Emily Mulrine is excited to have the name she chose, "Limpio," emblazoned in green letters on the side of a school bus.

But, more importantly, she's excited about what's inside the bus: a hybrid-electric powertrain.

She's also excited about what will be coming out of the bus: an estimated 90-percent less emissions.

And, of course, she's excited about what will be going into the bus: far less diesel, due to a projected 70- to 100-percent increase in fuel economy.

Mulrine, a sixth-grader in the Manatee County (Fla.) School District, was one of the winners of a contest to name the district’s two new plug-in hybrid school buses — which are among the first school buses of their kind in the country.

Mulrine's moniker, "Limpio," means "clean" in Spanish. Her co-winner, high-schooler William Ung, submitted the name "Wouk" in honor of Victor Wouk, a pioneer in developing hybrid technology.

"It's exciting that the name I chose is going to be on the hybrid bus, and it's great that these buses are going to help the environment," Mulrine says.

In all, 19 of these new buses have been awarded to school transportation providers across the nation by Advanced Energy, a non-profit corporation that initiated the groundbreaking Plug-In Hybrid Electric School Bus Project.

How they work
The plug-in hybrid Type C school buses, which were built by bus manufacturer IC Corporation and hybrid drivetrain maker Enova Systems, have a charge-depleting system that draws on energy stored in the batteries during the driving cycle to optimize fuel economy.

With this type of system, more energy can be taken from the batteries than in a standard, or charge-sustaining, hybrid —but for a limited distance (about 44 miles). Once the batteries have been "pulled down," the system will maintain itself in sustaining mode. Then the batteries are plugged in for recharging.

The hybrid buses also use regenerative braking to capture some of the energy created when the driver applies the brakes, helping to charge the battery. Another advantage of regenerative braking is that it leads to longer brake life due to decreased wear. With the frequent stops of typical school bus routes, the vehicles stand to benefit significantly from this technology.

IC Bus Marketing Manager Randall Ray says that plug-in hybrids on certain routes can achieve huge boosts in fuel economy.

"If we assume 7 miles per gallon, which is heavily dependent on the route, buses using a charge-depleting system could see up to 12 to 14 miles per gallon during the deep discharge phase of operation," Ray says. "Again, this is dependent on a route with multiple stops per mile."

The buses couple an International VT365 V8 diesel engine with the hybrid-electric powertrain, which comprises a transmission, batteries and an electric motor. The battery pack consists of 28 individual batteries. Ray says that the pack should last 6 to 8 years in a charge-depleting system using lithium-ion batteries.

The system is based on parallel architecture, allowing it to efficiently use both diesel and electric power.

Ewan Pritchard, hybrid program manager for Advanced Energy, says that most of a vehicle's emissions are created when it is used at peak power — i.e., when it accelerates. With these hybrids, the electric motor does most of the acceleration, which reduces emissions peaks and saves fuel.

Also, biodiesel can be used in these hybrids, further reducing emissions and use of petroleum diesel. Manatee County is using a B20 biodiesel blend in its two hybrids.

Hybrid arrivals
Manatee County received its two hybrid school buses in March. Don Ross, the district's supervisor of vehicle maintenance, says that the vehicles' performance has been promising so far.

"There has been no down-time whatsoever on either bus," Ross says.

The main difference in operating the buses is that drivers engage or disengage the hybrid system as they feel necessary by flipping a switch on the dash. Also, drivers notice stronger acceleration off of idle, and they feel regenerative braking occurring when they step off of the accelerator.

[PAGEBREAK]From a maintenance standpoint, there is an isolated cooling system for the hybrid components, so the coolant in that reservoir needs to be checked regularly. For safety, high-voltage cabling is clearly marked in orange, and there is a manual high-voltage cutoff switch.

The hybrids are equipped with International's AWARE GPS system, allowing operators to track their performance as well as their location.

