IC Ignition Sytems

[JorgensenSteam]

Here is a continuation of the discussion about IC ignition coils/capacitors.

 

[kustomkb]

I don't know much about ignition systems, but for my twin I went with these guys,

http://www.rcexl.com/

They run off of a couple of AA's, auto advance/ retard and the works. Available from your local hobby shops.

 

[kustomkb]

I hear ya Pat, I really miss my Rocket 350. Like you said, now ya gotta pull the whole engine to get at anything, that is if you know what it is you want to get at.

I picked up the electronic ignition for its compact size. Also because I happened to have 70 bucks burning a hole in my pocket and not enough time to spare to educate myself with the alternatives.

 

[Ken I]

At the risk of boring everone....

A capacitor's impeadance decreaces with increacing frequency - at 0 Hz (DC) it is non conductive (apart from the initial space charge) an inductor's impeadance increaces with increacing frequency - at 0 Hz it is fully conductive (apart from its innate resistance).

The role of the capacitor on a conventional coil igniton......

If you were to simply switch off the coil the slowly collapsing magnetic field would generate voltage in both in primary and secondary windings (like a transformer) - the presence of the capacitor across the switch conducts this voltage - this imparts a load which causes the field to collapse faster which increaces the voltage and its slope (a'la frequency) which increaces the conductance of the capacitor - AVALANCHE ! (and of course once the arc occurs at the plugs - more load - more rapid collapse of the field).

This causes the field to SLAM down generating much higher voltages than the simple winding ratios would suggest.

Without the capacitor (condensor) the voltage is generally not high enough to generate a spark at the plugs and the system will not work - the condensor is not there to protect your points from arcing - it does do so but this is secondary.

Regards,
Ken

 

[Lakc]

Ok, so now you see the topic.

Steve:

I looked at the info in your link. Although it is very informative, it makes several statements that are incorrect, so I have to take the entire narrative with a grain of salt.

I wouldnt be too hard on the guy. There is a genuine effort there to translate some tricky electronic principles to an easier to understand level, a little artistic license can be granted.

For instance, he says that the fact that the current is being switched on and off to the coil makes if flow back and forth like AC current. This is not true, the currrent flows in one direction only in DC circuits.

Ignition systems are tricky, the important stuff actually occurs on an AC level. Coils (transformers) dont convert DC voltage and capacitors dont pass DC current but both occur here. Thus the sytem itself is actually converting DC to AC by nature of its switching.

The he states that when the coil is powered up, it stores near 30,000 volts.
This is incorrect. Coils don't store voltage. Voltage is a measure of the difference of potential between two points. Coils store power in the magnetic field.

Coils store power, most correctly measured in Joules of energy. Since the voltage those joules of energy are capable of are what were most interested in, ill give him a little license here on this one.

120 VAC is much more dangerous than high voltage. They actually use high voltage in all the defibrilator machines to save you.

I dont want to comment too far into the medical realm and away from my profession, but again its the joules of energy that are important, as well as what part of the body becomes the path of least resistance. Technically, anything over 32 volts is considered high voltage in many automotive circles.

The capacitor does not slow the colapse, it actually makes it much faster. That is just physics there.

Once you have the spark breakdown across the points you create an ionized atmosphere that lowers the gap resistance. At that point you have more energy discharging across the points that is taken from the energy of the collapsing magnetic field in the coil. The capacitor does slow down the collapse somewhat, but keeps the total energy in the collapsing magnetic field higher.

The capacitor is not designed to tune the harmonics of the system. They used tuned L/C, L/R, and L/R/C (inductive/resistive/capacitive) filters in your car radio to be able to separate channels.

Yes, but tuned radio circuits are for entirely different reasons and frequencies. Capacitors are "tuned" to an ignition system by selecting the appropriate value that keeps point wear to a minimum.

He says "a magnet passing by a coil wire creates a voltage current".
Wow, this guy is making up terms as he goes.

He could have done a bit better with that one, I agree.

Although much of the article appears to be correct, there is a serious amount of "voodoo science" mixed in there, and while it certainly sounds like serious science, he is not telling it straight.

If one does not really understand electricity, then one should not try and explain it in a confusing and incorrect way.
Not trying to argue, but the facts have to follow the laws of physics.

