Measuring device and method for a fuel injection system Field of the invention

The present invention relates to a device and a method according to the preambles of the independent claims, specifically for detecting faults in a fuel injection system, and in particular for detecting faults in a piezoelectric fuel injector.

Background to the invention

In directly injected combustion engines, fuel injectors are used to deliver a charge of fuel to a combustion chamber before ignition. The fuel injector is typically situated in the cylinder head with respect to the combustion chamber so that its tip protrudes somewhat into the chamber in order to deliver the fuel charge in the chamber. A type of fuel injector particularly suited to directly injected engines is so-called piezoelectric injectors.

A piezoelectric injector comprises a piezoelectric means adapted to controlling the position of an injector valve needle relative to a valve seat. The piezoelectric means comprises a stack of piezoelectric elements, also called a piezostack, which have capacitive electrical characteristics. The piezostack is adapted to expanding and contracting in response to a varying voltage applied to the piezoelectric means. The expansion and contraction are thus used to vary the axial position of the valve needle and thereby control the amount of fuel injected. The piezoelectric injector is controlled by an injector control unit which is often an integral part of the vehicle's control unit. The injector control unit comprises typically a microprocessor and a memory.

US 7,140,353 refers to an example of a fuel injector with a piezoelectric means which comprises a piezoelectric stack.

If any of the elements in the piezostack disintegrates or becomes damaged, or if the lines to any of the stack elements disintegrate or become damaged, this will affect the physical size of the stack. Examples of faults might be an individual stack element not abutting correctly against neighbouring elements, presence of microcracks in stack elements, or errors in the arrangement of the electrical connections on the stack element.

In such cases the injector may continue partly to function but may deliver a different amount of fuel from that desired at a certain active time and a certain fuel pressure.

Changes depend on the injector being subjected to reduced or increased needle lift.

Accordingly, to ensure that the injector functions as intended, it is necessary to be able to test the function. This may currently be done indirectly by measuring the torque delivered by the cylinder in which the injector is situated and comparing it with the expected torque. Any major discrepancy may be due to defects of the piezostack.

An example of how testing may be done is known from US 2007/0001545, which refers to a method for diagnosing a fuel injection device which comprises a piezoelectric means. A diagnostic unit is connected to the fuel injection device and is used to apply a defined voltage to the piezoelectric means. The capacitance of the piezoelectric means is then determined by the diagnostic unit, enabling it to deliver a diagnosis for the device. During the measurement of the capacitance, the piezoelectric means is thus provided with a charge which has, after the measurement, to be led away during a discharge phase. US 2004/0036479 describes a method for detecting cracks in a piezoelectric element, which method is applied in conjunction with a so-called micromotor. This is done by conducting a frequency sweep and detecting the response signal, which may provide information about any cracks. In the event of break or short-circuit on the connecting lines to the injector, it is normal practice to measure the current, and any major deviation from the normal value may suggest a possible break or short-circuit.

The object of the present invention is to indicate an improved and quicker method and device for diagnosing a fuel injection device which comprises a piezoelectric means both with regard to faults in the connecting lines and of the piezoelectric means. Summary of the invention

The above objects are achieved with the invention defined by the independent claims.

Preferred embodiments are defined by the dependent claims.

The invention thus comprises a measuring device intended to be used for diagnosing a piezoelectric injector means in a fuel injection system.

The measuring device comprises a signal generator adapted to generating a measurement signal in the form of an AC voltage with a varying frequency with a predetermined frequency content and a predetermined voltage amplitude. The measurement signal is applied to the piezoelectric injector means via one or more connecting lines which are also adapted to receiving a response signal which is then passed on to a detector adapted to detecting the resonance frequency of the injector means. The measuring device further comprises a calculation unit adapted to comparing the detected resonance frequency with one or more reference values for the resonance frequency and to generating on the basis of the comparison a diagnosis signal which contains information about the function status of the piezoelectric injector means.

The invention is based on the insight that the resonance frequency of the piezostack is affected inter alia by the physical dimensions of the stack and the material constant of the piezomaterial. This means that if any of the stack elements disintegrates or becomes damaged, or if the lines to any of the stack elements disintegrate or become damaged, this will affect the physical size of the functioning stack and therefore its resonance frequency. To diagnose such a piezostack, a voltage frequency sweep is applied to the piezoinjector and the resonance frequency is determined, e.g. by measuring the voltage across a resistance connected in series with the stack.

At the resonance frequency of the stack its impedance will reach a minimum, causing the voltage across the resistance to reach its maximum. The diagnosis is preferably done when the vehicle is not in operation, although it is also possible for it to take place during operation.

