In a car ignition how does a circuit work?

[Herbert11]

As I get it is that after the contact breaker breaks the contact then the current flows from plus of the battery through the primary winding then through the capacitor and to the minus of the battery. But then what happens to the secondary current? It goes from secondary coil and then what? Does it go through capacitor? Does it go back through primary winding to the plus of the battery? I don't get it.

 

[davenn]

As I get it is that after the contact breaker breaks the contact then the current flows from plus of the battery through the primary winding then through the capacitor and to the minus of the battery

not quite, the capacitor is only for RF noise suppression. If you removed the capacitor, the system would still work, except that there would be lots of noise on AM radio in particular.

But then what happens to the secondary current? It goes from secondary coil and then what? Does it go through capacitor? Does it go back through primary winding to the plus of the battery? I don't get it.

So it rather goes down via the contact breaker to ground ( negative). This pulsing voltage in the primary winding of the coil induces a voltage into the secondary winding. A much higher voltage is produced because of the turns ration of the pri. and sec. of the coil giving around 25 kV to each of the sparkplugs in the appropriate firing order via the distributer

That's about the easiest description
Dave

 

[Tom.G]

Here is an article with a more detailed explanation, a video, and an interactive simulation you can download. You may notice that the schematic you posted has an error in the connection of the secondary winding.

http://www.learnmeche.com/simulations/systems/kettering-ignition
EDIT: Another difference is that the above article shows a resistor in the primary circuit. It is called a Ballast Resistor. It was introduced when 12V electrical systems replaced the 6V system in cars and serves to limit the coil current in normal operation. However the resistor is bypassed during engine starting to supply the full battery voltage during engine cranking.

Cheers,
Tom

 

[dlgoff]

Here's a good explanation also: Auto ignition coils

... opening a switch quickly in the primary circuit to drop the current to zero will generate a large voltage in the secondary coil according to Faraday's Law.

 

[Asymptotic]

not quite, the capacitor is only for RF noise suppression. If you removed the capacitor, the system would still work, except that there would be lots of noise on AM radio in particular.

True enough about lots of RF without the capacitor, but it isn't the whole story. Years ago my Datsun B-210 died suddenly, and produced a very weak spark when turned over. Lifted the condenser lead, and got it running (cap must have shorted), but it ran like crap, and nearly ate up the points in the twenty mile drive home. Part of what the capacitor does is reduce arcing across the points, which (in addition to reducing RFI) prevents them from failing prematurely, and (by not dissipating energy in the point gap) frees it to do useful work across the spark plug gap.

 

[jim hardy]

Part of what the capacitor does is reduce arcing across the points, which (in addition to reducing RFI) prevents them from failing prematurely, and (by not dissipating energy in the point gap) frees it to do useful work across the spark plug gap.

Indeed that capacitor is the secret to this type of ignition.

The capacitor provides a current path around the points for that brief instant when they first open.
Almost immediately the coil voltage at points rises to a high value, like a couple hundred volts,
because coil's inductance is now trying to push the current that got established while points were closed through the capacitor instead.
The capacitor slows that voltage rise just enough for the points to move apart a few thousandths of an inch without forming an arc.

Since the coil is really a step-up transformer,
a much much higher voltage appears at its secondary
and that's the voltage they send on to the distributor and plugs.

Without the capacitor a set of points will last less than an hour,
and the engine will run progressively worse as the points burn away from the arcing.

Good anecdote there , @Asymptotic !

old jim

 

[Averagesupernova]

You may notice that the schematic you posted has an error in the connection of the secondary winding.

How so?
https://www.popularmechanics.com/cars/how-to/a6382/4338376/

 

[Averagesupernova]

True enough about lots of RF without the capacitor, but it isn't the whole story. Years ago my Datsun B-210 died suddenly, and produced a very weak spark when turned over. Lifted the condenser lead, and got it running (cap must have shorted), but it ran like crap, and nearly ate up the points in the twenty mile drive home. Part of what the capacitor does is reduce arcing across the points, which (in addition to reducing RFI) prevents them from failing prematurely, and (by not dissipating energy in the point gap) frees it to do useful work across the spark plug gap.

I am surprised you got it to run at all without a condenser. I have fiddled with more points ignition systems than I can count and have yet to see anything run at all without a condenser. I took the liberty of putting in bold in the above quote what I always though to be the case.

 

[Asymptotic]

I am surprised you got it to run at all without a condenser. I have fiddled with more points ignition systems than I can count and have yet to see anything run at all without a condenser. I took the liberty of putting in bold in the above quote what I always though to be the case.

