running type of the determined vehicle;

the engine is associated with an exhaust line comprising exhaust gas depollution means, the engine being adapted to switch in the at least one operation implementing a late injection of fuel for the regeneration of these, the means for estimating the dilution ratio comprise means for calculating an increase in this rate when the motor is switched in the at least one operation from a variation law according to the relationship: t

Δτ (t) = JA _REP (t) xS (t) dt x o where Δτ is the increase in dilution ratio, t is time, A ^^ is a coefficient of proportionality representative of the rate of increase of the rate of dilution depending on at least one operation and the type of running current of the vehicle, and S (t) is a time function representative of the instantaneous overconsumption of fuel by the engine in the at least one operation, and the value of the coefficient of proportionality AREP being selected by the selection means according to the current running type of the determined vehicle;

the engine is associated with an exhaust line comprising exhaust gas depollution means, the engine being adapted to switch in the at least one operation implementing a late injection of fuel for the regeneration of these, the means for estimating the dilution ratio comprise means for calculating an increase in this rate when the engine is switched in the at least one operation from a law of variation according to the relation:

Where Δτ is the increase of the dilution ratio, t is the time, V is a rate of increase of the oil dilution ratio depending on the at least one operation and the type of current running of the vehicle, and the value of the speed V being selected by the selection means according to the type of running of the determined vehicle;

the at least one operation has a low application frequency, and in that the means for estimating the dilution ratio comprise means of calculating a decrease thereof by evaporation of the fuel contained in the oil of lubrication from a law of variation according to the relation:

Δτ (t) = τ (D) xt - (b + (l -b) e- ^E ( ^t - ^D ))) where t is the elapsed time since said stop D, Δτ (t) is the decrease in the rate of dilution obtained by evaporation of the fuel at time t from the stop time D of the second or third engine operation, τ (D) is the value of the dilution ratio at the instant of this stop D, b is a predetermined parameter dependent on the type of fuel and the type of lubricating oil used in the engine, and E is a decay coefficient; and

the at least one operation has a low application frequency, the dilution ratio estimation means comprise means for calculating a decrease thereof by evaporation of the fuel contained in the lubricating oil from 'a law of variation according to the relation:

where t is the time elapsed since said stop D, Δτ (t) is the decrease in the dilution ratio obtained by evaporation of the fuel at time t from the stop time D of the second or third operation of the engine, τ (D) is the value of the dilution ratio at the instant of this stop D, b is a predetermined parameter depending on the type of fuel and the type of lubricating oil used in the engine, and E is a dependent decay coefficient the type of running current of the vehicle, and the value of the decay coefficient E is being selected by the selection means according to the current running type of the determined vehicle. The invention will be better understood on reading the following description, given solely by way of example, and with reference to the accompanying drawings, in which: FIG. 1 is a schematic view of a system according to the invention; ;

FIG. 2 is a schematic view of an embodiment of a module for determining the type of running current of the vehicle forming part of the system of FIG. 1; and

FIG. 3 is a schematic view of the embodiment of a dilution ratio estimation module forming part of the system of FIG. 1.

FIG. 1 diagrammatically shows a system 10 for determining the dilution ratio of a motor vehicle engine whose exhaust line is equipped with pollution control means, and more particularly with a NOx trap and a particle filter.

This motor is periodically and / or regularly switched from standard operation in lean mode in different modes of operation in rich mode implementing late fuel injections into its cylinders res, for example post-injections, for the regeneration of the means of pollution.

More particularly, the engine is switched:

with a high frequency and for a short duration, for example 5 seconds every minute, in a first operation in rich mode for purging the NOx trap; and

with a low frequency over a longer period, for example 400 seconds every 1000 km, in a second and third operation in rich mode for the desulfation of the NOx trap and the regeneration of the particle filter respectively.

The system 10 for determining the dilution ratio comprises a module 12 for estimating it from a mathematical model of variation of this rate, taking into account the characteristics of the various types of operation of the engine and the duration that it it goes in each of them. The mathematical model used by the module 12 depends on parameters whose current values are a function of the type of current operation of the engine and the type of current running of the vehicle.

