Long range without time-consuming opportunity charging: Mercedes-Benz eCitaro Range Extender with fuel cell

Sep 18, 2022
  • Next milestone on the road map of the eCitaro for 2023,
  • eCitaro Range Extender: consistent, practical and cost-conscious
  • 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 kilometers for average requirements without recharging for an all-electric solo bus in urban traffic, around 350 kilometers with the corresponding articulated bus – the Mercedes-Benz eCitaro Range Extender makes it possible. The powerful and locally zero-emissions, 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 vehicle's premiere and the first deliveries are planned for 2023.

eCitaro Range Extender: Consistent, practical and cost-conscious

Vehicle drivetrains with 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 generating electricity, 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 gas stations.

Fuel cell: Highly efficient, compact and durable

The fuel cell of the eCitaro Range Extender comes from Toyota and is already part of the 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 is already 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. On 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 through 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. There water is the only emission resulting from a chemical reaction between 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 hold five kilograms each. Having an inner container made of plastic and an outer sheath made of carbon fiber, they belong to Type°4, and are both lightweight 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 for fire hazard prevention - and others - which will only be binding from 2024.

The tank system has a modular design. The solo bus 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 solo bus 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, they boast 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 in 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 safeguard 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.

Owing to the fuel cell serving as a range extender, opportunity battery charging en route is neither necessary nor planned. Charging exclusively takes place 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 achieves a range of around 400 kilometers as a solo bus under average requirements without opportunity charging or refuelling. The range of the articulated bus will be around 350 kilometers. 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 optimization of the weight distribution of batteries, fuel cells and hydrogen tanks, down to the last detail, safeguards a high number of passenger seats. Two models can be used by way example: Daimler Buses calculates the two-door solo bus with two battery packs and six hydrogen tanks at around 88°passenger seats. For 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

Through its fuel cell drive integration, once again the Mercedes‑Benz Citaro has 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. 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 R134a refrigerant, 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 of the passenger compartment and, separately, the driver cockpit, is controlled automatically. As with the eCitaro, airflow regulation and the heating energy used also depend on the number of passengers.

Proven low-floor drive axle

The power is transferred as usual to the ZF°AVE°130 low-floor portal axle with motors near the wheel hub. They generate a maximum output of 125°kW per wheel and achieve a torque of 485 Nm. Thanks to a fixed ratio, this means a maximum torque of 11,000°Nm per wheel. In articulated buses, the centre and rear axles are driven as standard, while the driven axle of the rear section is also sufficient for light terrain.

Key advantages of the Mercedes-Benz eCitaro Range Extender

The new, locally zero-emissions Mercedes-Benz eCitaro Range Extender achieves long ranges. At the same time it offers a surprisingly high number of passenger seats due 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.