PowerHybrid for Future Plug-In Drives

In European hybrid vehicles, the dominant trend still is to integrate transmissions with the full scope of functions for the combustion engine power path. They keep the same number of speeds and mechanical structures and simply have an electric motor added to the transmission input shaft. Elsewhere, manufacturers are taking a different approach. Asian OEMs, for example, are increasingly using dedicated hybrid transmissions (DHT) in their vehicles. This trend could soon assert itself in Europe: the dedicated hybrid transmission offers numerous advantages and can be designed for compact space and high performance, as demonstrated by a proprietary development at IAV.

Efficient, powerful and yet compact, inexpensive and versatile for flexible use in different vehicles – those are the main challenges to be met by future DHTs. “Two approaches currently dominate this area: electric power-split transmissions such as the Hybrid Synergy Drive in the Toyota Prius, and combined hybrid transmissions as in the Mitsubishi Outlander”, reports Dr. Jörg Müller, Head of the Hardware Development for Transmission and Hybrid Systems department at IAV. “They generally stand out with a relatively simple mechanical structure. Even so, these DHTs need two electric motors plus power electronics, which has implications in terms of package and costs.”

The aim at IAV was to develop a new DHT for a hybrid powertrain with torque addition. This system combines low fuel consumption with high performance as well as compact package requirements; it is also relatively cheap to produce. Furthermore, it is suitable for a wide range of vehicle categories right through to high-torque, high-power combustion engines. This makes it particularly interesting for the European and American markets.

The main difference to the established solutions is that the IAV DHT manages with just one e-motor and one power electronics unit, without making any compromises in terms of power or efficiency. On the contrary: the entire electric power is available for effective hybrid functions such as boosting, all-electric driving and recuperation at nearly all operating points, regardless of the coupled combustion engine. In power-split or combined DHTs, this is restricted because of the operating principle.

The mechanical advantages are still retained: the electrified powertrain in the IAV Power- Hybrid needs far fewer speeds than a conventional transmission. While today’s torque converter transmissions have up to ten speeds and double-clutch transmissions up to eight, a DHT normally manages with four to six speeds. “We arrived at this number of speeds using our IAV Powertrain Synthesis tool, under the condition that the electric motor has an output of about 100 kilowatts”, says Müller. “The necessary spread is thus reduced from about eight to just four to five.”

Using IAV Powertrain Synthesis, the developers compared around 5.1 million powertrain configurations and opted for four speeds and an e-motor with 90 kilowatts output for the IAV PowerHybrid. “Greater output would have offered almost no further consumption benefits in globally typical consumption cycles or improvements in acceleration, at least up to 100 km/h”, says Müller. The DHT has a maximum input torque of 750 Newton meters and an output torque of up to 6,500 Newton meters. Measuring just 380 millimeters in length, it fits in the space typically available in front transversal applications, despite the high output. The IAV PowerHybrid is thus suitable for a wide range of vehicle categories, from the A-segment through to the SUV.

A simulation-based comparison with a typical electric power-split system shows that the IAV solution offers clear advantages for a Csegment vehicle with comparable electric output. For example, the IAV PowerHybrid in combination with a 1.8-liter combustion engine in hybrid mode takes just six seconds to accelerate from 0 to 100 km/h, instead of more than ten seconds. About three seconds can be saved in all-electric mode. Despite the improved performance, WLTC consumption is reduced by a further 0.2 liters.

“We have shown that current hybrid systems can be further developed for the pending requirements of entire vehicle fleets”, says Müller. “We are currently discussing our solution with several customers and have already received enquiries about possible development projects for volume production.”

Number of speeds: 4 Electric motor output: 90 kW Electric motor torque: 300 Nm Length including e-motor: 380 mm DHT input torque: max. 750 Nm DHT output torque: max. 6,500 Nm