Engineering:Dual-mode bus

Boston Neoplan DMA-460LF dual-mode bus, operating in diesel mode (with its trolley poles lowered)

Esslingen dual-mode bus of the type Mercedes-Benz O 405 GTD, also operating in diesel mode.

A dual-mode bus, also known as a duobus or bimodal bus, is a bus that can operate using two independent power sources. Most commonly, this refers to a vehicle that combines the features of an electric trolleybus, drawing power from overhead lines, and a conventional bus, powered by a diesel engine.

The term is primarily used for buses with two full-fledged, powerful propulsion systems, designed for regular service on routes that are only partially equipped with overhead wires. This distinguishes them from trolleybuses equipped with a small auxiliary power unit (APU), which is only intended for short-distance, low-speed maneuvers such as bypassing obstacles or navigating depots. A separate term, IMC Electric bus (for In-Motion Charging), is used for modern vehicles that combine overhead wire operation with a large battery pack, allowing for significant wireless travel without a fossil fuel engine.

Characteristics

Advantages

The primary advantage of a dual-mode bus is its flexibility. It can operate as a zero-emission electric vehicle in city centers or on heavily used corridors where overhead lines are installed, offering benefits like high acceleration, low noise, and no local air pollution. On suburban sections of the route or in areas where installing overhead lines is impractical or too expensive, it can switch to its diesel engine and operate with the same range and flexibility as a standard bus.

This capability allows for one-seat rides on routes that transition from wired to unwired sections, eliminating the need for passengers to transfer. It also provides operational redundancy; the bus can continue to run during power outages or when overhead lines are damaged by switching to its diesel engine. Furthermore, routes can be extended into new areas before the wire infrastructure is built.

Disadvantages

Despite their flexibility, dual-mode buses have several drawbacks that have limited their widespread adoption. The inclusion of two complete powertrains (an electric motor and control systems, plus a diesel engine, fuel tank, and transmission) significantly increases the vehicle's weight. For example, the electrical equipment on a Mercedes-Benz O 405 GTD added six tons to its weight. This extra weight leads to higher energy consumption in both modes of operation.

Dual-mode buses are also more expensive to purchase and maintain than either a standard trolleybus or a diesel bus. The complexity of the dual systems can lead to lower reliability and higher maintenance costs. The space required for the additional equipment can also reduce passenger capacity, impacting the vehicle's economic efficiency. Finally, the process of switching between power modes requires the bus to stop, which can add to overall journey times.

History and decline

Early precursors to the modern dual-mode bus concept include the "All-Service Vehicle" in New Jersey and dual-power vehicles in several European cities. However, the term "Duo-Bus" was first popularized in Esslingen am Neckar, Germany, initially for a battery-trolleybus hybrid in the 1970s. When that proved unsuccessful, Esslingen introduced a diesel-trolleybus model in 1979, which became the template for the modern dual-mode bus.

The concept saw peak interest in the 1980s and 1990s, with several cities in Europe and North America adopting fleets. However, due to the operational challenges and high costs, many of these systems were short-lived. A significant number of dual-mode buses were prematurely retired, converted into either diesel-only buses or electric-only trolleybuses, or sold to other operators who did not use their dual-mode capability.

Since the 2000s, advances in battery technology have led to the rise of in-motion charging (IMC) electric buses. These vehicles offer the same operational flexibility as diesel dual-mode buses but without the emissions, weight, and maintenance complexity of a combustion engine, effectively making the classic diesel duobus concept largely obsolete. As of 2018, only two cities, Fribourg, Switzerland, and Boston, USA, were still operating diesel dual-mode bus systems, and Boston has since retired its fleet.

List of models

Excluding early precursors, a total of 438 series-production dual-mode buses have been built. The vast majority of these were articulated vehicles.

Production models of dual-mode buses

Manufacturer	Type	Production years	Electrical equipment	Units built	Primary operators
Breda	ADPB 350	1988–1991	AEG / Westinghouse	236	King County Metro (Seattle)
Renault	PER 180 H	1982–1984	(various)	64	Nancy, Saint-Étienne, Grenoble (France)
Daimler-Benz	Mercedes-Benz O 405 GTD	1986–1995	AEG	47	Esslingen, Essen (Germany); Bergen (Norway), and others
Neoplan	AN460LF	2004–2006	Škoda	32	Massachusetts Bay Transportation Authority (Boston)
Neoplan	N 6121	1998	Vossloh Kiepe	27	TL (Lausanne)
Volvo / Hess	B10M	1987–1989	BBC-Sécheron / ABB	12	TPF (Fribourg)
MAN / Hess	NGT 204 F	2004	Kiepe	9	TPF (Fribourg)
Daimler-Benz	O 305 GTD	1983	AEG	4	Esslingen, Essen (Germany)
Daimler-Benz	O 305 D/E (rigid)	1979	Bosch / Dornier	2	SVE (Esslingen)
Daimler-Benz	O 305 G D/E (articulated)	1979	Bosch / Dornier	1	SVE (Esslingen)
MAN	SG 240 H Duo	1983	Kiepe	1	Essener Verkehrs-AG (Essen)
MAN / Voith	SL 200 (prototype)	1984	BBC-Sécheron	1	Demonstrator vehicle
Daimler-Benz	O 405 GNTD	1996	Kiepe / ZF	1	Verkehrsbetriebe Zürich (Zürich)
Van Hool	AG300TD	1993	Kiepe	1	Connexxion (Arnhem)

Examples of use

• Seattle, Washington, USA: Operated 236 Breda ADPB 350 buses in its Downtown Seattle Transit Tunnel from 1990 to 2004. The buses ran on electricity in the tunnel and switched to diesel power for surface street routes. The fleet was retired and replaced by hybrid electric buses.
• Boston, Massachusetts, USA: Used 32 Neoplan AN460LF buses on the Waterfront portion of its Silver Line from 2005 to 2023. Electric power was used in the tunnel under the Fort Point Channel, while diesel power was used for surface streets and the Ted Williams Tunnel to Logan Airport.
• Esslingen am Neckar, Germany: One of the pioneering systems, operating various Mercedes-Benz models from 1979 until 2008.
• A hydrogen-powered bus was developed by Tokyo City University in 2009. It received a license plate from the Ministry of Land, Infrastructure, Transport and Tourism, becoming the first hydrogen engine bus in Japan to be legally permitted to operate on public roads. It was used as a shuttle bus between the university's campuses.

See also

• Electric bus
• Electro-diesel locomotive, the railway equivalent
• Guided bus
• Hybrid electric bus
• IMC Electric bus
• List of buses