WO2015154949A1

WO2015154949A1 - Method for the control and diagnosis regarding the operation a fuel injector

Info

Publication number: WO2015154949A1
Application number: PCT/EP2015/055497
Authority: WO
Inventors: Jean-Francois Berlemont
Original assignee: Delphi International Operations Luxembourg S.À R.L.
Priority date: 2014-04-09
Filing date: 2015-03-17
Publication date: 2015-10-15
Also published as: EP3129637A1; GB201406358D0

Abstract

In a piezo-electrically actuated fuel injector, a method of determining or providing a measure the gap between the end of the piezo electric stack/actuator and the valve, and/or the point at which the gap is zero, comprising energising the stack/actuator, and measuring one or more characteristics of oscillation.

Description

Method for the control and diagnosis regarding the operation a fuel injector

Technical Field

The invention relates to fuel injectors for delivering fuel into a combustion chamber such as a cylinder of an internal combustion engine. It relates in particular, but not exclusively, to fuel injectors including piezoelectric actuators used to control movement of a needle in an injector valve. Background of the Invention

The use of piezoelectric actuators is known as an alternative to solenoids for controlling injections in a fuel injector. Typically, a stack of piezoelectric elements can be arranged to control fuel pressure within an injector fuel chamber so as to consequentially control the movement of an injector needle away from a valve seat so as to inject fuel. In operation the distal portion of the piezo stack abuts/contacts the proximal (upper end) portion of an injector needle (typically via intermediate components), and actuation of the piezoelectric

stack/actuator pushes against the needle to force the needle downward so as to displace the needle away from its seat to dispense fuel. In alternative designs fuel may be dispensed on retraction of the needle due to retraction of the piezoelectric stack. In all cases, the needle needs to be in contact with the end of the stack (albeit sometimes via intermediary

components) during fuel injection process.

Components of the injector have different thermal expansion/contraction characteristics. Often there are also variation in manufacturing or measurement tolerances, clamping/clamp nut load tolerances, and there is also valve seat wear. For these reasons injector designs provide for a gap between the end of the stack./actuator (including any associated

components) and the needle(respective needle components), which is required for a fully discharged piezo stack to overcome such variations in thermal expansion/contraction, aging, wear in the valve seat, valve , head, shims and piston wear and well as to cope with variation in clamping. However the necessity of providing a gap makes certain aspects of the control of fuel delivery difficult and many fuel injector systems require consequential gap compensation strategies to be performed dynamically to minimise delivery drift, minimise injector to injector scatter allow accurate closed loop closure feedback. It is particularly advantageous to be able to estimate the gap and/or when it is closed so that closed loop detection of needle closure can be performed, as it is desirable for the gap to be reduced to zero beforehand.

It is an object of the invention to provide a method to measure the gap and to determine when the gap is reduced to zero.

Statement of the Invention

In one aspect of the invention is provided, in a piezo-electrically actuated fuel injector, a method of determining or providing a measure of the gap between the end of the piezo electric stack/actuator and the valve, and/or the point at which the gap is zero, comprising energising the stack/actuator, and measuring one or more characteristics of oscillation.

A method of operating a piezo-electrically operated actuated fuel injector in an internal combustion engine, the fuel injector comprising a piezo-electric stack actuator, one end of which provides an actuation force on a fuel injection needle valve member when the stack is energised to move the valve between open and closed positions, the method comprising measuring one or more of the

characteristics of oscillation of the stack when a voltage is supplied across the stack in order to provide a measure of a gap between the end of the stack and the valve member.

The phrase "stack is energised to move the valve between open and closed positions" means from an open position to a closed position or vice versa

The method may include the steps of: i) applying a current pulse to the stack; ii) measuring the open circuit voltage; and determining from ii) one or more oscillation characteristics of the stack.

