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HYBRID TRANSIT BUS
CERTIFICATION WORKGROUP
Engine Certification Recommendations Report
Northeast Advanced Vehicle Consortium
NAVC0599-AVP009903
September 15, 2000
Submitted to
U.S. Department of Transportation
U.S. Environmental Protection Agency
California Air Resources Board
by
Northeast Advanced Vehicle Consortium
112 South Street, Fourth Floor
Boston, MA 02111
September 15, 2000
Agreement No.: NAVC0599-AVP009903
Prepared By
M.J. Bradley & Associates, Inc.
Manchester, NH
Transient Operation Analysis By
West Virginia University
Department of Mechanical Engineering
Morgantown, WV
Copyright 2000, NAVC, DOT, All Rights Reserved
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Agreement No.: NAVC0599-AVP009903 09/15/00 ii
About AVP and NAVC
The NAVC Hybrid Transit Bus Certification Project was generously supported by the United
States Department of Transportation’s Advanced Vehicle Technologies Program (AVP). The
AVP combines the best in transportation technologies and innovative program elements to
produce new vehicles, components, and infrastructure for medium- and heavy-duty transportation

needs. The primary objectives of AVP are to:
• reduce vehicle emissions beyond 2004 standards,
• improve vehicle fuel efficiency by 50 percent,
• make the United States globally competitive in advanced vehicles, components and
infrastructure, and
• increase public acceptance of advanced transportation technology.
The AVP program continues the approach developed by the Defense Advanced Research
Projects Agency (DARPA) Electric and Hybrid Vehicle (EHV) Technologies program of
forming partnerships with other federal agencies, private companies, research institutions and
state and local governments to expedite technology development vital to the nation’s interests.
The Northeast Advanced Vehicle Consortium (NAVC) is a public-private partnership of
companies, public agencies, and university and federal laboratories working together to promote
advanced vehicle technologies in the Northeast United States. The NAVC Board of Directors
includes a representative of the New England Governors’ Conference and the Northeast States for
Coordinated Air Use Management and representatives appointed by the eight Northeast
governors and the mayor of New York City. Its participants have initiated numerous projects,
spanning a wide range of technology areas including electric, hybrid-electric and fuel cell
propulsion systems, electric and natural gas refueling, energy storage and management, and
lightweight structural composites. The NAVC receives funding from federal agencies and private
members.
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Agreement No.: NAVC0599-AVP009903 09/15/00 iii
Acknowledgements
The Northeast Advanced Vehicle Consortium (NAVC) thanks the U.S. Department of
Transportation (DOT) Advanced Vehicle Technologies Program for the funding and support of
this project. We recognize Shang Hsiung of DOT for his personal assistance. The project was
initiated by Sheila Lynch, NAVC Executive Director, and organized and lead by Thomas Webb,
NAVC Project Director.
The NAVC thanks M.J. Bradley & Associates for their excellent work on the project, particularly
Thomas Balon, the lead author; Paul Moynihan, MJB&A staff engineer; and Amy Stillings,

MJB&A staff analyst.
The NAVC thanks West Virginia University (WVU) for sharing and carrying forward the wealth
of knowledge they possess with regard to engine certification testing. We personally thank Dr.
Nigel Clark for his oversight and providing expertise on interactions between an engine’s
operating conditions and emissions and the rest of the WVU staff for their participation.
In addition, the NAVC thanks the United States Environmental Protection Agency (EPA) and
California Air Resources Board (CARB) for its participation. In particular, we thank Dennis
Johnson (EPA), Tom Stricker (formerly EPA), Jack Kitowski (CARB), Tom Chang (CARB) and
Fernando Amador (CARB) for expressing a deep interest in the project from the beginning and a
desire to explore alternate means to certify hybrid transit buses.
The NAVC would also like to thank the electric drive manufacturers, specifically Allison
Transmission, Lockheed Martin Control Systems and ISE Research for allowing access to
proprietary data that is at the heart of this report. In addition, the ongoing participation of other
interested parties and all workgroup participants was extremely valuable, including hybrid
component suppliers, engine manufacturers, bus equipment manufacturers, environmental
organizations and other governmental agencies.
Finally, the NAVC thanks the American Public Transportation Association for getting the hybrid
bus certification ball rolling several years ago. Frank Cihak and Jerry Trotter provided valuable
insight and contacts.
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Disclaimer
The viewpoints expressed in this report are those of the authors. While the report was prepared
and reviewed by a broadbased and representative group of people from industry and government
(listed in Appendix A), none of the participating organizations were asked to, nor have they
necessarily, endorsed or adopted the findings and recommendations included in this report.
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Executive Summary
The purpose of this report is to help facilitate the introduction of hybrid-electric drive technology

