The result of the discussion on the range of all-electric city buses: The Mercedes Benz eCitaro Range Extender with fuel cell is on its way

• Next milestone on the road map of the eCitaro for 2023,
• eCitaro Range Extender: Consistent, practical and cost-conscious
• The fuel cell: Highly efficient, compact and durable
• Modular hydrogen tanks, light and safe
• High-performance batteries of the latest generation
• Long range, high number of passenger seats
• New thermal management using the waste heat from the fuel cell

A range of around 400 kilometres for average requirements without recharging for an all-electric solo bus in urban traffic, around 350 kilometres with the corresponding articulated bus – the Mercedes Benz eCitaro Range Extender makes it possible. The powerful and locally emission-free low-floor bus can cover even demanding routes, and almost 100 percent of all transport operators' requirements for maximum range. Thus it can to accelerate the transport transition towards e-mobility with city buses in cities. In addition, the eCitaro Range Extender once again demonstrates the fascinating possibilities of the Citaro family with tailor-made solutions. The first deliveries are planned for 2023.

eCitaro Range Extender: Consistent, practical and cost-conscious

Vehicles driven by fuel cells are considered to be the answer, and all-electric city buses with a very long range are in demand. After all, the energy for the electric motors is produced on board. But not all drive systems are the same: They can be configured in very different ways.

A buffer battery is always necessary as fuel cells are continuously working on electricity production, in contrast to the typical behaviour of a city bus with its continuous changes in acceleration and braking action. Individual variants range from a very powerful fuel cell with a compact battery to a compact fuel cell with high-performance batteries.

Daimler Buses has chosen the second route for good reasons. The eCitaro Range Extender is systematically designed for operation as a city bus in a practical and cost-conscious manner. Electricity is a significantly cheaper fuel than hydrogen – which is why it is a matter of a battery-electric low-floor city bus, and the hydrogen-powered fuel cell therefore serves only to extend its range and not as the main onboard energy source.

For the fuel cell, Daimler Buses relies on a tried-and-tested unit, but in its latest version. Finally, hydrogen is used in gaseous form and at a pressure of 350 bar. This means that there is no need for a high degree of compaction and correspondingly high expenditure at filling stations.

The fuel cell: Highly efficient, compact and durable

The fuel cell of the eCitaro Range Extender comes from Toyota and is already in its second generation. It is a heavy-duty unit with a maximum output of 60 kW. In the eCitaro Range Extender, it is operated very efficiently at around 30 kW at its best point. It operates in a voltage range of from 400 to 750 volts.

The fuel cell used has numerous advantages. Its flat and compact design makes it ideal for mounting on the roof of the eCitaro. It is highly efficient, which results in a comparatively low consumption of hydrogen for power generation. A voltage converter has already been integrated. The very long service life of around 40,000 hours in use as a range extender is also impressive. Even then it does not suddenly stop running. Instead, its efficiency is slowly decreases, and a gradual aging of the components can be expected.

The fuel cell module, which weighs around 240 kilograms, is mounted on the roof of the solo bus in a position just behind the front axle. In the articulated bus it is positioned at the front on the roof of the rear section.

How the fuel cell drive works

The fuel cell generates electrical current and heat from the reaction of hydrogen and oxygen – also known as “cold combustion”. The individual cell consists of a polymer electrolyte membrane (PEM), a negative and a positive electrode, as well as two separators. Fuel cells are galvanic cells.

The hydrogen passes from the tanks on the roof via pipes to the negative electrode where it reacts with oxygen. Electrons are released and migrate to the positive electrode. This results in an electrical current. The hydrogen atoms are transformed into hydrogen ions by the release of the electrons. They flow through the polymer electrolyte membrane to the negative electrode. At this stage, water is the only emission resulting by a chemical reaction from oxygen, hydrogen ions and electrons. Several hundred fuel cells are required to power an eCitaro Range Extender. Together they form a fuel cell unit, the so-called fuel cell stack.

Since the drive system of the eCitaro Range Extender is based on a very powerful battery, strictly speaking it is an all-electric hybrid bus that combines batteries and fuel cells as a power source.

Modular hydrogen tanks, light and safe

The hydrogen tanks for supplying the fuel cell of the eCitaro Range Extender each hold five kilograms. With an inner container made of plastic and an outer sheath made of carbon fibre, they correspond to the so called Type 4, and are both light and highly resilient.

