The Radial Turbine for Small Turbocharger Applications: Evolution and Analytical Methods for Twin-Entry Turbine Turbochargers

In 1917, French Prof. Rateau built and operated the first turbocharger. He used an axial turbine because of existing experience from the steam turbine to drive a centrifugal compressor. From then on the axial turbine was improved regarding temperature capability and performance and is still the first choice for larger turbochargers today. From the beginning, multi-entry turbines, as discussed in the patent of Buch in 1925,were applied to make use of the pulse effect and, thus, to improve the engine performance at low speed and during transient operation. Between 1936 and 1945, the radial turbine for gaseous substances was proposed and finally implemented for smaller units, mainly because of lower complexity and cost. From the beginning, variable nozzle turbines were designed and built but without entering mass production. Up until 1963, multi entry turbine housings for radial turbines, like the axial turbine variants, were solely segment controlled. The twin entry turbine patent for radial turbines was filed by Garrett in August 1963 and the turbochargers went into production the same year at Caterpillar. Today there is an increasing share of 4 cylinder gasoline engines that make use of the twin entry turbine feature. This paper proposes a gas stand analysis process to measure the behavior of these turbines under unequal flow conditions in an extended pressure ratio operating range. A special turbocharger test rig for low turbocharger speed was set up in addition to a friction test rig to extend the turbine map for part load and transient optimization and simulation. Friction measurements as function of speed and thrust load were performed to separate mechanical from aerodynamic performance. A method to describe and analyze twin flow turbines is presented. In addition, backflow in the turbine was measured. These methods should help to improve the turbocharger behavior and to achieve refined turbocharger-engine interaction.