The resonance frequency f of the piezostack is given by the relationship

where L is the inductance and C the capacitance of the piezostack.

The signal generator which generates the frequency sweep is an inexpensive and robust product. A number of A/D converters are also needed for measuring the voltage across the resistance.

An advantage of the solution according to the present invention is that the measurement can be done quickly, e.g. in a time of the order of 1 second. The present invention makes it possible to diagnosis intermittent and permanent faults on piezoinjectors and then generate fault codes which identify the injector and indicate types of fault. By using these fault codes, a service mechanic can easily deal with a fault by replacing the relevant component. Brief description of drawings

Figure 1 is a schematic block diagram illustrating the present invention.

Figure 2 is a schematic block diagram of the calculation unit according to an embodiment of the present invention.

Figure 3 is a flowchart illustrating the present invention.

Detailed description of preferred embodiments of the invention

The invention is described below in more detail with reference to the attached drawings.

Figure 1 is a schematic block diagram illustrating a measuring device for diagnosis of a piezoelectric injector means in a fuel injection system. The measuring device comprises a signal generator adapted to generating a measurement signal in the form of an AC voltage with a varying frequency with a predetermined frequency content and a predetermined voltage amplitude.

The measurement signal preferably comprises frequencies within the range 10-50,000 Hz with a voltage amplitude (absolute value) within the range 5-50 volts. The primary factor in the choice of voltage amplitude is that it has to be so low that the piezoelectric injector means is not affected, i.e. the injector does not open.

In addition, one or more connecting lines are provided to apply the measurement signal to the piezoelectric injector means and to receive a response signal from the injector means. This signal is then passed on to a detector adapted to detecting the resonance frequency of the injector means.

The measuring device further comprises a calculation unit adapted to comparing the detected resonance frequency with one or more reference values for the resonance frequency and to generating on the basis of the comparison a diagnosis signal which contains information about the function status of the piezoelectric injector means.

Before the measuring device can be used, one or more reference values for the resonance frequency of the injector means have to be measured on a non-defective injector means. This is preferably done at the time of manufacture of the injector means and the resulting values are then fed into a memory unit of the calculation unit. These reference values may take the form of a range of values for the resonance frequency which indicate that the injector is functioning satisfactorily. There may also be frequency values and/or frequency ranges which represent maximum and minimum resonance frequency limits for the injector to be regarded as functioning satisfactorily.

According to an embodiment, the measuring device comprises a detector comprising a resistance and the resonance frequency is determined by measuring the voltage across the resistance and identifying the frequency at which the voltage reaches its maximum value across the resistance. According to another embodiment, the detector comprises a tuned filter consisting of one or more coils and capacitors which can be adjusted to the resonance frequency of the response signal, which can thus be identified. To make it easy for an operator to ascertain the injector's function status, the diagnosis signal contains information about the identity of the injector means and, if a fault is found, the function status information comprises one or more fault codes which indicate types of fault. As mentioned above, the calculation unit comprises a memory unit (see Fig. 2) for storing inter alia the reference values, but also other data, e.g. historical resonance frequency values. The calculation unit is adapted to detecting changes in stored historical resonance frequency values and to generating a second diagnosis signal which reflects such changes. With reference to the flowchart in Figure 3, the invention comprises also a method for diagnosis of a piezoelectric injector means in a fuel injection system. The method comprises

- generating a measurement signal in the form of an AC voltage with a varying frequency with a predetermined frequency content and a predetermined voltage amplitude,

- applying the measurement signal to the piezoelectric injector means,

- detecting the resonance frequency of the injector means,

- comparing the detected resonance frequency with one or more reference values for the resonance frequency, and

- generating on the basis of the comparison a diagnosis signal which contains information about the function status of the piezoelectric injector means.

The resonance frequency is preferably determined by measuring the voltage across a resistance and identifying the frequency at which the voltage reaches its maximum value across the resistance.

The measurement signal comprises frequencies within the range 10-50,000 Hz and voltage amplitudes within the range 5-50 volts. The diagnosis signal according to a preferred embodiment contains information about the identity of the injector means, and said function status comprises one or more fault codes which indicate the type of any faults found. Said reference values for the resonance frequency of the injector means will have been measured on a non-defective injector means.

The present invention is not restricted to the preferred embodiments described above. Sundry alternatives, modifications and equivalents may be used. The above embodiments are therefore not to be regarded as limiting the invention's protective scope defined by the attached claims.