<title>High-temperature performance analysis of automotive combustion pressure sensor</title>

Abstract

A theoretical approach is developed to evaluate pressure detection sensitivity and its temperature dependence for diaphragin-t ype fiber optic combustion pressure sensors. Temperature-induced mechanical response variations and diaphragm optical reflectivity degradation, particularly at high temperatures, are identified as the two major factors that produce errors in sensitivit y, and hence in pressure measurement. Experimental results using hermetically sealed sensor construction prove the feasibifit y of maintaining diaphragm optical reflectivity under high temperatures. This analysis predicts that simple temperature compensation could reduce temperature-induced errors in sensor output, and obtain desired pressure measurexnent accuracies. Engine tests performed with the present fiber optic sensors demonstrate good signalto-noise performance and temperature stability. ThITRODUCTIONCombustion pressure is the fundamental variable that determines the operating state of internal cornbustion engines. Real-time combustion pressure measurement can provide iuformatio n which leads to optimum control of engine speed, throttle, air/fuel ratio, gas recirculation and optimum response to ambient conditions . Closed-loop controls based on combustion pressure history enable engine operation at an opthnuin air/fuel ratio that results in significant improvement of fuel efficiency, and reduces emission levels of polluting gases2. Combustion pressure waveforms can be used to directly identify engine knock and misflre. Effective knock controLs and lean combustion become practical only if engine optimization is based on individual in-cylinder pressure information.The importance of combustion pressure for engine controls has long been recognized. Yet, practical applications have been largely limited due to lack of suitable pressure sensors that meet performance, cost, and size requirements. The major obstade in developing viable and stable combustion pressure sensors has been to overcome sensor performance degradation caused by adverse operating conditions, which include high combustion temperatures and strong electromagneti c interference (EMI). Extensive effort has been devoted to the development of piezoelectric-type combustion pressure sensors5. However, these sensors cannot operate at over 200°C without water cooling due to inherent material liinitat ions. In addition, signal conditioning electronics must usually be located near the sensor head in order to combat strong EMI and stray capacitance loading effects of electrical cables. This further complicates sensor design, and increases cost because of the need to compensate thermal drift in both sensor head and electronics.Fiber optic sensors prove themselves operationally viable in high temperature conditions. EMI immunity uniquely suits them in applications such as engine combustion pressure monitoring. Among various types of fiber optic pressure sensors reported6, the high cost and technical complexity of interferometric-type sensors prohibit their use in large scale commercia...