Controller and control method for internal combustion engine

1. A controller for an internal combustion engine comprising at least one processor configured to implement:

a knock signal calculator that calculates a knock signal based on an output signal of a combustion condition detection sensor which outputs a signal in accordance with a combustion condition of the internal combustion engine;

a BGL calculator that calculates a background level based on the knock signal;

a knock decision threshold calculator that calculates a knock decision threshold based on the background level;

a knock intensity calculator that calculates a knock occurrence intensity based on the knock signal and the knock decision threshold; and

a knock controller that changes a control parameter of the internal combustion engine so as to suppress occurrence of knocking in accordance with the knock occurrence intensity,

wherein the BGL calculator calculates the background level by processing a low pass filter to the knock signal; and performs a low side frequency increase which makes a low side frequency, which is a cutoff frequency of the low pass filter in the case where the knock signal is smaller than an output value of the low pass filter, higher than a high side frequency, which is a cutoff frequency of the low pass filter in the case where the knock signal is larger than the output value of the low pass filter.

2. The controller for the internal combustion engine according to claim 1 , wherein in the low side frequency increase, in the case where the knock signal is smaller than the output value of the low pass filter, the BGL calculator sets the low side frequency to infinite frequency, and coincides the output value of the low pass filter with the knock signal.

3. The controller for the internal combustion engine according to claim 1 , wherein the BGL calculator uses a discretized first order lag filter as the low pass filter; the first order lag filter is a processing which calculates a total value of a value, which multiplied an output value of the first order lag filter of the last time calculation cycle to a filter gain, and a value, which multiplied a value which subtracted the filter gain from 1 to an input value of the first order lag filter of this time calculation cycle, as the output value of the first order lag filter of this time calculation cycle;

in the low side frequency increase, in the case where the input value of the first order lag filter is smaller than the output value of the first order lag filter of the last time calculation cycle, the BGL calculator sets the filter gain as the low side frequency to 0; in the case where the input value of the first order lag filter is larger than the output value of the first order lag filter of the last time calculation cycle, the BGL calculator sets the filter gain as the high side frequency to a value which is larger than 0 and smaller than 1.

4. The controller for the internal combustion engine according to claim 1 , wherein the BGL calculator calculates the background level by processing the two or more steps of low pass filters to the knock signal, and performs the low side frequency increase about at least the first step low pass filter.

5. The controller for the internal combustion engine according to claim 4 , wherein the BGL calculator processes the three or more steps of low pass filters, and performs the low side frequency increase about the two or more consecutive steps of the low pass filters from the first step.

6. The controller for the internal combustion engine according to claim 4 , wherein the BGL calculator performs a high side frequency increase which makes the high side frequency higher than the low side frequency about at least the final step low pass filter.

7. The controller for the internal combustion engine according to claim 6 , wherein in the BGL calculator, the low side frequency and the high side frequency in each of the low pass filters are set so that a time constant of change of the background level to positive side step change of the knock signal and a time constant of change of the background level to negative side step change of the knock signal become equivalent.

8. The controller for the internal combustion engine according to claim 1 , wherein the combustion condition detection sensor is a knock sensor which detects vibration of the internal combustion engine, or a cylinder internal pressure sensor which detects pressure in cylinder of the internal combustion engine.

9. The controller for the internal combustion engine according to claim 1 , wherein the control parameter is any one or more of ignition timing, phase of an intake variable valve timing mechanism, intake air amount, and rich amount of fuel injection.

10. A control method for an internal combustion engine comprising:

calculating a knock signal based on an output signal of a combustion condition detection sensor which outputs a signal in accordance with a combustion condition of the internal combustion engine;

calculating a background level based on the knock signal;

calculating a knock decision threshold based on the background level;

calculating a knock occurrence intensity based on the knock signal and the knock decision threshold; and

changing a control parameter of the internal combustion engine so as to suppress occurrence of knocking in accordance with the knock occurrence intensity,

wherein in calculating the background level, calculating the background level by processing a low pass filter to the knock signal; and performs a low side frequency increase which makes a low side frequency, which is a cutoff frequency of the low pass filter in the case where the knock signal is smaller than an output value of the low pass filter, higher than a high side frequency, which is a cutoff frequency of the low pass filter in the case where the knock signal is larger than the output value of the low pass filter.