- Citaro: from low-emission bus to zero-emission bus
- Flexible charging technology: from a power socket as standard, via current collector as an option
- Thermal management: new approaches for heating and climate control
- Viable range under difficult conditions
- Intensive summer and winter testing
It emits zero local emissions and runs almost silently. It combines the platform of the world’s best-selling city bus with new technological solutions. The all-electric Mercedes-Benz Citaro takes electric mobility using city buses to a new level altogether. The 12-metre variant will celebrate its world premiere in September at the IAA Commercial Vehicles, with the start of series production coming before the end of the year. The new Citaro offers an impressive and hitherto unknown standard of energy efficiency, courtesy of its innovative thermal management system. The intelligent modular concept for its battery and charging technology is a similarly compelling proposition. The electric Citaro is also more than simply a city bus: Mercedes-Benz takes a holistic view of electric mobility, embedding the Citaro firmly within its overall eMobility system. This comprises services that range from individual and very practically oriented advice ahead of any purchase and recommendations relating to charging infrastructure through to taking on servicing work at the customer’s own workshop.
Urbanisation demands low-emission and locally emission-free mobility
The process of urbanisation marches on: for the last ten years, for the first time in the history of the earth, more people have been living in cities than in the country. According to forecasts from the United Nations organisation (UN), this proportion is set to grow to two thirds of the world’s population by 2050. Whether we are talking about a megacity, a large city or a small town, the challenges and problems are similar: a growing population means a growing need for mobility, for both work and recreational purposes. And thereby both an opportunity and a risk for already overburdened traffic systems, along with potentially greater environmental pollution from exhaust emissions and noise. Both the populace and politicians are, however, becoming more and more acutely aware of the associated disadvantages and limitations.
One possible solution in this respect is a well-developed local public transport infrastructure, with low-emission and locally emission-free buses. The current buzzword, therefore, is electric mobility. The new all-electrically powered Mercedes-Benz Citaro is an important element here, running as it does both locally emission-free and almost silently.
Thanks to new technological solutions, the Citaro is able to demonstrate outstanding performance capability. At the same time, its variability means that it is perfectly suited to meet the stringent demands of the transport operating companies with their complex systems of route networks and timetables. It also brings the advantage of being based on a tried and tested vehicle: with more than 50 000 units sold, the Citaro is the world’s best-selling city bus.
Citaro: from low-emission bus to zero-emission bus
With a wide range of low-emission and, going forward, emission-free city buses, Mercedes-Benz has the appropriate answers to questions about environmentally friendly local public transport. All variants are based on the global best-seller, the Citaro. Back in 2012, this became the first city bus in the world to meet what is currently the most stringent emissions standard, Euro VI.
The Citaro hybrid variant then became a fully recognised low-emission bus. Launched last autumn, it features a hybrid module that further optimises the performance of the combustion engine and reduces the already low fuel consumption by as much as 8.5 percent. Compared with a city bus meeting the Euro V standard, the CO2 emissions have thus fallen by almost 20 percent and the nitrogen oxide emissions by as much as 98 percent in just a few years, while particulate emissions have even reached the limits of detection. The Citaro hybrid, featuring innovative 48-volt technology, dispenses with the need for a high-voltage network and is available as an option for the majority of Citaro variants, including for the natural gas-powered Citaro NGT. This model is particularly quiet-running and its emissions low, while when fuelled by biogas it is virtually CO2-neutral.
The Citaro and the large-capacity CapaCity bus, as well as the Citaro hybrid and the Citaro NGT, provide practical evidence, thousands of time every day, of the highly sophisticated level of development of drive systems with combustion engines. They are characterised by both their efficiency of operation and their low emissions. The all-electric Citaro is now taking the next step from low-emission bus to locally emission-free bus. In doing so, it adds to the range a variant that sets new standards in terms of electric mobility.
Proven electric axle, new modular battery concept
The drive system of the new all-electric Citaro is based on the tried and tested ZF AVE 130 electric portal axle with electric motors at the wheel hubs, as previously deployed in other variants of the Citaro. Their peak output of the motors is 2 x 125 kW, while torque is 2 x 485 Nm. It is an inherent feature of such motors that this is fully available right from the start, so ensuring appropriate dynamic performance even with a full complement of passengers.
Lithium-ion batteries with a total capacity of up to about 243 kWh are responsible for providing the power. These are modular in design: the batteries are split between up to ten modules, each supplying around 25 kWh. As well as two battery modules on the roof, the standard equipment includes four battery modules in the rear of the bus. In the Citaro these are located on the left-hand side in the direction of travel, in the place of the current drive system combination of combustion engine and transmission. Depending upon customer requirements, a further two or four battery modules may be mounted on the roof of the Citaro.
