Hybrid concepts require maximum efficient combustion engines to achieve a significant reduction in CO2 emissions. Miller/Atkinson-cycle gasoline engines are showing progress in striving for the reduced CO2 levels. BorgWarner's VTG turbochargers are emerging as one of the preferred charging systems for this process.
In addition to the electrification of the powertrain, alternative concepts based on highly efficient combustion engines are necessary to achieve strict future CO2 targets. The hybridistion of the powertrain offers possibilities for integrating the electrical support of the charging system and at the same time new design strategies for exhaust turbocharging.
The common goal of these concepts is to extend the spread from increasing efficiency in fuel-relevant operating areas of the engine and to display the stoichiometric motor operation in the entire motor characteristic field at full load. This increase in engine efficiency is presented by Miller/Atkinson combustion, for which the turbocharger with variable turbine geometry (VTG) is a viable charging solution.
BorgWarner’s VTG turbocharger offers additional efficiencies through improved flow geometry in the turbine and reduced friction performance by using ball bearings instead of journal bearings.
Gasoline engine development
A number of adjusting screws are available to increase the efficiency of the gasoline combustion engine: higher geometric compression ratio, charge dilution, Miller/Atkinson cycle and various combinations of these measures. The growing range of full and plug-in hybrids justifies the development of internal combustion engines optimsed for hybrid operation. Turbocharged gasoline engines with Miller cycles achieve a higher specific power density with improved consumption compared to hybrid-optimised naturally aspirated engines.
In addition to the higher requirements for charging pressure due to charge dilution and intermediate expansion, the increase in efficiency of internal combustion engines also brings new challenges. BorgWarner claims that the VTG turbocharger for the gasoline engine offers a better system efficiency compared to the classic Wastegate technology.
In addition to an increase in performance from 15 to 20 kW compared to the Wastegate turbocharger, the VTG turbocharger for gasoline allows a reduction in fuel consumption. This is done by the possibility of increasing the Miller rate using the entire exhaust gas enthalpy. The VTG turbocharger can achieve a temperature gradient between the engine outlet and the entrance into the exhaust aftertreatment system, which is about 25 °C higher compared to Wastegate turbochargers. At the same time, it remains at a constant low-speed torque, enabling RDE (Real Driving Emissions, RDE) compliance up to power classes above 100 kW/l.
The optimised design of the aerodynamic components and the use of ball bearings instead of the classic oil lubricated journal bearings ensures an increase in turbocharger efficiency of up to five percent compared to the Wastegate turbocharger. In addition, the use of ball bearings helps to compensate the delayed load build-up caused by the new combustion process. Compared to traditional Wastegate technology, the combination of an optimised Miller cycle with cooled exhaust gas recirculation and reduced dynamic support of the P2 module can reduce CO2 emissions by up to 3 percent.
Advanced performance design
Considerable advancements in material and cost, along with the Miller/Atkinson combustion technique at exhaust gas temperatures of up to 950°C, have combined to make VTG technology very attractive for use with gasoline engines now being developed for use in next generation hybrid systems.
The enhanced aerodynamic efficiency and control of the VTG turbine is achieved through adjustable inlet guide vanes, which allow a flow rate spread of more than twice that of fixed-housing turbines with wastegate control at comparable wheel diameters. The wide efficiency plateau is made possible by a unique combination of blade angles of the inlet guide vanes and wheel. The turbine provides high flexibility in creating turbine backpressure and turbine power output, ensuring boost pressure control. This contributes to substantial CO2 reductions while also effectively adapting to efficiency characteristics for the engine in use.
Design of the aerodynamic components is especially important in effectively influencing the VTG efficiency parabola. BorgWarner turbines are adapted to the position of the connecting flanges of engine and exhaust gas system to optimise flow for each customer application. The volute is designed to have a surface that is wetted minimally by exhaust gas and to fit in a small installation space. Both components are aligned aerodynamically to produce a directed flow to the inlet guide vanes.
The innovative flow guidance of the VTG turbocharger reduces thermomechanical deformation of the cartridge and, along with the design of the vane itself, works to minimise the inlet guide vane side clearances. This allows for high efficiencies at the knee torque of the engine. The turbocharger’s inlet guide vanes feature BorgWarner's patented S form to ensure that low-loss flow deflection and the torque on the guide vanes is as low as possible, while maintaining the fail-safe function to open at all times.
Because gasoline engines require a lower turbine backpressure and an enlarged flow rate to avoid knocking, BorgWarner uses radial wheels in VTG turbines. Higher efficiencies are also necessary at small turbine flow rates, around 30-40 percent of its maximum, for improved engine response and knee torque. The response behavior of the VTG turbine, similar to fixed-casing turbochargers with small inertia-optimised wheels, is achieved through improved efficiencies and a 10-20 percent reduction in the moment of inertia of the turbine wheel compared to earlier BW VTG generations for diesel applications. The VTG turbine wheel is designed for high flow rates, with the mechanically optimised wheel disk reducing stresses to provide long service life at a reduced moment of inertia.
More efficient turbochargers with ball bearings
Overall efficiency of a ball bearing turbocharger is further increased by enhanced rotor stability which allows contour clearance reduction on both the compressor and turbine sides. This unique ball bearing concept optimises the acoustic transmission path and shaft motion stability during both ramp-up and ramp-down of the rotor. The higher stiffness of the ball bearings requires optimised vibration transmission paths which are effectively transferred to the surroundings.
BorgWarner uses a patented system of decoupling rings to ensure build-up of the damping oil films, while allowing the cartridge to be centered in the bearing channel, improve the balancing and consequently acoustic behaviour. The rings also seal off the pressure area of the damper from the rest of the bearing housing.