CRDI 디젤엔진의 NOx 및 PM 저감을 위한 EGR과 VGT시스템의 동적 decoupler기반 다변수 제어알고리즘과 연료분사 제어인자의 적응제어전략에 관한 연구

Abstract

디젤엔진은 질소산화물(NOx)과 입자상 물질(PM)같은 유해 배기배출물 저감을 위한 다양한 기술들이 개발됨에 따라 승용차량에 널리 적용되고 있다. 배기배출물 저감을 위한 대표적인 기술로 커먼레일 시스템(Common rail system), 배기 재순환(Exhaust gas recirculation, EGR) 및 가변용량 터보차저(Variable geometry turbocharger, VGT) 시스템이 개발되었다. 이 기술들을 효과적으로 엔진에 적용하여 배기배출물 저감과 연비향상을 동시에 달성하기 위해서는 정밀 제어알고리즘 설계가 필수적이다. 제어알고리즘 목표 값은 배기 및 연비와 같은 엔진성능 극대화를 위해 최적화 되어 있기 때문에, 빠르고 정밀하게 목표 값 추종이 가능한 제어기 설계기술은 매우 중요하다. 이 논문은 커먼레일 시스템과 EGR 및 VGT 시스템을 위한 제어알고리즘 설계 기법을 제시한다. 커먼레일 시스템의 정밀 제어를 위해서는 고압 펌프 및 연료분사 장치로 인한 불연속적인 연료 흐름을 효과적으로 상쇄하여야 한다. 불연속적인 연료흐름은 정상상태 운전조건에서 레일압력 맥동을 야기하고, 이 맥동은 정밀 연료 분사량제어에 어려움을 발생시키기 때문에 엔진성능 저하를 피할 수 없다. 따라서 이 연구에서는 연료 유량 밸브(metering unit, MeUn)와 레일압력 제어 밸브(pressure control valve, PCV)의 협조 제어알고리즘 설계를 통해 레일압력 맥동을 감소하고 과도구간 응답성능 향상을 위한 제어알고리즘을 설계 하였다. 제시한 제어알고리즘을 적용함으로써 레일압력 맥동은 최대 20 bar에서 8 bar로 감소하였으며, 엔진의 다양한 과도구간 운전조건에서 제어기의 강건성을 검증하였다. 두 번째로, EGR과 VGT 시스템의 강한 비선형성과 각각의 시스템들이 서로 영향을 미치는 상호작용특성을 상쇄하기 위하여 분산화된 다변수 제어 알고리즘(decentralized multivariable)을 제시하였다. 제어기 구조는 직관적인 캘리브레이션이 가능하고 연산시간이 적은 PI 제어기를 사용하였다. EGR 및 VGT 시스템의 강한 비선형성을 선형 PI 제어기로 극복하기 위해서 제어이득 스케줄링 전략을 제시하였다. 제시한 제어이득 스케줄링 전략은 EGR 및 VGT 시스템의 물리적 특성을 반영할 수 있는 흡기 및 배기 매니폴드 압력비와 배기 공연비 인자를 이용하여 설계하였다. 그리고 EGR 및 VGT 시스템의 강한 상호영향을 감소하기 위하여 simplified decoupler 설계기법을 적용하였다. Simplified decoupler는 모델오차에 강건한 특성을 갖고 있어서 운전조건에 따라 다양한 특성을 나타내는 디젤엔진 흡배기 시스템에 적합하다. 제시한 EGR과 VGT 시스템의 다변수 제어알고리즘은 다양한 엔진운전조건에서 검증하였으며, EGR과 VGT 시스템의 상호영향이 14-66 %까지 감소함을 보여주었다. 제시한 커먼레일 압력과 EGR 및 VGT 시스템 제어알고리즘은 엔진 성능을 위해 최적화된 목표 값의 빠르고 정확한 추종이 가능하다. 그러나 제어 목표 값은 엔진의 정상상태에서 최적화 되었기 때문에, 과도구간 운전에서 최적성능을 보장하지 못한다. 엔진의 과도구간에서는 응답 특성이 빠른 연료분사 시스템과 느린 흡배기 시스템이 농후한 연소 조건을 형성하기 때문에, 과도한 입자상 물질이 배출된다. 따라서 이 논문에서는 과도구간에서 연료 및 흡배기 시스템의 제어 목표 값을 보정하기 위한 적응제어 전략을 제시하였다. 제시한 목표 값 적응제어 전략은 엔진의 과도구간에서 연료 및 공기의 응답특성 차이를 상쇄하여 입자상 물질 감소에 기여한다. 또한, 엔진 출력의 빠른 증가를 통해 운전성능 향상이 가능하다. 제시된 적응 제어전략은 연료분사량 제한 알고리즘과 EGR 가스 및 커먼레일 압력의 목표 값 적응 알고리즘으로 구성하였다. 연료 분사량 제한 알고리즘은 농후한 연소를 피하기 위하여 연료 분사량을 조절한다. 연료 분사량 제한으로 인한 엔진 출력 감소를 보상하기 위하여, EGR 가스 저감과 커먼레일 압력 증가를 위한 적응 알고리즘이 설계 되었다. 과도구간에서 EGR 가스 저감은 VGT 효율을 향상시키고, 신기의 응답속도를 높인다. 그리고 커먼레일 압력 증가는 연료 미립화 촉진을 통하여 엔진 토크를 증가시키는 동시에 입자상 물질을 저감하는 역할을 한다. 제시한 제어알고리즘은 오직 두 개의 캘리브레이션 변수를 사용함으로써 과도구간 엔진의 배기 성능과 운전 성능 사이의 균형을 쉽게 조절할 수 있는 장점을 가진다. 엔진의 과도구간에서 EGR 및 레일압력 적응 제어전략을 적용함으로써, 입자상 물질은 11 % 감소하였고, 엔진 토크는 5.44 % 증가하였다.|Diesel engines have widely applied to passenger cars along with advanced technologies such as common rail system and exhaust gas recirculation (EGR) and variable geometry turbocharger (VGT) systems to reduce nitrogen-oxides (NOx) and particulate matter (PM) emissions. In order to obtain advantages of the technologies, design of precise control algorithm for the technologies is one of the most important tasks for modern diesel engines. Since set-points of the controller for fuel and air systems such as rail pressure and EGR rate are optimized to cope with a trade-off relation of NOx and PM emissions, precise control algorithms to follow the optimized set-points are preferentially required to satisfy a stringent emission regulation. This thesis proposes precise control algorithms for the common rail pressure and the EGR and VGT systems for passenger car diesel engines. Control of common rail pressure has a difficulty caused by discontinuous inlet and outlet fuel flows of the common rail, resulting in a large rail pressure fluctuation. In order to reduce the rail pressure fluctuation, a coordinated rail pressure control algorithm is proposed by using the metering unit (MeUn) and the pressure control valve (PCV). The MeUn control algorithm is designed based on a simplified fuel flow model to reduce the rail pressure fluctuation under a steady state. Since the MeUn operation adjusts the inlet fuel flow of the