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

Abstract

The invention relates to a method to control a fuel pump for a direct injection system of a heat engine provided with a common rail comprising the steps of determining a minimum threshold based on the pressure in the common rail and on the speed of the heat engine, on the temperature of the high-pressure pump and on the inlet pressure of the high-pressure pump; calculating the objective fuel flow rate to be fed by the high-pressure pump to the common rail instant by instant in order to have the desired pressure value inside the common rail; comparing the objective fuel flow rate with the minimum threshold; and controlling the high-pressure pump based on the comparison between the objective fuel flow rate and the minimum threshold.

Claims

1. A method to control a fuel pump (4) for a direct injection system of a heat engine (1) provided with a common rail (3) comprising the steps of: determining a minimum threshold (Q.sub.MIN, Q.sub.TEMP) of fuel to be fed by the high-pressure pump (4); calculating the objective fuel flow rate (M.sub.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 (P.sub.TARGET) inside the common rail (3); comparing the objective fuel flow rate (M.sub.ref) with the minimum threshold (Q.sub.MIN, Q.sub.TEMP); and controlling the high-pressure pump (4) based on the comparison between the objective fuel flow rate (M.sub.ref) and the minimum threshold (Q.sub.MIN, Q.sub.TEMP); the method is characterized in that the step of determining a minimum threshold (Q.sub.MIN, Q.sub.TEMP) comprises the sub-steps of: determining a first contribution (Q.sub.MIN_COLD) and a second contribution (Q.sub.MIN_HOT) based on the pressure (P.sub.RAIL) in the common rail (3) and on the speed (n) of the heat engine (1); wherein the first contribution (Q.sub.MIN_COLD) is the minimum threshold of fluid to be pumped under cold conditions that are far from the triggering of cavitation phenomena for given values of the pressure (P.sub.RAIL) in the common rail (3) and of the speed (n) of the heat engine (1), and the second contribution (Q.sub.MIN_HOT) is the minimum threshold of fuel to be pumped under hot conditions that are close to the triggering of cavitation phenomena for given values of the pressure (P.sub.RAIL) in the common rail (3) and of the speed (n) of the heat engine (1); determining a coefficient (K) based on the temperature (T.sub.PUMP) of the high-pressure pump (4) and on the inlet pressure (P.sub.LOW) of the high-pressure pump (4); wherein said coefficient (K) expresses the closeness of the high-pressure pump (4) to the condition of triggering of cavitation phenomena; and determining said minimum threshold (Q.sub.MIN, Q.sub.TEMP) based on the first contribution (Q.sub.MIN_COLD), on the second contribution (Q.sub.MIN_HOT) and on the coefficient (K).

2. A method according to claim 1 and comprising the further steps of: determining a third contribution (Q.sub.EEff) to increase energy efficiency based on the pressure (P.sub.RAIL) in the common rail (3) and on the injected fuel quantity (Q.sub.F_INJ); determining a fourth contribution (Q.sub.DAM) to decrease possible risks of damaging the high-pressure pump (4) based on the pressure (P.sub.RAIL) in the common rail (3) and on the speed (n) of the heat engine (1); and determining said minimum threshold (Q.sub.MIN) based on the third contribution (Q.sub.EEff) and on the fourth contribution (Q.sub.DAM).

3. A method according to claim 2, wherein the third contribution (Q.sub.EEff) is determined depending on a driving mode (DV) chosen for the vehicle provided with the heat engine (1); preferably, depending on the position of a hand lever among a plurality of possible positions.

4. A method according to claim 2 and comprising the further steps of: determining a fifth contribution (Q.sub.TEMP) to contain the temperature variation generated during the pumping phase in the high-pressure pump (4) based on the first contribution (Q.sub.MIN_COLD), on the second contribution (Q.sub.MIN_HOT) and on the coefficient (K); and determining said minimum threshold (Q.sub.MIN) based on the comparison among the fifth contribution (Q.sub.TEMP), the third contribution (Q.sub.EEff) and the fourth contribution (Q.sub.DAM).

