The engine community has a number of strongly held ignition beliefs that are just not true. Three of the many examples are touched on here. We are told we must replace a bad capacitor/condenser in a high-tension ignition with the exact same size when, in fact, the system will be happy with a large deviation from the original size (Another Bad Condenser). Size here meaning capacitance value, not physical size.
For a second, we are told to measure the output voltage of a low-tension magneto. The 10V or 12V reading you get is simply how your meter will read a 50V to 100V narrow, distorted pulse. No meter on planet earth can read that narrow pulse and, as a result, your neighbor’s meter will read it differently. That voltage is a nonparticipant in the spark. Current (amps) is what counts and is what should be measured (What Does Your Magneto Voltage Tell You). Finally, we are told not to remove the wire going from the magneto to the igniter or spark plug as it will damage the magneto, yet I would guess we have all done that, maybe by accident.
I’ve been told to never remove the magnet from its magneto. In fact, the Wico EK manual says that if you remove it, you will immediately destroy the magnet. Should we question that belief? It seems the reasoning for this statement is that air is thousands of times more difficult for a magnetic field to pass through than the iron-based magneto pole pieces and the armature core. Once the magnet is removed from the magneto, the argument seems to be that the large air gap and more difficult magnetic path produced will rapidly degrade the magnet.
Testing the magnets
The normal degradation of old permanent magnets is a long, slow process. Most hit-and-miss engines will run for many years before they become hard to start due to a weak magneto magnet. Although early magneto repair shops had magnet chargers, seldom will you see a magneto manufacturer recommending regular magnet maintenance or recharging. But science does tell us that there are quick magnet killers: external magnetic fields, excessive heat, shock, loss of volume and radiation. There is no mention of changing its environment. If we remove the magnet from a magneto and don’t get too close to a magnet charger, don’t hit it with a propane torch, don’t bang around on it with a hammer, don’t shorten its legs with a hacksaw or don’t X-ray it, we shouldn’t expect a rapid drop in magnetic strength, according to science.
To test the belief that one should never remove the magnet from a magneto, three magnetos and two bare magnets were set up: a low-tension rotary (John Deere), a low-tension oscillator (Webster), a high-tension (Wico EK), an IHC magnet and a magnet of unknown origin that had been repaired with a weld across a crack, shown in Figure 1. Each magnet was recharged on a strong commercial charger (Weidenhoff 818). Immediately after the magnets were recharged, the output of each magneto was measured, and the field strength of the magnets was measured. A subjective measure such as hot spark, blue spark, voltage or “it tickles” was not used to measure the three magneto outputs. The real measure of a magneto’s output is energy delivered and is directly related to its short-circuit current (amps) at the time the igniter trips or the points open. More about that later.
The magnets were then removed from the John Deere and Webster magnetos, while the armature was removed from the Wico EK. Without the armature, the Wico magnet sees a large air gap, similar to removal from the magneto. They were then placed on a wood workbench as seen in Figure 1. Daily for a week, then once a week, each Monday morning, the magnetos were reassembled, quickly tested and then disassembled again. The magnetic field strength of the two standalone magnets was also measured each Monday. The field strength was measured across a standard air gap using a Tesla meter.
I won’t take up much print space with all the data, but let me pick some random dates with the output for the Wico EK (A) and the welded magnet (mT).
- Nov. 15 – 2.2A – 137mT
- Nov. 16 – 2.0A – 136mT
- Dec. 14 – 2.0A – 132mT
- Jan. 25 – 2.0A – 133mT
- Feb. 15 – 2.0A – 128mT
- Mar. 1 – 1.8A – 127mT
Those results were consistent across the three magnetos and the two magnets. Immediately after a strong recharge, all three magnetos saw a 5% to 10% drop in energy output in the first day. Further experiments with other magnetos showed that the 5% to 10% drop in the first few hours occurred under all conditions: recharging the magnet on the magneto, recharging off the magneto and immediately putting the magnet back on, or recharging off and immediately placing a keeper on the magnet. After 15 weeks, there appeared to be a trend of another small drop. Reading the two magnets’ field strength is difficult and sensitive to external conditions, so readings bounced around a bit from week to week. That said, the magnets lost less than 10% of their strength over the period of the experiment. This 10% drop included the decrease in the first few hours. The experiment ended after 18 weeks.
For the skeptics among us, Figure 2 is a picture of a John Deere 3hp engine running on the tested magnet, and Figure 3 is a picture of a Stover running on the tested Wico EK, each after the 18th week.
