Mercedes-Benz will introduce its 12m battery-electric Citaro bus in September at the IAA Commercial Vehicles show, with the start of series production also taking place this year. (Earlier post.) The company has now given a preview of the new locally emission-free 12-meter variant.
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. Peak motor output is 2 x 125 kW, while torque is 2 x 485 N·m.
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 comprises 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 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 maximizes 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 opportunity charging necessary.
A separate battery cooling device is 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.
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.
Since the development of battery technology is progressing at a rapid pace, provision has already been made for the transition to the next generation.
Charging technology. 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, 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.
Thermal management. At an outside temperature of -10 ˚C (14 ˚F), the energy consumption of a city bus doubles by comparison with journeys where no heating is required—thereby halving the range. Because 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 meters or so, assuming normal bus-stop intervals.
The engineers have therefore put a considerable amount of thought into the issue of thermal management. Compared with the current Citaro with combustion engine, the energy requirement for heating, ventilation and climate control has fallen by about 40%. 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 unfavorable conditions, the company said.
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.
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 utilization of the bus is measured via its axle load sensors.
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; Mercedes-Benz is one of the first bus manufacturers to use CO2 as a coolant.
Integrated eMobility system. The new all-electric Mercedes-Benz Citaro is a key part of Daimler Buses’ integrated eMobility system. This encompasses the pre-purchase advisory service eMobility Consulting, including an in-depth analysis of the transport operating company and its various routes. Omniplus Service offers companies tailor-made service concepts designed for electric mobility, right through to services provided in the customer’s own workshop.