Automobile Ignition Question

[Bob Scott]

Automobile ignition systems used to use points and a capacitor, known as a "condenser" in wrenchhead-speak. We electronics types stopped using the word "condenser" back in the '60s.

The inductance of the ignition coil primary winding and the capacitor form a tuned circuit. Does anyone know, offhand, what the value of the "condenser" is in uF? Also the typical inductance of the coil's primary winding?

I'd like to calculate the resonant frequency using the equation f = 1 / (2*pi*(L*C)^0.5).

I think that the period of oscillation causes a spark delay and is compensated for by using the vacuum advance.

 

[Jon Wilder]

I think that the period of oscillation causes a spark delay and is compensated for by using the vacuum advance.

Let's think about this a moment. If this were true, then why do most performance engines NOT use a vacuum advance?

The delay is NOT in the spark itself. The delay has to do with the burn time of the fuel air mixture. The fuel/air mix does not immediately burn out once the spark ignites it. It takes time to burn, and at some point in the burn cycle it reaches its maximum combustion point, at which maximum cylinder pressure is made.

From ignition time to maximum combustion time, the piston has already started on its downward travel down the bore on the power stroke. For optimum performance the piston has to be so far down the bore on the power stroke at the point at which maximum cylinder pressure is made. At higher RPMs the fuel/air mix has to be ignited earlier in order for the maximum cylinder pressure to be made when the piston is at the same point of down travel on the power stroke.

Higher octane fuels have a slower burn rate, which requires more advance than would be needed with a lower octane fuel.

 

[#12]

I think I saw .047 on an ancient car capacitor.

 

[MikeMl]

I have seen ~0.05uF.

Follow this link to a post where I ran a simulation of a point ignition.

 

[tcmtech]

I just went though about a dozen condensers I have from a multitude of engines and ages and the numbers I got range from .25 uf to 3.5 uf. Most where around .75 to 1.25 uf though.

You wont find a standard condenser size just as you will never find a standard ignition coil inductance. There are countless variations of both. 6 volt, 12 volt, with external resistor, non resistor, CD systems, and so on and so forth.

You are correct on matching the condenser size to the coil though. Too much or too little capacitance gives weak sparks and burns out points faster than having them correctly matched.

 

[WestOzXJR]

As already stated no one all-round ideal capacitor value exists for all engine variations. It is possible though to tune the capacitor value for any given engine combination by observing the pattern of arc pitting that forms on the points and adjusting the cap value accordingly.

The function of vacuum advance is to advance the spark under part throttle cruise cruise conditions which aids in both improved pat throttle fuel consumption and low rpm throttle response.

The design theory is to do with incomplete cylinder filling and optimization of the subsequently reduced combustion efficiency at part throttle.

A vacuum source from the engine side of the carburettor primary butterfly is relatively high vacuum under low throttle conditions. When this vacuum is applied to the actuator it pulls/holds the baseplate on which the points are mounted in an "ignition advanced" state.

When the throttle blade is opened - corresponding to a higher load condition, the vacuum level decreases so the actuator diaphragm relaxes allowing the points baseplate to return to a more retarded position.

High performance race engines don't usually have this feature - they don't spend much time at part throttle cruise conditions.

 

[ronv]

The capacitor is there to act as a snubber so the inductance of the coil doesn't cause to much arcing of the points, not to create a resonant circuit. You can start with a nominal value suggested above and then use this to fine tune:
A condenser with to much capacitance will cause a build up (metal transfer)on the mounting side of the points. A condenser to little capacitance will cause a build up (metal transfer) on the arm side of the points.

 

[Jon Wilder]

A vacuum source from the engine side of the carburettor primary butterfly is relatively high vacuum under low throttle conditions. When this vacuum is applied to the actuator it pulls/holds the baseplate on which the points are mounted in an "ignition advanced" state.

When the throttle blade is opened - corresponding to a higher load condition, the vacuum level decreases so the actuator diaphragm relaxes allowing the points baseplate to return to a more retarded position.

This is incorrect. Vacuum advance is supposed to run on ported/venturi vacuum...not manifold vacuum. Ported/venturi vacuum does the opposite of manifold vacuum (i.e. it increases as the throttle is opened and vice versa).

This being said, it's supposed to supply an immediate advance upon throttle transition from idle to part throttle in a dead stop situation. Since the mechanical advance is controlled by engine speed, it cannot provide this instant advance as the RPMs have to increase before the mechanical advance can be applied. Vacuum advance is able to provide this instant advance before the RPMs increase since venturi vacuum instantly increases as venturi airflow increases.

Another little known secret in regards to vacuum secondary carburetors...most think they work on manifold vacuum. However they too operate on venturi vacuum.

 

[MikeMl]

The capacitor is there to act as a snubber so the inductance of the coil doesn't cause to much arcing of the points, not to create a resonant circuit. You can start with a nominal value suggested above and then use this to fine tune:
A condenser with to much capacitance will cause a build up (metal transfer)on the mounting side of the points. A condenser to little capacitance will cause a build up (metal transfer) on the arm side of the points.

The capacitor is absolutely there to resonate with the coil primary. You obviously have never hung a scope probe across the points of a running engine.

 

[The Electrician]

Also the typical inductance of the coil's primary winding?

I measured 4 coils that were lying around and got values from 3.5 millihenries to 7 millihenries.

