Patent Publication Number: US-11041447-B2

Title: Method to control a high-pressure fuel pump for a direct injection system

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This patent application claims priority from Italian patent application no. 102019000012300 filed on Jul. 18, 2019, the entire disclosure of which is incorporated herein by reference. 
     TECHNICAL FIELD 
     The invention relates to a method to control a fuel pump for a direct injection system. Preferably (though not necessarily), the control method is used for a direct injection system in a spark-ignition internal combustion engine, which, thus, works with gasoline or similar fuels. 
     PRIOR ART 
     As it is known, a fuel—in this specific case gasoline—direct injection system of the common rail type for an internal combustion heat engine comprises a plurality of injectors, a common rail, which feeds pressurized fuel to the injectors, a high-pressure pump, which feeds fuel to the common rail and is provided with a flow rate adjusting device, a control unit, which causes the fuel pressure inside the common rail to be equal to a desired value, which generally varies in time depending on the engine operating conditions, and a low-pressure pump, which feeds fuel from a tank to the high-pressure pump by means of a feeding duct. 
     The control unit is coupled to the flow rate adjusting device so as to control the flow-rate of the high pressure pump, so that the common rail is supplied, instant by instant, with the amount of fuel needed to have the desired pressure value in the common rail; in particular, the control unit adjusts the flow rate of the high pressure pump by means of a feedback control, which uses, as a feedback variable, the value of the fuel pressure inside the common rail. 
     The operating cycle of the high pressure pump substantially comprises three phases: an intake phase, in which to allow the passage of a fuel flowing into a pumping chamber of the high-pressure pump; a reflux phase, during which a respective intake valve is kept open and there is a passage of fuel flowing out of the pumping chamber towards the low-pressure circuit; and a pumping phase, during which the respective intake valve closes and the fuel pressure inside the pumping chamber reaches a values that is such as to cause a fuel flow flowing out of the pumping chamber towards the common rail. 
     Experiments have shown that, during the pumping phase, there is a significant increase in the temperature of the high-pressure pump. In particular, when there is a pressure increase from 200 to 600 bar, the temperature variation ranges from 30 to 50° C. in the different points of the high-pressure pump, whereas, in case there is a pressure increase from 600 to 800 bar, the temperature variation assumes much more significant values in the range of 80° C. A temperature variation ranging from 30 to 50° C. could already lead to cavitation problems, which cause the high-pressure pump to become unstable and scarcely reliable, namely incapable of making sure that the common rail is supplied, instant by instant, with the quantity of fuel needed to reach the desired pressure value inside the common rail. 
     It has been proved that this phenomenon worsens in case the high pressure pump does not work with a full load, i.e. in case the fuel quantity needed to have the desired pressure value inside the common rail and fed by the high pressure pump is lower than the maximum flow rate that can be delivered by the high-pressure pump. In case the high-pressure pump operates with a full load (namely, in case the fuel quantity needed to have the desired pressure value inside the common rail and fed by the high-pressure pump is equal to the maximum flow rate that can be delivered by the high-pressure pump), the heat generated during the pumping phase is removed through the fuel flow rate flowing out of the high-pressure pump and the removal of the heat generated during the pumping phase is proportional to the fuel flow rate flowing of the high-pressure pump. 
     Furthermore, in case the high-pressure pump does not operate with a full load, but with a partial load, the operation of the high-pressure pump is characterized by negative effects, in particular in terms of energy efficiency, and by potential damaging risks. 
     In particular, the energy used (and, as a consequence, the heat generated) during the compression phase is proportional to the mass of fuel trapped by the respective intake valve (considering both the adjusted fuel flow rate and the dead volume), whereas the heat removed is proportional to the sole flow rate delivered (since the dead volume does not flow out of the high-pressure pump and, clearly, cannot disperse heat). As a consequence, the smaller the flow rate delivered, the greater the thermal overload. The useful energy transmitted by the system to the fuel is also proportional to the sole flow rate delivered. 
     On the other hand, as far as the potential damaging risks of the high-pressure pump are concerned, closing the intake valve far from the top dead centre and from the bottom dead centre of the high-pressure pump, namely when the speed of the piston of the pump is other than zero and when the engine operates at a high speed, leads to quick and significant pressure increases, which cause, in turn, mechanical oscillations with consequent potential damaging risks. 
     In order to avoid the triggering of cavitation phenomena or the damaging of the high-pressure pump, different solutions were suggested over the years, which, in particular, are aimed at limiting the temperature increase of the high-pressure pump during the pumping phase. 
     For instance, in order to solve the cavitation problem, it is possible to increase the pressure of the fuel flowing into the high-pressure pump, but this solution is also affected by negative effects in terms of energy efficiency. Alternatively, the high-pressure pump can be provided with a fuel recirculation circuit, which is provided with a draining duct, which transfers a fuel portion from the pumping chamber to the tank, so that the heat generated during the pumping phase is disposed of through the fuel flow rate flowing out of the high-pressure pump; this technical solution, though, suffers from significant drawbacks in terms of overall dimensions of the injection system and is disadvantageous from and economic point of view. 
     DESCRIPTION OF THE INVENTION 
     Therefore, the object of the present invention is to provide a method to control a fuel pump for a direct injection system, said method not suffering from the drawbacks described above and, in particular, being easy and economic to be implemented. 
     According to the invention, there is provided a method to control a fuel pump for a direct injection system according to the appended claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will now be described with reference to the accompanying drawings, showing a non-limiting embodiment thereof, wherein: 
         FIG. 1  is a schematic view, with some details removed for greater clarity, of a fuel direct injection system; 
         FIG. 2  is a block diagram showing a first variant of the operating logic of the method according to the invention; and 
         FIG. 3  is a block diagram showing a second variant of the operating logic of the method according to the invention. 
     
