Understanding the High-Tension Ignition System

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Photo by Dr. David Cave
1952 Clinton engine.

Recently, a friend’s 1952 Clinton engine wouldn’t start. After some analysis, we narrowed the cause to spark. The component values may vary, but all high-tension ignitions are schematically identical. That made it easy to quickly determine that the condenser was bad. In his parts drawer, he had an old automotive condenser, likely for a 1949 Chevy. Could it be used to get the Clinton running again?

Before answering that question, it is very helpful, as well as necessary, to understand the high-tension ignition system and why the condenser exists. Figure 1 is the schematic for all high-tension ignitions. In these systems, a changing magnetic field strength causes a current to flow in the primary coil through the closed points to ground. Changing is the key word in that sentence.

The amount of voltage that drives the current in the primary coil is directly related to the rate of magnetic field strength change that the coil experiences. A weak magnet will create a good spark if the system manages to change the field strength rapidly. On the other hand, a strong magnet will fail to deliver a good spark if the magnetic field strength changes slowly. It is easier to get a big change in the magnetic field strength quickly if you start with a strong magnetic field.

All high-tension ignitions use the same electrical schematic. The capacitor value may change and the number of turns in each coil may vary, but the base schematic never changes. Where they differ is the method used to create the changing magnetic field. Clinton and Maytag engines have a circular shaped magnet (or magnets) attached inside the flywheel, see Figure 2.  As the flywheel turns, no magnetic field is generated. The N pole approaches and suddenly switches to the S pole, and then back to no field.

The popular Wico EK adds two large air gaps to the magnetic loop as it trips. Air is the worst possible medium for magnets; adding those air gaps causes the magnetic field strength to drop like a rock. The Wico Series A and many tractor magnetos, such as the Fairbanks Morse RV-2B, have a compact magnet that spins sitting adjacent to the primary coil.

Those are the three common methods of creating the changing magnetic field strength. Each creates a different looking and seemingly different behaving magneto, but from there on, all of them are identical. The Wico EK is rectangular and boxy in appearance, tractor magnetos are shaped like a loaf of bread, Maytags don’t appear to have a magneto, but underneath they all use the electrical schematic of Figure 1. It is a rather simple schematic, consisting of the condenser, a set of points, a magnetic core used to route the magnetic field, and two coils on that core. The left coil is called the primary or sometimes the low-voltage coil. The other is called the secondary or high-voltage coil. In operation, the changing magnetic field is going to cause the primary coil to store up energy in the form of high current and low voltage. The system then transfers that energy to the secondary coil at a higher voltage, but much lower current.

Condenser function experiment

Before we try to understand why the condenser is there, we need to understand what it does and how. A mechanical analog will help. We will start with a sealed (no air can get in), full, 50-gallon water drum. Both the left and right ends have an inlet/outlet pipe with a shut-off valve. Attached to the right inlet pipe is an energy source, a pump. What makes this tank interesting is a large, stretchable membrane in the center that completely separates the left half from the right, see Figure 3. We want this tank to store energy and be capable of doing work for us.

With the left valve closed, we open the right valve and attempt to pump in three gallons. Nothing happens. In order to push water into the right side, the membrane would need to stretch leftward to make room, but it can’t because the left side is full. Now, let’s open the left valve and pump three gallons into the right side. The membrane stretches leftward and pushes exactly three gallons out the left side. We have put three gallons in, but the drum capacity is still 50 gallons. If we close the right valve, we will read a pressure on the gauge as the membrane tries to return to its neutral position, see Figure 4.

If we open the right and left valves and pump in another three gallons, three gallons will leave the left side as the diaphragm stretches more. Closing the right valve, we would now read higher pressure. Note, at this point the ground water on the left has an excess of six gallons while the ground water on the right is reduced by the same amount. If we set up the same experiment with a drum of the same length but a smaller diameter, the extra six gallons would stretch the membrane more and we would read a higher pressure, see Figure 5.

With the right valve closed, and six extra gallons on the right side under pressure, the system is storing energy initially from the pump. We could use that energy. We could close either valve, disconnect the tank at the shutoff valves, wheel it across town to the local mill, release it slowly, and turn a waterwheel to grind flour.

Practical application

Now, we can apply those principles to the condenser. The symbol used to represent a condenser is similar to its actual construction: two electrically conducting plates separated by a nonconducting layer, similar to the two halves of the water tank separated by a membrane. In the days of our old engines, a common implementation was two sheets of aluminum foil separated by a sheet of waxed paper, rolled up like a jelly cake roll.

