US Patent 7185621 Glow plug with combustion pressure detecting function

A glow plug comprising: a cylindrical housing; a heater member as defined herein; a conductive center pole as defined herein; a first piezoelectric element as defined herein; and a second piezoelectric element as defined herein, wherein each of said first piezoelectric element and said second piezoelectric element is subjected to a preload for compressing it in said axial direction and is arranged such that, when said center pole is displaced to a root-end side, a load to be applied to one of said first piezoelectric element and said second piezoelectric element for compressing the same in said axial direction increases whereas a load to be applied to the other for compressing the same in said axial direction decreases.