From earlier findings on P2 and P3 arrangements of hybrid drivetrains, Oerlikon Graziano developed the concept for a P3 drive system in the form of a compact hybrid axle module. By removing the input gear stage and the angular drive, an independent electrical axis in P4 architecture is created. The additional traction thus gained, increases the vehicle dynamics of powerful vehicles.
CHARACTERISTICS OF SMALL PRODUCTION VOLUMES
The legislation limits for pollutant and CO2 emissions are strongly pushing vehicle manufacturers toward electrification of their vehicle fleet. Even high performance car OEMs are taking on the challenge of using electrification, a trend observed for premium and exotic vehicles. This electrification is seen by some manufacturers as an opportunity both to increase the vehicle performance, while moving toward more environmentally-friendly drivetrains.
It means that the installation of electric power in the car should certainly not affect the traditional performance of only an internal combustion engine, but increase the usable power, driving pleasure and vehicle dynamics, while cutting the local emissions during driving. System layout is not an easy task, as the hybrid components (battery, inverter, and electric motor) strongly affect the overall vehicle weight and thus its driving dynamics as well as system cost.
All these arguments factor in even stronger high performance cars due to the challenges posed by low to medium production volumes. Most manufacturers look for synergies between hybrid and standard model series in order to minimise cost and investment. Because of this, modularity of the sub-systems plays a key role during concept selection.
The Oerlikon Graziano electric drive development started with insights and experiences gained during transmissions production and internal development programs, like the OGeco transmission , a development inspired by the idea of designing a robust and efficient transmission with an electrical drive. Analysing different hybrid architectures, the multi-speed P3 configuration was selected as the best solution to maximise the performance benefit of the additional electric power in combination with a high-efficiency mechanical layout, which is beneficial especially in Battery Electric Vehicle (BEV) and hybrid mode. In order to maximise the system efficiency and reduce the overall weight and mechanical complexity vis-à-vis automatic transmissions (ATs and DCTs), a P3 hybrid module was integrated in an automated
During the OGeco development, the efficiency advantages of the P3 architecture versus a comparable P2 architecture were measured on a HiL test rig . Experimental results showed up to 8 % of energy saving of the P3 compared to the P2 architecture during Worldwide harmonised Light Duty Test Cycle (WLTC) run in electric mode.
Following these earlier activities, Oerlikon Graziano developed a concept for a patented P3 hybrid powertrain in form of the compact hybrid axle module presented in (1). This product comprises a transmission input module powered by the internal combustion engine, and a bevel gear set driving the differential. That way, the output from the conventional gearbox is connected to the input flange of the Oerlikon Graziano rear axle hybrid module through the prop shaft.
The force is transferred via a cylindrical gear set to the differential gear, providing an additional gear ratio offering flexibility regarding powertrain adaptation and packaging before finally driving the differential via the bevel gear set. Hybrid functionality for this fully integrated system is provided by the electric motor with a hollow rotor shaft connecting to the differential, allowing a drive shaft to pass torque from the differential through the electric motor to the wheel.
Removing the mechanical link between the prop shaft and the final drive, the system can be re-configured as a standalone electric axle in P4 architecture, suitable for both front and rear wheel drive applications. The modularity makes the technology suitable for a P3 hybrid, P4 “through-the-road” hybrid or BEV architecture with one or two electric drives.
This arrangement of an axle forms the basis of the hybrid module, which can optionally be equipped with the electrical Limited Slip Differential (eLSD) made by Oerlikon Graziano, a mechanical LSD or a traditional open differential (standard option) within the same package space. Using the eLSD, (2), combines the necessary differential functionality with performance enhancing traction control for an improved driving experience in pure electric and hybrid mode.
The eLSD is capable of 2,500 Nm locking torque being actuated in less than 100 ms, both in closing and opening the clutch. Thus, it is suitable for all sports car applications.
Due to the chosen approach, the electric motor size can be selected according to certain boundary conditions and performance classes for the e-drive in the available vehicle package. The electric motor is connected directly via a planetary gear set module to the differential housing, bypassing the bevel gear set, for maximum efficiency. In addition, two gear ratios shown here provide for a wide vehicle speed range.
The planetary gear set module in two-stage compound architecture allows high transmission ratios between the electric motor and the wheels in a compact package at maximum motor speeds of up to 18,000 rpm. Furthermore, it allows for the possibility of having two selectable transmission ratios for a higher torque-to-speed map and better efficiency in relation to the target vehicle and driving situation. In (3), a view of the used planetary gear set is presented.
