GB2472828A

GB2472828A - Correcting the charge current profile of a piezoelectric injector of an i.c. engine

Info

Publication number: GB2472828A
Application number: GB0914565A
Authority: GB
Inventors: Tommaso De Fazio; Ignazio Dentici
Original assignee: GM Global Technology Operations LLC
Current assignee: GM Global Technology Operations LLC
Priority date: 2009-08-20
Filing date: 2009-08-20
Publication date: 2011-02-23
Also published as: GB0914565D0

Abstract

The invention relates a method for controlling a piezoelectric injector for metering fuel inside the cylinder of a combustion engine, wherein the injector comprises an injection needle operated by means of a piezoelectric actuator and a charge current profile is applied to the piezoelectric actuator. The method comprising the following steps: (i) calculate a charge setpoint value in function of an engine operating parameter, (ii) sample, during at least a time interval of the fuel injection cycle, the charge stored in the piezoelectric actuator, (iii) determine the difference between the charge setpoint value and the charge stored in the piezoelectric actuator and (iv) use the difference between the charge setpoint value and the charge stored in the piezoelectric actuator to generate a correction index to be applied, during the next fuel injection cycle, to the charge current profile.

Description

METHOD FOR CONTROLLING A PIEZOELECTRIC INJECTOR

TECHNICAL FIELD

The present invention relates to a method for controlling a piezoelectric fuel injector for an internal combustion engine.

BACKGROUND OF THE INVENTION

The fuel injection system of internal combustion engine is provided, for each cylinder, with at least a fuel injector comprising an injection needle allowing the injection of fuel from a distribution pipe, known as rail, into the cylinder. The injection needle works as a valve member opening and closing a valve seat.

The movement of the injection needle can be operated by means of a piezoelectric actuator comprising a stack of piezoelectric elements whose displacement is proportional to the charge transferred to the piezoelectric elements according to a current profile.

During charging and discharging phases, piezoelectric actuator can be considered as a capacitor, which equivalent electric value is strictly depending on voltage across the stack of the piezoelectric elements, the stack's temperature, and the injector aging. Hence, in order to control and guarantee that injector is open for the whole injection duration, it's necessary to control the relevant electric parameters across the piezoelectric actuator.

The electrical parameter that is typically used by piezoelectric injector control systems, is the voltage across piezoelectric actuator between the end of the charging phase, and the beginning of the discharging phase.

Prior art control systems perform, during an injection cycle, a stable voltage closed loop control on piezoelectric injector comparing the voltage measured across the stack of piezoelectric elements with a reference value in order to apply a corrected current profile during the next injection cycle. This operation mode produces good outcomes when applied during long duration injections, such as for instance the main fuel injection in common rail i.c. engine where a large quantity of fuel has to injected in the combustion chamber, while it does not produce sure results when applied during short duration injections for metering in the combustion chamber small quantity of fuel.

This behavior is due to the voltage pattern variation across the piezoelectric actuator which is disclosed in Fig. 1. As can be noticed, voltage across piezoelectric actuator, just after the charging phase, can be seen as a sine wave which damps in function of the injection time and fuel injection system high pressure.

During long duration injections, for instance for the time T2 indicated in Fig.l, the voltage value damps up to reach a steady state value and therefore a voltage measure, performed when the voltage value is quite stable, gives a sure and repeatable value which can be used in voltage closed loop control to guarantee the correct displacement of the injection needle during the next injection cycle.

On the contrary, during short duration injection, for instance for the time Ti indicated in Fig.l, corresponding to small quantity of fuel to be injected in the combustion chamber, the damping of the voltage does not reach a stable value, and hence a measure of the voltage across the piezoelectric elements does not give a sure value which can be compared to a reference value to guarantee the opening of the injection needle during the next injection cycle.

As a consequence, during short duration injections, it's not possible to perform a stable voltage closed loop control on piezoelectric injector, and to guarantee precision of small injected fuel quantity.

A wrong injected quantity of fuel has effects on engine emission of polluting substances and on combustion noises.

This is particular relevant in multi-jet diesel combustion engine where multiple small fuel quantity injection are required.

Aim of the present invention is to solve, or at least to positively reduce, the above mentioned drawbacks with a simple, rational and inexpensive solution.

The aim is attained by the characteristics of the invention as reported in independent claim 1. The dependent claims recite preferred and/or especially advantageous features of the invention.

DISCLOSURE OF THE INVENTION

The invention provides a method for controlling a piezoelectric injector for metering fuel inside the cylinder of an internal combustion engine, wherein the injector comprises an injection needle operated by means of a piezoelectric actuator and a charge current profile is applied to the piezoelectric actuator.

The method comprises the following steps: -to calculate a charge setpoint value in function of an engine operating parameter, -to sample, during at least a time interval of the fuel injection cycle, the charge stored in the piezoelectric actuator, -to determine the difference between the charge setpoint value and the charge stored in the piezoelectric actuator, -use the difference between the charge setpoint value and the charge stored in the piezoelectric actuator to calculate a correction index to be applied, during the next fuel injection cycle, to the charge current profile.

