45 replies to this topic

[gtrboyy]

You could always use 2 coils,fill 1 in with epoxy resin,wire 1 normally & the second pos to pos,neg to neg from the 1st coil.Apparently it was done on old jags,I read about a long time ago in a mag.But I think a normal electronic dissy & single coil would be enough for most engines.Anyone else seen or read about this?

Edited by gtrboyy, 01 February 2006 - 05:03 PM.

[Tiny]

Constant spark cannot work. You need teh spark to be timed at exactly the right moment otherwise you'll ignite the mixture at is comes in!

Plus you'll burn the plugs out i reckon.

MSD type units amplify the power going to the plugs so that the sparks are more bright ( Higer amperage), and they also release multiple sparks per cycle, but begun at the right time so as not to do what i mentioned above.

I have no doubt that anyone with some electronics skill could make one, infact i think Jaycar have a HEI type kit for cars?

Cheers.

[antelopeslr5000]

I'll try and simplify the operation of a MSD.

To keep it simple let's say that the MSD basically performs 2 functions.

1. Acts as a separate battery, or power supply, for the coil
2. Effectively becomes the points system for the coil (I'll explain this later)

Now, where to start....

While the MSD acts as a separate battery for the coil, it's definitely no "ordinary" battery. The MSD is a battery designed to discharge rapidly, a battery of very little internal resistance. A battery with high internal resistance cannot supply a useful amount of power to an external load. So the fact that the MSD has low internal resistance is a good thing and the fact that the MSD has to power only one external device (i.e. the ignition coil) makes it even better.

But wait, there's more...

The output voltage of the MSD is not the same as the car's battery voltage. The voltage of a normal car battery maybe in the range of, I don't know, let's say 10-15 volts for the sake of argument. The output voltage of the MSD is in the range of 450 volts. A big difference, don't you agree?

So the primary winding in the coil is now receiving 450V rather than the normal 10-15V. This results in peak voltage on the secondary winding (a good quality coil can really take advantage of this) and a much stronger spark and more successful combustion of the air/fuel mixture in the cylinder.

Just a word of warning, if you intend on using a MSD don't connect anything to the positive side of the coil other than the orange wire from the MSD! (it's not +12V as you are aware of now)



Moving on to the next point...

The MSD replaces the points in the distributor too, well as far as the coil is concerned anyway! The MSD still needs to receive a signal from the distributor as the MSD still needs to know the correct time to discharge so that the cars timing will not be affected (i.e. when the points open). So once the MSD receives the signal from the distributor, it discharges its 450 volts of fury to the coil!

So as you can see the distributor no longer commands the coil when to discharge, that is now the job of the MSD. All the distributor does is instruct the MSD the correct time to fire, it doesn't instruct the MSD to fire the coil. It's up to the MSD when it decides to discharge the coil. I guess what I'm trying to establish here is the coil is now independent of the distributor.

As the MSD is rather efficient at its job, it performs the task of firing the spark plug extremely fast, so fast in fact that it can perform this task multiple times before the next firing stroke (also know as Multiple Spark Discharge!). At low rpm, the MSD is able to achieve multiple discharges before the next firing stroke. As the engine's rpm increases it has less and less time between firing strokes so less multiple discharges are achievable per firing stroke. Once the engine has exceeded a certain rpm, the MSD is no longer able to achieve multiple sparks as the time between the firing strokes is now not long enough. (even for an efficient MSD!)

Well, that it basically. I hope you have all enjoyed the read!

[Tiny]

Thank you VERY much for that!! Certainly learned alot i did!!

Cheers!

[antelopeslr5000]

Well no, it wouldn't, because the coil is an inductor!

The internal inductance of the coil limits the current. As you are probably aware, inductance can be simply defined as a coil of wire resisting any changes in electric current flowing through the coil.

Let's say you were to increase the current in the coil. The increasing current flowing through the wire in the coil would in return create a counter electromotive force (emf) which would oppose current flow (Lenz's Law). So you can see, the coil regulates its own current flow (as all good inductors should!).

Hope that clears things up.

[antelopeslr5000]

Well I'm not too sure what you are trying to say here but I try to explain in more depth...

A coil which has no voltage applied to it has no current flow. If a voltage is then applied to the coil then the current must change from zero state to non zero state. Now because there is current flowing though the coil, the current induces a magnetic field. The change in this magnetic field induces a emf that is in the opposite direction of the change in current (i.e. a force that opposes current flow). The strength of the emf is proportional to the change in current and the inductance. So when these opposing forces are in balance, the result is a current that increases linearly in time where the rate of change is determined by the applied voltage (& inductance).


The current changes in a linear manner at a rate proportinal to the applied voltage.

So what that means is that by applying a higher voltage to the coil, the coil current reaches peak value at a faster rate.

Does that explanation help at all?

[antelopeslr5000]

OK, I think I'm on the same wave length now!

Using the formula V=IR

Assuming resistance remains unchanged then increasing the voltage would increase the current.

That's all well and good for a DC steady state because the inductor is seen a short circuit.

However the coil is subjected to a switching DC voltage so the impedance of the inductor (or coil) comes into play. The impedance of the coil is dependent on frequency or in this case the speed of the 'switching' DC voltage to the coil.

So now V=IR + EMF (where EMF opposes the applied voltage)

Therefore as applied voltage increase so does the counter EMF.

If V is increased, so does EMF.


The output voltage to the coil from the MSD is very unlikely to reach a DC steady state with a discharge time between multiple sparks of about 1 millisecond. Even if the points are closed in the distributor there will not be a constant current flow through the coil. As I explained in my earlier post, the coil is independent of the distributor once the MSD is installed.

