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Knock Detection |
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Knock (or detonation) is the sound of uncontrolled burning of the air/fuel mix causing the pressure to suddenly increase within the cylinder. Many factors can cause this, air temperature, coolant temperature, air / fuel mix, air density, etc, etc.
A knock sensor can be used to detect this sound, but it can be very hard to setup or tune as most manufactorures will have spent a lot of time and effort tuning their sensor for a specific frequency. For example the sensor on the Corvette (GM PN 1997562, 1997699) is designed to pick up around 5.2Khz and operates around 4800 - 5600 RPM. So just by bolting a sensor to your engine block you are not necassarily going to detect any harmful knocks.
The best option as far as tuning goes, is to use a pair of air defenders without any wadding in them. Simply get a peice of copper pipe (brake pipe) and flatten one end of it enough to bolt it to the cylinder head. Then attach a peice of plastic tube (fish tank air pipe) on the other end of the copper pipe. Drill a small hole in one of the ear peices and push the other end of the pipe into it. Now any knock you will be able to hear clearly whilst tuning.
If you still wish to detect knock whilst running, then there are 2 options;
1 ) Use an LED on the dash to show you have knock.
2 ) The MS ECU can have the option of knock reaction added to it.
Both options will need a conditioner unit to work! These are available from HERE or from certain cars in scrap yards.LED on Dashboard Option:
MS ECU option:
If using them from a scrap yard e.g. GM's Electronic Spark Control (ESC) module (PN 16022621, 16052401), you will need to fit a zener diode and a resistor to protect the MS ECU input, as the input is only designed to work at 5 - 0V, anything higher than 5V will blow the uP.
The ECU's I supply are ready to work with the conditioner unit from http://www.viatrack.ca/ if you requested a knock input, I dont add the resistor or diode so if you want to use another conditioner youll need to add the resistor and diode as below.
Please remember that this is not a magic pill, if you have detonation then you have issues you need to address and cure!!
MS1 Settings:
As long as the engine speed is below the Ignore Knock RPM setpoint and the MAP Kpa is below the Ignore Knock setpoint then the code will control the advance in the following manor:
If a knock is detected (The input goes switches to ground) the ignition will be retarded by the “First Knock” retard value. Any more knocks are then stored until the timer “Wait time between steps” has passed. If any more knocks where seen then the ignition is retarded by the “Subsequent Knock” value. This allows a big first step then smaller steps after that if knock continues.
If no knocks are then seen after the wait period it will advance the ignition by the “Advance when no knock“ value (this should be smaller than the retard value). After the wait period is over, as long as no knocks are seen, it will advance again until it runs the same advance as the main ignition map.
If a knock is seen whilst its advancing then the timer is reset and the ignition is retarded immediately and the whole process starts over.
There is also a limit to how much retard can be applied.
The boost can also be linked into this system if using boost control.
MS2 Settings:
Note, the explanation is for V2.1.0 and above, the V2.0.1 code was slightly different, try using Threshold direction "Below threshold" , Threshold(V) "1.0V" and Peak(V) "5V" for that older code, I would recommend upgrading to V2.1.0 if you have issues with the older version when using knock.
Knock Control:
Disabled: do not use knock feedback for ignition advance control
Safe Mode: use knock retard, but keep the advance below that which caused knock. This backs the advance 1 small step back and leaves it at that until TPS or MAP changes - or knock comes back. This is "safe mode" scheme is the safest thing for a DIY set-up.
Aggressive Mode: use knock retard, but keep advance at threshold of knock occurring. That is, the program advances (up to the timing table value) if it doesn't see knock, and retards if does see knock. The difference from safe mode is that the timing can be advance all the way to the table value after knock, not just up to one step below knock. This may result in the knock returning, in which case the timing is retarded again, then advanced slowly, and so on.
Knock Indicated by: This sets whether MegaSquirt recognizes a knock with a low signal or a high, for most conditioners this should be set to low (Input goes to ground for a knock).
Pull up/down on input: The input pin can be held high (5V), Low (0V) or left floating (None) using the microprocessors internal circuit. This setting is dependant on how your conditioner works, measure the output voltage of the conditioner when it is powered up but not connected to the MS ECU. If it measures nearly 0V then select Held High, if it measures 5V then select None, if it measures 12V then DO NOT connect it directly to the MS ECU, see the above circuits!!
Knock Count (knocks): number knock detects required for valid detection
No Knock Above MAP (kPa): no knock retard is implemented above this MAP
No Knock Below RPM (rpm): no knock retard is implemented below this rpm
No Knock Above RPM (rpm): no knock retard is implemented above this rpm, which may be desirable if valve train noise triggers the knock sensor when knock isn't actually occurring.
Maximum Retard (deg): maximum total retard when knock occurs. This can be useful to prevent timing from being excessively retarded (avoiding potential overheating issues) if the senor malfunctions or there are other problems with the knock sensing system.
Retard Check Time (sec): this is the time between knock retard corrections, allows short time step to quickly retard
Retard Step Size (deg): ignition retard step size when 1st knock or after stopped, make it large to quickly retard the timing and stop knock
Advance Check Time (sec): this is the time between knock advance correction (I.e., timing return to 'normal')
Advance Step Size (deg): ignition advance steps after knock has stopped
Recovery Advance (deg)): this is the change in table advance required to restart advance until knock or reach table value (0 knock retard) process. This only applies in 'Safe Mode'
