The 48 V P2 Hybrid – a useful bridging technology

In addition to the current forecasts, which predict a more rapid growth in purely electric mobility, there is also an increasing trend towards efficient entry-level hybridization in order to be able to meet the 2020 CO2 targets. Nearly all these drives currently use a 48 V system. This means that gasoline engines can now achieve a comparable performance in terms of CO2 emissions to that of diesel engines. Various technical concepts have arisen from the current P0 to P4 topologies for a 48 V hybridization of the powertrain in vehicles driven by an internal combustion engine.

The possible and useful performance range of 48 V systems and the achievable functions vary depending on the relevant topology. It has been shown that, by using both high-performance P2 and P3/P4 systems, there is considerably greater potential for reducing CO₂ compared to the current, established P0 systems with 8 to 10 kW output. Depending on the driving cycle used, the potential reductions range from up to 20% (NEDC) or up to 16% (WLTC) compared to vehicles with a conventional powertrain. At these consumption values, Schaeffler estimates that, gram for gram, the complete P2 system is more economical in terms of CO2 than a P0.

Based on these consumption figures, Schaeffler estimates that the complete P2 system is more economical per gram C0₂ saved than a P0.

Additional reductions in CO₂ can be achieved in P2 systems, particularly when compared to P0 systems, due to the option of decoupling the engine using the additional disconnecting element (K0) between the electric machine and the combustion engine.

The option of using the gear ratio spread of the transmission enables the electric motor of the P2 system to be operated in its most efficient range and therefore offers an advantage compared to a P3/P4. Furthermore, as far as their installation position and function are concerned, P2 systems are also not limited to an output of approximately 10 kW. This raises a significant question in terms of system design: What electrical output should be installed in a 48 V system in order to ensure that the maximum potential reduction in both consumption and CO₂ emissions is achieved within reasonable costs? These considerations should obviously also take into account the demands on the electrical output required to generate perceptible drive functions from the customer's perspective.

In order to generate these perceptible drive functions, Schaeffler believes that it is advisable to increase the electric machine output to approximately 20 kW, depending on the vehicle class. Simulations show that, at this output level, it is possible to achieve comfortable, purely electric driving in typical inner-city traffic at up to 30 km/h in a form that customers will embrace - and this is in addition to the existing functions of creeping, electrically driven parking, crawling and boost. The P2 architecture allows the combustion engine to be restarted after this purely electric driving as well as after a sailing phase by using the K0 with the existing electric motor which means that an additional starting system is not necessary in the P2 configuration.

Both the coaxial and the parallel axis designs have a dry, multi-disk clutch. This is fully integrated into the rotor or the belt pulley together with the actuating slave cylinder. The current design can transmit torques up to 300 Nm with very low drag torques when the clutch is open during electric driving, recuperation and sailing phases. The increase in length of the powertrain in current modules is 80 mm for the coaxial variant and 30 mm for the parallel axis variant.

An automated drive clutch, i.e. one which can accommodate different driving strategies, is required for functional reasons when combining the P2 hybrid module with a manual transmission. This allows the driver interface to be either a clutch-by-wire system, which includes a clutch pedal, or an electronic clutch management system (ECM) without a clutch pedal, depending on the degree of customer acceptance. The ECM provides the driver with other benefits, such as a creep function while driving in traffic jams.

Furthermore, the 48 V P2 module can be combined with all automatic transmissions. This means that the housing, damper, disconnect clutch, EE system and starter element subsystems can all be optimized in terms of their design. The starter element can therefore have a double clutch, a converter (CVT, automatic transmission) or a wet clutch as a replacement for the existing converter. New solutions are available for applications with a wet double clutch in which K0/K1/K2 clutches can be fully integrated into the rotor of an electric motor requiring only 45 mm active length. This technology can therefore be combined with the majority of four-cylinder engines in small vehicles.

As mentioned in the introduction, the design of an electrical system with a motor output of 15 to 20 kW is initially based on the possible CO₂ savings. There are some further basic requirements for the subsystem, including a very high efficiency as well as a high torque for quick warm starts and reliable cold starts. Designs with a maximum torque of 180 Nm allow modern four-cylinder engines (both gasoline and diesel) to be started at temperatures as low as -30 °C. Due to the extremely high demands on the available space between the engine and the transmission, a PMSM has to be used in the coaxial design. More particularly, a concentrated coil is used with the extremely compact electric motor with an active length of approx. 45 mm. This results in not only the reduced overall length of the electric motor which the short end windings allow, but also the high optimum peak efficiency of more than 96% and the relevant consumption map ranges of more than 95%. The resulting efficiency at system level (including integrated power electronics) is up to 93% in the relevant driving range.

As with all hybrid vehicles, starting the combustion engine and the subsequent torque build-up is one of the core acceptance criteria. The systems shown here start comfortably using the 48 V system via the disconnect clutch so the conventional starter motor is no longer required. Starting from a coast phase, the driver receives the initial reaction from the reduced recuperation torque. The K0 closes in less than 100 ms and the crankshaft is accelerated and coupled to the powertrain (time to acceleration typically 600 ms) using the rotational energy in the electric motor and clutches. An additional comfort start system connected directly to the combustion engine, as is required in the P3 and P4 concepts, is not included in this configuration. Further proof of this acceptance when combined with a manual transmission is part of the focus of the current optimizations on the GTC2 concept vehicle which was developed with Continental and Ford. The results from this will be presented as part of the VDI Transmissions Symposium 2017.

Authors of the article

Dierk Reitz, Andreas Englisch, Thomas Eckenfels

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