    
    
     PREFERRED EMBODIMENTS OF THE INVENTION 
     In  FIG. 1 , number  1  indicates, as a whole, a fuel direct injection system, in particular using gasoline as a fuel, of the common rail type for an internal combustion engine. 
     The direct injection system  1  comprises a plurality of injectors  2 , a common rail  3 , which feeds fuel under pressure to the injectors  2 , a high-pressure pump  4 , which feeds fuel to the common rail  3  by means of a feeding duct  5  and is provided with a flow rate adjusting device  6 , an electronic control unit  7 , which causes the fuel pressure inside the common rail  3  to be equal to a desired value, which generally varies in time depending on the engine operating conditions, and a low-pressure pump  8 , which feeds fuel from a tank  9  to the high-pressure pump  4  by means of a feeding duct  10 . 
     The electronic control unit  7  is coupled to the flow rate adjusting device  6  so as to control the flow rate of the high-pressure pump  4  in order to feed to the common rail  3 , instant by instant, the quantity of fuel needed to have the desired pressure value inside the common rail  3 . Furthermore, the electronic control unit  7  is connected to a pressure sensor  11 , which detects in real time the fuel pressure P RAIL  inside the common rail  3 . 
     Hereinafter we will describe the strategy implemented by the electronic control unit  7  to control the high-pressure pump  4 . 
     The strategy entails determining a minimum threshold Q MIN  of fuel to be pumped with every operating cycle of the high-pressure pump  4 , according to  FIG. 2 . 
     The minimum threshold Q MIN  is basically determined based on a plurality of parameters, such as pressure P RAIL  in the common rail  3  detected by means of the pressure sensor  11 , temperature T PUMP  of the high-pressure pump  4 , inlet pressure P LOW  of the high-pressure pump  4 , speed n of the heat engine  1  and engine load C. 
     The temperature T PUMP  of the high-pressure pump  4  can either be detected by means of a dedicated temperature sensor housed on the high-pressure pump  4  (T PUMP_SENSOR ) or be estimated by means of an estimation model (T PUMP_VIRTUAL ). 
     More in detail, inside the electronic control unit  7  there is stored a map COLD, which provides an (open loop) contribution Q MIN_COLD  to determine the minimum threshold Q MIN . The contribution Q MIN_COLD  represents the minimum threshold of fluid to be pumped under cold conditions, i.e. under conditions that are far from the triggering of cavitation phenomena for given values of the pressure P RAIL  in the common rail  3  and of the speed n of the heat engine  1 . Indeed, the map COLD receives, as an input, the values of the pressure P RAIL  in the common rail  3  and of the speed n of the heat engine  1 , respectively, and, based on said input values, provides the contribution Q MIN_COLD . 
     Similarly, inside the electronic control unit there is stored a further map HOT, which provides an (open loop) contribution Q MIN_HOT  to determine the minimum threshold Q MIN . The contribution Q MIN_HOT  represents the minimum threshold of fluid to be pumped under hot conditions, i.e. under conditions that are close to the triggering of cavitation phenomena for given values of the pressure P RAIL  in the common rail  3  and of the speed n of the heat engine  1 . 
     Finally, inside the electronic control unit  7  there is stored a map VAPOR PRESSURE, which provides a coefficient K (expressed as percentage), which is also used to determine the minimum threshold Q MIN . The map VAPOR PRESSURE receives, as an input, the values of the inlet pressure P LOW  of the high-pressure pump  4  (also known as “low pressure”) and of the temperature T PUMP  of the high-pressure pump  4 , respectively, the latter being expressed either by the temperature (T PUMP_SENSOR ) detected by means of the temperature sensor housed on the high-pressure pump  4  or by the temperature (T PUMP_VIRTUAL ) estimated by means of an estimation model. Said map VAPOR PRESSURE contains the curves of the fuel vapour pressure depending on the temperature T PUMP  of the high-pressure pump  4 . Based on the temperature T PUMP  of the high-pressure pump  4  and on the inlet pressure P LOW  of the high-pressure pump  4 , the map VAPOR PRESSURE provides said coefficient K, which expresses (as a percentage) how far or close the high-pressure pump  4  is from or to the condition of triggering of cavitation phenomena. 
     Therefore, the minimum threshold Q MIN  is calculated as follows:
 
