FR2751702A1 - Internal combustion engine fuel injection system with piezo- electric fuel atomiser - Google Patents

Abstract

The fuel injection system has a body (2) with injector (1) inside of which is formed a space (4) connected to a pressurised fuel supply circuit. The space (4) outputs to an injection bus (12) at the end of which is fuel injection orifice (14) co-operating with movable vanes (3) which move between an open position and a closed position. The vanes are actuated by a drive (5,6) operated by an electronic system which controls the engine operating conditions. The system has device (8) for cyclically pressurising the fuel filling the bus (12). The frequency and intensity of the over-pressure being adjusted by a control driven by the engines electronic system so as to produce fuel atomisation in the form of drops of a pre-determined size during opening of the vanes (3).

Description

FUEL INJECTION DEVICE FOR
INTERNAL COMBUSTION ENGINE
The present invention relates to a fuel injection device for an internal combustion engine intended in particular to equip a motor vehicle. The invention relates more particularly to a fuel injection device using a piezoelectric actuator to atomize the fuel injected in the form of very fine droplets.

 The fuel injection devices used today on internal combustion engines fitted to motor or road vehicles, conventionally operate on the model of a tap whose open or closed state is continuously controlled, the dosage of the injected fuel then done directly by the opening time of the injectors.

 Such injection systems include an electric fuel supply pump which supplies, via a distribution manifold, all of the injectors under a pressure having a constant difference with the pressure prevailing in the intake manifold. through a pressure regulator. By electronically controlling the electromagnet actuating the valve of each injector, one controls the beginning and the duration of opening of this one and one then determines a precise flow of fuel for each one of the injectors, thus the quantity of fuel injected depends only the opening time of the electro-injectors.

 The injectors of the electromagnetically controlled needle type, which are the most commonly used, however have limits which slow down the improvement in engine performance, particularly in terms of pollution control. In particular, the times taken to open or close the needles are still too high, of the order of 1 to 2 ms, which prevents the injection from being distributed correctly over the entire opening time of the valve. In addition, the minimum opening time, which determines the minimum dose of fuel that can be injected, is still too long for certain engine operating points.

 Known needle injectors also have injection holes of relatively large diameters to allow the delivery of the required quantities of fuel for operations at full load and high engine speeds. This arrangement generates fuel jets having drops of large dimensions, which slows down the vaporization of the fuel (and therefore the preparation of the fuel mixture) and is capable of promoting the phenomenon of wall wetting.

 In fact, the non-vaporized fuel tends to settle on the walls of the intake duct or the combustion chamber (by direct injection).

Such a deposit causes metering problems, particularly acute in transients due to a lack of knowledge of the quantity of fuel which actually enters the corresponding combustion chamber. This wall wetting phenomenon is one of the major causes of high pollutant emissions during cold engine starts.

 In addition, with a conventional needle injector, when the needle opens when the latter begins to leave its seat, a liquid bulb forms which disappears when the needle is fully raised, the fluid flow regularizing then. This change in the nature of the flow makes any precise control of the instantaneous injector flow impossible.

 Some have sought to solve these various problems, by developing injectors using piezoelectric actuators to maneuver the needle so as to reduce the duration of opening and closing of the needle, but such systems which always operate according to the principle of the valve, retain significant drawbacks related in particular to the significant dispersion affecting the size of the drops in the fuel jet leaving the nozzle nose.

 All of the problems mentioned above therefore result in: a vaporization of the fuel which may be incomplete and not homogeneous during the preparation of the fuel mixture in the combustion chamber, inaccurate dosages, with the result of incomplete combustion resulting in the formation a high amount of polluting gases and an energy deficit affecting engine performance.

 The object of the present invention is therefore to solve all of these problems by proposing an injection device capable of delivering a cloud of fuel drops whose sizes are perfectly calibrated to ensure precise metering and sufficiently small to ensure the complete and homogeneous spraying of the injected fuel.

 The fuel injection device for an internal combustion engine with spark ignition according to the invention is of the type comprising an injector body inside which is arranged a capacity connected to a fuel supply circuit under a suitable pressure , this capacity opening into an injection nozzle at the end of which is formed a fuel injection orifice provided with shutter means movable between an open position and a closed position under the action of piloted maneuvering means by an electronic engine control system according to the engine operating conditions.