Each battery module is made up of 15 cell modules, together with a control unit for monitoring purposes and as a means of balancing the charge of the battery cells. Each separate cell module houses twelve battery cells. Mercedes-Benz uses easily manageable prismatic cells with a capacity of 37 Ah each. With a minimum of six and up to a maximum of ten battery modules possible, transport operators can adapt their usage and charging strategy very precisely to individual needs. Opting for the largest number maximises the range of the buses, while a smaller number reduces the weight as well as the cost of purchase and allows more space for passengers – but potentially makes time-consuming interim, or ‘opportunity’, charging necessary.
With the maximum complement of ten battery modules, the all-electric Citaro with standard equipment weighs around 13.7 tonnes. In conjunction with a permissible gross vehicle weight of 19.5 tonnes, this corresponds to a payload of 5.8 tonnes or around 80 passengers – even in the rush hour, as is commonly necessary.
The Citaro’s engineering has been future-proofed. Since the development of battery technology is progressing at a rapid pace, provision has already been made for the transition to the next generation.
Flexible charging technology: charging at the depot as standard, via current collector as an option
The Citaro’s charging technology also allows it to adjust to the individual wishes and requirements of the transport operators. For the start of series production, charging via connector is envisaged. The city bus features as standard a socket for a Combo 2 connector above the front wheel arch on the right-hand side of the vehicle in the direction of travel. This corresponds to the usual position for the tank filler neck on a Citaro with diesel engine.
If, in order to extend the range of the vehicle, there is a requirement for opportunity charging, the all-electric Citaro can also be charged via a current collector. This option will be gradually phased once series production has started. There will be two possible variants: in phase 1 an integrated vehicle collector on the vehicle roof; in phase 2 the fitting of charging rails that will allow charging via a fixed-installation current collector at a charging station. In both cases the equipment will be mounted in line with the front axle.
The Citaro will thus allow for all charging variants currently in use. This intelligent modular concept for the battery and charging technology means that Mercedes-Benz is able to offer transport operators the opportunity to configure the Citaro precisely to the individual requirements of the company, or even of each separate route.
The Citaro is also able to generate electrical energy through a process of recuperation. In this case the two electric motors at the wheel hubs of the drive axle act as alternators during braking, transforming the kinetic energy of the vehicle into electricity.
Thermal management: new approaches for heating and climate control
The battery capacity on its own, however, provides little indication of the actual performance capability and, above all, the range of an all-electrically powered city bus – the true measure is that of energy consumption. In the case of a city bus, this is impacted significantly by climatic conditions, through the need to cool and, above all, heat the interior.
At an outside temperature of minus ten degrees Celsius, the energy consumption of a city bus doubles by comparison with journeys where no heating is required – thereby halving the range. The cause: the extreme efficiency of an electric motor means that, compared with a combustion engine, the amount of usable waste heat is negligible. The heating system must therefore be fed from the vehicle’s own energy supply. An added factor is the sheer volume of the interior space, particularly when up to three double-width doors are opening regularly and letting outside air in every 400 metres or so, assuming normal bus-stop intervals.
The engineers have therefore put a considerable amount of thought into the issue of thermal management. It is one of the outstanding features of the Citaro and has been honed and refined in every detail: compared with the current Citaro with combustion engine, the energy requirement for heating, ventilation and climate control has fallen by about 40 percent. Achieving this exceptional level of energy efficiency has been a complex process, but it provides the basis for the Citaro’s very viable operational range, even under unfavourable conditions.
Batteries at the ideal temperature: maximum performance and service life
Here too, thermal management is important. Mercedes-Benz, for example, cools the batteries to ensure that they remain at the ideal temperature, thereby ensuring maximum performance capability and service life. This cooling is undertaken by a separate battery cooling device mounted on the roof. At extreme outside temperatures, the standard passenger-compartment climate control system is used to boost the cooling of the batteries.
Further flexibility is possible by exploiting the discharge depth of the batteries. This can be extended, although at the expense of range and service life.
Heating: even the passengers help with heating
The passenger compartment of the Citaro is heated in an energy-efficient manner by a heat pump. An even distribution of temperature is ensured by use of the familiar side-wall fan heaters. The conventional heater at the front is boosted by the addition of a double heat exchanger. For use in extreme weather conditions, or to extend the vehicle’s range, a fuel-powered auxiliary heater can also be used as an option.
A series of examples makes clear the care and detailed attention that have gone into the thermal management system. 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. Since the human body likewise gives off heat, the heating on a bus carrying a full complement of passengers can be turned down earlier. Furthermore, Mercedes-Benz varies the output of the heating and climate control systems according to the number of passengers on board: the intake of fresh air in the bus is matched to the current number of passengers. The capacity utilisation of the bus is measured via its axle load sensors.
