Before the knock solution, a standalone engine management system was in some sense a step backwards for many people when they removed their knock-detecting factory ECM to control the freshly turbo'd engine.
Knock frequency estimation formulas: Start at page 20, paragraph 2.6. For diesel engines , but chapter 4.1 gives the equation: f=const*c/pi/B. Where c is the sound velocity (c=sqrt(kRT), k=1.4, R=287, T is the average gas temperature in the cylinder, c=630m/s if T=1200K; B is the bore; const is the so called mode constant, and can be: 1.841 4.4 kHz 3.054 7.2 kHz 3.832 9.1 kHz 4.201 10.0 kHz 5.332 12.7 kHz However according to this paper, exact calculation won't be possible: It won't matter much, as we won't know the exact temperature anyway, and the filter has a quite wide bandpass frequency range.
The circuit starts in hold mode. When we fire a cylinder, we pull the hold/integrate line high to start listening to the knock sensor. When out of the knock window again, we release it and check the voltage on the chip's output. The knock sensor is sampled cylinder per cylinder and data are stored for each cylinder, thus individual adjustment of each cylinder will be possible. Example of sampling (a single ignition event): For a 4 cylinder engine, the knock sensor is sampled in a J-degrees window starting at K degrees ATDC, the noise is correspondingly sampled in a J-degrees window following the first window. The resulting data is referred to as knock_val and noise_val.
The required chip and 4MHz crystal is factory installed on v3.3 and later boards if your board was ordered with the knock kit. You will only need to connect the sensor input through the 3 pin header (pads) at the northwest corner (looking at the "top")
The knock sensor frequency must be manually configured with any reasonable acoustic-knock system. Each engine is different, and tailoring the knock frequency to your setup will only enhance its capabilities.
Mik measured his knock freq to be around 7.4 kHz: MembersPage/MichaelKristensen/KnockExperiments
MembersPage/Fero's Mazda 1.8L 4cyl: expected to be higher frequency than Mik's, but the knock3.wav shows 6.2-6.7 kHz which is decimal 39 (knock1_frequency=0x27) according to
| Variable Name | Starting point value | Units | Description |
|---|---|---|---|
| knock_conf | 0x00 | 0x00=Disable knock detection and ignition retard. 0xFF=Enable knock detection and ignition retard. | |
| knock_sampling_window | 0xFF (255) | ||
| knock_minrpm | 0xFF (255) | 0xFF=No minimum RPM limitation on knock detection enable. 0x01=100rpm | Minimum RPM before knock detection is enabled. |
| knock_maxrpm | 0xFF (255) | 0xFF=No maximum RPM limitation on knock detection enable. 0x01=100rpm | Maximum RPM knock detection is enabled to. |
| knock1_frequency | 0x29 (41) | TODO: 0x01=? | Listen specifically for knocks in this frequency range. |
| knock1_gain | 0xFF (255) | ||
| knock1_integrator | 0xFF (255) | ||
| knock2_frequency | 0x29 (41) | ||
| knock2_gain | 0xFF (255) | ||
| knock2_integrator | 0xFF (255) | ||
| knock_threshold | 0xFF | ||
| knock_noise_scale | 0xFF | ||
| knock_max_retard | 0xFF (255) | 0x01=0.25 crank degrees. | Maximum amount to retard ignition. |
| knock_default_retard | 0x00 | 0x01=0.25 crank degrees. | Default ignition retard. |
| knock_retard_step | 0xFF | 0x01=0.25 crank degrees | Retard step. |
| knock_retard_delay | 0xFF | 0x01=15ms | Retard delay. |
| knock_advance_step | 0xFF | 0x01=0.25 crank degrees. | Ignition advance step size. |
| knock_advance_delay | 0xFF | 0x01=15ms | Ignition advance delay. |
Table 9.7. Knock sensor config variables