 According to the invention, the fuel injection device is characterized in that it comprises means for pressurizing the fuel cyclically filling said nozzle, the frequency and the intensity of the overpressure thus generated being adjusted by means of control controlled by the electronic engine control system to produce an atomization of the fuel in the form of droplets of predetermined sizes and allow the injection of a desired quantity of fuel during the opening of said shutter means.

 According to the invention, the injection of fuel through the injection orifice is done by application of an overpressure in the injection nozzle. Each overpressure corresponds to the injection of an elementary quantity of fuel in the form of a beam of droplets, to inject the total quantity required, it is therefore necessary to generate an appropriate succession of overpressures. The number of cycles is very simply calculated as the ratio of the total quantity required by the elementary quantity injected at each pressure pulse.

 This cyclic variation in the pressure of the fuel contained in the injection nozzle, also called dynamic pressure, is caused according to a particular embodiment by deformation of the walls of the cavity. The variation in volume thus obtained is adapted to make it possible to generate the overpressure necessary for injecting the fuel.

 To control the deformation of the walls of the cavity, it is possible to use piezoelectric or magnetostrictive materials activated by an electric power source controlled by the engine control system. Such materials deform under the influence of an electric or magnetic field and therefore allow the application, by direct contact or remotely by indirect action, of stresses on the walls, thus generating the desired displacements.

 Such an electronically controlled injection device allows very precise control of the size of the drops injected and which is completely independent of the flow rate and the duration of injection.

 It also offers the possibility, by being placed in the vicinity of an intake valve, of injecting the fuel into the combustion chamber during the entire opening time of the valve, and thus ensuring precise control of the dosage. and good vaporization of the fuel directly in the chamber thanks to the small diameter of the drops injected.

 Such an arrangement makes it possible to take advantage of the air flow entering the combustion chamber during this intake phase to assist the separation of the series of drops formed on the injection orifice and prevent the drops from settling on the neck of the valve by entrainment effect. In addition, this air flow causes abrasion of the injected fuel drops, thus contributing to their vaporization as soon as they are formed.

 According to another characteristic of the fuel injection device for an internal combustion engine object of the invention, the passage section of the injection orifice is shaped so as to form fuel drops of substantially uniform dimensions. The size of these drops is preferably between 20 to 40 microns in diameter.

 According to different embodiments, this injection orifice is in the form of a straight or circular oblong slot, of transverse dimension reduced to a few microns, for example of the order of 15 to 20 microns.

The objects, aspects and advantages of the present invention will be better understood from the description presented below of various embodiments of the invention, given by way of nonlimiting examples, with reference to the accompanying drawings, in which
Figure 1 is a sectional view of the cylinder head of an internal combustion engine and of a first embodiment of the fuel injection device according to the invention
Figure 2 is a view similar to Figure 1 showing a second embodiment of the injection device according to the invention
FIG. 3 is a partial detail view of the injection device shown in FIG. 1.

 In accordance with the appended drawings, only the elements necessary for understanding the invention have been shown. In addition, to facilitate reading of these drawings, the same parts have the same references from one figure to another.

 Referring to Figure 1, we see in cross section the end of an intake duct 30 formed through the cylinder head 31 of an internal combustion engine of the multicylinder type with spark ignition. This conduit 30 opens into the combustion chamber 33 defined on the underside of the cylinder head 31 by means of an inlet orifice 32 serving as a seat for a stem valve 9 slidably mounted in a guide 34 housed in the cylinder head 31. The opening of the valve 9 is conventionally controlled by suitable means which are not shown, such as a camshaft.

 The fuel injection device according to the invention is of the multipoint type with electronic control. Each combustion chamber 33 of the engine is supplied with fuel by at least one specific injector 1. According to the particular layout shown, the injector 1 which is the subject of the present invention is shaped to come and surround the intake valve 9.

 This injector 1 which extends into the intake duct upstream of the combustion chamber 33 in the direction of the flow of the oxidizing fresh air, is essentially formed of a body consisting of a tubular body 2 substantially cylindrical arranged coaxially to the valve stem 9.

