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Automotive ignition coil: firing primary of coil for spark on RISING or COLLAPSING mag-field?

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LAN_403

Signification

Sun Dec 02 2018, 06:55AM

Registered Member #54278 Joined: Sat Jan 17 2015, 04:42AM
Location: Amite, La.
Posts: 367

I have always heard that the high voltage generated by the automotive ignition coil (IC)generates the plug fire on the COLLAPSING field of the primary winding of the IC-this sounds better, as firing on the rising field of the primary would encounter more self inductance and therefore be slower.

However, I have searched, and can seem to find only firing initiated on the RISING edge. Also, I have built this circuit (rising edge triggered demo circuit), and it works wonderfully...cutting way down of some complexity of the COLLAPSING trigger-type. Can anyone shed some light on this?

This may help:

klugesmith

Sun Dec 02 2018, 08:44AM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1714

Nice link, but I think over-complicated -- it has a magneto power source and solid-state switching of the coil.
How about a plain old battery powered ignition with breaker points and no semiconductors? Here's a link with some typical current and voltage waveforms:

The primary and secondary voltages can be much higher during the "collapsing" phase than the "rising" phase. Ordinary designs get primary voltage more than 10 times higher than battery voltage that way, and that greatly reduces the required turns ratio for a given spark voltage.

Think of the coil as two coupled inductors, much like a CRT flyback "transformer".
The primary isn't driven by alternately forcing two different voltages, it's driven by alternately closing and opening one switch.
Let's say one end of coil is permanently connected to battery voltage, perhaps through a resistor, and the other end is periodically grounded when the breaker points are closed.

"Rising" phase: switch closed. That end of coil is forced to 0 volts, so voltage across the coil = battery voltage. Primary current (and magnetic flux) ramps up according to di/dt = V/L. Secondary voltage is battery voltage times the turns ratio Ns/Np, and that had better NOT be enough to fire a spark in compressed gas. If points stay closed long enough, current (and magnetic flux) eventually stop increasing and have values determined by primary circuit resistance.

"Collapsing" phase: switch open. That doesn't change primary voltage to 0, it changes primary _current_ to 0 (or tries to). The voltages across primary and secondary windings immediately reverse sign and assume a much larger magnitude, enough to fire the spark. Now the magnetic flux (from primary ampere-turns + secondary ampere-turns) collapses at a much higher rate that it rose. Rate controlled by voltage in spark circuit.

Did that answer the right question?

Sulaiman

Sun Dec 02 2018, 09:45AM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

In the '70's I worked for Lucas designing electronic ignition systems,
there are a few patents in my old name that belong to Lucas.
just to add to the above;

There are a few basic types of ignition circuitry,

Inductive energy storage, spark formed by COLLAPSING magnetic field.
the Kettering type ignition system,
where energy is stored in the magnetic field of the ignition coil while the contact points are closed,
and released when a spark is required by opening the contact points.
An improvement was to use the contact points to operate a power transistor, reducing wear on the points,
A further improvement was the use of a magnetic pickup/sensor to control the power transistor, removing the need for mechanical contacts.
Finally a microprocessor was used to control timing and switching.
The ignition coil operates as a 'flyback' transformer.

Capacitive energy storage, spark formed by RISING magnetic field.
Generally known as Capacitive Discharge ignition (CDi).
Energy is built up and stored in a capacitor, typically around 300V+,
when a spark is required the capacitor is connected accross the ignition coil primary winding giving a near instantaneous spark.
The ignition coil acts as a 'pulse' transformer.

There are many small variations and details in both systems,
but essentially most ignition systems use either inductive or capacitive storage/ discharge.
_______________________________________ __________________________
Some tales of the electronic ignition department;

For the transistor assisted points type of ignitin we needed a suitable power transistor, we had our own semiconductor foundry so all I had to do was to specify the transistor parameters required,
I needed 3.5A 300V peak, so I specified 5A 350V - a little more costly but the safety margin would cover me for any mis-calculations.
My boss did not want to be blamed so increased the specifications,
his boss did not want to be blamed either,
nor the manager of the foundry,
nor the actual fabrication designer.
If you ever find a TO3 ignition transistor from Lucas, manufactured during the early '70's - keep it
... they are almost indestructible !

