EP1463882A1

EP1463882A1 - Injector control method

Info

Publication number: EP1463882A1
Application number: EP02801120A
Authority: EP
Inventors: Lanig Garo
Original assignee: Renault SAS
Current assignee: Renault SAS
Priority date: 2001-12-17
Filing date: 2002-12-12
Publication date: 2004-10-06
Also published as: FR2833654B1; FR2833654A1; WO2003054370A1

Abstract

The invention relates to a fuel injection device comprising: a fuel injector; a tube which is used to supply fuel to the injector; a needle which moves and thereby opens or closes the injector; and a valve which controls a fuel leak in order to cause the needle control pressure to drop below a switching pressure threshold, said tube being supplied with fuel at a source pressure (Ps). The invention comprises the following two steps, namely: the valve is opened, outside of the injection phases, for short periods at an excitation frequency in order to create an oscillatory phenomenon in relation to the pressure in the tube around the source pressure (Ps), the opening duration being shorter than a predetermined period which is necessary for the control pressure to reach the switching pressure; and the fuel injection is started at a moment when the pressure in the tube is increasing and greater than the source pressure (Ps).

Description

Injector control method.

The invention relates to an injector control method, in particular for common rail engines under high pressure.

Fuel injectors for engines are known, supplied by a common rail under a very high source pressure Ps, for example under 1600 bars. In these injectors, a needle has a conical end and cooperates with a seat to close or free the passage of fuel to a nozzle. In a particular embodiment, as disclosed in document WO 00/55490, the movement of the needle is controlled by a difference in axial pressure exerted by the fuel on surfaces of the needle. More specifically, the needle comprises a flange on the side of the conical end, the flange and part of the conical end of the needle being subjected to the pressure of the fuel before passing through the nozzle. Furthermore, the other end of the needle is subjected to the pressure of the fuel prevailing in a bypass coming from a tube coming from the common rail for the supply of fuel. The bypass communicates with the tube through a restricted orifice. In addition, a valve makes it possible to connect the bypass on command to a fuel return line.

Thus, when the valve is open, a fuel leak is established between the tube and the pipe. Due to the restricted orifice, the pressure in the bypass drops to a level significantly lower than that in the tube. The force resulting from this pressure on the needle therefore decreases and the balance of forces on the flange and on the end of the needle is modified so that, after a switching delay after opening the valve, the needle leaves the seat. The passage of fuel is therefore possible and injection takes place. When the valve closes, the leak is stopped and the pressure in the bypass rises and causes the needle to move against the seat, which ends the injection of fuel.

When it is desired to increase the quantity of fuel injected, it is possible to simply increase the duration of injection. However, this duration is limited by considerations on the injection conditions. Indeed, the fuel must be injected when the piston is in top dead center, without extending too much during the expansion phase. It is then conceivable to increase the injection pressure. For this, the most obvious solution is to increase the source pressure delivered by the capacity of the injection pump supplying the common rail. However, this increase in capacity dramatically increases the cost of such equipment. It is also conceivable to increase the size of the injector orifice. However, the quality of the fuel spraying in this case is less good and leads to a reduction in the cleaning performance.

It is therefore an objective of the invention to increase the quantity of fuel injected without modifying the source pressure delivered by the pump. the duration of the injection.

With this objective in view, the invention relates to a method for controlling a fuel injection device, the device comprising a fuel injector, a tube for supplying the injector with fuel, a needle movable between a position opening and closing position of the injector, a valve controlling a fuel leak to drop a control pressure of the needle below a switching pressure threshold, the tube being supplied with fuel under a source pressure, characterized in that: apart from the injection phases, short valve openings are piloted at an excitation frequency to create an oscillatory phenomenon of the pressure in the tube around the source pressure, the duration of the openings being less than a predetermined time necessary for the control pressure to reach the switching pressure, the start of fuel injection is controlled at a time when the pressure in the tube is increasing and greater than the source pressure.