In Figure 6, I have partially unrolled a typical early 1900s condenser. On the left is a layer of waxed material and under that is the upper plate. Mid picture, I have pulled up the upper plate to reveal the lower plate. The brass plate is the contact to one of the aluminum sheets.

Keeping in mind the water tank, in Figure 7, the condenser is in its initial state (tank is full, no water pumped in), and both plates at the atomic level have exactly the same number of positive and negative charges – they are electrically neutral. I have shown four on each plate, but the actual number of those positive and negative charges is huge: It would be a one followed by zeros across the page.

In Figure 8, we have a 0.2µF (gallon) condenser with switches on each contact as well as a voltage source (pump) and a voltmeter (pressure gauge). The left switch goes to ground and is open (nonconducting). If we close (conducting) the right switch and apply a voltage, nothing happens. This is represented by the tank with the left valve closed. This is the second-most common failure mode of condensers, one of the leads becomes disconnected from its brass plate. We call that an open.

Now we close both the right and left switch and apply a voltage. Depending on the condenser size and voltage used, a given number of positive charges will flow onto the right plate. Figure 8 shows the excess positive charges on the right plate. I have shown six excess charges – for even a small voltage, the number of charges is gigantic. With this excess charge, an electric field similar to the rubber membrane develops between the plates, which causes an interesting thing to happen: the exact same number of positive charges leave the left plate.

So, for every positive charge that is accumulated on the right plate, a positive charge departed the left plate. The condenser is still electrically neutral. It still has the same total number of positive charges; however, some are not where they would like to be. Like the water tank, on a larger scale, the whole world is still neutral. The battery took those extra charges on the right plate from ground on the right, while those missing on the left plate can be found at ground on the left.

The device has not condensed charge, it is still neutral. Therefore, for the rest of this explanation, we will call it by its correct name, capacitor. With the capacitor charged, if we open the right switch, the voltmeter will read a voltage. If we increase the stored charge by pushing in more current, the capacitor voltage will increase. In a similar manner, if we have the same number of charges on a smaller capacitor, the voltage will be higher. In either case, an important point is that the capacitor is storing energy that can be used. For example, if we attached a small LED bulb to the capacitor and closed the right and left switch, we would see a flash as the capacitor discharged, dumping its stored energy into the bulb.

In a more notable example, I received some capacitors from Lighting Magneto for evaluation. The last test was a 600-volt leakage test, following which the units were placed on the bench. After more than 15 minutes, during cleanup, the first capacitor was accidently picked up by its leads — wow, it had 600 volts worth of stored energy still onboard. Like the water tank, for a charge to leave one side of a capacitor, another charge must enter the other side. The experience would have been less painful had I picked that capacitor up by only one lead.

It is difficult to count those excess positive charges, but easy to measure current in amps. Earlier, I said there would be a gigantic number of charges. To be more specific, 1 amp is 6,240,000,000,000,000,000 positive charges-per-second. If we force 1 amp of current into the capacitor, that many charges-per-second will be accumulating on the capacitor plates and the voltage will be rising at a rate dictated by the capacitor value. Each excess charge in Figure 8 represents more than a hundred trillion charges.

The most common failure mode, and one you have likely experienced, is for some form of conductive path to develop through or around the waxed film separating the plates. Those positive charges we have stored on the right plate will migrate to the left side. The capacitor is “leaky,” similar to our water tank having a small hole in the membrane. I learned the Lightning Magneto capacitors don’t leak. Modern capacitors are still parallel plates, but the dielectric (separation material) and the lead connections are greatly improved, making today’s versions smaller and more reliable.

Coils and inductors

In order to put the high-tension system all together, a discussion of how coils and inductors behave is needed. Refer to The Low-Tension Ignition System, Gas Engine Magazine, October/November 2020 issue. They are like big flywheels, it takes time and effort to get a flywheel spinning to 500rpm. Once spinning, it’s storing a huge amount of energy and will not stop spinning and come to rest until it has dissipated all stored energy. If coasting to a stop, most of that energy is converted to heat in the bearings, cylinder wall, etc. With a lot of effort, it can be made to stop more quickly, but it cannot be stopped instantly.

