Unwanted spark suppression from an automotive spark coil

Thread Starter

AGray

Joined Jan 18, 2021
8
Hi All,
I'm currently working on a project designing an experimental engine control unit which ignites a flammable mixture with a standard spark plug and standard ignition coil (the old style https://en.wikipedia.org/wiki/Ignition_coil). The primary is first charged to a nominal voltage (12V I think) then to generate the spark the primary voltage is quickly dropped to zero and the collapsing magnetic field in the secondary inductor causes a voltage spike across the spark plug. I'm controlling the primary voltage with a microprocessor and MOSFET and charge the primary when the engine is "armed" and turn off the coil when "arm" is switched off.

I'd like a way of making the spark coil fail-safe, i.e. if there is a sudden power cut, no spark will be created due to the sudden voltage drop, but be able to generate a spark by intentionally dropping the voltage whenever needed. This needs to be 100% reliable as it could be safety-critical if there is a build up of explosive gasses and an emergency shutdown is needed. I understand usually you'd connect a flyback diode (plus Zener/resistor) in reverse bias across the coil to stop the voltage spike but the voltage spike is needed to generate a spark...
I looked at make-before-break relays to connect the flyback diode before the primary current is switched off but couldn't really find any available and I'm not sure how they could be connected to stop the spark in the event of a power cut or when the "arm" switch is turned off. Likewise with using solid-state switches to include a diode in the loop, I'm not sure if it could be connected quickly enough to beat the voltage spike.

Thanks in advance for any input, this has been puzzling me for months now. I'm a mechanical engineer at heart so please be patient if I've misunderstood anything in the above!

Andrew
 
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Danko

Joined Nov 22, 2017
1,041
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What you are proposing is keeping power on to the coil constantly which is around 6A drain, and then get a single spark when you remove power? The coil only needs a few msec for the magnetic field to build up, any longer is a waste of energy and heats up the coil. At atmospheric pressure (not a combustion chamber on the compression stroke) I think you need more than one spark.

You would instead drive an ignition coil with an oscillator (search for 555 timer ignition coil driver circuits, also on youtube- dozens of examples) to give you a spark at say 3,000RPM equivalent. It's a little circuit to build. I've used them for lighting alcohol fumes (bird cannon).

But your safety aspect makes little sense, what's the problem with the on/off switch and an ARM switch?

I would delete the 1N5400 diode: http://chemelec.com/Projects/Car-Coil/Car-Coil.htm
youtube video of the build:
 
Is this an experimental engine, or controller, or both?

How about using a capacitor or battery backup to keep the ECU powered long enough after the power is cut to keep things in order? After the power is cut the ECU could keep the MOSFET on and let everything decay away gracefully.
 

Marley

Joined Apr 4, 2016
404
The fact is that when you connect 12V across the primary of the ignition coil, energy is stored in the magnetic field in the coil.
Normally, the current is interrupted and the energy is released to create the spark. So before interruption, the energy is there in the coil and has to be dissipated somehow. Difficult to dissipate this energy slowly when the power fails.
It would be better to change to a different system so that no energy is stored in the coil. I would use a capacitor discharge system (CDI). Now, the energy is stored in a capacitor instead. Still stored energy but will be retained if the power fails and it is easier to slowly discharge (into a resistor) without causing any sparks!
 

crutschow

Joined Mar 14, 2008
26,436
I'm controlling the primary voltage with a microprocessor and MOSFET and charge the primary when the engine is "armed" and turn off the coil when "arm" is switched off.
As noted by prairiemystic, it only takes a few ms for the coil current to reach its full value so, if you can tolerate a small delay when the spark is generated, use the microprocessor to generate a short pulse which energizes and turns-off the coil when the "arm" switch is turned off.
Then if the power is cut, there is nothing to energize the coil, so it is inherently fail-safe.

The capacitive discharge approach is not inherently fail-safe, since the capacitor is already charged and a stray pulse during power shut down could trigger the spark.
 
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Thread Starter

AGray

Joined Jan 18, 2021
8
Yes, a group of strangers on the internet is the perfect place to get life-supporting and mission-critical advice.
Haha I hear your point but it's not like I'm giving strangers on the internet free reign to install whatever components they like, I know just enough to vet out bad ideas... I might think twice if someone suggested strapping a pole pig to the engine and shorting the contacts to create the spark.
 

Thread Starter

AGray

Joined Jan 18, 2021
8
Is this an experimental engine, or controller, or both?

How about using a capacitor or battery backup to keep the ECU powered long enough after the power is cut to keep things in order? After the power is cut the ECU could keep the MOSFET on and let everything decay away gracefully.
It's an experimental pulsed detonation rocket engine, with a new controller replacing the old one as our requirements have evolved. I did think about a capacitor or UIPS but adds complexity and expense, I'm not sure where I could place a capacotor such that it only powered the coil and didn't arc across contacts when intentionally making a spark?
 

