Restoration of 1952 MG TD 2

Author: Bob McCluskey
First posted: 1 Sept 2000
Last amended: Dec 2015
Please email Bob McCluskey
Car No TD/11935
Engine No XPAG/TD2/12333
Body Type 22381
Body No 11301/78948

P HE COIL This would be perhaps the least understood part of the system. A good deal of confusion surrounds the entire ignition system, and especially the subject of the polarity of ignition coils in cars with positive earth systems. Most cars these days have -ve earth, but our cars still have +ve earth. Coils these days are marked + and -. Conventional wisdom is that, in the case of positive earth, you should connect the + terminal to ground and the - to the live (negative) terminal of the battery. In this chapter I argue, with some diffidence, that the conventional wisdom is wrong, that a number of papers which I believe are poorly researched and misleading have been printed and disseminated, and, worse, believed, and that the errors have been reproduced and reprinted so many times that they have acquired the patina of truth. I am aware of the dangers of contradicting the dominant paradigm, because "... in the multitude of counsellors there is safety" (Proverbs 11:14 KJV). On the other hand, "The simple believeth every word; but the prudent man looketh well to his going" (Proverbs 14:15 KJV). Moreover, "There is nothing so absurd that it cannot be believed as truth if repeated often enough" (William James 1842-1910), and "The fact that an opinion has been widely held is no evidence whatever that it is not utterly absurd; indeed in view of the silliness of the majority of mankind, a widely spread belief is more likely to be foolish than sensible" (Bertrand Russell, 1872-1970). These philosophical insights have been verified experimentally in scholarly works: have a look at Solomon Asch's work on conformity, and Hasher and Goldstein's work on how the repetition of more or less plausible statements affects one's judgement of their truth and validity.

So I thought I would look at the whole issue again from first principles.

Long ago, when I first when I first had to come to grips with the mysteries of these ignition systems, some cars had positive earth systems and some had negative. All coils were marked SW and CB: SW went to the battery via the ignition switch, and CB went to earth via the contact breakers. Easy. They didn't have separate coils for +ve or -ve earth, and it didn't matter whether you had positive or negative earth: you bought your coil, and you wired SW to battery via the switch, and CB to earth via the contact breaker. If you had a positive earth, the current in the primary went one way, and if you had negative earth, it went the other way. It didn't matter. What did matter was how the internal windings of the coil were connected relative to the battery and contact breakers.

Nowadays, although some cars, mostly British classics like MGs, still have positive earth, almost all cars have negative earth, and instead of ignition switches and contact breakers they have things called engine management systems. If the coils were still marked SW and CB, the engineers wouldn't know how to connect them, so to make it easy for them modern coils are simply marked + and -. All the manufacturers have done is replace the SW symbol with +, and the CB with -. This has lead to the confusion that exists for those of us who still have cars with positive earth (see eg mgaguru's 2009 paper and about 1000 others). When I went to buy my new coil, I was told by everyone either that it didn't matter how it was connected, or that the - terminal should go to the negative battery terminal via the ignition switch and the + terminal should go to ground via the contact breaker. Unanimously. But they were wrong: if it didn't matter, the manufacturers wouldn't have bothered to mark the coils at all, either with + and -, or with SW and CB. There is clearly a difference, and you can see the difference by drawing the wiring inside and outside the coil, and consider how it would look if it were connected firstly - to earth, and then + to earth.

The coil is a step-up transformer. It consists of a laminated iron core surrounded by two coils of copper wire wound in series. The primary winding has relatively few turns of heavy wire. The secondary winding consists of thousands of turns of smaller wire, with a turns ratio of perhaps 100:1. The two coils are wound in the same direction, and the top end of the primary and bottom end of the seondary are connected internally and brought out to the - terminal. The windings are insulated from the high voltage by enamel on the wires and layers of oiled paper insulation. The entire assembly is usually sealed in a metal can with insulated terminals for the high voltage and low voltage connections. The can itself isn't part of the circuit, and although it's usually grounded by its fixing, this is not necessary. One of the low-tension leads is connected to the battery via the ignition switch, and the other to the contact breaker points in the distributor - which lead goes where is a vexed matter for those of us with positive earth cars.