5. A method according to claim 4, wherein the fifth contribution (Q.sub.TEMP) is calculated as follows:
Q.sub.TEMP=(1−K)*Q.sub.MIN_COLD+K*Q.sub.MIN_HOT  [6] Q.sub.TEMP fifth contribution; K coefficient; Q.sub.MIN_COLD first contribution; and Q.sub.MIN_HOT second contribution.

6. A method according to claim 4, wherein the minimum threshold (Q.sub.MIN) corresponds to the greatest value among the fifth contribution (Q.sub.TEMP), the third contribution (Q.sub.EEff) and the fourth contribution (Q.sub.DAM).

7. A method according to claim 1 and comprising the further step of controlling the high-pressure pump (4) so as to deliver the objective fuel flow rate (M.sub.ref) only in case the objective fuel flow rate (M.sub.ref) is greater than the minimum threshold (Q.sub.MIN, Q.sub.TEMP); and controlling the high-pressure pump (4) so as not to deliver fuel in case the objective fuel flow rate (M.sub.ref) is smaller than the minimum threshold (Q.sub.MIN, Q.sub.TEMP).

8. A method according to claim 1, wherein the step of determining a minimum threshold (Q.sub.MIN, Q.sub.TEMP) comprises the sub-steps of: calculating an energy index (I), which gives an indication of the closeness—or lack thereof—to the triggering o cavitation phenomena in the high-pressure pump (4) based on the intensity of the perturbation of the signal concerning the pressure (P.sub.RAIL) in the common rail (3) detected in real time by a pressure sensor (11), wherein the perturbation is assessed by means of an integral within an observation time window; and calculating the minimum threshold (Q.sub.MIN, Q.sub.TEMP) based on said energy index (I).

9. A method according to claim 8 and comprising the further step of decreasing the desired pressure value (P.sub.TARGET) inside the common rail (3) by a first quantity (ΔP.sub.TARGET) and for a first amount of time in case the energy index (I) exceeds a first threshold value.

10. A method according to claim 9, wherein the first quantity (ΔP.sub.TARGET) is equal to at least 10 bar and preferably is independent of the difference between the energy index (I) and the first threshold value.

11. A method according to claim 8 and comprising the further step of increasing the minimum threshold (Q.sub.MIN, Q.sub.TEMP) by a second quantity (ΔQ.sub.MIN) in case the energy index (I) exceeds a first threshold value.

12. A method according to claim 11, wherein the second quantity (ΔQ.sub.MIN) is equal to at least 20 mg and preferably is independent of the difference between the energy index (I) and the first threshold value.

13. A method according to claim 8, wherein the energy index (I.sub.1) in case the objective fuel flow rate (M.sub.ref) is delivered is calculated as:
I.sub.1=∫.sub.t.sub.1.sup.t.sup.2(P.sub.TARGET−P.sub.RAIL).sup.2dt  [2] wherein t.sub.1, t.sub.2 instants defining an observation time window; P.sub.RAIL actual pressure in the common rail (3); P.sub.TARGET desired pressure value in the common rail (3).

14. A method according to claim 8, wherein the energy index (I.sub.2) in case the objective fuel flow rate (M.sub.ref) is delivered is calculated as:
I.sub.2=∫.sub.t.sub.1.sup.t.sup.2(P.sub.RAIL_M−P.sub.RAIL).sup.2dt  [3] wherein t.sub.1, t.sub.2 instants defining an observation time window; P.sub.RAIL actual pressure in the common rail (3); and P.sub.RAIL_M actual mean pressure in the common rail (3) and within the observation window.

15. A method according to claim 8, wherein the energy index (I.sub.3) is calculated as:
I.sub.3=∫.sub.t.sub.1.sup.t.sup.2(INT.sub.M−INT).sup.2dt  [4] wherein: t.sub.1, t.sub.2 instants defining an observation time window; INT value of the integral component of the closed loop of the pressure control; INT.sub.M mean value of the integral component of the closed loop of the pressure control within the observation window.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The invention will now be described with reference to the accompanying drawings, showing a non-limiting embodiment thereof, wherein:

(2) FIG. 1 is a schematic view, with some details removed for greater clarity, of a fuel direct injection system;

(3) FIG. 2 is a block diagram showing a first variant of the operating logic of the method according to the invention; and

(4) 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

(5) 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.

