Tech Stuff

Fire in the hole! Spark plugs: past and present

Weber is president of Virginia-based Write Stuff. He is an award-winning freelance automotive and technical writer and photographer with over two decades of journalism experience. He is an ASE-certified Master Automobile Technician, and has worked on automobiles, trucks and small engines. He is a member of the Society of Automotive Engineers (SAE) and numerous other automotive trade associations. He has worked as an auto service technician, a shop manager and a regional manager for an automotive service franchise operation.

Fire in the hole. That, as you know, is how the internal combustion engine works. Mixing fuel and air then igniting it causes a rapid rise in pressure. This is often called an explosion, but in an engine, it is in reality a controlled burn.

Igniting that air/fuel mixture has been a challenge since the first engine was built. The challenge continues to find better, more durable, more accurate, more timely and more precise ways of burning the fuel.

This is especially true as engineers strive to meet the goals of better fuel economy and reduced emissions. That was of little concern back when the internal combustion, gasoline engine was born.

The earliest motor vehicle engines used a system called a “hot tube” to provide the ignition. (Prior to the hot tube, slow spinning engines used a slide plate the exposed the cylinder mixture to a flame, but these typically ran at about 100 rpm). Gottlieb Daimler’s early engines spun up to 600 rpm thanks to the hot tube.

The hot tube was kind of like a glow plug. The heated tube was closed at one end similar to an inverted test tube and was heated by an external flame. During the compression stroke, some of the air/fuel mixture got pushed into the hot tube where ignition began. Clever, yes, but not very precise. But these were also extremely low compression engines.

Meanwhile, Robert Bosch was fiddling with electricity and inventing something called the magneto. It, too, was a clever device that generated voltage using coils of wire and permanent magnets. The magneto is extremely reliable and durable — so durable and dependable that its magnetos are still used in not only small engines such as lawn mowers, but in small personal airplanes.

At the same time another new device was under development that used the magneto’s electricity to make a spark jump a gap to ignite the mixture for a stationary engine. Gottlieb Daimler was the first to test this thing we call a spark plug in non-stationary, automobile and truck engines in 1898.

The spark plug has been the way to make fire in the hole ever since.

Through the years, despite the fact that it remains recognizable, the spark plug has gone through enormous changes.

The latest innovation is the use of iridium which may replace platinum as the rare metal of choice. Incidentally, one of the earliest spark devices was a pair of platinum wires affixed to a couple porcelain insulators.

It was platinum center electrodes that permitted spark plugs to go up to 100,000 miles and they have dominated the market since their introduction in the mid-1980s.

Just about a decade later, the first iridium plugs were introduced. Iridium has the advantage of having a higher melting point than platinum, is six times harder and is more corrosion resistant, reports the Automotive Aftermarket Service Association (AASA), of which the major spark plug manufacturers are members.

Another reported benefit is cost, but both are currently selling for over $1,000 per ounce. As of this writing, platinum was $1,596 an ounce and iridium was $1,025 an ounce. According to AASA, “...industrial demand is driving the cost of iridium more than platinum.” About 20% of the iridium is being used in spark plugs.

The corona ignition spark plug developed by Champion produces a high energy, high frequency electrical field to produce multiple streams of ions up to 25mm long to provide stronger ignition for the fuel mixture.
<p>The corona ignition spark plug developed by Champion produces a high energy, high frequency electrical field to produce multiple streams of ions up to 25mm long to provide stronger ignition for the fuel mixture.</p>

Although a tiny bit of precious metal is used on each plug, millions of them are sold every year. Additionally, some spark plugs have not only a platinum or iridium center electrode; they have a bit of the metal welded to the ground electrode, as well. They are often used on waste spark ignition systems since the plugs fire twice as often as a conventional system or a COP (coil on plug) system.

There are numerous ground electrode designs aimed at decreasing the possibility of misfiring while increasing the likelihood of producing a good spark with a large kernel to light off the air/fuel mixture. For instance, the groove design of some of Denso’s plugs effectively create a wider gap in the channel while providing a smaller gap for the initial spark to bridge.

NGK’s V-Power spark plugs have a grooved center electrode that helps achieve the same goal.

One-hundred-thousand-mile replacement intervals are now the norm. Prior to the platinum plug, it was common to replace spark plugs in as little as 20,000 miles.

Not only are extended change intervals attractive to the modern car owner who is often lax about maintenance, those intervals may save money and hassle.

As you know, with many vehicles, it is nearly impossible to reach one or more of the plugs due to engine design, layout or under hood space. Dropping the engine cradle to change one of the plugs is labor intensive.

Although most spark plugs will probably work fine, even well beyond the 100,000 mile mark, the gradually eroding gaps can put a strain on the rest of the ignition system. That’s something worth explaining to your customers. And remember that short trip driving, worn mechanical parts, such as rings and valve guides, can lead to plug fouling and the need for replacement.

Tags: Bosch  spark plugs 
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