In this diagram below, the battery is marked + and - in red; the + and - in black refer to the markings on the coil, and the red arrows show the direction of conventional current in the primary. The first diagram below shows what most people will recognise: the battery connected -ve to earth, the + terminal on the coil connected to the ignition switch and the - terminal connected to the contact breaker points.The second figure shows how it looks with the battery reversed to give a +ve earth, but the coil still connected with its + terminal connected to the ignition switch; and the final figure shows the coil also reversed, with its - terminal connected to the ignition switch.

Consider the "conventional" case first, with negative earth, and the + symbol on the can connected to the positive battery terminal, via the ignition switch. While the contact breaker points are closed, current flows from the battery through the primary winding of the ignition coil. This current builds up relatively slowly, due to the inductance of the coil, and builds up a magnetic field in the core and in the air surrounding the core. At some stage, once the primary current has built up to its full level, the contact breaker points open and the current is interrupted, and as we've seen the timing of this interruption is critical. The magnetic field collapses rapidly, thanks to the capacitor, and generates a back emf in the primary of about 300V, that is to say the voltage across the now open points increases rapidly to 300V. But here's the thing: because the magnetic field is collapsing, not expanding, it cuts the the primary coil in the opposite sense, and the back emf is negative, not positive. Because the turns ratio in the coil is about 100:1, it generates a much bigger voltage at the secondary - about -30kV (30,000 volts) (source: Wikipedia). And something else: because the secondary winding is wound in the same direction as the primary, the voltage at the primary is added to the 30kV on the secondary. But because the turns ratio is about 100:1, the voltage increase is only about 1% - not really material.

This diagram shows the possibilities for connecting a coil marked + and - into a conventional ignition system. Figure A shows what has become the conventional connection, with negative earth and the + terminal connected to the live positive battery via the ignition switch; Figure B shows the battery reversed, so the + terminal on the coil is connect to the -ve battery terminal,and Figure C shows the coil also reversed, so the - terminal on the coil is connected to the -ve terminal on the battery, again via the ignition switch. Conventional current is marked with a red arrow.
Now consider Fig B - the first possibility facing TD owners. The battery is reversed to give positive earth, but the coil is left unchanged. In this case, everything is the other way. When the points are closed, conventional current flows the other way in the primary - from earth, through the contact breaker, through the primary the other way, and into the negative battery terminal through the ignition switch. The magnetic field has opposite polarity, and when it collapses, it cuts the coils in the opposite sense, generating a positive pulse at the output of about 30kV plus the voltage from the primary. If you were able to look at the waveform with an oscilloscope, it would be exactly inverted.

And Fig C (the second choice facing TD owners), keep the battery with positive earth, and invert the coil so the the + terminal goes to ground via the contact breaker, and the - terminal goes to the live negative battery terminal via the ignition switch. Now the original polarity is restored: conventional current flows from graound via the contact breaker, through the primary coil to the negative battery terminal via the ignition switch. The magnetic field is established with the same polarity as originally, and when the points open and the field collapses, it will produce a negative pulse. But in this case, the primary voltage is not added to the secondary voltage: the voltage available for the spark is about 1% lower - not really material.

If you wire it one way, the current in the primary goes in one direction, and if you wire it the other way, the current is reversed. In one case, when the spark forms electrons flow from the HT lead, and in the other, when it forms, electrons flow towards the HT lead (in his paper, mgaguru describes a test with a graphite pencil, which he concedes is hard to interpret, to decide which way the electrons are flowing. I came across references to the same test in a few places (eg, none of them attributing the reference to another.

Conventional wisdom is that you want a negative going high tension pulse - ie you want the spark to be initiated by electrons flowing from the HT lead - because the initial spark is supposed to be caused by electrons, and they are supposed to flow more easily from the hot centre electrode to the cooler surrounding earth. (In fact mgaguru thinks that because the electrons flow more easily from the hot centre electrode, the spark forms with a 15-30% lower voltage. You can find many references to this, none of them giving a reference to support the assertion - mgaguru's paper is only one example. I could find no reference to measurements supporting the 15-30% reduction, just the assertion, often repeated, that it happened: you would think that such an important difference would have been well documented, but I could find no evidence.)