(6) 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.

(7) 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.sub.RAIL inside the common rail 3.

(8) Hereinafter we will describe the strategy implemented by the electronic control unit 7 to control the high-pressure pump 4.

(9) The strategy entails determining a minimum threshold Q.sub.MIN of fuel to be pumped with every operating cycle of the high-pressure pump 4, according to FIG. 2.

(10) The minimum threshold Q.sub.MIN is basically determined based on a plurality of parameters, such as pressure P.sub.RAIL in the common rail 3 detected by means of the pressure sensor 11, temperature T.sub.PUMP of the high-pressure pump 4, inlet pressure P.sub.LOW of the high-pressure pump 4, speed n of the heat engine 1 and engine load C.

(11) The temperature T.sub.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.sub.PUMP_SENSOR) or be estimated by means of an estimation model (T.sub.PUMP_VIRTUAL).

(12) More in detail, inside the electronic control unit 7 there is stored a map COLD, which provides an (open loop) contribution Q.sub.MIN_COLD to determine the minimum threshold Q.sub.MIN. The contribution Q.sub.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.sub.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.sub.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.sub.MIN_COLD.

(13) Similarly, inside the electronic control unit there is stored a further map HOT, which provides an (open loop) contribution Q.sub.MIN_HOT to determine the minimum threshold Q.sub.MIN. The contribution Q.sub.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.sub.RAIL in the common rail 3 and of the speed n of the heat engine 1.

(14) 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.sub.MIN. The map VAPOR PRESSURE receives, as an input, the values of the inlet pressure P.sub.LOW of the high-pressure pump 4 (also known as “low pressure”) and of the temperature T.sub.PUMP of the high-pressure pump 4, respectively, the latter being expressed either by the temperature (T.sub.PUMP_SENSOR) detected by means of the temperature sensor housed on the high-pressure pump 4 or by the temperature (T.sub.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.sub.PUMP of the high-pressure pump 4. Based on the temperature T.sub.PUMP of the high-pressure pump 4 and on the inlet pressure P.sub.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.

(15) Therefore, the minimum threshold Q.sub.MIN is calculated as follows:
Q.sub.MIN=(1−K)*Q.sub.MIN_COLD+K*Q.sub.MIN_HOT  [1]

(16) Q.sub.MIN minimum threshold;

(17) K coefficient;

(18) Q.sub.MIN_COLD “cold” contribution of the minimum threshold; and

(19) Q.sub.MIN_HOT “hot” contribution of the minimum threshold.

(20) 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.

(21) Furthermore, it should be pointed out that both inside the map COLD providing the contribution Q.sub.MIN_COLD and inside the map HOT providing the contribution Q.sub.MIN_HOT to determine the minimum threshold Q.sub.MIN there are embedded both the contribution to increase the energy efficiency and the contribution to decrease potential damaging risks.

(22) In other words, both the contribution Q.sub.MIN_COLD and the contribution Q.sub.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.

(23) 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.sub.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.sub.1 and t.sub.2, as described more in detail below.

(24) According to a first variant, the energy index I.sub.1 is expressed as follows:
I.sub.1=∫.sub.t.sub.1.sup.t.sup.2(P.sub.TARGET−P.sub.RAIL).sup.2dt  [2]

(25) According to a second variant, the energy index I.sub.2 is expressed as follows:
I.sub.2=∫.sub.t.sub.1.sup.t.sup.2(P.sub.RAIL_M−P.sub.RAIL).sup.2dt  [3]

(26) According to a third variant, the energy index I.sub.3 is expressed as follows:
I.sub.3=∫.sub.t.sub.1.sup.t.sup.2(INT.sub.M−INT).sup.2dt  [4]

(27) wherein:

(28) t.sub.1, t.sub.2 instants defining an observation time window;

(29) P.sub.RAIL actual pressure in the common rail 3;

(30) P.sub.TARGET pressure target in the common rail 3;

(31) P.sub.RAIL_M actual mean pressure in the common rail 3 and within the observation window;

(32) INT value of the integral component of the closed loop of the pressure control;

(33) INT.sub.M mean value of the integral component of the closed loop of the pressure control within the observation window.

