승용 디젤 엔진에서의 실린더 압력 정보를 이용한 저압 EGR의 고장 관리 시스템

Abstract

The dual-loop exhaust gas recirculation (EGR) system is one of the technologies that can effectively cope with the strict emission regulations. Although the control strategy of the dual-loop EGR system is various, in general, the low pressure exhaust gas recirculation (LP EGR) system is controlled by a look-up table (LUT) based feedforward controller considering the complexity of control. In this case, the LP EGR system is sensitive to disturbances. Particulate matters (PM) accumulation through the LP EGR path may hinder to obtain the desired LP EGR rate and thus causes an increase of nitrogen oxides (NOx). The degree of lack of the LP EGR rate should be detected, i.e. an LP EGR fault, and a remedy to compensate for the lack of LP EGR rate via the offset generation of the exhaust throttle valve position should be a mandate to suppress NOx emission, i.e. a fault management.

In order to accomplish those objectives, this study developed the LP EGR rate model via a modified orifice valve model by applying in-cylinder pressure information. Using this LP EGR rate model as a virtual sensor, the LP EGR fault diagnosis algorithm and fault tolerant control algorithm is proposed. The fault management system was implemented on the in-house engine management system (EMS) and validated at the fault condition in real-time. This study confirmed the reduction of NOx emissions by managing the LP EGR fault through the proposed fault management system.

A modified orifice valve model was designed by applying in-cylinder pressure information to the conventional orifice valve model. The problem of the conventional orifice valve model which was the poor estimation performance in the fault condition was improved using the proposed model. The combustion parameter which had correlation with the LP EGR mass flow rate was derived, which modified the effective area of the conventional orifice valve model. Consequently, the LP EGR rate estimation was improved to the maximum error of 2.38 % and RMSE of 1.34 % at various operating condition even under the fault condition compared to that of the previous model with the maximum error of 7.46 % and RMSE of 5.39 %. The proposed LP EGR rate model was suitable to be used as a virtual sensor for the fault management system since the LP EGR fault was defined as an error of more than 5 % from the set-point.

The fault diagnosis algorithm determines an LP EGR fault state through a residual generation and its evaluation. Based on the fault state, the fault tolerant control determines whether or not to generate the offset of the exhaust throttle valve position. This offset combines with the LUT based feedforward controller to control an LP EGR rate. As a result of real-time verification of the fault management system in the fault condition, the RMSE between the set-point and measured LP EGR rate was within 1.5 % and the NOx emission also decreased by up to about 15 %.