Spark plug ionisation

[M HILL]

I am starting to do some work on looking at spark plug ionisation for 
measuring instability in engines running at very lean AFRs.  I was 
just wondering if anybody else had done any work in this field.  If 
so, what sensors were used for measuring the ionisation currents.  
The system I am looking at is going to use a second spark at around 
15degrees after TDC and use a transient suppressor and capacitor on 
the secondary of the coil to store voltage for measuring ionisation.

Martin.