How a point and condenser ignition works...

Raner · Oct 7, 2016

Sometimes I see people asking questions about point ignitions...
I ran across this little video and thought this is pretty darn good.
It's simple but yet it shows every part and function of a point ignition. So I figured this site would be a good place for someone needing help to view it...
https://www.youtube.com/watch?v=W94iksaQwUo

john *.?-!.* cub owner · Oct 7, 2016

Pretty good video, but he has a couple things wrong. Current flow from the coil is to the moveable point, not the fixed one. The fixed one is directly grounded.

The condenser does not block back EMF, it actually produces it and causes current to flow backward in the coil producing a hotter spark than if no condenser.

Also the diagram implied that there was high voltage to the rotor all the time rather than mentioning that the points and rotor are tied together so that the points open and produce high voltage in the coil just as the rotor reaches the terminal for the plug wires.

John T · Oct 7, 2016

Fun little video even if not 100% technically accurate, but hey Billy Bob and Bubba might learn something from watching. For example (among other over simplifications) the coil doesn't necessarily and absolutely INSTANTANEOUSLY turn 12 volts into 22,000 volts as shown. When the points break open interrupting coil current the coils magnetic field collapses and voltage is induced over into the HV secondary and its NOT INSTANTLY 22,000 VOLTS, it ramps up higher and higher but ONLY high enough until current arc jumps the gap and then it collapses. Subject to the plug gap and the medium (fuel and compression) in which its located, the plug may fire at say 10,000 volts but if the gap is wider and/or compression changes, it may be less or maybe it is 22,000 volts.

Oh well still a cute little video and mostly correct and good visuals by which a lay person may gain knowledge and there's nothing wrong with that.

John T

Janicholson · Oct 7, 2016

Pretty good, but the last part of it shows the high voltage being continuous out the coil and trying to get out of the rotor all the time. It does not, it only
is delivered it perfect time to jump that rotor to terminal gap. Thus I give it a nope. Jim
clear dietribution demo.

Janicholson · Oct 7, 2016

We do Know that of which we speak. Good Job, To John as well. Jim

khouse6 · Oct 7, 2016

coffee shop talk says that a spark plug that fires in open air, may not fire at all when placed under compression (such as in a engine cylinder)
is this true?

jeffcat · Oct 7, 2016

Yes. There have been many plugs from different brands that are natorius for doing just that. Most every small engine I have runs on NGK Plugs with out a lick of trouble. If you have a question about a spacific plug this is where to ask.

John T · Oct 8, 2016

Its my understanding that can (under certain circumstances) be TRUE. As compression rises (in the engines combustion chamber) the voltage that must be achieved before current arc jumps across the plug gap increases. Just because a faulty say partially fouled or leaky insulator plug can fire in open air, that don't mean it couldn't fail due to being carbon fouled or insulator breakdown WHEN VOLTAGES RISE HIGHER AS UNDER COMPRESSION IN THE COMBUSTION CHAMBER. Rising compression causes higher voltage which can cause plug breakdown (via fouling material or bad insulator/ceramic) which lower open air voltages may not.

John T

Raner · Oct 8, 2016

Well Guys... If someone looks at this post for help in the future...
With your comments and both videos they should be able to
figured out what there problem is...
Or have an understanding of where to look to correct an ignition problem...

teddy52food · Oct 8, 2016

The condenser does not produce back EMF. That comes from the primary winding as the field collapses. The condenser absorbs it and releases it causing the coil to ring like a bell. Thus making multiple sparks that we see as one.

Janicholson · Oct 8, 2016

Correct it is. That fact is the specific reason I believe the resistor should not be placed between the coil and points. Causing that interaction to be even slightly dampened seems wrong. An example of the circuit on a massive scale is used to start a General Electric Dash 9 CW44 Locomotive. A massive (near one farad) capacitor is charged by the main generator to ~600 volts. This charged state is isolated when the engine is shut off. To start the engine, heavy duty solid state switches are used to condition the output of the capacitor while it is fed to the generator as a alternating current "signal" to turn the generator into a motor. The repurposed "motor" spins the 6000 hp. diesel. The current involved, passes into a giant inductor where it builds up a very substantial field. The field collapses and that emf is also fed into the "motor" to spin it. that voltage recharges the capacitor, Ringing the system through phase matching electronics. The diesels start in very cold weather due to several engine system assists, and that capacitors like cold. Jim

