AHEAD - Advanced Hybrid Electric Autobus Design

This project aims at optimizing the powertrain of a serial hybrid electric bus for public transportation. Mathematical models and advanced optimization techniques are used to find the optimal component sizes of the powertrain. Further, a predictive energy management strategy that takes into account the altitude profile of the line is implemented. This cuts fuel consumption and CO2 emissions by 25% compared to a conventional, combustion-engine based vehicle.

This project is a cooperation of HESS AG (Bellach, Solothurn) and ETH Zürich. It is sponsored by KTI. This project was awarded the Hans Eggenberger Preis 2010, and won the second prize in the European Satellite Navigation Competition.

Hardware of a serial hybrid powertrain: Two electric traction motors propel the bus (see upper part of figure). Electric energy is delivered by an engine-generator set in the back of the vehicle. During downhill driving and braking maneuvers, the traction motors regenerate kinetic and potential energy, which is then stored in the electric double-layer capacitors (EDLCs) on the roof of the bus. This "recuperated" energy can be used for propulsion later on.

Information about future elevation profile allows saving energy: If the route is unknown (consider the scenario "without preview" in lower part of figure), the system is not aware of the upcoming descent and the energy content of the EDLCs has to be maintained on a medium level to provide enough power. Later on, during the long descent, the EDLCs cannot absorb all recuperation energy, since they are already half full, and the excess energy has to be wasted. With a preview of the altitude profile (scenario "with preview"), the control system is able to react prior to the long descent: during uphill driving, the EDLCs are discharged almost completely, which provides enough free capacity to accept recuperation energy. Overall, the scenarios starts and ends with an equal EDLC energy level, but in the case "with preview" less fuel was used.

Components have to fit vehicle and mission: The components of the powertrain have to be adjusted to the primary use of the bus. If for example the route is a flat and straight road without many stops the components will be very different from the case where the bus is driven in a hilly city. This project develops methods and tools to find the optimal design for a vehicle's primary use.

Project goals: In public transportation, where buses run only on very few routes, the design of the vehicle can be specialized and optimized. This project seeks
i) to find the optimal component sizes for a given vehicle on a given route;
ii) to implement a predictive energy management strategy that takes into account the future elevation profile.

Practical outcomes: HESS benefits from
i) reduced time and cost for development and design;
ii) the possibility to demonstrate and compare the performance of powertrain designs.
The costumer of a HESS bus benefits from
iii) reduced CO2 emissions;
iv) reduced fuel cost.

Scientific research topics: Hybrid electric vehicle design and optimization and predictive control of hybrid electric vehicles.

Project Impressions:

First tests and measurements with HESS's lighTram Hybrid on Buochs Airfield.