by Peter Cole and Eric Worpe
The ignition system used in our cars was invented by Charles Kettering in 1908. Fig 1 shows part of his original patent submission. His ignition system was used in nearly every car made worldwide exactly as he described it from soon after its invention until the mid-1980s. The Kettering ignition is an extremely simple and reliable arrangement consisting of an ignition coil, a condenser (see Note 1 at the end of this article) and a contact breaker, more commonly referred to as ‘the points’. Prior to that time cars used a magneto to produce the ignition voltage. Whilst the magneto was significantly more expensive to manufacture, it had the advantage that it could produce a spark without a battery. More recently, refinements were made to Kettering’s ignition system such as adding a transistor to extend the life of the points or adding a resistor in series with the coil primary, which could be shorted out to aid starting. Essentially however, even with a few tweaks, these ignition systems are directly attributable to Kettering.
The ignition coil consists of two windings, the low voltage primary and the high voltage secondary winding. The ratio between the primary turns and secondary turns was typically between 1:50 on early coils and up to 1:150 on modern coils. The coil produces the spark voltage by transforming the 400-500 volts across the condenser and the open points up to the 10 – 20kV required to produce a spark at the plug.
Our cars were originally wired with the positive terminal of the battery connected to the chassis, which is termed ‘Positive Earth’ or ‘Positive Ground’ in the States. The coil terminal marked ‘SW’ is connected to the negative 12V battery supply via the ignition SWitch and the terminal marked ‘CB’ is connected to the Contact Breaker which, in turn, connects the CB terminal to earth when the points are closed. The primary and secondary windings are connected together inside the coil and are designed to produce a negative spark voltage (see Note 2 at the end of this article). A negative spark voltage is desirable because the spark plug gap will break down at a lower negative voltage than it will with a positive voltage. This makes use of the ‘thermionic’ effect whereby the hotter central electrode of the spark plug will emit electrons more freely than the cooler spark plug body. Hence a negative spark voltage will provide a stronger spark. However, some cars such as the Citroen 2CV use a coil where both ends of the secondary are accessible. Here, one end of the secondary is connected to one plug and the other end is connected to the other plug. The benefit, at least in terms of cost, is that there is no need for a distributor, but one plug has a negative spark voltage and the other a positive spark voltage. So, whilst a negative spark voltage is desirable it is clearly not essential.
So, this raises several questions: Does a T-Type with positive earth, using an original Lucas Q12 coil, produce a negative spark (see Fig 3)? What happens if the battery polarity is reversed? Does a modern car, with negative earth produce a negative spark (see Fig 4)? What happens if a modern coil with terminals labelled + & – is used to replace an original coil with terminals marked SW & CB in a positive earth T-Type (see Fig 5)? Will it still produce a negative spark? What happens if the coil connections are then swapped (see Fig 6)?
Any of these possibilities may be encountered in a T-Type today and possibly more than one. Swapping the polarity of the battery is commonly done to accommodate modern electronics such as a Sat Nav or LED light bulbs. Swapping to a modern coil is becoming increasing likely as original Lucas coils, some of which may be more than 75 years old now, reach the end of their useful life.
To answer these questions, it is necessary to look at the ignition circuit in more detail and to consider some of the coil, coil connection and battery polarity permutations most likely to be encountered. In doing so it might be helpful to refer to Fig 2 which shows the internal construction of a typical ignition coil and how the primary and secondary windings are arranged. Fig 2 is reproduced by kind permission of Robert Bosch GmbH and is taken from their publication Ignition Systems for Gasoline Engines. (ISBN 3-934584-63-2)
Key to Fig 2:
1 High voltage output
2 Inter-layer paper insulation
3 Coil terminal insulator
4 Interconnection between secondary start and high voltage output
6 Wide strap to fix coil to the car and aid cooling
7 Outer magnetic core
8 Primary winding
9 Secondary winding
10 Filling, usually asphalt
11 Base insulator
12 Inner magnetic core
1) T-Type with Positive Earth, using the original Lucas Q12 Coil wired as intended.
Referring to Fig 3, the finish of the primary winding SW is permanently connected to the negative 12 volt battery supply via the ignition switch. At the start of the ignition cycle the points close connecting the start of the primary winding CB to earth. Current starts to rise in the primary towards a level determined by the winding inductance, the winding resistance and the duration of the dwell period. As the engine speed increases the magnitude of the primary current at the end of the shorter dwell period will be lower, and hence there is less energy stored in the coil to produce a spark.
At the end of the dwell period the points are opened by the cam on the distributor drive shaft. The primary current then charges the capacitor across the points. The primary inductance forms a tuned circuit with the capacitor to produce a voltage across the points of around 400-500 volts. This is transformed by the coil to provide the voltage to necessary to create a spark at the plug. The spark voltage will be in the range 10 – 20kV depending on conditions inside the cylinder. The spark voltage will be NEGATIVE and the voltage at the points augments the secondary voltage.
