BACKGROUND TO THE INVENTION Users of utility vehicles, particularly trucks, e. g. transport companies, often have a large number of vehicles with varying equipment. It is also usual that each vehicle is often used by a succession of different drivers. This entails a need for each driver taking over the vehicle to know how much fuel is already on board. This information is needed in absolute terms, i. e. the remaining volume of fuel expressed in, for example, litres. The information may also be used for other functions, e. g. computer-aided itinerary planning.

The state of the art provides arrangements for presenting the remaining fuel volume in a vehicle. One problem of such systems is that when there is a change in the vehicle's fuel capacity, e. g. when the fuel tank is replaced or the vehicle is provided with an extra tank, information about the new fuel capacity has to be put into the vehicle's computer.

FR-2710743-A1 describes an arrangement for measuring in absolute terms the fuel quantity on board a vehicle. With that arrangement, fuel consumption while the vehicle is travelling can be calculated by means of information from a flowmeter. Information can also be obtained on how much fuel is taken on at each refuelling. A disadvantage of that system is that it requires special arrangements for supplying information about how much fuel is taken on at each refuelling. It is proposed that a computer unit on the fuel pump at the filling station is connected to the vehicle's computer via a connection on the outside of the vehicle. This solution entails increased complexity and cost of the system,

because of involving further components. Another proposal is that immediately after refuelling the driver puts manually into the vehicle's computer information about the quantity taken on. This proposal has the disadvantage of increasing the driver's workload, and the risk of incorrect or no input is also not insignificant.

OBJECTS OF THE INVENTION One object of the invention is to provide a method and a system by which it is possible to identify a change in the fuel volume capacity of a utility vehicle.

Another object of the invention is to improve the accuracy of information based on signals from a fuel level sensor in a vehicle.

BRIEF DESCRIPTION OF THE INVENTION The objects stated above are achieved with a method and a system according to the characterising features in patent claims 1 and 5 respectively.

Making an assumption in the manner proposed in patent claim 1 eliminates the need for information on the fuel volume capacity to be put into the vehicle's computer from an external source. This results in significant time and cost savings during vehicle manufacture and on the occasion of subsequent replacement or addition of fuel tanks.

The system according to the invention preferably comprises means of volume change determination for determining the aggregate volume capacity of at least one fuel tank when there is a change of capacity. This eliminates the need, on the occasion of tank replacement or addition, to put corresponding information into the vehicle's computer from some external source.

A correlation is preferably determined between information from the fuel level sensor and information about the ratio between residual quantity of fuel and full quantity of fuel, on the basis of information from the fuel level sensor and information about how much fuel is consumed between a full refuelling of the vehicle and a subsequent refuelling.

This means that the system can itself improve the accuracy of one of its own functions.

The presentation to the driver of the residual fraction of full fuel quantity is also improved.

DESCRIPTION OF THE DRAWINGS The invention is described below in more detail with reference to the attached drawings, in which -Fig. 1 depicts schematically an embodiment of a system according to the invention, - Fig. 2 depicts schematically a cross-sectional view of a fuel tank, - Fig. 3 shows two scales for residual fraction of full fuel quantity, and - Fig. 4 shows two scales for residual fraction of full fuel quantity.

DETAILED DESCRIPTION Fig. 1 depicts a system for fuel volume calculation for a utility vehicle, such as a truck.

This system can also be used on other types of vehicles such as buses, passenger cars and work vehicles. The system incorporates a fuel level sensor FS situated in a fuel tank FT of the vehicle. The vehicle may be equipped with one or more fuel tanks. The fuel level sensor FS is connected to a central computer unit CU which has calculation means and data storage means. In the vehicle's driving cab there is a fuel level indicator FLM and a fuel information display FID, both of them connected to the computer unit CU. The system also comprises a torque sensor TS connected to the computer unit CU and arranged to send signals corresponding to the engine's torque.

The vehicle is designed to be capable of being equipped with various tank alternatives for fuel storage. Some of these tank alternatives may refer to cases where the vehicle is equipped with one fuel tank which may be of various sizes. Another tank alternative may refer to cases where the vehicle is equipped with two or more tanks, with certain size alternatives and in various combinations. The computer unit CU stores information about a number (n) of possible values VI, V,,...., V, each of which corresponds to the value of the aggregate fuel volume capacity of one of the tank alternatives.

