U.S. DEPARTMENT OF ENERGY HYBRID 

ELECTRIC TRANSIT BUS EVALUATIONS

The role of AVTA is to bridge the gap between R&D and 

commercial availability of advanced vehicle technologies that 

reduce U.S. petroleum use while improving air quality. AVTA 

supports the U.S. Department of Energy’s FreedomCAR and 

Vehicle Technologies Program in moving these technologies 

from R&D to market deployment by examining market factors 

and customer requirements, evaluating performance and 

durability of alternative fuel and advanced technology 

vehicles, and assessing the performance of these vehicles in 

fleet applications. 

The main objective of AVTA projects is to provide comprehensive, unbiased evaluations of advanced technologies. Data 

collected and analyzed include the operation, maintenance, 

performance, safety, cost, and emissions characteristics of 

advanced technology fleets and comparable conventional 

technology fleets operating at the same site. By comparing 

available advanced and conventional technology vehicles, 

AVTA evaluations help fleet owners and operators make 

informed purchase decisions. 

The Fleet Test & Evaluation team, which is composed of 

National Renewable Energy Laboratory and Battelle personnel, conducts AVTA medium- and heavy-duty vehicle 

evaluations. Hybrid electric systems are one of the many 

medium- and heavy-duty advanced vehicle choices available 

today. The Fleet Test & Evaluation team is conducting 

several evaluations of hybrid electric transit buses (Table 1). 

For information on Fleet Test & Evaluation projects, visit 

www.nrel.gov/vehiclesandfuels/fleettest. 

HYBRID ELECTRIC BUS TECHNOLOGIES

Hybrid electric vehicles typically combine an 

energy storage device, a power plant, and an 

electric propulsion system. Energy storage 

devices are most often batteries, but other 

possibilities include ultracapacitors and flywheels. Power plant options include internal 

combustion engines, gas turbines, and fuel 

cells. The efficiency of a hybrid system depends 

on a number of factors, such as the combination 

of subsystems, how the subsystems are 

integrated, and the control strategy employed.

The Metropolitan Transit Authority’s New York City Transit (NYCT) is the largest public transportation system in the United States 

and a leader in the use of clean bus technologies. NYCT launched a pilot fleet of hybrid electric buses in 1998. A larger fleet of nextgeneration production hybrid electric buses began entering service in 2004. The U.S. Department of Energy’s Advanced Vehicle 

Testing Activity (AVTA) evaluated NYCT’s pilot fleet and is evaluating the production buses. This report provides an update on the 

evaluation of NYCT’s production hybrid electric buses and reviews the evolution of hybrid electric bus technologies pioneered by 

NYCT and other progressive transit agencies.

New York City Transit Drives Hybrid Electric Buses into the Future

Table 1. Ongoing AVTA Hybrid Electric Transit Bus Evaluations

Fleet Location Vehicle Technology

IndyGo Indianapolis, IN Ebus 22-ft transit bus Series hybrid, Capstone 

MicroTurbineTM (diesel)

King County 

Metro

Seattle, WA New Flyer 60-ft 

articulated transit bus

Parallel hybrid, GM-Allison 

EP 50 SYSTEMTM (diesel)

Knoxville 

Area Transit

Knoxville, TN Ebus 22-ft trolley bus Series hybrid, Capstone 

MicroTurbineTM (propane)

New York 

City Transit

Manhattan, Bronx, 

Queens, NY

Orion VII 40-ft 

transit bus

Series hybrid, BAE SYSTEMS 

HybriDriveTM system (diesel)

NREL/PIX 13793

NEW YORK CITY TRANSIT ◆ DIESEL HYBRID ELECTRIC BUSES
There are two basic strategies for hybrid propulsion: series 

and parallel (see sidebar). Hybrid electric vehicles also can 

use regenerative braking, which converts some of the energy 

dissipated during braking into electrical energy that can be 

used to power the vehicle.

Hybrid electric propulsion systems are being developed for 

a wide range of medium- and heavy-duty vehicle sizes and 

applications, including transit buses. Hybrid propulsion 

systems offer several advantages to transit agencies: 

• Increased efficiency, potentially lowering fuel cost

• Better acceleration, allowing quick merging into 

heavy traffic

• Decreased emissions

• Quieter operation.

More than 30 organizations in the United States are currently 

operating hybrid electric bus technologies. Promising results 

from early projects have led several agencies to place large 

orders for hybrid buses. More than 600 hybrid buses could be 

placed into service across the United States in the next several 

years. NYCT is one of the fleets that have made a commitment 

to hybrid technology. 

THE BAE SYSTEMS HYBRIDRIVETM SYSTEM

NYCT’s hybrid electric buses are built by Orion Bus Industries 

(DaimlerChrysler Commercial Buses North America) and use 

the BAE SYSTEMS HybriDriveTM propulsion system (Figure 1). 

