Fiber optic-based in-cylinder pressure sensor for advanced engine control and monitoring

This paper describes the design and performance of a miniature cylinder pressure sensor packaged either as a stand-alone device or integrated with a cylinder head gasket, glow plug, fuel injector, or spark plug. Benefitting from a fiber-optic based design the sensing element can operate at ultra-high temperatures and is not affected by EMI. This low cost device utilizes the principle of light intensity changes transmitted by two optical fibers upon reflection from a metal diaphragm deflecting under the effect of pressure. When the diaphragm thickness and shape are optimized, the sensor can operate up to 5 billion pressure cycles. The device is compensated for all temperature effects encountered in combustion engines, resulting from the thermal shock, engine load changes, and under-hood temperature fluctuations. Due to the diaphragm small thermal mass the sensor is subject to the thermal shock error if no heat shield is employed. While a suitable shield can almost eliminate the thermal shock error, it can get clogged in engines fuelled by diesel or landfill gas. For such engines a dual diaphragm construction offers a robust solution against soot and other combustion deposits as well as minimum thermal shock error. In comparison to a water cooled piezoelectric quartz transducer the present sensor offers the accuracy of ±1.5% of reading at pressures above 5 bars, ±0.1 bar error at pressure below 5 bars during compression, and the thermal shock error ranging from 0.1 bar to 0.3 bar dependent if a single or dual diaphragm is used. Such accuracy is possible with both stand-alone sensors as well as those 1.7 mm in diameter used in the “pressure sensing” cylinder head gaskets, glow plugs, fuel injectors, or spark plugs. This accuracy is maintained under all combustion conditions, sensor tip continuous temperatures up to 380 oC, signal conditioner temperature range of –40 oC to 140 oC, pressures up to 350 bar, and over frequency range of 0.1 hz to 30 khz. Such remarkable accuracy allows advanced engine controls based on highly accurate values of the Indicated Mean Effective Pressure, Mass Fraction Burned, Maximum Pressure Gradient, and Peak Pressure. Benefitting from sensor’s high accuracy at both low and high pressures the device enables closed-loop control of fuel injection as well as in-cylinder prediction of mass air flow and engine NOx emission levels.