Daimler : On course for the future: the drive systems of tomorrow from Daimler Buses

The era of testing future drive systems is drawing to a close. Daimler Buses, Europe's market leader in urban regular-service buses and the manufacturer of global best-seller the Mercedes-Benz Citaro, is the industry's technology trailblazer and is now defining the future drive system for such urban buses. Equal attention is being given to further enhancing and perfecting the diesel engine and to electric power with future models of the Citaro E-CELL and Citaro F-CELL due to be built on a modular platform through to the year 2020.

Daimler Buses has thoroughly checked all drive systems

Selecting the drive system for the urban bus of the future is like perusing the menu in a good restaurant. In both cases, there are different possibilities to choose from at a range of prices and to suit different tastes: combustion engines such as diesel and gas engines, electric drives, hybrid solutions, fuel cells to generate power, different ways of storing energy and a host of fuels in the form of fluids, gases and electricity.

There is virtually no drive system that Daimler Buses has not subjected to thorough checking, development and testing in urban buses in close cooperation with its customers over the years and decades. Diesel and natural-gas drive, electric drive, fuel cells, various hybrid drives - all have been thoroughly considered and studied in every conceivable combination. Likewise, there is no fuel which has not been carefully tested for usefulness.

As yet the electric drive is not able to replace the diesel engine

However varied the transport operator's requirements may be, the demands placed on an urban bus are always considerable: in use more than 300 days a year, these vehicles need to handle over 200 km every day in tough city traffic with extremely high reliability. Typically they will be going 16 hours a day and stopping and starting around a 1000 times.

In view of these requirements, it is clear that an electric drive is not yet sufficiently mature in technological and economical terms to replace the diesel engine in urban buses. Range, passenger capacity and cost are some way off the levels offered by diesel-powered buses. Air conditioning presents an additional challenge because it consumes a lot of energy.

The future: tailored drive systems for transport operators

On the basis of extensive experience it is evident that there is no single ideal drive system of the future for urban buses. As from a carefully composed menu, transport operators are able to put together the right drive system for them from the various individual components that Mercedes-Benz offers.

Just like the best-selling Mercedes-Benz Citaro urban bus with its platform strategy for low-floor, low-entry, rigid, articulated or large-capacity buses fulfils a diverse range of needs worldwide, the drive system must be designed for different customer requirements too.

The aim of this platform strategy is to deliver maximum operational suitability and the ambition to lead on both cost and consideration for the environment. Daimler Buses' drive strategy for tomorrow's urban buses is based on three pillars: ongoing development of the diesel engine, the Citaro E-CELL with a fully electric drive system and the Citaro F-CELL as a hybrid bus running on fuel cells.

Diesel drive system to become even more economical and thus cleaner

The leap from emissions standard Euro V to Euro VI demonstrates that, even more than 90 years since it was first used in commercial vehicles, development of the diesel drive system is by no means over yet. On the Mercedes-Benz Citaro, Euro VI is like trying to square the circle: cut exhaust gas emissions significantly while at the same time lowering fuel consumption considerably along with achieving a reduction in CO2 emissions.

The first proof came in the 2012 Record Run when consumption was down by 8.5 percent for the Citaro Euro VI by comparison with its predecessor. With some 4500 Citaro Euro VI buses out on the roads, a large number of transport operators have corroborated in day-to-day use that Euro VI is significantly more economical.

Progress will continue in this regard with the fuel consumption of diesel-powered urban buses set to fall further in the years ahead. For the Citaro with diesel drive system, Daimler Buses is aiming for a further drop in consumption by a low double-digit percentage amount over the next few years.

Promoting the hybrid solution with a due sense of proportion

The hybrid drive is one of various measures on the way to achieving this. In future, however, this system will not be an intermediate step prior to going electric but will rather support the diesel engine in attaining maximum efficiency. The pressure to deliver maximum economy does call for a sense of proportion. There is an argument against complex diesel-electric strong hybrid drive systems with plug-in technology which, although resulting in advantages in terms of consumption, involve considerable extra costs which can only be amortised over a very long timeframe.

