To detect engine knock we need to know three things.
- The frequency that it occurs at (or more specifically, the frequency our knock sensor outputs when the engine is knocking).
- The threshold of the knock ie the “volume” the knock sensor outputs when the engine is knocking.
- The RPM of the engine
(it would also be nice to know engine load but not essential)
It is not enough to simply measure the voltage output of a knock sensor (unless the sensor is a resonant type of knock sensor designed for that specific engine), we need to know the frequency of the vibrations and the amplitude. NGK have a page about knock sensor types here.
There are also standalone engine management systems with in-built knock detection. They connect directly to a knock sensor and the ECU has the internal hardware to measure the frequency and amplitude of the signals. An example of this would be the Motec M1.
What to look for in a knock detection product?
Each product, dedicated or not, will have its own setup procedure. Fundamentally, we are looking for a product which uses a knock threshold that adapts to RPM, be that through user inputted data and/or by interpolation.
Detect Engine Knock – Setup Procedure
As I have said above, each product will have it’s own setup procedure but generally speaking we are looking for a procedure that works like this.
To detect engine knock we need to know what frequency to monitor.
There are online calculators which will tell us the theoretical knock frequency of an engine, which is nice, but the specific installation, the specific sensor, the way a sensor is mounted, the way the measuring unit is installed (etc etc etc) will all play a part in affecting the signal we receive from the sensor. This is especially true if the sensor and/or wiring is old.
In short, the knock calculators can be a guide but nothing beats actually measuring the frequency ourselves.
So the device we are looking at should have a way of helping us identify the real knock frequency outputted by our sensor.
Motec and MoviChip products have built-in setup tools which monitor a range of frequencies. The tools show the amplitude of different frequencies and how the amplitudes change between no-engine-knock and engine-knock.
We run the engine in a known no-knock configuration and compare the amplitudes of different frequencies to the amplitudes when the engine is knocking.
The frequencies where amplitude has changed (increased) are the frequencies that represent knock (all other things being equal).
The frequency we get from these tools may or may not match the “theoretical” knock frequency.
By retarding ignition timing to a point we know the engine will not knock and then measuring amplitudes, we can be sure that the thresholds we set are correct (all other parameters being correct).
When we have set up our knock threshold we need a way of alerting the driver and/or the ECU of the knock condition.
We are looking for a product that can output a signal to an ECU ( 5 volts) and also output a signal to trigger an audible or visual indicator (5 or 12 volts).
Detecting engine knock may seem straightforward but we need to measure the knock signal in a certain way to get the data we need. We also need to know what data is normal and what isn’t.
A knock detection device should not be used as the sole protection against overlay advanced ignition timing and/or detonation.
No knock device should be used as a safeguard for a “bad tune”.
It is not recommended to constantly run an engine with too much ignition and rely on a knock sensor to save the engine from detonation.
Knock detection products are a tool to help tune an engine calibration.