Abstract: The Turbine-Based Combined-Cycle Engine (TBCC) is a favored solution for powering both the future long-range high-speed vehicle and the Two-Stage-To-Orbit (TSTO) reusable launch vehicle. Mode Transition from the low-speed propulsion system to the high-speed propulsion system is one of the most critical TBCC enabling technologies. In this paper，the principles of the Mach 4 over-under TBCC engine were analyzed，and a flexible TBCC simulation model，which considered the inlet/engine mass flow matching and improved the iterative technique for engine model solving，was developed for the steady-state and transient performance analysis of all modes of operation，including mode transition. Through comparing the variations of net thrust and SFC along the flight trajectory between turbo mode and ram mode，the mode transition Mach number is selected as 3.0. Based on the basic principle of smooth thrust and air mass flow variations during mode transition，a three-phase mode transition strategy according to the turbine engine operation was proposed，and control schedules of the regulated parameters during mode transition was designed. The simulation results of mode transition dynamic performance show that the proposed strategy can basically realize the stable mode transition from turbo mode to ram mode，but there exists a short-time decrease of total thrust by about 12.5% when the turbine engine afterburner shuts down during mode transition to ensure that the total air mass flow maintains constant.

Key words: Over-under TBCC；Mode transition strategy；Dynamic performance；Numerical simulation

摘要： 涡轮基组合循环(TBCC)发动机是未来远程高速飞行器和可重复使用双级入轨(TSTO)飞行器第一级运载器的理想动力，而模态转换是实现TBCC发动机工程实用所必须解决的关键技术之一。针对Ma4一级内并联式TBCC发动机，分析了其工作原理，发展了相应的总体性能计算模型，该模型考虑了进气道与发动机的流量匹配关系，改进了发动机模型的迭代求解方法。通过对比涡轮模态与冲压模态的净推力、单位燃油消耗率沿飞行轨迹的变化规律，确定模态转换马赫数为3.0。根据模态转换期间发动机推力、空气流量连续变化的基本要求，提出了一种根据涡轮发动机工作状态分三阶段进行的模态转换策略，确定了模态转换过程的参数调节规律。模态转换动态性能模拟结果表明，基本实现了涡轮模态至冲压模态的平稳转换，但在涡轮发动机加力关闭时，为保证发动机空气流量连续变化，发动机总推力将出现短暂的下降，降幅约为12.5%。

关键词: 内并联式TBCC；模态转换策略；动态性能；数值模拟