common rail, the varying inlet fuel flow is considered as uncertainties of plant parameters when the PCV control algorithm is designed. The PCV control algorithm is designed with quantitative feedback theory (QFT) which is a robust controller design technique within an uncertainty range of plant parameters. The proposed PCV and MeUn control algorithm secures robust control performance within the uncertainty boundary as well as reduction of the rail pressure fluctuation. The experimental results showed that the proposed rail pressure control algorithm reduced the rail pressure fluctuation from 20 bar to 8 bar under various steady state conditions. Furthermore, step test results of desired rail pressure and engine operating conditions showed a good degree of consistency under various engine operating conditions. Air path control of diesel engines suffers from strong nonlinear and coupled characteristics of the EGR and VGT systems. In this thesis, the difficulties are handled by designing a decentralized multivariable control algorithms for EGR and VGT systems. The strong nonlinearity is solved with a proposed gain scheduling strategy. The gain scheduling strategy determines controller PI gains in run-time by using a new scheduling parameter calculated from pressure ratio of the intake and exhaust manifolds and exhaust air-to-fuel ratio. The coupling characteristics of the EGR and VGT systems are compensated by applying a simplified decoupler design method. Engine experimental results in various engine operating conditions proved that the gain scheduled controller obtained stable control performances without any oscillatory behavior or overshoot even when there is a change of engine operating conditions. Furthermore, the proposed decoupler reduced the coupling effects 14-66 % in various engine operating conditions. Unfortunately, although the controllers for common rail system and EGR and VGT systems perfectly follow the optimized set-points, it does not guarantee satisfactory emission performance during transients. A dynamics difference between fuel and air systems leads to a PM emission peak under a transient state. Limitation of fuel injection quantity is widely used to reduce the transient PM emission. However, since limiting the fuel injection quantity leads to loss of engine torque, this study propose an adaptation algorithm of reference variables for the common rail pressure and EGR rate to minimize the torque loss under a transient state. The proposed adaptation algorithm is designed to reduce PM emission as well as enhance torque response during transients while it manipulates set-points for the EGR and rail pressure controller. Reduction of EGR gas and increase of rail pressure during transients respectively improve charging efficiency of the fresh air and combustion efficiency by well mixed fuel-air mixture, resulting in improvement of torque response while reducing PM emissions. The step test results of engine operating points represented that the maximum 23 % reduction of PM emission is achieved and the IMEP is increased by 5.2 % under a transient state.; Diesel engines have widely applied to passenger cars along with advanced technologies such as common rail system and exhaust gas recirculation (EGR) and variable geometry turbocharger (VGT) systems to reduce nitrogen-oxides (NOx) and particulate matter (PM) emissions. In order to obtain advantages of the technologies, design of precise control algorithm for the technologies is one of the most important tasks for modern diesel engines. Since set-points of the controller for fuel and air systems such as rail pressure and EGR rate are optimized to cope with a trade-off relation of NOx and PM emissions, precise control algorithms to follow the optimized set-points are preferentially required to satisfy a