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My engine has coils on plugs in a waste spark configuration. The coil driver is in the oem ecu.
So I need to add drivers to my MRE to fire the oem coils. I don't want to change the coils because they seal the plug pits which are exposed to rain in my car (funny not ?) . This topic will report what I am doing and the results obtained it might be useful to over users of Rusefi.

First thing done : isolating the ignition wires from the loom. Two coils are connected in series, so then one fails you loose half of the engine!

The coils are rated for 0,5 ohms. I don't have precise information about current rating but find some data saying 6 amps.
It will be necessary to have a current limiting device in the driver because relying only on dwell time to limit the current will be to tricky and a major risk of frying the coil if something goes wrong.

I will try these bosch drivers from an old hall distributor I have in storage. They have an internal 7,5 amps limiter implemented and have no dwell adjustment inside.

Next step was to have the drivers running with the board.

Finally it's easy because the default setup works, but needed to be checked.

This driver was not known to be compatible with ecu's.

Result with bosch coil (old style) matched with the driver : You can see that the driver is curent limited because the trace goes flat after 6,5 ms.
The dwell time was set to 7ms in MRE. So this dwell time is ok for this coil with this battery voltage (13V).

The mesurement is very easy to make : you just put your scope in parralel with the + wire from the coil, so it is used as a shunt without changing the working wiring. Be careful to disconnect all groud connections from the scope not to make a short.

Mesuring current with a calibrated shunt :
Shunt data : 60 mV for 15 A

So the current is more or less 7,5 amps which is coherent with bosch specs.

Now I am safe to switch to the real OEM coils from my engine.

Now with the shunt and the OEM Béru coil :

The driver works well, it limits the current to the same 7,5 amps with the new coil.
The dwell was set to 3ms (thanks Gwendal) and it is correct but quite a bit long for 13,5 V.

The ignition is now validated and should work as OEM driven.
But the setup is poor :
The dwell time is too short for à 7000 RPM engine with sequential COPs. We can accumulate more energy with a higher inductance by charging longer. 12ms won't be a problem at these RPM. We have 17ms cycle time, to divide between charging and burning. 2 ms burn is enough, so with 12 ms we are safe. Doing this way reduces the time the driver dissipates the energy to keep the 7,5A limit.

You can store more energy by driving higher currents, it's more efficient because the current is squared in the formula.
But, it will need an other driver and other coils.
I will report you what I will test.

JPh wrote: ↑

Tue Mar 24, 2020 12:39 pm

The dwell time is too short for à 7000 RPM engine with sequential COPs. We can accumulate more energy with a higher inductance by charging longer.

If the inductor is saturated (current has leveled off), you will not gain more energy by charging longer. Are you running a pair of coils in series?

That's not the point. I wanted to say I need an other coil with more inductance because I have time to charge it. I my older design I used 10ms coils that worked fine.

My coils will be driven by individual wires. Other is crap.

Just my 2 cents about using oem drivers or just igbt to drive the coils :

Don't forget oem drivers have important safety features : current limiting and on time limiting.
These two features are a must have for me.
Just limiting this two parameters with the software dwell control is not safe enough.
If your ecu freezes in the ignition on state you will need your fire extinguisher!