The hybrid bus developed by Kabus Oy and VTT ready for testing

Exhaust gas volumes and fuel consumption reduced by up to 45%

VTT and the bus manufacturer Kabus Oy have together investigated the opportunities for reducing the fuel consumption and exhaust volumes of buses. The joint research project in hybrid technology has now reached a stage where the hybrid bus – the only one of its kind in Finland – has been given its first test drives. The goal is to reduce exhaust volumes and fuel consumption by some 30%. Thanks to the lightweight structure of the hybrid bus, the attainable reduction in exhaust volumes and fuel consumption is up to 40–45% compared with traditional buses with the same passenger volumes.

The power source of a hybrid bus combines an electric motor with a combustion engine. The joint project offers VTT an opportunity to investigate the potential and advantages of the new technology. The bus has already been given its first test drives on the road as well as on VTT’s chassis dynamometer. Initial results from the chassis dynamometer tests showed a reduction of some 20% in fuel consumption. The team is still fine-tuning the system, and the parties are confident that the targeted reduction of some 30% is realistic. More detailed comparisons with traditional buses are scheduled for the near future. Should the results be satisfactory, Kabus is considering plans to commence serial production.

Parallel hybrids have good fuel efficiency

The development of hybrid buses has primarily focused on recovering the braking energy and reusing it as kinetic energy. While this reduces carbon dioxide emissions in proportion to reductions in fuel consumption, hybrid solutions have the potential for even greater reductions in particle emissions since the use of the combustion engine in disadvantageous situations can be reduced. Structurally, the hybrid system is a parallel hybrid, which retains the traditional powertrain and integrates an electricity system parallel thereto. The advantages associated with this solution are high efficiency and a relatively simple structure. Thanks to the traditional powertrain, parallel hybrids remain functional even if there is a malfunction in the electricity system. In a serial hybrid system energy transmission is solely based on electricity, which reduces the efficiency on the road. The parties considered parallel hybrid to be more viable than a serial hybrid, also with a view to potential serial production.

No need for batteries

In the hybrid bus the recovered energy is stored in supercondensators. This version omits the need for battery units associated with electrical drives, except for the battery required by the vehicle’s traditional 24V electricity system. The electric motor and gearbox were designed to order, and electric power is controlled by means of a state-of-the-art frequency converter. The vehicle logic unit was also specifically designed for the bus. The unit determines when each motor will be used and in which proportion.

The hybrid bus was built at Kabus’ plant in Lahti, Finland. A new Kabus city bus was taken direct from the production line and equipped with all the elements of the hybrid bus.

The project steering committee included representatives of Kabus, VTT and TEC TransEnergy Consulting. The project also involved the cooperation of Tekes, Vacon Traction Oy, Epec Oy, Katsa Oy, Dynawatt Powersystems, the Helsinki University of Technology and OEM Finland.


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