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Glow Plug Technology

It probably hasn’t escaped your notice that new car buyers are continuing to change their allegiance from spark ignition to compression ignition vehicles, and it won’t be long before diesel cars start outselling petrol cars – a shift that few would have predicted as little as 15 years ago.


In the first six months of 2008, diesels accounted for 43% of new car registrations. In 2006 that figure was 38%, so that’s an additional 5% of buyers swapping to oil burners in the last two years alone.

Continuous Improvement

The diesel engine is continuously being improved, in terms of its fuel consumption, power outputs and, ultimately, refinement. Drivers of modern diesel vehicles expect to be able to just jump in, turn the key and go. It’s no longer acceptable to have to wait 8 to 10 seconds for a glow plug light to go out before you can start the engine. One of the reasons this is possible is because of the technical advances that have been made in glow plug design, in particular with the development of both twincoil technology and the latest third generation of glow plug – the New High Temperature Ceramic type, or NHTC. In this article we take a closer look at these latest types of plugs in more detail; the manufacturers spearheading the new technologies, and the major motor manufacturers adopting them as OE.

Twin -coil Technology

Using twin-coil technology, a glow plug can be made to heat more quickly and then self-stabilise at the correct temperature. This type of plug is more resistant to misuse, as it will prevent overheating of the main coil and allows longer post glow periods, giving cleaner exhaust emissions.

Each twin-coil glow plug has been designed and tested to 7,000 heating cycles before being approved for production. This means that if the engine is started from cold twice per day, the glow plug could have a life expectancy of 10 years or more.

Glow plugs are probably far more intricate than you may imagine them to be. It takes precision laser welding to join the heating coils, which are then tested for maximum temperature and rate of temperature rise. This is to ensure that the glow plugs reach 850°C within a specified time.

NHTC Plugs – The 3rd Generation

Another significant advance in glow plug design concerns the development of NHTC, or New High Temperature Ceramic, plugs. This third generation glow plug is becoming increasingly attractive to the major European vehicle manufacturers.

The Volkswagen Audi Group (VAG) was the first European vehicle manufacturer to install NGK’s NHTC glow plugs as Original Equipment and, today, this new generation glow plug is used on all VW diesel engines with a diesel particle filter. DaimlerChrysler is also now installing NHTC glow plugs in diesel versions of their new Mercedes C- and E-Class models, whilst outside of Europe; Mitsubishi has also been installing NHTC glow plugs since 2005.

Why is the introduction of NHTC ceramic glow plugs such a breakthrough?

Ceramic glow plugs have the ability to reach high temperatures much more quickly than metal glow plugs. This third generation of ceramic glow plugs replaces the second generation High Temperature Ceramic glow plugs (HTC) which has been available to the market since 1998. The first generation – Self-Regulating Ceramic glow plugs (SRC) – was originally introduced as far back as 1985.

 The concept behind the NHTC generation of ceramic glow plugs is to offer a long element with a small diameter, which will heat up much more quickly – typically reaching 1000°C in under two seconds - and thus improve cold starting and reduce emissions. They also have a longer post starting operation time of approximately 10 minutes and are also able, for the first time, to assist in reducing emissions by their ability to operate intermittently during engine running – for example, when the throttle is eased back.

This type of plug also produces a high stabilised temperature of approximately 1300°C, thus ensuring excellent starting from a lower compression ratio. A benefit of this is a reduction in noise, engine stress and emissions, and improved mechanical resistance to eliminate vibration.

New Developments

In developing its new range of diesel engines – and in particular a V10 TDI, subsequently voted ‘Engine of the Year’ – Volkswagen insisted on a glow plug with a fully ceramic heating element. The advantages of this type of plug include a long service life, an extremely short pre-heating time and an extended post-heating time. Research and development undertaken by Volkswagen on the NHTC, as supplied by NGK, has confirmed that the high glow temperature considerably improves cold-start performance as well as reducing the emission of pollutants. In their formal presentation, Volkswagen also stressed that the glow plug is designed for minimum ageing over its entire service life, thereby enabling intermediate heating. This is important because it prevents the engine from cooling during coasting and guarantees a reduction in emissions.

AQGS glow plugs

A relatively recent development is the (Advanced Quick Glow System) AQGS glow plug – the very latest in metal technology. Their very high performance characteristics allow even faster heating times. These double coil glow plugs are designed to operate at battery voltage during the very short pre-heating stage – approximately 6.5 volts during cranking and 5.0 volts in the post-glow period. Due to the high in-rush current and the specific stages of applied voltage, they require very precise control by (pulse width modulation) PWM via the ECU. This strategy enables strict control of their long post glow performance, as the ECU can take data from various sensors around the engine. As such, the glow plug performance is at its optimum, thus allowing further reduction of harmful emissions.

Except at very low ambient temperatures the glow plug is primarily used to control smoke and other emissions after the start up and, just as importantly for the driver, good performance from cold.

As a result of these and other developments the performance of the glow plug has changed dramatically over recent years. You may know that NGK Spark Plugs is the clear brand leader in the UK spark plug market – with a market share of around 65% outselling there nearest competitor by more than two to one. What you may also not know is that NGK are the clear brand leader in the UK glow plug market. Recent independent research at installer level has shown that NGK now supplies more glow plugs into the UK Aftermarket than any other manufacturer – at least 50%+ – and growing!

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