Abstract:
Methods, a fuel supply system, and a computer readable medium embodying a computer program product are provided for controlling rail pressure in a fuel supply system comprising a fuel pump, an injector and a rail connecting the injector to the pump. At least one of the methods includes, but is not limited to establishing a relationship between said rail pressure and a leak rate of the injector, estimating a fuel drain rate from said rail based on a fuel injection rate, the rail pressure and said rail pressure/leak rate relationship, estimating a desired intake flow rate of said pump based on said fuel drain rate, and controlling the pump to operate at said desired intake flow rate.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority to British Patent Application No. 0915644.9, filed Sep. 8, 2009, which is incorporated herein by reference in its entirety. 
     TECHNICAL FIELD 
     The present invention relates to a method for controlling supply rail pressure in a fuel supply system, in particular for a Diesel engine, and to devices for carrying out the method. 
     BACKGROUND 
     Conventionally the fuel supply system of a Diesel engine comprises a fuel pump capable of delivering high output pressures of up to 1600 bar, an injector associated to each cylinder of the engine and a rail connecting the injector to the pump. The injector comprises a solenoid or a piezo element for electrically controlling a pilot valve. The pilot valve controls a flow of fuel to pressure-receiving surfaces of a valve piston, so that a tip of the valve piston is either pressed against ejection nozzles of the injector and blocks these or is withdrawn, allowing fuel to be ejected from the nozzles. Due to this principle of operation, only a fraction of the fuel that flows into the injector is actually injected into the cylinder. Fuel that has been used for driving the valve piston flows back to the tank, and so does fuel which escapes through internal leaks of the injector. 
     Fuel efficiency and pollutant emission rates depend critically on fuel injection timing. Not only must a predetermined quantity of fuel be injected into the cylinders at each engine stroke, but it must also happen at the right time interval (or intervals) during a stroke. Since the flow rate through the injector depends on the rail pressure (and other quantities), injecting the predetermined quantity of fuel may take longer than desired if the rail pressure is too low, or injection may stop earlier than desired if the rail pressure is too high. Further, atomization of the fuel depends on rail pressure. Non-optimal atomization may cause pollutant emission to increase and/or fuel efficiency to decrease. The fuel pressure that yields ideal atomization depends on the operating conditions of the engine, so that when these vary, the fuel pressure has to be adapted. For these reasons it is very important to control the fuel pressure. This must be done by controlling the operation of the pump so that at any time its delivery rate equals the rate at which fuel is drained from the rail by the injectors. The fuel drain rate is a rather complex function of operating conditions, since not only the engine speed, i.e., frequency of fuel injections may vary, but also the amount of fuel injected per engine stroke, and the leak rate of the injector depends on the duration of its excitation phases. Further, even if the fuel drain rate from the rail was exactly known, a pump can generally not be straightforwardly controlled to deliver this drain rate, since the pump also has internal leakage rates depending on input and output pressures and on fuel temperature, so that there is no one-to-one relationship between pump speed and delivery rate. 
     Conventionally, this problem is handled by experimentally analyzing the behaviour of the complete fuel supply system under a variety of operating conditions and tuning the control of the pump so that an appropriate fuel rail pressure is maintained in all operating conditions. This analysis and tuning has to be redone every time when the fuel supply system is modified, e.g., by replacing an injector or the fuel pump by one of a different type, requiring considerable amounts of labour. 
     At least one object of the present invention is to provide a control method and devices for carrying out the method which facilitate the integration of components having different characteristics into the fuel supply system. A further object of the invention is a controller for carrying out the control method and another object of the invention is a data processor program product. Furthermore, other objects, desirable features, and characteristics will become apparent from the subsequent summary and detailed description, and the appended claims, taken in conjunction with the accompanying drawings and this background. 
     SUMMARY 
     The at least one object is achieved by a method for controlling rail pressure in a fuel supply system comprising a fuel pump, at least one injector and a rail connecting the injector to the pump, the method comprising the steps of a) establishing a relationship between said rail pressure and a leak rate of the injector; c) estimating a fuel drain rate from said rail based on a fuel injection rate, said rail pressure and said rail pressure-leak rate relationship, d) estimating a desired intake flow rate of said pump based on said fuel drain rate; and e) controlling the pump to operate at said desired intake flow rate. 
