Auto Service World
Feature   March 17, 2010   by CARS Magazine


The spark plug continues to improve and evolve.

–Jeff Taylor

The humble spark plug is a key component to the internal combustion engine, even if it’s hard to credit one particular individual as being the founding father of this crucial automotive part.

Indeed, spark plugs may have been invented as long ago as the 1850s by either Edmond Berger or Oliver Lodge. The trouble is neither of these men received a patent. Later, Nikola Tesla, Robert Bosch, Richard Simms and Karl Benz all applied for patents or were responsible for the development of this vital engine component in 1898.

However, the real hero in the early development of the working spark plug (for high voltage) was Gottlob Honold, an engineer for Robert Bosch in 1902. Honold’s developments lead to advancements in early internal combustion engines. Today, spark plugs still perform the same basic task: ignite the compressed air/fuel mixture that forces the piston down.

Modern engine designers now excel at squeezing more power (yet releasing fewer emissions) out of every drop of fuel that is burned. And they accomplish this feat via better spark plug technology. 

 Spark plugs have many parts and pieces that function together as one cohesive unit. When we look at a plug, we see the metal/alloy base, the insulator, the electrode tip and the terminal where the spark plug wire or ignition coil attaches. What we don’t see is all the time, research and developments that were invested in this vital engine component. For example:

          The metal case (or housing) allows the spark plug to be threaded into the cylinder head and provides the ground path (side electrode). It also provides the sealing surface which mates to the cylinder head (gasket or tapered seat.) It holds and seals the insulator/terminal electrode assembly and removes the heat transferred to the insulator to the cylinder head. The case/housing thread size will get smaller. Engine designs are demanding tighter combustion chambers with larger valves and fuel injectors (GDI) for better efficiency and that means a smaller, more compact spark plug design is needed.

Spark plugs will continue to get longer and thinner as a result. The current M14 plug with a thread length of about 19mm is giving way to the thinner M12 with a longer thread length of about 26mm. We’re going to see it get smaller as well, down to a size of M10 or even M8 in the not-too-distant future.

The traditional housing is a one-piece unit made of a nickel-chrome or a nickel-plated alloy that resists corrosion both inside and outside the cylinder head. Some manufactures have been using a two-piece design that extends the tip deep into the cylinder head (this has created a new set of concerns, although that’s a story for another day.)

          The insulator assembly is a complex component. It has to hold the centre electrode, and the wire/coil attaching terminal end. It contains a resistor for radio interference control (5KΩ). It provides a seal that restrains the combustion chamber pressures. It has to provide dielectric properties to resist voltage loss and flashover and withstand the mechanical forces of vibration and heat shock (as much as 3000°C difference between the terminal end and the spark tip).

Aluminum oxide-based ceramic insulators provide excellent dielectric strength and are able to withstand 60,000V at temps of 1000°C. The thermal value of a spark plug is reflected in its ability to handle heat at the exposed insulator tip inside the combustion chamber. A hot plug has a longer insulator base, can absorb more heat and transfer it slowly away to the cylinder head. A cold plug does the opposite: it doesn’t absorb much heat and when it does it transfers the heat away quickly.

The thermal performance of the insulator tip is very important and is dependent on engine type and the type of fuels that will be used. Heat transfer is regulated not only by the insulator tip’s length but by the gas volumes around it and the material composition of the electrode inside it (usually copper that has been cored for better heat transfer).

The ideal insulator tip temperature is in the range of 500°C to 850°C. Anything lower than 500°C and the tip can’t reach a self-cleaning temperature to burn off the carbon deposits. The accumulation of these deposits leads to fouling, misfire, increased exhaust emissions and poor fuel mileage.

When temperatures exceed 850°C, we start to see increased electrode wear and 1000°C is sufficient to melt or blister the electrode and cause mechanical damage not just to the plug but the engine due to pre-ignition and detonation.

          The centre and ground electrodes: A massive engineering effort has gone into the design, shape, composition and configuration of the center electrode and the ground or side electrodes. The engine type/style, cylinder head design, fuel type, injection method and ignition configuration all affect the design and style of electrode required. Current tip design uses a corrosion resistant nickel alloy electrode (gold has also been used) with a copper core for good heat transfer and a combination of precious metals for durability.

