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 - apart from the voltage regulator. 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. Long ago, 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. Most cars these days have -ve earth, but our cars still have +ve earth. Coils these days are marked "+" and "-". Commonly accepted wisdom these days 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.

The coil is simply 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, and the secondary winding consists of thousands of turns of smaller wire, with a turns ratio of perhaps 100:1. The windings were originally insulated from each other by enamel on the wires and layers of oiled paper insulation and sealed in oil. Nowadays many coils are encapsulated in epoxy resin. 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 end of the primary and one end of the secondary are connected internally - the common point - and brought out to one of the external low-tension connection. The other end of the primary is brought out to the other low-tension terminal, and the other end of the secondary is brought out to the high-voltage insulated terminal on the top. One of the low voltage terminals is connected to the battery via the ignition switch, and the other the contact breaker points in the distributor and thence to ground - which lead goes where and what it means is the vexed matter for those of us with positive earth cars.

Originally the low-tension terminals on all coils were marked SW and CB: the common point was connected to the terminal marked CB, and the other end to the terminal marked SW. SW went to the battery via the ignition switch, and CB went to earth via the contact breakers on the distributors. 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.

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 "-". The common point is connected to the terminal marked "-", and the other end of the primary is connected to the terminal marked "+". All the manufacturers have done is replace the SW symbol with "+", and the CB symbol with "-". But this change is what 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 many 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 first diagram below comes from my copy of the Work Shop Manual (page N22) and shows how the manufacturers expected their coil to be connected. SW is the free end of the primary coil and goes to the -ve battery terminal, via the ignition switch; the common point is brought out to the terminal marked CB and goes to the +ve earth, via the contact breaker points. The second diagram shows a modern coil. The coil is identical, except that the terminal to which the common point is brought out is marked "-". The second diagram here shows how a system with +ve earth would look if you bought a coil today, with "+" and "-" markings, and hooked it up with "+" going to earth via the points, and "-" going to the battery via the switch. Different. Not what the manufacturer intended.

It seems so simple and obvious - why would anyone wire it the second way? Apart from the obvious confusion caused by connecting the "+" terminal to the negative pole of the battery and the "-" terminal to the +ve earth via the contact breakers, there is an argument concerning the polarity of the spark.

When the contact breaker points are closed, a current flows through the primary winding and establishes a magnetic field which also intersects the secondary. When the points open, the current ceases to flow, and the magnetic field collapses and induces the large voltage - about 30kV - in the secondary. If you wire it one way, the current in the primary goes in one direction, and the magnetic field has one polarity; and if you wire it the other way, the current is reversed and the magnetic field has the opposite polarity. In one case, when the spark forms, electrons flow initially 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 (and often the same text) in a few places (eg, none of them attributing the reference to another. A better test is described in US Patent4740745A.

Today's 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 eg, 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 electrons leaving 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. It's probably beyond the scope of this paper to try to explain thermionic emission; but have a look at this paper.

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 increase this electron emission; 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. For nickel, commonly used as the centre electrode in spark plugs, electron emission is not significant until about 2500 deg k (2200degC) - bright white hot. Sufficient to say that for electrons to be emitted, the surface has to be hot. The operating temperature for plugs is typically at the low end of 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. But if the plugs were red hot, pre-ignition would be a problem. At white hot there would be serious engine problems. Moreover, the car starts even on cold winter mornings when the plugs are cold, and that alone should tell us that the thermionic effect is not the most important factor in initiating the spark.

Nevertheless, you can find many papers claiming that "it has been shown" that you get this advantage from this hot centre electrode phenomenon. I wrote to several of these authors, asking for references to the original experiments, or indeed to any data supporting it. I received only one answer, from an author who strongly favoured electron emission, and who said, in part: "For as many times as people have claimed there is a difference, I have yet to see any documentation to back it up. Not that it can't be true, just that I have never seen the data. So far no one I know has ever proved to my satisfacton that there is or is not a bias... Without the data it ends up being a religious war. I may be chatting with someone who has religious conviction that could not possibly be swayed by facts and sound logic. No matter how much I may encourage this person to present a logical explanaton of his view there will never be any such expanation...people with a religious conviction will never see it that way (since they can never be wrong)". This from one of the major protagonists!

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, and that is where the spark will be initiated. 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.

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

The dielectric breakdown strength in air is about 30kV/cm. So it takes about 1.5kV to reach the dielectric breakdown strength across the 20thou sparkplug gap. It takes rather more - about 10-14kV - to fire the mixture under operating conditions, but still 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 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 probably connect it with the "-" terminal to battery negative terminal, via the ignition switch - but I think you will be disappointed if you expect to see any advantage, still less a 10-15% or 15-30% advantage. Otherwise, you will connect it with the "+" terminal to the ignition switch, and the "-" terminal - the common point - to the contact breakers, like the manufacturers. 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 go and have another beer.

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