The present invention relates to a fuel-injection system for an internal-combustion engine and to the corresponding method for controlling fuel injection.
In the engine sector, there is felt the need to make injections of fuel in which the instantaneous flow rate of injected fuel as a function of time presents an evolution comprising at least two stretches with levels that are substantially constant and different from one another; i.e., it can be represented schematically with a curve of the stepwise type. In particular, there is felt the need to inject an instantaneous flow F of fuel having an evolution in time T similar to the one represented by the curve of
In an endeavour to obtain such a flow-rate curve, it is known to provide injectors of a dedicated type, in which opening of the injection nozzle is caused by the lifting of two mobile open/close pins or needles, co-operating with respective springs, or else by the lifting of a single open/close needle co-operating with two coaxial springs. The two springs are differently preloaded with respect to one another, and/or present characteristics of force/displacement that are different from one another, for opening the nozzle with lifts such as to approximate the required flow-rate curve.
The known solutions just described are far from altogether satisfactory in so far as it is somewhat complex to calibrate the springs in an optimal way to obtain a first flow-rate level or step L1 lower than the level L2 of the maximum flow-rate from the nozzle and, hence, to approximate a flow-rate curve like the one of
Known from the document FR 2 761 113 A is an injection system comprising a control unit designed to control the injector. in such a way that, for each cycle, a pre-injection is first performed, followed by a main injection, which starts before the pre-injection has ended. This system presents the disadvantage of allowing situations in which it is not possible to obtain a pre-injection.
The aim of the present invention is to provide an injection system for an internal-combustion engine and a method for controlling injection of fuel which will enable the drawbacks set forth above to be solved in a simple and inexpensive way.
The above aim is achieved by a fuel-injection system for an internal-combustion engine, as defined in claim 1, and by a method for controlling fuel injection, as defined in claim 14.
For a better understanding of the invention, a preferred embodiment is now described, purely by way of non-limiting example, with reference to the attached drawings, wherein:
FIGS. 4 to 6 are graphs regarding the operation of an electroinjector according to preferred embodiments of the invention;
Designated as a whole by the reference number 1 in
The electroinjector 1 terminates with an atomizer, which comprises a nozzle 5 communicating with the inlet 4 through an injection chamber 6. The nozzle 5 has a conical tip 5b provided with holes 5a for injection of the fuel into a combustion chamber of the engine. The nozzle 5 is normally held closed by an open/close needle 7, having a conical tip 7a designed to engage the conical tip 5b. The needle 7 is mobile in an axial seat 9 for opening/closing the nozzle 5 under the control of an electroactuator device 8, which will be described in greater detail hereinafter. In particular, the conical tip 7b of the needle 7, by engaging the conical tip 5b of the nozzle 5, closes the holes 5a.
The needle 7 has an active surface subject to the pressure of the fuel in the chamber 6, said active surface being formed by a shoulder or annular surface 7a (
The electroinjector 1 carries out metering of the fuel by modulating opening of the needle 7 of the atomizer in time as a function of the supply pressure of the electroinjector 1 itself, i.e. of the pressure of the fuel at the inlet 4 (
The shell 2 has an axial seat 13, made as a prolongation of the seat 9, in which a rod 14 is housed, engaged with the needle 7 for transmitting to the latter an axial thrust under the action of the pressure of the fuel. Set between the needle 7 and a shoulder of the seat 13 is another spring 21, which contributes to keeping the needle 7 in the position for closing the nozzle 5. In particular, fixed in an intermediate stretch of the seat 13 is a metering solenoid valve 16, comprising a valve body 13a, which is coupled to the shell 2 in a fixed and fluid-tight position. The valve body 13a has an axial seat 13b, in which a top portion 14a of the rod 14, having a diameter D3, slides in a fluid-tight way. The diameter D3 of the top cylindrical portion 14a is larger than the external diameter D1 of the active surface 7a of the needle 7. In addition, the end of the portion 14a of the rod 14 defines, with the end portion of the seat 13b, a control chamber 15 of the rod 14, associated to the metering solenoid valve 16.
The control chamber 15 communicates permanently with the inlet 4, through a calibrated inlet duct 18 (
The calibrated radial passage 24 has a diameter D5 and is designed to be opened/closed by an open/close element defined by a sleeve 35 fixed to the armature 11 of the electromagnet 10. The sleeve 35 is fitted on the pin 29 and is axially slidable under the action of the electromagnet 10 for varying the pressure present in the chamber 15, and hence for opening/closing the nozzle 5.
Normally, the electromagnet 10 is de-energized, and the spring 12 keeps the sleeve 35 of the armature 11 in contact with the flange 20 of the distributor body 17, so as to close the annular chamber 34. In the control chamber 15 there is fuel under pressure, as in the injection chamber 6 and in the annular chamber 34 itself. The action of the pressure in the control chamber 15 acting on the rod 14, assisted by the action of the spring 21, prevails over the action of the pressure on the annular surface 7a so that the needle 7 keeps the nozzle 5 closed.
