The Mixed Flow Forward Swept Turbine for Next Generation Turbocharged Downsized Automotive Engines

Abstract

In automotive turbochargers, the nature of the performance characteristics of a conventional radial turbine are such that it does not make good use of the exhaust gas energy of the engine, because the efficiency is lowest when the exhaust manifold pressure is highest, i.e. at the peak of the exhaust pulse, and the point at which most exhaust gas energy is theoretically available. The turbine design is also seriously compromised by requirements of size and inertia to improve the transient response of the engine. In this study, the use of forward swept rotor blading to improve the efficiency characteristic is investigated. Stress considerations mean that a mixed flow turbine geometry is required for this purpose. By comparing a baseline radial and two mixed flow turbines in engine simulations, it is shown that under steady-state conditions, a large increase in engine torque at low speed (before the wastegate opens) is obtained with the mixed flow turbines. The simulated response of the engine to a load step also shows that the same transient torque (and therefore vehicle response) can be achieved with the mixed flow turbine, even allowing for large increases in rotating inertia. The use of forward swept blades, and the improvement in exhaust energy recovery that stems from it, compensates for increases in inertia required by the mixed flow geometry and increases in overall turbine size.