into the heavy-duty bus market in the United States today. Recent chassis testing of hybrid buses
by the Northeast Advanced Vehicle Consortium (NAVC) demonstrated the potential of hybrid
technology to significantly reduce emissions and greenhouse gases compared to conventional
drive buses. However, hybrid buses can present a certification quandary for industry and
regulators. Historically, engines for heavy-duty transit buses have been certified using the federal
transient procedure (FTP), but engines that may be optimized for today’s series hybrid buses may
not be capable of following the behavior prescribed in the FTP cycle.
While chassis testing may ultimately resolve this dilemma, new rulemaking for certifying transit
bus chassis (instead of engines) is a long way off. A short-term, alternate engine certification
procedure would help this viable, clean bus drive technology enter the market now. Hybrid-
electric transit bus engines have to meet the same emission standards as conventional urban bus
engines, however they should be tested on cycles that are representative of their actual operating
characteristics.
The NAVC formed the Hybrid Transit Bus Certification Workgroup to help industry and
regulators elect an appropriate existing engine cycle as an alternate to the FTP cycle. During the
course of several well-attended meetings, the government-industry group exchanged the latest
information on hybrid-electric drive bus technology and identified an approach to near-term
hybrid bus engine certification. The group collected and analyzed data from a representative
sample of state-of-the-art series hybrid buses operating in normal revenue service in New York,
Boston and Los Angeles. The data represents three leading electric drive manufacturers active in
the market at this time.
This report summarizes the analysis of in-use hybrid-electric bus engine data and compares it to
conventional bus, marine and off-road engine test cycles. The analysis indicates that the hybrid
engines have substantially less aggressive transient behavior than the FTP prescribes. Since
extreme transients cause the formation of particulate matter and carbon monoxide in diesel
engines, and precipitate air/fuel ratio deviation in spark ignited engines, hybrid engine emissions
are better represented by steady state operation.
A modal data analysis reveals that the Euro III 13-Mode Test Cycle is the most inclusive and
representative cycle for hybrid engines. Furthermore, it is widely used by regulators and engine
manufactures, making it practical for implementation in the near term. At this time, we

recommend the use of the Euro III to certify engines for use in series hybrid buses only.
Additionally, we recommend a sunset date of 2004 to allow industry and regulators to reevaluate
the cycle in light of advancements in hybrid technology and engine emission controls. The
recommended sunset date also coincides with the significantly reduced emission levels that will
be instituted in California in 2004.
Chapter 1 provides an overview of the project, its goals and objectives and reason for focusing on
near term alternate cycle for engine based certification of heavy-duty hybrid transit buses.
Chapter 2 offers a more detailed discussion of hybrid drive systems including generator set
operation and its affects on fuel economy, emissions and test procedures. Chapter 3 provides a
description of existing transient and steady state cycles for certifying engines. Chapter 4 lays out
a general in-use methodology for analyzing hybrid engine operation. This chapter may be of use
for future test programs. Chapter 5 provides the results of the analysis of three different hybrid
engines in use in New York, Boston and Los Angeles. Chapter 6 draws conclusions and
recommends the Euro III test cycle for near term certification of engines for series hybrid transit
buses.
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Table of Contents
About AVP and NAVC
Acknowledgements
Disclaimer
Executive Summary
1.0 Hybrid Bus Overview
1.1 Introduction
1.2 Proven Hybrid Emissions Reductions
1.3 The Certification Challenge
1.4 The NAVC Hybrid Transit Bus Certification Workgroup
1.4.1 Special Test Procedures
2.0 Series Hybrid-Electric Buses
2.1 Hybrid-Electric Drive Definition

2.1.1 Drive System Design Variations
2.1.2 Engine Sizing
2.1.3 Batteries and Regenerative Braking
2.2 Engine Operation and Control
2.3 Emission Implications
3.0 Existing Engine Test Cycles
3.1 The FTP Transient Cycle
3.2 Steady-State Cycles
3.2.1 Generator Set Test Cycle
3.2.2 Off Road Equipment Test Cycle
3.2.3 Marine Engine Test Cycles
3.2.4 Other Steady-State Test Cycles
3.3 Conclusions
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4.0 In-Use Data Collection
4.1 Data Collection Overview
4.2 Modal Data Analysis Methodology
4.3 Transient Data Analysis Methodology
4.4 Modal FTP Baseline
4.5 Conclusions
5.0 Manufacturers Data Analysis
5.1 NAVC Workgroup Data Collection
5.1.1 NAVC Workgroup Data Reduction
5.2 Engine Data—Transient Analysis
5.3 Engine Data—Modal Analysis
5.3.1 Modal Test Cycle Comparisons
5.4 Conclusions
6.0 Conclusions and Recommendations
6.1 Key Findings