     Instead of analyzing the fuel supply system as a whole, according to the present invention the experimental analysis is carried out separately for the components of the fuel supply system. The relationship between the rail pressure and the injector leak rate is easier to analyze than the behaviour of the entire system since the former is independent of all characteristics of the pump. If an injector has to be replaced, the rail pressure-leak rate relationship has to be established again for the new injector, but characteristics of the pump remain unchanged. Vice versa, if only the pump is exchanged, there is no need to update the rail pressure-leak rate relationship. Preferably, a relationship between the rail pressure and an efficiency of the pump is also established experimentally prior to steps c) to e), and the thus determined relationship is taken into account for estimating the desired intake flow rate in step d). 
     Since viscosity of the fuel depends on its temperature, the rail pressure-leak rate relationship should be established as a function of fuel temperature. Although the fuel is heated when decompressed in the leaks of the pump and the injector, a single measure of the fuel temperature, e.g., at the pump input, may be sufficient since for a given input temperature the amount of fuel temperature increase is determined by the rail pressure. 
     The leak rate of the injector varies depending on the excitation state of the pilot valve. Since the duty cycle of the pilot valve is a function of engine speed, the rail pressure-leak rate relationship should preferably specify the leak rate as an engine speed-weighted sum of at least a static leak rate associated to the closed state of the injector and a dynamic leak rate associated to its open state. 
     Preferably, the rail pressure-leak rate relationship, in particular the dynamic leak rate, should be established as a function of injector excitation time, since the instantaneous leak rate of the injector in the excited state of the pilot valve is often found not to be constant but to be a function of how long the pilot valve has been excited. 
     At an excitation time of zero, i.e., for the static component of the leak rate, it is surprisingly found that the leak rate increases more than linearly with the rail pressure. This is surprising since due to the small clearance through which the fuel flows, the leak flow through the injector should be laminar and the leak rate G st  should therefore be described by Poiseuille&#39;s formula 
                 G   st     =       K   γ     ⁢   Δ   ⁢           ⁢   p       ,         
Where K denote a geometry-dependent factor and γ the viscosity of the fuel, i.e., the leak rate G st  should be directly proportional to the pressure drop Δp (which substantially equals the rail pressure. In practice, the relationship between the leak rate G st  and the rail pressure p is not described correctly be this formula, probably due to the viscosity of the fuel being reduced while heating up due to decompression in the injector.
 
     As pointed out already, the dynamic leak rate may depend on excitation time. In particular, the dynamic leak rate may be found to increase with the excitation time at a first, high rate if the excitation time is below a given threshold and to increase with the excitation time at a second, low rate if the excitation time is above said given threshold. This can be attributed to the fact that while the excitation time is below the threshold, a displaceable member of the pilot valve is being displaced by fuel flowing through the pilot valve and does as such not obstruct the flow of the fuel. When the displaceable element has reached an abutment (and the injector is fully open), the displaceable element becomes an additional obstacle to the fuel flow through the pilot valve, so that the instantaneous flow rate through the pilot valve is reduced. 
     According to an alternative embodiment, the at least one object is achieved by a method for controlling rail pressure in a fuel supply system comprising a fuel pump, at least one injector and a rail connecting the injector to the pump, comprising the steps of b) establishing a relationship between said rail pressure and an efficiency of said pump, c) estimating a fuel drain rate from said rail based at least on a fuel injection rate, d) estimating a desired intake flow rate of said pump based on said fuel drain rate and said efficiency; and e) controlling the pump to operate at said desired intake flow rate. Due to the fuel viscosity depending on temperature, the rail pressure-leak rate relationship is preferably established as a function of fuel temperature, too. 
     Although the estimate obtained in step d) will be rather close to the actual intake flow rate of the pump required to maintain the rail pressure at a desired constant value, small deviations may cause the rail pressure to drift slowly. Such a slow drift can be compensated by step e) comprising e1) inputting to said pump a control parameter determined based on said desired intake flow rate, e2) detecting a deviation between a current rail pressure and a target rail pressure, and e3) correcting said control parameter depending on said deviation. In this way, the control parameter input in step e1) is obtained in an open control loop in a very short time, enabling to react quickly to variations of the fuel drain rate caused by the variations of engine load and/or speed, whereas a fine control of the pump operation is carried out in a closed loop in steps e2) and e3). 
     A controller is provided in accordance with an embodiment of the invention for carrying out the method as described above, the controller comprising a feed-forward unit for carrying out steps a) to d) and a feedback unit for carrying out step e). A data processor program product comprising program code means for enabling a data processor to form at least the feed-forward unit of the above described controller or to carry out the method as described above. This data processor program product may further comprise a data carrier in which said program code means are recorded in machine readable form. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will hereinafter be described in conjunction with the following drawing figures, wherein like numerals denote like elements, and. 
