The popularity of diesel engines in passenger cars continues to fuel a number of innovations in the glow plug sector, writes Matthew Beecham.

The main influences on the design of glow plugs centre on the design of the diesel engine itself. Over the last decade, 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.

In terms of the factors influencing the design of a glow plug, Tim Howes, technical services manager, NGK Spark Plugs (UK) Ltd believes that the volume of the combustion chamber is important as glow plugs are unsuitable for large capacity diesel engines of the type found in HGVs, plant and marine applications. He told us: “The shape of the cylinder head and geometry of the combustion chamber will have a direct bearing on the dimensions of the glow plug metal shell. The prevailing emission regulations in the territory of operation will dictate the heating performance and characteristics so that smoke and other unwanted emissions are kept within limits under cold start conditions and, of course, cost to the OEM is undeniably a major factor.”

Glow plug design has evolved along with improvements in the diesel engine. Jeff Boehler, chief engineer, fellow, Autolite believes that instant start like a spark ignition engine is desirable, but not possible because it takes time for the glow plug to heat. “Years ago, batteries were 6-volt, so the glow plug was turned on and it took one minute or more for the glow plug to heat. When 12-volt batteries became popular, the same plug would heat much faster, but a controller was required to mimic the heat up time and turn the power off before meltdown. Today the glow plug itself limits its own temperature by incorporating a second coil of wire whose resistance increases rapidly with temperature. This provides fast warm-up without melt down, eliminating the controller. The size and length of the heated portion is determined by the amount of heat (watts) and surface temperature needed by the engine. Thread size, diameters, lengths, terminal style, etc. are all defined by the desires of the engine developer.”

“The desire for instant starting, like a spark ignition engine, is always present,” added Sean Lyon, senior product manager, Autolite. “The most recent major design improvement is the incorporation of a second coil of wire to limit the maximum temperature. This self-compensating glow plug eliminates the need for a controller, which is a welcome improvement in system reliability. In the near future, there will be small improvements to extend the life and reduce the cost of the glow plug. In the far future, changes in glow plug design will follow ingenious ideas of the diesel engine developers.”

In terms of technical specification of glow plugs, Howes sees longer and thinner plugs with slimmer heater tubes, smaller thread diameters and longer bodies as the norm for many recent designs. “Again, significant changes in the dimensions will become increasingly more challenging and probably unnecessary. Glow plugs can also be designed to function as pressure sensors to provide essential information to the management system.”