 This body 2 is anchored at its upper axial end, identified by the point G, directly on the guide 34 of the valve 9 which is adapted accordingly while, on the other hand, its lower axial end, identified by the point A, extends in the vicinity of the seat 32 of the valve 9.

The body 2 has a wall of changing thickness defining two internal bores 22 and 24 of distinct diameters. The bore 22 is intended to receive sliding, in the zone located axially between the reference points E and
D, a plunger 3 consisting of a sleeve 13 also extending coaxially to the stem of the valve 9.

 The appropriate adjustment of the respective diameters of the bore 22 and of the sleeve 13 makes it possible to obtain centering and precise translational guidance of the latter relative to the body 2 of the injector. Preferably, the clearance between the facing walls is a few microns.

The bore 24, of diameter substantially greater than the bore 22, delimits with the walls facing the sleeve 13 located between the points
D and C, a substantially cylindrical cavity 4. The cavity 4 thus defined is intended to be permanently supplied with fuel by several supply channels 15 arranged radially through the sleeve 13. These channels are connected to a supply circuit fuel under a given pressure not shown.

 The dimensions of this cavity 4 and more particularly its height (in the axial direction) are adjusted as a function of the quantities of fuel to be delivered, its thickness (in the radial direction), namely the distance separating the walls of the bore 24 from the body 2 and those opposite the sleeve 13, is of the order of a few hundred microns.

 The lower end walls of the body 2, like those opposite the sleeve 13, located between points C and BA, terminate in the vicinity of the head of the valve 9, flaring and approaching one of the 'other until it comes into contact with their axial ends, identified respectively by points B and A.

 As a result, the cavity 4 narrows downward to form a fuel injection nozzle 12 which in turn ends in a circular slot 14 comprised between the two axial ends of the body 2 and of the plunger 3, slot 14 which constitutes the injection port of injector 1.

 The opening of this slot 14 is controlled by the axial movement of the plunger 3 which is movable relative to the body 2 which remains fixed.

The end B of the plunger 3 therefore constitutes a shutter valve for the slot 14, coming to rest against the corresponding seat which is formed by the end A of the body 2.

 The plunger 3 is able to move between the closed position where the respective ends A and B of the body 2 and of the plunger 3 are in contact and an open position where the end B is offset axially from the end A of preset value. These movements are made by means of an appropriate operating mechanism located in the upper part of the body 2 between the points G and E.

 This operating mechanism of the plunger 3 essentially consists of a tube element 6 with a thin wall extending the sleeve 13 and a stack of piezoelectric ceramics 5.

The tube element 6 forms elastic return means forcing the plunger 3 into the closed position of the slot 14, while when activated, the piezoelectric actuator 5 tends to push the plunger 3 from its seat At a given height against the antagonistic action of the tube 6.

The tube element 6 is fixed at its upper axial end, directly on the guide 34 of the valve 9. I1 also has a rigid tapped zone 16, extending between points G and F, on its external face so to receive a nut 7. This threaded portion 16 of the tube 6 is extended by a flexible thin wall of concave shape, extending between the points F and
E, to then join the sleeve mounted to slide in the body of the injector 2.

 The junction between the tube element 6 and the sleeve 13 defines a radially projecting shoulder which serves as an axial support at the lower end of the annular multilayer stack of piezoelectric ceramics 5. The other end of the stack 5 is prestressed by nut 7 with a suitable tightening force.

 The stack 5 is connected to a suitable electrical power supply not shown, controlled by an electronic motor control system also not shown. By applying an electrical voltage between the electrodes of the stack 5, this stack is deformed, resulting in an axial translation of the sleeve 13, by elastic deformation of the concave walls of the tube 6, by a predetermined value e and therefore also a translation along the axis of the opposite ends A and B of the body 2 and of the plunger 3 defining the circular slot 14 terminating the cavity 4, thus creating a circular opening of width e.

 The adjustment of the body 2 and of the plunger 3 in their central parts located between the points E and D allows an easy displacement of the plunger 3 and makes it possible to limit the angle between the two axes of symmetry of the two end walls A and B to preserve uniformity of the slot 14 along its circumference.

