Control apparatus for internal combustion engine

1. A knock control apparatus for an internal combustion engine, comprising:

a knock sensor for detecting knock of an internal combustion engine;

a signal processing section for performing frequency analysis using discrete Fourier transform on a knock sensor signal output from the knock sensor for each cylinder and each ignition cycle to calculate a knock intensity; and

a knock determination level setting section:

for calculating an average value of the knock intensity;

for calculating, based on the average value, an overall variance corresponding to a variance of the knock intensity of an entirety of a frequency distribution, a higher variance corresponding to a variance of the knock intensity above the average value, and a lower variance corresponding to a variance of the knock intensity below the average value;

for calculating a standard deviation of the knock intensity from the overall variance;

for presetting a value allowing the frequency distribution of the knock intensity to be a predetermined confidence interval as a confidence coefficient and correcting the confidence coefficient based on the higher variance and the lower variance to calculate a corrected confidence coefficient; and

for setting a sum of the average value and a value obtained by multiplying the standard deviation by the corrected confidence coefficient as a knock determination level.

2. A knock control apparatus for an internal combustion engine according to claim 1 , wherein the knock determination level setting section performs smoothing processing on the calculated overall variance, the calculated higher variance, and the calculated lower variance before calculating the standard deviation.

3. A knock control apparatus for an internal combustion engine according to claim 2 , wherein the knock determination level setting section calculates the corrected confidence coefficient according to a formula: 2×the confidence coefficient×the higher variance/(the lower variance+the higher variance).

4. A knock control apparatus for an internal combustion engine according to claim 3 , wherein the knock determination level setting section calculates the corrected confidence coefficient to allow a minimum value of the corrected confidence coefficient to be the confidence coefficient.

5. A knock control apparatus for an internal combustion engine according to claim 1 , wherein the confidence coefficient is experimentally preset to allow a probability of erroneous detection of a state where the knock does not occur as a state where the knock occurs to be a predetermined value.

6. A knock control apparatus for an internal combustion engine according to claim 1 , wherein the confidence coefficient is set on a map using a parameter representing at least one of an engine revolution and an engine load on an axis.

7. A knock control apparatus for an internal combustion engine, comprising:

a knock sensor for detecting knock of an internal combustion engine;

a signal processing section for performing frequency analysis using discrete Fourier transform on a knock sensor signal output from the knock sensor for each cylinder and each ignition cycle to calculate a knock intensity; and

a knock determination level setting section:

for calculating an average value of the knock intensity;

for calculating, based on the average value, an overall variance corresponding to a variance of the knock intensity of an entirety of a frequency distribution, a higher variance corresponding to a variance of the knock intensity above the average value, and a lower variance corresponding to a variance of the knock intensity below the average value to calculate a higher standard deviation and a lower standard deviation based on the higher variance and the lower variance;

for calculating a standard deviation of the knock intensity from the overall variance;

for presetting a value allowing the frequency distribution of the knock intensity to be a predetermined confidence interval as a confidence coefficient and correcting the confidence coefficient based on the higher standard deviation and the lower standard deviation to calculate a corrected confidence coefficient; and

for setting a sum of the average value and a value obtained by multiplying the standard deviation by the corrected confidence coefficient as a knock determination level.

8. A knock control apparatus for an internal combustion engine according to claim 7 , wherein the knock determination level setting section performs smoothing processing on the calculated overall variance, the calculated higher standard deviation, and the calculated lower standard deviation before calculating the standard deviation.

9. A knock control apparatus for an internal combustion engine according to claim 8 , wherein the knock determination level setting section calculates the corrected confidence coefficient according to a formula: 2×the confidence coefficient×the higher standard deviation/(the lower standard deviation+the higher standard deviation).

10. A knock control apparatus for an internal combustion engine according to claim 9 , wherein the knock determination level setting section calculates the corrected confidence coefficient to allow a minimum value of the corrected confidence coefficient to be the confidence coefficient.

11. A knock control apparatus for an internal combustion engine according to claim 7 , wherein the confidence coefficient is experimentally preset to allow a probability of erroneous detection of a state where the knock does not occur as a state where the knock occurs to be a predetermined value.

12. A knock control apparatus for an internal combustion engine according to claim 7 , wherein the confidence coefficient is set on a map using a parameter representing at least one of an engine revolution and an engine load on an axis.