Study on matching and altitude adaptive characteristics of multistage turbocharging for gasoline engine at high-altitude conditions

High-altitude long-endurance unmanned aerial vehicles (HALE UAVs) technology is an important technology for the exploration of high-altitude aerospace environments. However, existing researches on multi-stage turbocharged gasoline engines, an ideal power unit for HALE UAVs, show inconsistent conclusions on the strategies of turbocharger-engine match over the different altitudes, indicating the lack of understanding of the multistage turbocharging matching and limiting the advancement of this technology. This paper presents, for the first time, the discovery of a non-monotonic variation in the turbine effective flow area matching across altitudes, which causes the instability of the turbocharged gasoline engines in altitude adaptive characteristics. Firstly, a theoretical model between the reduced mass flow rates of the cylinders and the turbine is deduced, and it is found that the reduced flow rate of cylinders exhibits a maximal value. A turbine pressure ratio corresponds to this maximal value point is greater than the turbine choked pressure ratio when the product of nondimensional exhaust temperature and efficiency exceeds approximately 1.8. As the turbocharging system approaches this critical point, multiple stable operating points emerge, leading to dynamic instability in the altitude adaptive characteristics of the system. The presence of multiple stable points leads to sudden changes in the boosting capability of turbocharged gasoline engines, potentially destabilizing the system and forming a ‘trap box’ of the system state. The effect is amplified in multi-stage boosting systems, which offer higher efficiency but exacerbate this instability.