6.2 Future Research Needs
Appendix A: Attendance List for NAVC Hybrid Transit Bus Certification
Workgroup Meetings
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List of Tables
Table 1.1: Results of NAVC Hybrid Testing Project
Table 1.2: Current Hybrid-Electric Engine and Turbine Applications
Table 1.3: Hybrid Bus Certification Pathways
Table 1.4: EPA Urban Bus Engine Standards
Table 1.5: CARB Urban Bus Diesel Engine Standards
Table 3.1: Steady State Test Cycles
Table 5.1: Commercially Available Hybrid Buses
Table 5.2: Hybrid Bus Specifications
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List of Figures
Figure 1.1: U.S. Hybrid Bus Market
Figure 1.2: Comparison of Tailpipe Emissions between a Conventional and Hybrid Diesel Bus
Figure 2.1: Vehicle Energy Requirements
Figure 3.1: The FTP Transient Cycle
Figure 3.2: Five-Mode Steady-State Test
Figure 3.3: Eight-Mode Steady-State Test
Figure 3.4: E4 and E5 Marine Cycles
Figure 3.5: Thirteen-Step Japanese Steady-State Test
Figure 3.6: Thirteen-Mode Euro III Test
Figure 4.1: FTP Load-Point Analysis
Figure 4.2: FTP Cycle Histogram
Figure 4.3: FTP Horsepower Variations
Figure 4.4: FTP Cycle Engine Torque Distributions

Figure 5.1: LMCS Bus #6352 In-Use Speed
Figure 5.2: Comparison of Speed Behavior on FTP Cycle and In-Use Hybrid Test Data
Figure 5.3: Comparison of Horsepower on FTP Cycle and In-Use Hybrid Test Data
Figure 5.4: Comparison of Torque on FTP Cycle and In-Use Hybrid Test Data
Figure 5.5: Combined Hybrid Engine Torque Speed Hp-Hr Weighted, 1% Intervals
Figure 5.6: Combined Hybrid Engine Torque Speed Hr-Hp Weighted, 10% Intervals
Figure 5.7: Combined Hybrid Engine Torque Speed Hp-Hr vs. 8178 5-Mode
Figure 5.8: Combined Hybrid Engine Torque Speed Hp-Hr vs. 8178 8-Mode
Figure 5.9: Combined Hybrid Engine Torque Speed Hp-Hr vs. 8178 Marine E4 and E5
Figure 5.10: Combined Hybrid Engine Torque Speed Hp-Hr vs. 13 Step Japanese
Figure 5.11: Combined Hybrid Engine Torque Speed Hp-Hr vs. Euro III 13-Mode
Figure 6.1: Comparison of Horsepower on FTP Cycle and In-Use Hybrid Test Data
Figure 6.2: Combined Hybrid Engine Torque Speed Hp-Hr vs. Euro III 13-Mode
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1.0 Hybrid Bus Overview
Chapter 1 outlines the need for a special test procedure for certifying heavy-duty hybrid transit
buses in the United States. It gives an overview of the Northeast Advanced Vehicle Consortium
(NAVC) certification project including a brief status of the hybrid bus market, results of recent
emission testing of hybrid buses, and explanation of the hybrid bus certification challenge. The
history of the NAVC Hybrid Transit Bus Certification Workgroup is summarized, as well as the
reason for recommending an alternate, existing engine cycle for near-term certification of engines
for series hybrid transit buses.
1.1 Introduction
The growing need to reduce fuel consumption and lower emissions in the United States
transportation sector has spurred urban transit bus operators to pioneer the adoption of alternate
fuels and new drive system technologies. One of the most promising technologies to receive
attention is hybrid-electric drive, which consists of two or more onboard fuels that supply energy
to electric traction motors that in turn drive the wheels. By contrast, conventional drive employs
an internal combustion (IC) engine to generate rotational force that is then only mechanically

