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Ignition systems

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paulca:
For some reason I was trying to work out how a car ignition work and found myself stumbling.  It doesn't help that automotive people use completely different terms for things, like high tension instead of high voltage, condenser rather than capacitor and coil rather than transformer/inductor?

So here's how far I got... a lot of this is probably wrong and left me with questions.

In an old engine, you would see a coil of wire on the crank or the housing and a magnet on the other.  Each time the magnet passes the coil it creates a pulse of current.  That is then stepped up to high voltage and creates the spark across the plug.  The questions are, how is it stepped up and is this type of ignition void of "points" or does the condensor/capacitor hold the charge from the coil pulse and then release it when the points close? 

In a modern engine with CDI ignition I assume the alternator/battery circuit charges the capacitor constantly and electronics decide when to discharge it through the coil to get a spark.

This "coil" they refer to, I get the feeling it can be multiple things, like the generator coil on the crank or the step up transformer... or an inductor.

Can anyone help clarify for me in more "normal" electronics terms?

jpanhalt:
The magnet passing a coil is a magneto ignition.  Uncommon in most cars today but still used in piston powered aircraft as it will work without a battery.  The original Ford model T ignition used a "buzz" box.  They are still used today for such things as igniting model pulse jets and giving shocks.  There are lots of variations.  The attached picture is the one I use for my pulse jet.

A brief history:
https://www.modeltcentral.com/Model-T-Ford-Electrical-Specifications.html
https://en.wikipedia.org/wiki/Ignition_system

Ohioans are proud of Charles Kettering whose system was the standard automotive system for decades.  Kettering, Ohio is said to be just outside of Dayton, Ohio (see: Wikipedia).

Benta:
The "old" ignition system you mention is called a "magneto ignition". It has the advantage that no outside excitation is necessary (eg, battery). It's still used in lawn mowers etc.

Since the 1950s it's no longer used in cars. Cars today use ignition coil(s), which is basically a flyback transformer. The primary is energised by the 12 V system, and when ignition is desired, the primary circuit is broken, leading the secondary voltage to peak to around 20 kV.

Earlier, you'd have one or more external ignition coils (large cylindrical thingies), today the coils are much smaller and placed directly on the spark plugs, but the principle is the same.

nigelwright7557:
When the points are closed the coil primary charges up.
When points open there is a large change of voltage on primary which is stepped up in the secondary coil and that gives around 25,000 volts pulse.

HighVoltage:
In layman terms on points ignition:

12V system
3 Ohm primary winding of the coil
3 mH primary inductance
1:100 Transformer ratio

When the points are closed, 12V is applied to the primary winding and the current slowly increases until it reached 4A
12V/3Ohm = 4A

At this time you have stored the maximum energy in the coil
W=1/2*L*i^2
W= 1/2*3*4*4 =24mJ (Energy)

Once the points are opened, the magnetic field collapses and produces a voltage of around 300V peak at the primary windings
This 300V is transformed with the transformer ratio of 1:100
300V * 100 = 30.000 Volts peak secondary high voltage

The reality is a little more complicated but the general picture is correct.

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