For a bit over a hundred years now, sparkplugs fed with high voltage have been the preferred ignition method for the majority of internal combustion engines. Over the century, billions of sparkplugs have been made in all manner of variations by thousands of manufacturers.
Many of the variations created were attempts to address flaws in both materials and engine designs. Many such flaws would manifest themselves in difficulty of starting or fouling of the plugs during operation–usually by carbon or oil from overly rich fuel mixtures or poor oil control rings on the pistons.
To address hard starting, often caused by not being able to get sufficient fuel pulled into the cylinder from the updraft carburetors that were common at the time combined with slow cranking speeds (often by hand) Manufacturers would fit priming cups to the engines…where this was not feasible, sparkplugs were fitted with such cups. Simply fill the cup with fuel, open the cock to let it flow into the cylinder, and close the cock afterward. By the time you completed this process on all the cylinders and got back to the hand-crank, it would have likely had enough time to vaporize into a good combustible mixture that would allow for an easy start. Provided everything else was in order and no crucial step (like turning on the ignition power circuit) had been omitted. Otherwise you got to try again. And sometimes again. …and maybe a third time. By now your arm would be good and tired and the four letter words readily flowing into audibility. Hopefully you did not flood the engine……else you got to crank it with the plugs removed and wait to let the excess fuel evaporate and clear before trying all over again from the beginning. You didn’t need to get there on time anyway.
Another variation of the day was the so-called visible plug. There were actually two types of this. One was fitted with an extra gap outside of the cylinder. This was adjustable…and could often be viewed. The idea was that you could get an idea of the firing conditions based upon how the secondary arc looked. Adjusting it would have the so-called effect of “intensifying” the spark. It actually had the effect of retarding the timing on that plug a bit because this extra total gap length caused the voltage on the lead to have slightly more time to build up prior to jumping the gaps. These plugs are often referred to simply as intensifier plugs and may or may not have the gap in a visible housing. The other variation is a truly visible plug. The insulator, rather than porcelain or mica, was made of glass. These are especially rare but especially cool. You can actually see the color of light from the combustion within the cylinder while the engine runs! Run your engine in a darkened area…night time is best. That way you can see it to full effect. And being in the dark will also make it harder for you to adjust the carburetor or timing to get it right, thus giving you the “full benefits” of the experience. …..keep a flashlight handy.
Another category would be the take-apart plugs. These can be disassembled to allow for quick cleaning or replacement of the electrodes. In many cases, this design also incorporated other features–you will find primer plugs that can come apart, for example. Most screwed together, but some were even made with a quick release lever. Run them until dirty, then take them apart to clean, and reinstall.
The next category is not so much a category but just another way that was used to address various issues. All manner of different electrode designs were used. Some with multiple points (still done today, especially in aircraft engines)….some had oddly shaped electrodes, some just a smooth disc. At least one was made with a ball in a cage that would dance around, ostensibly cleaning itself as it ran. One of the odd ones was the “Fan Flame” …yes, you guessed it, there was an actual spinning fan shaped electrode. This was intended to give a better spark dispersal and also keep itself clean. More than likely what it did was get gummed up if run too rich….or if run too lean it probably burned off of its little mount and became a piece of foreign debris banging around and scraping up the cylinder walls. Better get that hone ready.
The “Twin Fire” was a series gap plug. The spark would jump from center electrode to a metal ring on the insulator, then from there across another gap to the body.
Other materials were sometimes used…today we see iridium plugs, but once upon a time Polonium was used–and its radioactive properties were touted as beneficial to achieving better ignition and longer life. With such low intensity and a short half-life, it likely made no difference by any measure other than ad copy….but the box and pink insulator looked good if nothing else. For a time, sparkplugs were the quack medicine of the automotive world. Just as carburetors would become during the fuel crunch of the early 70’s.
There are those who are avid sparkplug collectors. A great deal of knowledge of automotive history can be gleaned from an understanding of the history of sparkplug developments. You can join the Spark Plug Collectors of America if you are serious about it. http://www.spcoa.net/ Automotive museums often have their own collections on display. My collection is miniscule by comparison, it fits in a box. I have purchased a few just because they were unique and of interest to me….but usually I end up with them because I collect, restore, and use vintage engines. Some stay in my collection, others are fixed and sold or redone for a client’s project. Hit and Miss types, old outboard motors, antique cars, early aircraft engines, etc., etc. Often these old engines come to me with equally old plugs in them. Sometimes the plugs are good and get cleaned and reused, and sometimes they end up in my collection.
Cleaning and testing of sparkplugs used to be commonplace. It is still done in the aviation industry where the plugs are particularly costly to replace. Up until the 1960’s and 70’s, many automotive shops would clean, test, and reinstall plugs. Today, like everything else, they are mostly just discarded when fouled so that they can be used to help increase the size of the landfill and generate another sale at the store.
Cleaning machines were basically little sand blasters that would knock away the carbon deposits. Usually they were purpose built and had a design that allowed a plug to be placed, cleaned, and removed quickly.
The testers were an interesting contrivance. A plug might appear to fire just fine in air. But in the cylinder, under the heat and pressure of compression, it might not. In a service shop, the closest way to approximate this is to put the plug into a tester that allows you to apply air pressure and while firing the plug, observe it through a little window. These machines are still used in aircraft shops…and were once common in garages across the country.
About ten years ago I built one for the shop I worked in. (we did aerial survey work and the A&P mechanic wanted one for testing plugs when servicing the aircraft engines) The photo should be self-explanatory.
I used an oil burner ignition transformer for the high voltage source, a few bits of basic plumbing, and a piece of Lexan sandwiched between two blocks of aluminum to form the porthole window and threaded area to place the plug under test. The mirror is a safety feature. If a plug were screwed in that had some fuel or solvent on it, the combination of pressurized air and the fuel vapors could result in an ignition when the button is pushed. Unlikely, but it is possible. IF this were to happen, the window could possibly be blown out…..best to have it face another direction and be viewed via a mirror. It works well and was far less expensive than purchasing a new unit. It has been a couple of years since I worked there, but they were still using it when I visited a while back.
This pretty red tester is a commercially made model from Champion. It dates to the 1930’s and uses a buzz coil for the high voltage, much like the model T units. All other features work pretty much like the one I had built. This one, however, has multiple adapters and sizes to accommodate a wide range of sparkplug types–something unnecessary in the aircraft shop where all the plugs we tested were the same size threads but very important in a garage where any sort of car could have shown up. As you can see, it has the same sort of mirror arrangement as well.
Use of the tester to get a look at the firing patterns of some of the early plugs is an interesting exercise. While not a real-world test of the validity of a plug maker’s claims it does give some insights into what would have made a difference and what would not. If nothing else the resulting photographs can be interesting or even artistically pleasing to look at and shooting these with a macro lens on film could be a fun project in and of itself.
Ok Robert, I gotta ask… Is there any subject you don’t have knowledge about? Is it all stuck in your brain? Even after years knowing you, you still amaze me.
Hahaha! Many. But really, there is a common thread to much of what I know. Yes, much of it is direct from experience and memory. I may not remember what I walked into the next room to get….but I can remember what happens if you open Fibber McGee’s closet.