When a high-tension ignition engine won’t start, it’s common practice to pull the sparkplug, lay it on the engine, turn the flywheel and look for a spark. I often hear if it sparks, maybe the mag is weak and won’t fire the plug under pressure. If that statement is correct and pressure affects ignition so strongly, what can I do to ensure the magneto is OK?
Extreme is often attached to pressure in that saying. If there is extreme pressure, ignition has already occurred and there is no longer need for a spark. The pressure when spark is needed is just the pressure of compression. I have measured many engines over the years, they all have had compression ratios between 3:1 and 4.5:1 with the majority around 4:1. Compression ratio is the total volume inside the cylinder when the piston is at the bottom divided by the volume that is left when the piston is at the top. The universal gas law conveniently says that the pressure goes up as the compression ratio goes up. In other words, the pressure at the top, in a 4:1 compression engine will be 4X that at the bottom. The confusing part of that is that pressure gauges are calibrated to read zero at the bottom, they neglect the atmospheric pressure of 14.7psi (pounds per square inch). Worst case then at top dead center the pressure is 4 times atmospheric pressure or 4X14.7= 59psi, but your gauge will read 44psi. Measuring the actual pressure is not easy as most compression gauges have modern sparkplug metric threads while many old engines have pipe threads. After a pair of brass adaptors were made, Figure 1, I measured the pressure in the cylinder of four engines. A ¾ hp Hercules with a 3:1 compression ratio produced 30 psi, John Deere engines have a compression ratio a bit less than 4:1, a 1.5 hp engine produced 44 psi while a 3 hp produced 40 psi, a Stover with unknown compression ratio produced 50 psi.
Figure 1. Measuring the Hercules compression pressure.
But all engines run with ignition advance, typically they fire 20 to 30 degrees before top dead center (BTDC). With advance the piston has traveled about 90% of the way to TDC, giving a perfect 4:1 compression ratio engine a pressure of about 40 (gauge) psi at the time spark is needed. It’s likely then that an unrestored engine with “good compression” will have, at spark time, a pressure similar to a car’s tire pressure, 35 to 45psi.
Most often, a weak magneto is blamed on not enough voltage. So, what does 4:1 compression ratio or 50 psi do to the required voltage at the spark plug? A very old physics equation, loaded with conditions and restrictions, says that the voltage required to arc between two flat electrodes is 76 Volts multiplied by the gap in thousands of an inch multiplied by the pressure, V= 76 X gap X pressure. The nice thing about that expression is that pressure is measured in atmospheres which is exactly the compression ratio. In a 4:1 compression engine the voltage required to make a spark (at TDC) goes up 4X. For example, a perfect 4:1 engine (neglecting advance) with a plug gap of 0.025” requires 1,900V (1.9kV) at no pressure and 7,600V (7.6kV) at pressure to create a spark. That sounds like a lot, but in actuality anything but a totally dead high-tension mag can deliver that much voltage. The problem is when that voltage needs to drive some current for some period of time. There needs to be enough voltage to jump the gap with enough current to create a hot channel lasting long enough to cause combustion.
All high-tension magnetos have two phases they go through in a single spark cycle. The first is the rather long energy collection phase, called dwell. Energy, in the form of high current and low voltage, is stored in the magnetic field around the primary coil. In a Wico EK, this phase begins when the armature just begins to leave the magnet poles and ends when the points open. It is important to understand that this stored energy will be the spark. Think of a basket that the primary coil chucks energy into during dwell or this first phase. The second phase begins when the points open, the low voltage, high current stored energy is converted to high voltage, low current and delivered to the sparkplug. The second phase ends when the stored energy is depleted and the spark ceases. During this phase, energy is snatched chunk by chunk out of the basket and converted to spark. The time the spark arcs is called the plug burn time and ends when the energy basket is empty. Energy is the product of voltage (V), current (I) and time (t), E = V X I X t. A high-tension magneto cleverly uses a little voltage a lot of current over a long time to load the energy basket then converts that to high voltage low current over a short time, emptying the energy basket quickly. When I say long time, I’m using an electronics clock. A Wico EK typically gathers and stores energy for 20 ms (0.02 second) then creates a spark that lasts 1 or 2 ms (0.002 second).