According to the invention the engine operating parameter used to calculate the charge setpoint value can be the temperature of the piezoelectric actuator or the fuel pressure measured in the rail. However alternative different engine parameters can be taken into consideration provided that they are relevant for the operation of the piezoelectric actuator.

The charge current profile is defined in terms of current setpoint values and charging times.

The step to apply a correction index to the charge current profile provides to multiply the correction index for the charge current profile.

Since the charge, stored in the piezoelectric actuator during an injection cycle, has a stable value just after the charging phase and up to the discharging phase, the use of the charge value, as relevant electric parameter in a closed loop control method, is particularly advantageous because it guarantees the correct opening of the injection needle and an accurate fuel delivery into the combustion chamber.

BRIEF DESCRIPTION OF THE DEAWINGS

The present invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a schematic illustration of the voltage pattern across the piezoelectric actuator during an injection; Figure 2 is a schematic illustration of a flow chart of the circuit allowing the method according to the invention; Figure 3 is a schematic illustration of the charge variation across the piezoelectric actuator.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Fig. 2 shows a piezoelectric injector 1 comprising a needle 2 operated by a piezoelectric actuator 3 which is electrically connected to a control circuit 4.

The control circuit 4 comprises a charge circuit 5 which is able to apply to the piezoelectric actuator 2 a charge current profile, which is defined in terms of current setpoint values and charging times.

The control circuit 4 comprises also a memory module 6 in which, as a map, charge setpoint values are stored in function of an engine parameter. As reference engine parameter, to calculate the charge setpoint, can be chosen the temperature of the piezoelectric elements or the fuel pressure measured in the rail.

The charge setpoints are values of the charge which guarantee the opening of the injection needle for all the injection duration.

A sampler circuit 7 is connected to the piezoelectric injector with the function of sampling, during a fuel injection cycle, the charge stored in the piezoelectric actuator. The sampling of the charge values store in the piezoelectric actuator is performed during at least one specific time interval after the start of the injection event.

The charge stored in the electric actuator is determined by an usual integrator 8, connected between the piezoelectric actuator 3 and the sampler circuit 7. The integrator 8 performs an integral calculus of the current flowing through the piezoelectric actuator during an injection cycle: Qm Ji dt where I is the current flowing through the piezoelectric actuator.

The sampled values of the charge are sent to an adder 9 with which is also connected the memory module 6 storing the setpoint charge. The adder 9 calculates the difference (e) between the charge setpoint value(Qs) and the sampled charge (Qrn) stored in the piezoelectric actuator: (e)=Qs-Qm The difference (e) is supplied to a controller 10, for instance a P1 controller, which in function of the above named difference, generates a correction index, which is used as output signal of the controller 10, to correct the applied charge current profile in the next injection cycle.

According to the present disclosed embodiment of the invention the correction of the charge current profile is performed by means of a multiplier 11 which multiplies the correction index for the charge current profile and supplies the corrected charge current profile to the piezoelectric actuator 2 of the injector.

Fig. 3 shows the variation of the charge stored in the piezoelectric actuator during an injection cycle. As it can be notice the charge stored in the piezoelectric actuator has a stable value just after the charging phase and up to the discharging phase. Since the charge stored in the piezoelectric actuator is linked to the physical displacement of the stack of the piezoelectric elements, and consequently to the opening (lift up) of the injection needle, the use of stable charge value, in the control ioop system, guarantees a correct opening of the injection needle and an accurate fuel supply into the combustion chamber also during small fuel quantity injection.

While the present invention has been described with respect to certain preferred embodiments and particular applications, it is understood that the description set forth herein above is to be taken by way of example and not of limitation. Those skilled in the art will recognize various modifications to the particular embodiments are within the scope of the appended claims. Therefore, it is intended that the invention not be limited to the disclosed embodiments, but that it has the full scope permitted by the language of the following claims.

Claims

CLAIMS1. A method for controlling a piezoelectric injector for metering fuel inside the cylinder of a combustion engine, wherein the injector comprises an injection needle operated by means of a piezoelectric actuator and a charge current profile is applied to the piezoelectric actuator, the method comprising the following steps: -to calculate a charge setpoint value in function of an engine operating parameter, -to sample, during at least a time interval of the fuel injection cycle, the charge stored in the piezoelectric actuator, -to determine the difference between the charge setpoint value and the charge stored in the piezoelectric actuator, -use the difference between the charge setpoint value and the charge stored in the piezoelectric actuator to generate a correction index to be applied, during the next fuel injection cycle, to the charge current profile.
2. Method according to claim 1, wherein the engine operating parameter used to calculate the charge setpoint value is the engine temperature.
3. Method according to claim 1, wherein the engine operating parameter used to calculate the charge setpoint value is the fuel pressure measured in the rail.
4. Method according to claim 1, wherein charge current profile is defined in terms of current setpoint values and charging times.
5. Method according to claim 1, wherein the correction index is a correction value which is multiplied for the current charging profile.