transferred to drive the wheels.
The electric drive improves drive system efficiency, reduces energy consumption, recovers
energy, reduces emissions, and improves driveability. Pure battery-electric transit buses do not
appear feasible in the near term because the power and energy requirements associated with
typical urban transit bus drive cycles exceed the performance (primarily range) capabilities of
current battery technologies. However, hybrid-electric drive, or electric drive that uses two
sources of onboard motive energy (typically, an IC engine and traction battery), can easily meet
and exceed the urban transit bus drive cycle requirements while still dramatically improving fuel
economy and emissions. Thus, hybrids have emerged as a future direction for transit as well as
other light and heavy-duty vehicles.
Rapid technological progress has occurred in electric drive components and system integration
during the last five years. A growing number of companies are developing and beginning to
supply commercial hybrid-electric drive products to the truck and bus markets. In 1998 the Orion
VI Hybrid bus became North America’s first commercial hybrid product offering from a major
transit bus manufacturer. Other products are being tested and offered for sale by NovaBUS, New
Flyer, Advanced Vehicle
Systems and others. Hybrid
buses are being used in
revenue service in a number of
cities including Cedar Rapids,
Chattanooga, Los Angeles,
New York City, Tampa, and
Tempe.
While the population of hybrid
buses is relatively small today,
the demand is growing, as seen
in Figure 1.1. In a study
recently prepared by the
NAVC for the Transportation
Source: NAVC, based on actual and planned purchases.

US Hybrid Bus Population
0
50
100
150
200
250
300
350
1994 1995 1996 1997 1998 1999 2000 2001
Delivery Year
Annual
Cumulative
Figure 1.1: U.S. Hybrid Bus Market
Delivery Year
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Agreement No.: NAVC0599-AVP009903 09/15/00 2
Research Board,
1
the NAVC estimated there were about 70 hybrid buses delivered and another
230 or so on order in the United States as of December 1999. The potential exists for thousands
of hybrid transit bus orders over the next several years to meet customer demand for cost
effective pollution reduction strategies. Hybrid-electric drive technology is particularly well
suited to meet this need in the near term.
1.2 Proven Hybrid Emissions Reductions
Hybrids have been recently shown
to significantly lower overall
emissions and improve fuel
economy when compared to
conventional drive. An in-depth

test program was performed in 1999
by the NAVC for the Defense
Advanced Research Projects
Agency (DARPA) to evaluate fuel
economy and emissions
performance of state-of-the-art
hybrid-electric buses as well as
conventional and alternatively
fueled mechanically driven transit
buses.
2
Using the West Virginia
University (WVU) chassis
dynamometer, the NAVC tested six
heavy-duty hybrid transit buses on
multiple drive cycles, measuring emissions and fuel economy. The particulate matter (PM)
results for the diesel-electric hybrids were 50 percent lower than for a conventional state-of-the-
art mechanically driven diesel bus,
3
and oxides of nitrogen (NOx) emissions were 30-40 percent
lower. The hybrids exhibited the lowest carbon monoxide (CO) emissions of any bus tested (up
to 70 percent lower), and the hybrids demonstrated significantly lower total greenhouse gas
emissions than either conventional diesel or compressed natural gas (CNG) buses. Table 1.1
shows the results of the NAVC hybrid vehicle emissions chassis-based test results.
4

1
“Hybrid-Electric Transit Buses: Status, Issues and Benefits,” Transportation Research Board (TCRP
Report 59), National Academy Press, 2000, is available at www.nationalacadamies.org/trb/bookstore.
2

The comprehensive report that details the results of this project, NAVC, MJB&A and WVU, “Hybrid-
Electric Drive Heavy-Duty Vehicle Testing Project,” February 15, 2000, is available at www.navc.org.
3
Hybrid-electric diesel buses tested were equipped with particulate filter traps, whereas the conventional
diesel buses were not. Although the particulate filter traps contributed largely to the PM reductions, the
less aggressive transient operation by the hybrid engine also contributes significantly to the PM reductions.
Based on engine certification data the authors predict that a hybrid vehicle equipped with a 0.1 g/bhp-hr
engine would likely have the same PM emissions as a 0.05 g/bhp-hr engine in a conventional bus without
the benefit of a particulate filter trap.
4
NovaBUS completed emissions testing of its RTS model hybrid transit bus at Environment Canada on
May 26, 2000. The bus was equipped with the LMCS hybrid system and was tested at 34,500 lb on D1
fuel. The bus was equipped with a particulate filter designed to suppress sulfate production. Emission
results on the CBD-14 driving cycle were: CO = 0.0 g/mi; NOx = 14.54 g/mi; THC = 0.07 g/mi, PM =
0.0048 g/mi; CO
2
= 2304 g/mi and fuel economy of 4.40 mpg.
Source: NAVC, MJB&A and WVU, 2000.
Figure 1.2: Comparison of Tailpipe Emissions between a
Conventional and Hybrid Diesel Bus
PM10
0.24
0.12
0
0.05
0.1
0.15
0.2
0.25
0.3