For an axial displacement of the plunger 3 of the order of twenty microns and a clearance <ed of ten microns between the parts in their central parts, we obtain, for substantially identical lengths on either side of their central zone (distance DG next to DA), slight deviations possible in the opening of the slot 14 limited to a maximum of:
[e2 + d2] 1/2 ~ e = 2.5 ssm.

One can obtain an axial displacement <ee of twenty microns typically by applying fifty volts on a stack of twenty millimeters in height containing one hundred and thirty layers of annular piezoelectric ceramics, the axial force generated reaches several kilo
Newtons and is exercised in times of less than a hundred microseconds. These stacks are available industrially. This makes it possible to ensure a circular opening of the slot 14 of twenty microns in a time less than 0.1 millisecond to initiate the ejection of the fuel from the nozzle 12 through the slot 14.

Indeed, consider that the elastic wall 6 exerts a restoring force F, for a total weight of the inner plunger 3 of one hundred grams. To move the plunger 3 axially from twenty microns to one hundred microseconds, the stack must develop a maximum force T allowing the desired acceleration G of the plunger 3 and the deformation of the wall 6 to be ensured, ie
T = (mx G) + F
In the hypothesis where the displacement of the plunger 3 takes place according to a sinusoid of the type uOsin t, we then have
-at? 2
G = u0
either taking into account the numerical values retained
mx G = 250 Newtons.

 The restoring force F exerted by the elastic wall 6 is therefore defined to exert a stress of the same value to have a closing time which is substantially identical to the opening time, ie two hundred and fifty Newtons.

The ceramic stack 5 must therefore develop, upon opening, a stress T of the order of five hundred Newtons, which is widely admissible with the actuators currently on the market.

 The injection orifice of the nozzle 12 that constitutes the slot 14 is therefore suitable for delivering a cloud of fuel drops whose sizes are perfectly calibrated. In fact, thanks to the restricted radial dimension of the passage cleared during the lifting of the plunger 3, between fifteen and twenty microns, the fuel drops have a uniform diameter of between twenty and fifty microns.

 This reduced radial dimension of the injection orifice however extending over a suitable perimeter, the total section of the orifice is sufficient to allow the injection of the large quantities of fuel required for operations at full load and at high speed of the engine. The drops thus generated are then small enough to ensure complete and homogeneous vaporization of the injected fuel.

 During the time of application of the voltage between the electrodes of the stack 5, the opening of the circular slot 14 defined between the ends A and B, remains constant or varies according to the value of the applied voltage.

When this voltage drops to zero, the elastic return wall 6 ensures the closing of the circular slot 14 between A and B with a closing time of the same order of magnitude as that necessary for opening.

 According to a particularly original aspect of the injection device which is the subject of the invention, the fuel supply pressure filling the cavity 4 and the nozzle 12 is chosen as a function of the dimensions of the injection orifice formed by the slot 14 of so that when it opens, the surface tension of the liquid causes the formation of a meniscus preventing any flow of the latter out of the slot 14.

 The ejection of the liquid from the slot is therefore not generated by the lifting of the plunger but by the pressurization of the liquid contained in the cavity and therefore in the injection nozzle.

This pressurization is not continuous but cyclical. Each pressure pulse causes the ejection of an elementary quantity of fuel in the form of a beam of calibrated droplets, and it is the repetition of these elementary ejections at high frequency which produces the ejection of the total quantity desired in the given period of time, taking into account the operating conditions of the engine. The elementary quantity of fuel which is determined by the cross section of the injection orifice and the value of the overpressure is chosen to be small enough to satisfy all the operating conditions of the engine.

 To do this, the wall of the body 2 of reduced thickness in the zone of the cavity 4, that is to say between the points C and D, is surrounded by a cylinder made of piezoelectric material 8. This cylinder 8 is directly attached to the corresponding outer face of the injector body 2.

 The internal and external surfaces of the cylinder 8 which make up the electrodes are connected to a suitable electrical supply, not shown, controlled by the electronic engine control system which also controls the stack 5.

 With a radial polarization, when an alternating voltage is applied between the electrodes of the cylinder, one is able to vary its internal diameter which simultaneously causes a variation in diameter of the corresponding walls of the body 2, the nature and thickness of which are adapted to allow such elastic deformation.

