German version

The arrival of the Citaro hybrid sees Mercedes-Benz open a new chapter in the evolution of city bus drive systems. This marks the first time anywhere in the world that hybrid technology has been offered as optional equipment for an extremely wide range of city buses with both diesel and gas engines, rather than in the form of stand-alone models. In this way, a large number of models can benefit from its advantages: depending on the operating profile and vehicle version, the Citaro hybrid combines with the electrohydraulic steering, another new feature, to lower the fuel consumption of the famously efficient Citaro by as much as 8.5 percent again. Thanks to the intelligent combination of existing components from the Daimler Group, the Citaro hybrid is able to offer a wealth of benefits: it is highly efficient, light, inexpensive to purchase and unproblematic for drivers and workshops alike. The new Citaro hybrid is not an interim technology on the road to the all-electric drive system, rather it is being used by Mercedes-Benz in a unique way to hone the internal combustion engine for maximum efficiency.

Up to 8.5 percent lower consumption, quick amortisation

The key strength of the new Citaro hybrid is its remarkable economy that enables it to clearly stand out from the competition – the Citaro hybrid pays for itself. This is all rooted in a clearly defined development goal: to make minimal modifications to the basic vehicle while maximising fuel efficiency. The Citaro with combustion engine is already acknowledged to be very efficient, yet the intelligent combination of various components lowers the fuel consumption of the Citaro hybrid by as much as 8.5 percent by comparison, depending on the route it operates on and the model version.

Straightforward drive technology to assist the combustion engine

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 discshaped 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 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.

In addition to this, a slight boost effect from the electric motor at idling speed increases the combustion engine's efficiency. These two functions together translate into a major reduction in fuel consumption.

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. The two modules together have a total capacity of 2 Ah and are mounted in a space-saving location at the back of the roof. 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. Both the inverter and the electric motor are water-cooled. The additional radiator for this low-temperature cooling at a maximum of 65 degrees Celsius is installed in a space-saving manner on the left side of the vehicle directly in front of the main radiator.

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, height largely unchanged

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.

Proven bus, passenger car and truck components

The development team for the Citaro hybrid have relied on proven components and made the most of the close links and wide-ranging experience within the Daimler Group. These components have been tried and tested, their comparatively high production numbers lower costs and they also make it easier to supply replacement parts.

The electric motor in the Citaro hybrid, for instance, is already fitted on the new Mercedes-Benz S-Class as a starter generator. The additional radiator for the electric motor and inverter is also used at Mercedes-Benz Trucks, and the water pump can be found in passenger cars bearing the three-pointed star.

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.

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.

Besides the Citaro hybrid, Intelligent Eco Steering can also be ordered as an option for the city bus family's entire model range, including the CapaCity high-capacity bus.

Exceptionally 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.

The hybrid drive is likewise available for the Citaro NGT with the M 936 G gas engine – the combination of gas engine and hybrid drive is rarely found anywhere in the world, yet the particularly low environmental impact of the gas engine makes this a highly attractive option. Once again, the hybrid drive can be ordered for both the Citaro NGT rigid bus and the Citaro G NGT articulated bus.

The hybrid drive for the Citaro is completely compatible with the familiar automatic transmissions from ZF and Voith. The only models for which hybrid drive is not available are Citaro buses fitted with the Mercedes-Benz OM 470 diesel engine due to a lack of installation space, as well as the high-capacity buses in the CapaCity series for the same reason.

Citaro hybrid in practice: straightforward integration into fleets

In contrast to many other city buses with electric drive components that have a more experimental character, the Citaro hybrid can be easily integrated into companies' existing vehicle fleets, routes and timetables. Due to the low additional weight, the total passenger capacity is virtually the same. There are absolutely no restrictions in terms of journey profiles or range either. The vehicle height is unchanged compared to the Citaro with air conditioning. Due to this flat design, there is no need to modify vehicle routes or adapt either entrance gates in halls and workshops or parking areas.

And bus drivers will also not notice any difference when sitting at the wheel of the Citaro hybrid, either when operating its controls or driving it. Even the instrumentation is identical. Training is therefore unnecessary. The quick driver changes during regular-service operation that are common in many places do not pose any problem at all either, and the same is true of drivers changing between different buses.

A safe long-term investment for transport operators

The Citaro hybrid represents a safe long-term investment as far as transport operators are concerned. All components are designed to last for the duration of the vehicle's service life, which is around twelve years for a city bus. The hybrid drive's supporting function furthermore helps to conserve the combustion engine when driving at high loads. The use of the electric motor as a power generator has the additional effect of relieving the strain on the brakes, which can greatly extend the service life of both brake discs and pads.

The Citaro hybrid also makes things easy for the after-sales service and workshops. Besides the tried-and-tested production parts used for the electric motor, power storage units and cooling, the absence of high-voltage technology is also a crucial factor here. The 48 volt system is classified as low-voltage technology, meaning that it can be handled without any risk of injury. As a result, there is no need for either extensive training courses for the workshop personnel or special tools or special replacement parts. All auxiliary assemblies in the Citaro hybrid remain unchanged and are driven conventionally.

The maintenance intervals for the Citaro hybrid are also identical, at 60 000 km or once a year, and the same applies to all maintenance points on the conventional part of the drive system. Caring for the vehicle is likewise free of risks, including the use of automatic washing facilities.

Companies will notice the positive effects of the unproblematic drive system on the Citaro hybrid throughout the vehicle's operating life and even when the time comes to resell it: its many practical benefits mean they will not have any trouble selling the city bus as a second-hand vehicle.

Photos and text:
Daimler AG