The MSD also contains a step up tranformer to achieve the 450V output. So I guess the MSD can also be seen as a 'pre-coil' to the coil (that sounds confusing!). It steps up the voltage from the car's battery from 12V to 450V and then stores this voltage in a capacitor. When the MSD receives the timing signal from the distributor it then discharges its capacitor (in only 1 millisecond) to the primary winding of the coil.

Edited by antelopeslr5000, 17 February 2006 - 04:21 AM.

[Dangerous]

Antelope, I think your explanation is mixing the concepts of a couple of different types of ignition system. Ohm's law (V=IR) tends not to be applicable with inductive and capacitive loading either, but the general concept is still sound.

"MSD" is as you say a multi spark discharge system. The benefits of this are that because the spark is turned on and off many times in a single combustion, if the combustion mixture blows out a spark, there will be another one created immediately to ignite the mixture. It can be done with a conventional electronic ignition by simply rapidly turning on and off the main power transistor that takes the place of the points. It's not a worthwhile modification to a conventional electronic ignition though, as the whole system has to be designed with the MSD concept in mind.

Most of what you have described is a capacitor discharge ignition, or CDI. These systems make a very high voltage spark, and can recover spark energy quickly enough to allow them to be used in very high revving engines. I would say that most motorbikes these days would be CDI ignition, to help raise the upper rpm redline reliably. A CDI can be added to a conventional points system, or designed into an aftermarket electronic ignition system. The reason that CDIs are better at high revs is that they use a capacitor to store the energy which will be used to make a spark, not a rather large inductive coil. The capacitors can be charged more quickly than building up the magentic field in the coil. CDI units usually take the car's 12 V DC, convert it to high frequency AC, convert it back to high voltage DC, around 400 to 500 volts, and charge a capacitor with the high voltage DC.

A CDI system uses the ignition coil in a very different fashion to a conventional electronic or points system. Where a conventional system builds up energy in the coil in between sparks, then discharges it for a spark, a CDI system builds up energy in a capacitor, or more than one, and leaves the coil 'empty'. When it becomes time to fire a spark, the capacitor is discharged very rapidly into the coil, which amplifies the discharge voltage of the capacitor, and creates a very high voltage spark. The CDI spark is usually inherently a multi spark, as the energy from the capacitor resonates between the capacitor and coil, and this can be enhanced by rapidly connecting and disconnecting the capacitor from the coil.

Although CDI's are very good for high revs, sometimes the spark voltage is so high that it breaks down the insulation on the other ignition components that are designed for a lower spark voltage electronic or points system, and a misfire occurs. It was also found during the initial emergence of CDIs in road cars in the 80s and 90s that too much voltage caused the spark to arc all the way to the piston crown, not just to the spark plug electrode. I haven't looked a modern high end CDI systems for a while, but I suspect that they would be managing these problems by tailoring the spark voltage to only what is needed, not what the system is capable of.

It's only my opinion, but I wouldn't use a CDI system on a "road" car - it's not suited to it. It would be a different matter on a high revving, high compression, performance vehicle though.

[antelopeslr5000]

That's right Torana428HP. All the MSD needs is a trigger signal to charge the coil so it is possible that the engine may fire if the ignition is on.



Thanks for the info Dangerous.

True, V=IR is not really applicabe with inductive and capacitive loading, but I was trying to keep it simple.

I'm just a little confused when you say I'm mixing the concepts of different types of ignition systems?

[antelopeslr5000]

Antelope, I take it this battery you are talking about is the capacitor in a CDI ignition system? 

Yes it is. Sorry for the confusion! Like you said I did 'dumb' it down for ease of explanation. Probably too much which can cause problems for the more technically minded folk such as yourself.

The MSD is a capacitive discharge ignition (CDI) system. Well it is actually a multiple spark system combined with a capacitive discharge ignition.

I guess that's why I was a little confused when Dangerous said that my explanation was mixing the concepts of a couple of different types of ignition systems.

Edited by antelopeslr5000, 17 February 2006 - 10:12 PM.

[Dangerous]

Antelope, I take it this battery you are talking about is the capacitor in a CDI ignition system?�

Yes it is. Sorry for the confusion! Like you said I did 'dumb' it down for ease of explanation. Probably too much which can cause problems for the more technically minded folk such as yourself.

The MSD is a capacitive discharge ignition (CDI) system. Well it is actually a multiple spark system combined with a capacitive discharge ignition.

I guess that's why I was a little confused when Dangerous said that my explanation was mixing the concepts of a couple of different types of ignition systems.

The reason that I said your explanation seemed to be mixing concepts of different types of ignition is that your explanation started with

"I'll try and simplify the operation of a MSD."

and then went straight into

"To keep it simple let's say that the MSD basically performs 2 functions.

1. Acts as a separate battery, or power supply, for the coil
2. Effectively becomes the points system for the coil (I'll explain this later)"


An MSD, ie a generic non brand specific "multi spark discharge" ignition system does not perform the first of the above two functions. A CDI, or capacitive discharge ignition system does not necessarily perform the second of the above two functions either. Your explanation implied that all MSD systems are also CDI systems, which they aren't. Only the higher spec MSD (as in MSD the company) ignition systems do perform both.

Fair 'nuff?

[antelopeslr5000]

Oh, OK, now I understand.

I was explaning the function of the MSD manufactured by Autotronics, not a generic non-brand. MSD is a brand name of the Autotronics Controls Corporation. I should have made that clear from the start.

Sorry again for the confusion.