 Q   MIN =(1− K )* Q   MIN_COLD   +K*Q   MIN_HOT   [1]
 
     Q MIN  minimum threshold; 
     K coefficient; 
     Q MIN_COLD  “cold” contribution of the minimum threshold; and 
     Q MIN_HOT  “hot” contribution of the minimum threshold. 
     It is evident that, for example, a value of the coefficient K equal to 1 provided by the map VAPOR PRESSURE indicates that the high-pressure pump  4  is working under conditions that are close to the triggering of cavitation phenomena; on the other hand, a value of the coefficient K equal to 0 or to 0.2 provided by the map VAPOR PRESSURE indicates that the high-pressure pump  4  is working under conditions that are very far from the triggering of cavitation phenomena. 
     Furthermore, it should be pointed out that both inside the map COLD providing the contribution Q MIN_COLD  and inside the map HOT providing the contribution Q MIN_HOT  to determine the minimum threshold Q MIN  there are embedded both the contribution to increase the energy efficiency and the contribution to decrease potential damaging risks. 
     In other words, both the contribution Q MIN_COLD  and the contribution Q MIN_HOT  are determined so as to contain the temperature variation of the high-pressure pump  4  and, simultaneously, increase the energy efficiency and decrease potential damaging risks. 
     According to a preferred embodiment, the strategy entails determining an energy index I, which gives an indication of the closeness—or lack thereof—to the triggering of cavitation phenomena of the high-pressure pump  4 . The energy index I is preferably based on the intensity of the perturbation of the signal concerning the pressure P RAIL  in the common rail  3  detected in real time by the pressure sensor  11 . Said perturbation is assessed by means of an integral within an observation time window between time instants t 1  and t 2 , as described more in detail below. 
     According to a first variant, the energy index I 1  is expressed as follows:
 
 I   1 =∫ t     1     t     2   ( P   TARGET   −P   RAIL ) 2   dt   [2]
 
     According to a second variant, the energy index I 2  is expressed as follows:
 
 I   2 =∫ t     1     t     2   ( P   RAIL_M   −P   RAIL ) 2   dt   [3]
 
     According to a third variant, the energy index I 3  is expressed as follows:
 