In a similar manner, it takes effort and time to get current flowing in a coil; once the current is flowing, the coil is storing energy in a magnetic field that builds up around it. If the force (voltage) that caused the current to flow is removed, the coil current will continue flowing until that stored energy is dissipated.  Energy is always conserved; it cannot just disappear. In our case, it is converted to heat as current flows through the coil’s internal and external resistance. The current can be stopped more quickly with effort, resistance, or an air gap, but it cannot be stopped instantly because the stored energy cannot be dissipated instantly.

Think of a coil as giving current momentum. It takes effort and time to get it rolling, but once rolling, it is difficult to stop. Stopping it more quickly, for example, in the case of a low-tension rotary magneto, means tripping and opening the igniter. The current cannot stop instantly, it will only stop after all the energy is dissipated, so it jumps across the igniter points quickly dissipating a lot of energy.

High-tension ignition

In Figure 1, when the points are closed, we have a changing magnetic field creating small voltage and high current, flowing out of the primary winding through the points to ground. The capacitor is uncharged, sitting at zero volts because the closed points have it shorted out. Primary coils will typically have a resistance of less than 1 ohm, often near 0.25 ohm, so a small voltage will create lots of current. We know then that the coil is storing lots of energy. The stored energy is ½LI² where L is the coil’s inductance (number of turns, core size, core material, etc.). Because energy goes as current squared, doubling the current creates four times more energy. The primary coil’s stored energy will become the totality of the spark energy, so it needs to be maximized by increasing the current.  Everything in the spark plug spark originates as energy stored (current) in the primary coil.

Figure 9 is a stripped-down version of Figure 1. For discussion, the secondary coil is left off as well as the capacitor. The circuit is identical to a low-tension rotary magneto and igniter. Initially the points (igniter) are closed, the magnetic field is changing, current is flowing in the coil, and energy is being stored in the primary coil.  The path of the current is in red. When the igniter trips (points open), the system has asked the primary coil current to go to zero instantly. From previous discussion, we know this is not going to happen. Current is going to continue to flow until the coil has dissipated all its stored energy. The current has momentum. If it’s going to continue for an instant, it must jump (arc) across the points. That arc will dissipate a huge amount of energy quickly. If that happens, the energy that was going to be transferred to the spark plug just went up in smoke, in the form of a spark. Wasting the collected energy on the primary side as a spark before it can be transferred to the spark plug doesn’t seem like a good idea.

Figure 10 adds the capacitor back in. Again, with the points close, current is flowing and energy is being stored in the primary coil. With the points closed, the current follows the path shown previously in Figure 9. When the points open, the coil starts dumping its stored energy by shoving current (positive charge) onto the capacitor, illustrated by the red path of Figure 10. Rather than going up in smoke as a spark across the points, the primary coil energy has now moved over and is sitting on the capacitor (voltage and positive charge).

The situation now is very similar to the water tank with six gallons pushed in. The capacitor is not really happy being loaded up with all this energy. Using the voltage it has developed, like the water tank pressure, it shoves everything back to the coil. The coil in turn shoves it back to the capacitor. The system oscillates. Figure 11A shows the voltage on the capacitor and coil as the points open. The energy (in the form of current and voltage), first heading toward the capacitor, turns around and heads back toward the coil, then, back toward the capacitor again. The process continues, with each pass, decreasing in magnitude, until all the energy is dissipated.

Now to answer the question of whether the 1949 Chevy capacitor can be used to get the Clinton running. As the value of the capacitor is lowered, similar to reducing the water tank’s diameter, both the voltage and frequency of the oscillation will increase. Conversely, if the capacitor’s size is increased, the voltage and frequency of the oscillation will go down. Nothing else changes. Figure 11A and 11B compare the voltage and frequency of a Wico EK magneto with 0.06µF and a 0.78µF capacitor. The larger capacitor, Figure 11B, clearly has less amplitude and is a lower frequency.

Now we are ready for the final step, transferring this oscillating energy over to the spark plug. The primary coil and the secondary (high-voltage) coil are wound on the same magnetically conductive core and become a simple transformer.  As the name implies, a transformer transforms voltage and current. A typical Wico EK magneto has 196 turns of 22-gauge wire in each of the primary coils and 8,400 turns of 40-gauge wire in each high-voltage secondary coil for a ratio of 43:1. The voltage available at the spark plug will be 43 times larger than the voltage on the primary coil and capacitor, but the current will be reduced by 43. So, the transformer moves the primary coil’s energy over to the spark plug at a much higher voltage, but with a lower current.

