The Asymmetric Twin Scroll Turbine for Exhaust Gas Turbochargers

Current turbocharged diesel engines use exhaust gas recirculation (EGR) to effectively meet emission standards. With exhaust gas recirculation it is possible to keep the nitrogen oxide (NOx) emissions to a minimum, largely by lowering the local peak temperatures in the combustion chamber. Exhaust gas transportation from the exhaust side to the air side can be realized in different ways. All have in common that, a drop of pressure from the exhaust to the air is needed. In this paper the high pressure exhaust gas recirculation concept will be discussed, where the exhaust gases are transported from the upstream side of the turbocharger turbine to the downstream side of the charge air cooler. In this concept a negative pressure difference between turbine inlet and engine intake is needed, leading to inefficient gas exchange and, in the end, increasing fuel consumption. In order to keep the overall fuel consumption increase as low as possible, some of the current 6-cylinder Mercedes-Benz truck engines, that have EGR, are equipped with the so-called asymmetric twin scroll turbine to provide the most efficient exhaust gas transportation. In this design concept the negative pressure difference between engine intake and turbine inlet is generated in just one of the two exhaust branches. Thus, whilst some cylinders are operated with a high exhaust gas backpressure, others are operated with a fuel-saving low exhaust gas back-pressure. The different back-pressures in the two exhaust branches are created by designing each flow path of the twin scroll turbine differently. The exhaust branch with the higher back-pressure needs a turbine scroll with a much smaller flow parameter than the exhaust branch with the lower back-pressure. As both flow paths are coupled to the same turbine wheel, the flow parameter is modified using the design parameters of the scrolls. This produces two totally different turbine concepts in one turbine housing. The turbine path with the higher flow parameter has a classical radial turbine reaction value of 0.5. This flow path can thus be optimized for maximum efficiency in comparison with other radial turbines. In contrast, the turbine path with the lower flow parameter combined with the turbine wheel is operated with a reaction value approaching zero. This flow path tends to need an axial turbine with a high flow direction change like an impulse turbine, even if a radial turbine wheel is used. Operating a radial turbine wheel under this boundary condition needs new development steps to improve the turbine with regard to mechanical feasibility and thermodynamic efficiency. This paper describes the principle mechanism of the asymmetric twin scroll turbine. Detailed engine cycle simulations give a brief introduction into the main advantages of asymmetric turbines in combination with exhaust gas recirculation. Hot gas test stand studies show the principle characteristics of this turbine type and the numerical flow simulations give a detailed insight into the flow phenomena in the turbine. The key design values will be discussed and the future outlook indicates the next development steps that will be required.