ON THE CHOICE OF TURBINE TYPE FOR A TWIN-TURBINE HEAVY-DUTY TURBOCHARGER CONCEPT

In this study, a fundamental approach to the choice of turbocharger turbine for a pulse-charged heavy-duty diesel engine is presented. A standard six-cylinder engine build with a production exhaust manifold and a Twin-scroll turbocharger is used as a baseline case. The engine exhaust configuration is redesigned and evaluated in engine simulations for a pulse-charged concept consisting of a parallel twin-turbine layout. This concept will allow for pulse separation with minimized exhaust pulse interference and low exhaust manifold volume. This turbocharger concept is uncommon, as most previous studies have considered two stage systems, various multiple entry turbine stages etc. Even more rare is the fundamental aspect regarding the choice of turbine type as most manufacturers tend to focus on radial turbines, which by far dominate the turbochargers of automotive and heavy-duty applications. By characterizing the turbine operation with regards to turbine parameters for optimum performance found in literature a better understanding of the limitations of turbine types can be achieved. A compact and low volume exhaust manifold design is constructed for the turbocharger concept and the reference radial turbine map is scaled in engine simulations to a pre-set AFR-target at a low engine RPM. By obtaining crank-angle-resolved data from engine simulations, key turbine parameters are studied with regard to the engine exhaust pulse-train. At the energetic exhaust pressure pulse peak, the reference radial turbine is seen to operate with suboptimum values of Blade-Speed-Ratio, Stage Loading and Flow Coefficient. The study concludes that in order to achieve high turbine efficiency for this pulse-charged turbocharger concept, a turbine with efficiency optimum towards low Blade-Speed Ratios, high Stage Loading and high Flow Coefficient is required. An axial turbine of low degree of reaction-design could be viable in this respect.