 These variations in diameter of the cylinder 8 generate radial vibrations of the outer wall of the cavity 4. These vibrations generate, in turn, a dynamic pressure in the liquid fuel filling the cavity 4 and the injection nozzle 12.

 The wall of the plunger 3 is dimensioned so as to have sufficient rigidity to withstand the pressure peaks in the cavity 4 and in the nozzle 12 while undergoing only deformations causing variations in diameter of the second order relative to the variations in diameter of the walls body 2.

 At each tension cycle applied to the piezoelectric cylinder 8, a pressure pulse is generated in the cavity 4. The overpressure then generated is sufficient to break the surface tension of the meniscus of liquid forming in line with the annular slot 14 between A and B, when the latter is open.

 The suddenly pressurized liquid is then ejected in the form of a ring of drops whose diameter is directly a function of the dimensions of the opening of the slot 14.

The inclination of the end part of the cavity 4 defining the injection nozzle 12 between B and C which results from the flaring of the end walls of the body 2 and of the plunger 3, makes it possible to form a conical beam of drops suitable for be directed directly into the combustion chamber 33 when the valve 9 is opened.

 By way of example, the injection device comprises a cavity 4 of a hundred and fifty microns thick comprised between a body 2 made of brass one millimeter thick and a plunger 3 also made of brass twelve millimeters in diameter having a thickness of one hundred microns, the body 2 being associated by bonding to a piezoelectric ceramic cylinder 8 one millimeter thick over a length of fifteen millimeters (between points D and C). We then obtain with an alternating voltage of seventy five volts at eighty kiloHertz, the ejection of a beam of fuel drops. The diameter of the drops depends on the setting of the opening and can be chosen between twenty and fifty microns. The ejection speed of the drops can be adjusted between one to three meters per second for a given diameter.

 Thanks to the particular embodiment of the injection device according to the invention, represented in FIG. 1, it is possible to initiate the injection of the fuel when the valve 9 is lifted and to inject the fuel directly into the chamber combustion through the opening 10 thus released.

 Furthermore, during the opening of the valve 9, the intake air flow filling the combustion chamber 23 has a speed of the order of 70 meters per second in the vicinity of the valve neck. This flow assists the ejection of the fuel and drives the drops which are ejected from the circular slot 14 in the form of a conical beam to propel them directly into the chamber 23 through the opening 10.

 The difference between the ejection speed of the drops, close to 3 meters per second and that of the air flow, causes abrasion of the fuel drops and contributes to their evaporation.

 The opening of the ejection slot 14 can be synchronized with the opening of the valve 9 by simple voltage control of the actuator 5 and the ejection of the drops is then produced by application of an alternating voltage on the electrodes of the cylinder 8 for a predetermined time allowing the injection of the required fuel flow according to the operating points of the engine.

 Each pressure pulse producing the ejection of a determined number of drops, identified 11, it is therefore possible to measure very precisely the quantity of fuel injected by electrically controlling the corresponding number of pulses.

 FIG. 2 illustrates a second embodiment of the mechanism for operating the plunger 3 and opening the slot 14. In this version, the piezoelectric actuator 5 is replaced by an entirely mechanical system. The valve 9 has a curved part 19 projecting around the rod, this part 19 coming to bear on the elastic wall element 6 when the valve is lifted 9. The pressure caused by the pressing of the surface 19 against the wall 6 results in an elongation of the latter which in turn produces a translation of the lower part of the plunger 3 and a corresponding opening of the slot 14.

 In this second particular embodiment of the invention, the opening of the circular slot 14 is directly determined by the diameter of the curve 19 and by the rigidity of the flexible wall 6.

 FIG. 3 details a piezoelectric system capable of ensuring the deformations of the walls of the cavity 4 generating the cyclic overpressures of the fuel which it contains. The piezoelectric cylinder 8 is subdivided into several curved plates 13. Each piezoelectric plate 13 is secured to the wall of the body 2 by bonding and the electrodes are formed by the two facets. When a voltage is applied between these electrodes, by necking effect, each piezoelectric plate 13 stretches in its two planar directions; the state of stress on the two faces being different, this has the effect of creating a bending of the plate which results locally in a change of curvature of the corresponding wall of the body 2. Each plate 13 exerts the same deformation in phase with the applied electrical voltage. The resulting effect is a variation in volume appearing at each tension cycle, which makes it possible to generate the desired overpressure at each cycle in the cavity 4 delimited between the body of the injector 2 and the plunger 3.

