Method for determining the amount of fuel leaving an injector

1. A method for determining a quantity of fuel leaving an injector of a direct-injection diesel engine, said quantity of fuel supplying a cylinder of said engine in which a piston moves, the injection of fuel being performed by a pilot injection during which a first quantity of fuel is injected into the cylinder followed by a main injection during which a second quantity of fuel, greater than the first quantity of fuel, is injected into the cylinder, said method comprising:

a) operating the engine at an idle speed;

b) determining a maximum pressure in the cylinder; and

c) determining, from said maximum pressure, the quantity of fuel injected into the cylinder during the pilot injection, by means of a predefined one-to-one relationship, at the idle speed, between a maximum pressure value in the cylinder and a quantity of fuel injected into the cylinder during the pilot injection.

2. The method as claimed in claim 1 , further comprising;

determining the quantity of fuel injected into the cylinder during the main injection from the quantity of fuel injected into the cylinder during the pilot injection, by means of a predetermined correlation relationship linking said main quantity and said pilot quantity.

3. The method as claimed in claim 2 , wherein the piston reaching its top dead center point between the pilot injection and the main injection, the maximum pressure is determined by measuring the pressure in the cylinder when the piston is at the top dead center point.

4. The method as claimed in claim 2 , wherein the maximum pressure is determined by performing a plurality of pressure measurements in the cylinder then by selecting the pressure measurement that has the highest value.

5. The method as claimed in claim 1 , wherein the piston reaching its top dead center point between the pilot injection and the main injection, the maximum pressure is determined by measuring the pressure in the cylinder when the piston is at the top dead center point.

6. The method as claimed in claim 5 , wherein the pressure measurement(s) is/are carried out by a cylinder-pressure sensor mounted on the cylinder head.

7. The method as claimed in claim 6 , further comprising:

prior to the step a), a step of dismantling a preheating plug mounted on the cylinder head and a step of mounting the pressure sensor in place of said preheating plug.

8. The method as claimed in claim 1 , wherein the maximum pressure is determined by performing a plurality of pressure measurements in the cylinder then by selecting the pressure measurement that has the highest value.

9. The method as claimed in claim 8 , wherein the pressure measurements are carried out during a measurement interval encompassing the instant corresponding to the top dead center point of the piston.

10. The method as claimed in claim 9 , wherein the measurement interval is centered on the instant corresponding to the top dead center point of the piston.

11. The method as claimed in claim 8 , wherein the pressure measurements are carried out between the instant of the pilot injection and the instant of the main injection.

12. A diagnostic method for an injector of a direct-injection diesel engine, comprising the following steps:

determining the quantity of fuel leaving the injector in accordance with the method as claimed in claim 1 ;

comparing said determined quantity with a reference quantity; and

preparing a diagnostic report according to the result of the comparison.

13. The method as claimed in claim 1 , wherein the pilot injection is performed before top dead center of the piston between −15° and −12° angular positions of a crankshaft.

14. The method as claimed in claim 1 , wherein the main injection is performed after top dead center of the piston between 6° and 9° angular positions of a crankshaft.

15. The method as claimed in claim 1 , wherein combustion between −9° and −5° angular positions of a crankshaft.