STATE OF THE ART Injection systems for today's diesel engines are dimensioned for a certain predetermined fuel quality, which means that a certain quantity of fuel is supplied to the injection pump for injection into the engine at a given accelerator position. There are however very great local and regional variations in fuel quality, with the result that a diesel vehicle operated with a fuel which deviates from the expected fuel quality will deviate in performance.

It may be mentioned by way of example that a driver of a vehicle which is refuelled in southern Europe, where typically fuel of higher energy content is supplied, will experience significantly inferior performance of the vehicle when it is operated with a fuel of lower energy content such as is typically supplied in northern Europe. Such performance differences may also be experienced locally on particular markets where, for example, different fuel qualities occur at different times of the year. It may also happen that different suppliers on the same market supply different fuel qualities.

The effects of different fuel qualities on vehicle performance constitute a very undesirable problem which many drivers may perceive as a defect of the vehicle. This problem is caused by the fact that fuel qualities can vary by up to 5% of energy content during the year in a given market and by up to 10-12% between markets in, for example, northern and southern Europe. Corresponding performance differences may be experienced during vehicle operation with the different fuel qualities.

As a diesel engine is in principle dimensioned for fuel with the highest conceivable energy content, optimum operation will be impossible with a fuel of lower energy content. This problem may even result in truck owners finding themselves forced to invest in vehicles with more powerful engines or to run vehicles at below maximum load in order to compensate for loss of engine performance.

OBJECTS AND MOST IMPORTANT CHARACTERISTICS OF THE INVENTION One object of the present invention is to eliminate or at least alleviate the problems of the state of the art and to indicate a method whereby vehicles can be operated more optimally on various qualities of fuel.

This object is achieved according to the invention with a method and an arrangement of the kind indicated in the introduction by means of the features in the characterising parts of claims 1 and 7 respectively.

The result is that the resistance which the valve element meets in its movement is related to the fuel quality as regards energy content. On the basis of that movement and, in particular, the fuel's resistance to the movement it is possible to make a fuel quality estimate which can be utilised for controlling the quantity of fuel supplied to the injection pump for each injection.

In particular, it may generally be stated that a fuel with low viscosity, which therefore exerts low resistance to movement of the valve element, is of lower density and hence lower energy content. To achieve the same results, such a fuel has therefore to be supplied to the engine in larger quantities than a fuel of higher viscosity which is of higher density and hence higher energy content.

It is of great advantage to use the valve of the injection pump for said fuel quality detection, since it is a component already present in the injection system.

The injection pump is also arranged in such a way adjacent to the engine that its temperature will depend greatly on the engine temperature. The ambient temperature's

effects on measurements will thus be negligible. Fuel quality measurement will therefore not be appreciably affected by whether the vehicle is operated in a warm or a cold climate. The overall result will be reliable measurements at minimal economic cost.

It is preferable that the measurements be taken by sensing in a solenoid belonging to the injection pump and, in particular, that the length of the period between the time when the current activating the solenoid is applied and the time when the valve element reaches its seat and the valve is therefore closed be used as a description of the movement of the valve element. These times are easy to detect by means of components already present in the system, which means that their utilisation will be easy and cost-effective.

Further advantages of the invention are indicated by the following detailed description of an exemplifying embodiment with reference to the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS Fig 1 depicts schematically a control system according to the invention in conjunction with a fuel injector to a diesel engine, Fig. 2 depicts on a larger scale part of a valve for an injection pump for use with the invention, Fig. 3 depicts graphs to illustrate signals used for valve movement detection, and Fig. 4 depicts a block diagram of a method according to the invention.

DESCRIPTION OF AN EXEMPLIFYING EMBODIMENT Fig. 1 depicts schematically an injection system according to the invention, which is denoted generally by ref. 1. The system depicted is intended for a combustion engine of piston-and-cylinder type, e. g. a diesel engine, for a vehicle. An injection pump 3 is designed to inject in specified situations a measured quantity of fuel into a cylinder 2 of a combustion engine for a vehicle.

The injector depicted is of PDE type, i. e. a unit injector with the fuel injection nozzle combined with the pump unit, and the pump unit is electronically controlled.

The injection system further comprises in a conventional manner a pump P, which via its outlet works against a pressure regulator R and against a solenoid 5.

A control unit 4, which may take the form of a separate unit or be integrated in an engine control unit, is designed to control the operation of the injection system. To this end, an activity current is led to the solenoid 5 via electrical lines 7 to activate the solenoid. The same lines 7 may also be used to register, by the valve element's induction in an associated coil, the time when the valve element reaches its seat and the solenoid is thus closed.

Fig. 2 shows the solenoid 5 in more detail with a valve element 9 which is movable towards a seat 10 in response to delivery of activity current from the control unit 4 (Fig.

1) to a coil 11. The lower position of the valve element 9 corresponds to valve closed.

The valve element 9 is situated in a space full of fuel, so its movement depends on the viscosity characteristics of the fuel.

In Fig. 3, the curve 12 represents in graphic form the activation current as a function of time, and the curve 13 the movement of the solenoid's valve element as a function of time. In this diagram, to indicates initiation of the activation current and ts indicates the valve element impinging on the seat. This point ts is easy to detect by detecting an extreme point 14, the position of which may be defined by when current induced by the valve element at that point reaches a local minimum due to the cessation of the valve element's movement when it reaches its seat. The period of time between to and ts thus describes the movement of the valve element in the solenoid housing, and it has been found that this period depends on the viscosity of the fuel. The higher the viscosity of the fuel, the longer the period of time between to and tS. As the viscosity is also a measure of the density and energy content of the fuel, it is possible according to the invention to use the value obtained for adjusting the injection to cater for different fuel qualities.

Fig. 4 illustrates in simple block diagram form a sequence performed in a method according to the invention, in which the notations are as follows: Ref. 20, beginning of sequence.

Ref. 21, delivery of activation current at time to.

Ref. 22, detection of the time ts when the valve element impinges on the seat, detected by any conventional mathematical method.

Ref. 23, calculation of the value ts-to by, for example, any simple time measurement method.

Ref. 24, delivery of a signal from the control unit to the injection system and, in particular, to the solenoid, indicating the fuel quantity to be injected.

Ref. 25, end of sequence.

It is possible according to the invention for the measurements to be carried out at shorter or longer intervals of time. It is preferable that the measurements be initiated automatically, but they may also be done after manual initiation.

The connection between measured period of time ts-to-via fuel viscosity-via fuel density-via fuel quantity intended for injection-and delivery of the control signal is made, for example, automatically by means of an algorithm. This means that there is no need to calculate the intermediate values, since the values of ts-to can instead be caused to lead directly to control signals from the control unit to the solenoid resulting in the correct quantity of the fuel being injected into the cylinder.

It is also possible to incorporate in the system a limitation whereby an insignificant adjustment can be suppressed, thereby avoiding adjustment for fuel qualities whose energy content is close to a normal value.

The invention has been described against the background of a diesel engine with direct injection but is also applicable to diesel engines with indirect injection, e. g. those of HCCI type.