Glow plug for use in diesel engine

Claim 1

In a glow plug for use in a diesel engine comprising a first helical heater, a second helical heater serial connected to said first helical heater, said second helical heater being made of material having a larger positive resistance temperature coefficient than the material constituting said first helical heater, and a sheath covering said first and second heaters and being electrically insulated along said first and second heaters, characterized in that said first and second heaters are separated axially by an axial gap G larger than winding pitches P of the first and second heaters and electrically connected by a straight connector in said gap so that said gap G is 1.5 P to 12 P, said first and second heaters being wound about a guide rod made of insulating material so as to maintain the axial layout of the first and second heaters with said straight connector and gap providing a thermal impedance to conductive heat flow from said first helical heater to said second helical heater, and said second helical heater being made of low carbon steel containing less than 0.25% by weight of carbon.