FR2841940A1 - METHOD FOR CONTROLLING THE INJECTOR OF A COMMON RAMP COMPRESSION IGNITION ENGINE - Google Patents

Abstract

A fuel injection device comprises at least one fuel injector, a tube for supplying the injector with fuel, a needle whose movement opens or closes the injector, a valve controlling a fuel leak to drop a pressure of control of the needle below a switching pressure threshold, the tube being supplied with fuel under a source pressure (Ps). According to the invention, the following steps are carried out: - outside 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 than a predetermined time necessary for the control pressure reaches the switching pressure, - the start of fuel injection is controlled at a time when the pressure in the tube is lower than the source pressure (Ps).

Description

Method for injector control of a

  common rail compression ignition.

  The invention relates to a method for injector control of a compression ignition engine, in particular for common rail engines under high pressure, making it possible to optimize the constraints of noise and pollutant emissions. Fuel injectors for engines are known, supplied by a common rail under a very high source pressure Ps, for example up to 1600 bars. In these injectors, a needle has a conical end and cooperates with a seat to block or free the passage of fuel to a nozzle. In a particular embodiment, as disclosed by 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 has 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 on command the

  diversion 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 movement of the needle against

  the seat, which ends the fuel injection.

  In a compression ignition engine, intake air is compressed in a cylinder before the injector sprays fuel into the cylinder. During compression, the air heats up to reach the temperature and pressure conditions at which the fuel injected into the chamber ignites spontaneously. From the start of the injection, the fuel is sprayed in the form of vaporizing droplets. Fuel in the gaseous state mixes with the moving air in the cylinder. As soon as a fraction of volume comprises a mixture of air and fuel in proportions close to the stoichiometric ratio, self-ignition occurs in this volume and combustion propagates to all of the fuel sprayed into the cylinder. Then the combustion is concentrated around the jet of fuel injected

  until the end of the injection phase.

  The formation of a fraction of the volume where auto-ignition occurs is not immediate and occurs for a period of time before auto-ignition, known as auto-ignition delay. Said self-ignition delay essentially depends on the pressure and temperature conditions in the cylinder, and on the movement of air. Self-ignition is a source of noise for diesel engines. The higher the noise level, the greater the quantity of fuel injected before auto-ignition. One solution to reduce this noise is therefore to reduce the flow rate of fuel injected, or introduction rate, during the start of the injection phase. It is possible to reduce the introduction rate uniformly throughout the duration of the injection. However, as the duration of injection is limited, limiting the rate of introduction amounts to limiting the power of the

engine.

  Document US 4,333,436 discloses a fuel injection system in which the rate of fuel introduction can be controlled during the injection phase. However, this system does not apply to common rail injection systems. Another constraint for engines is to reduce polluting emissions. In particular, for common rail diesel engines, it is desirable to increase the rail pressure for supplying fuel to the injectors, so that the fuel is injected by forming very fine droplets, which reduces the mass total of particles emitted. This constraint is

  therefore contradictory with that of the sound level.

  It is therefore an objective of the invention to propose a fuel injection method making it possible to obtain, for a compression-ignition engine with common rail, a low level of emission of pollutants and a low noise level.

during auto-ignition.

  With this objective in view, the invention relates to a method for controlling a fuel injection device for a compression-ignition engine, the device comprising a fuel injector, a tube for supplying the injector with fuel. , a needle moving 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, 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 the injection is controlled tion of the fuel at a time when the pressure in the tube is

  lower 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 comprising 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 lower than the source pressure, which makes it possible to reduce the rate of fuel introduction during the start of the injection, therefore during the self-ignition delay. The amount of fuel injected before self-ignition is reduced, and the noise generated by the engine is therefore reduced. On the other hand, during the continuation of the injection, the average pressure is close to the source pressure, and therefore the average introduction rate is preserved. The maximum engine power is therefore retained. Similarly, the pollutant emission rate is close to that

  determined by source pressure.

  Thanks to the invention, it is possible to reduce the noise level at self-ignition while retaining the same level of pollutant emission and the same engine power. According to another choice of engine parameters, it is possible to increase the source pressure to decrease the pollutant emission rate while maintaining the same noise level

to auto-ignition.

  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 resonant frequency.

  The resonance is then the fastest.

  Preferably, the small valve openings are piloted from an instant preceding the injection of a predetermined duration

  comprising an integer of full periods.

  Thus, as the first half-period is a pressure drop phase, the pressure is also lower than the source pressure at the time of injection start. Thanks to this feature, the start of the injection takes place at a selected time

  compared to the oscillatory phenomenon.

  A subject of the invention is also a fuel injection device for a compression-ignition engine, the device comprising an injector control unit, at least one fuel injector, a tube for supplying the injector with fuel, a needle moving 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 at source pressure, characterized in that the injector control unit includes means for

  implement the method described above.

