Effects of a variable geometry turbocharger on the energy and exergy distributions of a light-duty diesel engine at various altitudes

Variable geometry turbocharging is widely used to mitigate diesel engine performance deterioration at high altitudes, yet systematic experimental quantification of how variable geometry turbocharger (VGT) regulation redistributes energy and exergy at various altitudes remains limited. This study investigates a light-duty diesel engine equipped with a single-stage VGT using a plateau-environment simulator and first- and second-law thermodynamic analyses. The effects of VGT vane opening on energy and exergy distributions were quantified at altitudes of 0, 1000, and 2000 m under full load conditions at 2200 r/min (peak torque) and 4000 r/min (rated power). The results show that an optimal VGT vane opening exists at each altitude for maximizing brake thermal efficiency (BTE) and exergy efficiency, and that the optimum decreases with increasing altitude. Appropriate reductions in VGT vane opening increased BTE by 0.61–1.39% and exergy efficiency by 1.09–1.59%, with greater benefits at higher altitudes. The exhaust energy percentage decreased with both altitude and VGT vane opening, whereas the exhaust exergy percentage decreased with altitude but increased with VGT vane opening; nevertheless, more than one-third of the exhaust energy at the turbine outlet remained recoverable. With increasing altitude, the percentages of heat energy and exergy transferred to the charge air cooler and coolant increased. Appropriate VGT regulation also reduced combustion losses, miscellaneous losses, and exergy destruction. These findings clarify the altitude-dependent redistribution of energy and exergy induced by VGT regulation and provide a thermodynamic basis for altitude-adaptive VGT calibration and exhaust energy utilization in high-altitude diesel engines.