The module 12 is connected to a module 14 for determining the fuel injection characteristics in the engine according to the current operation of the engine, and more particularly the instantaneous overconsumption of fuel associated with each type of operation in rich mode.

The module 12 is also connected to a module 16 for selecting, and for delivering to the module 12, current values of the parameters of the mathematical model as a function of the type of current running of the vehicle and the running operation of the engine, as will be explained more in detail thereafter.

The module 16 is furthermore connected to a module 18 for determining the type of running current of the vehicle as a function of the speed V thereof and the indicated engine torque C, for example measured by acquisition and / or calculation means. known in the field of technology equipping the vehicle.

The module 18 comprises means 20 for calculating a criterion CTR representative of the type of running current of the vehicle receiving, as input, measurements of the speed and the engine torque and suitable for calculating this criterion, for example according to the relation:

CTR = Vx (l + kxC) where k is a predetermined parameter.

The means 20 are connected to means 22 for determining the type of rolling according to the calculated criterion.

Figure 2 is a schematic view of an embodiment of the means 22 for determining the type of running current of the vehicle.

These means 22 comprise means 24 for forming a sliding average of the rolling type criterion calculated by the means 20, for example over a time window corresponding to 1 second. The means 24 are connected to mapping means 26 adapted to determine, according to the average criterion CTR, an instantaneous rolling type Trins from a predetermined mapping of rolling types.

For example, the mapping means 26 comprise comparison means 28 connected to the sliding averaging means 24 and storage means 30 storing predetermined threshold values CTR1, CTR2, CTR3, CTR4. These threshold values CTR1, CTR2, CTR3, CTR4 define ranges of values, each of these ranges being representative of a predetermined type of rolling of the vehicle. For example, if the average criterion CTR delivered by the means 24 is smaller than a first threshold value CTR1, then the comparison means 28 deliver a type of instantaneous rolling TRins equal to a first predetermined type of rolling, if the average criterion CTR is between the first threshold value CTR1 and a second threshold value CTR2, then the comparison means 28 deliver a type of instantaneous rolling TRins equal to a second predetermined type of rolling, etc.

The threshold values CTR1, CTR2, CTR3, CTR4 and the rolling types associated with them are determined experimentally during a test campaign, for example in a manner similar to that used for determining the types of rolling used by an anti-lock wheel system.

Mapping means 26 are connected to counter means 32 adapted to determine the percentages of time% TR1,% TR2,% TR3,% TR4,% TR5 that the vehicle passes in each of the types of instantaneous rolling TRins determined by the means 26. These means 32 are connected to comparison means 34 which receive the percentages of time% TR1,% TR2,% TR3,% TR4,% TR5 to compare them with a predetermined threshold value P, for example equal to 60 %. These comparison means 34 then deliver the current running type TR of the vehicle which corresponds to the percentage of time greater than the threshold value P. The percentages of time% TR1,% TR2,% TR3,% TR4,% TR5 are for example calculated by the means 32 over a predetermined time window corresponding for example to 1 minute.

With reference to FIG. 3, an embodiment of the module 12 for estimating the oil dilution ratio will now be described.

The estimation module 12 comprises first calculating means 36 adapted to calculate the increase in oil dilution induced by the application of the first operation of the engine in rich mode for purging the NOx trap. These calculation means 36 are adapted to calculate an increase in the dilution ratio induced by the application of the first operation from a variation law according to the relationship: t where Ax ₁ is the increase in the dilution rate induced by the application of the first operation, t is the time, AREP is a coefficient of proportionality depending on the type of current running of the vehicle and the first operation of the engine, and S is a representative function of the instantaneous overconsumption of fuel by the engine in its first operation.

As can be seen, because of the high frequency of application of the first operation, the dilution ratio increases, on average, continuously. Thus, advantageously, the AREP coefficient also takes into account both the increase in the dilution ratio by fuel injected late into the cylinder during the application of the first operation and the evaporation of the fuel contained in the lubricating oil between two successive applications of the first operation.

