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’. Over a century later, the key factors driving spark plug innovation include emissions legislation, engine design and longer service intervals. Still plugging away, Matthew Beecham reports on the latest developments in spark and glow plugs.

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 1988 with the introduction of double copper spark plugs with copper core centre and earth electrodes. Platinum plugs were the next development, with Bosch launching its version in 1994. Ranges became more diverse with features such as V-grooves and multi-ground electrodes being introduced. 

Today, the main technical drivers of spark plugs are long service life and functionality, protection of the catalytic converter and cold-start reliability.  For its part, Federal-Mogul is developing high energy spark-plug technologies and enabling advanced combustion concepts like stratified charge, lean burn and diluted charge; and the supplier sees significant opportunities in this area. 

“Direct injection engine designs are requiring new designs to deal with the change in thermal cycle imposed on those spark plugs,” said Mark Whitehead, product marketing director engine and sealing, EMEA, Federal-Mogul.  “Direct injection and the general downsizing of the automotive engine have a side effect on all components on or within the engine.  Engine size and weight reduction will continue to force the reduction in size of spark plugs.  Most new engine designs are with M12 thread diameter, replacing the M14 of the last decade.  We can expect M10 to supplant M12 in the not too distant future.”

Even though spark plug technology is improving, can it really catch-up with engine life?  Whitehead points out that engine life has been a moving target for many years, and today’s engine has possibly twice the life of an engine from 25 years ago before it would require major servicing.  “Spark plugs of today can come very close to meeting this need, however there is no consensus as to whether or not this is a good idea as  other engine component failure (such as a fuel injector) can adversely impact the function of the spark plug.  Serviceability is still a major requirement for today’s designs.  Ultimately, though, the spark plug needs to be part of the cylinder head design for space and packaging efficiencies.”

As far as the spark plug aftermarket is concerned, most manufacturers concede a shrinking market. Aftermarket  volumes are declining mainly due to longer service intervals and the fact that fewer gasoline-powered vehicles are being built in Europe. The same forces apply in North America.

Indeed, the popularity of diesel engines in passenger cars is fueling a number of innovations in the glow plug sector.

The main influences on the design of glow plugs centre on the design of the diesel engine itself. Over the last ten years, there has been a trend toward smaller and longer glow plugs which have come about from the technology change from the indirect injection diesel engine to the direct injection common rail diesel engine.  But this trend is maturing.

“As diesel engines continue to develop,” says Whitehead, “it’s clear that glow plugs will have to endure higher and higher in-cylinder pressures and this will undoubtedly influence technical specifications. And, as diesels downsize, it is clear that glow-plugs will have to also. Cylinder head packaging is a real issue in smaller engines.

“The big difference will be in the energy-efficiency of powertrain components. Lower-voltage designs that place less demand on the vehicle’s electrical system and battery will contribute to fuel savings. There has been some talk in the industry of integrating glow plugs’ control units into the powertrain management system more closely, but the benefits are relatively slim. Consequently, optimising the mass and the friction between moving parts in the engine is a higher priority for powertrain component suppliers.”

We are also seeing certain engines with low compression ratios requiring ceramic glow plugs.    As ceramic glow plugs are more expensive than metal glow plugs, this technology will be applied to some engines only.

In future, we expect to see additional functions such as sensors being integrated into the glow plug combustion chamber driven by the latest emission regulations. Indeed, manufacturers say stringent forthcoming exhaust regulations will spur further demand for more environmentally-friendly automotive parts.