NovaBUS RTS
(DDC50)
Orion Hybrid
VI (DDC30)
NOx
30.1
19.2
0
5
10
15
20
25
30
35
NovaBUS RTS
(DDC50)
Orion Hybrid
VI (DDC30)
D1 fuel used in both buses. DDC50 certified to PM 0.05g/bhp-hr. DDC30 certified to
PM 0.10g/ bhp-hr equipped with a particulate filter trap.
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The reasons for the reductions are severalfold. Regenerative braking contributes significantly to
reducing fuel consumption and thereby improving efficiency. Regenerative braking takes
advantage of the energy storage system to capture the kinetic energy of the vehicle during
braking. This is accomplished by using the drive motors as generators during braking to
recapture the vehicle’s kinetic energy and restore a portion of this energy back to the energy
storage device to be used later, for example during acceleration.
Another contributing factor to the reductions is the fact that, on a series hybrid, the engine is not

directly coupled to the vehicle drivetrain (i.e., the electric drive motor alone drives the wheels).
This allows the auxiliary power unit (APU) to operate independently from the vehicle. This
would theoretically allow the engine/generator to operate at peak efficiency and optimized
emission load points. Series hybrid control strategies typically prevent the engine from operating
in zones where its efficiency may be low and its emissions high.
Reduced air pollutant emissions from hybrid-electric vehicles is an important consideration for
transit agencies, especially those in congested urban areas. The emission reduction of hybrids is
directly tied to reduced fuel consumption; when less fuel is used, fewer emissions are produced.
In addition, when an engine in a conventional vehicle is under heavy load, such as acceleration, it
operates in areas of the engine map that are more heavily emissive. By supplementing the engine
with electric drive motors as the hybrid does, and operating the engine in a narrow zone, heavy-
load operation may be avoided altogether resulting in lower emissions.
1.3 The Certification Challenge
Industry and regulators have recognized for some time the unique challenge posed by hybrids in
the emissions certification process compared to traditional transit buses. Current series hybrid
transit buses often use new or unconventional engine technology that is smaller and different in
design, control and operation from conventional engines. Table 1.2 shows the variety of engines
and turbines used in hybrid vehicles today. Some of the hybrid engines in use today do not meet
current EPA urban bus standards on the FTP cycle, however many demonstrate superior
emissions performance in chassis testing of hybrid buses (see Figure 1.2). As the technology
evolves, system designers will want to develop hybrid engine and system controls to achieve
further emissions reductions and fuel economy improvements. All of these factors make
certification of hybrid engines a challenge.
Table 1.1: Results of NAVC Hybrid Testing Project
Emission Rate (gram/mile) Fuel Economy
CO NOx NMOC PM CO
2
CH
4
(mpg)

Orion-LMCS VI Hybrid Diesel 0.1 19.2 0.08 0.12 2,262 0.0 4.3
Orion-LMCS VI Hybrid Diesel (no regen.) 0.04 22.0 0.12 0.24 2,625 0.0 3.7
Orion-LMCS VI Hybrid MossGas 0.1 18.5 0.03 0.02 2,218 0.0 4.2
Nova-Allison RTS Hybrid LS Diesel 0.4 27.7 bdl* bdl* 2,472 0.0 3.9
Nova-Allison RTS Hybrid LS Diesel (no regen.) 1.0 32.1 0.03 0.07 3,010 0.0 3.1
NovaBUS RTS Diesel Series 50 3.0 30.1 0.14 0.24 2,779 0.0 3.5
NovaBUS RTS MossGas Series 50 1.0 32.2 0.05 0.09 2,816 0.0 3.3
Neoplan AN440T CNG L10 280G 0.6 25.0 0.60 0.02 2,392 14.6 3.1
New Flyer C40LF CNG Series 50G 12.7 14.9 3.15 0.02 2,343 17.4 3.1
Orion V CNG Series 50G 10.8 9.7 2.36 0.02 2,785 23.7 2.6
* bdl = below detectable limit
Source: NAVC, MJB&A and WVU, 2000.
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The Engine Compliance Program at the Environmental Protection Agency’s (EPA’s) Office of
Transportation and Air Quality is responsible for certifying engines for heavy-duty practices. The
California Air Resources Board (CARB) Mobile Sources Control Division performs a similar
function for certification in the state of California. Both EPA and CARB use the same test
procedures for urban bus engine certification.
Emission certification of trucks and buses is presently done using the engine only. Chassis based
emissions testing in the United States only occurs on light duty vehicles and light duty trucks,
except in California where chassis based certification of medium-duty vehicles is allowed. In a
conventional mechanically driven vehicle, the engine performs the work and its speed and torque
varies according to the demands of the transient drive cycle. The Federal Code of Regulations
(40 CFR Part 86, subparts I and N) prescribes an engine dynamometer test for heavy-duty
certification. In California, the comparable regulation can be found in Title 13, California Code
of Regulations, Section 1956.1. Specific lab equipment and test protocols are also described.
Engines are certified on the Federal Test Procedure (FTP) transient cycle. Emissions are
measured and reported in units of grams of emissions per brake horsepower hour (g/bhp-hr)
delivered by the engine under specific load regimes. The emissions are not allowed to exceed