In this series hybrid electric system, a relatively small diesel 

engine running at an optimal controlled speed is connected to 

a generator that produces electricity for the electric drive motor 

and batteries. The electric motor drives the vehicle and acts as 

a generator to capture energy during regenerative braking. 

The batteries supply additional power during acceleration and 

hill climbing and store energy recovered during regenerative 

braking and idling. The battery optimization subsystem 

monitors and maintains the charge of each individual battery. 

The propulsion control subsystem manages the entire system 

and optimizes performance for emissions, fuel economy, and 

power. 

NYCT’S CLEAN FLEET COMMITMENT 

NYCT is the largest public transportation system in the United 

States. It currently has more than 4,500 buses operating from 

18 depots and serving more than 2,100 miles of routes daily. 

More than 2 million passengers use its 219 bus routes every day. 

Since 1992, NYCT has displayed its commitment to cleaner 

vehicles by testing and evaluating a variety of clean fuel buses 

in revenue service. NYCT’s Clean Fuel Bus Program is a 

continuing effort to lower bus fleet emissions, reduce traffic 

congestion, and improve air quality. The plan includes using 

new clean diesel engines with particulate filters and ultra-low 

sulfur (30 parts per million sulfur content) diesel fuel for the 

entire diesel fleet (starting in 1998), as well as purchasing 

compressed natural gas (CNG) and hybrid electric buses.

SERIES HYBRID: In this configuration, the power plant provides 

power to the electric motor(s), which drives the wheels. There 

is no mechanical connection between the power plant and the 

wheels. An advantage of this configuration is the ability to set 

the power plant to operate at its maximum efficiency.

PARALLEL HYBRID: This configuration has two power paths. 

The wheels can be driven by the power plant, the electric 

motor(s), or both. An advantage of this configuration is the 

combined power due to the electric motor and power plant 

driving the wheels simultaneously.

Figure 1. The BAE SYSTEMS HybriDriveTM System

A: A 5.9-liter diesel engine runs at an optimal controlled speed 

and is connected to a generator to produce electrical power

for the drive motor and batteries.

B: The electric motor drives the vehicle and acts as a generator 

to capture energy during braking.

C: The batteries supply power during acceleration and hill 

climbing and store energy recovered during regenerative 

braking.

D: The propulsion control system manages the entire system 

and optimizes performance for emissions, fuel economy, 

and power.

A

B

D

C

Companies Developing Hybrid Technology 

for Medium- and Heavy-Duty Applications

BAE SYSTEMS: Series hybrid system for 40-foot transit buses 

and military applications.

Eaton: Parallel hybrid system for medium- and heavy-duty 

trucks.

Ebus: Series hybrid system for 22-foot buses using a Capstone 

MicroTurbine.

GM-Allison: Parallel hybrid system for 40- and 60-foot transit 

buses.

ISE Research: Series hybrid system for 40-foot transit buses, 

over-the-road coaches, and military applications (diesel, 

gasoline, and hydrogen fueled).

Oshkosh: Series hybrid system for heavy-duty trucks such as 

refuse haulers and military applications.
NYCT’S FIRST-GENERATION PILOT 

HYBRID ELECTRIC BUSES

NYCT’s experience with hybrid electric buses began in 1998 

with the purchase of a pilot fleet of 10 Orion VI buses with 

the BAE SYSTEMS HybriDriveTM propulsion system. At the 

time these buses were ordered, a hybrid configuration was 

not an option for transit buses. BAE SYSTEMS, Orion, and 

NYCT collaborated to design and build this technology and 

implement it into transit service. Understanding that this was 

an investment in unproven technology, the partnership set 

specific goals for the project to measure success, including 

the following: 

• Reduce emissions 

• Significantly increase fuel economy 

• Show hybrid buses can operate in regular service with 

no restrictions and perform as well as or better than 

conventional buses 

• Significantly increase brake life 

• Demonstrate positive perception of hybrid electric buses 

by drivers and passengers 

• Improve the hybrid electric bus design to promote development and aid in commercializing the technology. 

All 10 prototype buses were in service by mid-2000. The 

AVTA Fleet Test & Evaluation team conducted a yearlong 

evaluation of this fleet in operation*. The prototype hybrid 

electric buses met or exceeded the project goals. The evaluation 

also established a benchmark for hybrid electric technology 

and assessed the progress of commercialization to that point.

NYCT’S NEXT-GENERATION PRODUCTION

HYBRID ELECTRIC BUSES

NYCT’s early experience with the pilot bus fleet helped the 

project team improve and optimize the system to increase 

efficiency and maximize the potential benefits to the agency. 