 I   3 =∫ t     1     t     2   ( INT   M   −INT ) 2   dt   [4]
 
     wherein: 
     t 1 , t 2  instants defining an observation time window; 
     P RAIL  actual pressure in the common rail  3 ; 
     P TARGET  pressure target in the common rail  3 ; 
     P RAIL_M  actual mean pressure in the common rail  3  and within the observation window; 
     INT value of the integral component of the closed loop of the pressure control; 
     INT M  mean value of the integral component of the closed loop of the pressure control within the observation window. 
     The indexes I 1  and I 2  are clearly calculated in case the objective fuel flow rate M ref  is delivered (as described more in detail below), namely under “normal” operating conditions (without deactivation). 
     The energy index I is used inside the electronic control unit  7  to obtain an adaptive function aimed at optimizing the strategy, so that it can be adapted to high-pressure pumps  4  with different production tolerances. 
     In particular, the adaptive function entails storing a threshold value inside the electronic control unit  7 . The threshold value preferably is variable based on the load (and, namely, on the injected fuel quantity Q F_INJ ). The threshold value preferably is variable also based on the speed n of the heat engine. Furthermore, the threshold value is variable based on the difference between the quantity Q F_INJ  of fuel injected by the injectors  2  and the actual fuel flow rate of the high-pressure pump  4 . 
     The threshold value is preferably determined in an experimental set up phase. The threshold value is continuously compared with the energy index I under stationary conditions of applied load, speed n of the heat engine and pressure target P TARGET . 
     The threshold value is determined in such a way that, when the energy index I exceeds the threshold value, this indicates that the high-pressure pump  4  is working under conditions that are close to the triggering of cavitation phenomena. Therefore, when the electronic control unit  7  detects that the energy index I exceeds the threshold value, the electronic control unit  7  is designed to increase the minimum threshold Q MIN  by a quantity ΔQ MIN  and to decrease the pressure target P TARGET  in the common rail  3  by a quantity ΔP TARGET  and for a given amount of time. 
     According to a preferred variant, the quantity ΔP TARGET  is equal to at least 10 bar (the quantity ΔP TARGET  is independent of the difference between the energy index I and the respective threshold value). In case the energy index I remains greater than the respective threshold value, the quantity ΔP TARGET  is increased to 20 bar. The quantity ΔP TARGET  is increased by 10 bar as long as the energy index I does not go back to a value that is smaller than the respective threshold value. 
     Therefore, in this case, the minimum threshold Q MIN  is calculated as follows:
 
 Q   MIN =(1− K )* Q   MIN_COLD   +K*Q   MIN_HOT   +ΔQ   MIN   [5]
 
