System and method for knock detection based on ethanol concentration in fuel

1. A knock detection system for an engine, comprising:

an intensity module that determines an engine vibration intensity during a first period of engine operation;

a knock threshold module that determines a knock threshold based on an ethanol concentration in a fuel supplied to said engine; and

a detection module that determines whether engine knock has occurred during said first period by comparing said knock threshold and said engine vibration intensity.

2. The knock detection system of claim 1 , wherein said knock threshold module determines a correction value based on said ethanol concentration and determines said knock threshold based on said correction value and a predetermined knock threshold.

3. The knock detection system of claim 1 , further comprising a statistics module that determines vibration intensity statistics based on engine vibration during a second period prior to said first period when said detection module has determined engine knock has not occurred, wherein said knock threshold module further determines said knock threshold based on said vibration intensity statistics.

4. The knock detection system of claim 3 , wherein said vibration intensity statistics include an average value and a standard deviation value.

5. The knock detection system of claim 3 , wherein said knock threshold module determines a correction value based on a first ethanol concentration during said first period and a second ethanol concentration during said second period and further determines said knock threshold based on said correction value.

6. The knock detection system of claim 5 , wherein said knock threshold module determines said correction value based on a difference between said first ethanol concentration and said second ethanol concentration.

7. The knock detection system of claim 1 , further comprising a statistics module that periodically updates vibration intensity statistics based on engine vibration during second periods when said detection module has determined engine knock has not occurred, wherein said statistics module refrains from updating said vibration intensity statistics when a change in said ethanol concentration is greater than a predetermined change, and wherein said knock threshold module further determines said knock threshold on said vibration intensity statistics.

8. The knock detection system of claim 1 , further comprising a statistics module that selectively updates a vibration intensity profile when a change in said ethanol concentration is less than a predetermined change, wherein said knock threshold module further determines said knock threshold based on said vibration intensity profile.

9. The knock detection system of claim 8 , further comprising:

an ethanol module that outputs an ethanol signal indicative of said ethanol concentration; and

a diagnostic module that detects an error in said ethanol signal, wherein said statistics module selectively updates an ethanol concentration value of said vibration intensity profile to a predetermined value based on said error.

10. The knock detection system of claim 1 , wherein said first period corresponds to a knock window of a cylinder of said engine and at least one of a duration and a phase of said knock window is based on said ethanol concentration.

11. A method for an engine, comprising:

determining an engine vibration intensity during a first period of engine operation;

determining a knock threshold based on an ethanol concentration in a fuel supplied to said engine; and

determining whether engine knock has occurred during said first period by comparing said knock threshold and said engine vibration intensity.

12. The method of claim 11 , wherein said determining said knock threshold includes determining a correction value based on said ethanol concentration and determining said knock threshold based on said correction value and a predetermined knock threshold.

13. The method of claim 11 , further comprising determining vibration intensity statistics based on engine vibration during a second period prior to said first period when engine knock has not occurred, wherein said determining said knock threshold further includes determining said knock threshold based on said vibration intensity statistics.

14. The method of claim 13 , wherein said vibration intensity statistics include an average value and a standard deviation value.

15. The method of claim 13 , wherein said determining said knock threshold includes determining a correction value based on a first ethanol concentration during said first period and a second ethanol concentration during said second period and determining said knock threshold based on said correction value.

16. The method of claim 15 , wherein said determining said correction value includes determining said correction value based on a difference between said first ethanol concentration and said second ethanol concentration.

17. The method of claim 11 , further comprising:

periodically updating vibration intensity statistics based on engine vibration during second periods when engine knock has not occurred; and

refraining from updating said vibration intensity statistics when a change in said ethanol concentration is greater than a predetermined change, wherein said determining said knock threshold further includes determining said knock threshold based on said vibration intensity statistics.

18. The method of claim 11 , further comprising selectively updating a vibration intensity profile when a change in said ethanol concentration is less than a predetermined change, wherein said determining said knock threshold further includes determining said knock threshold based on said vibration intensity profile.

19. The method of claim 18 , further comprising:

outputting an ethanol signal indicative of said ethanol concentration; and

detecting an error in said ethanol signal,

wherein said selectively updating said vibration intensity profile includes selectively updating an ethanol concentration value to a predetermined value based on said error.

20. The method of claim 11 , further comprising determining a knock window of a cylinder of said engine based on said ethanol concentration, wherein at least one of a duration and a phase of said knock window is based on said ethanol concentration, and wherein said first period corresponds to said knock window.