Optimization of Simultaneous Control of High Pressure EGR and Turbocharger Variable-Geometry during Diesel Engine Speed and Load Transients

Modern diesel engines incorporate increasingly complex subsystems such as electronically-controlled high-pressure fuel injection, exhaust-gas recirculation (EGR), variable-geometry turbochargers (VGT) and elaborate after-treatment devices. The challenge to control such devices escalates significantly when they function interactively. Simultaneous control of high-pressure EGR (HPEGR) and VGTs becomes particularly challenging within transient speed and load change events because the turbo geometry change correspondingly changes the exhaust back pressure which drives the EGR flow across the control valve. HPEGR, where the exhaust gas is extracted upstream of the turbine inlet and recirculated downstream of the compressor exit, is desirable because it eliminates the flow of corrosive and sooty exhaust through the compressor and reduces the losses associated with expanding and recompressing the recirculated exhaust gasses. Variable-geometry turbocharging can help with both NOx reduction and efficiency improvements. This paper describes an effort to optimize simultaneous control of HPEGR and a VGT. A medium duty diesel engine has been so equipped and a control method has been established to optimize the overall performance of the engine in transient steps. Fast response NOx measurements, fast response particulate matter measurements and instantaneous fuel consumption were used to quantify the transient performance. An optimization objective function was applied using data acquired under steady state operating conditions to find boost and EGR levels which meet emission goals while minimizing fuel consumption. The associated steady state optimum VGT and EGR actuator positions were then applied at corresponding speed and load conditions during speed, load and combined speed-load transients while observing the effects on the emissions and fuel consumption. Anticipatory actuator strategies designed to minimize transient performance degradation were then conceived and evaluated.