The shifting of the gears is based on a dog clutch, where a sleeve is hydraulically actuated from neutral position into gear. The speed synchronisation is ensured electrically by the electric motor enabling an overall shifting time of less than 100 ms. (4) shows an example of a shifting process. Typical values for shifting times are less than 70 ms for neutral to first gear shifts and less than 95 ms for first to second gear shifts. Thanks to the system specifications and design, the gear shifting can take place at vehicle speeds above 150 km/h, making it possible to drive the WLTC without shifting.
The concept’s special characteristic is its compactness that allows OEMs to package the complete hybrid module within the sub-frame of the body, without modifying the standard powertrain (internal combustion engine, transmission and prop shaft). It allows using all the advantages of a P3 hybrid configuration. This includes for example a higher efficiency from the electric motor to the wheels as well as the potential of a very strong e-boost power at low velocities, when in high gear of the conventional transmission. The P3 hybrid’s independence from the internal combustion engine transmission offers important advantages over the P2 hybrid in e-boost torque, as the conventional transmission components are not overloaded by the electric drive. For this purpose, (5) shows a comparison of the P2 and the P3 hybrid. Here, the increased e-boost percentage can be seen as clearly being independent from the internal combustion engine transmission at low driving velocities of the hybrid vehicle.
From a drivability point of view, the e-boost functionality with regard to wheel torque availability is advantageous, since it can deliver torque directly to the wheels without the need for downshifting on the transmission side of the combustion engine, providing immediate boost during acceleration and kick-down manoeuvers.
Other interesting features are electric and hybrid driving, for CO2 emissions reduction and independent recovery of kinetic energy, especially state-of-charge control, depending on driving strategy. In addition, there are the compact external dimensions and a small package. The first prototypes of P3 hybrid modules, (6), have been developed, built, tested and shipped in a period of only ten months, and have been successfully installed in vehicles, enabling development and system application activities.
The hybrid module is equipped with a hydraulic power pack control in order to deliver all the actuation needed for functionality and high power density for the specified shifting powers. With the aim of providing a small installation space, the engineers have developed a system that can operate without the pressure accumulator, while still maintaining fast actuation times and precision while shifting.
The hydraulic power pack is composed by a brushless DC motor driving a gear pump, which feeds the working pressure oil line via control valves. The nature of brushless DC motors allows the use of the pump in a (constant) 100 % duty cycle, which can supply the working pistons with the proper choice of pressure oil on demand, when an actuation is needed, without any delay. There is no need to wait for the acceleration of the electric motor that is already spinning. Thanks to this arrangement, the dynamic response of the hydraulics is excellent. Related to the differential functionality and the requested locking torque, a pressure control valve is then able to regulate the pressure in the eLSD.
The hybrid module can facilitate a parking brake mechanism if required, developed, tested and patented by Oerlikon Graziano. This mechanism is integral and can be linked to the hydraulic control unit for actuation.
In addition and depending on the vehicle specifications and application strategy, a thermal management for transmission cooling was also developed and implemented, when needed by the vehicle application. For that, an oil-to-water heat exchanger would be fitted to the hybrid module and connected downstream to the coolant circuit of the electric motor, without the need for a separate connection to the vehicle coolant circuit.
Oerlikon Graziano with its new rear axle hybrid module offers a solution for P3 and P4 hybrid architectures on the path to electrification, with high power density, a compact package and full hybrid function. This efficient drivetrain enables P3 hybrid functionality and is equipped with an active differential. It makes the system suitable for OEMs, who want to use powerful drives without compromising on performance.
 Cavallino, C.: OGeco concept: Hybrid AMT with torque infill for sports car application. CTI Transmission Symposium, Berlin, 2012
 Vacca, F.; Capilli, G.; Sorniotti, A.; Fracchia, M.; Remondin, T.; Cavallino, C.: A Novel Hybridized Automated Manual Transmission for High Performance Cars. IEEE Conference Advanced Motion Control (AMC) 2018, Tokyo, March 9-11, 2018
DIPL.-ING. CARLO CAVALLINO is Head of System Development at Oerlikon Graziano S.p.A. in Turin (Italy).
DIPL. SERGIO DE SANTIS is Project Engineer at Oerlikon Graziano S.p.A. in Turin (Italy).
DIPL.-ING. (FH) PETER RIEMER is Senior Project Engineer at Oerlikon Graziano S.p.A. in Schorndorf (Germany).