stringent emission regulation. This thesis proposes precise control algorithms for the common rail pressure and the EGR and VGT systems for passenger car diesel engines. Control of common rail pressure has a difficulty caused by discontinuous inlet and outlet fuel flows of the common rail, resulting in a large rail pressure fluctuation. In order to reduce the rail pressure fluctuation, a coordinated rail pressure control algorithm is proposed by using the metering unit (MeUn) and the pressure control valve (PCV). The MeUn control algorithm is designed based on a simplified fuel flow model to reduce the rail pressure fluctuation under a steady state. Since the MeUn operation adjusts the inlet fuel flow of the common rail, the varying inlet fuel flow is considered as uncertainties of plant parameters when the PCV control algorithm is designed. The PCV control algorithm is designed with quantitative feedback theory (QFT) which is a robust controller design technique within an uncertainty range of plant parameters. The proposed PCV and MeUn control algorithm secures robust control performance within the uncertainty boundary as well as reduction of the rail pressure fluctuation. The experimental results showed that the proposed rail pressure control algorithm reduced the rail pressure fluctuation from 20 bar to 8 bar under various steady state conditions. Furthermore, step test results of desired rail pressure and engine operating conditions showed a good degree of consistency under various engine operating conditions. Air path control of diesel engines suffers from strong nonlinear and coupled characteristics of the EGR and VGT systems. In this thesis, the difficulties are handled by designing a decentralized multivariable control algorithms for EGR and VGT systems. The strong nonlinearity is solved with a proposed gain scheduling strategy. The gain scheduling strategy determines controller PI gains in run-time by using a new scheduling parameter calculated from pressure ratio of the intake and exhaust manifolds and exhaust air-to-fuel ratio. The coupling characteristics of the EGR and VGT systems are compensated by applying a simplified decoupler design method. Engine experimental results in various engine operating conditions proved that the gain scheduled controller obtained stable control performances without any oscillatory behavior or overshoot even when there is a change of engine operating conditions. Furthermore, the proposed decoupler reduced the coupling effects 14-66 % in various engine operating conditions. Unfortunately, although the controllers for common rail system and EGR and VGT systems perfectly follow the optimized set-points, it does not guarantee satisfactory emission performance during transients. A dynamics difference between fuel and air systems leads to a PM emission peak under a transient state. Limitation of fuel injection quantity is widely used to reduce the transient PM emission. However, since limiting the fuel injection quantity leads to loss of engine torque, this study propose an adaptation algorithm of reference variables for the common rail pressure and EGR rate to minimize the torque loss under a transient state. The proposed adaptation algorithm is designed to reduce PM emission as well as enhance torque response during transients while it manipulates set-points for the EGR and rail pressure controller. Reduction of EGR gas and increase of rail pressure during transients respectively improve charging efficiency of the fresh air and combustion efficiency by well mixed fuel-air mixture, resulting in improvement of torque response while reducing PM emissions. The step test results of engine operating points represented that the maximum 23 % reduction of PM emission is achieved and the IMEP is increased by 5.2 % under a transient state.