Here is an important point; the transformer is like the magnetic field, it only works when things are changing. Transformer action won’t work if the primary coil has a steady DC voltage. The primary coil voltage needs to be changing or oscillating. Adding the capacitor to the primary coil causes that side to oscillate, enabling transformer action.

Figure 12 demonstrates the transformer action when a Wico EK is tripped. In order to keep the high voltage in the range of the oscilloscope, the coil and capacitor voltage were purposely lowered by increasing the capacitor to 33µF. The yellow trace is the capacitor/primary coil voltage displayed at 20 volts-per-vertical-box and peaks at 60 volts. The blue trace is the open circuit’s (no spark plug) output of the high voltage coil displayed at 500 volts-per-vertical-box, peaking at 2,640 volts. The voltage multiplication is 44, very close to the turn’s ratio of 43. The error here is probably in reading the scope voltages or my counting the 8,400 secondary turns.

A quick summary of the purpose of each component in Figure 1 might be helpful. The primary coil’s job is to store up as much energy as possible during the time the magnetic field is changing, while the points are closed. Its second job is to transfer that energy to the secondary coil by transformer action, when the points open. The points’ function is to short the primary coil so it can maximize its current and thus maximize its energy collection. The points also precisely time when the spark occurs at the spark plug. The secondary coil’s job is, by transformer action, to accept energy from the primary coil at such a high voltage that a spark at the spark plug occurs, dissipating all the energy.

The purpose of the capacitor

First, the capacitor keeps the energy built up and needed in the primary coil from being destroyed as a spark across the points. A useful advantage, although not its purpose, is that it also reduces the pitting of the points.  Second, it causes the primary side to oscillate, thus enabling transformer action to move the energy to the spark plug. Changing the value of the capacitor only changes two things: the voltage of the oscillation and the oscillation frequency. Neither change significantly, so it’s likely the 1949 Chevy capacitor will work just fine for my friend’s Clinton engine.

Capacitor value experiment

After getting the Clinton back up and running, it seemed that an experiment involving capacitor value was in order. A real test might be to upsize and downsize the standard 0.2µF capacitor two times, but three times or four times would make a point. In addition, a 1949 Chevy going 5,000 miles prior to changing the points and condenser would have mechanically bounced down gravel roads and open and closed the points more than 40 million times. That capacitor might be a fair test also.

In the end, the original in each magneto (generally about 0.22µF), a 0.06µF, a 0.78µF, and a 1949 Chevy capacitor were compared. The magnetos tested were a Maytag on a testbed, Figure 13; a Wico EK on a testbed, Figure 14; the now-running Clinton; a Wico EK running on an Hercules, Figure 15; a Wico Series A running on a 3hp  John Deere EP Northern, Figure 16; and a Fairbanks Morse RV-2B. This resulted in data on all three types of changing magnetic fields mentioned earlier.

There are several questions to be answered.  Would some or all of those capacitor combinations start and run an engine smoothly and do so without excessive arcing across the points, which would lead to pitting? There are several parameters specified on capacitors, but unless you dig deeply, most will be specified by only two parameters; capacitance value and operating voltage. So, determining the maximum voltage on each capacitor would be helpful. Model T buzz coils are electrically noisy with high-frequency noise spikes so another capacitor specification, maximum rate of voltage change, must be watched with those coils. Although buzz coils use the same schematic as Figure 1, they are not addressed in this article. Yes, buzz coils and all pre-electronic ignition automobiles use the Figure 1 schematic. Rather than using a changing magnetic field to create the current in the primary coil, they use a battery.

Before looking at the results, let’s review what should be expected. First, the energy collected and stored in the primary circuit is unaffected by capacitor size. The primary coil and changing magnetic field determine the collected energy, therefore the amount of energy available to be transferred to the spark plug is unaffected. The size of the capacitor will affect the frequency and amplitude of the primary circuit oscillation. Frequency has little effect, but voltage needs to be looked at. If too high, the max capacitor voltage rating could be a problem; if too low, the transformer turns ratio might produce an inadequate voltage for the spark plug. Arcing of the points is a race, as air arcs at about 70-volts per 0.001 inch. As the points open, they begin to move apart at a rate determined by the cam profile and the engine’s rotations-per-minute. As this happens, the capacitor begins to take on charge (current) from the primary coil. Its voltage starting at zero begins to rise upward at a rate determined by its size and the amount of current coming from the primary coil. The rate that it rises, in volts-per-second, is I/C. For example, forcing 1 amp into a 0.2µF capacitor will cause the capacitor voltage to rise at a rate of 5 million volts-per-second or 5 volts-per-microsecond. If the points get to 0.0005 inch before the capacitor gets to 35 volts (7 µs in our example) or the points get to 0.001 inch before the capacitor gets to 70 volts, all is OK. If the capacitor voltage goes up too fast, a spark will occur across the points. That spark is difficult to see with the naked eye, but fortunately easy to observe on an oscilloscope trace as an instantaneous drop in the capacitor voltage.