(34) The indexes I.sub.1 and I.sub.2 are clearly calculated in case the objective fuel flow rate M.sub.ref is delivered (as described more in detail below), namely under “normal” operating conditions (without deactivation).

(35) 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.

(36) 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.sub.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.sub.F_INJ of fuel injected by the injectors 2 and the actual fuel flow rate of the high-pressure pump 4.

(37) 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.sub.TARGET.

(38) 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.sub.MIN by a quantity ΔQ.sub.MIN and to decrease the pressure target P.sub.TARGET in the common rail 3 by a quantity ΔP.sub.TARGET and for a given amount of time.

(39) According to a preferred variant, the quantity ΔP.sub.TARGET is equal to at least 10 bar (the quantity ΔP.sub.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.sub.TARGET is increased to 20 bar. The quantity ΔP.sub.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.

(40) Therefore, in this case, the minimum threshold Q.sub.MIN is calculated as follows:
Q.sub.MIN=(1−K)*Q.sub.MIN_COLD+K*Q.sub.MIN_HOT+ΔQ.sub.MIN  [5]

(41) Q.sub.MIN minimum threshold;

(42) K coefficient;

(43) Q.sub.MIN_COLD “cold” contribution of the minimum threshold;

(44) Q.sub.MIN_HOT “hot” contribution of the minimum threshold; and

(45) ΔQ.sub.MIN quantity.

(46) Preferably, the quantity ΔQ.sub.MIN is variable and at least equal to 20 mg (the quantity ΔQ.sub.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.sub.MIN is increased to 40 mg. The quantity ΔQ.sub.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.

(47) Once the minimum threshold Q.sub.MIN has been calculated, the strategy entails controlling the high-pressure pump 4 based on said minimum threshold Q.sub.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.

(48) According to a further variant shown in FIG. 3, the strategy entrails calculating a contribution Q.sub.TEMP to contain the temperature variation generated during the pumping phase in the high-pressure pump 4 according to the description above.

(49) More in detail, inside the electronic control unit 7 there is stored a map COLD, which provides an (open loop) contribution Q.sub.MIN_COLD to determine the contribution Q.sub.TEMP. The contribution Q.sub.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.sub.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.sub.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.sub.MIN_COLD.

(50) Similarly, inside the electronic control unit there is stored a further map HOT, which provides an (open loop) contribution Q.sub.MIN_HOT to determine the contribution Q.sub.TEMP. The contribution Q.sub.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.sub.RAIL in the common rail 3 and of the speed n of the heat engine 1.

(51) 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.sub.TEMP. The map VAPOR PRESSURE receives, as an input, the values of the inlet pressure P.sub.LOW of the high-pressure pump 4 (also known as “low pressure”) and of the temperature T.sub.PUMP of the high-pressure pump 4, respectively, the latter being expressed either by the temperature (T.sub.PUMP_SENSOR) detected by means of the temperature sensor housed on the high-pressure pump 4 or by the temperature (T.sub.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.sub.PUMP of the high-pressure pump 4. Based on the temperature T.sub.PUMP of the high-pressure pump 4 and on the inlet pressure P.sub.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.

(52) Therefore, the contribution Q.sub.TEMP is calculated as follows:
Q.sub.TEMP=(1−K)*Q.sub.MIN_COLD+K*Q.sub.MIN_HOT  [6]

(53) Q.sub.TEMP contribution to contain the temperature variation generated during the pumping phase in the high-pressure pump 4.

(54) K coefficient;

(55) Q.sub.MIN_COLD “cold” contribution of the minimum threshold; and

(56) Q.sub.MIN_HOT “hot” contribution of the minimum threshold.