certain standards set by EPA and California. Engine manufacturers are responsible for complying
with exhaust emission standards.
Hybrid engines can meet the urban bus PM standard on the FTP with additional modifications as
demonstrated by the 2000 model year DDC S30 engine. However, research and development of
hybrid engines in the near future may result in engine optimizations for specific operating ranges,
which may be prohibited by current FTP certification protocol. Pursuing special test procedure
options or developing a hybrid-specific engine test protocol would allow the engine optimization
to be realized during the certification procedure resulting in lower engine emissions.
The challenge is for industry and regulators to find an acceptable cycle on which to test hybrid
engines for purposes of emissions certification. If the FTP transient cycle alone is used will
hybrid engines be able to meet the urban bus standard and at what cost? Will viable engine
technologies be excluded if the FTP is used? Will the FTP allow flexibility to improve and
optimize hybrid engine controls for further emission reductions in the future? Is there another
engine cycle that would better represent hybrid engine operation that could be used for
certification purposes? These questions were being debated in the hybrid industry when the
NAVC formed the Hybrid Transit Bus Certification Workgroup.
Table 1.2: Current Hybrid-Electric Engine and Turbine Applications
Engine Manufacturer / Model
DDC S30 Cummins ISB DDC 642 GM V-8 Capstone
Engine Type
1
CI CI DI SI Turbine
Peak Power (hp) 230 275 160 n/a 43
Displacement (l) 7.3 5.9 4.2 5.7 n/a
Weight (lb) 930 962 662 n/a 163
PM Emission
Rate (g/bhp-hr)
0.05 0.10
2
0.10 n/a n/a

EPA Urban Bus
Compliant?
yes no
2
no no exempt 49
states
1 – Compression Ignition (CI), Direct Injection (DI), Spark Ignition (SI)
2 – In process of certifying to EPA Urban Bus PM standard of 0.05 g/bhp-hr on the FTP
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1.4 The NAVC Hybrid Transit Bus Certification Workgroup
The primary goal of the NAVC Hybrid Transit Bus Certification project was to develop a
comprehensive protocol for the testing and certification of heavy-duty hybrid-electric vehicles.
To this end, the NAVC put together the Hybrid Transit Bus Certification Workgroup of
government and industry stakeholders to determine the best course of action. The project was
coordinated by the NAVC and supported by the Advanced Vehicle Program under the
administration of the United States Department of Transportation.
The NAVC hosted two meetings of the Workgroup in the spring of 2000. A variety of
presentations were given at each meeting to share the latest emissions and fuel economy data with
all participants and to explore certification concepts. The meetings drew a large and
representative body of participants from all aspects of the industry including manufacturers of
engine and gas turbines, aftertreatment, hybrid drive systems and bus equipment, as well as
transit operators, the American Public Transportation Association, EPA, state representatives,
other environmental advocacy groups and industry consultants. A complete list of participants
appears in Appendix A. The Workgroup objectives were to balance industry and government
interests, provide current and unbiased information pertinent to hybrid certification, explore the
feasibility of alternate means to certification, validate those means, and publish and distribute its
recommendations widely.
At its first meeting, the
Workgroup identified

three pathways to
hybrid bus certification
(see Table 1.3). The
first (immediate)
pathway represents the
status quo. Engines for
hybrid bus orders placed
through 2000 will most
likely be certified on the
current federal transient cycle. The second (short-term) pathway is to certify using an existing,
alternate cycle that better represents in-use hybrid engine operation. The Workgroup decided its
resources were best spent on selecting and justifying the alternate, existing cycle. The third (long
term) pathway is to develop new, hybrid-special test cycles for engines and/or chassis.
5
This
pathway is considered long term because it requires rulemaking that is expected to take several
years from start to finish. The third pathway is beyond the present scope of the NAVC
Workgroup.
6

5
The arguments for and against chassis-based emissions certification will have to be resolved before a
chassis emission certification protocol can be adopted. Some regulators favor chassis testing because it
will more accurately reflect real-world emissions. Some heavy bus equipment manufacturers (OEMs)
oppose chassis based certification as unfair and onerous because they have little control over subsystems
that determine emissions, and because their products are varied and markets are small.
6
The newly formed Society of Automotive Engineers (SAE) Truck and Bus Hybrid and Electric Vehicle
Committee is working to identify the appropriate standards for electric and hybrid-electric trucks and buses
including modification of existing standards or development of new ones, as required. The NAVC