Pat J

As I mentioned earlier, the important stuff in an ignition system functions at an alternating current level, thats why they are tricky. He did do a wonderful job of pointing out the differences of a Kettering sytem from a CDI system. While most model ignition systems are CDI I only know of one OEM automotive use back in the 60's. When you have the space for a big oil filled ignition coil it doesnt make dollars and sense to use a more complicated CDI, so his comments on their popularity must refer to motorcycles.
As I begin to wrap up my boxxer twin, I will move back to the electronic side of things and probably bore a bunch of people with a thread on developing an ignition system.

 

[Lakc]

The definition of an AC circuit is that the voltage reverses polarity.
This is not the case in an automotive ignition circuit.
You can call a duck a chicken all day long, but a duck will always be a duck.
You can easily see the difference between AC and DC on an oscilloscope.
An AC circuit has some type of alternating waveform, either sinusoidal, square, or some other shape where the polarity is actually reversing on a regular basis.

AC has been described as varying DC, which is true, but DC circuits never cross the zero voltage line where the polarity would reverse.

Pat,
AC stands for alternating current, where the current flow reverses direction. Voltage is all relative here, I guess on this point we will have to agree we disagree.

 

[seagar]

I had one but the wheel fell off. :shrug: :wall: *club* :redface2: *bang*

 

[jpeter]

So place a voltage acrossed the coil primary and current flows through the coil building up the magnetic field in the core, right. Then the points open hoping to reverse the flow to collapse the field to generate flow in the secondary. The problem though is the primary circuit is not complete so the charges have no place to go. Thus comes the role of the capacitor. Acrossed the coil it acts like an accumulator for coil charges (coulombs) giving them a place during the discharge to collect. On top of that, while the coil was being charged with electrons the capacitor was being depleted of same so when the points open the capacitor sucks the charges out of the coil, collapsing the magnetic field even faster.

Notice, the current first flowed into the coil, then later flowed out of the coil. Sounds like AC to me.

Who can answer the question as to why the cheap electronic ignitions we use don't need a capacitor to work.

 

[Lakc]

Who can answer the question as to why the cheap electronic ignitions we use don't need a capacitor to work.

Every model igntion I have seen so far has been a capacitive discharge type. It uses an LC (Inductor(coil)-capacitor) tank circuit to first build an alternating charge which is dumped across the ignition coil to cause the spark.
As to why a transistor does not need the capacitor to work has to do with its structure, with some additional help from a few other components like zener diodes.

 

[Ken I]

Not all systems work the same way.

In most currenty used automotive ignition systems the LT side of the coil is permanently on (in some cases transistor switched) via a ballast resistor (part of the "ignitor" circuit) - the pulse signal to fire triggers a transistor which "shorts" the primary to earth (incoming power short current limited by ballast resistor) - this causes the magnetic field to collase rapidly (almost instantaneously) thereby generating the HT in the HT winding. This is of very short duration - if you try triggering these systems with points then the on time short circuit is far too long causing it to overheat and burn out in a minute or two.

Pat J - great vid of the HT switchgear - scary stuff.

The arc suppression chamber in larger circuit breakers (and some small ones) contains a series of metal plates (insulated from one another) to divide up and collapse the arc. An arc divided into 5 smaller arcs would require 25 times the voltage to sustain it, plus the plates cool and kill the ionising - normally zink plated as well - the zink oxide gas acts as an arc suppressor.

It follows that one should never install a circuit breaker upsidedown as the arc suppression will not work (I wonder what they do in spacecraft ?)

Regards,
Ken

 

[Admiral_dk]

Who can answer the question as to why the cheap electronic ignitions we use don't need a capacitor to work.

I experimented quite a bit with electronic ignition systems as a youngster (for my Moto-X racing bikes) and I can assure you that putting a high voltage capacitor across the ignition coil on the "simple" electronic ignition will increase the power of the spark. I got about 7mm. sparks in normal air from the system I build back then without the capacitor and a nice hot blue spark in a sparkplug - But with a 680nF 630v capacitor across the coil (value WILL depend on coil) I got spark in a sparkplug that could ignite heavy paper by putting it into the gap.

I can only guess that those who builds most of those simple electronic ignition systems you find on the net, don't have much actual knowledge of electronics and therefore are happy to share their experience with you, so you can get your IC engine running too, in order to save you from most of their own grief, from before they found a solution.

