Mercedes-Benz unveiled the first Citaro with diesel/electric hybrid drive in autumn 2007. The highly sophisticated serial hybrid-drive system allows emission-free operation under battery power alone on appropriate sections of bus routes. The configuration is equally ambitious: the system is installed in a Citaro G articulated bus, and drives the centre and rear axles using four wheel hub motors. This combination is currently unique to the Citaro Hybrid.
Rather than providing continuous driving power, the diesel engine in the Citaro G BlueTec Hybrid operates as a generator to produce electrical power as and when required. Typically of a serial hybrid-drive system, the engine has no direct mechanical connection to the driving axles. This gives more freedom for innovative configuration of the interior.
The electrical power is stored in maintenance-free lithium-ion batteries mounted on the roof of the Citaro. Notable features of this battery technology are its high power density and storage capacity. The battery system has an output of 180 kW and with a weight of less than 350 kg, including cooling, is relatively light. The batteries are charged not only by the diesel generator but also by fuel-saving energy recuperation from braking.
Four electric wheel hub motors drive two axles
The actual drive power to the wheels of the Citaro Hybrid is provided by four electric wheel hub motors on the centre and rear axles of the bus. These motors develop a maximum output of 80 kW each and are liquid-cooled. The total wheel hub motor output of 320 kW is ample for an articulated bus, even under strenuous operating conditions. Two-axle drive, meanwhile, provides better traction. Also, the fact that in principle it would be possible to control the four wheel hub motors individually opens up interesting possibilities in terms of traction control and the use of electronic safety systems.
A 650-volt high-voltage intermediate circuit handles the power transmission between the generator, battery and wheel hub motors. The fact that the diesel engine is mechanically decoupled from the drive train and that energy can be recuperated from braking allows new control strategies to be implemented in order to reduce emissions. For example, the braking energy recuperated when pulling into a bus stop or coming to a stop at traffic lights can be used to provide electrical power when the vehicle is stationary or when moving off.
Emission-free stopping, standing and starting at the bus stop
When drawing into, standing at and accelerating away from bus stops, the hybrid bus can operate in electric-only mode, which is virtually emission-free. A further plus point is that the vehicle can hardly be heard stopping and starting at bus stops or traffic lights.
A big advantage of the serial hybrid drive in the Citaro is downsizing: instead of the large 12-litre six-in-line OM 457 hLA engine normally used in the articulated model, the hybrid bus is fitted with a compact OM 924 LA unit which develops maximum power of 160 kW (218 hp) from a displacement of 4.8 litres. This reduces the weight of the engine from approximately 1000 kg to around just 450 kg.
Since the diesel engine of the Citaro G BlueTec Hybrid does not drive the vehicle directly, it is not required to produce peak outputs and is therefore able to operate very economically and with low environmental impact over a narrow rpm range at or close to its peak efficiency. The torque curve of the hybrid-bus engine is therefore tailored predominantly to steady-state operation, for optimal emissions performance and fuel efficiency.
Electrically powered auxiliaries
This hybrid bus is able to dispense with a conventional automatic transmission. This results in improved efficiency and weight savings. The auxiliary units have also been optimised: the air-conditioning compressor, power-steering pump and air compressor are no longer belt or gear-driven from the internal combustion engine but are individually driven by electric motor.
This leads to a further improvement in efficiency, since drive is only supplied to the various auxiliary units when necessary. There are virtually no constraints on the positioning of these electrically driven auxiliary units. For example, they can be placed on the roof of the bus.
A new layout for the passenger compartment
The sophisticated drive technology used in the Citaro Hybrid also provides the basis for a new interior layout. Since no transmission or differential are required, and since there is no rigid connection between the individual drive train components, the rear passenger compartment layout in the Citaro G BlueTec Hybrid can be reorganised. Despite the engine's vertical tower design, the engine cowl can be situated low enough to be used as a luggage shelf by passengers, who enjoy an excellent all-round view.
Diesel/electric hybrid bus: a logical step on the way to fuel-cell drive
A close look at the hybrid-drive Citaro reveals it to be a logical step on the way to an emission-free fuel-cell-powered urban regular-service bus of the future. For example, the hybrid-drive Citaro already features a suitable electric drive unit, along with an energy store in the form of batteries. In principle, the diesel engine would simply need to be replaced by fuel cells when the time comes. The generator and the diesel and AdBlue tanks could then be dispensed with and the hydrogen tanks could be mounted on the roof in the usual way in the area over the front axle.
These are relatively minor changes from the design and manufacturing point of view but they would allow the bus to operate with virtually zero emissions in urban areas. This Zero-Emission Vehicle would emit no particulates, nitrogen oxides or CO2, and would also be even quieter in operation.
Deliveries scheduled from 2009
With the Citaro G BlueTec Hybrid having successfully completed regular winter testing in the Arctic Circle, deliveries of customer vehicles are scheduled to begin from the end of 2009. An ambitious and closely related project is to make the diesel/electric urban hybrid bus an economical proposition for the operator. With the present small production volumes, the cost-effectiveness threshold has not yet been reached, despite the improvements of up to 30 percent in fuel efficiency. Ultimately the success of hybrid vehicles will depend not only on advances on the manufacturing side but also on the willingness of the public to recognise and support the ecological benefits of the concept.
Daimler: 40 years of hybrid buses
Mercedes-Benz and the Daimler Group have more extensive, and longer, experience with hybrid buses than any other manufacturer in the world. Daimler-Benz AG presented the world's very first hybrid bus at the Frankfurt Motor Show in 1969. It was based on an eleven-metre urban regular-service version of the Mercedes-Benz OE 302. The DC electric traction motor developed a steady-state output of 115 kW (156 hp) and a peak output at low speeds of 150 kW (204 hp). The traction motor was powered by five underfloor battery blocks, whose 189 cells produced a total system voltage of 380 V and had an energy capacity of 91 kWh. This gave an operating range in regular service of approximately 2.5 hours. The batteries had a weight of 3.5 t.
