In the decade I’ve been with Motor Age, I’ve written dozens of how-to articles on a variety of topics. Many of them have been on common service procedures, like the one I’m going to talk to you about today. And in the process of researching these articles, I often find that I’ve been doing it wrong (in some way) and I’ll bet that, after reading this article, you’ll find out that you have to. Ready?
|(Image courtesy of Robert Bosch LLC) Today’s high compression, turbocharged, direct injection engines are placing even higher demands on the spark plug.|
First, some fun facts about spark plugs
Many claim to be the first to patent the idea of a “spark plug,” with patent records going back as far as the late 1800s. Names you recognize today were among them: Robert Bosch and Albert Champion, to name a few. There was even a patent submitted by Kenelm Lee Guinness, of the Guinness brewing family. Who knew beer and spark plugs had anything in common?
Albert, it seems, had a rough time in the early years, losing the Champion Ignition Company he founded in Flint, Mich. to investor drama. Shortly after, though, he was named president of the AC Spark Plug Company, formed with backing from Buick. This company’s products were used in a variety of applications including the aircraft used in Lindbergh’s and Earhart’s trans-Atlantic flights and even more cool, to fire off the second- and third-stage rocket engines that powered Neil Armstrong’s mission to the moon.
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But it seems that most historians agree that the credit of designing what would become the modern spark plug goes to a Bosch employee, Gottlob Honold. Assigned the task by Robert Bosch in 1901, Honold engineered a magneto ignition system based on what was then known as “electric arc” ignition. This design is credited with making the modern spark ignition engine a possibility.
|(Image courtesy of Robert Bosch LLC) By means of two coils on the armature, Bosch’s Gottlob Honold generated a high-voltage current. This was conducted to a spark plug via a simple cable connection.|
How does it work?
The terminal (top) end of the spark plug is connected to a high voltage supply source, the ignition coil. While the coil is incorporated in a variety of ways consistent with different ignition system designs, it still serves the same purpose Gottlob’s magneto did. As current flows from the coil, a voltage potential builds between the center (positive) electrode and the side (ground) electrode. Now, of course, the two electrodes do not touch and that means it’s an open circuit, right?
Not entirely. There is something there – air. Or more precisely, the air/fuel mixture that was injected into the cylinder micro-moments before. Initially, this mixture acts as an insulator, but as the voltage potential grows, the gasses begin to change. When the dielectric strength of the gasses is reached, the gasses ionize and become a conductor instead. No more open circuit!
As the current flows across the now-bridged gap, the temperature in the spark channel rises significantly and quickly. This causes the ionized gasses to expand just as quickly, like a small explosion, and results in the “click” you hear when a spark jumps outside of the combustion chamber. Same idea between lightning and the resulting thunder you hear during a summer storm. The heat and pressure force the gasses to react with one another and ideally, near the end of the spark event, a small ball of fire (or kernel) has formed as the gasses burn on their own. This tiny little fireball then expands to consume the surrounding gasses and combustion is realized.
Small part, big job
Based on what I’ve shared so far; you can see that the spark plug plays a critical role in the combustion process. Extending that idea, then, it is easier to understand that even minor discrepancies in the ability of the plug to function can result in inefficiencies of the combustion process.
For example, consider a gap that is too big or too small for the application. Too small, and the resulting spark may be too weak to ignite the air/fuel mixture. The spark will fire each ignition cycle, but still be the root cause of an elusive miss. Too big, and the voltage requirement goes up, perhaps to the point that the plug doesn’t “fire” at all. Excessive gaps, either due to improper initial setting or wear, can also stress ignition coils to the point of failure.
Ever drop a spark plug during a routine service? What did you do with it after you dropped it? The correct answer is “threw it away and got another one!” The drop could cause cracks or internal separations that will lead to premature, if not immediate, failure.
And what about something as simple as removing and reinstalling a plug? Let’s find out from a few men who deal with spark plugs on a daily basis.
What the experts say
Early in my writing career, I learned (the hard way) that I should never rely on my preconceived notions on even the simplest of topics. As I mentioned from the start, I have often learned that I was doing something wrong even in the most routine repairs. I strive to be as professional as possible, and share what I learn in the hope that you will be too. With that said, let’s see what some of the experts in the spark plug segment have to say about the proper service of this common component.