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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.

Where to look when a vehicle suffers an MOT emissions failure

Tim Howes, NGK’s Deputy General Manager – Supply Chain & Technical Service.

Due to the hostile environment in which they work, sensors are subject to a degree of wear and tear and ageing. It would be impossible to say definitively what the service life of a Lambda sensor should be due to the very different conditions that each vehicle experiences under different drivers. City driving with lots of stop/start operation will have a different influence on the life expectancy of a Lambda sensor compared to steady motorway use.

We recommend that the function of the sensor is checked every 20,000 miles or annually. The emissions check as part of the current MOT test samples the exhaust gases to monitor the efficiency of the engine, exhaust system and engine control systems. The Lambda sensor is a vital part of this system and its function is therefore influenced by many other components.

A malfunction of an associated part may directly affect the performance of a sensor. An oscilloscope and gas analyser is a much more accurate way of assessing sensor performance than relying on fault codes alone. Look for slow response times, output range and heater function

Contaminants from poor quality oils and fuel which remain in the exhaust gasses can become deposited on the sensor element, affecting its operation. Even coolant from a leaking head gasket can reach the element.


‘Universal’ or ‘Bespoke’?
A ‘universal’ Lambda sensor is designed to cover as many applications as possible by splicing in the connector from the unit being replaced. In theory this sounds like a good idea however, in practice, the potential for a mismatch with the vehicle’s system or subsequent premature failure can be high. Apart from the more obvious choices of number of wires and whether it’s a zirconia or titania type, there are several base sensor designs to be considered with differing characteristics that includes heater element performance, response time performance and even internal electrical grounding.

Once the correct choice has been made you need to worry about making sound electrical connections and ensuring that you marry the lead wires correctly. Sensor manufacturers’ wiring colours differ so you can’t always simply join the matching colours

For some sensors, soldering the leads together is out of the question as they may need to ‘breathe’ through the tiny gaps in the lead wires and poor quality crimping can lead to water or oil ingress which can damage the sensor or affect its performance

Apart from all that, if you have not even disconnected the multiplug when splicing in the new unit you run the risk of missing such problems as corroded or damaged connector pins. In addition, many vehicles now use wideband sensors and for this type there is no substitute other than the correct OE specification item.

OE experience
NGK Spark Plugs (UK) manufactures Lambda sensors under the NTK brand name and supplies a comprehensive range of Lambda sensors to the UK aftermarket. The experience of supplying major vehicle manufacturers allows the company to offer the right sensor for every vehicle application.

The importance of fitting correct specification glow plugs

Diesel has obviously undergone a huge amount of change in the recent past. Although technology has moved on, the glow plug is essentially doing the same job that it always has done in raising the temperature inside the combustion chamber so that we can achieve stable combustion. However, the demands placed on the glow plug have become extremely severe and the performance that we expect is significantly higher than we ever had in the past. The technologies that have evolved have been a direct response to those demands.

We are reaching the point where we are trying to get the temperatures up in the combustion chamber so very, very quickly that there is no discernible delay between putting the key in the ignition and actually starting the engine with little or no smoke emission – even at extremely low ambient temperatures. You have therefore now got several technologies – from the standard metal glow plugs to the latest generation NHTC (New High Temperature Ceramic) types and the AQGS (Advanced Quick Glow System) plugs.

Ceramic glow plugs, though a little more expensive, are increasingly being selected by the OEs because of their long service life, extremely short pre-heating time and long post-heating time.

Stick to specification
Because the glow plug is now a vital part of the Engine Management System (EMS), as with most other components, it has never been more important to ensure that the correct specification item is fitted. Nowadays the EMS can pick up very quickly the fact that it is the wrong item, so that often rules out the ‘cheap’ alternative. The OBD system will find fault with the item if it is not to the correct specification and the performance of the vehicle will suffer.

Our advice is always to fit a plug that was manufactured to OE specification – as all NGK plugs are. Our ‘Fit & Forget’ philosophy means that we will only test specific parts against specific applications when we are absolutely 100% sure that they are entirely suitable for the vehicle. This philosophy is one of the reasons why NGK has been so successful – not only with spark plugs but with glow plugs as well.

Technical developments
Reduction in available space to install the plug in new engine designs will continue and plugs like the NHTC ceramic types – which are available with an unbelievably small thread diameter of M8 – will continue to assist engine designers in meeting forthcoming emissions legislation. Although not at the same level as the ceramic types, the small probe diameter AQGS plugs will become more widespread.

At moderate ambient temperatures some modern direct injection engines are less reliant on the glow plug to start up. However we need to reduce emissions still further. One particular way of achieving a reduction of NOx is by lowering compression ratios. However this makes the engine more difficult to start and therefore the cars are once again more reliant on the glow plug to start the engine.

Whatever challenges face engine designers you can rest assured that NGK will, as always, invest massively in research and development to provide them with a solution.

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.

What causes black, brittle and broken filter media?

Occasionally an oil filter element, removed from its housing during servicing, may show visible damage to the pleated filter media.

At first glance, the affected filter may be considered to be defective, as often the pleats can be broken (as illustrated below, left), or may be ‘brittle’ and disintegrate as a result of slight finger pressure when attempting to remove the element.

This abnormal condition is known as media embrittlement.

The root cause
Due to severe operating conditions – such as those experienced by short distance or stop/start driving, delivery vehicles, Mom’s taxis and driving schools – the engine lubricant can become extremely acidic and may attack the protective resin impregnation of the filter media, leaving the media strands open to degradation by acids and chemicals in the oil. This turns them black and brittle.

Related causes
Unsuitable, poor quality or aggressive oil – A depleted TBN (Total Base Number) lubricant is unable to neutralise acidity.

Underservicing of oil and filter – This is commensurate with severe operating conditions.

Non-OE or inferior quality filter A sub-standard filter media is unable to cope with OE standard or extended service intervals.

The solution
1. Maintain oil and filter service intervals as per the vehicle’s operating conditions.

2. Install the correct lubricant grade for the vehicle application.

3. Install a Genuine OE quality oil filter from a trusted supplier, such as MANN-FILTER.

Oil filters – Why choose premium quality?
Filter media – Engineered and impregnated with special resins that defend against oil acidity and aggressive conditions.

Efficiency – Meets the OE specification for optimum contaminant retention.

Service life – Efficient protection of engine components during the entire OE recommended service interval.

Seals – Specifically matched to each application and operating conditions and durable for filter service life.

How to ensure optimum sealing of the oil filter element

Installation of certain oil filter elements can feel tight in comparison to alternative filter element designs. Rest assured, any tightness is normal and ensures optimum sealing of the element.

Tight is right!
Many original equipment MANN-FILTER oil filter elements have end caps which seal radially to the relevant matching part in the oil filter housing (usually a plastic centre tube). This is achieved through the precise fit of the inside diameter bead of the filter end caps.

How to ensure optimum sealing of the oil filter element

 Installation tips
The following steps will help you with installation of the new MANN-FILTER elements:
■ Clean the sealing surface on the centre tube/housing.
■ Lightly oil the centre tube and the inner surface of the new filter element end cap.
■ Twist the filter element slightly when installing.

How to ensure optimum sealing of the oil filter element

■ Push the filter element onto the centre tube as far as it will go, to align the sealing surfaces of the element and centre tube correctly.
■ No worries!

How to ensure optimum sealing of the oil filter element