Note that if the connections to the SW and CB terminals of a Q12 coil or the battery polarity are swapped the spark voltage will be POSITIVE. Swapping the SW and CB connections of the Q12 coil after the car has been converted to negative earth would revert to a negative spark but don’t do it! Note the warning in the conclusion later in this article (after the commentary on Fig 6)
2) Modern car with Negative Earth, using a modern coil.
Referring to Fig 4, the primary start (labelled +) is connected to the positive 12 volt battery supply via the ignition switch. At the start of the ignition cycle the primary finish (labelled -) is connected to ground via the points. As the points open the secondary spark voltage is NEGATIVE but the primary voltage does not augment the secondary voltage.
3) T-Type with Positive Earth, using a modern coil.
Referring to Fig 5, the primary finish (labelled -) is connected to the negative 12 volt battery supply via the ignition switch. At the start of the ignition cycle the primary start (labelled +) is connected to ground via the points. As the points open the secondary spark voltage is NEGATIVE and primary voltage augments the secondary voltage.
4) T-Type with Positive Earth, using a modern coil, with the connections swapped.
Referring to Fig 6, the primary start (labelled +) is connected to the negative 12 volt battery supply via the ignition switch. At the start of the ignition cycle the primary finish (labelled -) is connected to ground via the points. As the points open the secondary spark voltage is POSITIVE but the primary voltage does not augment the secondary voltage.
So in conclusion, we can see that no matter which type of coil you use, no matter how it is connected and no matter how your battery is wired you will end up with a spark that will run your car. Some combinations produce a less effective positive spark voltage, so are not ideal. This may be evident during starting when the battery voltage can sag to 7 or 8 volts, so starting may be difficult or even impossible. It may also be evident at higher engine speeds when the energy stored in the coil is less so the engine may start to mis-fire. However, none of the combinations of coil type, battery wiring, and coil connection will result in a situation that will overheat the coil, nor stop the car running, at least in the short term, but there are two possible areas of concern.
The first is when the connections to an original Lucas Q12 coil are reversed. Under these conditions the SW terminal is exposed to the voltage across the points (up to 500 volts) rather than the intended 12 volts battery voltage. This may cause the insulation inside the coil between primary start and the outer magnetic core to fail.
The second is when a modern coil intended to be used with a ballast resistor is used without one, or with one of too low a resistance. This will overheat the coil causing it to fail and quickly burn out the points. The way to avoid this is to check the coil primary resistance (between + and – terminals) of the coil using a digital ohmmeter. The original Lucas Q12 coil has a primary resistance of 4.2 ohms and the period accessory Lucas ‘Sports’ coil has a primary resistance of 3 ohms. This primary resistance defines the ultimate primary current whilst the points are closed, at least at low engine speeds. As long as a modern replacement coil has a primary resistance in this range it can be used as a direct replacement for an original coil. A modern coil intended for use with a ballast resistor may have a primary resistance of less than 1 ohm, so must be used with a series ballast resistor. The value of this should be chosen so that the total ballast resistor + coil primary resistance is around 3 to 4 ohms and must be rated at 50 watts.
Note 1.The purpose of the condenser, which today is more correctly termed ‘capacitor’, is to slow the rate of rise of the voltage across the opening points. Without it the barely open points would arc and the energy stored in the coil’s magnetic core would be dissipated before a spark voltage is produced at the plug. Many capacitors sold today for classic cars are notoriously unreliable. Anything with ‘Lucas’ printed on it is either at least 50 years old now, or more likely a poor quality Far Eastern copy. Eric has recognised this and can offer a reliable modern capacitor encased in a copper sleeve ready soldered onto a base plate ready to fit into your distributor. These are offered on an exchange basis. A description of the problems he has found with early capacitors, and some modern replacements too, was published in TTT 2 Issue 31 (August 2015). A copy of the article can be downloaded from the TTT 2 website by selecting the relevant issue from the dropdown box. The article is well worth reading as it gives a fuller description of the operation of the T-Type ignition system.
2.The polarity of the spark voltage can be checked by using back to back LEDs in series with one of the spark plug leads as described by David Heath and others. By using separate red and green LEDs or a single bi-polar red/green LED in series with the plug lead the spark voltage polarity can readily be detected. Examples follow:
Fig 7A shows a LED spark polarity tester using a bi-polar Red/Green LED which fits onto one of the spark plugs in series with the ignition lead. Fig 7B shows the tester in use with a Lucas Q12 coil, wired as intended in a positive earth car. This demonstrates the car has a negative spark voltage as indicated by the green LED. Fig 7C shows the same car, but with the connections to the SW & CB coil terminals temporarily reversed resulting in a positive spark voltage as indicated by the red LED.