When the electromagnet 10 is energized, this attracts the armature 11, so that the sleeve 35 opens the chamber 34. The fuel of the control chamber 15 is discharged through the radial passage 24, and the pressure of the fuel in the injection chamber 6 pushes the needle 7 along the opening stroke upwards, opening the nozzle 5 and thus determining injection of the fuel. When the electromagnet 10 is de-energized, the spring 12 brings the armature 11 back downwards, so that the sleeve 35 recloses the annular chamber 34, and the fuel entering from the inlet duct 18 restores the pressure of the control chamber 15. The action of said pressure on the surface of the portion 14a of the rod 14, assisted by the action of the spring 21, prevails again over the pressure of the fuel on the annular surface 7a, so that the needle 7 performs its stroke for closing of the nozzle 5.
It is evident that, when the sleeve 35 closes the chamber 34, it is subjected to a zero resultant of pressure of the fuel along the axis 3, with consequent advantages from the standpoint of stability of the dynamic behaviour of the mobile parts of the electroinjector 1. In particular, the displacement of the needle 7 along the opening stroke and along the closing stroke is practically constant between one injection and the next, in response to a given electrical command sent to the device 8.
In other words, it is possible to correlate the position of the needle 7 in a biunique and repeatable way with the electrical commands sent to the device 8. The position of the needle 7 along the opening and closing strokes, in response to an electrical command, can be obtained by means of theoretical calculation, as a function of constructional parameters of the electroinjector 1 (for example, the diameters D1 and D2 of the needle 7, D3 of the rod 14, D4 of the inlet duct 18 and D5 of the outlet passage 24 of the control chamber 15) and as a function of known operating parameters (for example, pressure of supply of the fuel to the inlet 4). At the same time, the section of opening of the nozzle 5, and hence the evolution of the instantaneous flow rate of the fuel can be determined in a unique way as a function of the axial displacement of the needle 7, in particular on the basis of the dimensions of the passages of the nozzle 5 itself and on the basis of the supply pressure of the fuel.
In particular, the law of axial displacement of the needle 7 depends not only upon the spring 21 but also upon the ratio D3/D1 between the diameter D3 of the portion 14a and the external diameter D1 of the active surface, i.e. of the shoulder 7a, and upon the ratio D1/D2 between the external diameter D1 and the internal diameter D2 of the active surface, which in the case under examination coincides with that of the shoulder 7a. The value of said ratios renders the injector more or less sensitive to the evolution of the pressure in the control chamber 15. As the ratio D3/D1 tends to unity and/or as the ratio D1/D2 increases, the displacement of the needle 7 becomes very sensitive to said pressure, so that a small drop in pressure in the control chamber 15 brings about opening of the nozzle 5. Preferably, the ratio D3/D1 can be comprised between 1.05 and 1.2, and the ratio D1/D2 is comprised between 1.85 and 2.35, whilst the diameter D1 of the needle 7 can be comprised between 3.2 and 4.8 mm.
In turn, the pair of values of the diameters D4, D5 of the inlet duct 18 and of the radial outlet passage 24 affects the curve of the pressure of the fuel in the control chamber 15, both during opening of the solenoid valve 16 and during the subsequent closing. As the ratio D5/D4 increases during the opening stroke of the sleeve 35, the pressure in the control chamber 15 decreases more rapidly, thus reducing the transient of opening of the needle 7. Furthermore, as the ratio D5/D4 increases, during the closing stroke of the sleeve 35, the pressure in the control chamber 15 increases more slowly, thus causing the delay in closing of the needle 7. Preferably said ratio D5/D4 is chosen between the values 0.7 and 1.4, whilst the diameter D5 of the radial passage 24 can be chosen between 0.22 and 0.35 mm.
In
According to the method of the present invention, to obtain a fuel injection, supplied to the device 8 is at least a first and a second electrical command (
With reference to
In particular, at the instant T1 the first command C1 is issued, the evolution of which increases with the ramp R1, then remains substantially constant for a short stretch M1, then decreases along the stretch D1, presents a stretch N1 that is substantially constant, and finally decreases with a stretch E1. The evolution of the command C1 causes displacement of the needle 7 starting from an instant TQ0, with TQ0>T1 on account of the delay in the response of the device 8, with a profile P comprising a stretch A1, which increases up to a value H1, and a decreasing stretch B1. On account of the short duration of the stretch N1 of the command C1, the lift H1 of the needle 7 is limited and has the purpose of controlling a pre-injection of a fixed amount of fuel.
The second command C2 is issued at an instant T2 such as to start the second lift, i.e. the stretch A2, in a point Q1 of the stretch B1 before the needle 7 has reached the position of end of closing stroke of the nozzle 5. In particular, the instant T2 is smaller than the theoretical instant in which the first command represented by the curve C1, which prolongs the stretch E1, would reach a zero value. The curve C2 has a stretch N2 of duration longer than the stretch N1, which depends in a known way upon the operating conditions of the engine, so that the lift of the needle 7 reaches a value H2 higher than H1, causing a degree or cross-section of opening of the nozzle 5, and/or a duration of said opening, greater than that reached at the end of the stretch A1. There then follows a closing displacement defined by the stretch B2, up to complete closing of the nozzle 5, after which the needle 7 remains stationary until the subsequent injection.
