Method for determining the injection law of a fuel injector using a roller-test bench

1. A method for determining an injection law of a fuel injector ( 4 ) to be tested in an injection system ( 3 ) that includes a plurality of fuel injectors ( 4 ), a common rail ( 5 ) feeding fuel under pressure to the injectors ( 4 ), and a fuel pump ( 6 ) that keeps the fuel under pressure inside the common rail ( 5 ), said method being performed by an engine control unit and comprising the steps of:

completely interrupting the feeding of the fuel from the fuel pump ( 6 ) to the common rail ( 5 );

avoiding opening of all of the fuel injectors ( 4 ) except for one of the fuel injectors ( 4 ) to be tested;

measuring initial pressure (Pi) of the fuel inside the common rail ( 5 ) before starting the opening of the fuel injector ( 4 ) to be tested;

opening the fuel injector ( 4 ) to be tested for a number (N) of consecutive openings with a same test-actuation time (T);

measuring final pressure (Pt) of the fuel inside the common rail ( 5 ) after ending the opening of the fuel injector ( 4 ) to be tested;

determining a pressure drop (ΔP) in the common rail ( 5 ) during the opening of the fuel injector ( 4 ) to be tested, which is equal to a difference between the initial pressure (Pi) and final pressure (Pf);

estimating, according to the pressure drop (ΔP) in the common rail ( 5 ), the total fuel quantity (Q) that has been actually injected by the fuel injector ( 4 ) to be tested during the openings with the same test-actuation time (T);

calculating a fuel quantity (Q) that is actually injected by the fuel injector ( 4 ) to be tested when the fuel injector ( 4 ) is opened for the test-actuation time (T) by dividing the total fuel quantity (Q) by the number (N) of openings; and

causing an internal-combustion engine ( 1 ) using the injection system ( 3 ) to rotate by an external actuator during the openings of the fuel injector ( 4 ) to be tested to allow execution of a high number of consecutive openings of the fuel injector ( 4 ) to be tested with the same test-actuation time (T).

2. A method as set forth in claim 1 , said method comprising a further step of keeping the internal-combustion engine ( 1 ) at a substantially constant rotational speed that is predetermined by the external actuator.

3. A method as set forth in claim 1 , said method comprising a further step of using a motorized roller-test bench ( 29 ) to cause driving wheels ( 30 ) of a vehicle ( 28 ) incorporating the internal-combustion engine ( 1 ) to rotate.

4. A method as set forth in claim 1 , said method comprising further steps of:

carrying out, in sequence and for the fuel injector ( 4 ) to be tested, a series of tests for different predetermined test-actuation times (T); and

carrying out, in sequence, the series of tests for each fuel injector ( 4 ) of the injection system ( 3 ).

5. A method as set forth in claim 1 , wherein said method comprises further steps of:

waiting for a first predetermined time interval between interruption of the feeding of the fuel from the fuel pump ( 6 ) to the common rail ( 5 ) and measurement of the initial pressure (Pi) inside the common rail ( 5 ) to obtain a pressure stabilization; and

waiting for a second predetermined time interval between the ending of the opening of the fuel injector ( 4 ) to be tested and measurement of the final pressure (P 0 inside the common rail ( 5 ) to obtain a pressure stabilization.

6. A method as set forth in claim 1 , wherein said method comprises further steps of:

estimating a lost-fuel quantity that is lost by the common rail ( 5 ) due to either of leakage and actuation during the openings of the fuel injector ( 4 ) to be tested;

estimating, according to the pressure drop (ΔP) in the common rail ( 5 ), a gross-fuel quantity that has come out the common rail ( 5 ) during the openings of the fuel injector ( 4 ) to be tested; and

calculating the total fuel quantity (Q) that is actually injected by the fuel injector ( 4 ) to be tested during the openings of the fuel injector ( 4 ) to be tested by subtracting the lost-fuel quantity from the gross-fuel quantity.

7. A method as set forth in claim 6 , wherein said method comprises a further step of estimating the lost-fuel quantity according to the fuel pressure inside the common rail ( 5 ).

8. A method as set forth in claim 6 , wherein said method comprises further steps of:

determining a first contribution that is directly proportional to a duration of a time interval that elapses between the two measurements of the fuel pressure inside the common rail ( 5 );

determining a second contribution that is directly proportional to the number (N) of openings of the fuel injector ( 4 ) to be tested; and

estimating the lost-fuel quantity by adding the first and second contributions.

9. A method as set forth in claim 1 , wherein said method comprises further steps of:

establishing in a design phase a set of characteristic actuation times (t 1 , t 2 , t 3 , t 4 ) that allow an accurate reconstruction of the injection law of the fuel injector ( 4 ); and

choosing the test-actuation time (T) from the set of characteristic actuation times (t 1 , t 2 , t 3 , t 4 ).

10. A method as set forth in claim 9 , wherein said method comprises a further step of establishing four characteristic actuation times (t 1 , t 2 , t 3 , t 4 ), two first characteristic actuation times (t 1 , t 2 ) belonging to a ballistic-operation area (B) and being used to approximate the ballistic-operation area (B) with a first straight line (R 1 ) and two second characteristic actuation times (t 3 , t 4 ) belonging to a linear-operation area (D) and being used to approximate the linear-operation area (D) with a second straight line (R 2 ) intersecting the first straight line (R 1 ).

