The Citaro hybrid celebrated 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, even for the gas-engined Citaro NGT. Together with the new electrohydraulic steering, the Citaro hybrid further reduces the fuel consumption of the Citaro, by up to 8.5 percent. Benefit of the additional hybrid drive is, it takes up little space; the outer contours, like the passenger compartment, remain unchanged; the extra weight is low.
The basic way in which the Citaro hybrid works could not be simpler: 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, especially when moving off. The drive technology behind this has a remarkably 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. It generates a maximum output of 14 kW and 220 Nm of peak torque.
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 first commercial vehicle with 48 volt technology
Another big plus point of the Citaro hybrid is that it does without an elaborate high-voltage network with all the accompanying restrictions and safety requirements. 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, takes 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.
Citaro enthusiasts will already be familiar with the energy-storing supercaps as a recuperation module. These supplement the vehicle batteries by storing the electric power generated by recuperation and then making it available when moving off or on hilly sections. This relieves the load on the alternator, and therefore the engine, which brings about a reduction in fuel consumption. This same technology is also employed in the Citaro hybrid.
Wide range of Citaro hybrid models available
The hybrid drive is available as optional equipment for all Citaro models in conjunction with the Mercedes-Benz OM 936 G in-line six-cylinder diesel engine in both vertical and horizontal versions. The remarkably wide Citaro hybrid model line-up therefore comprises the compact Citaro K, the Citaro rigid bus, the Citaro G articulated bus, as well as the low-entry Citaro LE in all variants.