The coefficient A p is for example equal to the average rate of increase per unit of time of the dilution ratio corresponding to 1% overconsumption induced by the application of the first operation of the NOx trap purging motor. In this case, the time function S is equal to the instantaneous actual overconsumption of fuel induced by the application of the first operation. The instantaneous value of this function S is determined by the module 14 (FIG. 1) which delivers it continuously to the first calculation means 36.

As a variant, the module 14 is adapted to calculate a sliding average of the instantaneous overconsumption of the motor induced by the application of the first operation, for example over a time window of 1 second, in order to reduce the noise level of the overconsumption caused by a sudden dispersion of the characteristics of the injection.

The value of the AREP proportionality coefficient at the instant t is for its part selected by the selection module 16 as a function of the type of running current of the vehicle that it receives from the module 18 (FIG. 1).

The values of this coefficient are for example tabulated in modules 16 according to the different types of driving of the vehicle. These values are previously calculated after a test campaign on a vehicle serving as a reference, for example.

The estimation module 12 also comprises second calculation means 38 adapted to calculate the increase in the oil dilution rate induced by the application of the second operation of the engine in rich mode for the desulfation of the NOx trap. These second calculation means 38 are structurally similar to the first calculation means 36. They are activated and initialized during the switching of the engine in its second desulfation operation of the NOx trap and are adapted to calculate this increase in the dilution ratio from a law of variation according to the relation:

T

Ax ₂ = | _REF (t) x S (t) x dt O where Ax ₂ is the increase of the dilution ratio induced by the application of the second operation, ÂREP and S are respectively a proportionality coefficient dependent on the second operation and the type of taxi current of the vehicle and a time function representative of the overconsumption of fuel induced by the application of this second operation respectively, and T is the time spent by the engine in the second operation.

The coefficient λ p is for example equal to the instantaneous rate of increase per unit of time of the dilution ratio corresponding to 1% of overconsumption induced by the application of the second operation of the NOx trap purging motor.

In this case, the time function S is equal to the instantaneous actual overconsumption of fuel induced by the application of the first operation.

The values at time t of the coefficient _REF and of the function S are determined in a manner analogous to that of the coefficient A p and the function S of the calculation means 36 by the modules 16 and 14 respectively. The values of the coefficient _{REF REF} are also determined experimentally in a similar manner to the values of the coefficient _{n n} ^.

In a variant, when the over-consumption induced by the application of the second engine operation is constant over time, the means 38 calculate the increase in the oil dilution ratio according to the relation:

T

Δτ ₂ = {v ₂ (t) dt 0 where V ₂ is a rate of increase of the oil content whose value at time t is selected by the module 16 according to the type of running of the vehicle. The values of this speed are for example tabulated in the means 16 according to the different types of running of the vehicle. This then makes it possible to save the tabulation of the function S in the means 16.

Third computing means 40 are also provided for calculating the increase in the dilution ratio induced by the application of the third regeneration operating mode of the particulate filter. In a similar manner to the first and second means 36 and 38, these means 40 are activated and initialized during the switching of the motor in its third operation and are adapted to calculate such an increase from a law of variation according to the relation: f

Δτ ₃ = | Â _REF (t) xS (t) xdt 0 where Δτ ₃ is the increase of the dilution ratio induced by the application of the second operation, ÂREP and S a coefficient of proportionality depending on the third operation and the type of current running of the vehicle and a time function representative of the overconsumption of fuel induced by the application of this third operation respectively, and f is the time spent by the engine in the third operation.

For example, ÂREP and S represent the same types of magnitudes as the coefficient ^ ^^ p and the function S.

As a variant, when the over-consumption induced by the application of the third operation of the motor is constant over time, the means 40 calculate the increase in the oil dilution ratio according to the relation:

where V ₃ is a rate of increase of the oil dilution ratio whose value at time t is selected by the module 16 according to the type of running of the vehicle. The values of this speed are for example tabulated in the means 16 according to the different types of running of the vehicle. This then makes it possible to save the tabulation of the function S in the means 16.

The estimation module 12 also comprises means 42 for calculating a reduction in the dilution ratio by the evaporation of the fuel contained in the oil.

