Citaro is electrified

Daimler Buses has unveiled its first phase all-electric Mercedes-Benz Citaro. As Andy Izatt reports, it’s part of a measured, two-pronged response to growing political and social pressure for emissions-free transport

In preparation for the public debut of the phase one version of the 12m left-hand drive Mercedes-Benz Citaro all-electric at IAA Commercial Vehicles, Hanover in September, Daimler Buses has given this important new addition to its product range its trade press launch. Right-hand drive will be available in two years, by which time a significantly greater capacity battery pack is anticipated (second phase) and articulated versions of the vehicle will also be introduced.

“Indications of operating range for all-electric powered city buses are often difficult to compare and caution is advised, since reference values can be missing and the figures have often been calculated under ideal conditions,” says Daimler Buses. “In the interests of reliable data, Mercedes-Benz prefers to consider a ‘worst-case scenario’ and therefore takes its direction from the challenging ‘Standardised On-Road Test cycle (SORT2). Mercedes-Benz also adds the energy requirements of the ancillary consumers into the equation. According to SORT2, the Citaro with a full complement of batteries can achieve an operating range of around 150km in the summer.”

By the time series production starts, a dozen prototypes will have completed a gruelling, comprehensive evaluation programme that has included trials at below minus 15 degrees Celsius in the Arctic Circle and at more than 30 degrees Celsius in the Sierra Nevada mountains of Spain. All components will have been tested individually and once integrated both under laboratory conditions and in the field.

“It’s built on the platform of the proven Citaro, which provides an excellent basis for the powertrains of today and tomorrow,” explained Gustav Tuschen, Head of Product Engineering, Daimler Buses. “The layout of the passenger compartment remains the same. This means no change for transport operators or passengers. We use the electrically powered rear axle with wheel hub motors already proven in our buses. Modular battery packs are installed on the centre of the roof, and in the rear. As standard, the bus is charged at the depot via a Combo 2 connector. On the road it can be optionally charged by means of an on-board or stationary pantograph.” [wlm_nonmember][…]

[/wlm_nonmember] [wlm_ismember] The introduction of the all-electric Citaro will be supported by a pre-purchasing advisory service, eMobility Consulting provided by Daimler Buses, which undertakes an in-depth analysis of operating requirements for customers. Precise recommendations will then be made covering aspects such as charging infrastructure, energy consumption, the load capability of the power supply at the depot and charge management.

As would be expected, Daimler’s Omniplus aftersales will provide comprehensive tailor-made support covering service, maintenance and repairs ranging from supervision of a customer’s workshop through to contracted workshop support provided by the Daimler dealer network.

“We are working hard not only on the fully electric city bus,” said Hartmut Schick, Head of Daimler Buses. “Partially automated driving also continues to have high priority. We will invest approximately €200m by 2020 alone in the implementation of the CASE strategy in order to shape the future mobility. We offer solutions enabling passengers to travel safely, flexibly and free of emissions.”

CASE stands for the future areas of connectivity (Connected), automated driving (Autonomous), flexible use (Shared & Services) and electric drive (Electric).

Drive in detail

The drive system of the all-electric Citaro is based around the ZF AVE130 electric portal axle with water-cooled asynchronous electric motors at the wheel hubs that is already used in other variants of the Citaro. Peak output is 2 x 125kW with torque of 485Nm x 2. The gearing ratio turns that into a drive torque of approximately 2 x 11,000Nm.

Up to 10 high-voltage 25kWh Akasol nickel-manganese-cobalt lithium-ion modules make up a battery pack with a power capacity of 243kWh. Four modules are in the rear overhang of the vehicle where a diesel engine and gearbox would otherwise have been located while two, four or up to six more are on the centre of the roof. The minimum option Daimler Buses offers is a total of six modules delivering 150kWh.

Each module comprises a control unit for monitoring and balancing and 15 Samsung cells. Within each of those are 12 37Ah prismatic cells.

Daimler Buses says depth of discharge can be tailored to specific applications. The battery pack is guaranteed for five years, but actual life beyond that will dependent on utilisation and could realistically be significantly longer. Different options for disposal are currently being explored.

With a maximum complement of 10 battery modules, the electric Citaro has an unladen weight of 13.7 tonnes – 2.4 tonnes more than its diesel equivalent. Utilising higher loading front wheels and tyres, it has a front axle plated at 8 tonnes rather than the 7.5 tonnes used on diesel variants.

The passenger saloon layout and maximum seating capacity remains unchanged at 42 while right-hand drive versions for the UK and Ireland will have only a small reduction in overall capacity. That’s achieved by taking advantage of the 19.5-tonne GVW (Gross Vehicle Weight) now available. Diesel variants continue to be plated at 18 tonnes.

From the start of series production, charging will be via a standard-fit, plug-in, DC Combo 2 connector positioned above the front wheelarch on the right hand side in left-hand drive versions – where the fuel tank filler is currently located on diesel variants. Suitable for both high charging loads and quick charging, Combo 2 can be used in conjunction with mobile chargers up to 80kW, stationary charging stations up to 150kW and currents of 200A. Based on outputs of between 20-80kW, charging times could be anything from three to 11 hours.

To be phased in once series production starts are an integrated vehicle collector (pantograph) on the vehicle roof and the installation of charging rails that facilitate charging via a fixed-installation charging station. In both instances the installation will be in line with the front axle.