Workgroup is preparing to work in partnership with the SAE to revise SAE J1711, the recommended
practice for measuring emissions and fuel economy of light-duty hybrid-electric vehicles using a chassis
dynamometer.
Table 1.3: Hybrid Bus Certification Pathways
Immediate Short-Term Long-Term
Certify on the
current FTP
Other options
within the current
regulatory
structure (i.e.,
special test
procedures)
Rulemaking for
hybrid technology
(possibly to
include chassis-
based certification)
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The general consensus
among participants in the
Workgroup was that
short-term certification
testing to help early
market penetration should
remain engine based and
the responsibility of the
engine manufacturers. A
sunset date of 2004 was

selected due to new
emissions standards that
go into effect, and the
need to re-evaluate the
Workgroup’s
recommendations in light
of technology
advancement expected by
then. Table 1.4 provides a
summary of past and future
Federal engine certification
standards for urban buses.
Table 1.5 provides a
summary of the CARB
Urban Bus Engine Standards
and the proposed urban bus
engine
conventional/advanced
technology path standards,
which diesel-hybrid buses
would follow.
To help build consensus for
short term hybrid engine
certification, the NAVC
Workgroup turned to the
Special Test Procedures
provisions in the code of
regulations for Federal and
California certification.
1.4.1 Special Test Procedures

The Administrator may, on the basis of written application by a manufacturer,
prescribe test procedures, other than those set forth in this part, for any light-
duty vehicle, light-duty truck, heavy-duty engine, or heavy-duty vehicle which the
Administrator determines is not susceptible to satisfactory testing by the
procedures set forth in this part. - 40 CFR 86.090-27
Engine Emission Standards (g/bhp-hr)
Model Year PM NOx HC CO
1991 0.10 5.0 1.3 15.5
1994 0.07 5.0 1.3 15.5
1996 0.05 5.0 1.3 15.5
1998 0.05 4.0 1.3 15.5
2004 0.05 2.0* 0.5 15.5
2007** 0.01 0.20 0.14 15.5
* Nominal NOx level based on emission standards of 2.4 g/bhp-hr
NOx plus non-methane hydrocarbons (NMHC) or 2.5 g/bhp-hr plus
NMHC with 0.5 g/bhp-hr cap.
**Proposed, with fuel sulfur limitations and a NOx/NMHC phase-in
Source: U.S. EPA.
Table 1.4: EPA Urban Bus Engine Standards
Engine Emission Standards (g/bhp-hr)
Model Year PM NOx NMHC CO
1991 0.10 5.0 1.2 15.5
1994 0.07 5.0 1.2 15.5
1996 0.05 4.0 1.2 15.5
1998 0.05 4.0 1.2 15.5
2000 0.05 4.0 1.2 15.5
2002 0.05 2.0* 1.2 15.5
2004** 0.01 0.5 0.05 5.0
2007** 0.01 0.2 0.05 5.0
* Nominal NOx level based on emission standards of 2.4 g/bhp-hr

NOx plus non-methane hydrocarbons (NMHC) or 2.5 g/bhp-hr plus
NMHC with 0.5 g/bhp-hr cap.
**Proposed
Source: CARB
Table 1.5: CARB Urban Bus Diesel Engine Standards
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Agreement No.: NAVC0599-AVP009903 09/15/00 7
In January of 1998, EPA granted permission to Navistar to use a special test procedure in
certifying the T444E engine, which had been designed for light heavy-duty market, for use in a
limited number of heavy-heavy duty hybrid transit buses. In its request to EPA, Navistar argued
that the quasi-steady state D-2 cycle was more representative of actual engine operation in the
hybrid bus than the transient FTP cycle. Furthermore, Navistar showed that the engine could
meet the urban bus emission standards on the D-2 cycle. EPA approved Navistar’s request to use
the D-2 cycle and on-highway deterioration factors, but limited it to one model year only and
required the engine be properly labeled for hybrid use only.
EPA also requested that Navistar share in-use hybrid engine operation and chassis data with EPA
in the future to help it better understand the benefits of the hybrid engine and to determine the
suitableness of the alternate cycle used for certification. The 2000 NAVC hybrid emissions study
(see section 1.2) reported the results of a side-by-side chassis comparison of hybrid to
conventional drive technology to help address regulators’ concerns. The NAVC Hybrid Transit
Bus Certification Workgroup set out to analyze in-use hybrid engine data in order to determine
the most representative, existing engine cycle for certifying hybrid engines. While it does not
appear that the D-2 cycle is representative of the hybrid application, the use of a steady state cycle
appears acceptable.
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2.0 Series Hybrid-Electric Buses
Series hybrid-electric vehicles are an outgrowth of pure electric vehicles, combining the
developments in electric drive systems and energy storage devices with conventional technology
to produce the next generation of vehicles with low emissions and high fuel economy, while

