Designed to be combined with a high-tension magneto ignition system, the spark plug solved what Carl Benz had described as 'the most fundamental obstacle to early motoring'. A century later, the key factors driving spark plug innovation include emissions legislation, engine design and longer service intervals. Matthew Beecham reports.

The spark plug is the last step in the ignition circuit. It transfers the high-tension ignition voltage powered within the coil into the combustion chamber, where the arc that this voltage produces between the electrodes serves to ignite the air/fuel mixture. The spark plug thus plays a decisive role in ensuring reliable, optimal engine performance.

The technical development of the spark plug has been quite marked over the last 20 years. Copper core electrodes became widespread during the 1980s. While NGK had developed such a product almost a quarter of a century before, it was not until 1984 that Champion, under pressure from Bosch, launched its own copper core range. It followed this in late 1988 with the introduction of double copper spark plugs with copper core centre and earth electrodes. Platinum plugs were the next development. Bosch was the first to introduce platinum-tipped plugs in North America in 1985. It features a centre electrode made entirely of platinum that is sintered, or heat fused, into a ceramic insulator.

Driven by federal legislation, the fitment of precious metal -- mainly platinum-electrode and iridium -- spark plugs has risen dramatically since the mid-1980s. Most newly-registered cars in North America are fitted with platinum-tipped spark plugs. Following the US trend, the Mercedes-Benz and Jaguar led the European demand for platinum-tipped plugs.

These days, some spark plug manufacturers observe how OEMs tend to use the extended life of a precious metal spark plug more as a marketing tool than factual empirical data point. "In the small print, they will typically state that such plugs will last more than 100,000 miles provided all the usual maintenance and check points are passed," said Brent Berman, Product Manager, Ignitions, Champion Spark Plugs, Federal Mogul. "But if your car is fitted with a bad fuel injector then that might necessitate spark plug replacement regardless of the mileage."

"The main factors pushing innovation in the spark plug market are extended life, plug size, efficiency, ignitability and cost," added Mark Sullivan, Vice President, Global Ignition & wiper Operations, Federal Mogul. The high levels of power output, fuel efficiency and low emissions have created higher technical demands on spark plug makers. Two trends are especially evident. The first is that spark plug designs are being closely matched to each engine, and the second is the use of precious metal electrodes to provide higher levels of resistance to wear, together with high performance and low emissions. Because precious metals resist erosion so well, plug manufacturers are developing smaller diameter electrodes thereby lower the voltage requirement and providing stronger and more consistent ignitability.

While spark plugs are now required to have a longer service life, they are also required to be higher in ignitability, as exhaust gas regulations have been tightened recently. However, the effort to miniaturize the centre electrode is reaching a limit. While ignitability improvement is important, another critical point in the development of spark plugs is to reduce required voltage to accommodate a higher engine compression ratio, to keep up with future engine design trends. The most effective approach to reducing required voltage is to narrow the spark gap; however, a narrower spark gap might lower ignitability and it is difficult for conventional spark plugs to realise high ignitability and lowered required voltage.

As far as the aftermarket is concerned, most manufacturers concede a shrinking market due to the spread of longer-life precious metal plugs increasingly fitted as standard to new cars. "We see the entire [European] aftermarket volume of spark plugs, including long-life spark plugs as well as nickel spark plugs, declining due to the increase in OE installation," said Marko Wowczyna, manager, Denso Aftermarket. "We estimate this amounts to about a 1 - 2% decline each year on average in Europe. We are also seeing the most marked decrease in middle and northern Europe where the installation ratio of longer-life spark plugs is high, and the smallest decrease in southern and eastern Europe where the installation ratio of longer life spark plugs is low."

"On a global basis, we're definitely seeing a trend to flat and mild decline," added Berman. "But in North America, the popularity of large pick-ups and SUVs is leading to exponential growth of V8 engines. That trend is really helping to stabilise the unit decline of spark plugs in the aftermarket. Also, the introduction of premium-priced precious metal plugs in 1999 fitted as OE in North America is causing an upward trend in dollars per unit."

Other manufacturers share this view. "Whilst [spark plug aftermarket] volume is declining, however, it is fair to say that the value of the overall spark plug market remains strong due to the growth of higher technology plugs, maintaining sales revenues," added Wowczyna.

Although it has become a mature product, NGK perceives the spark plug aftermarket as increasingly profitable. Although aftermarket volumes are shrinking, NGK reports that the increasing complexity, technical quality and bespoke nature of spark plugs for newer vehicles means that the real value of the spark plug market in monetary terms is actually increasing.

Another factor dampening the European spark plug aftermarket has been the popularity of diesel engines in passenger cars. According to Bosch, nearly 52% of all newly-registered cars last October were equipped with diesel engines.

Future spark plug designs will evolve in response to changes in ignition and fuel injection systems, decreases in engine size, the addition of multi-valved heads, push by vehicle makers to design for longer replacement intervals, and pressure to increase fuel economy and further decrease emissions.

Expert Analysis

A global market review of spark plugs and glow plugs - forecasts to 2010 - 2nd edition (download)

In this second edition review of vehicle spark plugs, we extend the analysis in chapter two by forecasting market volume of OE glow plugs as well as spark plugs. The key factors pushing innovation in spark and glow plugs include longer service intervals, emissions legislation and engine design. Chapter three sets out some recent innovations. Chapter four provides brief profiles of the major spark and glow plug manufacturers, namely AC Delco, Autolite, Beru, Bosch, Champion, Denso and NGK Spark Plug.

To find out more about this report and to order your copy, please follow this link.