As I recall it was a near thing, white knuckle, praying "please don't stall" all the way home. Guess I got lucky. :)

 

[Guineafowl]

How so?
https://www.popularmechanics.com/cars/how-to/a6382/4338376/

Yes - the ignition coils I’ve dealt with have all had a common negative like that.

The high voltage side needs a chassis ground reference so the spark jumps to ground, but why it’s done inside the coil, rather than with an external strap, I’m not sure.

 

[jim hardy]

Yes - the ignition coils I’ve dealt with have all had a common negative like that.

Ones I've encountered were autotransformers where there's only three wires brought out as in first post of the thread
so spark current flows through the LV part of the windings(or perhaps through the condenser/capacitor).
Think about it -
spark current is so small that its amp-turn contribution in the LV windings is miniscule
it does its work in the copious turns of the HV winding.

old jim

 

[Guineafowl]

Ones I've encountered were autotransformers where there's only three wires brought out as in first post of the thread
so spark current flows through the LV part of the windings(or perhaps through the condenser/capacitor).
Think about it -
spark current is so small that its amp-turn contribution in the LV windings is miniscule
it does its work in the copious turns of the HV winding.

old jim

Would this autotransformer configuration reduce stress on the winding insulation, and save a bit of copper?

Apparently the newer-type coil packs have separate windings. I guess that modern electronic ignition systems appreciate proper isolation between primary and secondary.

 

[tech99]

The transformer has a ratio of perhaps 30:1 so that is not the whole story, as it gives 25kV from a 12V battery. It is the rate of change of current in the primary, which creates such a high voltage. V = -L dI/dt. The capacitor speeds up the rate of change.

 

[jim hardy]

It is the rate of change of current in the primary, which creates such a high voltage. V = -L dI/dt. The capacitor speeds up the rate of change.

Indeed it's di/dt that causes high voltage in both the LV and HV windings.
Current can't drop to zero immediately for that would make infinite voltage(were the coil a perfect inductor)
and without a capacitor the voltage rises so fast that it establishes an arc across the points before they've moved far enough to make a decent air gap.
That's why @Asymptotic's anecdote is so relevant.

Adding a capacitor let's flow of current continue, into the capacitor instead of through the points,
giving the points time to separate far enough they'll withstand a couple hundred volts.

Last car condenser i measured was 0.2 microfarads (my kids' 65 Buick)
and coil current was set to ~2 amps by the ballast resistor.

Now,
pumping 2 amps into 0.2 μF raises voltage by 10 volts per microsecond ($~i = c\frac{dv}{dt}~$ )
giving the points a fraction of a milisecond to establish a small air gap before voltage across them rises high enough to strike an arc.

Dielectric strength of air is maybe 75 volts per mil (3 megavots per meter https://hypertextbook.com/facts/2000/AliceHong.shtml )
and perhaps 30 to 40 megavolts per meter at very small gaps, see Paschen's Law

so using the lesser dielectric strength
a point gap of 0.018" should hold off at least 1350 volts
which with a 30::1 step up could hand 40kv to the sparkplug.
A plug with gap of 0.030 should require less than that to fire.

New CD systems charge a capacitor to high voltage (400 volts in Evinrude outboard motors I've worked on)
and connect that across the primary of a stepup transformer 'coilpack' .
They'll make higher voltage than these old systems we're discussing
and that's why new engines keep running when the spark plug electrode wears away and plug gap exceeds a hundred mils.

my observations - corrections welcome.

old jim

 

[OCR]

my observations

Our observations are almost identical, except for. . .

a point gap of 0.018" the thickness of a matchbook cover should hold off at least 1350 volts

Lol. . . j/k

Carry on. . . .

.

 

[hutchphd]

Our observations are almost identical, except for. . .

a point gap of 0.018" the thickness of a matchbook cover should hold off at least 1350 volts

Lol. . . j/k

Carry on. . . .

.

my observations - corrections welcome.

No one has mentioned that the primary circuit (with capacitor) is a high Q tank (series RLC) circuit that rings like a bell when the points open. I think this is a major design feature that allows a series of AC pulses be delivered to the plug when the circuit gets "hit" . I remember being astounded by what a nice oscillation actually ensues.

 

[sophiecentaur]

There seem to be a couple of points missing in this thread - or perhaps I missed them. The value of the "Condenser" (as it was called, years after the accepted term was Capacitor) is chosen to maximise the rate of change of current in the primary the the contact breaker opens - it's a Tuned circuit. That gives as high secondary Volts as possible. Also, I seem to remember that the series connection of the Primary and Secondary windings was to add the primary Volts to the Secondary Volts when applied to the plugs.
Some early systems used a Positive Earth (The 6V Ford Popular electrics, for example). I'm not sure why but it was something to do with the feeble spark and the need to give the plugs the best chance of actually lighting the fuel so the polarity was chosen appropriately.

