Dedicated CVT-Hybrid – Low Emission, High Performance
Maximilian Podschwadt, Program manager for CVT, Schaeffler
- Dedicated plug-in hybrid based on a CVT
- Transmission concept for front-transverse applications
- Detailed overall powertrain modeling
- Evaluation of fuel efficiency and vehicle performance
The core objectives in the future development programs of vehicle manufacturers and suppliers are the reduction of fuel consumption and therefore reducing CO2 emissions. On the one hand, this will be achieved through continuous optimization of conventional powertrains. On the other, most recent developments indicate a clear trend towards plug-in hybrids (PHEV) as a bridging technology to electric vehicles. PHEV vehicles provide not only greater electric range but also a more enjoyable driving experience due to the dynamic of the electric motor.
This paper presents a PHEV concept study based on a CVT. The transmission is designed for the vehicle volume market i.e. the B and C segments. The electric motor with a peak power of 80 kW has been highly integrated into the transmission due to demands on space in front transverse applications. The 4-cylinder gasoline engine under consideration has a displacement of 1.4 L with a maximum power of 110 kW. A maximum vehicle speed of 130 km/h is therefore possible in purely electric mode. The center distance of the chain variator is 169 mm with a ratio spread of 7.0.
A particular feature of the transmission is the parallel arrangement of the variator and a direct gear with fixed ratio. By this, two power flows to the wheels are possible. In electric mode, the direct gear transmits the power of the electric motor up to mid-range speeds and power. As a result high overall efficiency is achieved.
The efficiency is further enhanced in the CVT stage by using a LuK chain with low friction losses, optimum design of the hydraulic surfaces of the pistons and an intelligent pump concept. This concept makes use of both a mechanical and an electrically driven pump.
The transmission has a compact axial length of 390 mm due to the use of the CVT resulting in a low transmission weight. It belongs to the new category of “Dedicated Hybrid Transmissions” as driving in reverse is only possible via the electric motor. The energy consumption in EV mode is simulated and the value is 10.8 kWh/100 km in the New European Driving Cycle due to the high level of efficiency. In addition, the fuel consumption in other driving cycles as the new WLTP has also been investigated. Based on this cycle the component losses will be discussed.
The overall powertrain model as well as the sub-models for the components as e.g. chain variator and electronic components will be described in detail. Furthermore, the operation strategy which is of cruical importance for plug-in hybrids is presented.
The vehicle performance has also been evaluated. This yields that the acceleration time from 0 to 100 km/h is less than 8 s for a typical vehicle in the C segment.
Program manager for CVT