Learn the ins and outs of spark plug/high-tension ignition system basics.
Editor’s note: This is the third in a series of articles by Andrew K. Mackey examining antique engine fuel and ignition system basics.
High-tension ignition is electrically complicated but mechanically simple. A set of contact points opens or closes at the correct time in an engine’s operating cycle, completing a circuit and causing a field collapse in what is basically a transformer. That field failure is accelerated by a condenser, causing a secondary winding to generate a high-voltage surge. That high voltage jumps across a gap on a spark plug mounted in the combustion chamber, thus igniting a fuel/air charge in the combustion chamber. The result of that ignition is the rotational power of the engine. There are several kinds of high-tension ignition.
A buzz coil uses the iron core in the transformer assembly to attract a set of contacts known as a trembler. At rest, the trembler closes the primary circuit, causing the iron core to become an electromagnet. The magnetism attracts the trembler, opening the electric circuit via a set of points, which in turn causes a field collapse and a resultant spark through the high-tension side of the coil. As the field collapses, the trembler contacts again close, making the iron core again magnetic, opening the contacts attached to the trembler and repeating the cycle, thus creating another spark, and so on until the timing points open, breaking the electromagnetic circuit.
One of the best known buzz coils was made by Ford Motor Co. for their Model T automobile. Buzz coils can be charged by several means: Either a battery or, in the case of the Model T, a low-tension magneto can be used. The power itself is timed, either by a set of points that close when spark is supposed to happen or by what is known as a “wipe spark,” a circuit that is closed by the action of a brush against an isolated contact. An example of wipe spark timing is found on the Maytag upright engine or the early Fairmont railway engines. Learn how to make your own buzz coil.
The single spark high-tension coil is the most modern of the old engine ignition systems. Unlike a buzz coil, it makes only one spark each time it is needed. This can be battery and coil or magneto inducted. A set of contact points opens at the proper time, and a single spark occurs as the primary voltage current collapses the magnetic field in the coil. On this ignition, a condenser helps the collapse in order to make a high-tension spark in the secondary circuit. High-tension ignition systems are electrically complicated by the fact that there are two windings in the coil, a low-tension and a high-tension. A high-tension coil is a transformer — the windings create a higher voltage through a secondary coil when the primary field coil circuit opens. With older coils, winding insulation break down or corrosion of the wiring often lead to poor spark or outright failure to make spark. The Maytag Model 72 D and DA magnetos, for example, are well-known for coil failure. There are now several people who will either make new coils or will rewind your old ones in order to make these engines run again — but they aren’t cheap!
The high-tension coil needs a condenser to force rapid field collapse and to ensure the points contacts don’t pit from voltage arcing. If the condenser is bad, you will not get reliable spark. On a high-tension set up, you also must have clean points set at the proper gap. This is especially true of high-tension magnetos. Improperly gapped or dirty point faces are the cause of more than 75 percent of high tension coil malfunctions, especially in magneto-fired engines.