Compared with the Orion VI hybrid electric buses, the next 

generation Orion VII buses are expected to be more fully 

optimized for fuel economy and emissions. Table 2 shows the 

Orion VII specifications. Modifications included the following: 

• The bus platform was updated to the Orion VII model, a 

step-up low-floor bus; the Orion VII features a removable 

engine cradle for ease of maintenance. 

• The change to a rear step-up design (as opposed to a true 

low floor) allowed more seats and use of a standard rear 

axle configuration, which should reduce brake repair costs. 

• The engine-generator connection was modified to a “T” 

configuration to allow easier placement in the bus and 

better access for maintenance. 

• The power plant was switched to a Cummins ISB engine 

with an Engelhard DPXTM diesel particulate filter. 

• All hybrid components were improved for better 

performance, reliability, availability, and serviceability.

NYCT increased its commitment to hybrid electric technology 

by purchasing 325 production Orion VII hybrid electric 

buses in two orders, the first for 125 buses and the second for 

200 buses. These large orders mark the continued progress of 

this hybrid electric technology from a demonstration toward 

commercialization. 

STATUS OF NYCT’S NEXT-GENERATION 

HYBRID ELECTRIC BUSES

As of early March 2005, all of NYCT’s first 125 Orion VII 

hybrid electric buses have been delivered and are being 

integrated into fleet operations. Mother Clara Hale Depot in 

Manhattan has 65; staff at this depot have experience with 

hybrid electric buses from operating the pilot Orion VI buses. 

Queens Village Depot in Queens has 60 Orion VII hybrid 

electric buses. To date, NYCT’s experience with the hybrid 

electric buses has been positive. Drivers like the quick 

* To obtain publications related to this evaluation, visit the Alternative Fuels Data Center at www.eere.energy.gov/afdc or call the National Alternative Fuels Hotline at 

1-800-423-1363. Related publications include NYCT Diesel Hybrid Electric Buses: Final Results, NYCT Diesel Hybrid Electric Buses: Final Data Report, NYCT Diesel 

Hybrid Electric Buses: Program Status Update, and Advanced Technology Vehicles in Service: Diesel Hybrid Electric Buses.

NEW YORK CITY TRANSIT ◆ DIESEL HYBRID ELECTRIC BUSES

Table 2. Orion VII Bus Specifications

Buses: Orion VII

Model Years 2002, 2004

Length/Width/Height 40 ft/102 in/132 in

GVWR/Curb Weight 42,540/31,840 lb

Seats 38-44 passengers

Service Manhattan, Queens, Bronx, NY

Engine: Cummins ISB 5.9 liter*

Rating 270 bhp @ 2,500 rpm

660 lb-ft @ 1,600 rpm

Fuel Storage: Diesel 100 gal

Fuel Ultra-low sulfur diesel

(less than 30-ppm sulfur content)

Propulsion: BAE SYSTEMS HybriDriveTM

Motor and Internal

Gear Reduction

Type: AC Induction, high-power density

Horsepower: 250 hp continuous (320 hp peak)

Torque: 2,700 lb-ft @ 0 rpm

Generator Type: Permanent magnet

Horsepower: 160 hp continuous

Energy Storage Type: Sealed lead acid

Voltage: 520-700 VDC

Controls Type: Integrated power electronics and controls 

Traction motor/generator horsepower: 410 hp 

continuous (480 hp peak), J1939 CAN interface

Regenerative Braking Yes

Auxiliary Power Diesel engine

Emissions Equipment

Catalyzed Diesel 

Particulate Filter

Engelhard DPXTM (on MY 2002 buses)

Johnson Matthey CRTTM (on MY 2004 buses)

* Engines in MY 2004 (200-bus order) buses equipped with exhaust gas 

recirculation (EGR).
acceleration of the buses, which makes it easier to merge 

into heavy traffic situations typical in New York City.

In addition to the order of 125 Orion VII hybrid electric 

buses, NYCT has agreed to purchase 200 similar, but 

slightly upgraded, hybrid electric buses. According to 

BAE SYSTEMS, four subsystems on this order of buses 

have been improved, including the engine, generator, 

propulsion control, and cooling and packaging.

The company expects these refinements to result in 

improved emissions, improved power, quieter operation, 

and improved reliability, durability, maintainability 

and performance. Table 3 summarizes improvements 

made from the Orion VI prototype to the most recent 

version of the bus.

Delivery of the 200-bus order began in early 2005, and 

the buses will be split between three NYCT depots. Once 

delivery is complete, NYCT will have 325 hybrid electric 

buses, about 7% of its entire fleet. NYCT also expects to be 

operating 476 CNG buses by 2005, bringing its combined 

clean fuel fleet to 801 buses, about 18% of its entire fleet. 

