SIEMENS

If it were up to the environment, the good old combustion engine would have been put out to pasture long ago — for a number of reasons. For example, the unbridled use of gasoline and diesel fuel is depleting oil reserves. And, of course, engine exhaust contains carbon dioxide, which is heating up the earth's atmosphere. And let's not forget the fact that fine particulates and oxides of nitrogen irritate the human respiratory system.

But there is another reason for retiring the combustion engine. More than half of the world's population already lives in cities, and traffic is becoming denser and denser. This, in conjunction with environmental concerns, explains why even more buses will have to take to the streets in the future. After all, fuel consumption per passenger in a full bus is as much as one-third less than the equivalent figure for a full car. Many people already use buses to get around big cities, and not just in developing countries, where a privately-owned vehicle is a luxury. Even in industrialized nations like Germany, buses account for roughly half of all public transportation — every second mass transit kilometre is driven by a bus.

The more densely populated big cities become, the greater the desire for clean and quiet vehicles. London, for example, has been restricting access to its downtown since 2003. There and in Stockholm, Sweden, cars have to pay a toll, and gas-guzzlers are charged an extra levy. In Munich, Germany, trucks are no longer permitted to drive in the inner-city zone. It's very plausible that many communities will decide to issue even stricter emissions regulations for inner cities in the future.

In such a case, only extremely fuel-efficient vehicles or vehicles with electric drives would be permitted to travel in city centre areas. But buses drive two to three hundred kilometres a day and thus require many times more energy than an electric car. “A battery capable of powering a bus all day long is still very heavy and expensive,” says Manfred Schmidt of Siemens Industry's Drive Technologies division in Nuremberg, Germany, where electric drives are developed.

That's why Siemens is putting its faith in the hybrid bus. Hybrid means the combination of a combustion engine with an electric drive. The bus doesn't have to be plugged in, though. Whenever the driver steps on the brakes, the energy that would otherwise be lost as heat is fed into an electrical storage system. This is the same principle that hybrid cars have been using since the late 1990s.

Schmidt is convinced that “hybrid technology makes even more sense in a bus than it does in a car.” Not only is a bus in operation all day long, it also spends between 25 and 40 % of its time standing still at bus stops and red lights. It is thus constantly braking and starting. For the latter, buses can use stored braking energy to quietly accelerate without producing any emissions.

MVG, Munich's public transport company, currently operates two hybrid buses on its routes. One of these is the City Hybrid from MAN, for which Siemens supplies the drive technology. “We want to test and compare different hybrid buses,” says Herbert König, who heads MVG. “By doing so, we are supporting the manufacturers as they strive to develop this innovative vehicle technology.” Drivers and passengers are enthusiastic everywhere hybrid buses are in operation. There is no revving up noise while the bus is starting off, and in contrast to the sometimes jerky ride typical of conventional vehicles, hybrid buses seem to glide.

What makes ELFA, as the Siemens drive technology is known, so special is its serial hybrid solution. With the parallel hybrids typically used today, both a combustion engine and an electric motor drive the axle via the drive shaft. But with a serial hybrid the drive shaft is turned solely by the electric motor that preferentially draws its energy from a storage device called an UltraCap — a high-performance capacitor installed on the roof of the bus. The UltraCap's high energy density and high efficiency make it superior to a conventional battery (see Pictures of the Future, Fall 2007, Piggybanks for Power).