Lucas used to hire graduates every year, each department getting a fresh electronics engineer every one to four years.
I joined when microprocessors were new, only one guy in my year did a microprocessor based project for the degree.
(not me, I discussed his project and I made the prophetic pronouncement;
microprocessors will not catch on - they are too complicated !)
Immediately on joi ning Lucas I was sent to Motorolla and Intel courses !

Our pre-releaseIntel processor cost Â£200, my salary was $28/week.

My colleague that graduated the previous year had almost completed a small rack of plug-in circuits to form an electronic ignition system with engine-mapping,
all cmos based.
- half of his woes were due to incomming goods inspection staff that were not briefed on electrostatic handling.
- the other half was me and the microprocessors - all that work wasted.

An aditional part of his troubles was also due to me,
his 'processor' could not cope with the emi from the coil on my bench :)

There was an electronic ignition system being manufactured by Lucas at the time I joined,
it relied on the weights and springs in the distributor for mapping and was so unreliable that only one person in the ignition department used it in their own car
- because he was involved with the design,
- and he kept a spare distributor in the boot (trunk).

Signification

Wed Feb 13 2019, 12:58AM

Registered Member #54278 Joined: Sat Jan 17 2015, 04:42AM
Location: Amite, La.
Posts: 367

Klugesmith / Sulaiman: Sorry for the late reply. I just wanted to thank you for the extreamly useful information. I have learned what I need to know based on your information. I wrote a tedious basic program based primarily on the Biot-Savart law. It told me that for a single layer solenoid that the B-field was ALWAYS strongest at the coil center closest to the wire coils (I am speaking of the B-field INSIDE the coil). The field is weakest on the center axis but not constant--strongest at the coil center. Likewise, adjacent to the wire, the strongest field inside the entire coil is at the center adjacent to the inner wire (forming a internal inner loop.

Proud Mary

Wed Feb 13 2019, 02:19AM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

If I remember correctly, the time constant of the simple, old fashioned ignition coils conceived as a first order LR circuit is quite high - i.e. it takes quite a long time to charge up the magnetic field - so there is no 'leading edge' but rather a ramp leading from zero to the maximum magnetic field

From memory of my experiments some years ago, the long time constant limits the coil to operation at 50Hz or so, if the magnetic field is to be built up to the maximum on each cycle.

I should stress that I know very little about all the sophisticated types of coil that Sulaiman described, so I hope I will be understood when I talk of the simple old-fashioned coil.

Sulaiman

Wed Feb 13 2019, 08:44PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

The old oil-filled ignition coils were wound with copper wire of cross-sectional area to give a primary current of three to six amps when connected to 12V.

Later the wire was thicker producing a lower resitance coil and an external (ballast) resistor was added,
the external resistance was shunted out during cranking to provide good sparks with the lower battery voltage durng cranking (starting)

To operate ignition coils at higher frequencies use higher voltages,
the ON (dwell) time needs to be reduced to prevent excessive current.
A 555 driven mosfet (or similar) with a 24 to 50 Vdc supply gives impressive (but terribly noisy) arcs for Jacobs Ladder etc.
Continuous operation is limited by oil temperature / heat removal.
Enough volts, amps or watts to burn skin or possibly kill you,
- from either the secondary or the primary - so take care.
(I've not heard of death-by-ignition-coil, but significant pain is common ;)

There are many designs for ignition coil based eht generators on the

Proud Mary

Wed Feb 13 2019, 11:53PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

I have read that the use of a peaking capacitor of 70pF or so across the spark gap can radically improve power transfer efficiency, and give a major boost to the spark current.

Ash Small

Wed Mar 13 2019, 09:56PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

When they were developing the Jaguar V12 engine they had to design new coils that could cope with the number of sparks (shorter dwell time).

Adding some capacitance can improve things.....

Sulaiman

Thu Mar 14 2019, 01:27AM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

:) The Jag V12 used the Lucas OPUS unreliable distributor/ignition system that I mentioned above :)

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