Twin entry turbines are widely used in turbocharging as a means of using the exhaust pulse energy of multi-cylinder engines. For modern engines where high levels of EGR are required, an asymmetric twin-entry turbine has been shown to have considerable advantages. Such turbines require a more developed approach to analysis and design than usual. A meanline model for a radial inflow turbine with twin-entry scroll has been developed. Different total pressures and total temperatures may be specified at each entry. Each volute passage is solved separately from the inlet to the splitter location, where the static pressures of both passages are assumed to be the same. From the volute splitter to the rotor inlet, the two streams mix into one uniform flow following conservation laws of continuity, momentum and energy. Experiments have been conducted on a test stand with a radial turbine with an asymmetric twin-entry scroll, where the inlet conditions can be varied independently for each entry. The test results are compared with the model prediction. A good accuracy of prediction is achieved with a realistic set of modeling coefficients. In the future, insights gained from test data and CFD analysis will be used to develop further the volute mixing model and include explicit partial admission losses in the rotor.

In automotive turbochargers, the nature of the performance characteristics of a conventional radial turbine are such that it does not make good use of the exhaust gas energy of the engine, because the efficiency is lowest when the exhaust manifold pressure is highest, i.e. at the peak of the exhaust pulse, and the point at which most exhaust gas energy is theoretically available. The turbine design is also seriously compromised by requirements of size and inertia to improve the transient response of the engine. In this study, the use of forward swept rotor blading to improve the efficiency characteristic is investigated. Stress considerations mean that a mixed flow turbine geometry is required for this purpose. By comparing a baseline radial and two mixed flow turbines in engine simulations, it is shown that under steady-state conditions, a large increase in engine torque at low speed (before the wastegate opens) is obtained with the mixed flow turbines. The simulated response of the engine to a load step also shows that the same transient torque (and therefore vehicle response) can be achieved with the mixed flow turbine, even allowing for large increases in rotating inertia. The use of forward swept blades, and the improvement in exhaust energy recovery that stems from it, compensates for increases in inertia required by the mixed flow geometry and increases in overall turbine size.

One of the more visible tasks when designing a turbocharger is the optimized design for a compressor and a turbine. The ultimate measure of a successful turbocharger design, however, is how well it works with a specific engine at various operating conditions. Final design decisions must be based on the engine-turbocharger system as a whole, rather than only on the individual component performance. This paper describes the effort to develop an integrated design system which allows the user to design and optimize a turbocharger on a system level. With the basic engine parameters specified, along with simple models for other commonly used components, such as the Exhaust Gas Recirculation (EGR), wastegate, and intercooler, the program may be linked to two powerful meanline programs that can handle the fast iterations of the design and analysis of a compressor and a turbine. The output is either a new compressor or turbine that best matches the operation of the engine or the performance of an existing turbocharger at a specific engine operating condition. A case study is presented where the program is applied in a real-life design situation to fit a new turbocharger for a large locomotive 18-cylinder diesel engine. The tool is extensively used in guiding the selection of the turbocharger and in the simulation of the overall system performance. The test data from the new design show close agreement with the simulation results, as well as an improvement over the original design.

In the process of evaluating a parallel twin-turbine pulse-turbocharged concept, the results considering the turbine operation clearly pointed towards an axial type of turbine. The radial turbine design first analyzed was seen to suffer from sub-optimum values of flow coefficient, stage loading and blade-speed-ratio. Modifying the radial turbine by both assessing the influence of “trim” and inlet tip diameter all concluded that this type of turbine is limited for the concept. Mainly, the turbine stage was experiencing high values of flow coefficient, requiring a more high flowing type of turbine. Therefore, an axial turbine stage could be feasible as this type of turbine can handle significantly higher flow rates very efficiently. Also, the design spectrum is broader as the shape of the turbine blades is not restricted by a radially fibred geometry as in the radial turbine case. In this paper, a single stage axial turbine design is presented. As most turbocharger concepts for automotive and heavy-duty applications are dominated by radial turbines, the axial turbine is an interesting option to be evaluated for pulse-charged concepts. Values of crank-angle-resolved turbine and flow parameters from engine simulations are used as input to the design and subsequent analysis. The data provides a valuable insight into the fluctuating turbine operating conditions and is a necessity for matching a pulse-turbocharged system. Starting on a 1D-basis, the design process is followed through, resulting in a fully defined 3D-geometry. The 3D-design is evaluated both with respect to FEA and CFD as to confirm high performance and durability. Turbine maps were used as input to the engine simulation in order to assess this design with respect to “on-engine” conditions and to engine performance. The axial design shows clear advantages with regards to turbine parameters, efficiency and tip speed levels compared to a reference radial design. Improvement in turbine efficiency enhanced the engine performance significantly. The study concludes that the proposed single stage axial turbine stage design is viable for a pulse-turbocharged six-cylinder heavy-duty engine. Taking into account both turbine performance and durability aspects, validation in engine simulations, a highly efficient engine with a practical and realizable turbocharger concept resulted.

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.