Something that not many people know is that the Citroen 2CV ignition system had two cylinders and no distributor. The Coil Secondary was connected in series with both plugs (and HT lead from each end of it). Every rotation of the engine applied a spark to both (series) plugs. The plug in the 'Exhausted' cylinder would strike easily with very small Volts drop because the pressure was low and the plug that was under pressure hogged nearly all the EHT Volts. I guess it meant fewer moving parts in an already simple engine. The contact breaker was on the end of the crankshaft (with centrifugal advance weights) but there was no vacuum advance - but hell, it wasn't Formula One'

Note the 12V battery which was only on 'later' models! I seem to remember that one plug consistently used to fail before the other, due to the different polarity. There were many lunatic features on the loveable little car. The ignition coil was positioned right where the rain could get in, just over the (Air Cooling) fan intake. WD40 was frequently needed.

 

[Guineafowl]

There seem to be a couple of points missing in this thread - or perhaps I missed them. The value of the "Condenser" (as it was called, years after the accepted term was Capacitor) is chosen to maximise the rate of change of current in the primary the the contact breaker opens - it's a Tuned circuit. That gives as high secondary Volts as possible. Also, I seem to remember that the series connection of the Primary and Secondary windings was to add the primary Volts to the Secondary Volts when applied to the plugs.
Some early systems used a Positive Earth (The 6V Ford Popular electrics, for example). I'm not sure why but it was something to do with the feeble spark and the need to give the plugs the best chance of actually lighting the fuel so the polarity was chosen appropriately.

Something that not many people know is that the Citroen 2CV ignition system had two cylinders and no distributor. The Coil Secondary was connected in series with both plugs (and HT lead from each end of it). Every rotation of the engine applied a spark to both (series) plugs. The plug in the 'Exhausted' cylinder would strike easily with very small Volts drop because the pressure was low and the plug that was under pressure hogged nearly all the EHT Volts. I guess it meant fewer moving parts in an already simple engine. The contact breaker was on the end of the crankshaft (with centrifugal advance weights) but there was no vacuum advance - but hell, it wasn't Formula One'
View attachment 241027
Note the 12V battery which was only on 'later' models! I seem to remember that one plug consistently used to fail before the other, due to the different polarity. There were many lunatic features on the loveable little car. The ignition coil was positioned right where the rain could get in, just over the (Air Cooling) fan intake. WD40 was frequently needed.

So my homemade electric fence energiser might benefit from a series capacitor then...Many modern cars use a single coil pack per two cylinders, a bit like the 2CV. It’s called a ‘waste spark’ system. Each cylinder is sparked near TDC whether on the compression stroke or the exhaust. This may be to save complication, but also reportedly helps burn any excess fuel left in the combustion chamber.

 

[sophiecentaur]

This may be to save complication, but also reportedly helps burn any excess fuel left in the combustion chamber.

Well well. 2CV engine had green credentials!

 

[sophiecentaur]

So my homemade electric fence energiser might benefit from a series capacitor then...

Are you sure there is none in there? Is it a simple mechanical type with a contact breaker? Perhaps it is designed to have a limited poke? You don't want to give cattle heart attacks.

 

[Guineafowl]

Are you sure there is none in there? Is it a simple mechanical type with a contact breaker? Perhaps it is designed to have a limited poke? You don't want to give cattle heart attacks.

It’s a 555 timer that pulses an ignition coil via a power transistor. Output voltage is limited by a 100V zener and standard diode opposed across the primary as suggested by @Tom.G . Ignition coils don’t like running open circuit (as a fence would) as the voltage peaks too high and damages the windings.

 

[sophiecentaur]

It’s a 555 timer that pulses an ignition coil via a power transistor. Output voltage is limited by a 100V zener and standard diode opposed across the primary as suggested by @Tom.G . Ignition coils don’t like running open circuit (as a fence would) as the voltage peaks too high and damages the windings.

Ahh well, that's a different kettle of fish altogether. Its a common or garden high voltage transformer system. I am surprised about the 100V figure because your average electric fence gives a noticeable tickle and sometimes 100V would hardly get through your rubber soled sneakers. I guess cows don't wear sneakers. I always imagined an electric fence would have higher volts but a high source resistance, to limit the current.
Another aside - I heard that gamekeepers use low voltage 'selective' electric fences to discourage raiding mammals with damp noses whereas the birds' feathers and beaks are far higher resistance and don't notice the tingle.