That is why Daimler Buses is pursuing the strategy of a more straightforward and cost-optimised hybrid concept in the guise of the compact hybrid. Although this will not be a zero-emission vehicle, it will yield significant consumption savings compared to the Citaro Euro VI diesel bus for a low additional cost, and will therefore pay for itself for customers within a few years.

Gas drive systems are a logical complement to the diesel engine as documented by the new Mercedes-Benz M 936 G engine. With a combustion engine powered by natural gas, urban buses are quieter and lower on emissions than when fitted with a diesel engine. When run on biogas, gas engines even approach being CO2-neutral.

Citaro E-CELL and F-CELL with modular platform

For Daimler Buses, the consequence of continuing to develop the diesel engine successfully will be that the diesel-electric strong hybrid drive can be skipped as a transitional technology. In parallel to further optimising the diesel engine, the next step is therefore fully electric urban buses, in this case the Citaro E-CELL powered by batteries and the Citaro F-CELL with its fuel cell drive system.

For both these models, Daimler Buses is presently developing a modular electric mobility platform. It meets all the requirements of an electric drive system on an urban bus. This electric mobility platform includes charging systems with plug-in technology as well as current collector systems on the roof of the vehicle, different sets of batteries and fuel cell system configurations. For many of the associated technologies, Daimler Buses is able to draw on components and experience from within the Group.

Emission-free driving only at reasonable additional cost

The objective behind the e-platform is to create an electric urban bus that can be customised based on largely standardised components. This will allow various market demands to be met while at the same time attaining maximum economy. A modular approach not only reduces direct vehicle costs but also the amount spent on repairs and maintenance. This drastically reduces the gap between the TCO (Total Cost of Ownership) of urban buses powered by diesel and those with electric drives. In this way, the TCO premium for emission-free driving becomes reasonable.

Next steps: Citaro E-CELL followed by the F-CELL, which is fit for series production

The next step towards bus electrification is the battery-powered Citaro E-CELL. It will be followed immediately by the fourth generation of the fuel cell bus, the Citaro F-CELL, as a series production vehicle.

As the pioneer of this technology - Mercedes-Benz unveiled the world's first fuel cell bus back in 1997 in the form of the Nebus - Daimler Buses is clearly the innovation leader for this powertrain system. The fuel cell drive system has undergone rapid development from being launched in the Nebus to series production in around just 20 years. Daimler Buses is not alone in this endeavour: last autumn, leading European bus manufacturers signed a joint undertaking to develop and roll out buses with the fuel cell drive system. In this context, several hundred fuel cell buses are expected to be produced by 2020. By that time the necessary infrastructure for hydrogen power should have been extended significantly. Development is thus proceeding in parallel with the EU's commercialisation strategy.

The upcoming Citaro F-CELL incorporates all the experience gained from the present third generation of fuel cell buses, which altogether comprises 27 buses. Introduced in 2011, the majority of them are used by customers on regular-service routes. This important testing and validation phase for the fuel cell drive system is therefore now complete.

The fourth generation of fuel cell bus ready for series production will include rigid buses as well as articulated and large-capacity vehicles. Hydrogen consumption for the rigid vehicles will decrease to less than 10 kg/100 km. There is another important point that will benefit transport operators: availability will increase to over 90 percent.

Both the Citaro E-CELL and the F-CELL are to enter series production by 2020. In 2030, electric drives in urban buses will dominate. They will not, however, replace the diesel drive system entirely.

Daimler Buses: a cross-disciplinary approach

The clearly defined steps on the way to the drive system of the future are based on Daimler Buses' unique experience with virtually every conceivable related technology. Developers have done pioneering work in almost all these areas. No other bus manufacturer has more years of experience or a broader knowledge of all manner of drive systems for urban buses.

Daimler Buses benefits from a unique constellation in this regard: developers of buses, trucks and passenger cars, as well as the central Group Research division, all work closely together. As well as ensuring a lively exchange of experience, this also means that suitable components are used alternately. Over the decades, Mercedes-Benz in particular has acquired a unique understanding of drive systems.