The all-electric Citaro also generates additional electrical energy through recuperation. The two wheel hub electric motors on the drive axle act as alternators during braking, transforming the kinetic energy created into electricity.

Sophisticated thermal management

Daimler Buses points out that the range of any electric bus can be limited by the need to cool and heat the saloon depending on climatic conditions. With an outside temperature of minus 10 degrees Celsius, energy consumption doubles compared to when no heating is required thereby halving the vehicle’s range. Unlike a combustion engine, electric motors create negligible heat waste so the heating system must be fed from the vehicle’s own energy supply. An additional factor is the size of the saloon.

It argues that an enhanced thermal management system is the solution and compared with a diesel Citaro, has managed to reduce overall energy demand for heating, ventilation and climate control by about 40% – 25% for climatisation and 50% for heating. Overall energy consumption for the vehicle that would otherwise be at least 2.8kWh per km in a diesel bus is reduced to just 2.0kWh per km.

All components that give off heat are linked together, so keeping the amount of energy required for their cooling while in operation to a minimum. Heat generated by passengers also contributes and the output of the heating and climate control systems is varied accordingly. Passenger numbers are measured via axle load sensors.

A heat pump is used to warm the passenger saloon with side-wall fan heaters helping to deliver an even temperature. A conventional heater at the front is boosted by the addition of a double heat exchanger.

During colder months, the roof-mounted HVAC (Heating, Ventilation & Air-Conditioning) system CO2 air-conditioning is additionally used as a heat pump to deliver climate control. Daimler Buses says that the use of CO2 as the standard coolant for Citaro climate control means it will work efficiently at temperatures as low as that minus 10 degrees Celsius benchmark. The coolant is also environmentally harmless, non-toxic and non-flammable. When temperatures outside are above 30 degrees, the energy consumption of the air-conditioning is just 1.4kWh per km.

Batteries are kept cool using a separate battery cooling device mounted on the roof. When outside temperatures are extreme, the standard passenger compartment climate control system is used to boost that cooling. Further flexibility is possible by exploiting the discharge depth of the batteries, which can be extended although inevitably at the expense of range and service life.

To compensate for energy drain resulting from extreme temperatures or to maximise vehicle range, a Spheros fuel-powered auxiliary saloon heater can also be specified. Climate control for the driver’s cab is controlled separately with a constant ambient temperature of 24 degrees Celsius being the objective.

Two-pillar strategy

Gustav predicted that by 2025 there will be as many electric buses registered as diesels. “By 2030 they will account for 75%,” he said. “Recharging along the route will have become a thing of the past because all-electric buses will have a range of 400km on one charge. TCO (Total Cost of Ownership) will be lower than diesel buses. The power they use comes from renewable energy sources. They are therefore not just locally emission-free, but usually totally emission-free. A vision? Yes, but we believe this to be a realistic one.

“Despite all enthusiasm for electrically powered buses, the era of the combustion-engined urban bus is far from over. It’s impossible for the combustion engine to be replaced overnight by the all-electric powertrain. What makes it impossible at the moment is the cost and the still limited range, the complex power supply and the necessary changes on the service front as far as workshop equipment and employee training are concerned. Electric mobility means totally rethinking local public transportation with buses.

“An abrupt end to the combustion engine is not possible – nor is it necessary. Last autumn we unveiled the Citaro hybrid. With that vehicle, we lowered fuel consumption and CO2 emissions by up to another 8.5%. No comparable combustion-engined regular-service bus is more economical. Even the gas-engined Citaro NGT is available as a hybrid.

“In 2012, our globally top-selling Citaro became the world’s first urban bus to comply with the currently most stringent Euro 6 emissions standard. With its hybrid variant, we now speak of a low-emission urban bus. Compared with a Euro 5 urban bus, we have reduced CO2 emissions by almost 20% and NOx by up to 98% in just six years. Particulate emissions have even reached the lower limit of detection. I’m not talking about measurements in the lab here. I’m talking about measurements in the real world, out on the road. These outstanding results cannot be achieved with retrofit solutions.

“We are talking here about a 0.5-litre vehicle per passenger in rush-hour traffic. As an alternative to the diesel, we have the gas engine. It has made similar strides and operated on biogas, it is almost CO2-neutral. As you can see, the combustion engine will continue to have its justification for quite some time when it comes to urban buses. It is not yesterday’s powertrain. It is totally up-to-date.

“This is the background against which we devised our well-known two-pillar strategy. On the one hand, we are optimising the combustion powertrain. On the other, we are developing the all-electric urban bus, which will gradually be able to replace buses with combustion engines. In the first phase, the range of the new all-electric will be sufficient to cover over 30% of bus services without recharging.

“Under ideal conditions, it is capable of travelling almost 250km. Unfortunately, such a peak value is irrelevant for the design of a transport system. Transport operators need a reliable range forecast for the challenging days. At plus 30 degrees in summer and at minus 10 degrees in winter and, on top of that, in rush hour. The range under these conditions is the one that is systemically relevant revenant for our customers, and that is the range we have especially set out to optimise.

“We have already defined the design of the next battery generation together with our development partners. Capacity will increase by over a third to more than 300kWh. Our aim for the medium term is a systemically relevant range of 250km. That is enough to cover more than 70% of all operations without having to recharge. If operators choose, they will even be able to swap (their current) battery packs for the more powerful ones in future.

“Until we finally have an entirely new form of urban transport in 2030 at the latest – largely autonomous, clean, safe and quiet – the way there has been paved with the new all-electric Citaro.”
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