The method may comprise the steps of i) applying a bias voltage to the piezoelectric stack/actuator; ii) applying a current pulse to the stack; iii) measuring the open circuit voltage; iv) determining one or more oscillation characteristics of the stack;, and v)repeating steps i) to iv) with varying initial bias voltage

The method may include ii) applying a current pulse to the stack; ii) measuring the open circuit voltage; iii) determining one or more oscillation characteristics of the stack;

iv)repeating steps i) to iv) with varying current pulse magnitude/duration. The method may include determining or estimating the bias voltage and/or current pulse characteristics that is required to provide oscillation indicative of zero gap.

The method characteristics of oscillation include frequency and/or damping coefficient.

Said measured voltage/current pulse characteristics may be processed in the time or frequency domain and/or processed using Fast Fourier Transform.

Preferably said current pulse is greater than 1 Amp and between 5 to 50 microseconds. In a further aspect is provided a method of compensating for a variation of said gap in the operation of said fuel injector comprising using the results of the methods above to vary the control signals to the actuator during operation.

Brief Description of Drawings Figures 1 a, b, c and d show sectional views of a fuel injector which includes a piezoelectric stack/actuator;

Figures 2a and b show a known design of fuel injector;

Figure 3 shows the sequence of events on actuation of the piezoelectric stack with respect to interaction between the end of the piezo-stack/actuator and the needle; Figure 4 illustrate the oscillation characteristics of the stack/actuator in a free mode;

Figure 5 shows the methodology and results of exciting a stack/actuator in a real system

Figure 6 shows the resulting nature of oscillation in another mode when the stack is in contact with the valve; and,

Figure 7 shows the nature of the oscillation of the stack/actuator in a mode where there is a small gap between the actuator and the needle. Figures 1 a,b,c and d show sectional views of a fuel injector 1 which includes a piezoelectric stack 2 located within a housing 4 along a generally common central axis. The stack operates to move in slidable fashion, an injection needle 3 so as to move the tip of the needle to/away from a valve seat so as to dispense fuel into a combustion space. Figure 1 b shows in enlarged view the end portion of the stack/actuator which includes actuator components 2a, 2b which in operation, contact the top end of the needle so as to actuate the needle on extension of the stack. Figures lc and d shows further expanded view showing the positions of components at stages with and without a gap between the end of the needle and the end component 2b of the stack arrangement.

Figure 2a also shows a known design of fuel injector. Figure 2b shows in more detail the spring installation portion 5. Figure 2c shows the equivalent portion of the fuel injector to figures la and c. Such a piezo-injector is intended to be designed without a lash adjuster between the actuator and the valve.

Thus as mentioned, in the unenergised state a gap is required between the actuator (stack) and the valve to ensure that the valve remains shut at different thermal conditions. The size of the gap can vary with temperature, piezo-aging, valve/actuator wear, clamping force etc. For this design, one option is to operate the injector control valve without any lash compensation or adjustment device, in direct contact between actuator and valve. In this configuration, due to thermal and ageing effects, there is a small gap between the actuator and the valve, when the actuator is not activated or energized. This gap varies as a function of the temperature, and other effects, like wear, piezoelectric ageing etc.

Figure 3 shows the sequence of events on actuation of the piezoelectric stack with respect to interaction between the end of the piezo-stack/actuator and the needle. The gap is reduced as the end of the stack/actuator (including end components) contact the top face of the needle. It should be understood that alternatively the top portion of the needle may also include associated components, the most distal (uppermost) contacting the stack components in operation. In the last of the sub-figures 32c, the needle is pushed downwardly from the downward force from the stack.

The presence of this gap influences strongly the performance of the injector, as it introduces some variation in the valve movement, for a given actuator movement. It is important, for good consistency in the injector performance, to control and compensate this gap. In addition, many injectors are intended to have closed loop control. It is important that the actuator and valve must be in contact for measurement of force change on the valve to enable closed loop control. Therefore, it is necessary to have a means of measuring or estimating the magnitude of this gap, or the point at which the gap is zero, during operation of the injectors , and allow a vehicle ECU to compensate for this gap via a correction on the injector drive signal.

