Recuperation

The hybrid Citaro’s developers believe they can achieve fuel savings of between 20 and 30 percent compared to a conventional diesel drive system, depending on how the vehicle is used. Moreover, because the new bus is equipped with a BlueTec exhaust aftertreatment system that also effectively reduces nitrogen oxide emissions, it will be one of the cleanest regular-service buses. That’s one reason why the bus received the DEKRA Environmental Award during the World Mobility Forum held in Stuttgart at the end of January 2008.

The electricity produced by the generator in the Citaro G BlueTec Hybrid is channeled either directly into the electric motor and the ancillary units, or stored in maintenance-free lithium-ion batteries on the vehicle’s roof. The actual propulsion for driving the bus is provided by four electric hub motors installed at the center and rear axles. The quartet delivers a combined output of 320 kilowatts (426 hp). That’s more than enough for even an articulated bus in uphill driving. Simulations of a hilly test route show that the diesel engine will only be engaged for roughly half the total time the bus is in operation.

In addition to receiving charge from the generator, the bus’s lithium-ion batteries also store energy recovered from braking (recuperation). Here, the four hub motors act as generators during braking, producing electricity that flows into a so-called intermediate circuit. Whether or not this electricity is then stored in the battery or tapped by a component or the electric motor ultimately depends on the situation at hand, whereby an intelligent operating strategy ensures that energy is optimally distributed among the various devices.

Broad acceptance

Instead of being driven by the diesel engine via belts, devices such as the air conditioning compressor, power steering pump, air compressor etc. are powered by the electric motors. This offers two advantages, according to Störmer: “The devices only receive power when they’re needed, which saves energy and improves efficiency. In addition, they don’t need to be placed in the engine compartment, but can instead be positioned in places like the roof.”

Because the hybrid Citaro is equipped with electric hub motors, it also doesn’t need a mechanical automatic transmission. This not only reduces the vehicle’s weight; it also guarantees that acceleration and braking are very smooth. “The prototype we just finished will now undergo test operations; we’re already planning production of a small series in 2009 for use by customers,” says EvoBus Development Managing Director Richard Averbeck, who also reports that the Citaro G BlueTec Hybrid has already met with broad acceptance: “Public transport companies are impressed by both the significant fuel savings and the extremely low noise levels.”

Frequent stopping

The EvoBus engineers decided to use a series hybrid for the new Citaro because this drive system is more suitable for vehicles that have to constantly stop after short intervals. “This technology is especially effective in supporting the so-called bus stop function, which enables zero-emission and low-noise stopping and starting up again at bus stops,” says Störmer.

Enlarge   The display screen shows various driving situations and the respective energy flows.

Enlarge   Lithium-ion batteries and ancillary parts.

Enlarge   Diesel engine in the rear area.

Enlarge   Testing the intermediate electrical circuit.