{HEAD ~ Knocking frequency components} passing through a band pass filter 32 and {TAIL ~ frequency components} lower than the knocking frequency and passing through a low load noise band pass filter 321 are picked from the ionic current detected by an ionic current detecting unit 17 
passing through
{ HEAD ~ Knocking frequency components } passing through a band pass filter 32 and { TAIL ~ frequency components } lower than the knocking frequency and passing through a low load noise band pass filter 321 are picked from the ionic current detected by an ionic current detecting unit 17
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