승용 디젤 엔진에서의 실린더 압력 센서를 이용한 EGR률 모델링

Abstract

나날이 강화되는 배기 및 에너지 규제를 만족시키고 위해 커먼레일직분사 (Common-Rail Diect Injection, CRDI) 시스템을 비롯한 가변용량터보차저 (Variable Geometry Turbocharger, VGT), 배기가스 재순환 (Exhaust Gas Recirculation, EGR) 장치 등의 기술들이 개발되어 왔다. 그 중에서도 EGR 시스템은 특히 NOx 배출물 저감의 대표적인 기술로, 배기가스의 일부분을 다시 흡기관으로 재순환 시켜 이로 인해 높아진 비열과 감소된 가용 산소농도로 연소 온도를 저감시켜, 온도에 민감하게 발생하는 NOx 배출량을 줄일 수 있다. 하지만 EGR gas의 비율이 너무 높아지면 NOx는 줄어드는 반면, 입자성 물질 (Particulate Matter, PM) 발생량은 증가하고 연비가 낮아지게 되므로, trade-off 관계를 최적화 시킬 수 있도록 EGR률 제어가 필요하다. EGR rate 제어를 위해서는 정확한 EGR률 정보가 필요하다. 하지만 EGR path에는 고온, 고압의 배기가스가 지나가고, 또한 배기가스의 불순물들이 많아 센서 파울링 현상이 빈번히 발생하는 등 센서 사용에 부적합한 환경이기 때문에, EGR률을 추정하는 모델이 필요하다. 이 논문에서는 실린더 압력센서를 이용한 EGR률 모델링 기법을 제시하고 있다. 기존의 EGR률 모델링 연구들은, 흡/배기관의 긴 air path에서 측정한 물리량들을 이용하기 때문에 시간 지연이 존재하고, 비선형적이고 측정 또는 추정이 어려운 물리량들을 이용하여 모델링을 진행하므로 모델링이 어렵고 복잡하다. 하지만 실린더 압력을 이용한 EGR률 모델은, 실린더 내부의 상태를 실시간으로 모니터링하고 이를 바로 모델에 사용하기 때문에 응답성능이 빠르고, 제한된 공간에서의 물리량을 이용하여 모델링을 하기 때문에 모델 구조를 단순화 시킬 수 있다. 실린더 압력을 이용한 EGR rate 모델은. 미리 지정된 두 지점에서의 실린더 압력 차를 이용하여 실린더 내부의 총 가스의 질량을 추정하는 ΔP method를 이용하여 최종적으로 EGR률을 추정한다. 이렇게 만들어진 모델을 ECU에 integration하여 여러 가지 조건에서의 엔진 셀 실험을 진행하여 검증하였다. 추가적인 case study로 EGR률 제어를 진행하여 feasibility를 확인해보았다. 실린더 압력을 이용한 EGR률 모델은 cycle-by-cycle로 정확한 EGR률 정보를 얻을 수 있기 때문에, 이를 이용하면 차후 진행될 배기 배출물에 대한 피드백 제어에서 분사 제어 전략과 흡배기 시스템 제어와의 협조제어도 가능할 것이다. |Emission and energy restriction have been strengthened as interest in environment has increased. In order to meet the strengthened emission and energy regulations, many technologies such as common-rail direct injection (CRDI), variable geometry turbocharger (VGT) and exhaust gas recirculation (EGR) have been developed. Among those technologies, the EGR system is a significant technology to reduce NOx emission. The EGR system recirculates a portion of the exhaust gas from the exhaust manifold into cylinders. When the amount of recirculated exhaust gas increases, the combustion temperature decreases due to reduced available oxygen concentration and raised heat capacity. As a result, NOx emission is decreased since it is formed at high temperature condition. Nevertheless, the higher rate of EGR gas, the less NOx is formed but the more particulate matter (PM) is generated and the fuel efficiency is degraded. Therefore, EGR rate should be controlled to optimize these trade-off relationships. In order to control the EGR rate precisely, accurate EGR rate information is required. However, it is hard to use sensors for measuring the EGR rate directly because of harsh environment condition of the EGR path. Not only the temperature and pressure of the exhaust gas are high, but also PM causes problems like sensor fouling. Therefore, using the sensor for directly measuring the EGR rate is not suitable because of its durability. Consequently, a model is required to estimate the EGR rate. In this paper, the modeling of the EGR rate based on in-cylinder pressure measurement is proposed. Traditional modeling approaches for the EGR rate use variables which are measured at long intake/exhaust air path so that time delay is inevitable. In addition, the modeling is complex and difficult because many non-linear or unmeasurable variables such as valve effective area or efficiency of each component are also used by the model. By using the in-cylinder pressure sensor, the information of combustion state of in-cylinder can be obtained directly with fast response. Therefore, there is no need to consider the complex variables such as effective area of valve and efficiency of each components so that the model can be more simplified. The proposed EGR rate model estimated the EGR rate based on the ΔP method. This method can determine the total mass of mixed gas using the pressure difference between predefined two crank angle positions during compression stroke. The model was integrated into ECU and validated through engine cell tests on the various operating conditions. In addition, experiments for EGR control were also conducted to validate the feasibility as a case study.; Emission and energy restriction have been strengthened as interest in environment has increased. In order to meet the strengthened emission and energy regulations, many technologies such as common-rail direct injection (CRDI), variable geometry turbocharger (VGT) and exhaust gas recirculation (EGR) have been developed. Among those technologies, the EGR system is a significant technology to reduce NOx emission. The EGR system recirculates a portion of the exhaust gas from the exhaust manifold into cylinders. When the amount of recirculated exhaust gas increases, the combustion temperature decreases due to reduced available oxygen concentration and raised heat capacity. As a result, NOx emission is decreased since it is formed at high temperature condition. Nevertheless, the higher rate of EGR gas, the less NOx is formed but the more particulate matter (PM) is generated and the fuel efficiency is degraded. Therefore, EGR rate should be controlled to optimize these trade-off relationships. In order to control the EGR rate precisely, accurate EGR rate information is required. However, it is hard to use sensors for measuring the EGR rate directly because of harsh environment condition of the EGR path. Not only the temperature and pressure of the exhaust gas are high, but also PM causes problems like sensor fouling. Therefore, using the sensor for directly measuring the EGR rate is not suitable because of its durability. Consequently, a model is required to estimate the EGR rate. In this paper, the modeling of the EGR rate based on in-cylinder pressure measurement is proposed. Traditional modeling approaches for the EGR rate use variables which are measured at long intake/exhaust air path so that time delay is inevitable. In addition, the modeling is complex and difficult because many non-linear or unmeasurable variables such as valve effective area or efficiency of each component are also used by the model. By using the in-cylinder pressure sensor, the information of combustion state of in-cylinder can be obtained directly with fast response. Therefore, there is no need to consider the complex variables such as effective area of valve and efficiency of each components so that the model can be more simplified. The proposed EGR rate model estimated the EGR rate based on the ΔP method. This method can determine the total mass of mixed gas using the pressure difference between predefined two crank angle positions during compression stroke. The model was integrated into ECU and validated through engine cell tests on the various operating conditions. In addition, experiments for EGR control were also conducted to validate the feasibility as a case study.