Thermodynamic cycle characteristics of twin-VGT diesel engine and its control method at variable altitudes

Abstract

Matching characteristics between two-stage turbocharging and fuel injection significantly affect energy flow distribution in diesel engine thermodynamic cycles at variable altitudes. This paper investigates thermodynamic cycle characteristics of the Twin variable geometry turbocharged (VGT) diesel engine and proposes a control method to optimize the engine’s energy flow at variable altitudes. First, a thermodynamic analysis model was built to investigate the influence mechanism of key thermal parameters on in-cylinder combustion, intake, and exhaust processes at variable altitudes. Furthermore, a principle of four maximum and three minimum was proposed to achieve thermodynamic cycle efficiency for the Twin-VGT diesel engine at variable altitudes. The maximum intake density is based on the interaction of two-stage compression and cooler, which corresponds to a maximum brake thermal efficiency (BTE). The maximum intake density, coupled with fuel injection strategy, would achieve the lowest fuel consumption under a part-load condition and the largest torque corresponding to the minimum air–fuel ratio under a full-load condition. Maximum global expansion ratio (GER) and isentropic efficiency can be achieved by reasonably controlling the effective flow area of high-pressure (HP) and low- pressure (LP) VGT to fully utilize exhaust available energy at variable altitudes. The minimum power consumption for the two-stage compressor is obtained based on the distribution method of global boost pressure (GBR) to reach the targeted boost pressure. This paper provides an energy flow control method to realize an efficient thermodynamic cycle for the Twin-VGT at variable altitudes.