Dual-mode vehicle
A dual-mode vehicle is a vehicle that can run on conventional road surfaces, a railway track or a dedicated track known as a "guideway". Dual-mode vehicles are commonly electrically powered and run in dual-mode for power too, using batteries for short distance and low speeds, and track-fed power for longer distances and higher speeds. Dual-mode vehicles were originally studied as a way to make electric cars suitable for inter-city travel without the need for a separate engine. More recently, starting in the 1990s, a number of dual-mode transit mass transit systems have appeared, most notably a number of rubber tyred trams and guided buses.
- See also Global Hybrid Cooperation for the General Motors/DaimlerChrysler/BMW hybrid vehicle technology often called "Dual-Mode"
The problem
The modern automobile is one of the most flexible forms of transit invented. It can operate in almost any weather, is instantly available on demand, carries hundreds to thousands of pounds of cargo and drives directly from origin to destination. The modern internal combustion engine is able to provide power with relatively good efficiency over a wide range of power demands. The fuel is simple to store and transport, is energy dense, and can re-fuel a vehicle in moments. This combination of features means that a car can be used to travel over any sort of distance, from intra-city trips at low-speed for shopping, to inter-city trips at high-speeds to visit distant relatives. No other form of transit offers the same variety of capabilities.
That said, there are numerous reasons why automobiles are not as efficient as they could be. Engines are normally sized to provide the acceleration performance or load-bearing capacity needed for operations in a city, which is far too much power than would be needed for operations on an expressway at high speeds. Suspension loads vary relative to the square of speed, so cars have systems able to deal with loads at highway speeds when a much lighter-duty system would be suitable for city driving. Electric motors offer all the advantages of engines, and more so, but their "fuel" (batteries) are only suitable for short and infrequent service.
If one was able to design cars for specific purposes, they can be tuned for much greater efficiency. The vast majority of car trips are short and low-speed; cars designed for this role can be far more efficient than the generalist vehicles generally used. However, the low ownership of specific-purpose vehicles, like motorcycles, is a good indication of the basic problem: people don't want to have to buy two vehicles to serve a single need: transportation. This has limited other forms of transit to specific roles: aircraft are used for long-distances, trains for inter-city freight and travel, and electric vehicles for known routes where power can be provided at all times.
Dual-mode solution
Dual-mode vehicles address the problem of providing two different types of service in a single vehicle: a low-speed short-distance vehicle for typical trips, and a high-speed long-distance vehicle for longer trips. They do this by tuning their performance for the short-distance portion, the easy problem, using electric motors for traction and batteries for power. The vehicle can be made more efficient because it will only be operating at low speeds; crash protection needs are reduced, and suspension loads are much lower. The result is a vehicle similar to the Neighborhood Electric Vehicles that have appeared in the 2000s.
For longer distances, custom routes are used. For car-like vehicles these generally look like U-shaped concrete roadways. These routes, or "guideways", also supply power, eliminating the need to use the batteries and allowing the motors to run at much higher power settings. Generally the systems also use this power to recharge the batteries while en route, so the vehicle will be fully charged when it reaches the end of the guideway. Since the guideway is tuned for serving vehicles of known size and weight, the capital costs are similar to those of conventional expressways.
The canonical example of a car-like dual-mode vehicle is the Alden staRRcar. The staRRcar looked like existing city cars of the era, using a steel chassis with fibreglass bodywork on top. Batteries gave the vehicle a 30 mph (48 km/h) top speed with a range of a few tens of miles. For longer distance trips, Alden envisioned concrete guideways similar to the existing interstate highways. When the staRRcar entered the guideway, small wheels on either side of the vehicle pressed against the sides of the guideways and handled steering, while an on-board guidance system prevented collisions with the vehicle in front - the only collision hazard in this mode of transit. Since the driving was automated, the vehicles could be operated with almost no distance between them, or "short headways, which increased the capacity of the lanes. A single lane of guideway could safely operate at loads twice or greater than a conventional expressway.
Dual-mode cars have never been installed operationally, but the same system has seen some use as the basis for guided bus systems worldwide. These systems do not necessarily provide power as well as guidance, using conventionally powered buses. Experiments with hybrid systems on trams as a way of recovering power otherwise lost in braking (about 25% of the load) led to the concept of using supercapacitors in place of batteries because they could handle the large currents being generated. This led to the idea of removing the overhead electric feeds for portions of the tramways, using the hybrid to power the tram through these gaps - this allowed the often messy cable runs over road intersections to be removed, improving aesthetics. The two types of vehicle have recently converged in the form of the rubber wheeled tram, which uses tram-like overhead power and a single road-embedded rail as a guideway, a few of which allow the vehicle to leave the guide rail and operate as a traditional electric bus.
Technology
The ground level power supply system is well known by children playing with cars at miniature racing tracks using vehicles with rubber tires and DC-power rails (which also have a guiding function). Because of the health risks with higher voltages in real systems, the power rail is only switched on when a vehicle is covering the section thus preventing pedestrians from being injured. This system is used in Bordeaux with trams and is called Alimentation par Sol .[1] The French Wikipedia article states that it is three times more expensive than the Catenary system.
Hybrid vehicles differ from dual-mode vehicles because they may not be fed by another energy source during operation (there might be an exemption for a hypothetical hybrid solar vehicle).
Logistics
On the main urban arterial streets a catenary system may serve both public transport and freight forwarders. This has great synergic effects making the operation of trolleybuses more efficient because of the additional income from freight forwarders. The operation of dual-mode trucks is not bound to the electric system. The distance from the logistics centre to the inner city is driven in a conventional way. Also there is the possibility to reach all clients aside the catenary system.
Motivation
Cities with slow air exchange (inversion) and high emission figures (particulate matter PM10, PM2.5, NOx, Ozone) caused by diesel-powered vehicles, need a way to reduce big pollution sources. Commercial diesel-fueled vehicles are prime targets because of their high NOx and PM emissions caused by the lack of sufficient pollution controls.