Some techniques (include using hydraulic lash adjustors/amplifiers) are using strategies based on detection of stress/force change on the actuator when there is contact between the actuator and the valve, measured by detection of a change in the actuator capacitance. Other methods are based on detection of pressure decay in the rail, when the actuator manages to slightly open/leak the control valve, indicating that the gap is closed.

Description of Examples

In a simple example, the gap is measured or estimated depending on the nature of oscillation of the actuator (stack). The phrase "measuring or estimating the gap" includes the notion of determining the point at which the gap is closed i.e. reduced to zero and so interpretation of the term "measuring the gap" should be interpreted to include this.

When the actuator (stack) is not in contact with the valve/ valve needle arrangements or components, it is basically, free to oscillate; it thus has a natural (free) oscillation. If it is excited by a specific signal, its response will be an oscillation (close to a sinusoidal signal), with the main frequency being the natural oscillation frequency of the first longitudinal mode, in line with the piezo-stack axis. This mechanical oscillation of the actuator is basically an exchange of energy between a deformation of the actuator (stress/strain), and velocity (movement). This oscillation will decay with time, in function of the damping of the system. This is illustrated in figure 4.

As this oscillation is linked to oscillation of stress in the actuator, and thus, in the piezo stack, this stress oscillation in the piezo stack can be measured or characterized via the

measurement of the electrical charge in the stack itself. With the actuator maintained in open circuit, for example, the oscillation can be measured as an oscillation of the voltage on the piezo stack; that is to say using the same leads/wires which are used to excite the stack. Figure 5 shows the results of exciting a piezo-stack in a real system. A sharp Dirac style pulse is sent to the actuator as shown in figure 5 a. Figure 5b and c show the actual resulting oscillation, as measured via the electrical terminals of the piezo-electric stack.

Figure 6 shows the resulting nature of oscillation when the actuator is in contact with the valve, i.e. where there is no gap, and where the valve needs a certain amount of force to be moved, and which thus has its own additional mass, the oscillation of the system will be different, both in frequency and in damping characteristics.

Figure 7 shows a condition with very small gap, same order of magnitude as the oscillation displacement, it is likely than the oscillation will be even further damped, due to intermittent contact between the valve and the actuator. In general, the actuator/stack can be extended by applying varying bias voltages and then excited as described above. The vibrational characteristics can then be determined.

Example 1 Now will be described one example of the implementation of the methodology according to one aspect. Firstly bias voltage, or a "start voltage", is applied to the actuator/stack. This voltage will affect the length of the stack/actuator. For a fixed configuration, with an initial gap present between the actuator and the valve, this start voltage, or bias voltage, will increase the length of the actuator, and basically, reduce the gap, between the actuator and the valve. Based on these considerations, the method to measure/estimate the gap is as follows :

A short current pulse is applied on the stack, to excite the system, typically, from 5 to 50 in duration, with current significantly above 1 Ampere. The stack is kept in open circuit, forcing the current to zero, and the voltage, i.e. oscillation, is monitored and recorded. The data pertaining monitored voltage oscillation is processed, so as to characterize the nature of the oscillation. For example the frequency and/or the damping coefficient may be determined. In a preferred embodiment such data processing uses FFT or other methods to identify the way the system oscillate, will allow the characterization of the oscillation, in time domain and/or frequency domain. This process is repeated with increasing amounts of initial voltage so as to vary the length of the stack, i.e. to extend the stack, until it is determined that a point has reached that the characteristics of oscillation are those when the gap is zero. The condition at which the oscillation goes through a significant change can then be used to determine the initial gap.