The present invention makes it possible to determine the fuel volume capacity of the vehicle. This is done without having to input any information from an external source.

An advantage of the invention is that replacing one tank by another, followed by automatic determination of the new tank volume, can be carried out without external operations.

We now describe an embodiment of the invention such as may be used for determining the fuel volume capacity of a vehicle. The vehicle is fully refuelled on the occasion of a first refuelling Tl. The functions of the system on the occasion of refuellings which are not full refuellings are described in more detail below. The system comprises means for determining how much fuel the vehicle consumes. When the vehicle is travelling after the first refuelling Fi, the computer unit CU receives from the torque sensor TS information on the basis of which the fuel consumption rate is calculated. Alternatively the vehicle may be equipped with a flowmeter which is situated in a fuel line between the tank FT and the engine and which is connected to the computer unit CU to provide the fuel consumption rate. Integration which takes place either continuously or at frequent intervals provides information on how much fuel Fe is consumed after the first refuelling Ti. Information on the fuel quantity consumed V, is updated and stored progressively as it is renewed.

The system incorporates means for determining the fraction of remaining fuel in the tank FT. This means may include the computer unit CU and the fuel level sensor FS, in which

case the computer unit CU continuously receives from the fuel level sensor FS signals on the basis of which a residual fuel quantity fraction R is calculated. The residual fuel quantity fraction R is merely a relative value, i. e. a fraction of the full fuel quantity, e. g. one-quarter, and information about it is updated and stored continuously by the computer unit CU.

On the occasion of a second refuelling T2, which is likewise a full refuelling, the computer unit CU determines that a refuelling is taking place. In this connection it is possible to utilise the fact that the signal from the fuel level sensor FS changes considerably more quickly when the vehicle is being refuelled than when the vehicle is travelling. For example, on the basis of the signal from the fuel level sensor FS the computer unit CU can calculate the rate of change of the signal and determine that refuelling is taking place if the rate of change exceeds a predetermined value.

The computer unit CU also determines the residual fuel quantity fraction R when the second refuelling T2 begins. This may be done by the computer unit comparing successive stored values for the residual fuel quantity fraction R.

The computer unit CU does a first calculation of the total volume capacity Vrl : Vil = Vc, / (l-RI), where Vl is the value of the fuel quantity consumed between the first refuelling T, and the second refuelling T2. Vl is matched by the respective value of Vc on the occasion of the second refuelling T2. The result of the first calculation of the total volume capacity V, is stored.

The computer unit CU selects, as a first assumption about the aggregate volume capacity of the vehicle's fuel tank, whichever of the stored possible values Vb V,...., V", is closest to the first calculation Vl.

On the occasion of a third refuelling T3, likewise a full refuelling, the initial stages above are repeated, resulting in a second calculation of the total volume capacity V2 : V2 = Vc2l (1

-R2), where Vc2 and R2 are new values of the fuel quantity consumed between the second refuelling T2 and the third refuelling T3 and the residual fuel quantity fraction R at the beginning of the third refuelling. The result of the second calculation of the total volume capacity V2 is stored.

A second assumption about the aggregate fuel capacity of the vehicle's fuel tank is made.

With a view to improved accuracy, the second assumption is made on the basis of both the first calculation of the total volume capacity Vl and the second V2. The assumption may be made by selecting whichever of the possible stored values VZ, VI----, V", is closest to the average value of V, and V2.

The result at each repeat of the method is a new calculation of the volume capacity.

Better accuracy of the assumption about the volume capacity of the fuel tank can thus be achieved.

Such stored calculated volume capacity values are used on the occasion of each new assumption. If an average of these values is used for the assumption, it is possible in the average value calculation for the calculated values to be weighted, e. g. depending on how large a volume is taken on, when calculating each volume capacity. A capacity calculated when taking on the relatively small refuelling volume may be assigned a smaller weight than one calculated on the occasion of a larger refuelling, since the accuracy may be inferior in the case of small tank volumes.

The volume capacity assumption results may be used, via the fuel information display FID, to inform the vehicle driver about the volume capacity. In Fig. 1, this appears in the top row of the fuel information display FID. Comparing the assumption about the total volume capacity with information about the fuel level makes it possible, e. g. while the vehicle is travelling, for information to be provided about the remaining fuel volume. In Fig. 1 this appears in the bottom row of the fuel information display FID.

