Knock sensor control (and other uses for ionization gaps)

[Matthew Lee Franklin]

>levels involved? I'm looking for something that will indicate a "lean" 
>condition faster than an egt thermocouple. Am I correct in deducing that 
>a lean mixture will burn slower, thus higher ion currents will flow? How 
>about applying this to detonation detection?

The SAE paper I'm thinking about was written by Brehob in '89 or '90.  I 
built the sensor for that QUAD-4 based on the schematics from A Cambridge 
PhD thesis.  I think the author's name was Peter Hand, but I'm not 
certain.  He used 10 volts, if I remember rightly.  I used 15 volts 
because that's what I had, and it helped the signal to noise ratio..  

To get yourself playing with a real system as soon as posible, I would 
suggest the following.  I hope I can remember.  Get a voltmeter, 1 to 10 
M-Ohm impedance.  Get a 9 volt battery.  Connect the one terminal of 
the nine volt to one leg of the DVM.  Connect the other leg of the battery 
to one side of the gas to be sensed.  Connect the free leg of the DVM to 
the other side of the gas gap.  Then pass this gap through a hydrocarbon 
flame.  Add salt to the flame to watch the ion concentrations increase.  
The 9 volt battery is your bias voltage, the internal impedance of the 
voltmeter is your current sensing resistor, the "ideal DVM" measures the 
voltage drop across the 10 Meg.  It sounds really bad, but it works well.  
I think the current ranges from pico amps to nano amps.  Maybe the ASCI 
gram will clear it up??

|-------- DVM----9VOLTBATTERY---------|
|                                     |   
|                                     |
|------------->  GAP  <---------------|

             \/\/\/\/\/
               FLAME

It really works.  A scope may be substituted for the DVM.

On the engine version, I had mega-trouble with 60 Hz noise (on the dyno in 
the lab) when I used an off-the-shelf 15 Volt supply, but a big metal box 
to shield the supply from the amplification seemed to help a lot.

Brehob used it to sense EGR induced misfire while Hand used it to sense 
lean misfire.

Have fun...
Matt