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Mercedes-Benz Citaro hybrid: a step towards the zero emission vehicle

Posted: 6 October 2007 | Paul Wolfe, Commissioning Editor | No comments yet

Ten years after the Mercedes-Benz Citaro urban regular service bus was unveiled at the UITP Congress 1997 in Stuttgart, Citaro number 17,500 was recently presented at the UITP Congress 2007 in Helsinki. The Citaro has been frequently updated over the previous years and the ultra-modern Citaro of today continues to set the standards in its application segment, just as it did at the launch a decade ago.

For the first time, annual production last year topped 3,000 units. Since it was unveiled, the Citaro has been operating very successfully in most major European cities, including London, Paris, Madrid, Stockholm, Berlin, Rome, Bucharest and Istanbul. It is also well established in overseas markets, such as Dubai, Israel and Mexico.

Ten years after the Mercedes-Benz Citaro urban regular service bus was unveiled at the UITP Congress 1997 in Stuttgart, Citaro number 17,500 was recently presented at the UITP Congress 2007 in Helsinki. The Citaro has been frequently updated over the previous years and the ultra-modern Citaro of today continues to set the standards in its application segment, just as it did at the launch a decade ago. For the first time, annual production last year topped 3,000 units. Since it was unveiled, the Citaro has been operating very successfully in most major European cities, including London, Paris, Madrid, Stockholm, Berlin, Rome, Bucharest and Istanbul. It is also well established in overseas markets, such as Dubai, Israel and Mexico.

Ten years after the Mercedes-Benz Citaro urban regular service bus was unveiled at the UITP Congress 1997 in Stuttgart, Citaro number 17,500 was recently presented at the UITP Congress 2007 in Helsinki. The Citaro has been frequently updated over the previous years and the ultra-modern Citaro of today continues to set the standards in its application segment, just as it did at the launch a decade ago.

For the first time, annual production last year topped 3,000 units. Since it was unveiled, the Citaro has been operating very successfully in most major European cities, including London, Paris, Madrid, Stockholm, Berlin, Rome, Bucharest and Istanbul. It is also well established in overseas markets, such as Dubai, Israel and Mexico.

The Mercedes-Benz Citaro range is available from 10.5 to 15 metres in length as a two or three-axle, low-floor solo bus, low-floor articulated bus, low-floor rural service bus, low-entry Citaro LE, or Citaro-derived Mercedes-Benz Capa-City high-capacity bus. These provide a host of answers to the requirements of regular service operators in the urban and urban-fringe segment.

Eco-friendly engines

The milestone vehicle on display in Helsinki was a Citaro LE Ü. This 12 metre low-entry bus is designed for rural-urban fringe service and is powered by a 220kW (299hp) horizontally mounted OM 457 hLA in-line six-cylinder engine coupled to a ZF torque converter and automatic transmission. The vehicle is fully compliant with the Euro 4 emissions standard; a mandatory requirement since Autumn 2006. Incidentally, this is the same time that Mercedes-Benz also supplied its first Euro 5-compliant diesel powered Citaro models, ahead of the actual deadline of 2008/2009. In addition, the Citaro range already contains a wide range of models which comply with the voluntary, even more stringent EEV (Enhanced Environment friendly Vehicle) emissions standard.

SCR technology: clean, economical and future-proof

The Mercedes-Benz Citaro meets the current and future emissions standards using SCR (Selective Catalytic Reduction) technology. This future-proof system has already proven itself extensively in practical operation across Europe, where a four figure number of SCR-equipped buses are already plying the road.

The BlueTec 4 and BlueTec 5 diesel engines are not only clean, they are also noticeably more economical on fuel than engines with other emission control technology. This will allow the Citaro to play its part in reducing CO2 emissions; blamed by experts for contributing to climate change.

Meanwhile, the EEV-compliant Citaro combines Euro 5 engines with a closed particulate filter. Whatever emissions rating customers are looking for, they can be confident that their Citaro will not only meet, but in most cases also significantly surpass all the relevant standards.