John T · Oct 8, 2016

Good post Professor, Indeed as we know and posted below its ONLY when the points break open does the coils HV secondary begin to ramp up to fire and that's when the rotor tip lines up with that plug wires pickup cap terminal to fire its plug at near TDC, if alls well that is. Then the whole process starts over, points close and coil conducts current to re build its magnetic field on and on and on. But we already know this........still fun chat

John T

John T · Oct 8, 2016

Youre a good man my friend. Yep the condenser is indeed in the LV primary circuit and inductors (coil) dont like to change current rapidly and the previously shorted discharged (when points were closed) capacitor acts like a shock absorber and accepts current (a short at time T = O+) and softens the energy blow that would otherwise arc at the points prematurely burning them. Its also does the ring thing and some energy is fed back to the coil as I understand it.

John T

John T · Oct 8, 2016

In addition they can read my Ignition Troubleshooting Procedure Ive posted a thousand times here lol

http://www.ytmag.com/cgi-bin/viewit.cgi?bd=farmall&th=5745

John T
John Ts Ignition Troubleshooting

khouse6 · Oct 8, 2016

struggling as to why, the voltage would have to increase to jump the spark plug gap, under compression. (same plug gap) only thing that has changed is inside the motor. seems like air outside cylinder and gas and air inside, plus compression. cannot get my pea brain around compression, raising the effort, to jump the gap. sigh, o k; lots of things we have to take on faith. so i put my faith in your answers.

Bob · Oct 8, 2016

kh, you must not be old enough to remember when every little "service station" had a sparkplug cleaner and tester, similar to the one in the top photo.

There were port(s) to screw sparkplugs in, and a high-tension wire to connect to them.

You screwed the plug in, powered it up and then raised air pressure the test chamber while viewing the spark though a little glass porthole.

The second photo (of a different machine) shows a plug under test.

<img src = "http://i167.photobucket.com/albums/u125/27Grainfield/Stuff/Champ3_zpshobimile.jpg">

khouse6 · Oct 8, 2016

yes, i am old enough to remember. guess where i lived, we were not that high tech. lol

John T · Oct 8, 2016

Good question, I cant get into high tech details and physics too much, but how about this for a seriously OVER simplified NOT exactly technically correct, but at least some sort of an "answer" BUT NO WARRANTY WHATSOEVER.

As you raise compression the fuel air mixture becomes far more dense with 10 times as many molecules in the way so it becomes more difficult to arc current through that gap???? Theres a lot more "stuff" in the way between that gap under high compression then only at atmosphere.

Does that make any sense at all?????????

John T

khouse6 · Oct 9, 2016

o k, that will do. thanks

john *.?-!.* cub owner · Oct 9, 2016

Had that proven to me once. Had a tractor (magneto fired) that would not start when hot. Switched magneto with one from another tractor, both ran great. Finally figured out coil was getting weak and spark voltage was slightly lower, just enough to not fire the one with higher compression when hot, and compression was slightly higher due to hot engine.

john *.?-!.* cub owner · Oct 9, 2016

(quoted from post at 08:14:45 10/08/16) The condenser does not produce back EMF. That comes from the primary winding as the field collapses. The condenser absorbs it and releases it causing the coil to ring like a bell. Thus making multiple sparks that we see as one.

t releases by pushing back against the flow, and that is back EMF

Tom Fleming · Oct 10, 2016

The simplest explanation I can provide on the higher compression requiring higher voltage to spark is this:

Electricity is the movement of electrons. Each electron has mass, and is also held in orbit around its atomic nucleus by electromagnetic attraction to the protons in the nucleus. It takes a finite amount of energy to get that electron away from its atomic nucleus (there are other factors as well, but let keep this focused on the base requirements).

When you increase the pressure (density) of the medium in which you want to spark in (i.e. Generate a plasma stream), the more density, the more atoms, the more energy required to get the electrons to move.

Now, we can factor in a number of different variables such as temperature, different combustion mixtures, and different flash temperatures of thos materials, and we can calculate down to the last microvolt what it takes to produce a spark in a given environment (combustion chamber).

Bottom line, in simplest terms: higher compression = denser gases. Denser gases require more energy to start the plasma stream (spark).

How a point and condenser ignition works...

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