Overview: Mercedes-Benz B-class F-Cell

The Mercedes B-class F-Cell is an electric vehicle with a hydrogen fuel-cell. It has been in small-scale production since the end of last year, with the first batch of 200 cars delivered in the U.S. and Europe.

The on-board fuel-cell stack generates electrical power for the drive motor: the cell’s feedstock is hydrogen gas from on-board tanks and oxygen from the air. The car’s direct emissions are limited to just water vapour, though hydrogen gas is very energy-intensive to produce.

The drive components are located in the B-class’s sandwich floor, where they are well protected and do not take up much space. The car’s interior and boot space are unaffected by the unusual drivetrain.

Mercedes-Benz B-class F-Cell, showing powertrain components hidden in sandwich floor.

The strength of fuel-cell cars — assuming refuelling infrastructure is availability — is that they are virtually as flexible to use as conventionally-powered vehicles. The Mercedes has a range of around 240 miles, and refilling the gas tanks takes roughly the same amount of time as refilling an ordinary fuel tank.

The B’s cell stack is 40 per cent. smaller than that of the experimental A-class F-Cell from 2004, but it develops 30 per cent. more power and consumes 30 per cent. less fuel.

The main drive system components are:

  • Fuel cell stack
  • Lithium-ion battery
  • Three 70MPa hydrogen tanks
  • Drive motor at the front axle.

Cold-starting

The B’s fuel cell stack offers a cold-start capability down to -25°C, which is pretty good for a system that effectively stops working altogether at low temperatures — one of the fuel cell’s traditional problems. The Mercedes features a humidification system using hollow fibres that stops water freezing in the stack. At -15°C, the B-class F-Cell starts in roughly the same time as a current diesel engine — in other words, with a couple of seconds’ wait.

The control system’s operating strategy helps the fuel cell stack reach its optimum operating temperature of around 80°C as quickly as possible each time the vehicle is started.

A liquid cooling system and intelligent temperature management maintain this temperature under all operating conditions.

Operation

The hydrogen used to run the fuel cell is stored in three tanks at a pressure of 700 bar (70MPa). Each tank holds 3.7kg of hydrogen. The tanks are hermetically sealed from the outside world, preventing the loss of hydrogen into the atmosphere even if the vehicle is left to stand for long periods. The B-class F-Cell can cover up to 400km. Refilling from empty takes roughly three minutes.

The electric motor is a permanently excited synchronous unit developing a peak output of 136PS and a maximum torque of 290Nm, the latter figure being available from the first revolution.

A lithium-ion battery is used; it is cooled by the air-conditioning system circuit and has a capacity of 1.4kWh. From 2012, the company intends to fit its vehicles with lithium-ion batteries produced by its joint venture company Deutsche Accumotive GmbH.

The electric motor receives its power during a cold start from both the lithium-ion battery-pack and the fuel cell system as it ‘powers up’. Battery power is sufficient as the fuel cell warms up.

In drive mode, the energy management system constantly maintains the drive system in its ideal operating range. The lithium-ion battery comes between the fuel-cell stack and the motor, acting as a ‘buffer’ to smooth out sharp variations in the power demands.

Whenever the driver brakes or as soon as they take their foot off the accelerator, the electric motor operates as a generator to capture spare kinetic energy as we have seen elsewhere.

While manoeuvring or on short journeys, the electric drive motor uses battery power. If the battery capacity is not sufficient, the fuel cell automatically kicks in. To ensure optimum efficiency and responsiveness, the drive management system deploys power from the lithium-ion battery, the fuel cell, or a combination of the two.

Sandwich safety

The key components for the electric drive with fuel-cell stack are housed inside the B’s sandwich floor. Interior space is not affected by the unusual drivetrain.

The hydrogen tanks are mounted inside the sanwich floor between the axles, with the battery pack beneath the boot floor. The battery’s casing is a robust steel structure, and its fitment behind the rear axle line places it inside a tough part of the car. We would rate this arrangement as considerably safer than a petrol-engined car with a full tank of fuel, though some folk will remain to be convinced about the integrity of the hygrogen filler pipework if it is subjected to a substantial side-impact. In the event of a crash, safety valves close the hydrogen supply lines to the fuel cell and decouple the tanks from the other system components. If a fire leads to excessive heat, a temperature-controlled valve provides controlled venting of the tank contents — a burn-off.

The lithium-ion battery is fitted with a blow-off vent and a rupture disc. An internal electronic controller signals any malfunctions.

All high-voltage components are connected by an electric loop. In the event of a malfunction, the high-voltage system is automatically shut off.

As soon as the ignition is switched off, or in the event of a possible malfunction, the high-voltage system is actively discharged. During an accident, the high-voltage system is completely and quickly switched off.