The time interval T1-TQ0 is the delay with which the needle 7 starts to move upwards and depends in the first place upon the ratio D5/D4 between the diameter D5 of the outlet passage 24 of the control chamber 15 and the diameter D4 of the inlet duct 18, which determines the rate of reduction of the pressure in the control chamber 15. Said delay depends not only upon the preloading of the spring 21 (see also
The curve F of the instantaneous flow rate obtained approximates in a satisfactory manner the desired curve of instantaneous flow rate illustrated in
The time interval TQ0-TQ1 depends also upon the ratio D3/D1 between the diameters of the aforesaid surfaces of the rod 14 and of the needle 7 and upon the ratio D1/D2 between the external diameter D1 and the internal diameter D2 of the active surface of the needle 7, and upon the ratio of the diameters D5/D4. As the ratio D3/D1 decreases and/or as the ratio D1/D2 increases, both the time interval TQ0-TQ1 and the displacements H1 and H2 increase because the needle 7 is more ready to open the nozzle 5 and slower in closing it, on account of the resultant of the pressures acting thereon. In turn, as the ratio of the diameters D5/D4 increases, both the time interval TQ0-TQ1 and the displacements H1 and H2 increase because the reduction of the pressure in the control chamber 15 is faster, so that the needle 7 is more ready to open the nozzle 5 and slower in closing it on account of the resultant of the pressures acting thereon.
From
According to the example of
In particular, the instant T4 is greater than the instant in which the stretch E3 of the curve C3 goes to zero. Albeit in a limit condition, the curve F′ of the instantaneous flow rate obtained comprises two consecutive portions S′ and U′ , which present respective maximum levels that are different from one another, and hence respective mean levels that are different from one another and once again approximate in a satisfactory way, respectively, the levels L1 and L2 of the desired curve of the instantaneous flow rate of
According to the example of
The values H5-H7 (relative maxima) reached by the needle 7 at the end of the first three lifts are substantially the same as one another so that the relative-maximum sections of opening of the nozzle 5 are substantially equal. In this case, the pre-injection is governed by the three electrical commands C5-C7. The value H8 reached at the end of the fourth and last lift (stretch A8) is higher and causes a greater degree or section of opening to determine the main injection, in so far as the stretch N8 has a longer duration than the stretches N5-N7.
There is consequently obtained a curve F″. of flow-rate which approximates the desired flow-rate curve of
According to variants (not illustrated), it is possible to approximate curves of instantaneous flow-rate of the stepwise type, in which more than two levels are present, by causing the needle 7 to be displaced with more than two consecutive lifts up to values H that are different from one another, and/or to approximate curves of instantaneous flow-rate in which a level L1 is followed by a low level L2, contrary to the levels L1 and L2 illustrated in
From the foregoing description, there clearly emerges the method for controlling fuel injection in an internal-combustion engine, in which an electroinjector 1 comprises:
The method for controlling fuel injection is characterized in that:
In addition, according to the method of the present invention for at least one injection, at least one of the following quantities is determined as a function of operating parameters of the engine:
In this way, it is possible to modulate the evolution of the instantaneous flow rate between the various injections by varying the amplitude and/or the duration and/or the number of the substantially constant levels of flow rate that it is desired to approximate.
From the foregoing description it is evident that the method for controlling fuel injection enables injection of an instantaneous flow-rate that approximates in an optimal way the flow-rate curve of a stepwise type and that is obtained in a relatively simple way. In fact, the control of injection according to the method described above does not require calibration of mechanical components and/or injectors built in a dedicated way. In addition, it is possible to vary easily the evolution of the flow-rate injected between one injection and the next so as to approximate as closely as possible the desired flow-rate curve and optimize the efficiency of the engine according to the specific point of operation of the engine itself.
From the above description it is evident that modifications and variations may be made to the injection system and to the control method described without thereby departing by the sphere of protection of the present invention. In particular, the control method could be performed with injectors that differ from the electroinjector 1 illustrated by way of example, but in which the displacement of the open/close needle element of the nozzle is always obtained as a function of the pressure of supply of the fuel and is repeatable in response to given electrical commands. In turn, the device 8 can be constituted by a piezoelectric actuator, instead of by an electromagnet.
Furthermore, as already mentioned, the diameter of sealing D2 between the conical tip 7b of the needle 7 and the conical tip 5b of the nozzle 5 may not coincide with the internal diameter of the annular shoulder 7a, for example on account of a different geometry of the bottom portion of the needle 7. Finally, the needle 7 can be displaced during lifting in one and the same injection for a number of times and/or by amounts different from the ones indicated by way of example.
Number | Date | Country | Kind |
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05425881.9 | Dec 2005 | EP | regional |