Glow plug

Abstract

A glow plug 1 includes a metallic tube 3 having a closed distal end and extending axially, a rod-like insulator 14 disposed within the metallic tube 3 so as to form a clearance therebetween, a coil member 5 disposed so as to encircle an outer circumferential surface of the insulator 14, and insulating powder 15 charged into the metallic tube 3 so as to fill the interior of the metallic tube 3. The insulator 14 is formed of a material having thermal conductivity lower than that of the insulating powder 15. By rendering the thermal conductivity within the coil member 5 lower than the thermal conductivity in a region between the coil member 5 and the metallic tube 3, conduction of heat generated by the coil member 5 to the inside of the coil member 5 is suppressed, so that the heat is effectively conducted to the metallic tube 3.

What is claimed is:  
     
         1 . A glow plug comprising a metallic tube having a closed distal end and extending axially, a rod-like insulator disposed within the metallic tube so as to form a clearance between the insulator and the metallic tube, a coil member encircling an outer circumferential surface of the insulator, and insulating powder charged into the metallic tube which fills the interior of the metallic tube, 
 wherein the insulator is formed of an insulating material having a thermal conductivity that is lower than that of the insulating powder.    
     
     
         2 . The glow plug as claimed in  claim 1 , wherein A≧0.6B, where A is a diameter of the insulator, and B is an inside diameter of the coil member.  
     
     
         3 . The glow plug as claimed in  claim 1 , wherein the insulating powder comprises magnesium oxide, and the insulating material comprises alumina.  
     
     
         4 . A glow plug comprising a metallic tube having a closed distal end and extending axially, a rod-like insulator formed of an insulating material and disposed within the metallic tube so as to form a clearance between the insulator and the metallic tube, a coil member encircling an outer circumferential surface of the insulator, and insulating powder charged into the metallic tube which fills the interior of the metallic tube, 
 wherein, as viewed on a cross section perpendicular to a longitudinal axis of the glow plug, the insulating powder present in a clearance between the metallic tube and the coil member has a greater occupancy rate per unit area than the insulating material of the insulator.    
     
     
         5 . The glow plug as claimed in  claim 4 , wherein the insulating powder has an average particle size that is smaller than that of the insulating material.  
     
     
         6 . The glow plug as claimed in  claim 4 , wherein the insulating material has a thermal conductivity that is lower than that of the insulating powder.