Piezoresistive pressure-measuring plug for a combustion engine

Method of manufacturing a calibrated piezoresistive pressure plug sub-assembly, the method comprising:

• providing a piezoresistive pressure sensor plug comprising a plug body and a sensing structure, the plug body configured for insertion into a cylinder of a combustion engine, the sensing structure arranged in the plug body between a pressure force interface part and the plug body, the sensing structure configured such that when acted upon by pressure prevailing in the combustion chamber of the cylinder the pressure force interface part transmits the pressure in the combustion chamber of the cylinder to the sensing structure via axial motion of the pressure force interface part relative to the plug body such that the pressure force interface part applies a force to the sensing structure, wherein a piezoresistive element mounted on the sensing structure provides a resistance change when the pressure force interface part is moved via axial motion, the plug body includes a body lower part and a body middle part, the body lower part is a sealing surface part that provides a seal between the body lower part and the combustion engine when inserted in the combustion engine, the piezoresistive pressure sensor plug further comprises an elongated hollow body part at an end linked to the sealing surface part, the elongated hollow body part extends friction free in the body middle part, the sensing structure is attached to an end of the lower body part opposite to the end linked to the sealing surface part, the pressure force interface part is an elongated interface body comprising a through hole for receiving a rod;
• providing a front membrane;
• positioning a temporary sealing member in the through hole;
• positioning the front membrane to contact the lower body part and the pressure force interface part;
• welding the front membrane to the lower body part;
• welding the front membrane to the pressure force interface part;
• calibrating sensor electronics by exerting a predetermined sequence of pressures on the assembly;
• and
• removing the temporary sealing member from the through hole.

A piezoresistive pressure-measuring plug for a combustion engine comprising:

• a plug body insertable into a cylinder of the combustion engine;
• a rod that is arranged in the plug body;
• a sensing structure that is arranged between the rod and the plug body, and in such a way that the sensing structure is acted upon, in use, by the pressure prevailing in the combustion chamber of the cylinder, whereby the rod transmits the pressure in the combustion chamber of the cylinder to the sensing structure due to the pressure in the combustion chamber leading to an axial motion of the rod relative to the plug body so as to apply a force to the sensing structure, wherein the sensing structure is integrated and located in the plug body and a piezoresistive element mounted on the sensing structure provides a resistance change when the rod is moved in axial direction;
• a Printed Wiring Board (PWB) carrying sensor electronics for measuring and conditioning the resistance change, wherein the PWB is arranged in the plug body;
• wherein the plug body comprises a threaded body part, and wherein the PWB is arranged in the threaded body part;
• wherein when in use the sensing structure is compressed in axial direction by the force applied to the sensing structure and the piezoresistive element mounted on the sensing structure provides a resistance change corresponding to the surface strain in axial direction;
• and
• wherein the sensing structure is a membrane structure that transforms axial movement of the rod in a surface strain in radial direction and the piezoresistive element mounted on the sensing structure provides a resistance change representing the surface strain in radial direction when the rod is moved in axial direction.