The Best Of Both Worlds?

Diesel Progress North American Edition, April, 2001 by Brent Haight

TransTeq's natural gas-fueled hybrid electric bus operating in Denver; "light rail on wheels"

As emissions standards for internal combustion engines become ever more stringent, public transportation operators are continually looking at alternatives that will allow them to meet the demands of their customers, and provide the required levels of performance, while maintaining clean air standards. Among the technologies that have sought to supplant the traditional diesel-powered bus in transit applications are alternative fuels such as natural gas, along with dedicated electric and/or hybrid vehicles.

Now one manufacturer is taking two of those nontraditional technologies and combining them in an interesting way. Transportation Techniques (Trans Teq) has developed the EcoMark bus, a compressed natural gas-powered hybrid electric bus, for the Denver Regional Transportation District (RTD).

"We've developed an electric bus with an onboard battery charger," said Paul Szilagyi, CEO of the Denver-based manufacturer. "In more technical terms, it is a series hybrid electric vehicle. By that, it means we don't have a mechanical connection between our onboard power generation -- the generator set or auxiliary power unit, depending on what you call it -- and the drive wheels.

"We create power using a four-cylinder, 70 hp Ford engine that we've adapted to run on compressed natural gas. The engine drives the generator. The generator is used to provide power to 26 lead acid batteries. That in turn provides power to two 220 hp, or 440 hp in total, electric drive motors. We developed both the generator and the electric drive motors with a specialized manufacturer with our specifications."

Power is also generated through regenerative braking and Szilagyi added that "we also capture energy that is normally lost during idle, because the engine, as it runs, serves only to charge the batteries."

The first prototype EcoMark bus from TransTeq hit the streets of Denver in July of 1999. The $15.4 million, 36-bus order is scheduled to be complete in 2001, giving the Mile High City what is reported to be the nation's largest fleet of hybrid electric vehicles.

"We started this project with a white sheet of paper," said Szilagyi. "There were a number of best-in-class features that were desired by drivers, mechanics, riders and the downtown business district that these buses service."

According to Szilagyi, the EcoMark hybrid electric bus was designed around serviceability and with more of a "remove and replace philosophy," rather than one that renders the entire bus out of service during repairs. The bus is designed so that one person can service any component on the bus. The engine is mounted on a skid that can be removed and replaced within 45 minutes, Szilagyi said, "allowing plug and play of a combustion engine so the vehicle can return to service while repairs are done in the shop."

The battery trays are similarly designed, so that one person can remove and replace the battery packs. There is also a consolidated master control panel for the advanced electronics that is readily available for servicing and troubleshooting.

The frame is constructed of welded, stainless steel tubing. TransTeq developed an independent trailing arm suspension that eliminates the rear axle. In front is a Dana S84-LFL axle, which is a derivative of the S75 axle. The gross vehicle weight is 45,000 lb.

The 45 ft. long x 8.6 ft. wide x 11.4 ft. high EcoMark bus can carry 116 passengers. It has a top service speed of 15 mph and a top on-road speed of 30 mph. Low floors that are only 14 in. above the street level provide easier boarding and exiting through the vehicles four doors, Szilagyi said.

Compressed natural gas is stored in two Lincoln Composite fuel tanks mounted on the roof. The 16 in. diameter x 10 ft. long tanks hold just over 3000 cu.ft. of CNG at 3600 psi. An Impco low-pressure regulator system is used to deliver the gas to the engine.

The engine is liquid cooled through an Active radiator and the cooling package includes an MP centrifugal pump and Aeroquip hoses. "The radiator is oversized for altitude considerations," said Josh Anderson, advanced research engineer for TransTeq. "We use high efficiency units that are about 30 percent over the needed capacity."

In designing the EcoMark, engineers at TransTeq factored in rapidly changing technology.

"We've designed this bus to be adaptive to any change in technology that happens in its design life," said Szilagyi. "And we expect them. Whether its energy storage, like batteries or capacitators or microturbines or fuel cells that are the source of power, this vehicle and its control system is adaptive to that.

"In essence, as you go throughout the schematics of the bus, you are really connecting your triad of energy creation, storage and usage by plugs. It is designed so that any enhancement of any of the components, whether its fuel cells or advanced energy storage, it can be readily adapted so that the new efficiencies can be gained."