Natural gas powered and fuel cell powered Citaro

As an alternative to the diesel-powered models, Mercedes-Benz can supply a natural gas powered Citaro. This drive system also complies with the EEV standard, and bears the Blue Angel seal of environmental quality.

The Citaro’s technology provides an opportunity for clean operation in the long term, too. In ten major European cities, as well as Perth, Australia and Peking, 36 fuel cell powered Citaro buses have clocked up approximately two million kilometres in everyday operation.

Following this successful performance demonstration for the fuel cell, attention at Mercedes-Benz is now focusing on two components; namely, a modern electric drive unit and an efficient electrical energy storage system. Both of these are required for a future production version of a fuel cell powered Citaro. Diesel-electric drive and an electrical energy storage system is therefore a logical step on the way to zero emission local public transport using a fuel cell powered bus.

Ambitious goals: the first Citaro with diesel-electric hybrid drive

The developers have set their sights high with this prototype; it embodies highly sophisticated diesel-electric series hybrid technology, allowing emission-free operation in battery only mode.

The configuration is equally ambitious; the system will be installed in a Citaro G articulated bus, and will drive the centre and rear axles using four wheel hub motors. This configuration is currently unique to the Citaro hybrid.

Rather than providing continuous driving power, the diesel engine in the Citaro series hybrid operates as a generator to produce electrical power as and when required. Typically of a series hybrid, the engine has no direct mechanical connection to the driving axles. This means that there is more freedom for configuring an innovative interior.

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 vehicle. These motors develop a maximum output of 80kW each and are liquid-cooled. The total wheel hub motor output of 320kW is ample for an articulated bus, even under strenuous operating conditions. Two-axle drive, meanwhile, provides better traction. This means that in principle, it would be possible to control the four wheel hub motors individually – opening up interesting possibilities in terms of traction control and the use of electronic safety systems.

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 batteries have an output of 170kW and weighing 450kg, they are relatively light.

A 650V high-voltage intermediate circuit handles the power transmission between the generator, battery and wheel hub motors. And control strategies to reduce emissions have been implemented. The diesel engine is mechanically decoupled from the drivetrain and energy can be recuperated from braking. For example, the braking energy used when pulling into a bus stop or coming to a stop at traffic lights can be utilised to provide electrical power when the vehicle is stationary or moving off. This is not only virtually emission-free, but also much quieter.

A big advantage of the clever series hybrid technology used in the Citaro is downsizing. Instead of the large 12.0 litre, six inline, 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 160kW (218hp) from a displacement of 4.8 litres. This reduces the weight of the engine from approximately 1,000kg to around 450kg.

Since the diesel engine of the Citaro hybrid does not drive the vehicle directly, it is not required to produce peak outputs. Therefore, it is 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 to frequent steady-state operation, for optimal emissions performance and fuel efficiency.

Electrically powered auxiliaries

The hybrid-drive Citaro is able to dispense with a conventional automatic transmission, resulting in improved efficiency and further weight saving. 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 motors.

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, so they can be placed on the roof of the bus, for example.

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 there isn’t a rigid connection between the individual drivetrain components, the rear passenger compartment layout in urban regular service buses can be re-organised.

Despite the engine’s vertical tower design, the engine cowl can be situated low enough to be used by passengers as a luggage shelf. Furthermore, the downsizing of the drivetrain components means that the weight penalty of the Citaro hybrid bus over a conventional diesel-powered articulated bus is only around one tonne.

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 a fuel cell powered urban regular service bus of the future. For example, the hybrid 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. 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 a 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 very quiet in operation.

Series production in 2009

While the fuel cell bus is a longer term proposition, the countdown to the hybrid diesel Mercedes-Benz Citaro is already well under way. Following the unveiling of the prototype, pilot operation will begin early next year and the series production is scheduled for 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 per cent 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.

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