Opening the valve outside the injection phase creates a leakage flow through the tube and a pressure drop. A wave is thus created and propagates in the fuel contained in the tube, according to the characteristic speed of the fuel. The wave is reflected at the tube connection on the common rail and returns to the injector. By exciting these oscillations in phase with the natural frequency of the fuel contained in a hydraulic line including the tube, the oscillatory phenomenon is created and amplified to an amplitude whose limit is determined by the viscosity of the fluid and the energy losses by damping the movement.

The injection is carried out over a period during which the pressure is more often greater than the source pressure than lower, and therefore the average of the pressure in the tube is greater than that which would be present without the oscillatory phenomenon. For a determined injection duration, the fuel flow rate obtained is therefore also higher by implementing the method of the invention.

The excitation frequency is preferably a frequency sub-multiple of the resonance frequency of the device comprising the tube. Advantageously, the excitation frequency is the resonance frequency.

The resonance is then the fastest.

Preferably, the injection duration extends over an odd number of half-periods of oscillation of the pressure in the tube, the pressure being greater than the source pressure during the first half-period. Thus, the number of half-periods during which the pressure is greater than the source pressure exceeds by one the number of half-periods during which the pressure is less than the source pressure.

The duration of injection being predetermined, the length of the tube will be determined to adjust the resonance frequency and so that the duration of injection coincides with an odd number of half-periods. Even more particularly, the duration of injection extends over three half-periods. This is a good compromise between the length of the tube and the resulting average pressure. To obtain that the injection lasts only half a period, it is necessary to lower the resonance frequency by increasing the length of the tube, in a way which is no longer realistic. On the other hand, by increasing the resonance frequency and the number of half-periods during the injection, the gain according to the method is less.

Advantageously, the small openings of the valve are piloted from an instant preceding the injection of a predetermined duration comprising an integer number of full periods and increased by a half-period.

Thus, as the first half-period is a pressure drop phase, the pressure is in the growth phase and greater than the source pressure at the time of injection start. Thanks to this characteristic, the start of the injection takes place at a selected time.

The invention will be better understood and other particularities and advantages will appear on reading the description which follows, the description referring to the appended drawings among which: FIG. 1 is a time diagram of the controls of the valve and the injection according to the invention; - Figure 2 is a time diagram of the pressure in the injector after the establishment of the oscillatory phenomenon, - Figure 3 is a time diagram of the pressure in the injector during injection according to the prior art and according to the invention; FIG. 4 is a time diagram of the comparison of the pressures according to the diagram in FIG. 3.

The control method according to the invention is implemented on a device identical to that of the prior art, described above. Only the operation is modified. The description of the device is not repeated here.

According to the method of the invention, small valve openings are controlled at a frequency which is the resonance frequency of the fuel contained in the hydraulic line which extends from the connection of the tube on the ramp to the needle of the injector. The leak created lets fuel escape, which returns to the tank through the return line. The resonant frequency is mainly determined by the geometric characteristics of the hydraulic line, in particular its length, and the characteristics of the fuel, in particular the speed of sound in the carburan. The duration of the openings is less than the switching delay, so that the opening of the injector is not controlled.

For example, when the engine is running at 4000 rpm, for a given cylinder, an injection is performed at 30 ms intervals. Frequency resonance is around 1000 HZ, and the opening time is 100 to 200 μs. These values correspond to a standard geometry and characteristics of an injection system. Figure 1 shows a diagram of which a curve 1 represents the opening of the injector, and a curve 2 shows the opening of the valve.

Each time the valve is opened, a vacuum wave is created in the hydraulic line. This wave oscillates in the tube and is amplified each time the valve is opened. After 8 to 10 valve openings, it can be seen that the maximum amplitude of the oscillations has been reached. The phase of establishment of the oscillatory phenomenon lasts on the order of 8 to 10 ms.