         FIG. 1  is a block diagram of a fuel supply system; 
         FIG. 2  is a section of an injector of the fuel supply system of  FIG. 1 ; 
         FIG. 3  is a block diagram of the controller of the fuel supply system; 
         FIG. 4  is an example of experimental leakage rate data on which control of the fuel supply system is based; 
         FIG. 5  illustrates static leakage rates as a function of rail pressure for various fuel temperatures; 
         FIG. 6  illustrates dynamic leakage rates as a function of excitation time for various values of fuel temperature and rail pressure; 
         FIG. 7  is an example of efficiency characteristics of the fuel pump as a function of engine speed at various values of the rail pressure and a fuel temperature of 40° C.; 
         FIG. 8  illustrates characteristics of the pump efficiency at various fuel temperatures and a rail pressure of 300 bar; and 
         FIG. 9  illustrates efficiency characteristics at various fuel temperatures and a rail pressure of 1600 bar. 
     
    
    
     DETAILED DESCRIPTION 
     The following detailed description is merely exemplary in nature and is not intended to limit application and uses. Furthermore, there is no intention to be bound by any theory presented in the preceding background or summary or the following detailed description. 
       FIG. 1  is a schematic outline of a fuel supply system of a Diesel engine in which the present invention is applicable. A fuel pump  1 , e.g., a gear pump or a pump having multiple pistons driven by a same rotating excenter, draws fuel from a tank  2  and supplies it at high pressure to a rail  3 . The rail  3  has an arbitrary number of injectors  4  connected to it for injecting fuel from rail  3  into cylinders of a Diesel engine, not shown. An electronic controller  5  controls the rotation speed of pump  1  and excitation times of injectors  4  based on fuel temperature T fuel  and rail pressure P detected by sensors  6 ,  7  at the fuel rail  3 , a rotation speed n of the diesel engine and a fuel injection quantity Q inj  to be injected per cylinder and per engine stroke, set by a higher level controller, not shown. 
       FIG. 2  is a schematic longitudinal section of one of injectors  4 . A high pressure fuel inlet  11  which receives fuel from rail  3  is connected to an injection nozzle  12  at the bottom end of injector  4  by a feed pipe  13 . In the configuration shown, output of fuel at nozzle  12  is blocked by a conical tip of a control piston  14 . At an end of control piston  14  opposite to said tip there is a control chamber  15  which communicates with fuel inlet  11  via a small feed orifice  16 . Pressurized fuel in control chamber  15  urges control piston  14  downward. The control piston  14  is shaped so that if pressures at the tip of piston  14  and in control chamber  15  are equal, a net downward force keeps the piston  14  pressed against injection nozzles  12 . 
     The control chamber  15  has a bleed orifice  17  which at rest is held blocked by a pin element  18  of a pilot valve. If the pin element  18  is allowed to recede by exciting a solenoid  19  of the pilot valve, fuel escapes from control chamber  15  through bleed orifice  17 , causing the pressure in control chamber  15  to drop, whereby control piston  14  is displaced upwards by the pressure acting on its bottom tip. The tip of the piston  14  is thus removed from the injection nozzles  12 , and fuel is ejected from nozzles  12  into a combustion cylinder. 
     When the excitation of the solenoid  19  stops, pin element  18  is pressed against bleed orifice  17  again by means of a spring. In consequence, the pressure in control chamber  15  rises again and finally becomes sufficient to press the control piston  14  against the injection nozzles  12  again. 
     While the injection nozzles  12  are blocked, fuel may escape from high pressure regions of the injector to a return port  20  thereof and from there back to tank  2  via clearings, e.g. along control piston  14 . In addition, when the solenoid  19  is excited, fuel that escapes through bleed orifice  17  will reach the return port  20 . Thus the total flow of fuel through injector  4  can be regarded as made up of three contributions, firstly a flow which is indeed injected into the combustion cylinder, secondly a static leakage flow which may be defined as that portion of a total leakage flow which exists regardless of whether the solenoid  19  is excited or not, and a dynamic leakage flow which is made up of the fuel used for driving the displacement of pin element  18  or which escapes through leaks inside the injector which exist only when the solenoid  19  is excited and the control piston  14  is displaced from its rest position shown in  FIG. 2 . 
