It was therefore absolutely vital that the entire electric powertrain should also be integrated in the underfloor unit. In other words, the customer was looking for an electric motor and power electronics featuring a compact design and a high power density. At the same time, high efficiency was stipulated, to allow the train to enter and leave a station in pure electric mode. The new technology moreover had to be implemented in such a way that rail vehicle operators of could improve their profitability.

It consists of two permanent magnet electric motors, each with embedded magnets and two traction converters with a CAN bus interface to MTU’s own powertrain control system. The electric motors have a rated mechanical torque of 150 Nm, a short-time maximum torque of more than 300 Nm and a specified maximum operating speed of 10,000 rpm. In this case the rated mechanical power is 200 kW – whether as a motor or as a generator.

During a journey the electric motor acts as an energy converter. When functioning as a motor, to accelerate the vehicle for example, it converts electrical energy from the energy storage system into mechanical energy. As a generator, when the vehicle is braking, it converts mechanical energy into electrical energy and charges the energy storage system. The traction converter, which supplies the necessary three-phase current for the electric motor, has a rated current of 240 A; it provides the full continuous electric power output of 185 kVA in an overload protected voltage range from 610 to 730 VDC. It is equipped with a CAN bus interface for communication purposes in accordance with the J1939 standard.

The combustion engine produces the mechanical energy which is needed to drive the vehicle. An electrically switched separating clutch decouples the two drive systems and enables locally emission-free driving in pure electric mode.

The drive control system manages power and energy within the drive system and is designed to implement traction and braking commands selected with the control lever with maximum energy efficiency. It decides when the diesel engine should cut in or out, when the electric motor releases power and, for example, how braking torque is distributed between the retarder and the electric motor throughout the braking process. The power electronics components additionally supply auxiliary electrical equipment. The energy storage system in the Hybrid PowerPack – several battery modules mounted on the roof, in the interior or underfloor – stores the energy recuperated during braking and makes it available to the electric powertrain.

Following extensive practical trials, MTU has confirmed that the hybrid traction drive holds considerable future potential for railway operators. Several significant benefits have been determined compared to the diesel reference model: fuel savings of up to 25 percent as well as correspondingly fewer CO₂ emissions, NOx emissions down 20 percent plus exhaust and – to a large extent – noise-free driving in railway stations speak for themselves. Taken over the lifecycle as a whole, the capital costs for a Hybrid PowerPack are first of all recovered in the form of fuel savings. The hybrid technology also has a clear lead over purely diesel powered rail vehicles when it comes to maintenance costs. The service intervals are longer because the vehicle runs partly in electric mode and so the diesel genset has fewer operating hours. Furthermore, the generator function of the electric motor prevents brake wear during the recuperation phases.

Constructive dialog sets the tone for success
Several different departments at MTU and WITTENSTEIN cooperated closely on this project and engaged in a regular exchange of ideas and experiences. All in all, nearly 40 people from development and shop-floor departments were involved. A core team of about eight, interacting very strongly, was formed. Apart from WITTENSTEIN’s expertise with regard to drives and controls, it was our willingness to assume responsibility for the project’s implementation that especially impressed MTU. From the outset, the entire electric drive system was designed in line with production requirements.