Patent ID: 7107819

Claim:
A pressure sensor unit ( 1 , 200 , 210 , 220 , 230 ) in which a ring-like pressure sensing element ( 3 ) is incorporated in a cylindrical unit body portion ( 2 ) having a spark plug insertion hole ( 4 ) formed in a bottom surface thereof, the pressure sensing element ( 3 ) surrounding the spark plug insertion hole ( 4 ); when a side where the pressure sensing element ( 3 ) is located in a direction of an axis (O) is set as a front end side, the unit body portion ( 2 ) is inserted into and fixed to a plug hole (PH) from the front end side in the direction of the axis (O); and in that state, a spark plug ( 100 ) is inserted into the spark plug insertion hole ( 4 ) to be thereby attached to an attachment hole (PF) at a bottom of the plug hole (PH), so that the pressure sensing element ( 3 ) is nipped and retained between a sensor support portion ( 102 ) projecting like a collar from an outer circumferential surface of a metal shell ( 101 ) of the spark plug ( 100 ), and an opening circumferential edge portion (GS) of the attachment hole (PF); wherein the pressure sensing element ( 3 ) is disposed in a clearance ( 7 ) formed between an outer cylinder member ( 5 ) forming an outer circumferential surface portion of the unit body portion ( 2 ) and an inner cylinder member ( 6 ) forming an inner circumferential surface portion likewise, and an output from the pressure sensing element ( 3 ) is extracted by a sensor output lead wire ( 9 ) via a sensor output terminal ( 8 ) led out into the clearance ( 7 ); and in a state where the spark plug ( 100 ) has been attached, a rear end of at least one of the inner cylinder member ( 6 ) and the outer cylinder member ( 5 ) in the direction of the axis (O) is located between a rear end of the pressure sensing element ( 3 ) and a rear end of an insulator ( 110 ) of the spark plug ( 100 ) (hereinafter, referred to as “in a standard section (SS)”), while at a rear of the standard section (SS), no component of the pressure sensor unit ( 1 ) exists in an area inside an inner circumferential surface ( 5 b ) of the outer cylinder member ( 5 ) or an extended plane (EP) thereof.