     Q MIN  minimum threshold; 
     K coefficient; 
     Q MIN_COLD  “cold” contribution of the minimum threshold; 
     Q MIN_HOT  “hot” contribution of the minimum threshold; and 
     ΔQ MIN  quantity. 
     Preferably, the quantity ΔQ MIN  is variable and at least equal to 20 mg (the quantity ΔQ MIN  is independent of the difference between the energy index I and the respective threshold value). In case the energy index I remains greater than the respective threshold value, the quantity ΔQ MIN  is increased to 40 mg. The quantity ΔQ MIN  is increased by 20 mg as long as the energy index I does not reach a value that is smaller than the respective threshold value. 
     Once the minimum threshold Q MIN  has been calculated, the strategy entails controlling the high-pressure pump  4  based on said minimum threshold Q MIN  so as to contain the temperature variation generated during the pumping phase in the high-pressure pump  4 , increase energy efficiency and decrease potential damaging risks. 
     According to a further variant shown in  FIG. 3 , the strategy entrails calculating a contribution Q TEMP  to contain the temperature variation generated during the pumping phase in the high-pressure pump  4  according to the description above. 
     More in detail, inside the electronic control unit  7  there is stored a map COLD, which provides an (open loop) contribution Q MIN_COLD  to determine the contribution Q TEMP . The contribution Q MIN_COLD  represents the minimum threshold of fluid to be pumped under cold conditions, i.e. under conditions that are far from the triggering of cavitation phenomena for given values of the pressure P RAIL  in the common rail  3  and of the speed n of the heat engine  1 . Indeed, the map COLD receives, as an input, the values of the pressure P RAIL  in the common rail  3  and of the speed n of the heat engine  1 , respectively, and, based on said input values, provides the contribution Q MIN_COLD . 
     Similarly, inside the electronic control unit there is stored a further map HOT, which provides an (open loop) contribution Q MIN_HOT  to determine the contribution Q TEMP . The contribution Q MIN_HOT  represents the minimum threshold of fluid to be pumped under hot conditions, i.e. under conditions that are close to the triggering of cavitation phenomena for given values of the pressure P RAIL  in the common rail  3  and of the speed n of the heat engine  1 . 
     Finally, inside the electronic control unit  7  there is stored a map VAPOR PRESSURE, which provides a coefficient K (expressed as percentage), which is also used to determine the contribution Q TEMP . The map VAPOR PRESSURE receives, as an input, the values of the inlet pressure P LOW  of the high-pressure pump  4  (also known as “low pressure”) and of the temperature T PUMP  of the high-pressure pump  4 , respectively, the latter being expressed either by the temperature (T PUMP_SENSOR ) detected by means of the temperature sensor housed on the high-pressure pump  4  or by the temperature (T PUMP_VIRTUAL ) estimated by means of an estimation model. Said map VAPOR PRESSURE contains the curves of the fuel vapour pressure depending on the temperature T PUMP  of the high-pressure pump  4 . Based on the temperature T PUMP  of the high-pressure pump  4  and on the inlet pressure P LOW  of the high-pressure pump  4 , the map VAPOR PRESSURE provides said coefficient K, which expresses (as a percentage) how far or close the high-pressure pump  4  is from or to the condition of triggering of cavitation phenomena. 
     Therefore, the contribution Q TEMP  is calculated as follows:
 
 Q   TEMP =(1− K )* Q   MIN_COLD   +K*Q   MIN_HOT   [6]
 
     Q TEMP  contribution to contain the temperature variation generated during the pumping phase in the high-pressure pump  4 . 
     K coefficient; 
     Q MIN_COLD  “cold” contribution of the minimum threshold; and 
     Q MIN_HOT  “hot” contribution of the minimum threshold. 
     Or, alternatively, the contribution Q TEMP  is calculated as follows:
 
 Q   TEMP =(1− K )* Q   MIN_COLD   +K*Q   MIN_HOT   +ΔQ   MIN   [7]
 
