Optimization of Hybrids from the Powertrain Level

Optimization of Hybrids from the Powertrain Level

In view of today’s primary energy mix along with infrastructure and storage limitations, it is important to develop technically and economically feasible short-term solutions based on real driving conditions and current powertrains, in addition to electrified solutions.  A vehicle with an electrified powertrain introduces many additional energy flows to consider in comparison with a conventional ICE vehicle. A high  level  of  vehicle  and powertrain expertise is now required to develop the  technically  and  economically  best  solutions for  incorporating  the  variety  of  energy  storage solutions available (fossil fuels, e-fuels, batteries  and  fuel  cell). To play an active role in this transformation of the automotive industry, Schaeffler has moved away from the classic approach separating the “engine” and “transmission” units and is now focusing on the development   of   the   overall   system. This allows consideration of the not only engine and transmission energy flows but also electrical, thermal, and chemical.  Understanding these flows, and especially where energy is lost,  supports innovation at the level of the powertrain as well as the entire vehicle while also enabling more sustainable analysis.

Dedicated Hybrid Powertrain as one system

Based on the degree of electrification – “micro-hybrid”, “mild/full hybrid”, “plug-in hybrid” or “xEV” – a powertrain matrix will be used to continue to develop tomorrow’s engine, transmission and electric drive subsystems.

What all approaches have in common is that the optimum solution can only be achieved if the entire powertrain is analysed, including all physical interactions  between  the  internal  combustion engine,  the  transmission,  the  electric  motor and the interaction with the vehicle; thus, not only the flow of forces but also acoustic and  thermal  phenomena  are  taken  into  account. (Fig. 1)

In this study, we considered a C segment plug-in hybrid vehicle. The consumption simulations were carried out based on the WLTC.  The battery for determining the electric range is 9.0 kWh (Fig. 2). As a result of this optimisation the WLTC rated fuel consumption is 3.5 % lower compared to a powersplit and 2.0% less than P2. The efficiency maps used in the simulation for DHE and MultiModeHybrid are basis for the further development on component level. The optimised energy management considers a good driveability.

Development of the MultiModeHybrid

The expectation of improving fuel economy means that optimization of the entire transmission is becoming a priority. This also includes the option of simplifying the mechanical part, possibly by removing the reverse gear and integrating the electric motor(s) into the transmission to take over this function completely.

In order to fulfil all future powertrain demands on a competitive cost level, Schaeffler developed the MultiModeHybrid. The transmission bases on a serial hybrid with the ability to lock the driveline to a parallel drive by closing a K0. The input from the engine has a first ratio of 3.5. The inner transmission input shaft is carrying the rotor of E-motor 1. E-motor 2 is connected to the hollow shaft and via an overall ratio of 8.3 in two steps directly coupled to the differential. Between the two motors is a disconnect clutch in a normally open design and actuated from a CSC. With this layout it will be identical for the FHEV and the PHEV version. The only remaining difference is a HV DC/DC converter for the FHEV version integrated to the double inverter.

With this layout the transmission has three operation modes. The first mode is E-drive. E-motor 2 is driving the vehicle via one fixed ratio of 8.4. There is no shift between any gears. The one speed transmission fulfils the required acceleration over the whole E-drive speed to 135 km/h with a maximum output power of 125 kW. The second mode is the serial drive. In this mode the vehicle is driven in the same manner as in mode 1. Additionally, the engine is running and producing the needed electrical power with the directly connected E-motor 1 which is able to run at 110 kW mechanical input power from the engine. The third mode is parallel drive. At a vehicle speed > 50 km/ h and low to mid acceleration request, the clutch K0 can be closed and with this, there will be a direct connection from the engine to the wheels. The overall ratio of this gear is 2.4, so it is a highly efficient overdrive for urban and motorway driving. (Fig. 3)

Future Dedicated Hybrid Engine

On the one hand, the engine sweet spot is optimized to a point of lower speed and torque. The low-end max torque can be reduced and will be compensated by the E-motor. As a further step, the max speed can be limited below 5.000 rpm as a result of the CVT character and high ratio of the MultiModeHybrid. Instead of using rigid connecting elements between the camshaft and engine valve, a defined oil volume encapsulated in the high-pressure chamber   transfers the cam (lobe) contour to the engine valve. A pump driven by the camshaft via a finger follower builds pressure in the high-pressure chamber. The control system, in addition to the electrohydraulic actuator technology, represents a key module of the UniAir system. Schaeffler has devised a development method that also makes it possible to quickly and easily integrate   the   UniAir   system   in   existing ICE designs. (Fig. 4)

The control algorithms developed by Schaeffler are provided to automotive manufacturers as a software module, which is then integrated as a control module in the engine control unit. With the cycle-specific control logic, the UniAir system opens up new possibilities for altering torque output via the air path as the air mass can be adapted almost as quickly as the ignition point. The next logical step will be to support the trend to further charge dilution (gasoline compression ignition, EGR) with Schaeffler technologies.

Especially in the hybrid driveline, the flexible valve timing can also be used to reduce the final compression pressure of the first compression. With this, the required torque for the ICE restart from E-drive can be reduced and the NVH behaviour of the restart will be significantly improved.

As a next step, the high level powertrain control will be integrated to the PEU of the MultiModeHybrid. Vehicle energy management, emissioning and the driver interface are part of this control. Demonstrator vehicles are in build up for verification of the new powertrain software.


Electrified powertrains offer more energy paths and therefore a more complex optimization problem.  By studying each path including losses, a particular powertrain can be optimised.  One can see that the architecture of transmission and engine are both important and must harmonize with each other as well as the other sub-systems in the vehicle.  This study concerns a FWD C-segment car, but the principles can be applied to mild hybrids, P2 hybrids, battery electric vehicles, and fuel cell vehicles.


Schaeffler: We Pioneer Motion

For more than 70 years, the Schaeffler Group has pioneered motion to advance how the world moves. As a leading automotive and industrial supplier, Schaeffler is dedicated to making motion and mobility more efficient, intelligent, and sustainable. Specializing in the development and production of solutions for the challenges facing the evolving mobility industry, Schaeffler manufactures high-precision components and systems for drive train and chassis applications, as well as products across the full bandwidth of electrification options.