Results and conclusion

The Maytag testbed, the Wico EK testbed, and the Fairbanks-Morse all showed good, consistent spark for all capacitors at all speeds (Maytag and Fairbanks-Morse). The Clinton, as well as the John Deere EP with the Wico Series A, started easily and ran well on all capacitors. The Hercules started and ran nicely on the 0.06µF, 0.22µF, and ’49 Chevy capacitors, but was difficult to start and ran poorly on the 0.78µF capacitor. It did start easily and run smoothly on a 0.68µF capacitor however (three times the standard). The condition of the magneto magnet and its points gap are unknown. It was also later learned that the Hercules was sensitive to trip mechanism setup. The capacitor oscillation voltage and frequency went up, in the four cases measured, with decreasing capacitor size as expected. For the 0.06µF capacitor, which is the worst case, the maximum voltages were as follows: for the Maytag 75 volts, for the Wico EK testbed 400 volts, for the Wico Series A 250 volts. The Maytag however had a noise spike, coming from the spark plug, of 125 volts. The noise spike was the same for all capacitor values, no other magnetos showed a significant noise spike.

The Maytag, Wico EK testbed, Wico Series A, and the Fairbanks BV all showed the points arcing under some conditions (running or starting) for all capacitors except the 0.78µF. Physical location made it impossible to get a scope on the Clinton and Hercules. It was later determined that above about 0.47µF, all points arcing ceased. Surprisingly, it did not appear that the 0.06µF capacitor caused significantly more points arcing than the base capacitor or the ’49 Chevy.

In Figure 17, the blue trace is the capacitor voltage at 50 volt-per-vertical-box. Immediately after the mag trips, the capacitor voltage starts moving, then twice it abruptly goes back to zero before getting off to its sinusoidal decay as energy moves to the spark plug. Those two abrupt returns to zero are each a small spark across the points. The yellow trace is the sparkplug voltage at 2kV (2,000 volts)-per-box. The plug arc is made up of short bursts of short bidirectional arcs. In all the magnetos studied here, one or two of these short duration points arcs were observed when the points opened. Those one or two arcs apparently give the points time to separate far enough apart to save even the 0.06µF capacitor from further arcing and wasting energy before it could be transferred to the plug.

Although primary coil current is unaffected by capacitor size, for reference, low-tension rotary magnetos tend to generate about 1 amp in their coil while in these high-tension experiments the Wico EK generated about 2 amps, the Maytag generated about 1.5 amps at running speeds while the Fairbanks Morse generated about 2.4 amps.

There is a lot of information and even more misinformation on various websites instructing us on the proper capacitor to put into a given high-tension magneto.

My conclusion, however, is that the high-tension ignition system is very tolerant of capacitor size. Although this discussion was nontechnical, a rigorous mathematical analysis of Figure 1 also puts little restriction on the capacitor value. The capacitor size sets the oscillation voltage and should not exceed the capacitor rating, but should be high enough to spark the plug. For reasonable deviations around the standard 0.2µF capacitor, engines will run and start fine. The energy collected and delivered to the spark plug is independent of capacitor size. The maximum voltage on the capacitor, again for reasonable deviations, is below 300 volts making the often quoted 600 volts specification for engine ignition capacitors very adequate. Unless the capacitor size is doubled to 0.47µF, all four magnetos tested with a scope showed minor points arcing under some conditions.

Dr. David Cave is a retired electrical engineer with a doctorate in electrical and computer engineering. He spent his entire career as a computer chip designer at Motorola Semiconductor. He had a gas engine in grade school that never ran. About 20 years ago, after retiring, the bug bit him again. David now focuses on collecting John Deere engines. His collection is made up of one of every model and every horsepower (of every model) John Deere made plus pulleys and other peripheral items. But, with his electronics background, he spends more time collecting and fixing magnetos. He can be contacted at jdengines@cox.net.

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