 Of course, the invention is in no way limited to the embodiments described and illustrated which have been given only by way of example.

 On the contrary, the invention includes all the technical equivalents of the means described and their combinations if these are carried out according to the spirit.

 Thus, the invention is not limited to an injector body 2 disposed concentrically with the axis of a valve 9. The body 2 of the injector can thus be disposed further upstream in the intake duct, or even outside the cylinder head in the intake manifold.

 Thus the present invention can be applied to all types of injectors and in particular to a conventional needle injector. However, it suffices that the injection nozzle can cooperate with an excitation source capable of creating pressure waves at very high frequencies.

 Likewise, it is possible, by adjusting the intake pressure of the fuel introduced into the cavity 4 accordingly, to operate with an ejection slot 14 permanently open during the entire operating time of the engine.

The injection is then controlled by the sole pressurization of the cavity 4 by the piezoelectric element 8. In the absence of excitation of the element 8, the surface tension of the fuel creates a meniscus which prevents any leakage of fuel through slot 14.

Claims (10)

[1] Fuel injection device for an internal combustion engine with positive ignition of the type comprising a body (2) of injector (1) inside which is formed a capacity (4) connected to a supply circuit in fuel under a suitable pressure, said capacity (4) opening into an injection nozzle (12) at the end of which is formed a fuel injection orifice (14) cooperating with movable shutter means (3) between an open position and a closed position under the action of maneuvering means piloted (5,6) by an electronic engine control system according to the engine operating conditions, characterized in that it comprises means for setting in cyclic pressure (8) of the fuel filling said nozzle (12), the frequency and intensity of the overpressure being adjusted by control means controlled by said electronic engine control system to produce atomization of the coach burant in the form of droplets of predetermined sizes and allow the injection of a desired quantity of fuel during the opening of said shutter means (3). [2] Fuel injection device for internal combustion engine with positive ignition according to claim 1, characterized in that the passage section of the injection orifice (14) during the opening of said shutter means (3 ) is shaped so as to form drops of fuel of substantially uniform dimensions. [3] Fuel injection device for internal combustion engine with positive ignition according to claim 2, characterized in that the passage section of the injection orifice (14) during the opening of said shutter means (3 ) is shaped so as to form fuel drops with a diameter between 20 and 50 microns.  [4] Fuel injection device for internal combustion engine with positive ignition according to any one of claims 2 to 3, characterized in that the injection orifice (14) is formed by an oblong slot whose dimension transverse is between 15 and 20 microns. [5] Fuel injection device for internal combustion engine with positive ignition according to claim 4, characterized in that the injection orifice (14) is formed by a circular slot. [6] Fuel injection device for internal combustion engine with spark ignition according to any one of claims 1 to 5, characterized in that the fuel supply pressure entering the cavity (4) is adapted to the dimensions of the injection orifice (14) so as to be insufficient to cause the fuel to be ejected from said orifice (14) when said shutter means (3) are opened. [7] Fuel injection device for internal combustion engine with positive ignition according to any one of claims 1 to 6, characterized in that the operating means of said movable shutter means (3) comprise a piezoelectric actuator ( 5) and elastic return means (6). [8] Fuel injection device for an internal combustion engine with positive ignition according to any one of claims 1 to 7, characterized in that said means for cyclic fuel pressurization filling said injection nozzle (12) include a piezoelectric actuator (8).  [9] Fuel injection device for an internal combustion engine with positive ignition according to claim 8, characterized in that said piezoelectric actuator consists of a plurality of piezoelectric plates (13) secured to the wall of thickness adapted to the body (2) of the injector (1) in line with the capacity (4). [10] Fuel injection device for internal combustion engine with spark ignition according to any one of claims 1 to 9, characterized in that said body (2) of the injector (1) is adapted to be mounted concentrically to an intake valve (9) controlling the introduction of the combustion air into a combustion chamber (33) of the engine.