Clear benefits: Series hybrid drives for urban buses

“Lithium-ion batteries are very expensive, and our customers therefore don’t want to replace them at any time during the service life of the bus,” says Susuki. That’s why the batteries have to withstand ten years of daily operation and more than 500,000 km (300,000 miles) of driving. The engineers had to come up with a clever strategy to ensure that charging and discharging processes put as little strain on the battery as possible.

That wasn’t easy, given that the battery is involved in every acceleration and braking maneuver. The brakes are the key here, as the bus recovers some of the energy invested in accelerating every time it brakes. By comparison, the braking process in conventional vehicles produces only useless heat.

Since city buses must make many stops, the recovery of kinetic energy can considerably reduce the amount of diesel fuel consumed. What’s more, the electric motors act as generators (large dynamos) as well. When the bus brakes, they convert some of the kinetic energy back into electricity, which is used to charge the battery. This energy is then available for the next time the bus begins to move. “In the Aero Star, the electric motors basically manage the entire braking process under normal conditions,” says Susuki.

The test driver now demonstrates how the commercial vehicle driving cycle for fuel consumption is measured in Japan. A curve appears on a screen in front of the steering wheel, and this curve tells the driver when he should accelerate, maintain speed, or brake. Series hybrid drives actually become less effective at both higher and more constant speeds, in which case the additional losses resulting from electrical energy generation by the diesel engine become noticeable. Parallel hybrid drives, on the other hand, are more suitable for vehicles that make fewer stops, which is why they’re generally used in hybrid passenger cars. In a parallel hybrid, power is supplied by both a combustion engine and an electric motor. Daimler Trucks uses such a system for most of its full-hybrid commercial vehicles, including in the Mitsubishi Fuso Canter Eco Hybrid, while series hybrids are reserved for urban buses, where they work most efficiently.