       FIG. 3  is a block diagram of the controller  5 . For ease of description, the controller  5  is shown divided into three controller units  22 ,  23 ,  24 , any of which might be implemented by hardware of its own. In most practical embodiments, however, it is to be expected that each control unit will be implemented as a software module, and that all modules are executed on a same hardware. 
     First open loop controller unit  22  receives from a higher level engine controller, not shown, data Q inj  specifying an amount of fuel to be injected into each cylinder of the engine during an engine stroke, and an excitation time ET specifying for how long an excitation current will be supplied to solenoid  19  during said stroke. It should be noted that both Q inj  and ET can be thought of as scalar quantities if there is just one fuel injection per stroke, or as vectors in case of multiple injections, the components of the vectors specifying injection amounts and excitation times of each injection. A current engine speed n is supplied to control unit  22  by a rotation speed sensor at an output shaft of the engine, or a target value of the rotation speed n is delivered by said higher level controller. Fuel temperature data T fuel  are provided by sensor  7 . 
     Control unit  22  comprises a storage  22 ′ in which a plurality of characteristics of static and dynamic leakage rate and, eventually, program instructions for controlling the operation of unit  22  are recorded. Such characteristics may be derived from experimental leakage rate data as shown exemplarily in  FIG. 4 . The curves shown in  FIG. 4  illustrate average leakage rates under equilibrium conditions observed as a function of excitation time ET for various values of rail pressure, from 300 bar to 1600 bar and of the fuel temperature, from 28° C. to 55° C., at a constant rotation speed of the engine of e.g. n=1500 rpm. Quite clearly, for ET=0 the curves of  FIG. 4  will give the static leakage rate. 
       FIG. 5  is a typical example of characteristic curves st 28 , st 40 , st 55  of static leakage rates G st  of an injector  4  as a function of rail pressure P for fuel temperatures 28° C., 40° C. and 55° C., as will be recorded in the storage  22 ′ of control unit  22 . It can be seen that the leakage rate G st  increases with fuel temperature T fuel  since viscosity of the fuel decreases when it is heated. What is unexpected is the pressure dependency of the static leakage rates. Theoretically, the flow rate of a laminar flow should be governed by Poiseuille&#39;s formula 
                 G   st     =       K   γ     ⁢   Δ   ⁢           ⁢   p       ,         
Where K denotes a geometry-dependent factor and γ the viscosity of the fuel, and the pressure drop Δp in the injector  4  can be regarded as equal to the rail pressure P, i.e., the leakage rate G st  should be directly proportional to the rail pressure P. It is quite clear from  FIG. 5  that this equation doesn&#39;t give a satisfactory description of the leakage rate G st . The actual increase of the leakage rate G st  with the rail pressure P is much more pronounced than any of the two formulas predicts. The reason for this is that decompression of the fuel in the injector is not isothermal. Diesel fuel has a negative Joule-Thomson coefficient, so that decompression will cause it to heat up. The amount of heating and its effects on the leakage rate depend in a complex fashion on the shape of the leakage paths, and on the speed at which the heat generated in the fuel is dissipated. Quite clearly, the dependence of the static leakage rate G st  of a given injector on fuel temperature T fuel  and rail pressure P is best determined by experiment.
 
     At any given fuel temperature T fuel  and rail pressure P, the discrepancy between the static leakage rates G st  of  FIG. 5  and the measurement data of  FIG. 4  corresponds to the dynamic leakage. Characteristics recorded in the storage  22 ′ of control unit  22  specify the dynamic leakage amount Δm dyn  in terms of the fuel mass leaking per injection event. The leakage amount Δm dyn  is straightforwardly calculated from the experimental data of  FIG. 4  by subtracting the static leakage rate G st  and dividing the result by the number of injections per unit of time, i.e., by n. 