To investigate the effect of pressure, a steel tube was made with pipe threads for the sparkplug and a plug for the air source. A 3095 sparkplug was gapped to 0.025” and put under pressure. An unrestored Wico EK using a spring trip, for consistency, was connected to the sparkplug, Figure 2. The spark was monitored with an oscilloscope looking at the sparkplug wire between the magneto and the sparkplug as well as looking at the primary coil. Although I could not see through the steel tube, the electrical signals on the plug wire and the primary coil are very clear indicators of what the sparkplug is doing. Your auto mechanic will check your car’s ignition the same way.
Figure 2. Putting a sparkplug under pressure.
What should I expect to happen? The collection of energy by the Wico in phase 1 is unaffected by the pressure, therefore the spark energy is the same for all pressures. The energy collected is primarily set by the magnet strength, rate at which the armature leaves the magnet poles, number of primary coil turns and the core material, none of which change as the plug pressure increases. Since energy is the product of voltage, current and time, if the required spark voltage goes up under pressure the time must get shorter. In other words, a sparkplug under 50 psi will burn energy at a faster rate than the same sparkplug under 30 psi. With a given amount of energy available in the basket the 50 psi spark won’t last as long as the 30 psi spark. Figures 3 A,B,C and D compare the results. In Figure 3A the Wico is driving the 3095 sparkplug under no pressure and results in a plug burn time, arc time, of about 2.75 ms (0.00275 second). Figure 3B shows an arc time of about 1.25 ms when 16 psi is applied. Figure 3C shows that at 29 psi I get less than 1 ms of arc time. At 45 psi in Figure 3D, the plug burn time, arc time, is in the order of 0.5ms.
Figure 3A. Wico driving a 3095 plug under zero pressure.
Figure 3B. The plug under 16 psi.
Figure 3C. The plug under 29 psi.
Figure 3D. The plug burn time at 45 psi.
I would expect then that a weak magneto will collect and store less energy in the basket in the first phase (dwell) and would have an even shorter spark duration. Figure 4 shows the same magneto purposely degraded driving the same sparkplug under 0 psi of pressure. The plug burn time has dropped from 2.75 to about 2 ms . I might suspect then that this degraded magneto might give the arc near zero duration at 50 psi.
Figure 4. Slightly degraded magneto driving the sparkplug under zero pressure.
Bottom line, a weak magneto may appear to deliver a decent spark on the bench but under the pressure of compression the spark duration may become too short to cause combustion. As an aside, automotive folks like to see plug burn time in the region of 2 ms (0.002 second) and will tell you that 0.5 ms (0.0005 second) will likely cause misfires. Those times may seem incredibly short but the spark channel remains hot longer and your eye has image retention allowing you to see the spark.
I have shown that all but dead magnetos can deliver the necessary voltage to arc but the issue is doing that with enough current long enough to cause combustion. I have also demonstrated on the bench that as the plug pressure goes up the arc duration decreases and that a weaker magneto results in even shorter spark times. At that point it seemed a real test was necessary.
A Wico EK was mounted on a Stover engine (2.5 hp CT-2) developing 50 psi and a Hercules ( ¾ hp) that developed 30 psi peak. The magnetos were degraded step by step until the engines no longer ran or could be started. At that point I had a magneto that fails to run smoothly both a low and high compression engine.