Every time that a spark is created, a handful of molecules comprising the electrode’s surface area are removed. This causes electrode erosion with the end result being increased spark gap.

As the engine manufacturer wants to keep the spark plug gap the same over its service life (up to 160,000 km) to optimize emissions and fuel mileage, this wear is unacceptable. The use of a precious/noble metal – i.e., platinum, iridium, rhodium, palladium, tungsten or yttrium – solves this problem. Platinum and iridium (iridium is harder and has a higher melting point than platinum) are the current favourites; both metals are very hard and very resistant to wear and are standard equipment on most new vehicles.

Tip design is also critical. A fine point is best at producing a spark, but wears quickly unless it’s protected. Laser technology allows the welding of a precious metal cap on top of the centre electrode as a protective cap. This has led to fine wire centre electrode tips that are as small as 0.4mm across. Such a diminutive small size requires less voltage to fire, thereby reducing the quenching effects of a larger electrode and providing better gas flow.

These plugs fire more easily and result in less misfires and fouling; there’s also lower thermal shock. These qualities result in extra power, better starting, improved acceleration, lower emissions and better fuel economy.

Of note, there are many ground electrode designs and variations. Ground electrode protection via the use of a precious metal is standard on most plugs. Subtle changes in the shape and placement all affect the spark kernel and its flame propagation properties. The use of multiple ground electrodes is common (there can be a combination of up to four.)

Multiple electrode design increases the service life as it provides more areas for the spark to occur, lowers the wear at any one point, and provides the same gap for a longer time. This design also features self-cleaning properties with the use of asemi-surface discharge that travels across the insulator tip before jumping to the ground (this burns off the deposits on the insulator tip.) Ground electrodes can be tapered, angle-cut, set back, split, U-shaped, multi-grooved, and at different heights. But the end result is to enhance the spark’s development and aid in flame proliferation.

          The future? At present, there are gasoline direct injection (GDI) engines in production and the spark plug requirements of this engine are different from anything we’ve encountered before.

The GDI engine has less room in the cylinder head for a spark plug because there’s now a fuel injector as well. The thermal shock on the insulator tip is extreme, and the effects of the charge density will mean a higher voltage to fire the mixture. The GDI engine’s combustion chamber uses a stratified charge. There are very rich areas and very lean areas in the combustion chamber increasing the demands on the spark plug. These factors increase the chances of the plug to foul, so multi-electrodes will be the norm and they will all incorporate some sort of semi-surface discharge ability to keep the insulator tip clean.

Meanwhile, the electrodes will have a longer projected spark plug gap to make sure the spark is applied to the correct region of the air/fuel mixture. These conditions will affect plug design as the increased firing voltages and smaller insulators are contradictory demands, newer materials are being developed, plugs bodies will get smaller, and the current six-point body shape will go to a 12-point design for better torque and sealing.

We may see more spark plug indexing that orient the ground electrode in relation to the intake valve, aiding in combustion efficiency. Hybrid and variable displacement spark plugs have to deal with the cooling affects of not igniting an air/fuel mixture and will have to resist fouling while dealing with the associated thermal loads.

For now, the question arises: are we always going to use spark plugs? The answer is a definite maybe. Some manufactures are looking at alternatives to the simple spark plug. With the demand to get the engine smaller and more efficient, the space for a spark plug is being limited so lasers are being toyed with. Ford has a working model that uses one laser and a series of fibre optic cables and reflectors to start the combustion event. This system can provide multiple ignition points in different areas of the combustion chamber; the result is better combustion with less fuel. Will it work? Probably. The technology isn’t new, dating back to 1978. But this is an expensive alternative, and the lasers need to be designed to function in the harsh environment of the modern automobile.

Still, thanks to advancements in electronics and given the ongoing demand for better fuel economy and fewer emissions, spark plug requirements will continue to change. And although change is a constant in our industry, expect the spark plug to hang around in the years to come.

Jeff Taylor is a technician at Eccles Auto Service in Dundas, Ont. 

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