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Diesel-electric hybrid Bus
For stop-and-go urban bus routes, nothing beats electrons.
By se Th MAsIA
Seth Masia is managing
editor of SOLAR TODA Y.
Dr. Paul Scott, chief scientific officer
at ISE Corp. ( isecorp.com), helped to
prepare this article.
The generator charges
an electric storage
system — either a bank
of batteries or of super-capacitors. Capacitors
are lighter and more
efficient. The storage
bay also contains
controller circuitry,
regulating power sent
to the drive motor.
Some 60,000 buses ply city streets in the United States, one block at a time, and 80 percent of them
burn diesel fuel (to meet clean-air regulations, most of the rest use compressed natural gas). About
5,000 new buses go into service each year, and roughly 20 percent of the new ones are hybrid diesel-electrics. American fleets now operate more than 3,000 diesel-electric hybrids.
By using stored electric power to accelerate, and recapturing some of that power in braking, hybrids get
roughly 30 percent better fuel efficiency than pure diesels — roughly 4. 5 miles per gallon compared to 3. 5
for a diesel bus. Where routes are very tight, the mileage gain is better: In London, 60 hybrids do twice as
well as the diesels they replaced.
A hybrid bus costs about a third more than a diesel one — about $200,000 more. Depending on the route
structure and the price of fuel, a hybrid may pay back that premium in eight to 10 years of service — well
within the typical 15-year service life. The federal loan program supporting bus fleet purchases requires a
minimum service life of 12 years. If, as forecast, diesel goes to $9 per gallon by 2020, the hybrid will look like
a bargain.
The diesel-hybrid drive will work well in any stop-and-go application. Eventually, construction equipment
(bulldozers, graders, front-loaders) will adopt the system.
The next step up in efficiency will be the hydrogen-powered bus, driving electric motors with a fuel cell.
If supported by a renewably powered hydrogen source, the hydrogen might sell for $4 per diesel-gallon-equivalent. And the exhaust would be pure water vapor. A hydrogen-electric drive can run at about 55 percent efficiency (that is, it turns 55 percent of the available power in the fuel into mechanical motion and 45
percent into heat). By contrast, a well-maintained diesel bus may operate at 37 percent efficiency. Another
potential bus technology is the hydrogen-burning internal combustion engine from a Ford/BMW partnership, which has achieved 48 percent efficiency. ST
A set of inverters turns
direct current from the
storage system into
alternating current for
the drive motor.
Batteries Fuel tank
ILLus TrATIon By Kur T s Truve
Cooling: A hybrid bus needs separate radiators to
cool the diesel engine and the electronic systems
(capacitors, inverters, motor controllers).
Controller
Like in a conventional bus, the
diesel engine sits at the back,
under the rear seats. It drives a
generator, not a transmission.
Electric drive motor
A separate circuit drives electrical accessories, which
may include hydraulic pumps for power steering
and brake systems, heating and air conditioning,
doors, lights and other normal vehicular systems.