 

[ronv]

The capacitor is absolutely there to resonate with the coil primary. You obviously have never hung a scope probe across the points of a running engine.

As a matter of fact I have. Guess I never made the connection. Learn something every day.

 

[Mosaic]

Back in the day, tuning a car involved cleaning/gapping/replacing plugs, and points, perhaps some adjustment of the timing around the 15 deg. btdc depending on the engine. Sometimes the condenser is replaced as a matter of it being cheap, no one seemed to test if it was good. The engine was tuned to run best around idle. Those of us who wanted more speed could have a tuned exhaust extractor installed and the head ported for more flow and carb. rejetted for more fueling. Too much head porting for top end flow resulted in poor air velocity at idle & poor air fuel mix and a rough idle with lots of backfiring.
Some altered cams for more valve lift etc, again this is an rpm tuned performance gain. Some distributors used a vacuum advance and a centrifugal advance to keep up with rpm and the changing advance requirements.

It can be quite a hassle with balancing all that to have a driveable performance car. I can recall that an electronic ign and a turbo simplified things a lot. Then fuel octane became quite significant though due to higher charge temps. Also, don't lean burn on a turbo....valve/piston melt will happen. I compensated for the turbo fueling by using an oil unit as an on/off pressure sensor which drove a fuel injector sitting on a sealed alum. air intake box mated to a weber 32/36 carb. Sort of like a throttle body fueling system. It was a hybrid as i had the weber jetted lean for city driving, but i had a fast spooling IHI turbo that gave a kick in the pants when the injector woke up. I had to put a 2500lb clutch plate to hold the torque and cling to the steering wheel due to fwd torque steering on launch. Increased top end by moving from 13" to 17" wheels & 5 fwd box. Added rear disc brakes too.

Lucky I didn't kill my self.....no abs, no air bags...never hit anything with it! Had the car for 17 years.

 

[john1]

Hi Bob,

I'm not at all sure about the equation you posted.
Here is a picture of the one i have always used.
Its a picture because i dont know how to put those symbols in a post.


Hi Mike,

You would have seen ringing on an oscilloscope,
which of course is from the cap and coil primary,
but i think this is just an incidental effect and not part of the ignition system.
I think the cap is there just as a snubber, as Ron describes.


Ive never met an ignition cap as large as 3 mfd,
i seem to remember them as mostly around 0.047 mfd although i could be wrong,
it was a while ago, and i only mean 12v car engines.
Very little knowledge of 2 stroke or magneto ignition systems,
which may be quite different.

I once made a capacitor discharge ignition system, driven by the points,
it did give better performance, and virtually no wear on the points,
and smoother running through the rev range.
Most noticeably nice and smooth at low revs,
i was able to get the engine to run smoothly at about 450 rpm.
The unit failed some years ago, i still have the car, its back on normal points ingnition now.
(tickover has to be up around 550 to 600 now.)

John

 

[Bob Scott]

Hi Bob,

I'm not at all sure about the equation you posted.
Here is a picture of the one i have always used.
Its a picture because i dont know how to put those symbols in a post.

It's the same equation John. The square root of a number is the same as raising its power to the (1/2)th. That is the same as the 0.5th. These rules are handy if you enter equations into a spreadsheet..... or this message base as you found out B-).

So,
f = 1 / (2*pi*(L * C)^0.5)

 

[#12]

Excuse the gray beard but, whether you had manifold vacuum or venturi vacuum depends on which decade you were in.

 

[tcmtech]

Manifold vacuum ported is from the pre emissions days venturi ported is from the post emissions compliance days.

However just moving the ignition advance vacuum hose from the venturi port to a full manifold port will allow for better fuel economy and a slight power increase in part throttle driving conditions.

 

[MikeMl]

...Hi Mike,

You would have seen ringing on an oscilloscope,
which of course is from the cap and coil primary,
but i think this is just an incidental effect and not part of the ignition system.
I think the cap is there just as a snubber, as Ron describes.

Sorry John, but I disagree. So does every tune-up manual I have ever read, including the 1955 Motor's Manual I have here.

Here is what Wiki says:

At the same time, current exits the coil's primary winding and begins to charge up the capacitor ("condenser") that lies across the now-open breaker points. This capacitor and the coil’s primary windings form an oscillating LC circuit. This LC circuit produces a damped, oscillating current which bounces energy between the capacitor’s electric field and the ignition coil’s magnetic field. The oscillating current in the coil’s primary, which produces an oscillating magnetic field in the coil, extends the high voltage pulse at the output of the secondary windings. This high voltage thus continues beyond the time of the initial field collapse pulse. The oscillation continues until the circuit’s energy is consumed.

 

[ronv]

Might be fair to say it does both - extend the spark duration and protect the points.

 

[crutschow]

Manifold vacuum ported is from the pre emissions days venturi ported is from the post emissions compliance days.

Only old engines with a carburetor have a venturi. It's what draws the fuel from the carburetor bowl into the air stream. Fuel injected engines don't need one since the fuel is injected under pressure.

 

[Jon Wilder]

Only old engines with a carburetor have a venturi. It's what draws the fuel from the carburetor bowl into the air stream. Fuel injected engines don't need one since the fuel is injected under pressure.

Furthermore, most multi port fuel injected engines also feature electronic advance rather than vacuum and mechanical advance. Exceptions for TB injection.