You may stop reading here, knowing that the answer to the title of this article is that nothing happens. You could also stop reading here being comfortable with the fact that, indeed, it is safe to remove the magnet from a magneto. As stated, permanent magnets of the early 1900s degrade very slowly with age and will eventually need recharging. Removing the magnet from the magneto most likely does accelerate that process, but it is not measurable over a period of a few weeks. Therefore, removing the magnet for a few hours while the magneto is being repaired will have little impact on its strength. The use of a keeper, a magnetic bar across the magnet poles shown in Figure 4, will actually put the magnet of a rotary magneto, with its two clearance air gaps, in a better environment than in the magneto. I find the use of a keeper not only gives the magnet a comfortable environment, but better yet, it keeps the magnet from collecting all the screws and metal filings on my bench.
The rest of this article will explain why the current delivered by the magneto was measured rather than the voltage or some other subjective measure.
Measuring magneto energy
I’ve mentioned that a high-energy spark is what we call a “hot spark.” So, what is energy? The physics guys define energy as the ability to do work. In the electrical world, energy can be viewed as hierarchical. Voltage is the first level and the first thing that comes to mind when thinking of a magneto. Why? I suspect because it’s easy to measure: put one lead of the meter on a wire or a point, and put the other lead on ground. Easy. But voltage is an enabler — by itself not worth much. A bird can sit on a 50kV uninsulated powerline without getting even a headache.
Current (amps) is the next level and is a bit more difficult to measure. The meter must go inline, requiring the disconnection of the path to be measured. Current will always be accompanied by a voltage that is pushing it along. Power is the third level, having voltage and current simultaneously produces power. But, of course, power is more difficult to measure, as both the voltage and current must be read simultaneously. The multiplication of voltage times current gives us power in watts. You likely know an old incandescent 75-watt light bulb gets hot or a kilowatt space heater does a good job, so maybe power produced would be a good way to measure a magneto.
Unfortunately, power is an instantaneous thing. If the kilowatt space heater is only on for a second, it won’t do much good. What we need to know is how long that power existed, that’s the fourth level, energy (power x time). For energy, we must measure voltage, current and time simultaneously. When the magneto delivers its load as a spark, it delivers voltage and current (power) for some period of time (energy). It might deliver a lot of power, but if the time involved is very, very short, we will not get ignition. Also, if the time is long but the power is too low, we won’t get ignition. The more energy in a spark, the more likely combustion will occur and the easier your engine is to start.
You may remember from a high school science class that energy is conserved. Energy cannot pop into existence nor can it just disappear. It can, however, be moved from one place to another, and it can change form. In our magneto, before the igniter trips or its points open, as the armature turns or as the Wico EK armature begins to move, the magneto develops a low voltage, which in turn builds up a large current in the armature coil. Using this voltage, current and time, the magneto has converted moving energy, which it received from the crankshaft, to magnetic energy stored in a magnetic field around the armature coil. That’s an important point. Before the spark, the magneto has been moving energy from the crankshaft and storing it in a magnetic field around its coil. No energy comes from the magnet. Running an engine does not weaken a magnet. All the energy in a spark was taken from the energy stored in the spinning crankshaft/flywheel. Using iron filings, we can see the magnetic field that builds up and stores energy around a coil (Figure 5). The physics guys also tell us that the energy being stored in the coil (called an inductor) must be dissipated (converted) elsewhere. In other words, that coil energy is going to move from the coil to another location, the spark.
When the igniter trips or the points open, the magneto’s stored magnetic energy is converted to heat energy as a spark. Measuring the voltage, current and time in a spark would be, at best, difficult, but the spark gets its energy from the coil’s built-up and
stored energy.
A way to measure the energy in a spark delivered by the magneto would be to measure the energy in the magneto coil when the igniter trips or the points open. For high-tension magnetos, the spark energy is initially stored in the primary or low-voltage coil.
Fortunately, there is a simple way to measure the energy stored in a coil: E= (1/2)LI²
L is the coil inductance and depends on the number of turns, coil length, coil diameter, core material and a host of other physical parameters. L is fixed and never changes after the armature is wound at the factory. The energy in the spark becomes, simply, a number (1/2 L) multiplied by the square of the current in the armature coil when the igniter trips or points open.
That is why the magneto current was measured. With the magnets lying on a wood bench, if week after week the magneto current does not change, then the energy in its spark will not change. Figure 6 shows the output current of the tested John Deere magneto at 1.0A, which can be read on a simple AC current meter. Figure 7 shows the current in the primary coil of the high-tension Wico EK, which requires a scope to read but is still much easier to read than reading the voltage, current and time of the spark simultaneously.
Dr. David Cave is a regular contributor to Gas Engine Magazine and Farm Collector. He can be reached at jdengines@cox.net.
