I often get asked how PicoScope can be used to tackle waste spark distributorless ignition secondary analysis.

PicoScope is not an engine analyzer and does not have raster and superimposed views, nor does it come with an octopus for DIS applications.  I have found that the important information in the secondary is in the detail, rather than trying to get them all on the screen at once.  I like to focus on one coil at a time and take a close look.  This is where PicoScope will outperform the rest.  Detail.

Quickly reviewing waste spark systems, the voltage is pumped from one plug to the other so that one fires side to center electrode and the other center to side on each coil.  The plugs fire in companion cylinders alternating exhaust and compression event firings.   The plug under compression has a higher firing KV than the one with no pressure on exhaust.  The exhaust plug fires first, followed by the compression plug a few millionths of a second later.  A lot of information can be gathered by observing these events simultaneously.

Lets just look at one single capture from a Subaru:



Here, channel A is on compression and B exhaust.  Note the detail in the spark line and how the firing line KV is actually displayed.  We have used the two included secondary pickups and selected ranges that make then both appear 'right side up'.  Remember they fire with opposite polarity.  Here the firing events appear simultaneous and the firing KV and spark time look pretty good.  How about if we take a closer look?  With over 15,000 voltage samples on each channel here there is a lot of detail yet to be examined.



Here we have zoomed in on the capture 100x and scrolled to the firing line area.  Note the exhaust firing occurs a little over 9 millionths of a second before the compression firing, as shown by the cursors (9327ns).  Our rotor notch is at almost 9KV.  But DIS don't have rotors!  We have a gap in the secondary after the pickup on the channel A side.  Not enough to raise the firing KV but surely adding resistance to the secondary flow and using up energy which shortens spark time.

Just try to do that with brand X.

Hi Neil,

As far as I know, the technique of observing the rotor notch in the firing line was discovered by Randy Bernklau. 

We are talking about a very fast event here.  As secondary voltage builds, in a distributor type ignition, the first gap is between the rotor and the cap terminal.  There is no voltage in the plig wire until this gap is bridged.  Once it is bridged, then voltage can build in the wire and plug until the spark plug gap ionizes and secondary current flows accross the plug gap.  The electrons flow from plug center to side electrode.



The pickup is on the coil wire so we see the voltage build behind the rotor gap dam.  When it breaks over the dam voltage falls a bit as current flows into the wire and plug.  Then voltage begins to rise again until the plug fires.  We can see this because our pickup is behind the dam in the direction of electron flow.  If we place the pickup on a plug wire, we would never see this rotor notch because it has already bridged it before reachiing the pickup. 

That's how we know the gap is after the pickup in the DIS picture.  Since channel A is negative fired, like conventional ignition, we know the gap is between the pickup and the plug tip.  Had the gap appeared on the channel B side it would be between the pickup and the coil because the electrons flow side to center plug electrode on the positive side toward the coil.

Make sense?