Fiber Optics-based Cylinder Pressure Sensors for Advanced Engine Controls

This paper presents the design, applications, and performance of the miniature fiber optic-based cylinder pressure sensor which has targeted advanced engine controls requiring high accuracy pressure information. Three types of sensor packages are described currently utilized in prototype closed loop control systems for diesel, gasoline, and dual fuel engines: glow plug with the heater-mounted sensor, spark plug with removable sensor, and a stand alone package. All these packages are based on the 1.7mm diameter sensor which has been validated in various combustion engines over the last ten years. This robust low cost sensor operates on the principle of light intensity changes due varying reflection from a flexing metal diaphragm, monitored by two optical fibres. The sensor is compensated for all major temperature errors encountered in internal combustion engines. (1) The auto-referencing technique maintains sensor calibration compensating for temperature effects on the sensor signal conditioner over the range of –40oC to +150C. (2) In a reliable design the sensor head materials and dimensions are optimized to compensate for the mid-term thermal errors associated with engine load changes over a temperature range of –40oC and 380oC, resulting in the thermal coefficient of sensitivity as low as 0.005%/C. (3) Due to minimal heat flux associated with the sensor’s small diameter the thermal shock error is minimized such that the total accuracy is typically in the range of 12%, dependent on sensor package and application. Close to a hundred glow plug-integrated sensors have been used in on-road tests over several hundreds of hours without sensor performance degradation or soot fouling. Tens of spark plug mounted sensors have been utilized to control engine knocking in aircraft engines fueled by automotive gasoline. Finally, several tens of stand alone sensors have been used to control heavily knocking diesel-natural gas engines operating in the Homogenous Charge Compression Ignited mode.