     Q TEMP  contribution to contain the temperature variation generated during the pumping phase in the high-pressure pump  4 . 
     K coefficient; 
     Q MIN_COLD  “cold” contribution of the minimum threshold; 
     Q MIN_HOT  “hot” contribution of the minimum threshold; and 
     ΔQ MIN  quantity. 
     Wherein the quantity ΔQ MIN  has the meaning described above, is variable and at least equal to 20 mg (the quantity ΔQ MIN  is independent of the difference between the energy index I and the respective threshold value). In case the energy index I remains greater than the respective threshold value, the quantity ΔQ MIN  is increased to 40 mg. The quantity ΔQ MIN  is increased by 20 mg as long as the energy index I does not reach a value that is smaller than the respective threshold value. 
     Furthermore, the strategy entails calculating a contribution Q EEff  to increase energy efficiency and a further contribution Q DAM  to decrease potential damaging risks. 
     More in detail, inside the electronic control unit  7  there is stored a map, which provides the (open loop) contribution Q EEff  to increase energy efficiency in order to determine the minimum threshold Q MIN . The contribution Q EEff  represents the quantity of fluid to be pumped in order to optimize energy efficiency for given values of the pressure P RAIL  in the common rail  3  and of the quantity Q F_INJ  of fuel injected by the injectors  2 . Indeed, the map receives, as an input, the values of the pressure P RAIL  in the common rail  3  and of the quantity Q F_INJ  of fuel injected by the injectors  2 , respectively, and, based on said input values, provides the contribution Q EEff . 
     The contribution Q EEff  is preferably determined based on a driving mode DV chosen by the driver of the vehicle provided with the heat engine  1 . Advantageously, the contribution Q EEff  is determined (weighed) depending on the position of the hand lever identifying the driving/operating mode DV chosen by the driver from among a plurality of possible driving/operating modes DV; for example, the possible driving/operating modes DV comprise the sports driving/operating mode DV (which enhances performances), the normal driving/operating mode DV, the eco driving/operating mode DV (which enhances the reduction of consumptions), etc. Each possible driving/operating mode DV corresponds to a weight (determined during a preliminary set up phase). 
     Furthermore, inside the electronic control unit  7  there is stored a map, which provides the (open loop) contribution Q DAM  to decrease potential damaging risks in order to determine the minimum threshold Q MIN . The contribution Q DAM  represents the minimum quantity of fluid to be pumped in order to decrease potential damaging risks for given values of the pressure P RAIL  in the common rail  3  and of the speed n of the heat engine  1 . Indeed, the map receives, as an input, the values of the pressure P RAIL  in the common rail  3  and of the speed n of the heat engine  1 , respectively, and, based on said input values, provides the contribution Q DAM . 
     Finally, the minimum threshold Q MIN  is calculated. Preferably, the minimum threshold Q MIN  corresponds to the greatest one among the contribution Q TEMP  to contain the temperature variation generated during the pumping phase in the high-pressure pump  4 , the contribution Q EEff  to increase energy efficiency and the contribution Q DAM  to decrease potential damaging risks. Alternatively, the minimum threshold Q MIN  corresponds to weighed mean of the contribution Q TEMP  to contain the temperature variation generated during the pumping phase in the high-pressure pump  4 , the contribution Q EEff  to increase energy efficiency and the contribution Q DAM  to decrease potential damaging risks. 
     Hence, the strategy entails calculating the objective fuel flow rate M ref  to be fed by the high pressure pump  4  to the common rail  3  instant by instant in order to have the desired pressure value inside the common rail  3 . 
     Then, the electronic control unit  7  is designed to compare the objective fuel flow rate M ref  with the minimum threshold Q MIN . 
     In case the objective fuel flow rate M ref  is greater than (or equal to) the minimum threshold Q MIN , the high-pressure pump  4  is controlled so as to deliver the objective fuel flow rate M ref . On the contrary, in case the objective fuel flow rate M ref  is smaller than the minimum threshold Q MIN , the high-pressure pump  4  carries out an idle operating cycle of the high-pressure pump  4 . In other words, in case the objective fuel flow rate M ref  is smaller than the minimum threshold Q MIN , the high-pressure pump  4  is not operated. 
     The control unit  7  is designed to adjust the flow rate of the high-pressure pump  4  so as to process objective fuel flow rates M ref  which are greater than the minimum threshold Q MIN . In other words, the control unit  7  is designed to control the alternation of operating cycles, in which the high-pressure pump  4  processes objective fuel flow rates M ref  which are greater than the minimum threshold Q MIN , and idle operating cycles. 
     Hence, the electronic control unit  7  is configured to control, with every activation cycle, the high-pressure pump  4  by means of a feedback control using, as feedback variables, the value of the fuel pressure inside the common rail  3 , which is preferably detected in real time by the pressure sensor  11 , and the comparison between the objective fuel flow rate M ref  to be fed by the high-pressure pump  4  to the common rail  3  instant by instant in order to have the desired pressure value inside the common rail  3  and the minimum threshold Q MIN , which is calculated according to formulas [1] or [5] described above. 
     The strategy implemented by the electronic control unit  7  to control the high-pressure pump  4  and described so far has some advantages. 
     In particular, even though it is advantageous in terms of costs, it is also easy and cheap to be implemented. In particular, the method described above does not involve an excessive computing burden for the electronic control unit  7  and, at the same time, allows manufacturers to avoid the triggering of cavitation phenomena, avoid damages to the high-pressure pump  4  and contain the temperature variation generated during the pumping phase in the high-pressure pump  4  as well as maintain the objective value of the fuel pressure inside the common rail  3 .