More particularly, the calculation means 42 are activated when the engine is switched to its standard mode since its second or its third operation. In this embodiment, only the evaporation of the fuel in the lubricating oil is specifically calculated as a result of this type of tilting. Indeed, the frequency of application of these operations is low (for example 400 seconds per 1,000 km) and the evaporation between two successive applications of these operations is not negligible. Moreover, because of the high frequency of application of the first operation (for example 5s every minute), evaporation of the fuel between two successive applications of this operation is assumed to be taken into account in the AREP coefficient, as has been explained. previously.

The means 42 are suitable for calculating this decrease in the dilution ratio caused by the evaporation of the fuel contained in the oil from a variation law according to the relation:

Δτ ₄ (t) = τ (D) x (l- (b + (l -b) e- ^E ( ^t - ^D ))) where t is the elapsed time since said stop D, Δτ ₄ (t) is the reduction of the dilution ratio obtained by evaporation of the fuel at time t from the stop time D of the second or third engine operation, τ (D) is the value of the dilution ratio at the instant of this stop D, b is a predetermined parameter dependent on the type of fuel and the type of lubricating oil used in the engine, and E is a predetermined decay coefficient.

As a variant, E is a time function whose value at time t is selected by the selection module 16 as a function of the current running type of the vehicle in a manner similar to that described for the selection of the proportionality coefficients described above. .

As a variant, the AREP proportionality coefficient associated with the calculation of the increase in the dilution by the application of the first operation does not take into account the evaporation of the fuel between two successive applications of this first operation, and the means 42 are adapted. to calculate the decrease of the oil dilution ratio by the evaporation of the fuel between two successive applications of the first operation in a manner similar to that described above. The outputs of the calculation means 36, 38, 40, 42 are connected to a summator 44 which calculates and thus delivers the dilution ratio τ of the lubricating oil as a function of the increases and decreases in the dilution ratio calculated by these means. .

Thus, the module 12 for estimating the dilution ratio B determines it according to the relationship: t na _/ ,, \ nb /, .. _\

X (O = JA _REP (t) xS (t) xdt + τia (b + (l - b) _e - ^{E (t} - ^Dia) ) ₊ τib (b ₊ (lb) _e - ^{E (t} - ^{Dib ))} oi = li = l where t is time, _n and rb are the number of regeneration of the NOx trap and the particle filter respectively at time t and τia τib are zero values of the dilution ratio to τ moment i ^emes stops the second and third operation respectively, and Dia and Dib are ^ith moments of stops second and third operations respectively.

In another embodiment of the means 42, these are adapted to calculate the determination of the decrease of the dilution rate by evaporation according to the relation:

Δτ ₄ (t) = τ (D) x (1 - b) x JE (t) xe ^{"E (t)} ^ ^{" D} ^ dt

Where E is a time function whose value at time t is selected by the selection module 16 according to the current running type of the vehicle in a manner similar to that described for the selection of the proportionality coefficients described above.

Although it has been previously described a determination of the oil dilution rate as a function of time, it will be understood that this determination can also be made as a function of the number of kilometers or determined according to a combination of the two.

Likewise, it will be understood that the mathematical relationships described below corresponding to continuous time relationships are discretized for actual application in a calculator.

Similarly, although a system for determining the dilution ratio in the context of an engine equipped with a NOx trap and a particulate filter has been described, the system according to the invention can also be used. apply to a engine equipped only with one or other of these means of pollution control, the corresponding calculation means in the estimation module comprising only the elements associated with the pollution control means equipping the engine.

Claims

1. A system for determining the dilution ratio of the lubricating oil of a motor vehicle engine by feed fuel thereof during at least one operation of the engine implementing a late injection of fuel in its cylinders, the system comprising means (12) for estimating the dilution ratio according to the current operating mode of the engine from a variation model of the dilution ratio, characterized in that it comprises means (18) for determining the type of running current of the vehicle and means (16) for selecting at least one parameter of the variation model of the dilution ratio according to the type of rolling determined.

2. System according to claim 1, characterized in that the means (18) for determining the type of running current of the vehicle comprises means for calculating a criterion representative of the type of running according to the speed of the vehicle and the torque. the engine and means (22) for estimating the type of vehicle running according to the calculated rolling criterion and a predetermined mapping of rolling types.

