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The supercharging of small-displacement gasoline engines requires high pressure ratios combined with a wide range of air flow rate. To resolve this conflict, two-stage turbo charging with two turbochargers or the combination of a turbocharger and a mechanical compressor is used. But this is associated with an increase in complexity. The highest potential for avoiding a multi-stage system is provided by the systematic modification of the turbo-machinery operating maps, e.g. on the turbine side by using variable turbine geometry. An additional promising approach is the implementation on the compressor side of a variable guide vane. The shape of the compressor map is directly affected and the requirements for highly boosted engines can thus be fulfilled. The present paper provides an assessment of the potential of a variable compressor in combination with a variable geometry turbine (VTG) and additional wastegate on a small-volume gasoline engine.

The analysis of the energetic losses in a combustion engine shows that one-third of the chemical energy is lost as heat through the exhaust gas. Prior investigations have shown that an exhaust-gas driven Organic Rankine cycle (ORC) is suitable for the recovery of some of that energy. One of the essential components in such an ORC is the expansion machine. An investigation of the suitability of a turbine for this application is presented in this conceptual study. The concept is investigated for a heavy-duty truck application and a passenger-car application. On the basis of predefined design points, a thermodynamic analysis is performed to determine a suitable working fluid and the best process parameters. A single-stage partially admitted impulse turbine shows the best performance at tolerable rotational speeds for the resulting thermodynamic boundary conditions.

A single-stage turbocharger turbine is developed with the objective of enabling a gasoline spark-ignition engine to operate under lean-burn conditions with an air-to-fuel ratio of λ=2 in the range of the Worldwide Harmonized Light-Duty Vehicles Test Cycle. For this purpose, extensive 1-D engine simulations are performed using a combination of a simple compressor and simple turbine model as well as a combination of the stock compressor and a simple turbine model. The results show that an isentropic turbine efficiency of more than 70% over a wide operating range is required for the desired engine operation - especially with regard to the low-end-torque. Based on the crank-angle-resolved engine simulation data, turbine requirements are determined. Their evaluation shows that an axial turbine is a reasonable alternative to conventional radial turbines for this application. Next, a preliminary axial turbine is designed using 1-D/2-D design approaches.