Mercedes fuel cell vehicles can use underground car parks, multi-storey car parks and tunnels with no restrictions.

Modular system

Using the same strategy adopted for developing hybrids, Daimler has also developed a modular system for electric vehicles with batteries and fuel cells. This enables, for example, the same parts to be shared efficiently across all electric vehicles. All key components of electric vehicles are ideally suited to modularisation — the electric motor and transmission, the battery and high-voltage safety system, the high-voltage wiring and the software modules. Specific components, such as fuel-cell stacks and hydrogen tanks, can be used as standard components for entirely different vehicles. For example, the Daimler fuel cell bus is powered by two passenger car systems of the same type used in a B-class F-Cell.

The close-to-production Concept Blue Zero series also use modularity. The Concept Blue Zero cars house their key drive components in the sandwich floor in a crash-resistant configuration, like the B-class F-Cell. Based on a single vehicle architecture, the modular concept paves the way for three variants with different drive system configurations. Mercedes’ Blue Zero cars are:

  • Blue Zero E-Cell: battery-electric drive, with a range of up to 200km;
  • Blue Zero F-Cell: fuel cell vehicle with a range of over 400km;
  • Blue Zero E-Cell Plus: series hybrid, with electric drive and additional combustion engine as a range extender. Total range up to 600km, electric range 100km.

The fuel cell

Mercedes uses PEM (Polymer Electrolyte Membrane) fuel cells. This design is presently regarded as the best for vehicles, though there are many other types of fuel cell. It operates efficiently at up to around 80 degrees.

A fuel cell is a voltaic cell that converts the reaction energy of an added fuel (e.g. hydrogen) and an oxidising agent (e.g. airborne oxygen) into electrical energy. A fuel cell is not an energy storage device like a rechargeable battery, but an energy converter.

The drive system on a fuel cell vehicle is more efficient in itself than a vehicle with a combustion engine. The cell stack converts the chemical energy of the fuel (hydrogen) directly into electrical energy. Well-to-wheel efficiency is harder to calculate, though, as hydrogen is energy-intensive to manufacture, and the chemical energy of diesel (for example) is converted directly into mechanical energy.

At the heart of the PEM fuel cell is the proton exchange membrane (PEM), a plastic film that conducts protons (hydrogen ions, H+), and which separates the reagents (oxygen and hydrogen gases).

The plastic film is a few tenths of a millimetre thick and is coated with a thin layer of platinum on both sides. This platinum layer acts as a catalyst for the chemical reaction that breaks down the hydrogen into protons and electrons.

While the protons flow through the membrane to the oxygen, the electrons are prevented from getting through by their electrochemical ‘aversion’ to oxygen.

The hydrogen ions react with the oxygen to create water, which is dissipated into the atmosphere. The excess electrons on the hydrogen side and the lack of electrons on the oxygen side induce an electrical voltage. If the two poles are connected through a load, an electric current flows. This is the current that drives the electric motor.

Apart from electrical energy, the reaction in the fuel cell also generates heat. This can be used to heat the vehicle.

To achieve an adequate voltage for the drive motor, individual fuel cells are electrically connected in series to create stacks. A control unit ensures the stack is supplied with the right amounts of hydrogen from the fuel tanks and oxygen from the air.

The hydrogen is fed into the stack by way of the anode module, while the air is added through the cathode module. A humidifier module keeps the stack moist to achieve optimum operating conditions within the stack. A cooling system always maintains the fuel cell at its optimum operating temperature of around 80 degrees.

The fuel cell stack for the B-class F-Cell was developed by the Automotive Fuel Cell Cooperation, based in Vancouver. The Company was founded in 2007 with Daimler as the majority shareholder; other partners include the Ford Motor Company and Ballard Power Systems.

NuCellSys GmbH developed the ancillary units to operate the fuel-cell stack and integrate the stack into the drive system. This wholly owned subsidiary of Daimler AG is responsible for system engineering and design, component and software development as well as system validation.

Background

History of the fuel-cell at Mercedes-Benz

1994: The NECAR 1 was launched. It was the world’s first vehicle fitted with an electric drive with fuel-cells. Since then, Mercedes-Benz has made enormous progress in developing this technology: local zero-emission fuel-cell vehicles have performed superbly in test fleets. In 2009, Mercedes-Benz took the decisive step towards mass production of the electric drive with fuel-cells as it started series production of the B-class F-Cell.

1999: NECAR 4 managed for the first time to house a 95PS electric drive with fuel-cells including the tank entirely in the sandwich floor of the A-class. The research vehicle was powered by compressed hydrogen and had a range of 200km.

2003: The first of 30 fuel-cell urban buses based on the Daimler Citaro went into regular service in Madrid and Stuttgart. Other European cities, as well as Perth (Australia) and Beijing, were to follow. By 2006 all the vehicles had clocked up over two million kilometres in around 135,000 operating hours.