1969: launch of the first hybrid bus, the Mercedes-Benz OE 302

Apart from the various experiments in propulsion conducted in the early days of the motor car, for Mercedes-Benz the era of alternative drive systems and alternative fuels started in 1969 with the OE 302, the world's first hybrid bus. With lead batteries weighing 3.5 metric tonnes in the underbody, it had a range of around 55 km in electric mode. The direct-current drive motor generated a peak output of 150 kW (204 hp). An additional compact diesel engine delivering an output of 48 kW (65 hp) acted as a supporting generator on the outskirts of towns.

Two years later, the Mercedes-Benz OG 305 urban regular-service bus went in a completely different direction with a natural-gas drive. Liquefied natural gas (LNG) was used as the fuel here for a modified six-cylinder in-line engine of type M 407 hG. Its output amounted to 127 kW (172 hp) while maximum torque was 677 Nm. Later Mercedes-Benz converted the bus to run on liquefied petroleum gas (LPG), thereby gaining experience with this fuel at an early juncture. In the mid-1980s Mercedes-Benz tested two LPG buses and two compressed natural gas (CNG) buses in Perth/Australia.

First minibus with hydrogen propulsion back in 1977

In the meantime Mercedes-Benz developed the first hydrogen-powered minibus. The Bremen van unveiled in 1977 with a petrol engine and output of 44 kW (60 hp) provided a suitable platform. For the purposes of testing, five different storage batteries were fitted, each made of different materials. Two years on, the brand presented an enhanced model with a more powerful engine and three storage batteries weighing a total of 400 kg. The hydrogen reserves of 5.5 kg were the equivalent of 20.5 litres of petrol.

In 1979 the OE 302 was succeeded by the OE 305 urban regular-service bus whose batteries were accommodated in two compartments running width-wise across the vehicle between the axles. The weight of the batteries ranged from 2.0 t to 3.5 t. In battery-only operation, they had a range of between 50 and 75 km. The drive 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 with an output of 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 drive motor could be powered either from overhead lines or by batteries. A further version of the duo bus had no batteries and was powered by either overhead lines or a diesel engine. Field trials of the overhead/battery duo bus began in Esslingen, Germany, in 1975. From 1979, 25 of the O 305 buses were operating with various types of hybrid drive in three German cities (Stuttgart, Esslingen and Wesel).

Towards the end of the 1970s, Mercedes-Benz also tested the gyro drive in the O 305 urban regular-service bus. Here a flywheel stored braking energy along with surplus power from the diesel engine. The diesel engine and flywheel worked together to power the vehicle. Over short distances, the flywheel was also able to drive the vehicle by itself, free of any noise or emissions.

Alternative fuels continued to be developed as well. In 1981 Mercedes-Benz presented an urban regular-service bus powered by methanol. It was the first methanol bus in the world and entered service in Auckland/New Zealand. Other methanol buses became operational shortly afterwards in Berlin/Germany and Pretoria/South Africa.

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 drive 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 duo bus carved out an international career early on

The O 305 GTD duo bus and its successor, the O 405 GTD, even carved out an international career for themselves. For example, over 200 units were supplied by Mercedes-Benz to the Ecuadorian capital Quito. Approximately 50 units of the O 405 GTD went into service in Europe.

By the mid-1980s Brazil had a dense network of ethanol filling stations. This alcohol fuel is made from sugar cane. In the early 1990s the Dutch city of Breda went on to test ethanol-fuelled vehicles successfully in Europe with three Mercedes-Benz O 408.

Development of gas-powered systems had made progress by this time too. In 1994 the first O 405 N 2 low-floor rigid bus entered into series production followed by articulated and rural-service buses. Engines of type M 447 hG were based on the corresponding six-cylinder diesel engines and were already being equipped with a three-way catalytic converter to treat exhaust emissions. As well as fully equipped buses for operation inside Europe, Mercedes-Benz supplied chassis to Australia.

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. The O 405 NÜH (a German abbreviation standing for low-floor hybrid rural-service bus) was the first hybrid bus with wheel hub motors, a diesel engine and traction batteries. The powerful sodium nickel chloride batteries weighed 800 kg, enough for a range of 10 km in all-electric mode. 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 was based on a diesel engine which powered a generator, which in turn fed an electric drive motor. Everything was fitted in the form of a 'power pack' in the rear of this compact low-floor urban regular-service bus. Again, no battery was used to store energy.

