Category Archives: Ignition

Spark plugs – Questions & Answers (Corona stain)

Q. What is the function of the corrugations on the insulator?

A. They are what we call current creep barriers and prevent flash-over. Flash-over is when there is a voltage discharge between the top terminal nut and the metal shell of the plug (as shown in the picture, left) and is highly undesirable because it weakens or prevents the spark occurring within the combustion chamber.

This will lead to misfiring and poor performance and to prevent this phenomenon taking place corrugations (ribs) are provided on the insulator which in effect extend the surface distance between the terminal and the metal shell. This design enhances the insulation needed for preventing flash-over.

Note: Always ensure that the spark plug insulator and the covers/caps are clean and in good condition as worn, perished or dirty covers/caps significantly increase the chances of flash-over occurring.



Q. Is a stain between the insulator and metal shell caused by gas leakage?

A. On occasions, when a spark plug is removed, a brownish stain that looks like a sign of combustion gas flow can be seen on the insulator just above the caulked portion ofthe metal shell. This discolouration, known as Corona stain, is the result of oil particles suspended in the air adjacent to the plug becoming attached to the surface of the insulator. It does not affect spark plug performance.




Q. Why does Corona stain occur?

A. Spark plugs have very high voltages applied to them in order to create a spark at the electrode gap. Under certain conditions this high voltage creates a phenomenon called Corona discharge (pictured, left) which occurs over the insulator just above the metal shell. It is formed due to the ionization of the gases around the plug. The oil particles are attracted by this discharge and adhere to the insulator causing the discolouration. In low light conditions this event may be observed as a pale blue glow around the high tension leads and plugs.

A quick guide to ignition coils

The operating principle of the ignition coil is essentially the same for all types – whether the classic can-type coil, or in a coil rail system. The device contains two copper wire windings and a laminated iron core, with the copper wires featuring insulating materials to prevent short circuits.

The battery current fed through the primary winding produces a magnetic field whose strength is further increased by the iron core.

When this circuit is opened, the magnetic field collapses, inducing a high voltage pulse in the secondary coil. This pulse is fed through the H.T. connection to the spark plugs. As an integral part of the ignition system, the coil produces the high voltage required to produce the electric spark to ignite the fuel. The relatively low battery voltage, nominally around 12V, is then transformed to up to 45,000V.

How can 12V produce a high-voltage pulse?

The secondary coil consists of a very fine wire with many more windings than the primary coil. The winding ratio is typically between 1:150 and 1:200. This has the effect of multiplying the voltage whilst reducing the current. The voltage output from the device depends upon:

  • The value of primary circuit current
  • The ‘turns ratio’ of the windings
  • The change time of the coil
  • The rate of magnetic field collapse

The different types of ignition coil

The last few decades have seen great improvements in ignition technology. As a consequence, various new ignition coil types have been developed. Depending on the age of the vehicle, the engine design and the ignition system, any of these ignition coil designs might be used:

Can-type ignition coils
In older vehicles and vintage cars, you might still find what is commonly known as a can-type ignition coil. Some older versions of this type are filled with oil, which acts as an insulator and a coolant, but most have a more modern dry insulation design.

Distributor coils
For this type, the induced high voltage reaches the individual spark plugs via a mechanically driven distributor mechanism.

Ignition blocks
Ignition blocks contain several ignition coils, which are connected by H.T. cables to each plug. This ignition coil type is available with single or dual spark technology. In single-spark ignition blocks, each ignition cable supplies the high voltage pulse to one cylinder. In dual-spark blocks, the high voltage pulse is fed simultaneously to two cylinders: one that is on the power stroke, and the other being on the exhaust stroke and, thus, has a “wasted spark”.

Pencil or coil on plug ignition coils
This ignition coil type is mounted directly on top of the spark plug. The high voltage pulse is fed straight to the spark plug, minimising power loss. As pencil ignition coils are mounted in the spark plug tunnel, they do not take up space in the engine compartment. Pencil ignition coils are used in vehicles with electronic ignition systems and are available as single-spark or dual-spark coils.

Ignition coil pack systems
So called ‘coil packs’ combine a number of pencil ignition coils mounted within a single component, known as a ‘rail’. This rail is then placed across a bank of several spark plugs.

Spark plugs – the ‘small giant’ of the powertrain

Expert: Tim Howes, Deputy General Manager – Supply Chain & Technical Service, NGK Spark Plugs (UK) 

When it comes to automotive components the spark plug is the small giant of the powertrain. If someone else has coined that phrase before I apologise, but it is an extremely good description of the component that is the pacemaker at the heart of every petrol engine, the spark plug.

Precise control of ignition  has always been a critical part of the process in extracting the energy from the fuel but the more we demand in terms of power, torque, starting performance, economy and reduction in emissions the significantly greater this requirement becomes.

All components within the ignition circuit are under a great deal of stress and have to be significantly higher performance than in years gone by but they all operate in relatively moderate conditions when compared to the spark plug. After all these years it’s still the intense heat energy released by the spark as it jumps the electrode gap that initiates the combustion process.

So what is actually happening at the electrodes? Fundamentally, just before TDC on every firing stroke the ignition coil is instructed to apply a high voltage between an anode and a cathode – the plug’s electrodes.

Now as there is a significant gap, usually between 0.8-1.0mm between these two, initially current cannot flow – the circuit is ‘open’. However because the potential available voltage from the coil is so high (tens of thousands of volts) the structure of the gasses between the electrode surfaces begins to change.

Without writing pages on ultraviolet radiation generating photoelectrons which collide and by impact particle breaking eventually generate a higher electron number … no I’ll stop there and say that the air/fuel gaseous mixture becomes ionised allowing current to flow between the two. This is the spark!

