A Short History of the Spark Plug
Spark plug historyfollows closely with the history of automotive development in America. Michigan, Ohio and Indiana were hot beds of automotive development and home to many of the people whose drive and ambition gave birth to the industry. This history is replete with such individuals as stoic and conservative Henry Ford, businessman Ransom Olds, flamboyant Alfred Sloan, and inventive Charles “Boss” Kettering.
As part of the combustion triumvirate of air, fuel and ignition, the spark plug lives a tough life. Its purpose is to provide a spark within the combustion chamber, igniting a great conflagration which rapidly moves across the chamber. The resulting heat and pressure moves the piston, thereby converting the heat energy available within a fuel mixture into mechanical energy. During the entire cycle of air/fuel intake, compression, ignition/ power and exhaust the spark plug is subjected to searing heat, noxious gas and then the cold burst as a new air/fuel charge enters the combustion chamber.
The basic design of a spark plug consists of a “side wire” which is welded to the steel shell and provides the ground for the electrical circuit. The center electrode is connected to the positive side of the circuit and therefore must be insulated from the steel shell. This is accomplished with a porcelain insulator which is necessary due to the heat generated within the combustion chamber. The problem arises when one attempts to provide a seal between the insulator and shell, a seal that must contain the combustion heat and gases yet not crack the insulator when assembled with the steel shell.
In the early development of the internal combustion spark ignited engine the combustion temperatures and pressures were relatively low. Fuels were of very low octane so engines were handicapped with low compression and generated low power and efficiency. The spark plug did not have to, and could not in all practicality, contain very high combustion pressure.
But while spark plug manufacturers, Albert Champion of Champion Spark Plug among them, were working on improving the manufacturing process, “Boss” Kettering was working on fuel improvement. With his development of Tetra Ethel Lead (TEL), Kettering gave engine designers the means to incorporate increased compression. While this provided a means to increased efficiency and power output it also put greater pressure on the spark plug.
As is typical of most human endeavors, each model year brought with it a demand for more horsepower and improved efficiency. The path to more power and efficiency is through the increase in cylinder filling with air-fuel mixture and greater compression. With the increase in power comes an increase in heat. It became more important to design spark plugs that could transfer excess heat to the cooling system. However, TEL also brought a new term to the spark plug business, “lead fouling”. Plugs that ran too hot would trigger the fuel mixture before the voltage was applied to the plug to create a spark. This was called “auto ignition” and would cause a loss in power and could damage the piston crown. Excessive heat would also melt the side wire and center electrode causing the plug to fail. Plugs that ran too cold would accumulate carbon and lead deposits and become lead fouled. The carbon deposits would retain heat and glow and could also cause auto ignition. The lead would bridge the spark plug gap and short the plug out so no spark was generated.
All of these issues had to be addressed and the means was a spark plug rating system. Through the auspices of the Society of Automotive Engineers the spark plug manufacturers joined to come to a common spark plug rating system. The means of rating a particular spark plug was the “spark plug rating engine”. The engine and the fuel to be used was standardized so that any manufacturer could establish a rating for its spark plugs and know where that plug fit relative to other manufacturers and the automobile manufacturers requirements.
In effect a spark plug to be rated was installed in the engine which had a means of altering its compression ratio. The engine would run with the plug to be tested and compression increased until auto ignition, sometimes called “ping” or “spark knock” was detected. The engine settings were analyzed and the plug was thus rated and a recommendation would be published for the spark plug application.
In the intervening years Albert Champion lost his company to his investors. He responded by starting AC Spark Plug Co. “Boss” Kettering had invented the electric starter in Dayton, Ohio and formed The Dayton Electric Company (Delco). Alfred Sloan was in the process of building General Motors and he absorbed both companies into GM, making AC-Delco a division. The other great American spark plug competitor was Autolite Spark Plug, which had been part of the Electric Autolite Company. Autolite was sold off to Ford Motor Company and ultimately to Bendix Co. There were small companies serving the aftermarket but Champion, AC and Autolite had the lion’s share of the market.
Spark plugs, of course, had long since moved beyond just the automotive market. They were now found in everything from lawn equipment, pleasure boats and aircraft. The plugs were designed for each application and were vastly different.
With the creation of the Environmental Protection Agency (EPA) automotive related pollution came under scrutiny. Government requirements began tightening the release of unburned hydrocarbons as well as oxides of nitrogen. At the same time the government also began the drive to improved fuel economy. The spark plug, being an integral part of the system, again came under pressure. Misfiring was not acceptable in a world of limited hydrocarbon emissions. Electronic ignition was introduced and helped eliminate misfiring. The entire system became more dependable with the elimination of the ignition point system. Spark plugs began to last longer than the ten thousand miles that used to signal time for a tune up.
With the advent of the catalytic converter came the end of TEL and lead fouled spark plugs. Spark plug life began to extend beyond fifty thousand miles. This was appreciated because engine compartments had become so very crowded.
The spark plug has evolved over the last 120 years and now has some new challenges. The new hybrid vehicles, and engines that shut off instead of idling when the vehicle stops, demand instantaneous starts. But as long as man depends upon the spark ignition engine the spark plug will have a place in the engine compartment.