 

[picclock]

Wow this is hot topic. Just to add my two cents.. .

When the points open the coil and capacitor form a series resonant circuit which results in the voltage and current being out of phase for the capacitor and coil. The primary side of the coil capacitor junction rings up to around 400 volts (collapsing magnetic field), with the frequency being controlled by the primary inductance and capacitor value. Making the capacitor bigger reduces the resonant frequency. This will generate a higher current at a lower voltage (looks better but may not be so), leading to increased plug wear. Under compression it is more difficult for a spark to jump across the plug gap, so it must be long enough to ignite the fuel mixture and of sufficient voltage to arc despite the increased pressure. The current rise in the inductor when the points are closed is limited by its inductance, which limits the frequency of ignition sequences (causing weaker sparks at high rpm).

CDI ignition systems bypass the primary resonant stage and switch a 400v charged capacitor across the coil directly turning it into a parallel resonant circuit. This results in far less power being wasted, and as the 400v is normally regulated against supply voltage variation it is available even at starter cranking voltages. The upper frequency of ignition sequences is also far higher as the time to charge the capacitor is set by the power of the charging circuit used.

Hope this helps a bit - if you don't believe me just get a scope and have a look.

Best Regards

picclock

 

[Lakc]

Wow this is hot topic. Just to add my two cents.. .

When the points open the coil and capacitor form a series resonant circuit which results in the voltage and current being out of phase for the capacitor and coil.

I had been avoiding using that terminology mainly because I was too lazy to think it through again. I am glad you did. Kudo's to you!

The current rise in the inductor when the points are closed is limited by its inductance, which limits the frequency of ignition sequences (causing weaker sparks at high rpm).

Indeed, and its the inductance that gives it the spark energy in the first place, hence the tradeoff. You know were talking science now, as it seems there is no science without a tradeoff somewhere.

CDI ignition systems bypass the primary resonant stage and switch a 400v charged capacitor across the coil directly turning it into a parallel resonant circuit. This results in far less power being wasted, and as the 400v is normally regulated against supply voltage variation it is available even at starter cranking voltages. The upper frequency of ignition sequences is also far higher as the time to charge the capacitor is set by the power of the charging circuit used.

I have very little practical experiance with CDI systems, but hope to be exploring that area soon.

Hope this helps a bit - if you don't believe me just get a scope and have a look.

In about 1.5 cylinder heads, 4 lifters, 2 spark plugs, and with a little help from some friends at Amidon, I hope to do just that.

If anyone is interested in the whacky world of voltage leading the current in inductors, current leading voltage in capacitors, and series or parallel resonant circuits, I would recommend picking up any old copy of the ARRL amateur radio handbook. Its the machinery handbook of the radio world, and published yearly for nearly as long.

 

[Ken I]

My prior explanations were simplistic, picclock's lucid L/C explanation is excellent but I would not consider it a resonant circuit (at least not in the normal sense), the ringing current between the inductor and capacitor is going to be an A.C. frequency determined by the LC relationship - however the first upslope is the one that fires the plug - subsequent waves diminish in amplitude by orders of magnitude and are not significant.

This link has a good explanation as well as osillcoscope traces.

http://mgaguru.com/mgtech/ignition/ig108.htm

The problem with complex subjects is that simple explanations are usually "wrong" in some way.

 

[Admiral_dk]

I thought that we where still keeping it at the simple level ....

If not, I start by stating that no system exist without a capacitor across the coil. You might not put one there, but it's there anyway, in the shape of what's called stray capacitance - mostly in the coil, but in the rest of the circuit too. So there's a parallel resonant circuit in the coil and most serious non CDI electronic ignition systems have a capacitor across it to tune this to max power in the spark gap. It's most certainly a resonant circuit, but as long as the spark fires, it's a very damped circuit - if it doesn't fire, it will ring for quite some time.

There's some good info on this page : http://www.daytona-twintec.com/tech_ignition.html

Go a head and have a laugh at the obvious error about external combustion ;D - he's talking about an external way of igniting the internal gas, but using the wrong term. Other than that and the fact that he' referring things to a Harley Davidson, it's the only place I found on the net where someone explains what goes on in the sparkgap in relation to the rest of the circuit + the fact that you need a scope of a much higher quality than mine, to measure what he's describes.