Once the spark arrives the heat it produces starts the air/fuel mixture in the adjacent area to burn which then spreads in a completely controlled manner throughout the combustion chamber.  And this all happens over 8 times a second for each spark plug whilst the engine of an average family car is idling. It becomes mind boggling when one thinks of what is happening at the firing end of the plug in an F1 car where engine speeds are approaching 20,000 rev/min.

To obtain this performance we have to employ some specific materials when manufacturing spark plugs. We need to have both good conductors and particularly good insulators of electricity to contain well in excess of 30,000 V.

We also need materials that have high mechanical strength and a great ability to withstand extremes of temperature. These changes in temperature occur extremely rapidly due to the large amount of heat generated by combustion being immediately cooled by the incoming fresh charge of air and fuel. Under extreme conditions the force of shock from combustion vibrations can reach 50G or 50 times the force of gravity.

The base material of the insulator –the white part – is aluminium oxide which we obtain from bauxite, one of the most common compounds found on earth. This high purity aluminium oxide powder is doped with other material to further enhance its mechanical, thermal and dielectric properties. After forming it is sintered and glazed to produce the familiar hard smooth form of the plug. The resultant insulator is so good for its intended purpose that it has not changed significantly for many years. The main electrode is formed from copper and a nickel alloy, offering good electrical and heat conduction with high wear resistance.

To increase wear resistance further, especially when very fine electrodes are required small chips of semi-precious metals such as platinum or iridium alloys are welded to the nickel alloy. Iridium for example is extremely hard, has a particularly high melting point and is probably the most corrosion resistant metal available. This allows diameters of 0.4mm to be employed at the centre electrode. This offers several advantages of improved ignition performance and increases service life.

The electrical noise suppression resistor is located within the insulator ‘in series’ with the main electrode and is formed from a mixture of conductive carbon and insulating glass powder. Varying the proportions of the two materials allow different target resistance values (as required by the OEMs) to be achieved easily without fear of degradation during the service life of the plug. The normal target value is 5 kilo ohms.

The metal shell that houses the insulator will have one or more electrodes welded to it. Depending on application these may nickel alloy or inconel. Inconel is often employed where special resistance to high temperatures is required and as a further refinement a layered copper design can be used. So even the small J shaped ground electrode that you pay little or no attention to when handling a plug has a lot of thought put into its design. The sealing washer that gets compressed upon installation is usually of folded mild steel construction but can be stainless steel which guards against vibration or even solid copper to promote good heat transfer.

So what goes wrong when experiencing a perceived spark plug problem? Well I have heard that plugs cause problems as varied as misfires to flat tyres… and I am not joking. It must be remembered that if the plug is the correct one specified for the application, is within its recommended service life and it has been installed correctly it is highly unlikely that the plug is the root cause of any problem. The plug produces no heat or deposits; it’s the combustion process that does that and the poor old plug has to suffer all that is thrown at it.

When cars get older all the equipment on board starts to tire, for instance weak coils often wreak havoc and renewing the plugs can temporarily take some of the load off these components leading to a misdiagnosis of the true fault. Whether you simply sell or actually install these little marvels of technology spare them a little more thought.

Ignition Coils – Everything you need to know


Look under the bonnet of a modern vehicle and there is no doubt that the scene appears different to that of one of yester-year. With regards to ignition, distributors and lead sets are now a rarity – replaced with ‘plug top’ coils and ‘rail’ coils.

The ignition coil sector is now a significant part of the business of NGK Spark Plugs (UK). Although we recognise the importance of still catering for the earlier vehicles, many of which utilise the old metal can type ignition coils which incorporate oil to provide insulation and cooling, we also supply coils for the modern vehicle models that are venturing out of the main dealer network for repair.

What causes the demand for ignition coils is the harsh environment in which they work, which in turn creates a greater possibility of failure. As a result, although not strictly service items as such, many technicians view them in that category.

Coil manufacture has to be of a very high standard these days, mainly due to the high temperature fluctuations they’re subjected to. Many are mounted directly on the spark plugs and the severe cooling/heating cycles that prevail are a test for even the best quality item.

It is worth investing in suitable coil removal tools, not only to make removal easier upon servicing, but to ensure that the body or housing is not twisted or distorted – which can cause unseen damage internally.

Strict quality processes
Compromises on coil quality due to choice of materials used or production costs should never be made without recognising that there is inevitably a significantly greater possibility of premature failure.

The ignition coils in the NGK range have been through strict quality processes, from the initial design stage to assembly and testing. The testing carried out prior to launch ensures the items meet or exceed the vehicle manufacturers’ OE items.

The quality processes also encompass the packaging in which the items are shipped. Attention to detail means that items are safe in transit and, to ensure correct fit first time, the NGK ignition coil packaging includes a label with a schematic diagram of the coil contained inside – so selection can be verified easily without removal from the box.

Coil selection can be made using NGK Partfinder found on the website and the current NGK ignition coils application catalogue is available in paper format, which includes enhanced coil images to further aid selection.

The most recent additions to the NGK range, which was launched in 2013, were 22 new coil types covering vehicles including the VW Up, Mini, Vauxhall Adam, Vauxhall Astra J, Vauxhall Mokka, Mazda 6, Renault Clio IV and Dacia Sandero II. Range expansion is on-going, with emphasis revolving around demand. In total, the range now comprises 340 ignition coils, thus offering a part for a high percentage of the UK car parc.

You can talk to the NGK Spark Plugs (UK) technical team and find out more about its ignition coils range by visiting Stand C28 at Donington Park.