The article Understanding The High-Tension Ignition System in Gas Engine Magazine February/March 2021 points out that all high-tension ignition systems use the same electrical schematic. There are several problems that will cause any high-tension magneto to be weak: weak magnet, sluggish or slow trip, low RPMs, shorted primary coil turns, improperly timed points and excess primary coil resistance to name some of the more common. From the external, these degradations all look the same because they all reduce the energy that is collected and stored in the energy basket in phase 1. In other words, a weak magnet and high primary coil resistance will behave the same. They all use the same schematic so I chose to use a Wico EK for these experiments because an easy way to degrade an EK is by adding primary coil resistance, Figure 5. This added coil resistance can be added step by increasing step until the engine runs rough or stops. After the experiment the resistor can be removed with no harm to the magneto. Figure 6 is the Stover under test.
Figure 5. Wico EK with series string of resistors to be patched in.
Figure 6. Stover running with resistance added to primary coil.
After degrading the magnetos step by step, I had a magneto that would not run a 50 psi engine with a 0.025” plug gap and another magneto that would not run a 30 psi engine. The interesting thing is that, as my automotive friends suggested, both the Stover and the Hercules stopped running smoothly when the spark duration got down to about 0.5 ms (0.0005 Second).
It became obvious that the reason a weak magneto won’t run an engine is not that it doesn’t produce enough voltage, under compression, rather the problem is that it doesn’t store enough energy in the energy basket to maintain the spark long enough to cause combustion. The higher voltage required under pressure causes the rate of energy consumption to go up.
After the mathematical manipulation of several energy equations, knowing that the energy basket was the same size and that more than 0.5 ms was needed for combustion, I could see that gap and pressure were reciprocal. This should mean that a 4X increase in pressure in the engine would correspond to a 4X gap increase on the bench. That is, if the magneto will create a spark under 4:1 compression it should create a spark 4 times the engine plug gap on the bench with no pressure. Spark, however, is a very complex phenomenon. When your eye sees a spark, it is actually many short duration sparks coming back-to-back, as seen in Figure 4. It is only the first one of those that follows 76 X gap X pressure equation, after the first the channel heats up and the gas atoms become ionized and the voltage drops. This would imply that a bench plug might not need to be gapped 4X for a 4:1 compression engine.
Figure 7. Some of the equipment used in these experiments.
I should point out that when the ignition was retarded for starting, the magneto needed to be stronger than in the advanced position. When retarded the piston is closer to TDC with more pressure at spark time. On the bench, the weak magnetos that would not run an engine were tripped with an increasing bench plug gap until the spark ceased. There was very little difference between a 0.5ms spark and no spark. For the 50psi engine, retarded with a 0.025” gapped plug the equivalent bench plug gap was 0.074”. The same conditions were run with the engine plug gapped to 0.037”, the bench plug equivalent was 0.119”. In both cases the bench plug was near 3X but I should point out some caveats: at these magneto degradation levels the engine was barely running, was very difficult to start and was very sensitive to the air fuel ratio. Sparking a plug on the bench gapped 4X your engine plug gap should ensure that your magneto will start with retard and run a high compression engine smoothly.
After this study was completed and written up, a lower compression (40 psi) throttle governed Cushman that could not be retarded, due to mechanical issues, was run thru the same process. Those results coupled with the Hercules suggest for lower compression engines with no ignition retard a bench plug gapped 3X the engine plug is sufficient.
For those who might question adding resistance to the primary coil, the 50 psi engine experiment was run again. In this run an EK magnet was carefully degraded, resulting in similar results. Also, for those that question that the problem is voltage not time, when the 50psi EK magneto was pulled off the engine it was still, in its degraded state, delivering 12,000V (12kV) pulses when it didn’t need to drive current for some period of time.
The run-of-the-mill unrestored EK used on the 50 psi Stover engine needed to be degraded to 35% of its original strength before the Stover stopped running. The fact that lower compression engines would run on even weaker magnetos leads me to believe that cylinder pressure is seldom the reason an engine won’t run.
I test all magnetos on the bench with a plug gapped to 0.125”. Also, when I pull a plug out of an engine to see if the magneto is working, I grab the 0.125” gapped plug. At 0.125” I believe I cover engines with up to 0.035 gapped plugs and 50 psi compression.
Cave 1/27/2021