Air conditioning system and heat pump: high efficiency through using CO2 as a coolant
During the colder months, the roof-mounted air conditioning system is used in addition as a heat pump, so ensuring effective and efficient climate control for the passenger compartment. The use of CO2 as a coolant brings further benefits. Its very efficient use of the heat pump, even at temperatures as low as minus 10 degrees Celsius, is particularly impressive. And, should there be a release of coolant, unlike some other materials it is environmentally harmless, non-toxic and non-flammable. Mercedes-Benz uses CO2 as a coolant as standard in the Citaro, making it one of the first bus manufacturers to do so.
Climate comfort tailored to the needs of passengers
Another boon: the interior can be conditioned up to and even beyond the desired temperature while the batteries are still being charged at the depot. The bus will therefore have been heated or cooled to reflect the season before it sets off.
The heating and climate control systems are configured in accordance with the requirements of the Association of German Transport Companies (Verband Deutscher Verkehrsbetriebe: VDV). If the temperature outside is extreme, comfort levels at either end of the scale are reduced somewhat in favour of energy consumption and thus range. Instead of the interior temperature being set to a consistent level all year round, it is adjusted according to situation to ensure the comfort of passengers. As passengers generally spend only a short time in the vehicle and are normally dressed in accordance with the time of year, the inside temperature is set higher on hot summer days and lower on cold winter days, without at any point compromising passengers’ sense of comfort.
When it comes to the driver’s workplace, Mercedes-Benz pursues a somewhat different strategy: since the driver spends all his or her working hours in the city bus, the expectations here are greater, while the physiological safety of the driver must be ensured at all times. The climate control system for the driver’s workplace is therefore controlled separately, with an ambient temperature of 24 degrees the aspiration at all times, whatever the extremes outside.
High efficiency means viable range under difficult conditions
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. Things are different with the Citaro: 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’, known as SORT2. To make things even trickier, 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 150 kilometres in the summer. In other words, it is already possible to serve some sub-networks within the daily coverage of a city bus without opportunity charging.
More space for passengers thanks to careful weight distribution
The downside of all-electric city buses is the extra weight added by their batteries. Even the intrinsically lightweight Citaro is unable to offset completely the 2.5 tonnes of the maximum battery set. However, with a carefully considered distribution of weight, the Citaro is able to exploit the permissible axle loads to secure the highest possible payload – and thus number of passenger places.
The engineers managed to balance the bus by installing four battery modules within the rear overhang and up to six further battery modules on the roof, on a line with the front axle. This axle is also characterised by a maximum load-carrying capacity of eight tonnes and thus, depending on the variant, space for a realistic complement of around 80 passengers.
Seating and optional extras: the customised city bus
Since the developers have adopted the tried and tested layout of the Citaro, the configuration of the passenger compartment remains unchanged compared with the conventional Citaro with vertically mounted engine. Passengers will find everything familiar. The vehicle is available with either two or three doors.
Another advantage of the Citaro platform: transport operators are able, as ever, to individualise the bus by selecting from a wide range of optional extras. Whether it’s the passenger seats, flooring, grab rails, communication systems or unseen details such as the door controls – the Citaro is, and remains, a fully customisable city bus, whatever its drive system.
Intensive summer and winter testing
The technology of the all-electric Citaro as it goes to the start is well proven. Its superb quality is guaranteed by the tried and tested Citaro that is already in such regular use. This is backed up by the fact that it is manufactured at the company’s bus plant in Mannheim, on the production line for the conventionally powered Citaro – where the test vehicles were also built. Key components such as the drive axle and the electrohydraulic steering system have already proved their worth in the challenging world of city bus transport. The design of the roof, with integrated heavy-duty rails for the batteries, is derived from the system used for the gas tanks on the Citaro NGT.
What is more, by the time of the series-production start-up, the all-electric Citaro will already have been through a comprehensive programme of testing. This bus has to pass all the same tests as any other bus that bears the three-pointed star, for the Citaro with all-electric drive system will offer the same high level of availability and the proverbial reliability of its stablemates with combustion engine.
All components are tested individually as well as in interaction with others, on test benches and in practice in the bus. Mercedes-Benz has tested the Citaro at temperatures below minus 15 degrees Celsius at the Arctic Circle and at more than 30 degrees Celsius in the searing summer heat of Spain. Winter testing also includes road tests on a slippery surface to test dynamic handling control systems and recuperation. Summer testing takes place in challenging urban traffic conditions and on steep up- and downhill gradients in the Sierra Nevada. The test programme also includes rough-road and endurance testing. All in all, a dozen or so prototypes are being very thoroughly put through their paces.
