Method for determining the valve opening moment in piezo servo-driven injectors

1. A method for determining the valve opening instant in piezo servo-driven injectors, the method comprising the following steps:

measuring a voltage U and a charge Q at an end of a charging process of the piezo actuator;

determining a provisional scaling factor C VORL by calculating Q/U;

determining the curve maximum of C VORL with respect to time and using the instant at which the maximum occurs as an approximation value of the valve opening instant;

measuring the charge Q and voltage U at the instant of the determined approximation value of the valve opening instant;

solving the equation

1.25× F 2 /E×C =( U×C−Q ) 2

(F=force acting on the piezo actuator, E=electrical parameter of the piezo actuator);

determining a new scaling factor C from the solution of the equation;

calculating the difference (U×C−Q) and determining a maximum of said value; and

using the instant at which said maximum occurs as valve opening instant.

2. The method as claimed in claim 1 , wherein the factor 1.25×F 2 /E is obtained from the rail pressure measurement.

3. The method as claimed in claim 1 , wherein the provisional scaling factor C VORL is determined in a preceding injection cycle N and is used in a subsequent injection cycle N+1 for the determination of the valve opening instant.

4. The method as claimed in claim 1 , further comprising:

calculating the idle travel based at least in part on the valve opening instant; and

adjusting an injection quantity of the injectors based on the calculated idle travel.

5. A method for controlling the injection of fuel into a combustion chamber of an internal combustion engine, the method comprising:

measuring a voltage U and a charge Q at the end of a charging process of a piezo actuator associated with the combustion chamber;

determining a provisional scaling factor C VORL by calculating Q/U;

determining a curve maximum of C VORL with respect to time and using an instant at which the maximum occurs as an approximation value of the valve opening instant;

measuring the charge Q and voltage U at the instant of the determined approximation value of the valve opening instant;

solving the equation

1.25× F 2 /E×C =( U×C−Q ) 2

(F=force acting on the piezo actuator, E=electrical parameter of the piezo actuator);

determining a new scaling factor C from the solution of the equation;

calculating the difference (U×C−Q) and determining a maximum of said value;

using the instant at which said maximum occurs as valve opening instant;

calculating the idle travel based at least in part on the valve opening instant; and

adjusting a control signal of the injector based on the calculated idle travel.