Spark Plugs

A spark plug (sometimes in British English , a '''sparking plug''') is an electrical device that fits into the Cylinder Head of some Internal Combustion Engine s and ignites compressed Aerosol Gasoline by means of an electric spark. Spark plugs have an insulated center electrode which is connected by a heavily insulated wire to an Ignition Coil or Magneto circuit on the outside, forming, with a grounded terminal on the base of the plug, a Spark Gap inside the cylinder. Early patents for spark plugs included those by Nikola Tesla (in US patent 609,250
for an ignition timing system], 1898 ), Richard Simms (GB 24859/1898, 1898), and Robert Bosch (GB 26907/1898). Karl Benz is also credited with the invention.

Internal combustion engines can be divided into ''spark-ignition engines'', which require spark plugs to begin combustion, and ''compression-ignition engines'' ( Diesel Engines ), which compress the fuel/air mixture until it spontaneously ignites. Compression-ignition engines may use Glow Plug s to improve cold start characteristics.

Spark plugs may also be used in other applications such as Furnace s where a combustible mixture should be ignited. In this case, they are sometimes referred to as flame igniters.

OPERATION

The spark plug is connected to thousands of Volt s generated by the Ignition Coil . As the electrons are pushed in from the coil, a voltage difference appears between the center electrode and side electrode. No current can flow because the fuel and air in the gap is an insulator, but as the voltage rises further, it begins to change the structure of the gases between the electrodes. Once the voltage exceeds the Dielectric Strength of the gases, the gases become Ionized . An ionized gas becomes a conductor and an ionized gas can pass electrons.

As the current of electrons surges across the gap, it raises the temperature of the spark channel to 60,000 K . The intense heat in the spark channel causes the ionized gas to expand very quickly, like a small explosion. This is the "click" you hear when watching a spark, similar to Lightning and Thunder .

The heat and pressure force the gasses to react with each other and at the end of the spark event there should be a small ball of fire in the Spark Gap as the gases burn on their own. The size of this fireball or kernel depends on the exact composition of the mixture between the electrodes and the level of combustion chamber turbulence at the time of the spark. A small kernel will make the engine run as though the ignition timing was retarded and a large one like the timing was advanced for that individual cycle.

SPARK PLUG CONSTRUCTION

A spark plug is composed of a shell, insulator and the conductor. It pierces the wall of the Combustion Chamber and therefore must also seal the combustion chamber against high pressures and temperatures, without deteriorating over long periods of time and extended use.

Parts of the plug

Terminal

The top of the spark plug contains a terminal to connect to the Ignition System . The exact terminal construction varies depending on the use of the spark plug. Most passenger car spark plug wires snap onto the terminal of the plug, but some wires have Spade Connector s which are fastened onto the plug under a nut. Plugs which are used for these applications often have the end of the terminal serve a double purpose as the nut on a thin threaded shaft so that they can be used for either type of connection.

Ribs

The physical shape of the ribs function to improve the insulator and prevent electrical energy from leaking from the terminal to the metal case along the side of the insulator. The disrupted and longer path makes the electricity encounter more resistance along the surface of the spark plug.

Insulator

The insulator is typically made from an Aluminium Oxide Ceramic as is designed to withstand 550° C and 60,000 V . It extends from the metal case into the combustion chamber. The exact composition and length of the insulator partly determines the heat range of the plug.

Seals

As the spark plug also Seals the combustion chamber of the engine when installed, the seals ensure there is no leakage from the combustion chamber.

Metal case

The metal case of the spark plug bears the torque of tightening the plug, serves to remove heat from the insulator and pass it on to the cylinder head, and acts as the ground for the sparks passing through the center electrode to the side electrode.

Insulator tip

The tip of the insulator surrounding the center electrode is within the combustion chamber and directly affects the spark plug performance, particularly the Heat Range .

Side electrode, or ground electrode

The side electrode is made from high nickel Steel and is welded to the side of the metal case. The side electrode also runs very hot, especially on projected nose plugs. Some spark plug designs use multiple side electrodes that do not overlap the center electrode.