Around the turn of the millennium gas-powered buses were due for a generation change. Now the Citaro provided the platform. Supercharged engines of type M 447 hLAG were used, featuring improved efficiency and considerably higher torque. On this basis, Mercedes-Benz delivered strikingly designed rigid and articulated buses with individual designs to Hanover for Expo 2000.

Experiences in North America and Asia

Daimler also acquired plenty of experience with hybrid buses internationally. The Orion VII HybriDrive urban regular-service bus made by former North American subsidiary Orion became the global market leader for hybrid buses at that time, moving more than 3000 vehicles as of 2003. These had a serial hybrid drive: the diesel engine is active all the time and transfers its power to a generator which is then available for the electric drive. Orion used a conventional drive axle with differential. Transversely mounted in the rear of the vehicle, the diesel engine output 191 kW (260 hp) from a displacement of 5.9 litres. The electric drive could briefly output 235 kW (320 hp). Orion was able to dispense with a transmission. As an alternative, Orion initially adopted lead batteries and then from 2008 also lithium-ion batteries to supply energy.

Likewise active in the field of hybrid-drive urban buses during that same period was Daimler's Japanese commercial vehicle subsidiary Fuso, with the low-floor Aero-Star model. The technical structure of the serial drive system was similar to the North American option: a 177 kW (241 hp) diesel engine, transversely mounted in the rear of the vehicle, drove a generator. This powered two electric drive motors, which together produced an output of 134 kW (182 hp) and supplied their torque to a portal axle via a summation gearbox. Lithium-ion batteries were used for energy storage. Vehicle testing was already underway in Japan at the time of the 2002 Football World Cup.

Direction future: powered by fuel cell

Unveiled in 1997, New Electric Bus (Nebus) was the name of an urban regular-service bus based on the Mercedes-Benz O 405 with a revolutionary drive system. In this case, fuel cells generated electricity from stored hydrogen to drive two wheel hub motors. The bus was almost noiseless to drive and completely emission-free: all that came out of the exhaust was steam. Thanks to experience acquired with natural gas-powered buses, there were now safe high-pressure tanks available. When filled up with 21 kg the bus could drive 250 km. Successful trials on regular-service routes followed in seven cities in Europe, the Americas and Australia.

Five years later Mercedes-Benz presented the next generation of fuel cell bus. It had no wheel hub motors and instead power was transmitted by an automatic transmission with torque converter. Newly designed containers on the roof stored 43 kg of hydrogen. In ten European cities, 30 Citaro fuel cell buses took part in a major on-road trial in 2003 as part of the EU-funded CUTE (Clean Urban Transport for Europe) project. Perth in Australia and Beijing/China subsequently followed suit in connection with the CUTE project.

Meanwhile the development of hybrid buses did not stand still - quite the opposite. In 2007 Mercedes-Benz presented the Citaro G BlueTec Hybrid articulated bus. This high-tech hybrid bus was delivered from 2010, and showed what could be achieved: with its serial hybrid drive the diesel engine served as a generator and was decoupled from the electric drive. A compact four-cylinder diesel engine with a displacement of only 4.8 litres and an output of 160 kW (218 hp) generated the electricity that was then stored in lithium-ion batteries weighing just 350 kg on the roof. They powered four electric wheel hub motors, each with an output of 80 kW. Stopping, standing and moving off from bus stops were emission-free. In all-electric mode the articulated bus could actually cover some 10 km quietly and without exhaust fumes.

Hybrid drive and fuel cell combined

Just two years on from the hybrid bus, the Citaro FuelCELL Hybrid celebrated its world première at the UITP congress in Vienna in 2009. As a hybrid with fuel cell, it combined two alternative drive systems simultaneously. This represented an enormous advance: a drastic reduction in hydrogen consumption by one half, an increase in fuel cell durability to six years, minimal maintenance requirements.

On-board the bus was 35 kg of hydrogen plus lithium-ion batteries with a capacity of 27 kWh and two fuel cell stacks. It was powered by two electric wheel hub motors. The Citaro FuelCELL Hybrid - today's urban regular-service bus with the drive system of tomorrow.