(57) Or, alternatively, the contribution Q.sub.TEMP is calculated as follows:
Q.sub.TEMP=(1−K)*Q.sub.MIN_COLD+K*Q.sub.MIN_HOT+ΔQ.sub.MIN  [7]

(58) Q.sub.TEMP contribution to contain the temperature variation generated during the pumping phase in the high-pressure pump 4.

(59) K coefficient;

(60) Q.sub.MIN_COLD “cold” contribution of the minimum threshold;

(61) Q.sub.MIN_HOT “hot” contribution of the minimum threshold; and

(62) ΔQ.sub.MIN quantity.

(63) Wherein the quantity ΔQ.sub.MIN has the meaning described above, is variable and at least equal to 20 mg (the quantity ΔQ.sub.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.sub.MIN is increased to 40 mg. The quantity ΔQ.sub.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.

(64) Furthermore, the strategy entails calculating a contribution Q.sub.EEff to increase energy efficiency and a further contribution Q.sub.DAM to decrease potential damaging risks.

(65) More in detail, inside the electronic control unit 7 there is stored a map, which provides the (open loop) contribution Q.sub.EEff to increase energy efficiency in order to determine the minimum threshold Q.sub.MIN. The contribution Q.sub.EEff represents the quantity of fluid to be pumped in order to optimize energy efficiency for given values of the pressure P.sub.RAIL in the common rail 3 and of the quantity Q.sub.F_INJ of fuel injected by the injectors 2. Indeed, the map receives, as an input, the values of the pressure P.sub.RAIL in the common rail 3 and of the quantity Q.sub.F_INJ of fuel injected by the injectors 2, respectively, and, based on said input values, provides the contribution Q.sub.EEff.

(66) The contribution Q.sub.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.sub.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).

(67) Furthermore, inside the electronic control unit 7 there is stored a map, which provides the (open loop) contribution Q.sub.DAM to decrease potential damaging risks in order to determine the minimum threshold Q.sub.MIN. The contribution Q.sub.DAM represents the minimum quantity of fluid to be pumped in order to decrease potential damaging risks for given values of the pressure P.sub.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.sub.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.sub.DAM.

(68) Finally, the minimum threshold Q.sub.MIN is calculated. Preferably, the minimum threshold Q.sub.MIN corresponds to the greatest one among the contribution Q.sub.TEMP to contain the temperature variation generated during the pumping phase in the high-pressure pump 4, the contribution Q.sub.EEff to increase energy efficiency and the contribution Q.sub.DAM to decrease potential damaging risks. Alternatively, the minimum threshold Q.sub.MIN corresponds to weighed mean of the contribution Q.sub.TEMP to contain the temperature variation generated during the pumping phase in the high-pressure pump 4, the contribution Q.sub.EEff to increase energy efficiency and the contribution Q.sub.DAM to decrease potential damaging risks.

(69) Hence, the strategy entails calculating the objective fuel flow rate M.sub.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.

(70) Then, the electronic control unit 7 is designed to compare the objective fuel flow rate M.sub.ref with the minimum threshold Q.sub.MIN.

(71) In case the objective fuel flow rate M.sub.ref is greater than (or equal to) the minimum threshold Q.sub.MIN, the high-pressure pump 4 is controlled so as to deliver the objective fuel flow rate M.sub.ref. On the contrary, in case the objective fuel flow rate M.sub.ref is smaller than the minimum threshold Q.sub.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.sub.ref is smaller than the minimum threshold Q.sub.MIN, the high-pressure pump 4 is not operated.

(72) 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.sub.ref which are greater than the minimum threshold Q.sub.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.sub.ref which are greater than the minimum threshold Q.sub.MIN, and idle operating cycles.

(73) 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.sub.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.sub.MIN, which is calculated according to formulas [1] or [5] described above.

(74) The strategy implemented by the electronic control unit 7 to control the high-pressure pump 4 and described so far has some advantages.

(75) 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.