Hybrid-Electric School Buses: Coming Around the Mountain?

School buses make many starts and stops. Thus, like refuse trucks, UPS trucks, FedEx trucks and transit buses, school buses are an ideal application for hybrid technology.
Fuel savings and reduced pollution can result from regenerative braking to recoup deceleration energy, shutting down the engine rather than idling at stops, and plugging into the electric grid to charge batteries. Hybrid electric school buses are already carrying kids. Developed by IC Corp. and Enova Systems, the first one was delivered to the Shenedehowa Central School in Deposit, New York.
 
According to IC Corp, 21 more will be placed into service in 11 states, from Florida to Washington state this spring as part of the Hybrid Electric School Bus (HESB) project managed by North Carolina’s Advanced Energy. Locations were chosen to evaluate their performance and feasibility under a variety of conditions.

“This project provides operational benefits to school districts, while also providing the reduced emissions desired by the U.S. Environmental Protection Agency and a valuable return on investment to school boards,” said Ewan Pritchard, Advanced Energy’s hybrid program manager.

How it Works

These CE Series hybrid electric buses feature Enova’s HybridPower system. Its Post-Transmission 80 kilowatt Hybrid Drive System is coupled with an International® VT365 V8 diesel engine and an Allison 2500PTS transmission. A motor/generator connected to the driveshaft acts as a generator to recover energy during regenerative braking that is used to charge the battery pack while slowing down.

In this parallel hybrid, energy from the battery pack supplies the motor/generator, now acting like a motor to provide additional power for starting, acceleration and hill climbing. While these first hybrid buses use advanced lead-acid batteries, NiMH (Nickel Metal Hybrid) batteries are also available. In the future, lithium-ion batteries may also be offered.

“We selected Valence’s Saphion phosphate batteries for their inherent safety, which addresses the limitations of traditional lithium-ion chemistries, particularly in large batteries like those needed for hybrid vehicles,” said John Dexter, director of operations and planning at Enova Systems. “In addition to safety, Saphion technology delivers the power and energy that these buses demand.”

The Valence technology eliminates the fire problem that has been recently experienced by laptop computers with Sony lithium-ion batteries.

Lean Green

School buses are ideal candidates for plug-in hybrid electric vehicle (PHEV) technology, as plugging into the electrical grid overnight for battery charging reduces the time the internal combustion engine has to operate. This further reduces fuel consumption and emissions.

With PHEVs, mobile emission sources are replaced by stationary power plants that are much cleaner and easier to control. Also, overnight charging moves stationary emissions from daytime to night time, thus reducing ozone and smog. Electric rates are also often lower at night.

“With fuel prices at all-time highs, new innovations in hybrid technology are needed to help customers keep their operating costs lower,” said Michael Cancelliere, VP/GM of IC Corp. “We feel that this technology could provide significantly improved fuel economy. In addition, even though current International diesel engines produce no visible smoke and low emissions, the hybrid program will reduce engine emissions even further.”

While the precise numbers are not in, Randy Ray, marketing manager for IC’s hybrid school bus program, predicts a “30 to 50 percent fuel efficiency improvement, plus maintenance savings on regenerative braking.”

Regenerative braking reduces wear and tear on brakes and saves money in brake maintenance costs. This increases to as much as 70-100 percent with PHEV capability when buses can run up to 44 miles on electricity alone. Additional savings could come from adding a start-stop capability that shuts down the engine, rather than idling at stops.

Ease of maintenance is facilitated by the post-transmission design, since no modifications are made to the engine, transmission, chassis, body or instrument panel.

“IC Corporation is scheduling diagnostics and hands-on repair sessions for dealers that will be supporting customers with hybrid school buses,” Ray added. There are two “new” pieces of equipment on the hybrid school bus that will require maintenance professionals to be updated on: an isolated cooling system, and for the computer, an electric drive motor and batteries. PHEVs also use traditional green coolant.

“On plug-in models, when you plug it in every night, the bus takes care of itself,” said Ray. “Without daily plug ins (or charge sustaining), you only need to plug it in every two weeks or so to balance the batteries out.”