Anyway, I was initially attracted to this idea.

But search as I would, the only explanation I could find for electron emission from a hot surface was the so-called thermionic effect, which was discovered in 1873 by Frederick Guthrie in Britain, developed into a patentable device by Thomas Edison in 1883, and finally explained by the British physicist Owen Richardson in 1901, for which he received the Nobel prize in 1928.

Thermionic emission arises because some of free electrons which exist inside all conductors will have more energy than others, and, under some conditions, usually when the conductor is heated, some will even have enough energy to escape from the conductor altogether. The energy needed to escape is called the work function. If the conductor is heated, more electrons will have energy that approaches the work function, but in most cases, the work function is prohibitively high, preventing escape. For example, for ordinary metals (eg tungsten, whose work function is 4.5 eV) the thermionic effect doesn't begin until about 2500 deg k (2200degC) - bright white hot. For nickel, commonly used as the centre electrode in spark plugs, the work function is even higher, at 4.6 eV.

Thermionic emission is the basis for operation of thermionic valves, such as those used in expensive valve-operated hi-fi amplifiers. The cathodes in thermionic valves have been coated with a metal oxide, usually barium oxide, to reduce the work function; even so, the thermionic effect is not significant for temperatures less than about 900-1000degC (bright red hot), which is why the valves glow red hot. Spark plugs are not so coated. The operating temperature for plugs is typically in the range 450 - 870 deg C, well below red hot, well below the temperature at which the thermionic effect is significant, even for coated surfaces.

Moreover, the car starts even on cold winter mornings when the plugs are cold, and that should tell us that the thermionic effect is not a factor in initiating the spark.

In fact, the spark is initiated by plasma, formed when the electric field exceeds the so-called Dielectric Breakdown Strength, which is when the field is strong enough to strip electrons from the atoms and molecules in the mixture. The polarity of the field is not important. It is only the field strength that matters: whether positive or negative, the field will be strongest near the centre electrode, where field lines will be more concentrated (compare this to lightning conductors, where the spark is also caused by dielectric breakdown of air, and where, like our spark plugs, the lightning spark is initiated where the field is strongest, near the tip). In other words, the spark is not initiated by electrons flowing to the central electrode, or from it; it doesn't matter whether whether the HT electrode on the coil is initially positive or negative.

Here's an interesting experiment: the dielectric breakdown strength in air is about 30kV/cm. The voltage from the coil is about 30kV. That means you should be able to draw out a spark about 1cm long. Try it. Try reversing the coil. No difference?

From consideration of the initiation of the spark, it doesn't matter which direction the current flows in the primary coil.

It takes only about 10-14kv to fire the mixture under operating conditions - less than half of the 30kV available, so even if it were real, the 10-15% advantage claimed for for reversing the coil would be immaterial. Once the spark is struck, its polarity doesn't matter, and it makes at least a few cycles of alternately positive and negative excursions before the energy initially stored in the coil's primary is dissipated. (This would explain why the graphite pencil test described in mgaguru's first paper is hard to interpret, since during one excursion electrons will be flowing in one direction, but in the next they will be flowing in the opposite direction: in one excursion the flare will be between the pencil and ground, and in the next, between the pencil and the HT lead. The excursions are so rapid that it looks as though the flare is going both ways.) This oscilloscope trace shows the ringing when the points open, causing the spark; the second, lesser, ringing is when they close again, to allow the current to build up in the primary. Other images and discussion can be found easily; the image on the right comes from Steve Maas's paper, referenced earlier. And although it's commercially driven, this one is a good resource. See also this thesis which was prepared for a MSc degree.

After all this, what is the correct way to connect the coil for a positive earth system? If after all you still believe in thermionic emission, you will connect it with the - terminal to battery negative terminal, via the ignition switch. Otherwise, you will connect it with the + terminal to the ignition switch. That's what I did. The car starts easily and works well, and any remaining problems must be due to some other cause. We will see.

"Man is a credulous animal, and must believe something; in the absence of good grounds for belief, he will be satisfied with bad ones". Bertrand Russell, Unpopular Essays (1950), "Outline of Intellectual Rubbish"

I happen to believe I've spent enough time on this, and I believe that now I'm going to have another beer.

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