Spark coils for gas engines

spark coil

Schematic drawing of vibrating jumb-spark coil.

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This antique gas engine repair article will speak mainly of primary or 'low-tension' coils and 'high-tension' or spark coils as commonly used on gas engines.

Probably the least understood and at the same time the most important function is that of gas engine ignition.

The effect of ignition on the working of an engine is remarkable. Nothing tempts an engine man to use a sledge hammer to fix things up permanently any quicker than an engine which will fire occasionally but refuse to run. Almost always this is due to a weak spark, and the weak spark can be due to a number of things. Corroded or dirty igniter points, faulty wiring and connections, weak or partially shorted coil, weak batteries or incorrect timing -- these are some of the common difficulties.

In low tension ignition, a spark is produced by snapping a pair of contacts within the cylinder. In order to produce a hot spark, a simple coil with many turns of rather coarse wire wrapped over a soft iron core is placed in series with the circuit. When the electric circuit is completed by closing the igniter points, the current passes through the coil and strongly magnetizes the iron core. When this circuit is then suddenly interrupted by opening the points, this magnetism reappears as current in the reverse direction which is proportional to the amount of magnetism stored in the core. On breaking of the circuit the resulting induced voltage from the magnetism in the core will rise several times the battery voltage; on some good coils as high as 150 volts. Although this high voltage lasts for only a fraction of a second, this is all that is necessary to provide a good hot spark for easy starting and running.

To further explain the electrical principle involved, let us use the following comparison: In a long water pipe running full stream, quickly closing the valve will make the pipe 'pound'. It we placed a gauge on this line we would see that for an instant the pressure would rise considerably over normal. The same principle is involved in either the high or low tension coil. Upon breaking the circuit the voltage or electrical pressure rises to a value much higher than normal due to the magnetism in the core, or one might say due to the 'electrical inertia'. Always think of voltage in a circuit as 'electrical pressure' and current as expressed in amperes as 'electrical volume'.

Incidentally, igniters set up for battery ignition always have the points open except when closed by the trip rod, then open at ignition and remain open till next time around. The igniter points on a magneto equipped engine always remain closed except when tripped by the push rod. This holds true for all rotary low-tension magnetos as well as all oscillating low tension magnetos such as the Webster Tri-Polar.

For a good hot primary coil, the following dimensions will work reasonably well: Take No. 16 soft iron wire, cut into 6 inch pieces enough to make a neat bundle about 1 inch in diameter. Anneal the wire so that it is dead soft. Hard wire is useless as it will retain some of the magnetism. A solid core is also worthless. Neatly bundle these wires together. Place the bundle into two wooden heads about one-half inch thick and four and one-half inches square. Glue together with shellac, then insulate the core by wrapping with two layers of varnished cambric and glue down. Cambric can be obtained at any electric motor repair shop.

Drill holes in one head for the terminals, then stick the end of the magnet wire through a hole and start winding. Use 14 ga. Formvar magnet wire. Wind carefully so the turns do not cross or overlap and keep the wire snug as you wrap. Chucking one end of the core in the lathe and running very slowly works nicely. Wrap all the way to the opposite end, then cover with one layer of cambric and glue down as before, taking special care at the ends of the coil. Repeat this process going from one end of the coil to the other until you have between 500 and 600 turns which will be between seven and nine layers depending on the type of insulation and the care you have used in winding.

Doubling the number of turns will reduce the battery current by doubling the resistance. The number of ampere turns will remain the same. The principle advantage in doubling the number of turns over the above figures is the saving of the battery.

Also a short coil of larger diameter will provide a much hotter spark than a long slim coil because the magnetic field decays much faster in the shorter core.

Thomas Edison, among others, spent a great deal of time and money in order to develop a coil which would put out the hottest spark with the least amount of current. In order to design a coil from scratch, several detailed calculations must be made which are of little practical value here.

From a practical standpoint it is easier and cheaper to obtain a primary coil from various sources rather than build one. In a pinch, using the primary side of an old automotive coil will do the job, but they are hard on batteries. Several individuals advertising in this magazine build primary coils which although they may differ in design than the one suggested here will produce excellent results.

High-tension or spark coils embody the same basic principle as the primary coil, that of using the reverse current induced by a decaying magnetic field on breaking the points, except that instead of relying on a single coil of coarse wire for the spark, a secondary winding of many hundreds of turns of very fine wire is wound over the comparatively few turns of coarse wire in the primary.

Vibrating coils such as the familiar one used on the Model T Ford are the most common in gas engine ignition. It consists of (1) a soft-iron core made up of a bundle of soft-iron wires, (2) a primary winding made up of about 16 ga. magnet wire and relatively few turns, (3) a secondary winding of about 36 ga. magnet wire and many hundreds of turns, (4) the vibrator points and (5) a condenser.

Referring to the drawing it is observed that the battery current flows from the (+) terminal to one of the vibrator points through the points to the primary wind ing and back to the (-) battery terminal. The current flow sets up an electromagnetic field, the core is magnetized and attracts the flexible vibrator point, the other point remaining stationary. The circuit is thus broken, the magnetic field decays and a very high voltage is induced in the secondary winding. We have seen in the primary coil that for an instant the voltage rises considerably higher than normal on opening the points, so let us say that in a random coil the primary voltage will rise to 100 volts. Then if there are 100 turns in the secondary to 1 turn in the primary, we will have a 100:1 increase over the primary voltage. We then conclude that we should have a secondary voltage of 10,000 volts then from the six volt battery. Of course, although this voltage or electrical pressure is very high, the amperes or electrical volume is infinitely small.

At the instant the points separate the core loses its magnetism, the points again close, this completes the circuit and so the action becomes continuous. The rapid vibration of the trembler induces a series of high-voltage currents and the secondary delivers a shower of sparks across the spark plug.

The condenser has two purposes, one being to hasten rupture of the circuit by helping to demagnetize the core so that magnetic decay takes place more quickly, thus producting a hotter spark; the other being to reduce arcing at the vibrator points.

Approximate design of the high-tension coil consists of a soft-iron core about 5/8' diameter by 6' long. Shellac thoroughly, then wrap with varnished cambric. Place oak heads on the core so that there is 4 3/4 inches between them. Wrap two layers of 16 ga. magnet wire on the spool, insulating between layers and passing the ends through the head. Coat the coil with shellac, then wrap with cambric and glue down with shellac.

The secondary coil is then wound using about eight ounces of 36 ga. magnet wire. Again wrap very carefully to avoid crossing over of turns. Wrap each layer with cambric and glue down with shellac. The cambric must fit nicely at the ends otherwise there will be a breakdown of the coil since the difference in potential is greatest at the ends of a coil.

This very basically describes the general proportions used. In order to make such a coil, a great deal of time and patience is required and even then there is a chance of failure. Modern day coils are dried under vacuum and impregnated with an insulating material; a newer method consists of encapsulating the entire coil in an epoxy resin thus rendering it impervious to the elements.

Many volumes have been written on ignition, and even yet there remains a lot of unsolved problems. For further information we direct you to the 'Gas Engine Guide' which you may obtain through GEM.