       FIG. 6  exemplarily illustrates such characteristics dyn 300 / 28 , dyn 300 / 55 , dyn 750 / 28 , . . . , dyn 1600 / 55  for various fuel pressures and temperatures as a function of excitation time ET. At low rail pressure values of 300 bar or 750 bar, the leakage amount Δm dyn  appears to increase linearly with excitation time over the entire range of ET shown. At a rail pressure of 1200 bars, the slope of the leakage amount curves dyn 1200 / 28 , dyn 1200 / 55  decreases above an excitation time of 1200 μs, and at 1600 bars, a decrease of the slope of curve dyn 1600 / 28  is seen at ET=approx. 1000 μs for a fuel temperature of 28° C., and at ET=approx. 900 μs for a fuel temperature of 55° C. in curve dyn 1600 / 55 . The reason for this is believed to be in the internal structure of the injector  4 : as long as the pilot valve pin element  18  is pushed upwards by the fuel escaping through bleed orifice  17 , it does not constitute an obstacle to the dynamic leakage at bleed orifice  17 . The dynamic leakage rate is therefore determined mainly be the width of bleed orifice  17  and the fuel temperature there. The time needed by pin element  18  to reach an abutment is the shorter, the higher the flow rate through bleed orifice  17  is, i.e., the higher fuel pressure P and temperature T fuel  are. When pin element  18  has reached the abutment, it forms a further obstacle to the flow of fuel, and the flow rate through bleed orifice  17  will decrease. The dynamic leakage amount Δm dyn  shown in  FIG. 6 , being an integral of the flow through bleed orifice  17 , will exhibit a reduced increase rate when the pin element  18  has reached its abutment. 
     In case of a fuel supply system with a single injection per stroke, control unit  22  will look up the dynamic leakage characteristics of  FIG. 7  at the values of excitation time ET, fuel temperature T fuel  and rail pressure P received by it, and will multiply the thus determined value of the leakage amount Δm dyn  by the rotation speed n in order to calculate a dynamic leakage rate G dyn  in terms of mass per time unit. 
     In case of a multi-injection system, leakage amounts may be looked up from the characteristics of  FIG. 6  for each injection of a same stroke, taking account of the individual excitation time ET which may be different for the various injections, and the sum of the leakage amounts of the individual injections gives a total leakage amount Δm dyn  per injector and stroke. 
     A dynamic leakage rate G dyn  is obtained in control unit  22  by multiplying the leakage amount Δm dyn  by the number of strokes per time unit, i.e. by the rotation speed n. The control unit  22  calculates a desired delivery rate Q out     —     pump  of pump  1  as the sum of specified injection flow rates Q inj  and total leakage rates G st  and G dyn  of the injectors  4  at given operating conditions n, T fuel  and P set . 
     A second control unit  23  receives the desired delivery rate Q out     —     pump , T fuel  and P set . Control unit  23  comprises a storage  23 ′ with efficiency characteristics of fuel pump  1  stored therein. Just like the leakage characteristics of the injectors  4 , these efficiency characteristics may be determined for a particular type of fuel pump by experiment.  FIGS. 7 to 9  show typical examples of such characteristics. In  FIG. 7 , the efficiency is shown as a function of pump rotation speed for different rail pressures P and a temperature T fuel  of 40° C. Quite expectedly, the efficiency η decreases with pressure P. Surprisingly, however, the efficiency η is observed to decrease with pump rotation speed at low values of the rail pressure P whereas at high pressure values it increases. This latter effect is quite independent of the fuel temperature as evidenced by  FIGS. 8 and 9 , which show the efficiency η as a function of pump rotation speed for different fuel temperatures T fuel  at a rail pressure P of 300 bar in case of  FIG. 8  and of 1600 bar in case of  FIG. 9 . 
     Based on the stored pump efficiency characteristics, control unit  23  outputs a control parameter to fuel pump  1  in order to deliver the desired flow rate Q out     —     pump  at its output side. In most practical embodiments, this control parameter will be a target rotation speed of the pump  1 . 
     Since this target rotation speed is determined in an open control loop, an updated value of it is available at minimum delay whenever the operating conditions of the Diesel engine change. Fluctuations of the rail pressure P due to changes of the desired injection quantity Q inj , the engine speed n etc. can thus be kept at a very low level. 
     In order to avoid long-term deviation between the target rail pressure P set  and the actual pressure P, the third control unit  24  establishes a closed loop control: a subtractor  25  determines a deviation P err  between the rail pressure P and its target value P set  and provides it to PID controller  26 . A correction term output by PID controller  26  is superimposed upon the control signal from control unit  23  by adder  27 , and pump  1  is controlled using the output of adder  27 . In this way, the high response speed of open loop control is combined with the precision and freeness from drift of closed loop control. 
     While at least one exemplary embodiment has been presented in the foregoing summary and detailed description, it should be appreciated that a vast number of variations exist. It should also be appreciated that the exemplary embodiment or exemplary embodiments are only examples, and are not intended to limit the scope, applicability, or configuration in any way. Rather, the foregoing summary and detailed description will provide those skilled in the art with a convenient road map for implementing an exemplary embodiment, it being understood that various changes may be made in the function and arrangement of elements described in an exemplary embodiment without departing from the scope as set forth in the appended claims and their legal equivalents.