Glow plug with combustion pressure sensor

#This application is a National Stage of International Application No. PCT/JP2012/054057 filed Feb. 21, 2012, claiming priority based on Japanese Patent Application No. 2011-039118 filed Feb. 25, 2011, the contents of all of which are incorporated herein by reference in their entirety.

#The present invention relates to a glow plug having a combustion pressure sensor (a glow plug with a combustion pressure sensor), adapted to promote ignition within a combustion chamber of an engine and to sense (detect) combustion pressure as well. Specifically, the present invention relates to a glow plug having a combustion pressure sensor which is mounted to an engine head such that its heater is exposed to the interior of a combustion chamber for the purpose of promoting ignition of fuel and receiving, through the heater, combustion pressure (combustion gas pressure) in the combustion chamber so as to sense the combustion pressure by means of a sensor, such as a piezoelectric element or a strain sensor (gauge), disposed within a housing which holds the heater.

#There are known various types of such a glow plug having a combustion pressure sensor (hereinafter, may be referred to merely as a glow plug) (refer to, for example, Patent Document 1). FIG. 9 is a longitudinal, partially sectional view showing a simplified sectional structure of a glow plug 901 of the same type. The glow plug 901 of FIG. 9 is configured such that a rodlike (circular columnar) heater (e.g., a metal heater or a ceramic heater) 10 which generates heat through energization is disposed within a tubular housing 110 with its forward end (an end on a side toward a combustion chamber, or a lower end in FIG. 9) 10a projecting from a forward end 136 of the housing 110. In the glow plug 901, in consideration of transmission of a received combustion pressure to a sensor disposed at the rear end of the heater 10, thermal expansion of the heater 10, etc., the heater 10 is disposed within the housing 110 in such a manner as to be displaceable in the direction of its axis G (axial direction), by a very small amount. That is, the heater 10 is disposed with a gap (annular gap) formed in cooperation with the inner circumferential surface of the housing 110. A sensing means; for example, a piezoelectric element 40, for detecting pressure generated as a result of combustion pressure pressing the heater 10 rearward from the forward end 10a is disposed rearward of the heater 10. This configuration is designed as follows: a force generated as a result of combustion pressure pressing the heater 10 rearward compresses the piezoelectric element 40, and an electric signal generated according to a change in compressive force applied to the piezoelectric element 40 is measured, whereby the combustion pressure is detected.

#FIG. 1 Partially sectional view showing a glow plug having a combustion pressure sensor according to an embodiment (first embodiment) of the present invention, and an enlarged view of essential members of the glow plug.

#FIG. 2 Further enlarged view of region A1 (showing, on an enlarged scale, essential members including a holding member (a seal member in the present embodiment) of FIG. 1.

#FIG. 3 Further enlarged view of region A2 of FIG. 2.

#A glow plug having a combustion pressure sensor according to an embodiment (first embodiment) of the present invention will be described with reference to FIGS. 1 to 5. A glow plug 101 of the present embodiment is composed primarily of a substantially cylindrical housing 110; a sheath heater 10 disposed in the housing 110 with its forward end (an illustrated lower end) 10a projecting from a forward end 136 of the housing 110; and a piezoelectric element 40 serving as a sensor and disposed at the rear end of the sheath heater 10. First, the overall configuration of the glow plug 101 will be described in detail with additional reference to FIGS. 4 and 5 showing an example assembly step.

#In the present embodiment, the housing 110 includes a substantially cylindrical housing body 111; a sensor-supporting inner housing 121 inserted and disposed in the housing body 111 in such a manner as to support the piezoelectric element 40 at the rear end of the sheath heater 10; and a forward-end housing 131 located at a forward end portion of the housing body 111. The housing body 111 includes a screwing polygonal portion 113 provided on the outer circumferential surface of its rear end portion; a screw 115 adapted to be screwed into a cylinder head and provided on an outer circumferential surface located forward of the screwing polygonal portion 113; and a cylindrical portion 117 located forward of the screw 115 and having a diameter slightly smaller than the minor diameter of the screw 115. The sensor-supporting inner housing 121 is inserted and disposed along the inner circumferential surface of a forward portion of the cylindrical portion 117.

#The sensor-supporting inner housing 121 is a cylindrical tube having an outside diameter slightly smaller than the inside diameter of the housing body 111 (see FIGS. 4 and 5) and includes a flange 123 projecting on the outer circumferential surface of its forward end portion. The flange 123 has the same outside diameter as that of the cylindrical portion 117 of the housing body 111, and, as shown in the enlarged view in FIG. 1 and others, the flange 123 is joined (hereinafter, referred to merely as “welded”) to the housing body 111 by, for example, welding (specifically, laser welding) such that a rear end surface 124 of the flange 123 is in contact with a forward end 118 of the housing body 111. Meanwhile, as shown in the enlarged view in FIG. 1, a cylindrical cap 127 having an annular bottom plate 126 with a center opening is welded to a rear end 125 of the sensor-supporting inner housing 121 via the forward end of a cylindrical portion 128 of the cap 127. The annular piezoelectric element 40 sandwiched between insulation plates 47 via respective electrode plates 43 and 44 is disposed, within the cylindrical cap 127, in contact with the annular bottom plate 126. Although unillustrated, signal output leads extend rearward from the respective electrode plates 43 and 44.