The Knock Sensor Circuit

     This is the general arrangement of the Knock Detector. In operation, the output signal from the knock sensor is first fed to the Knock Detector circuit for processing. This processed signal is then fed to the Display Light Circuit.  The level of knock is converted to an array of 1 to 10 Super-bright LEDs.  The Number of LEDs lit indicate the severity of the knocking.  The lights are automatically dimmed at night for night flying.  The signal will also be fed to my Electronic Ignition System to retard the advance.

     Signal processing is necessary because the knock sensor also detects all the other noises that the engine makes. This means that the wanted knock signal is buried amongst the sounds produced by piston movement, valves and tappets opening and closing, and by various other operating parts both inside and outside the engine.  This in turn means that some way of removing these unwanted signals is necessary. Fortunately, there are some strategies that can be used to separate out the knock signal from the rest of the noises.

     This shows the block diagram of the Knock Detector. As shown, the knock sensor output is first fed to an amplifier stage.  A trim pot is used to set the gain of this amplifier stage, to set the correct sensitivity for engine knock.  The amplifier has Automatic Gain Control to increase the signal when the engine is running slow.  ( The engine will make more noise at higher speeds. By adjusting the signal level you get better filtering of the signal.

     According to the car manufacturers, engine knock signals generally only cover a narrow frequency range from about 4.8-6.4kHz. This means that we can more readily detect engine knock if we remove signals outside this range. That’s the purpose of the bandpass filter stages.  These only allow the frequencies of interest – ie, between 4.8kHz and 6.4kHz – to pass through.  The resulting signal is then rectified and filtered to provide a DC signal voltage. The signal is then amplified and fed to the bar graph driver and then to the LED array as well as the ignition system Controller.

     Another problem at high engine RPM, is that the knock signal is often swamped out by engine noise. This can lead to incorrect knock sensing.  Since my Corvair engine will not exceed 3500 RPM this should not be a serious drawback.  Further signal processing will be done within the ignition system circuits to reduce noise in the system.

For discussions on this system please go to   Exp_Aviation net .