3. System according to claim 1 or 2, characterized in that the engine is associated with an exhaust line comprising means for cleaning the exhaust gas, the engine being adapted to switch in the at least one operation implementing a late injection of fuel for the regeneration thereof, and in that the means (12) for estimating the dilution ratio comprise means (36) for calculating an increase in this rate when the engine is switched into the at least one operation from a law of variation according to the relation: t

Δτ (t) = JA _REP (t) xS (t) x dt where Δτ is (increase of the dilution ratio, t is the time, AREP is a coefficient of proportionality representative of the average rate of increase of the dilution ratio , taking into account the evaporation of the fuel contained in the lubricating oil between two successive applications from least one operation and depending on at least one operation and the type of running current of the vehicle, and S (t) is a time function representative of the instantaneous overconsumption of fuel by the engine in the at least one operation, the value of the coefficient of proportionality A ^^ being selected by the selection means (16) as a function of the current driving type of the determined vehicle.

4. System according to claim 1, 2 or 3, characterized in that the engine is associated with an exhaust line comprising exhaust gas depollution means, the engine being adapted to switch in the at least one running operation. a late injection of fuel for the regeneration thereof, and in that the means (12) for estimating the dilution ratio comprise means β8; 40) for calculating an increase in this rate when the engine is switched into the at least one operation from a variation law according to the relation: t

Δτ (t) = JA _REP (t) xS (t) x dt where Δτ is the increase of the dilution ratio, t is the time, AREP is a coefficient of proportionality representative of the rate of increase of the dilution ratio depending on the at least one operation and the current running type of the vehicle, and S (t) is a time function representative of the instantaneous overconsumption of fuel by the engine in the at least one operation, the value of the proportionality coefficient AREP being selected by the selection means (16) as a function of the current running type of the determined vehicle.

5. System according to claim 1, 2, 3 or 4, characterized in that the engine is associated with an exhaust line comprising exhaust gas cleaning means, the engine being adapted to switch in the at least one operation implementing a late injection of fuel for the regeneration thereof, and in that the means (12) for estimating the dilution ratio comprise means (38; 40) for calculating an increase in this rate when the engine is tilted in the at least one operation from a law of variation according to the relation:

o where Δτ is the increase of the dilution ratio, t is the time, V is a rate of increase of the oil dilution ratio depending on the at least one operation and the type of running current of the vehicle, the value of the speed V being selected by the selection means (16) as a function of the current running type of the determined vehicle.

6. System according to any one of the preceding claims, characterized in that the at least one operation has a low application frequency, and in that the means (12) for estimating the dilution ratio comprise means (42) of calculating a decrease thereof by evaporation of the fuel contained in the lubricating oil from a law of variation according to the relation:

Δτ (t) = τ (D) xt - (b + (l -b) e- ^E ( ^t - ^D ))) where t is the elapsed time since said stop D, Δτ (t) is the decrease in the rate of dilution obtained by evaporation of the fuel at time t from the stop time D of the second or third engine operation, τ (D) is the value of the dilution ratio at the instant of this stop D, b is a predetermined parameter depending on the type of fuel and the type of lubricating oil used in the engine, and E is a decay coefficient.

7. System according to any one of claims 1 to 5, characterized in that the at least one operation has a low application frequency, and in that the means (12) for estimating the dilution ratio comprise means ( 42) for calculating a decrease thereof by evaporation of the fuel contained in the lubricating oil from a law of variation according to the relation: Δτ (t) = τ (D) x (lb) x JE (t) xe ^{"E (t)} ( ^{t" D} U

Where t is the time elapsed since said stop D, Δτ (t) is the decrease in the dilution ratio obtained by evaporation of the fuel at time t from the stop time D of the second or third operation of the engine, τ (D) is the value of the dilution ratio at the instant of this stop D, b is a predetermined parameter depending on the type of fuel and the type of lubricating oil used in the engine, and E is a decay coefficient depending on the type of running current of the vehicle, the value of the decay coefficient E being selected by the selection means (16) according to the current running type of the determined vehicle.