  The invention will be better understood and other features and advantages will appear on reading

  of the description which follows, the description

  referring to the accompanying drawings, among which: - Figure 1 is a time diagram of the valve controls 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; - Figure 4 is a time diagram of the comparison of pressures according to the diagram of Figure 3; - Figure 5 is a timing diagram of flow rates during injection according to the prior art and according to the invention; - Figure 6 is the difference in the amount of fuel injected between the methods according to the art

and the invention.

  The control method according to the invention is implemented on a device identical to that of the prior art, as disclosed for example by document WO 00/55490. Only the operation is

  amended. 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 resonant 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 essentially determined by the geometric characteristics of the hydraulic line, in particular its length, and the characteristics of the fuel, in particular the speed of the sound in the fuel. Fuel pressure is in particular a factor influencing the speed. 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. The resonance frequency is of the order of 1000 Hz, and the opening time is 100 to 200 lls. 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

opening 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 is found that the maximum amplitude of the oscillations is reached. The establishment phase of the

  oscillatory phenomenon lasts on the order of 8 to 10 ms.

  The start of the oscillations is synchronized with the start of the injection phase so that the needle is opened when the pressure in the tube is lower than the source pressure Ps. In FIG. 2, the pressure P in l 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 according to the method of the invention. 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 time Ti to time Tf to symbolize the duration Di of the injection. Note that the segment 11 extends substantially over a period of the oscillatory phenomenon of the pressure P. With the numerical values cited above, this duration is

around 1 ms.

  The graph in FIG. 3 makes it possible to highlight the pressures P in the injector with or without the oscillatory phenomenon. A segment 21 extends from instant Ti to instant Tf to symbolize the duration Di of the injection. 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 1000 bars, until an instant Tv o the control valve opens. The pressure P drops slightly until the instant Ti o 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 according to the method of the invention continues until an instant Tfa, not shown, where the needle passes from the open position to the closed position to stop the fuel injection. This instant Tfa slightly precedes the instant Tf so that the quantities of fuel injected are identical according to the methods of the prior art and of the invention. 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 a decreasing phase and drops below the source pressure Ps. The pressure also drops during the fuel injection more quickly than in the previous case. The needle returns to the closed position at time Tf. Self-ignition occurs at an instant Ta, of the order of 0.5 ms after

the start of the injection.

  The comparison between curves 20 and 22 is highlighted in the graph of FIG. 4, on which curve 30 is represented representing the difference DP between curve 20 and curve 22. A segment 31 extends from the instant Ti at instant Tf to symbolize the duration Di of the injection. of the

  oscillations appear until time Ti.

  During the injection phase, represented by segment 31, the pressure difference DP also presents oscillations over a period, even if these oscillations are weaker than those present before the start of injection Ti. At the start of the injection, it is found that the pressure difference DP is negative, which means that the pressure by applying the method according to the invention

  is lower than according to the prior art.

  The consequence of this pressure difference on the flow of injected fuel is highlighted by the diagram in FIG. 5. A segment 41 extends from instant Ti to instant Tf to symbolize the duration Di of the injection . A curve 40 represents the rate of introduction D (or flow) according to the method of the prior art. This curve has the shape of a bell between the instants Ti and Tfa and the upward slope and the downward slope are determined by the switching speed of the injector needle and the pressure level in the injector. A curve 42 represents the introduction rate (or flow rate) in the case of the method according to the invention. This curve 42 also has a bell shape between the instants Ti and Tf, but with a lower flow rate during the ascending phase, at the start of the injection, d at the slightest pressure in the injector. A curve 43 represents the difference between the introduction rate according to the invention and the introduction rate according to the prior art. It is noted that this difference is negative at the start of the injection, to become positive again until the end

of the injection.

  The consequence of this difference in flow rate is shown in FIG. 6. A curve 50 represents the difference in quantity of fuel injected DV between the methods according to the invention and according to the prior art. A segment 51 extends from time Ti to time Tf to symbolize the duration Di of the injection. In the end, the difference in quantity of fuel injected is substantially zero. On the other hand, since the flow rate is less at the start of injection in the case of the method according to the invention, the quantity of fuel injected before the auto-ignition instant Ta is less. This results in a reduction in the self-ignition noise, while maintaining the same

amount of fuel injected.

Claims (5)

  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 than a predetermined time required for the control pressure to reach the switching pressure, - the start of fuel injection is controlled at an instant (Ti) where the pressure in the tube is   lower than the source pressure (Ps).   2. Control method according to claim 1, characterized in that the excitation frequency is a frequency sub-multiple of the resonance frequency of the device comprising the tube. 3. Control method according to claim 2, characterized in that the excitation frequency is substantially the resonance frequency. 4. 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.   5. Fuel injection device for a compression ignition engine, the device comprising an injector control unit, at least one 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 (Ps), characterized in that the injector control unit includes means for implementing   the method according to one of claims 1 to 4 for each injector.