11. A method for determining an injection law of a fuel injector ( 4 ) to be tested in an injection system ( 3 ) that includes a plurality of fuel injectors ( 4 ), a common rail ( 5 ) feeding fuel under pressure to the injectors ( 4 ), and a fuel pump ( 6 ) that keeps the fuel under pressure inside the common rail ( 5 ), said method being performed by an engine control unit and comprising the steps of:

completely interrupting the feeding of the fuel from the fuel pump ( 6 ) to the common rail ( 5 );

avoiding opening of all of the fuel injectors ( 4 ) except for one of the fuel injectors ( 4 ) to be tested;

measuring initial pressure (Pi) of the fuel inside the common rail ( 5 ) before starting the opening of the fuel injector ( 4 ) to be tested;

opening the fuel injector ( 4 ) to be tested for a number (N) of consecutive openings with a same test-actuation time (T);

measuring final pressure (Pt) of the fuel inside the common rail ( 5 ) after ending the opening of the fuel injector ( 4 ) to be tested;

determining a pressure drop (ΔP) in the common rail ( 5 ) during the opening of the fuel injector ( 4 ) to be tested, which is equal to a difference between the initial pressure (Pi) and final pressure (Pf);

estimating a lost-fuel quantity that is lost by the common rail ( 5 ) due to either of leakage and actuation during the openings of the fuel injector ( 4 ) to be tested;

estimating, according to the pressure drop (ΔP) in the common rail ( 5 ), a gross-fuel quantity that has come out the common rail ( 5 ) during the openings of the fuel injector ( 4 ) to be tested; and

calculating a total fuel quantity (Q) that is actually injected by the fuel injector ( 4 ) to be tested during the openings of the fuel injector ( 4 ) to be tested by subtracting the lost-fuel quantity from the gross-fuel quantity

estimating, according to the total fuel quantity (Q), a fuel quantity (Q) that is actually injected by the fuel injector ( 4 ) to be tested when the fuel injector ( 4 ) is opened for the test-actuation time (T);

causing an internal-combustion engine ( 1 ) using the injection system ( 3 ) to rotate by an external actuator during the openings of the fuel injector ( 4 ) to be tested to allow execution of a high number of consecutive openings of the fuel injector ( 4 ) to be tested with the same test-actuation time (T);

wherein estimating the lost-fuel quantity comprises further steps of: determining a first contribution that is directly proportional to a duration of a time interval that elapses between the two measurements of the fuel pressure inside the common rail ( 5 ); determining a second contribution that is directly proportional to the number (N) of openings of the fuel injector ( 4 ) to be tested; and estimating the lost-fuel quantity by adding the first and second contributions.

12. A method for determining an injection law of a fuel injector ( 4 ) to be tested in an injection system ( 3 ) that includes a plurality of fuel injectors ( 4 ), a common rail ( 5 ) feeding fuel under pressure to the injectors ( 4 ), and a fuel pump ( 6 ) that keeps the fuel under pressure inside the common rail ( 5 ), said method being performed by an engine control unit and comprising the steps of:

completely interrupting the feeding of the fuel from the fuel pump ( 6 ) to the common rail ( 5 );

avoiding opening of all of the fuel injectors ( 4 ) except for one of the fuel injectors ( 4 ) to be tested;

measuring initial pressure (Pi) of the fuel inside the common rail ( 5 ) before starting the opening of the fuel injector ( 4 ) to be tested;

opening the fuel injector ( 4 ) to be tested for a number (N) of consecutive openings with a same test-actuation time (T);

measuring final pressure (Pt) of the fuel inside the common rail ( 5 ) after ending the opening of the fuel injector ( 4 ) to be tested;

determining a pressure drop (ΔP) in the common rail ( 5 ) during the opening of the fuel injector ( 4 ) to be tested, which is equal to a difference between the initial pressure (Pi) and final pressure (Pf);

estimating, according to the pressure drop (ΔP) in the common rail ( 5 ), a fuel quantity (Q) that is actually injected by the fuel injector ( 4 ) to be tested when the fuel injector ( 4 ) is opened for the test-actuation time (T); and

causing an internal-combustion engine ( 1 ) using the injection system ( 3 ) to rotate by an external actuator during the openings of the fuel injector ( 4 ) to be tested to allow execution of a high number of consecutive openings of the fuel injector ( 4 ) to be tested with the same test-actuation time (T);

wherein said method comprises further steps of: establishing in a design phase a set of characteristic actuation times (t 1 , t 2 , t 3 , t 4 ) that allow an accurate reconstruction of the injection law of the fuel injector ( 4 ); and choosing the test-actuation time (T) from the set of characteristic actuation times (t 1 , t 2 , t 3 , t 4 ).

13. A method as set forth in claim 12 , wherein said method comprises a further step of establishing four characteristic actuation times (t 1 , t 2 , t 3 , t 4 ), two first characteristic actuation times (t 1 , t 2 ) belonging to a ballistic-operation area (B) and being used to approximate the ballistic-operation area (B) with a first straight line (R 1 ) and two second characteristic actuation times (t 3 , t 4 ) belonging to a linear-operation area (D) and being used to approximate the linear-operation area (D) with a second straight line (R 2 ) intersecting the first straight line (R 1 ).