Thread Starter

AGray

Joined Jan 18, 2021
8
As noted by prairiemystic, it only takes a few ms for the coil current to reach its full value so, if you can tolerate a small delay when the spark is generated, use the microprocessor to generate a short pulse which energizes and turns-off the coil when the "arm" switch is turned off.
Then if the power is cut, there is nothing to energize the coil, so it is inherently fail-safe.

The capacitive discharge approach is not inherently fail-safe, since the capacitor is already charged and a stray pulse during power shut down could trigger the spark.
Thanks Danko, prairiemystic, Marley and Crutschow, very useful advice. I was copying the previous system by holding it at a high voltage and dropping it, but I think I will charge the coil just before (couple of ms, via the microprocessor code) it is needed to spark, then drop the voltage at exactly the right time as happens currently. I read about adding a capacitor (condenser) into the circuit which I will do if there is not one already to improve the strength of the spark.
If that poses problems I will move to a CDI system as they don't seem to be very expensive, but as Crutschow pointed out there is still stored energy that could cause problems.

If anyone has ideas about how to measure the time taken for an inductive coil to reach supply voltage without an oscilloscope I'm all ears, but that is a whole separate question I haven't looked into.

Thanks again!
 

crutschow

Joined Mar 14, 2008
26,436
I read about adding a capacitor (condenser) into the circuit which I will do if there is not one already to improve the strength of the spark.
That works only for a mechanical switch (points) switching the primary current.
The capacitor minimizes arcing across the contacts as they slowly (relatively) open.
A capacitor won't increase the spark voltage for a transistor driver.
If anyone has ideas about how to measure the time taken for an inductive coil to reach supply voltage without an oscilloscope I'm all ears
It's primarily a function of the coil primary inductance and resistance.
Do you know what those are?
 

MrSalts

Joined Apr 2, 2020
133
I would charge the capacitor through a diode and measure how long it takes to charge the capacitor.

if you have a microcontroller with ADC, you can use that to measure (sense) when the coil has charged voltage-wise but you can also add a small secondary coil around your existing inductor with a current sensing resistor across the two ends of the secondary. This allows you to indirectly measure the current snd induced magnetic field of the system which lag any voltage measurement of the primary.
 
It's an experimental pulsed detonation rocket engine.
Neat. I wasn't expecting that...

I was thinking since you normally open and close the ground side of the coil to make the spark if you keep the coil grounded while removing the power side of things it should normally decay away without issues. It sounds simple enough in my head, but making it actually happen is a different story.
 
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I would experiment with a soft turn off circuit ie 30ms for the mosfet and let it absorb the energy. FWIW some of the newer sicfet's would be much more efficient. I have played with quite a few ignition circuits and from experience not clamping down gate voltage is the surest way to destroy a mosfet or igbt. Gate to source with a zener very close to drive voltage prevents negative gate voltage as well as positive spike. A good coil to try is an Echlin IC676 from NAPA. This is my go to on these and point ignition systems. Also very important to use about .220-390 uf 600V would paper or poly film not a ceramic cap across the coil or just a condenser from a point ignition system. Another lesson from the trail of fried parts!
 
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shortbus

Joined Sep 30, 2009
8,344
I wish the TS good luck with the project. But from the big companies doing the research on this detonation jet engine after light off they don't use a spark, but a type of tube to delay from the last ignition to the next one.
 
You might want to drive the mosfet with a monostable, 3ms on to near saturate the coil then release. That way the excess spark is no longer generated and the gate voltage can be shunted to near ground all but 3ms of the ignition cycle. FWIW the coil I mentioned will give adequate spark at only about 3 volts across, or 2.-- amps draw. That was from an ignition simulator using a distributor machine, effectively a V8 at 4000 rpm. Note that the condenser across the coil is essential to both increase spark intensity and to protect the mosfet from dv/dt damage to gate. I blasted both 600V 20A igbt's and mosfets experimenting with it.
The CD ignition route is more complex than needed and the bike one shown earlier likely uses an engine driven exciter coil to charge cap.
 

shortbus

Joined Sep 30, 2009
8,344
It's not a normal engine. It's a detonation jet engine, a newer form of a pulse jet but used as a rocket. Normal forms of ignition don't work, or at least all of the big players couldn't get it to work.
It's an experimental pulsed detonation rocket engine,
 

Thread Starter

AGray

Joined Jan 18, 2021
8
That works only for a mechanical switch (points) switching the primary current.
The capacitor minimizes arcing across the contacts as they slowly (relatively) open.
A capacitor won't increase the spark voltage for a transistor driver.
It's primarily a function of the coil primary inductance and resistance.
Do you know what those are?
Thanks for clarifying that, and yes I'm comfortable calculating the inductance response using impedance etc., but off the top of my head I don't know the inductance and resistance of the coils. It shouldn't be too hard to simply measure their response to the supply voltage to work out the minimum time to energise them without ever having to measure/calculate the inductance.
 
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