The start of an injection phase is synchronized with the oscillations so that the needle is opened when the pressure in the tube is increasing and greater than the source pressure Ps. In FIG. 2, the pressure P in the injector downstream of the needle is represented as a function of a time scale T which extends from a period before the theoretical start of the injection Ti and until after the theoretical end of the injection Tf . The pressure represented by curve 10 is the pressure which prevails in the injector in the absence of injection, after the installation of the oscillatory phenomenon. A segment 11 extends from the instant Ti to the instant Tf to symbolize the duration Di of the injection. Note that the segment 11 extends over three half-periods of the oscillatory phenomenon of the pressure P. With the numerical values cited above, this duration is 1 order of 1.5 ms.

The graph in FIG. 3 makes it possible to highlight the pressures P in the injector with or without the oscillatory phenomenon. Curve 22 shows the pressure P conventionally present without the oscillatory phenomenon, while curve 20 shows the pressure with the implementation of the method according to the invention.

Curve 22 has a bearing 220 at the source pressure Ps, for example 1600 bars, until an instant Tv when the control valve opens. The pressure P drops slightly until the instant Ti when the needle passes from the closed position to the open position and allows the injection of fuel. The switching delay is therefore the difference Ti-Tv. ^'The injection continues until time Tf where the needle passes the opening to the closed position position to stop fuel injection. The pressure in the tube rises above the source pressure Ps because of the "water hammer" which occurs when the injector closes with the needle.

By comparison, the curve 20 has an oscillation before the needle passes into the open position at the instant Ti. At this instant, the pressure P is in the growth phase and reaches the source pressure Ps. The pressure also drops during fuel injection, but less quickly than in the previous case. The needle returns to the closed position also at time Tf. The comparison between curves 20 and 22 is highlighted on the graph of FIG. 4, on which the curve 30 is represented representing the difference DP between curve 20 and curve 22. Oscillations appear until now Ti. During the injection phase, represented by segment 11, the pressure difference DP also presents oscillations over three half-periods, even if these oscillations are weaker than those present before the start of the injection Ti. Over the injection period, it can be seen that the mean value of the pressure difference DP is positive. Indeed, this is related to the difference between the surfaces 31, 33 included between the curve 30 and above the abscissa axis, and the surface 32 comprised between the curve 30 and below the abscissa axis . The sum of the surfaces 31 and 33 is much larger than the surface 32. It is easily concluded that the average pressure during the injection phase is higher in the case of the application of the process than in its absence. The amount of fuel injected is therefore greater.

Claims

1. A method of controlling a fuel injection device, the device comprising a fuel injector, a tube for supplying the injector with fuel, a needle movable between an open position and a closed position of the injector, a valve controlling a fuel leak to drop a control pressure of the needle below a switching pressure threshold, the tube being supplied with fuel under a source pressure (Ps), characterized in that : apart from the injection phases, short valve openings are piloted at an excitation frequency to create an oscillatory phenomenon of the pressure in the tube around the source pressure (PS), the duration of the openings being less at a predetermined time necessary for the control pressure to reach the switching pressure, the start of fuel injection is controlled at an instant (Ti) when the pressure in the tube is increasing and s higher than the source pressure (Ps).

2. Control method according to claim 1, characterized in that the excitation frequency is a frequency submultiple of the resonance frequency of the device comprising the tube.

3. Ordering method according to claim 2, characterized in that the excitation frequency is the resonant frequency.

4. Control method according to claim 1, characterized in that the injection duration (Di) extends over an odd number of half-periods of oscillation of the pressure in the tube, the pressure being greater than the pressure source (Ps) during the first half-period.

5. Control method according to claim 4, characterized in that the injection duration (Di) extends over three half-periods.

6. Control method according to claim 1, characterized in that one controls the valve openings from an instant preceding the injection of a predetermined duration comprising an integer of full periods and increased by half -period.

7. Fuel injection device comprising at least one injector and an injector control unit, the control unit comprising means for implementing the method according to one of claims 1 to 6.