EVALUATION PLANS

The AVTA evaluation of NYCT’s Orion VII hybrid electric 

buses will include collecting operation, maintenance, and 

performance data from a selection of hybrid buses from 

the 125-bus order and the 200-bus order. Comparison of 

both hybrid versions and conventional diesel buses will 

help show the evolution of the product toward commercialization. Detailed data will be collected from a total of 20 

hybrid and 20 conventional diesel buses: 10 hybrid buses 

(125-bus order) and 10 conventional diesel buses at Mother 

Clara Hale Depot, and 10 hybrid buses (200-bus order) 

and 10 conventional diesel buses at Fresh Pond Depot. 

NYCT considers the hybrid electric buses to be commercial 

products and plans no special treatment for their operation. 

The depots will not have additional spare buses and will be 

responsible for maintaining bus availability at 85% or better, 

the same as required for conventional diesel and CNG buses. 

Recovery time for troubleshooting and repair of the hybrid 

electric buses is extremely important to NYCT. The ability 

of the maintainers to integrate the hybrid buses fully into the 

fleet will be a measure of the success of these bus orders.

NYCT is also receiving Orion VII CNG buses, which will be 

included as part of this evaluation. The CNG buses will be 

operated out of West Farms Depot in the Bronx and Jackie 

Gleason Depot in Brooklyn. Detailed data will be collected 

on a selection of CNG and conventional diesel buses operating 

from West Farms Depot. 

Table 3. Improvements in NYCT’s Hybrid Electric Transit Buses, 

 MY 1998–2004

Orion VI 

(MY 1998)

Orion VII

(MY 2002)

Orion VII

(MY 2004)

Number of Buses 10 125 200

Engine DDC S40 Cummins ISB Cummins ISB 

with EGR

Emission Certifi cation 

Level, g/bhp-hr

PM 0.05, 

NOx 4.0

PM 0.05, 

NOx 4.0

PM 0.05, 

NOx 2.5

Emissions Control Catalyzed 

DPF

Catalyzed 

DPF

Catalyzed

DPF

DPF/HEV Control Monitor only Actively 

managed

Actively 

managed

Batteries Lead acid Lead acid Lead acid 

(2 types)

Battery Control Generation 1 Generation 2 Generation 3

Motor/Controller 

Cooling

Oil/WEG Oil/WEG Oil/Oil

(integrated 

system)

Generator Mounting Coupling Coupling Direct mount 

(no coupling)

Generator Control Speed 

controlled

Electronically 

controlled

Electronically 

controlled

DPF—diesel particulate fi lter; EGR—exhaust gas recirculation; 

HEV—hybrid electric vehicle; WEG—water ethylene glycol.

Neither the United States government nor any agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, 

or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply 

its endorsement, recommendation, or favoring by the United States government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States government or any agency thereof.

Contacts

BAE SYSTEMS 

Thomas Webb, Business 

Development, Transit

Phone: 413-253-2564

Fax: 413-253-0732

thomas.webb@baesystems.com 

New York City Transit

Gary LaBouff, Director, Research 

& Development, Department of Buses

Phone: 718-566-3535

Fax: 718-927-8143

galabou@nyct.com

Orion Bus Industries 

Mark Brager, Vice President, Sales

Phone: 905-403-7806

Fax: 905-403-8600

mbrager@orionbus.com 

DOE

Lee Slezak, Manager, 

Advanced Vehicle Technology 

Analysis and Evaluation Team 

Offi ce of FreedomCAR and 

Vehicle Technologies

Phone: 202-586-2235

Fax: 202-586-2476

lee.slezak@hq.doe.gov

Battelle 

Kevin Chandler, Project Manager

Phone: 614-424-5127

Fax: 614-458-5127

chandlek@battelle.org

National Renewable Energy 

Laboratory

Leslie Eudy, Sr. Project Leader 

Phone: 303-275-4412

Fax: 303-275-4415

leslie_eudy@nrel.gov

Robb Barnitt, Engineer

Phone: 303-275-4489

Fax: 303-275-4415

robb_barnitt@nrel.gov

For more information contact:

EERE Information Center

1-877-EERE-INF (1-877-337-3463)

www.eere.energy.gov 

A Strong Energy Portfolio for a Strong America

Energy efficiency and clean, renewable energy will mean a stronger economy, a cleaner 

environment, and greater energy independence for America. Working with a wide array 

of state, community, industry, and university partners, the U.S. Department of Energy’s 

Office of Energy Efficiency and Renewable Energy invests in a diverse portfolio of energy 

technologies.

DOE/GO-102005-2009 • April 2005

Printed with a renewable-source ink on paper containing at least 50% wastepaper, including 20% postconsumer waste.

NEW YORK CITY TRANSIT ◆ DIESEL HYBRID ELECTRIC BUSES

Produced by the 

National Renewable Energy Laboratory (NREL)

NREL is a U.S. Department of Energy National Laboratory

Operated by Midwest Research Institute • Battelle