In particular, Manatee County is monitoring fuel usage, idle time, amount of time for brake depressions and number of stops. The hybrids are being alternated with non-hybrid control buses on certain routes to further gauge their success.

Ross couldn't yet report an average miles-per-gallon figure for the hybrids, but he says that they are definitely seeing a positive increase in fuel economy.

The district also installed meters to record the actual hours and costs of charging the hybrids. The buses are plugged into the electrical grid at night and between morning and afternoon runs. Ross says that it takes two-and-a-half to three hours to give the vehicles a full charge.

Later in March, Jennings Transportation, a contractor in Nazareth, Pa., received the third unit of the 19 plug-in hybrids that have been awarded in the Advanced Energy project. Like Manatee County, Jennings will be tracking the performance of its hybrid and will compare it to that of a control bus on the same route.

"We're going to make sure that all the information we're reporting is accurate," says Meghan Ochs, Jennings' communications and special projects director. "Plug-in technology is perfect for school buses, but we need it to be more commercially viable."

Weighing costs
For now, the prohibitive factor for the hybrids is their cost — about $200,000 — but officials involved in the project expect that to come down significantly.

The current price is about a $140,000 premium over the cost of a regular Type C school bus. Pritchard says that the goal is to reduce it to a premium of about $40,000.

IC Marketing Director David Hillman says that one of the keys to success for the hybrid school buses will be government agencies providing grants and tax incentives to help operators acquire the vehicles.

In the case of Jennings Transportation, the company received a grant of $112,000 from the Pennsylvania Department of Environmental Protection. Manatee County received a grant of $50,000 through Advanced Energy for its two hybrids. In addition to the bus costs, the district spent about $10,000 on installing special outlets for plugging in the vehicles and meters to record the electricity usage.

Of course, electricity isn't free either, but Pritchard says that it equates to buying diesel at just 40 to 80 cents per gallon.

Other options
In addition to the plug-in hybrid school bus, IC Corporation is offering a charge-sustaining hybrid, which recharges the batteries with regenerative braking and with the diesel engine. Company officials estimate that this type of hybrid can improve fuel economy by 30 to 55 percent.

Also, Enova offers its hybrid system for retrofits as well as for new vehicles. Executive Vice President Mike Staran says that one of the key advantages of the company's system is that it is a "non-invasive solution." Specifically, it requires little or no modification to the chassis, body and instrument panel, and it doesn't intrude on the engine control/communication system.

Odyne Corp., a developer of plug-in hybrid powertrains, is working on applying its technology to school buses. In fact, CEO Roger Slotkin says that the company considers the school bus "one of the three best applications" for its plug-in hybrid system.

A group of six school districts in California is working with Odyne to develop 12 plug-in hybrid Type Ds that would be fueled with compressed natural gas rather than diesel. The group is seeking $3.25 million from the California Air Resources Board, among other entities, to help fund the endeavor.

Sam Armentrout, director of transportation for Madera (Calif.) Unified School District, which is involved in the CNG-hybrid project, says that in addition to the emissions reductions that the buses could achieve, they would "reduce our dependence on foreign oil and save us money. What two things could be better?"

What's next?
The 19 hybrids in the current phase of the project will be evaluated for about two years. The next step is to work on a larger purchase.

"They've only produced 19 of them, so cost is high now," Pritchard says. "The holy grail for us is getting 1,000 of these purchased — to get volume."

Hillman says that it may take a few years for hybrid school buses to be in widespread use, but he believes that as more people become educated on the advantages this technology can provide, widespread use will come.

"Only through early adoption by users will this technology be able to be worked into a commercial model," Hillman says.

While the long-term success of the project still faces challenges ahead, it is certainly off to a remarkable start.

Particularly impressive, Pritchard says, is that while other modes of transportation are looking to build plug-in hybrids, none appears to be as far along as the school bus industry. Even Toyota's popular Prius hybrid car likely won't be available as an OEM plug-in version until 2010.

"The school bus industry has lapped the rest of the transportation industry," Pritchard says.