Method for detecting a misfire condition in an internal combustion engine

1. A method for detecting a misfire condition in an internal-combustion engine that includes a control unit, a driving shaft and phonic wheel that is splined to the driving shaft and coupled to a sensor, the method comprising steps of:

detecting intensity of a signal generated by passage of teeth of the phonic wheel by the sensor;

sampling the signal generated by the passage of the teeth;

processing the sampled signal by applying a discrete Fourier transform;

representing the sampled signal processed by a virtual phasor that represents in a vector form the signal that has been sampled and processed;

determining a corresponding theoretical phasor for the virtual phasor that represents theoretical evolution of the signal that has been sampled and processed over time;

comparing the theoretical phasor with the virtual phasor; and

determining presence of a misfire condition by comparing the theoretical phasor with the virtual phasor that represents the signal that has been sampled and processed; and

detecting the presence of the misfire condition according to an increase in amplitude of the signal that has been sampled and processed.

2. A detection method as set forth in claim 1 , wherein the theoretical phasor performs at least one rotation step with a predetermined, fixed amplitude equal to

α step =360/ N TDC ,

where α step =amplitude of the rotation step and N TDC =number of top dead centers that have been analyzed.

3. A detection method as set forth in claim 1 , wherein the method comprises further steps of:

determining a given threshold value in a preliminary step of setting up and adjusting; and

determining the presence of the misfire condition if the virtual phasor substantially coincides with the theoretical phasor for a time interval that is at least equal to the given threshold value.

4. A detection method as set forth in claim 3 , wherein the method comprises further steps of:

determining a given tolerance interval in the preliminary step of setting up and adjusting; and

determining the presence of the misfire condition if the virtual phasor is within a confidence interval that is centered on the theoretical phasor and has an amplitude substantially equal to the given tolerance interval.

5. A detection method as set forth in claim 3 , wherein, if the presence of the misfire condition is determined, the method comprises further steps of:

graphically representing the signal that has been sampled and processed by the virtual phasor for the determined misfire condition;

determining the theoretical phasor for the misfire condition that represents the evolution that the corresponding virtual phasor is supposed to have over time;

performing a vector addition of the virtual phasors;

performing a vector addition of the theoretical phasors associated with the virtual phasors;

comparing the vector addition of the theoretical phasors with the vector addition of the virtual phasors; and

validating the presence of the misfire conditions by comparing the vector addition of the theoretical phasors with the vector addition of the virtual phasors.

6. A detection method as set forth in claim 5 , wherein the method comprises a further step of determining a maximum number of the detectable misfire conditions in the preliminary step of setting up and adjusting.

7. A detection method as set forth in claim 1 , wherein the step of sampling the signal generated by the passage of the teeth is carried out by a substantially circular memory buffer.

8. A detection method as set forth in claim 7 , wherein a size of the substantially circular memory buffer varies as a function of rotation speed of the driving shaft.

9. A detection method as set forth in claim 8 , wherein the size of the substantially circular memory buffer is differentiated for engine speeds lower than 2500 rpm, between 2500 rpm and 4500 rpm, and higher than 4500 rpm.

10. A detection method as set forth in claim 7 , wherein a number of samples used to sample the signal generated by the passage of the teeth is variable.

11. A detection method as set forth in claim 10 , wherein the number of samples varies as a function of time required by the driving shaft to cover an angular measuring section defining a size of the angular measuring section that is equal to a number of the teeth ranging from 3 to 12.

12. A detection method as set forth in claim 11 , wherein the number of the teeth equals any of 3, 6 and 10.

13. A detection method as set forth in claim 7 , wherein a number of samples varies as a function of a number of analyzed combustions.