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As the engines of today decrease in displacement with unchanged power output, focus of today's research is on transient response. The trend of today is to use a turbocharger with high boost level. For SI-engines a regular WG-turbocharger has been used, but in the future, when the boost level increases together with higher demand on the transient response, a Variable Nozzle Turbine (VNT) will be used together with Variable Valve Timing (VVT). As the degree of freedom increases, the control strategies during a transient load step will be more difficult to develop. A 1D simulation experiment has been conducted in GT-Power where the transient simulation was “frozen” at certain time steps. The data from these time steps was put in a stationary simulation and the excessive energy was then bled off to obtain the same conditions for the engine in the stationary simulation as if the engine where in the middle of the transient.

The paper describes a measurement procedure to measure complete 2-stage turbo systems (TST - Two Stage Turbo) in a turbocharger test rig. The possibility to measure the complete range of operation of the series-sequential mode of the 2-stage system is proven. Also the performance is broken down for the two turbochargers individually; both when operating as a part of the TST system and measured as completely separated individuals. The maps measured individually were then used to calculate a composed TST-map as if there would be no losses between the two stages, and this composed map was compared to the measured TST-map. The difference between these two maps is interpreted as interstage losses when packaging the two turbochargers as closely as is necessary to fit in the car. The breakdown showed that the entire difference is not solely due to losses of total pressure interstage.

In this paper, the performance of a radial turbine working under pulsatile flow conditions is computed with two different modeling approaches, time resolved 1-dimensional (1-D) and 3-dimensional (3-D) CFD. The 1-D modeling approach is based on measured turbine maps which are used to compute the mass flow rate and work output from the turbine for a given expansion ratio and temperature at the inlet. The map is measured under non-pulsatile flow conditions, and in the 1-D method the turbine is treated as being a quasi-stationary flow device. In the 3-D CFD approach, a Large Eddy Simulation (LES) turbulence approach is used. The objective of LES is to explicitly compute the large scales of the turbulence while modeling the effects of the unresolved scales. Three different cases are considered, where the simplest case only consist of the turbine and the most complex case consist of an exhaust manifold and the turbine.

In this work a comparative study between 1-D and 3-D calculations has been performed on different junctions. The geometries are a 90° T-junction with an area ratio of unity and a 45° junction with an area ratio of 1.78 between the main pipe and the side branch. The latter case had an offset between the centerlines of the main and the branched pipe. The 3-D modeling framework uses the Reynolds Averaged Navier-Stokes (RANS) equations with the k-ε model both for the steady and the unsteady flow cases. The comparison is made both under steady and pulsating flow conditions. The aim has been to assess the 1-D/3-D differences in terms of measures for flow losses. For the steady flow cases it is shown that there is a large difference between the 1-D and 3-D computed losses for both junction geometries. The differences are largest in the junction and right downstream of it.