Evolution of Diesel Engine Performance Under the Coupling Effects of Altitudes and Miller Cycles

To clarify the impact of the Miller cycle on the performance evolution of diesel engines under varying altitude conditions and to address the operational demands of hybrid-specific diesel engines in high-altitude environments， a one-dimensional simulation study was conducted based on a turbocharged diesel engine. The research systematically examined the effects of different altitude levels， Miller-cycle strategies （early intake valve closing（EIVC） and late intake valve closing（LIVC）） on volumetric efficiency， thermal efficiency， power， fuel consumption and NO_x emissions at different engine speeds. The results show that under plain conditions， a slight advance in intake valve closing achieves the optimal compromise between torque and fuel consumption. Between the two Miller-cycle strategies， EIVC favors air charging at 1 600 r/min， while LIVC is more beneficial at 2 400 r/min. Elevating altitude from 3 km to 4 km leads to a notable decline in engine performance. Volumetric efficiency， thermal efficiency， and power decrease by 18.05%， 18.76% and 19.56%， respectively， with a concurrent increase in fuel consumption of 24.24%. NO_x emissions exhibit a ring-shaped distribution， with the peak emission zone located around 2 km altitude and Miller-cycle ranges of the intake valve closing advance angle 40°，30°，20°，10°， the original intake value closing angle， and the intake value closing angle delay 10°，20° and 30°. Deviating from the central region， whether by altering altitudes or Miller intensities， can effectively reduce NO_x emissions， albeit through distinct mechanisms. When applying deeper Miller-cycle strategies （either EIVC or LIVC） to lower NO_x emissions， a balance between dynamic performance and fuel economy must be considered.