# Design of an Axial Turbine for Highly Downsized Internal Combustion Engines
## 2 Background of Turbochargers
### Machine Types  
Conventional turbochargers implement radial rotors for both turbine and compressor rotors. This is mainly due to their relatively low cost, the wider range of volumetric flow they can operate under and the higher-pressure ratio per stage they can withstand. Other turbochargers feature axial and mixed flow impellers. The former is of particular interest because of its inherently lower inertia and higher efficiency for certain operative ranges, despite often being more expensive than conventional radial machines.  
In recent decades, many solutions have been proposed by OEMs and researchers in the field to achieve design improvements from the manufacturing or pure performance point of view. Among these, most recently in [2], it was shown that additive manufacturing techniques, inparticular Selective Laser Melting (SLM), offer advantages over conventional manufacturing techniques, such as design freedom, flexible processing and onsite manufacturing. This can reduce inertia on the turbines, allowing for very complex and efficient aerodynamics and mechanical designs (e.g., a honeycomb-like inner structure allowing for lower rotating mass, thus maintaining acceptable structural requirements). On the other hand, others, such as Volvo, use a separate electrically driven compressor connected to an air tank to blow the turbine with compressed air from the high-pressure tank when the driver is accelerating to reduce the spool-up time [3]. The most significative recent development in this field is, however, the DualBoost design proposed by Honeywell. Honeywell produced an innovative ultralow inertia system, called DualBoost, featuring an axial turbine and a double-sided compressor.  
The axial turbine presents excellent flow capacity and better efficiency compared to a conventional radial turbine under flow conditions typical of gasoline engines. Further, this layout allowed for a perfect match between the axial turbine's high speed (being necessary for higher efficiency) and that of the reduced-diameter double-sided (back-to-back type) compressor impeller, proving to be an effective choice for LMP1 cars, such as the 24 Hours of Le Mans winner, the Porsche 919 Hybrid.  
The DualBoost design is of interest to motorsport because of its fitness with the new 2021 F1 power unit regulations, which in removing the Motor Generator Unit-heat (MGU-H), will need ultraperforming low-inertia turbochargers. Further, the application of this kind of layout in an electric machine would be a novelty (to the best of the authors' knowledge) and would find application in the next-generation power units for the Federation Internationale de l'Automobile World Endurance Championship (FIA WEC) [4].  
Variable-Geometry Turbines (VGTs) are a category of machines allowing for a better match between turbine geometry and off-design flow conditions. They are among the other main research subjects, as well discussed by [5]. This review discussed the benefit of the implementation of different VGT techniques despite the increased cost of the systems. They showed that the most common and efficient VGT nozzle types to be implemented resulted in pivoting stators and sliding vanes, also pointing out a trend in implementing hydraulically and in particular, electrically actuated systems for a more precise geometry control than the standard pneumatic actuated types. VGTs have recently been used in other automotive-related applications, such as waste heat recovery [6].