This article is written by and © Copyright to Rob Skinner of www.rustyiron.com a fellow member of W.A.P.A. Thanks to Rob for generously making his article available for Steam & Engine. The article was first published on W.A.P.A.'s Hit'n'Miss magazine.
Three simple things are required to make an engine run: fuel, air and spark. Considering the simplicity of the formula, one would think that we wouldn't spend so much time building up our biceps by pulling on flywheels. Alas, there are many nuances that can adversely affect the trinity required to make an engine run.
Early in the development of spark ignition, it became obvious that there are factors that can prevent the spark from igniting the fuel-air mixture. One such factor is carbon fouling of the spark plug caused by incomplete combustion.
In the worst case, the carbon will create a direct short across the spark plug's insulator, preventing the engine from running. The solution is to clean the plug. But long before a spark plug becomes completely shorted, the carbon contamination slowly builds and slowly degrades the intensity of the spark.
George Meissner, in the early 1900's, fretted over this problem, as have most modem enginemen. He noticed, as many modem enginemen have, that by pulling off the spark plug wire and creating a small gap between the wire and the plug, that an engine with a contaminated plug would sometimes fire. Mr. Meissner, however, took the idea one step further, built a spark-gap attachment, and patented the idea.
Meissner's Spark Plug Intensifier is basically a glass ball containing two electrodes. As the spark plug fires, the engine operator can see the spark in the intensifier. Meissner's intensifier is unique in that it acts as a magnifying glass, making the spark easier to view by the operator.
In the standard high-tension ignition circuit, the spark is not really instantaneous. It's very fast, but many things happen as the voltage builds. If we were to monitor the voltage in slow motion, as the magneto fired, we would see the voltage rise from zero until it was sufficiently high to jump the gap on the spark plug. Depending upon many variables, this might happen at about 8,000 volts. The voltage then remains steady for a short period until the supply from the magneto is insufficient to maintain the spark. Clearly, a long duration spark has a better chance of igniting the fuel-air mixture.
In the ignition circuit with a contaminated plug, the contamination acts as a resistor, bleeding off the voltage as it builds. When the plug fires, it will be of shorter duration than if the spark plug was in good condition. If the contamination is bad enough, it will bleed off so much voltage that it never reaches the intensity to jump the spark plug gap.
The Spark Plug Intensifier adds another gap to the circuit. When there is another gap, the circuit is incomplete, and voltage is not able to bleed through the contaminated spark plug. However, when the voltage builds up sufficiently, it will jump both the the intensifier and spark plug gaps, creating a good spark of long duration.
At top is an original Meissner intensifier. At the bottom is a modern copy using an oiler sight glass.
There are drawbacks of such a system. Because there are two gaps, the voltage much be much higher before the spark will occur. If the magneto is weak, it might not be able to generate sufficient voltage to fire the system. Even if the magneto is in good condition, the higher voltage might cause it to degrade faster.
Are there benefits from a spark intensifier? Realistically, it's probably best to make sure you have a good magneto and a good spark plug. For the modem engineman, the primary benefit is that it's a neat little gadget to entertain observant spectators.
End of Rob's Article
This is a Rentz Lighthouse plug which is carries Meisner's idea into the plug itself. You see these appear on ebay from time to time, usually fetching rediculous prices. I'd love to have these on my engines for night running - the combination of the spark with flames from the exhaust (you did not know your old engine shot flames? Many of them do... run it in the dark :).
There were some discussions on spark intensifier plugs recently on the ATIS Stationary Engine List
I recently saw a rather odd spark plug. Conventional in every way except
that toward the top there was a glass tube with a spark gap in it, about a
.040 in gap. Top connector goes to the top electrode in the spark gap,
bottom electrode apparently goes to the bottom center electrode. What is
going on here, how does it work?
Jim and Diane
Hemet, CA USA
Could be some sort of test plug, or some people believe that making the spark jump a gap prior to the gap it jumps to fire the engine provided for a hotter spark. Perhaps the designer of this plug was "making hay" out of their belief.
PS, More intelligent people believe that if the spark is not "hot" enough to fire the engine the situation should be corrected!
All true enough. However, such a "booster gap" makes it a lot harder for the plug to foul out and stop firing due to carbon on the insulator leaking off the charge on the center electrode. A coil doesn't fire instantaneously when the points open. There's a very brief rise time for the voltage to get high enough to jump the gap. If there's enough current leaking through the conductive carbon on the insulator nose, the voltage won't ever get high enough, and there's no spark. With the booster gap there's no current flow until the voltage has risen high enough to jump both gaps, which it does. Many of these plugs have been used to keep an oil burning, plug fouling engine running.
That is a spark intensifier, and it really does work (assuming that the coil
has enough umpf to drive it.)
The concept is that the coil builds up to a higher voltage before jumping
the gaps. The tradeoff is reduced life of the coil. Also a LOT more radio