The bus was also fitted with a 3.8-litre four-cylinder diesel engine developing a maximum output of 48 kW (65 hp). This unit, which operated fuel-efficiently and at constant rpm and was transversely mounted in the rear of the vehicle, was activated to provide additional power when operating on the outskirts of town. This vehicle already featured an electric brake for braking-energy recuperation.
From the Mercedes-Benz OE 302 to the OE 305
The OE 302 was succeeded ten years later by the OE 305 urban regular-service bus, whose batteries were now accommodated in two compartments running width-wise across the vehicle between the axles. They had a rated voltage of 360 V and a capacity, depending on specification, of between 150 and 275 Ah. Their weight ranged from 2.0 t to 3.5 t. In battery-only operation, they had a range of between 50 and 75 km. The traction motor had the same output as before, namely 115 kW (156 hp) in steady-state operation, with a peak output of 150 kW (204 hp), while the diesel engine was now a six-cylinder unit developing 74 kW (100 hp).
From the hybrid bus to the Duo bus
The hybrid bus was also joined by an electric-only Duo bus, whose traction motor could be powered either from overhead trolley lines or by batteries. A further version of the Duo bus had no batteries and was powered either by overhead lines or a diesel engine. Field trials of the trolley/battery Duo bus began in Esslingen, Germany, in 1975. From 1979, twenty-five O 305 buses with various types of hybrid drive were operating in Stuttgart, Esslingen and Wesel.
By 1984, a limited-production Duo articulated hybrid bus, the O 305 GTD, was operating in everyday regular service. In Essen it even used a guided busway and some parts of the tram network. The drive system now comprised an electric traction motor, powered from overhead lines, in combination with a normal diesel engine. The two power sources alternated with each other to drive the third axle.
The O 305 GTD Duo bus and its successor, the O 405 GTD, even carved out something of an international career for themselves. For example, over 200 units were supplied by Mercedes-Benz to the Ecuadorian capital Quito. In Europe, approximately 50 units of the O 405 GTD went into service. In Esslingen, some of these are still operating in regular service today.
O 405 NÜH: the first hybrid bus with wheel hub motors
The mid-1990s marked a new milestone in the development of hybrid-drive buses. For urban operation, Mercedes-Benz began trials of the O 405 GNDE, a diesel/ electric articulated bus with wheel hub motors, but no batteries to store energy. A further landmark in hybrid bus development was the O 405 NÜH low-floor hybrid rural-service bus. This was the first hybrid bus with wheel hub motors, a diesel engine and traction batteries. The two asynchronous wheel hub motors at the rear axle each had an output of 75 kW (103 hp). The low-floor design of the front end of the bus led to the four traction batteries being mounted on the roof, to save space. The powerful sodium-nickel-chloride batteries weighed 800 kg and had a range of 10 km in continuous electric-only regular-service operation. After each such electrically-powered trip through an urban area, the batteries were recharged by the diesel engine.
The Mercedes-Benz Cito urban midibus, presented in 1998 and in production from 1999 to 2003, also featured diesel/electric drive. The drive system, comprising a diesel engine which powered a generator, which in turn powered an electric traction motor, was fitted in the form of a "power pack" in the rear of this compact low-floor urban regular-service bus. Here too no battery was used to store energy.
Approximately 1700 Orion hybrid buses already on the road
The Orion-brand buses built by Daimler's North American bus subsidiary are already well past the trial stage. Since 2003, Orion has won contracts to supply numerous US and Canadian cities, from New York to San Francisco, with a total of some 2800 Orion VII HybriDrive low-floor urban regular-service buses, around 1700 of which are already on the road. This makes Orion the world market leader in hybrid buses. The company also has extensive development experience: the first Orion hybrid buses went into service in the early 1990s.
The Orion VII HybriDrive was developed by Orion in association with US partners BAESystems and is a serial hybrid vehicle. The diesel engine operates continuously, powering a generator which in turn powers the electric motor. The driving axle is a conventional axle with a differential. The diesel engine is transversely mounted in the rear, develops 191 kW (260 hp) from a displacement of 5.9 litres and is equipped with a particulate filter. The traction motor delivers maximum power of 184 kW (250 hp), which can be raised for short periods to 235 kW (320 hp). A transmission is dispensed with.
Alternatively, the Orion uses standard lead batteries to supply energy, and since 2008 has also been able to use lithium-ion batteries. The lead batteries are housed in two containers on either side of the roof. They each comprise 23 modules and weigh more than 1.5 t. No provision has been made for recharging the Orion from an electric power outlet. Instead the batteries are recharged by braking‑energy recuperation and by using excess power from the diesel engine. Customer tests revealed that the Orion is approximately 25 percent more fuel-efficient than a diesel‑powered competitor vehicle.
Fuso Aero Star: Japan's first standard-production hybrid bus
Likewise active in the field of hybrid-drive urban buses is Daimler's Japanese commercial-vehicle subsidiary Fuso, with the low-floor Aero-Star model. Technically, the Aero-Star's serial hybrid-drive system is similar to that of the Orion vehicle in North America: a 177 kW (241 hp) diesel engine with application- optimised performance characteristics, transversely mounted in the rear of the vehicle, drives a generator which powers two electric traction motors, which together produce an output of 134 kW (182 hp). These motors supply their torque to a combining gearbox. The vehicle features a portal axle and roof-mounted lithium-ion batteries with a weight of just 323 kg.
A number of these first standard-production Japanese hybrid buses have already gone into regular-service operation. Testing was already under way in Japan at the time of the 2002 Football World Cup.