Cylinder pressure based Combustion Controls of Combustion Phase, Maximum Rate of Heat Release and IMEP for Common-rail Diesel Engines

Abstract

therefore, the dispersions of both the NOx and PM emissions were effectively reduced during engine parameter changes. Furthermore, to compensate for the variations of fuel injection, indicated mean effective pressure (IMEP) control was added to the MFB50 and ROHRmax control; Fundamentally, diesel engine combustion is a feedforward control system. For this reason, despite identical engine parameter conditions, unexpected combustion may occur due to production tolerance, engine aging and different circumstance such fuel quality, humidity or temperature. Unexpected combustion results from the deterioration of engine performance and enlargement of emission dispersions. Therefore, in order to have consistent performance and meet the stringent emission regulations during engine lifetime, combustion feedback and control are required to prevent unexpected combustion. In this study, the combustion state is monitored and controlled by feedback information obtained from in-cylinder pressure. To reduce the combustion variations and the dispersions of NOx and PM emissions regardless of different circumstances, engine aging, and production tolerance, mass fraction burnt 50 % (MFB50) and the maximum rate of heat release were selected as control variables. MFB50 is a highly related combustion phase to NOx emissions, and the maximum rate of heat release (ROHRmax) has close correlation with soot release and oxidation. As the combustion is controlled by MFB50 or ROHRmax, the effects of MFB50 and ROHRmax control on combustion and the dispersions NOx and PM were analyzed according to changes of engine parameters such as fuel rail pressure, boost pressure, mass air flow, swirl valve, main injecting timing, main injection duration, pilot injection timing, and pilot injection duration. As a result, NOx dispersions were reduced by MFB50 control and PM dispersions were decreased by ROHRmax control. In addition, the simultaneous MFB50 and ROHRmax control reduced the variations of the combustion phase and the gradient of heat release; as a sequence, emission dispersions NOx and PM emissions were further reduced under all parameter changes except boost pressure and air mass flow changes. The calculations of combustion indicators for combustion control such as MFB50, ROHRmax and IMEP demand extensive cylinder pressure data and high computational load. Therefore, the vast data acquisitions of cylinder pressure and the high computational load contribute toward making real-time implementation difficult. To implement the proposed cylinder pressure based combustion control in real-time, an estimation algorithm for MFB50, ROHRmax, and IMEP was developed using difference pressure between the firing pressure generated by combustion and the motoring pressure obtained by the piston’s movement. The proposed algorithm is able to estimate MFB50, ROHRmax, and IMEP with less cylinder pressure data and low computational load by simplification of the conventional heat release equation. The estimated MFB50, ROHRmax, and IMEP were validated by engine experiments under various operating conditions, and the combustion control with the estimated combustion indicators showed identical performance compared to the original combustion control with MFB50, ROHRmax, and IMEP|디젤 엔진은 동일한 엔진 파라미터 조건에도 불구하고, 생산공차, 엔진 노후 및 다른 연소환경 등에 의해서 예상치 못한 연소편차가 발생할 수 있다. 이러한 연소편차는 엔진 성능악화 및 배기가스 산포 증가를 초래하므로 엔진 수명 동안 동일한 성능을 유지함으로써, 배기규제를 만족할 수 있도록 연소상태를 모니터링하고 제어하기 위한 방법이 필요하다. 이 연구에서는 실린더 압력을 이용하여 연소위상을 나타내는 질량연소분율 50% (MFB50) 지점과 최대열발생율(ROHRmax)을 계산하고 연소위상과 최대열발생율 제어가 연소 및 배기 편차에 미치는 영향을 분석하였다. 또한 MFB50과 ROHRmax을 동시에 제어함으로써, 엔진파라미터 변경에도 MFB50과 ROHRmax 편차를 감소시킴에 따라 NOx와 PM편차를 감소시켰다. 연료분사 편차에 의한 연소 편차 보정을 위하여 도시평균유효압력(IMEP) 제어를 추가하였으며, 그 결과 과급압력 및 부스트압력 및 흡입공기유량을 제외한 모든 엔진 파라미터 변경 시에도 연소편차를 더욱 감소시켰다. 연소압력을 이용한 연소제어를 구현하기 위한 MFB50, ROHRmax, IMEP등과 같은 연소제어인자 계산은 많은 수의 실린더 압력 데이터와 높은 계산부하를 필요로 한다. 이러한 많은 데이터 양과 높은 계산 부하는 실시간 적용을 어렵게 하므로 실시간 적용을 보다 쉽게 하기 위하여 MFB50, ROHRmax, IMEP 추정 알고리즘을 제안하였다. 제안한 추정알고리즘은 연소압력과 모터링 압력의 차이인 차이압력을 이용하여 기존의 열발생율 식을 간소화 함으로써 적은 데이터 수집량과 계산부하로도 MFB50, ROHRmax, IMEP의 추정이 가능하다. 추정된 연소제어인자는 엔진 실험 검증을 통하여 다양한 조건에서 MFB50, ROHRmax, IMEP를 성공적으로 추정하는 것을 확인하였으며, 추정한 연소제어인자를 이용한 연소제어 시 엔진 파라미터 변경에 의한 NOx 및 PM편차가 감소되는 것을 확인하였다.; as a sequence, emission dispersions NOx and PM emissions were further reduced under all parameter changes except boost pressure and air mass flow changes. The calculations of combustion indicators for combustion control such as MFB50, ROHRmax and IMEP demand extensive cylinder pressure data and high computational load. Therefore, the vast data acquisitions of cylinder pressure and the high computational load contribute toward making real-time implementation difficult. To implement the proposed cylinder pressure based combustion control in real-time, an estimation algorithm for MFB50, ROHRmax, and IMEP was developed using difference pressure between the firing pressure generated by combustion and the motoring pressure obtained by the piston’s movement. The proposed algorithm is able to estimate MFB50, ROHRmax, and IMEP with less cylinder pressure data and low computational load by simplification of the conventional heat release equation. The estimated MFB50, ROHRmax, and IMEP were validated by engine experiments under various operating conditions, and the combustion control with the estimated combustion indicators showed identical performance compared to the original combustion control with MFB50, ROHRmax, and IMEP