Interview with Yuta Susuki, project leader development series hybrid drives for buses, Mitsubishi Fuso Truck & Bus Corporation
 

There are three main reasons. First of all, they eliminate the need for a large diesel engine to handle the frequent acceleration required in stop-and-go traffic. Instead, series hybrids allow the use of much smaller diesels operated at a constant and optimal speed, which reduces fuel consumption. Secondly, the smooth drive such hybrids offer without the jolts caused by gear-shifting makes things more comfortable for passengers, especially those who are standing. Finally, series hybrid buses can be retrofitted with fuel cells at any time in the future.

What has been the most important technological development for hybrid drives over the last ten years?

Battery technology: Today's lithium-ion batteries are much more powerful than before, and that's why hybrid drives have been so successful of late.

Unlike its predecessor model, the Aero Star Eco Hybrid is equipped with fully electrical ancillary devices. Was the main goal here to achieve the resulting capability of driving short distances in the pure electric mode?

No, the most important aspect was to reduce the fuel consumption of the diesel engine that powers the components. Take the air conditioning system, which is a primary consumer of electricity in the summer. In the Mitsubishi Fuso Aero Nonstep HEV predecessor model, the air conditioning system was powered mechanically by the diesel engine, which had more than eight liters of displacement. By contrast, the displacement of the engine in the new model is only 4.9 liters. The huge diesel engine in the predecessor model had to keep running when the vehicle was stopped in order to keep the interior cool. This resulted in excessively high fuel consumption on hot days. The air conditioning system in the new bus operates much more efficiently.

What are your developers focusing on at the moment?

One area of focus is the human-machine interface. To give you an example, there was once a very experienced bus driver who tested the Aero Star Eco Hybrid and complained that the bus wouldn't start. As it turned out, the driver was used to depressing the gas pedal completely when he turned the key to start the system, as in old diesel buses. If you did that with our new electric bus at that time, the system did not start up, since this kind of operation was not expected. Now we have taken into consideration every possible action on the part of the driver.

What will be the biggest challenge in the future?

Reducing costs. The electronic systems for controlling the hybrid drive are still very expensive, for example. However, if we cooperate closely within the Daimler Trucks organization and share components, we can reduce procurement expenditure by purchasing higher unit volumes. We're now working very closely on this with the developers of the Mercedes-Benz Citaro G BlueTec hybrid bus.