Effects of variable geometry turbocharging coupled with mechanical turbo-compounding on power recovery and thermal efficiency of a diesel engine under plateau conditions

ABSTRACT To clarify the effect of a mechanical turbo-compounding (MTC) system on power recovery and brake thermal efficiency (BTE) of a turbocharged heavy-duty diesel engine under plateau conditions, a one-dimensional simulation model was developed and validated using experimental data at an altitude of 2000 m. Based on the validated model, a variable geometry turbocharger (VGT) was matched with the MTC unit. Full load performance, including output power, brake specific fuel consumption (BSFC), and BTE, was compared between two configurations: waste-gated turbocharger coupled with MTC (WGT-MTC) and VGT coupled with MTC (VGT-MTC). The VGT-MTC system was further evaluated at altitudes of 0, 2000, and 3000 m. The results show that the benefit of a MTC system is essentially governed by the competition between recovered power and the backpressure-induced penalty. At low speeds, recovery cannot offset the induced power loss, yielding limited net benefit; at mid-to-high speeds, higher exhaust enthalpy allows recovered power to increase faster than the penalty, resulting in a stable positive net benefit region. Compared with WGT-MTC, VGT-MTC provides higher average recovered power, lower BSFC, and a larger net benefit region over a wider speed range. With increasing altitude, the power turbine recovery capability improves, but the total system output power remains constrained by the reduction in the engine’s baseline. Relative to the uncompounded baseline, VGT-MTC delivers a larger relative BTE gain at higher altitudes, indicating stronger compensation under plateau conditions. Therefore, VGT-MTC is recommended for plateau applications to achieve greater improvements in system power and efficiency.