2004: Mercedes-Benz handed over ten fuel cell cars to customers in Berlin. The A-class F-Cell filled up with hydrogen at the public filling station run by the Clean Energy Partnership (CEP).

2009: Mercedes-Benz unveiled its near-series Concept Blue Zero study, a modular drive concept for electric vehicles with a battery-electric drive system, fuel cells, or an electric motor and additional combustion engine as a range extender.

2009: Mercedes-Benz produced the first series production fuel cell vehicles with the small production series of the B-class F-Cell. Thanks to 700-bar hydrogen tanks — twice the pressure traditionally used — the range of the 136PS vehicle was extended to around 400km.

Filling up

Joint initiatives need to be taken to promote refuelling infrastructure. Daimler AG reached a significant milestone in terms of a sufficient supply of hydrogen in September 2009 by joining forces with EnBW, Linde, OMV, Shell, Total, Vattenfall and NOW GmbH (Nationale Organisation Wasserstoff- und Brennstoffzellentechnologie).

In a memorandum of understanding, the partners agreed a plan to set up a filling station network in two phases. Phase I will examine various options for setting up a nationwide hydrogen filling station network, as well as developing a joint, economically viable business concept. The aim is also to develop concepts for setting up new, additional hydrogen filling stations by 2011. If the business continues to perform well, the partners will then implement a suitable action plan in Phase II. This forms the basis for the nationwide roll-out of a hydrogen filling station network. This initiative receives funding as part of the German government’s Recovery Package II. Daimler AG, Ford Motor Company, General Motors Corporation/Opel, Honda Motor Co. Ltd., Hyundai Motor Company, Kia Motors Corporation, the joint venture Renault S.A. and Nissan Motor Co. Ltd. and Toyota Motor Corporation agreed previously in a letter of understanding to commercialise fuel cell vehicles from 2015 onwards.

Daimler AG has been committed to promoting this important issue for many years, reflected in its participation in joint projects, cooperation with government agencies, power utilities and oil companies in places such as Hamburg, Stuttgart and California. The City of Hamburg has become the centre for local zero-emission mobility on the basis of the electric drive with fuel cells. In the spring, the City teamed up with Daimler, Shell, Total and Vattenfall Europe to launch a major project for the use of passenger cars and buses using fuel cell technology. From the end of 2010 the first of a total of 10 latest-generation fuel cell buses will be on the roads in Hamburg. They will be joined by 20 B-class F-Cell vehicles. By 2014 a total of four hydrogen filling stations should be up and running. The aim of the joint initiative is to promote the development of a zero-emission vehicle fleet and the associated infrastructure. In cooperation with Linde AG and Daimler AG, OMV turned its Stuttgart Airport site into the first public hydrogen filling station in Baden-Württemberg in June 2009. In the USA, Daimler AG is promoting fuel cell technology in the car as part of the California Fuel Cell Partnership. Hydrogen as an energy source.

A decisive advantage of electric vehicles with a fuel cell drive system is that they generate zero emissions locally. But how much CO2 is generated during the production of hydrogen depends on the form of energy or the process used. The bulk of the hydrogen required today is generated by means of a steam reforming process. Natural gas and water vapour are processed in the steam reformer at high temperatures to produce hydrogen, carbon monoxide and carbon dioxide in the first instance. The following step involves converting the carbon monoxide component into carbon dioxide and hydrogen through the addition of steam. The highly efficient fuel cell means the overall CO2 emissions of 20 to 30 per cent. are already below those for ultra-modern diesel vehicles.

Furthermore, hydrogen can also be easily produced from renewable energy sources, such as wind and solar power. This allows hydrogen to be produced by means of electrolysis, as well as through the use of biomass. The synthesis gas (essentially carbon monoxide and hydrogen) produced in the initial process stage is converted into carbon dioxide and hydrogen. As the proportion of renewable energy is increased, so the process moves closer to a CO2-neutral energy chain.

Mercedes B-class F-Cell
Body type 5 door hatch
Occupants 5
Battery: type
— capacity
— output
— cooling
Li-ion
1.4kWh
35kW
liquid
Cold start limit -25°C
Motor PS 136
Motor Nm 290
Maximum speed 106
0-100km/h 11.3
CO2 equivalent
(indirect) as MPG
(l/100km)
85.6
(3.3)
Transmission (1)
Driven wheels Front
Fuel: load mass
— tank pressure
11.1kg
70MPa
Kerb mass * 1815
PS/t 75
Nm/t 159
Length 4272
Width 1775
Height 1618
Wheelbase 2776
Track: front
Track: rear
1552
1547
Cd 0.31
* U.S. homologation.
(1) Single-speed reduction gear.
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