Low-Tension Ignition Basics
Editor’s note: This is the second in a series of articles by Andrew K. Mackey examining antique engine fuel and ignition system basics.
There are two forms of electric ignition used on old engines: low-tension and high-tension. These in turn are divided into two subsets: magneto and/or battery supplied.
Low-tension or igniter-fired
A set of points are set within the combustion chamber. One point is fixed, isolated from the engine and attached to the power source and coil. The other is movable, grounded to the engine, usually through a ground joint, similar to a valve seat face. This ground face prevents loss of compression through the shaft.
This assembly is called an igniter. On a battery and coil system it works as follows: The battery supplies low voltage to a coil. As the engine nears TDC compression the points, usually open, are momentarily closed by a trip assembly on the engine exhaust pushrod, or by a trip rod attached or activated by a cam. A spring snaps the points back open and low voltage current is interrupted. The coil provides a power surge as the field generated collapses, and an arc flashes across the opening points in the combustion chamber. The rapid expansion of burning fuel drives the piston down the bore, the connecting rod turns the crank and the power generated runs the engine. Some magneto systems, like Webster, have the points normally shut, sparking upon opening.
Low-tension ignition is electrically simple but it is mechanically complicated, especially on magneto-fired engines. All mechanical linkages and trip mechanisms must be “just so” and the points must open at exactly the right time in order to get a quality spark needed to ignite the fuel charge.
Many early igniter engines used two forms of power to supply spark: A battery and coil were used to start the engine, and a magneto was used to actually run the engine once it warmed up. A double throw, single-pole switch directed the power source. A battery and coil is more reliable for starting, but a magneto gives a better spark at speed. The problem with battery and coil engines was the fact that the dry cell batteries used lost power rather rapidly over time and were not rechargeable.
The largest drawback to low-tension ignition is the fact that it is highly susceptible to failure in humid conditions. Any moisture on the igniter, its mica insulation or the wiring tends to short out the low-tension power enough to prevent spark within the combustion chamber under compression.
It is very frustrating to trip an igniter outside the combustion chamber and have what appears to be a good flash and then, once installed and under pressure, have no fire to light the charge. This is especially true in magneto-fired igniter engines, as a slow-turning magneto does not provide as much power as a battery and coil at starting speeds. Once a magneto-fired engine starts though, the magneto outperforms the battery and coil, as a magneto makes more current the faster it turns.
Engines run on today’s alcohol-containing fuels have an added problem — if fuel wets the mica insulation used on the igniter as insulation it will short out under compression, thus rendering the igniter ineffective. In that case, the igniter will have to be removed and thoroughly dried off in order to regain spark under compression.
More Gas Engine Basics
Andrew K. Mackey • firstname.lastname@example.org
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