The arrangement transverse to the direction of travel enables easy access to the valves. Safety is key: Each individual vessel has its own pressure and temperature sensor. The tanks already comply with the UN ECE R 134 standard amongst others for the prevention of fire hazards, which will only be binding from 2024.

The tank system has a modular design. The solobus can be equipped with five or six tanks with a total of 25 or 30 kilograms of hydrogen. The articulated bus can be equipped with six or seven tanks with 30 or 35 kilograms of hydrogen, as desired. They are mounted on the roof at the height of the front axle and the front overhang. Refuelling is carried out consistently in the direction of travel on the right above the second axle. Under optimal conditions, for example, the refueling time in the solobus is around ten minutes.

High-performance batteries of the latest generation

The eCitaro Range Extender uses the latest generation of high-performance batteries. These NMC 3 lithium-ion batteries (NMC = nickel-manganese-cobalt) will be introduced in the eCitaro from the end of 2022, and have a very high energy content and are very durable.

Similar to the hydrogen tanks, the batteries are also scalable. Two or three battery packs with a combined capacity of 196 or 294 kWh are used for the solo bus. In the case of the articulated bus, there are either three or four battery packs with a maximum energy capacity of 392 kWh. These battery packs alone ensure a considerable range.

As both the battery capacity and the quantity of hydrogen carried can be selected, each transport company will receive the eCitaro Range Extender tailored to its individual usage profile.

As a result of the fuel cell as a range extender, intermediate charging of the batteries en route is neither necessary nor envisaged. Charging takes place consistently via a plug at the depot, with a maximum charging capacity of 150 kW. As usual with the eCitaro, there are three plug positions to choose from, at the left and right above the front axle and at the rear. Up to two plug positions are possible per vehicle for flexible positioning in the depot or vehicle hall.

Long range, high number of passenger seats

Depending on the configuration of batteries and hydrogen tanks, the eCitaro will achieve a range of around 400 kilometres as a solo bus with average requirements without intermediate charging or refuelling in the meantime. The range of the articulated bus will be around 350 kilometres. These outstanding values completely cover the requirements of almost all transport operators. With this step, a fuel-powered auxiliary heating also becomes superfluous.

In addition, the precisely optimised weight distribution of batteries, fuel cells and hydrogen tanks ensures a high number of passenger seats. For instance, two models can be used: Daimler Buses calculates the two-door solo bus with two battery packs and six hydrogen tanks at around 88 passenger seats. In the case of a three-door articulated bus with one axle in operation, three battery packs and seven hydrogen tanks, the figure will be around 128 passenger seats.

New thermal management uses the waste heat from the fuel cell

In the integration of the fuel cell drive, the Mercedes-Benz Citaro has once again demonstrated its versatile concept. Although extensive adaptation measures were necessary, no complete redesign has been required. Transport companies therefore benefit from the familiar passenger compartment and a host of identical components and parts from the Citaro, eCitaro and eCitaro Range Extender.

The control units, for example, are new to it. The heat generated by the fuel cell also leads to a completely new thermal management system – its waste heat can be used to heat the passenger compartment. That is why Daimler Buses is now using the compact air-conditioning system with the refrigerant R134a, also with a heat pump. Together with the waste heat from the fuel cell, it can achieve higher efficiency at low temperatures than the eCitaro's CO₂ air conditioning system. The waste heat from the fuel cell can also be used to temper the batteries.

The heat-emitting components are also interconnected in the Citaro Range Extender. The temperature in the passenger compartment and, separately, the driver’s workplace, is controlled automatically. As with the eCitaro, airflow regulation and the heating energy used also depend on the number of passengers.

Tried and tested low-floor drive axle

As usual, power is transferred to the ZF AVE 130 low-floor drop centre axle via motors near the wheel hub. They generate a maximum output of 141 kW per wheel and achieve a torque of 494 Nm. Thanks to a fixed ratio, this means a maximum torque of 11,000 Nm per wheel. On articulated buses, the center and rear axles are driven as standard, while the driven axle of the rear section is also sufficient for easy topography.

The key advantages of the Mercedes‑Benz eCitaro Range Extender

The new, locally emission-free Mercedes-Benz eCitaro Range Extender achieves long ranges. At the same time it offers a surprisingly high number of passenger seats thanks to its weight distribution. Its concept also focuses on the total cost of ownership: The focus on battery-electric drive, the ingenious design of the fuel cell drive, and durable components keep costs comparatively low.