September 29, 2008 By Corey McKenna
Last week, Volvo announced the launch of its new entry into the diesel-electric hybrid bus market. The hybrid Volvo 7700 is a 12-meter low-floor city bus that gets up to 30 percent better fuel economy and emits 40 to 50 percent less nitrous oxide and 30 percent less carbon, the company said. In addition, a smaller engine and nearly a third of the buses components are developed in-house by Volvo.
The hybrid Volvo 7700 uses "parallel hybrid" technology which allows the bus to be powered by either the electric motor or the diesel engine or both at the same time.
I-SAM, the company's hybrid technology, is comprised of a combined start motor, electric motor, generator and an electronic control unit. I-SAM works together with a diesel engine and Volvo's I-shift gearbox. In addition, a lithium-ion battery is charged during braking via the electric motor/generator. This battery then provides energy to the electric motor for drive power.
The company plans to use the same technology in its trucks and construction equipment after the buses go into mass production in 2010.
As a result of the parallel hybrid technology, the bus has a smaller, 5-liter diesel engine and as a result, the entire driveline fits in the same space as in a diesel-powered Volvo 7700. The hybrid bus weights largely the same as a diesel-driven bus, but has a better weight distribution, which allows the hybrid bus to take more passengers than a comparable diesel bus, the company said in a statement.
With many of today's hybrid solutions, the bus becomes substantially heavier and, consequently, can carry fewer passengers. It is the opposite with the Volvo 7700 Hybrid. The bus weights only 100 kilograms more than a diesel version. As a result of better weight distribution, it can carry up to seven more passengers than its diesel counterpart.
"A common approach earlier was that bus manufacturers purchased hybrid components externally and attempted to adapt them to their own bus, but this is difficult," explained Håkan Karlsson, president of Volvo Bus Corporation. "Since we developed the components internally, we have been able to optimize the bus's fuel consumption fully. At the same time we could ensure very high reliability."
As a parallel hybrid, the bus can be powered by the electric motor or the diesel engine independently and well as by both engines simultaneously. As a result, the bus could be equipped with a smaller, 5-liter diesel engine compared with the 9-liter engine in the diesel version of the Volvo 7700. The bus's performance is enhanced, but fuel consumption is reduced.
"Another major benefit with Volvo's hybrid technology is that the diesel engine will be turned off at bus stops and traffic lights," Karlsson explained. "The bus starts moving driven by the electric motor and when the bus reaches 9-13 mph (15-20 kph), the diesel engine starts up automatically."
According to a report by United Press International, the first of Volvo's hybrid buses will begin testing on city streets in London and Gothenburg, Sweden, later this fall.
Volvo said it would start delivering hybrid diesel electric buses to customers in 2009 with mass production beginning in 2010.
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