Center electrode

The center electrode is connected to the terminal through an internal wire and commonly a ceramic series resistance to reduce emission of radio noise from the sparking. The tip can be made of a combination of Copper , Nickel - Iron , Chromium , or Precious Metal s. The center electrode is usually the one designed to eject the electrons (the Cathode ) because it is the hottest (normally) part of the plug; it is easier to emit electrons from a hot surface, because of the same physical laws that increase emissions of vapor from hot surfaces. In addition, electrons are emitted where the electrical field strength is greatest; this is from wherever the radius of curvature of the surface is smallest, ''i.e.'' from a sharp point or edge rather than a flat surface. It would be easiest to pull electrons from a pointed electrode but a pointed electrode would erode after only a few seconds. Instead, the electrons emit from the sharp edges of the end of the electrode; as these edges erode, the spark becomes weaker and less reliable.
At one time it was common to remove the spark plugs, clean deposits off the ends either manually or with specialized sandblasting equipment, and file the end of the electrode to restore the sharp edges, but this practice has become less frequent as spark plugs are now merely replaced, at much longer intervals. The development of precious metal high temperature electrodes (using metals such as Yttrium , Iridium , Platinum , Tungsten , or Palladium , as well as the relatively prosaic Silver or Gold ) allows the use of a smaller center wire, which has sharper edges but will not melt or corrode away. The smaller electrode also absorbs less heat from the spark and initial flame energy. At one point, Firestone marketed plugs with Polonium in the tip, under the questionable theory that the radioactivity would ionize the air in the gap, easing spark formation. ( See external link below )

Spark plug gap

Spark plugs are typically designed to have a spark gap which can be adjusted by the technician installing the spark plug, by the simple mechanism of bending the ground electrode slightly to bring it closer to or further from the center electrode. The somewhat common belief that plugs are properly gapped as delivered in their box from the factory is incorrect, as proved by the fact that the same plug may be specified for several different engines, requiring a different gap for each. A ''spark plug gap Gauge '' with round wires of precise diameters is used to measure the gap; use of a Feeler Gauge with flat blades instead of round wires, as is used on Distributor points or Valve lash, will give erroneous results, due to the shape of spark plug electrodes. The simplest gauges are a collection of keys of various thicknesses which match the desired gaps and the gap is adjusted until the key fits snugly. With current engine technology, universally incorporating
solid state ignitions and computerized fuel injection, the gaps used are much larger than in the era of carburetors and breaker point distributors, to the extent that spark plug gauges from that era are much too small for measuring the gaps of current cars.

This adjustment can be fairly critical, and if it is maladjusted the engine may run badly, or not at all. A narrow gap may give too small and weak a spark to effectively ignite the fuel-air mixture, while a gap which is too wide may be too wide for a spark to fire at all. Either way, a spark which only intermittently fails to ignite the fuel-air mixture may not be noticeable directly, but will show up as a reduction in the engine's power and fuel efficiency. As the plug ages and the metal of the tip erodes, the gap will tend to widen; therefore experienced mechanics often set the gap on a set of new plugs at the engine manufacturer's minimum recommended gap rather than in the center of the specified acceptable range, to ensure longer life between plug changes. On the other hand, since a larger gap gives a "hotter" or "fatter" spark and more reliable ignition of the fuel-air mixture, and since a new plug with sharp edges on the center electrode will spark more reliably than an older, eroded plug, experienced
mechanics also realize that the maximum gap specified by the engine manufacturer is the largest which will spark reliably even with old plugs, and will in fact be a bit narrower than necessary to ensure sparking with new plugs; therefore, it is possible to set the plugs to an extremely wide gap for more reliable ignition in high performance applications, at the cost of having to replace and/or regap the plugs much more frequently, as soon as the tip begins to erode.

Variations on the basic design

Over the years variations on the basic spark plug design have attempted to provide either better ignition, longer life, or both. Such variations include the use of two, three, or four equally spaced ground electrodes surrounding the center electrode. Other variations include using a recessed center electrode surrounded by the sparkplug thread, which effectively becomes the ground electrode. Also there is the use of a V-shaped notch in the tip of the ground electrode.

Sealing to the cylinder head

Most spark plugs seal to the cylinder head with a hollow metal washer which is crushed slightly between the flat surface of the head and that of the plug, just above the threads. If the torque used to install the plugs is not excessive, the washer can be reused when the plug is removed and reinserted, although this practice is, strictly speaking, not recommended and replacement washers are available.

Ford engines, however, were once distinct in using a tapered hole and a matching taper on the bottom of the plug above the threads, in order to seal the plug. The torque for installing and removing these plugs was higher, and it was easier to break them if the wrench were applied partially off axis.

More recently, some types of Ford Fiesta , and Ka also had a similar sealing system. The torque required to install these plugs is less than with the above type, and it is extremely critical that they not be overtightened, since overtightening can result in it being difficult or impossible to remove them. In addition, they have been known to corrode into
the cylinder head, particularly if left in too long between removals. In such a situation, it is not unknown for a plug to snap below the hexagonal nut, leaving just the threaded portion (and the outer electrode) in the cylinder head. Ford has on occasion issued Technical Service Bulletins reminding technicians to use the correct methods of installation.

Tip protrusion

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Spark Plugs