 

[GoatGuy]

not quite, the capacitor is only for RF noise suppression. If you removed the capacitor, the system would still work, except that there would be lots of noise on AM radio in particular.

So it rather goes down via the contact breaker to ground ( negative). This pulsing voltage in the primary winding of the coil induces a voltage into the secondary winding. A much higher voltage is produced because of the turns ration of the pri. and sec. of the coil giving around 25 kV to each of the sparkplugs in the appropriate firing order via the distributer

That's about the easiest description
Dave

The capacitor is NOT for ''noise suppression'', although it does serve that purpose.

It is to act as the C part of the LC tank circuit formed, allowing the current to ring between primary coil and the capacitor, back and forth like the sloshing of water in a bathtub. This back-and-forth ringing is what induces the high voltage discharge spike onto the secondary.

Without the capacitor, all you get are tiny discharges (often not large enough to ignite the gasoline-air mixture in a cylinder). Moreover, and with most negative impact, without a capacitor, the 'points breaker' switch's contacts are quickly eroded from arcing. With the capacitor, arcing nearly ceases, multiplying the life of the contacts many dozens of times.

 

[sophiecentaur]

back and forth like the sloshing of water in a bathtub.

Actually, the main part of the Energy of the spark is in the first peak in secondary volts because the arc is a low impedance load, once it has struck. I think the oscillating components of the arc current are more to do with the RF characteristics of the secondary circuit.

With the capacitor, arcing nearly ceases,

Yes. That's a really useful spin off and I remember that was given as the main reason by my Early Physics teacher. (But we hadn't 'done' LC circuits in early A level)

 

[GoatGuy]

While you are right about the energy-of-the-spark peaking just after strike, there is a substantial amount of potential energy at that same time in the condenser. As the (rapidly decreasing) energy sloshes between condenser and coil primary, it continues to excite the now-struck arc on the spark plug, expanding and heating it further.

All this happens in microseconds.

It was quite apparent when standing around the car-mechanics shop, watching them take inductive-pickup spark-plug-wire readings with their then-newfangled oscilloscope displays.

At 600 RPM, on a 8-cylinder (thank you Ford!) engine, the pulses are generated at

600 RPM × 8 cylinders ÷ 2 strokes ÷ 60 seconds = 40 pulses per second.
1,000,000 microseconds ÷ 40 = 25,000 µS/pulse.

I remember as a teen (1970s) using a ruler to measure the number-and-period of those pulses on the mechanic's oscilloscope screen; wickedly, it not being a proper instrument, there were no standardized base scan-rate numbers. Working with the tachometer tho, and seeing 2 discharge pulses, I worked out that the ringing was at about 15 kHz. It wasn't obvious, but given that the condenser was 0.22 µF, and the formula F₀ = 1/(2π√(LC)), the primary of the coil had to be about 0.5 millihenries.

Just saying,
GoatGuy ✓

 

[OCR]

...your average electric fence gives a noticeable tickle...

And I'll bet I'm not the only guy who made a noticeable shriek, from that
noticeable tickle. . Spark plugs seem to cause me to respond in the same embarrassing
moaner manner .

.

 

[Guineafowl]

Ahh well, that's a different kettle of fish altogether. Its a common or garden high voltage transformer system. I am surprised about the 100V figure because your average electric fence gives a noticeable tickle and sometimes 100V would hardly get through your rubber soled sneakers. I guess cows don't wear sneakers. I always imagined an electric fence would have higher volts but a high source resistance, to limit the current.
Another aside - I heard that gamekeepers use low voltage 'selective' electric fences to discourage raiding mammals with damp noses whereas the birds' feathers and beaks are far higher resistance and don't notice the tingle.

The 100V snubber is intended to limit the primary voltage, so that the secondary does not rise too high. The arcs from the output will jump around 25 mm, so I’d guess it’s in the 20 kV range, and certainly gives more than a ‘tickle’.

I once knew of a horse that would sniff at an electric fence to see if it was on or not - if not, the horse would reliably escape.Whether it was getting a tingle through the nose hairs, or sensing corona discharge or smelling ozone, we never knew.

 

[sophiecentaur]

The 100V snubber is intended to limit the primary voltage, so that the secondary does not rise too high. The arcs from the output will jump around 25 mm, so I’d guess it’s in the 20 kV range, and certainly gives more than a ‘tickle’.

I once knew of a horse that would sniff at an electric fence to see if it was on or not - if not, the horse would reliably escape.Whether it was getting a tingle through the nose hairs, or sensing corona discharge or smelling ozone, we never knew.

HAha. It's like in the POW camp - he has nothing else to think about so his mind goes towards ESCAPE. It's easy to under (and over) estimate the way other animals' minds work.