Effects of Miller cycle and variable geometry turbocharger on combustion and emissions in steady and transient cold process

•Miller cycle and two-stage turbocharge are applied to HD diesel engine.•The goal is to assess the potential of the techniques in the transient process.•Compared to steady-state map, soot peak emissions are reduced by 50%.•NOx peak emissions are reduced by 10% compared to the conventional cycle. The influence of Miller cycle and variable geometry turbocharger (VGT) on combustion and emissions was investigated in steady and transient operation of a heavy-duty diesel engine. The experiment was conducted on a six-cylinder diesel engine with common rail fuel injection, two-stage turbocharger and Miller cycle. The results revealed that Miller cycle decreased soot and nitrogen oxides (NOx) emissions and increased brake thermal efficiency (BTE) in certain operation. In the steady operation, the intake flow was reduced in a cold condition because the low oil temperature led to the hydraulic mechanism delay intake valve closing timing. However, by adjusting VGT, low emissions of soot and NOx could still be obtained with relatively high BTE. In cold transient process, dynamic response of torque depended on indicated work and gas pumping work. A good trade-off performance could be achieved by maintaining the initial VGT rack position for 0.5s. Compared to VGT variation under the steady-state map, soot peak emissions were reduced by 50% with NOx emissions nearly unchanged by VGT control strategy 3 and Miller cycle. Compared to diesel conventional cycle, the combination of Miller cycle and VGT control strategy 3 can not only ensure quick dynamic torque response, but also reduce NOx peak emissions by 10% with slightly increased soot peak emissions.