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.
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.
Spark plug of the future
What may become of the spark plug in the future? For one thing, it may get much smaller. Heads are becoming full of holes. Along with multiple valves, there is now direct injection. As displacement continues to shrink so does the space in the head for spark plugs so they may become as thin as pencils.
Another possible solution is lasers. They are already being employed on large stationary industrial engines.
Yet another possibility is plasma ignition. It is found on some race car engines and jet aircraft engines.
Bosch reports that homogenous charged compression ignition (HCCI), which is similar to that used by diesel engines, is a technology being investigated.
Corona discharge is another technology on the near horizon. Champion Spark Plugs has as patent pending on a spark plug that creates long streams of ionization to replace the small spark created across a conventional gap.
Then again, ignition may be supplied by something completely new and ingenious like what happened over a century ago.
Spark plug replacement
Spark plug replacement is such a routine job, that it is often assigned to a rookie in the shop. But it does not hurt anyone to review the steps for proper plug replacement.
• Start with a cold or cool engine. This helps avoid thread damage, especially in aluminum heads.
• Unless the plugs are protected from the environment such as COP plugs, used compressed air to blow any sand, dirt or debris from the spark plug wells.
• Crack the plugs loose using a spark plug socket. If they fight back, squirt a little penetrant into the well and allow it to soak in for several minutes. Ford Triton engines may need to soak as long as overnight. (See ASP May/June 2011.)
• If the plugs did not come out smoothly, you may want to chase the threads in the cylinder head (use a chaser tap to re-form the threads, not a cutting tap.)
• Check the gap on new plugs. Most spark plugs come properly gapped from the factory, but it never hurts to verify. If you need to re-gap do not pry on the center electrode as it will likely snap off.
• Do not use anti-seize compound even if those old timers say you must. It can lead to fouling and over-tightening. Today’s plugs are treated to prevent seizure. If you insist, apply anti-seize very sparingly to only the top few threads to prevent it from getting on the electrodes and reduce torque by about 30%.
• Install the new plugs by hand to avoid cross-threading. In deep, hard-to-reach places, put a length of vacuum hose on the plug to extend your reach.
• Use a torque wrench in order to properly tighten according to the engine manufacturer’s specs. Improperly torqued spark plugs can lead to costly damage.
• If you refuse to use a torque wrench, give flat seat (gasketed) plugs a 90-degree turn after they make seat contact. Give conical-seat plugs a 15-degree turn. Do not use a half-inch drive ratchet.
Spark plug heat ranges
There is a common misconception about spark plug heat ranges, even among some technicians. The error is in thinking a hotter spark plug creates a hotter spark, hence it must be a better spark. Nope. Heat range refers to the plug’s ability to dissipate heat from the tip. Hotter plugs dissipate the heat slower which helps prevent deposits. Colder plugs dissipate the heat quicker to reduce the likelihood of knock. Here is how NGK explains the conditions and solutions:
• Low quality and/or low octane fuel can cause knock which will elevate cylinder temperatures. The increased cylinder temperature will cause the temperature of the combustion chamber components (spark plug, valves, piston, etc.) to rise, and will lead to pre-ignition if the knock is uncontrolled.
• When using an ethanol blend fuel with high ethanol content in high performance applications, a colder heat range may be necessary. The spark timing can be advanced further because ethanol blend fuel has a higher resistance to knock (higher octane). Due to the decreased knock, there will be less audible “warning” from knock before the spark plug overheats and pre-ignites.
• Some types of fuel additives in lower quality fuels can cause spark plug deposits that can lead to misfires, pre-ignition, etc.
• Advancing ignition timing by 10 degrees will cause the spark plug tip temperature to increase by approximately 70 degrees to 100 degrees Celcius.
• A colder heat range spark plug may be necessary if the ignition timing has been advanced to near the knock level. Higher cylinder temperatures near the knock level will bring the spark plug firing end temperature closer to the pre-ignition range.
• Significantly increasing the static/dynamic compression ratio will increase cylinder pressures and the octane requirement of the engine. Knock may occur more easily. If the engine is operated near the knock level, a colder heat range spark plug may be necessary due to the resulting increased cylinder temperatures.
Forced induction (turbocharging, supercharging)
• A colder heat range spark plug may be necessary due to the increased cylinder temperature as boost pressure (manifold pressure) and subsequent cylinder pressure and temperature increase.
Ambient air temperature/humidity
• As the air temperature or humidity decreases, the air density increases, requiring a richer air-fuel mixture. If the air-fuel mixture is not properly richened, and the mixture is too lean, higher cylinder pressures/temperatures, knocking, and the subsequent increase in the spark plug tip temperatures can result.
• As the air temperature or humidity increases, the air density decreases, requiring a leaner air-fuel mixture. If the air-fuel mixture is too rich, decreased performance and/or carbon fouling can result.
• Air (atmospheric) pressure and cylinder pressure decrease as altitude increases. As a result, spark plug tip temperature will also decrease.
• Fouling can occur more easily if the air-fuel mixture is not adjusted to compensate for the altitude. Higher altitude = less air = less fuel. ●