The biggest new hazard is the much higher voltage used in hybrids, compared to the 12 volts found in traditional school buses. Voltages as high as 425 volts DC are found in the new HybridPower system. As little as 50 volts can be fatal.

“There are really no safety issues for high voltage,” Ray said. “Ground fault detectors are built into the hybrid bus, so the power disconnects in the event of an accident. Orange high-voltage lines are also clearly marked as per the universal color coding of wires.”

Technicians will need to know not to bump into orange cables or touch damaged cables or components, as they may be hot. Just like any vehicle, most of electrical systems—lights, heaters, etc.—still use 12 volts DC. There is still a 12-volt lead-acid battery and all the other normal hazards.

A similar prototype hydraulic hybrid shuttle bus using Eaton Corp.’s Hybrid Launch Assist™ (HLA®) technology was developed for the U.S. Army Hydraulic Hybrid, Advanced Materials and Multi-fuel Engine Research (HAMMER) program that is aimed at reducing fuel consumption in military ground vehicles. In this hydraulic hybrid, a 14,000-pound GVW Ford E-450 Cutaway chassis is used with a bus body supplied by Glaval Bus. A 235-horsepower, 6-liter Ford Power Stroke V8 diesel engine supplies 440 lb-ft of peak torque. Eaton’s Gen 2 HLA system boosts this up to 300 peak horsepower and 1,000 pounds per foot peak torque.

Higher Price Tag

One of the biggest obstacles to the widespread acceptance of hybrids is their higher initial cost that may take years to pay back through fuel and maintenance savings. According to Ray, “As production increases, we hope to bring the initial cost down from the current level of about twice the cost of a standard school bus. We are still working with customers to provide funding at the state and federal level to help defray the cost of a hybrid school bus. In addition to the environmental advantages the hybrid school bus provides, we also have a goal to provide a sensible financial payback to customers so that these buses can be an economical, viable choice.”

Advanced Energy’s Ken Dulaney adds, “We’ve had success with state-level environmental agencies and energy agencies. Also, the EPA Clean School Bus USA Program should now consider plug-in hybrid electric buses a proven.” The EPA has not been an advocate of hybrids, favoring instead battery electric vehicles. PHEVs are really BEVs because they can travel much farther on electricity alone.

With the large number of school buses in operation in the U.S., there could be a very large market for hybrid school buses. This includes hybrid buses from other competitors and incorporating other technologies.

A completely different type of hybrid, the hydraulic hybrid could provide competition needed to reduce prices. Here, electricity is replaced by high-pressure hydraulic fluid. The conventional powertrain is augmented by a hydraulic system consisting of a reversible hydraulic pump/motor coupled to the drive shaft through a clutch, a high-pressure accumulator and a low-pressure accumulator.

When the driver steps on the brake pedal, deceleration energy drives the pump/motor to force hydraulic fluid out of the low-pressure accumulator into the high-pressure one. The fluid compresses nitrogen gas in this accumulator to pressurize the system. When accelerating, the system switches from pump to motor mode. Nitrogen gas forces the hydraulic fluid back into the low-pressure accumulator, passing through the pump/motor to applying power to the driveshaft through the clutch.

Fuel savings result when stored energy is used to assist during initial, high-fuel consumption acceleration. According to the EPA, when manufactured in high volume, the added costs of the hybrid components could be recouped in under three years through lower fuel and brake maintenance costs. Additionally, the hydraulic components are designed to last the life of the vehicle, which is not the case with the batteries in hybrid electric vehicles. To date, hydraulic refuse trucks, shuttle buses, UPS trucks and military trucks have been built.

About the author: Siuru is an automotive journalist located in Temecula, California, specializing in green technologies. He has written for such publications as Diesel Progress, Diesel & Turbine International, Green Car Journal, and Mass Transit.

Reprinted from the March 2007 issue of School Transportation News magazine. All rights reserved.

Last modified onTuesday, 29 May 2018 15:12