Spark coils for gas engines

| May/June 1970

spark coil

Schematic drawing of vibrating jumb-spark coil.

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.