The Citaro hybrid will celebrate its world debut at Busworld Europe. With it, Mercedes-Benz is breaking new ground: the hybrid powertrain is optionally available for a large number of model variants of the best-selling urban bus. Together with the new electrohydraulic steering, the Citaro hybrid further reduces the fuel consumption of the Citaro, already famed for its efficiency, by up to 8.5 percent. Benefit of the additional hybrid drive: it takes up little space; the outer contours, like the passenger compartment, remain unchanged; the extra weight is low. Transport operators profit from rapid amortisation, while society and the environment benefit from even lower fuel consumption and emissions.
Straightforward drive technology to assist the combustion engine
The basic way in which the Citaro hybrid works is quiet simple: when the bus is decelerating, the electric motor acts as a generator and transforms braking energy into electric power. This power is stored so that it can be used by the electric motor to assist the combustion engine. The drive technology behind this has a straightforward design. One of the hybrid drive's core components is the electric motor. The disc-shaped unit with integral power electronics is a permanently excited synchronous motor, meaning it is mechanically very robust. The electric motor is fitted between the combustion engine and the automatic transmission.
The electric motor on the Citaro hybrid assists the combustion engine first and foremost when high levels of power are required, particularly when moving off. It does not serve to boost maximum power, however, which is why the output and torque figures for the bus remain unchanged. Instead, the electric motor reduces the burden on the engine while also improving the vehicle's start-off characteristics. This does not involve lowering the engine's speed – the peak output is simply throttled imperceptibly with the electric motor making up the difference.
Supercaps serve as temporary power storage units
The power for driving the electric motor is produced for free by means of recuperation, meaning that the electric motor acts as a generator during braking and overrun phases. The electric power generated in this way is stored in doublelayer capacitors, also known as supercaps. These electricity storage units are characterised by a high power density. They are resistant to high power peaks and have a long life. Unlike batteries, supercaps are ideally suited to the continuous quick changeover between charging and discharging that occurs when stopping and pulling away again in typical city bus operation. Braking to a stop from a speed of 50 km/h just once is enough to recharge the power storage units in the Citaro hybrid.
The power storage unit assembly in the Citaro hybrid is made up of two modules, with each module containing 16 doublelayer capacitors. A perforated aluminium cover protects the modules from external influences and direct sunlight. At the same time, this also makes use of the airflow to provide the necessary cooling. An inverter converts the stored direct current into alternating current in order to drive the electric motor. The city bus is the first commercial vehicle to come with a separate 48 volt network, similar to that found on hybrid passenger cars from Mercedes-Benz.
Low weight and taking up little space
A key benefit of the compact and technically straightforward design of the supplementary hybrid drive is that it takes up little space, so the vehicle's outer contours remain unchanged, as does the interior – there is no loss of passenger seating. The only change is a modified maintenance hatch in the floor area.
The hybrid drive adds just 156 kg to the vehicle's weight, which is a fraction of the weight of other electric drive systems. As a result, there is just a marginal reduction in total passenger capacity that is insignificant in practice: the standard-specification Citaro hybrid with diesel engine offers space for up to 105 passengers as a rigid bus with a permissible GVW of 18 t and 159 passengers as an articulated bus.
Electrohydraulic Intelligent Eco Steering System
The Citaro hybrid is equipped with a further brand new component as standard in the form of Intelligent Eco Steering, an electrohydraulic steering system. Whereas the power assistance mechanism is constantly at work on a conventional hydraulic steering system, with an electrohydraulic system it only operates on demand, in other words when the driver turns the steering wheel.