Each iteration, or renewed assumption about the volume capacity on the occasion of a refuelling preceded by a full refuelling, is conducted as above. If the previous refuelling was not a full refuelling, a calculation of the volume capacity V would be misleading and should not be used as the basis for a new assumption about the volume capacity.

Accordingly, if a refuelling is not a full refuelling, this fact has to be determined. Such determination may be based on the fact that the fuel level sensor does not indicate full when the refuelling ends. A new assumption about the volume capacity is preferably omitted on the occasion of a refuelling which follows a refuelling which was not a full refuelling.

If the fuel level sensor does not indicate full immediately after a number of successive refuellings, this may indicate that the sensor is faulty. The system may therefore include a function which, when there is no"full"indication on the occasion of a predetermined number of successive refuellings, informs the vehicle driver that the fuel level sensor is out of action.

If a volume capacity V value calculated after any refuelling deviates substantially from previous calculations and/or assumptions, this suggests one of the following alternatives: i) previous refuelling was not a full refuelling, ii) faulty fuel level sensor, or iii) a change in aggregate volume capacity.

In the case of alternative i), this was preferably already determined at the time of the previous refuelling, as described above. In the case of alternative ii), i. e. faulty fuel level sensor, this may be determined as described above. In the event of an assumption that neither alternative i) nor ii) applies, the computer unit CU preferably determines that alternative iii) applies. This may mean that the fuel tank or, if the vehicle is provided with two or more, one or more of them has/have been changed or a further fuel tank has been fitted.

If it is determined that the aggregate volume capacity has changed, a new series of assumptions of the new volume capacity begins as described above. It should be noted that no information from any external source is required in this respect. More specifically, there is no need for any reprogramming of the system when the volume capacity changes, since the system itself detects the volume change and determines for itself the new volume capacity.

The invention may be used not only for determining the fuel volume capacity but also for improving the accuracy of information on the basis of the fuel level sensor FS. This results in more correct presentation on the fuel level indicator FLM. On the occasion of each new assumption about the volume capacity during the repeated process described above, a value for the residual fuel quantity fraction and a value for the quantity of fuel consumed since the previous refuelling are obtained. Correction of the fuel level scale can be done on the basis of these two values.

Figs. 2,3 and 4 show a simple example to illustrate a function for improving the accuracy of information on the basis of the fuel level sensor FS. A vehicle is provided with a fuel tank FT which extends between two parallel walls and has a cross-section according to Fig. 2. A fuel level sensor FS situated in the tank is arranged to emit a signal which corresponds to a linear relationship between the fuel level and the fuel quantity, as illustrated by the upper scale in Fig. 3. The lower scale in Fig. 3 corresponds to the information obtained after correcting the signal from the fuel level sensor FS.

Let us suppose that three tank volume alternatives of 200,300 and 450 litres are stored in the vehicle's computer unit. Let us also suppose that at the beginning of a refuelling operation the value of the residual fuel quantity fraction R, is one-half. Let us further assume that the value of the quantity of fuel consumed Vl since the previous refuelling is 170 litres. The volume capacity calculated on this basis will then be V, = i/ (l-) = 170/ (1-1/2) = 340 litres. An assumption is then made that the volume capacity is 300 litres, this being the stored value which is closest to the calculated value. As the fuel

quantity consumed Vl since the previous full refuelling is 170 litres, the remaining fuel quantity is 130 litres. It may then be assumed that the residual fuel quantity fraction Rl is 13/30. The residual fuel quantity fraction scale may then be corrected as shown in the lower scale in Fig. 4. Here the value at the respective reading point has been altered from 1/2to 13/30, and linear adaptation is applied in both directions from the reading point. It is also conceivable for the whole scale to be adapted to a non-linear curve.

Progressively as new values for the residual fuel quantity fraction R and the fuel consumed Fe are obtained, new adjustments can be done at further points on the scale in the same manner as above. Subsequent adjustments are preferably applied only within the portion of the scale which contains the respective adjustment points and is bounded by earlier adjustment points situated closest to the relevant adjustment point.