Ignitor / amplifier tech questions

[Chris Wilson]

I have some general questions on coil on plug igniter modules (some call them amplifier modules, but as they seem to be a switching device i will call them igniter modules for now).

My questions are re failure modes and testing procedures. I will use for example the expensive, and now obsolete ignition igniters for the Nissan RB26-DETT twin turbo straight six engines. I and many others have experienced issues with these. Sometimes they will fail to fire on or sometimes more than one coil. The coil si not triggered at all. the thing's kaput. But I have had several experiences where the module only fails to fire one or more coils when the engine is around peak torque and cylinders pressures should be around their highest. Idle, starting and off very much boost running is unaffected. Take it up around peak torque and boost and one or more plugs are not fired, either at all, or adequately. Leave the status quo exactly as is, and renew the igniter, and all is fine again, so presumably neither a plug nor coil issue.

Now what is the failure mode of the igniter in this scenario? Are they FET's or BJT's that trigger the coils? Can they see different switching currents dependent on cylinder pressure? What is this interim failure mode?

As an aside, short wiring lengths apart, why the hell did Nissan think atop the head, in a very hot region, with a thermal path straight into the head casting, a good place to mount the damned things?

I fixed my failure issues ages ago on my M800 run RB-26DETT with the two 3 cylinder Bosch igniters from a Volvo V90 straight six, mounted on an alloy heat sink, well away from engine heat and vibration, but would like to get a technical handle on why these (and I mean not just Nissan ones) can falter whn the plug is asked to fire high cylinder pressures, yet be OK for a while when the engine is not put under appreciable loads.

Has there ever been a commercial test rig for igniters that trigger them from say a signal generator, and load their outputs either electronically, or by actually firing coil packs with variable spark gaps whilst monitoring them with a `scope? Some of the things are very expensive and I don't like the "let's just replace it and hope it cured the miss" routine with expensive gizmos....


Thanks for reading.

[Scott@FP]

Sounds like you need to employ a variable dwell setup for inductive ign on a boosted engine. Use an aux table to import into an axis on the dwell table so you can 3D RPM, voltage, and load. Use appropriate plug gaps, dependent on timing requirements and boost levels, the closer to TDC the spark instant and the higher the boost, the less gap you can get away with.

Some of these OEM modules employ current limiting.

Many modern IGBT's are available that you can build your own ignitor out of, that will fire 40 amps worth of coil or more, and you can fry coils at will with their current capabilities.