Thus, one solution can be to perform a series of oscillation measurement, changing progressively the start voltage or the bias voltage. When the bias voltage is too low to fully close the gap, the oscillation of the actuator will be related to the free oscillation

characteristics, when the actuator is not touching the valve. When the bias voltage is large enough so that the gap is almost closed during the oscillation, there will be a strong damping of the free oscillation. If we continue to increase the bias voltage, the system will oscillate again, but with a modified frequency as the system has changed (actuator + preloaded valve now), the bias voltage for which the damping of the free oscillation mode is maximum, can be related with the initial gap.

As mentioned the output will be a bias voltage. Thus the term "estimating or measuring the gap" should be construed as providing the bias voltage required to close the gap, which effectively is a measure of the gap.

The accuracy of the gap measurement will depend on the number of measurements, with different bias voltage. To handle the very small gaps, or the case where there is no initial gap, a negative bias voltage could be applied. In that case, the actuator retracts, so that we can measure the free oscillation mode anyway.

Another example, another solution, would be to vary the amplitude of the excitation pulse, progressively, and, on the same principle as described before, relate the magnitude of the pulse, to the point where a change in the oscillation response/characteristics indicate zero gap.

The time required to perform one measurement of oscillation is typically, between 5 and 10 ms. At engine idle conditions or low speed, the time between injection events is typically larger than 100 ms, so we have time enough to perform one or several oscillation

measurements, between combustion/injection events. The method may be applied on a regular basis, during a large number of engine operating conditions, and can then be used to refine/update the system that will compensate the presence of the gap in the injector, to obtain consistent injection performance, even for variation of gap that cannot be estimated by other indirect methods

This strategy of gap "measurement" can be implemented in the engine control unit (ECU), in the same way as the strategy to "compensate" the gap. The method can be used on designs where a gap between the piezo actuator and the valve (or what the actuator is supposed to move) exist and needs to be estimated/measured.

Claims

1. A method of operating a piezo-electrically operated actuated fuel injector in an internal combustion engine, the fuel injector comprising a piezo-electric stack actuator, one end of which provides an actuation force on a fuel injection needle valve member when the stack is energised to move the valve between open and closed positions, the method comprising measuring one or more the characteristics of oscillation of the stack when a voltage is supplied across the stack in order to provide a measure of a gap between the end of the stack and the valve member.

A method as claimed in claim 1 including the steps of:

applying a current pulse to the stack actuator;

measuring the open circuit voltage in the stack actuator;

determining from ii) one or more oscillation characteristics of the stack.

3. A method as claimed in claim 2 comprising the steps of

i) applying a bias voltage to the stack actuator;

ii) applying a current pulse to the stack actuator;

iii) measuring the open circuit voltage of the stack actuator;

iv) determining one or more oscillation characteristics of the stack;

v) repeating steps i) to iv) with varying initial bias voltage

4. A method as claimed in claims 2 or 3 including

ii) applying a current pulse to the stack actuator;

ii) measuring the open circuit voltage across the stack actuator;

iii) determining one or more oscillation characteristics of the stack actuator from step ii); iv) repeating steps i) to iv) with varying current pulse magnitude/duration.

5. A method as claimed in claims 1 to 4 including determining or estimating the bias voltage and/or current pulse characteristics that is required to provide oscillation indicative of zero gap.

6. A method as claimed in claims 1 to 5 wherein the characteristics of oscillation include frequency and/or damping coefficient.

7. A method as claimed in claims 5 or 6 wherein said determined or estimated measured voltage and/or current pulse characteristics are processed in the time or frequency domain.

8. A method as claimed in claim 7 wherein determined or estimated measured voltage and/or current pulse characteristics are processed using Fast Fourier Transform.

9. A method as claimed in claims 2 to 8 wherein said current pulse is greater than 1 Amp.

10. A method as claimed in claims 2 to 9 wherein said current pulse is between 5 to 50 microseconds.

11. A method of compensating for a variation of said gap in the operation of said fuel injector comprising using the results of the methods of claims 1 to 10 to vary the control signals to the actuator during operation.