maintaining good performance attributes. This chapter begins by defining what a hybrid-electric
drive system is, describes some design variations, and explains how hybrids differ from
conventional vehicles. We discuss the major design areas that affect hybrid engine operation and
emissions, including overall system design, engine type and size, engine controls, and
regenerative braking. These factors help explain why hybrid bus engine emissions do not
correlate to hybrid bus chassis emissions. In particular, the engine in a hybrid was hypothesized
to operate in more of a steady state than transient mode, which could provide a key to engine
certification procedures.
2.1 Hybrid-Electric Drive Definition
A hybrid-electric vehicle is one that has two motive power sources used either separately or in
combination. These two sources are the electrical energy storage device such as a battery pack,
supercapacitor or flywheel, and the auxiliary power unit (APU), such as an internal combustion
engine, turbine or fuel cell. Hybrid-electric vehicles also contain a single or multiple electric
motors that provide power to the wheels. Power to the motors is provided by either the energy
storage device or the APU, or in combination, depending upon the type of hybrid-electric vehicle.
Hybrid-electric vehicles use the signal from the accelerator pedal to determine how much power
will be provided by the APU and/or by the energy storage device. The vehicle’s computer
constantly monitors the battery state-of-charge (SOC) to determine if engine operation is needed
to recharge the batteries, independent of the driver signals. Because the bus does not rely on the
engine for its peak power output at the axles, the hybrid bus engine is sized based on a
combination of the average bus power demand and the peak power demand, rather than the peak
power demand alone. For the same power output, a smaller engine operated at high percentage
output will usually be more efficient than a larger engine operated at lower percentage output,
because the frictional and pumping losses of the smaller engine are lessened. In this way, the
engine in a hybrid vehicle can offer greater cycle average fuel efficiency, and hence can also offer
the potential to lower emissions.
2.1.1 Drive System Design Variations
There are several different kinds of hybrid-electric vehicles, which are categorized as series,
parallel or dual mode, engine or battery dominant, charge sustaining or charge depleting.
Currently charge-sustaining series and parallel hybrids have received the most attention. The

exception is the turbine hybrid built by Advanced Vehicle Systems and Capstone Turbine, which
is a battery dominant, charge depleting series hybrid with a micro-turbine as the APU.
In a series hybrid all of the power necessary to drive the wheels is provided by electric drive
motors or in other words, the engine is mechanically de-coupled from the wheels. This
configuration consists of an APU that is used to charge the batteries or provide electric power
directly to the drive motors. In this case, the APU does not necessarily operate in a load-
following manner and is basically independent so that the engine can be optimized to run in a
narrow range of operating conditions, in a zone of both high efficiency and low emissions.
Optimizing the engine to operate within a narrow band is achieved partially through engine
management software that prohibits it from moving out of the desired zone and partially by the
original design specifications of the attached generator.
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In a parallel hybrid, both the engine and the drive motors provide power to the wheels. Rather
than having to cycle the energy from the engine through a generator and then to the drive motors,
the mechanical energy from the engine can be applied through a differential directly to the drive
wheels. In stop-and-go applications, the engine is more load following than in a series hybrid and
is better suited to continuous operation at higher speeds (i.e., the engine and transmission
combination are speed following). If the application is known, the engine in a parallel hybrid can
be partially optimized for efficiency and emissions within a specific zone of operation.
Compared to a conventional bus, series and parallel hybrids offer the advantage of reducing the
amount of energy provided to the wheels by the engine. By supplementing engine energy with
energy from the batteries, the engine will be less load-following than in a conventional bus, and
will therefore use less fuel. Reducing the amount of fuel correlates directly to reduced emissions.
Also, since the engine is less load-following and does not necessarily require operation near
maximum power, a smaller engine can be utilized.
2.1.2 Engine Sizing
Hybrid engine sizing is affected by the size of the energy storage device, which contributes to
whether or not it will be of battery or engine dominant design. There is no clear-cut definition of
battery vs. engine dominance and in fact many of the current hybrid offerings are right in the

middle with a moderately sized battery pack and engine. With a smaller engine the hybrid can
still meet the acceleration demands with help from the batteries but with reduced fuel
consumption and reduced emissions.
Generally speaking, the determination of battery or engine dominance is actually better made
using the terms charge sustaining vs. charge depleting as these terms are easier to define. A pure
electric bus is obviously both charge depleting and battery dominant as it derives all of its motive
energy from the batteries. Several prototype fuel cell buses have been developed as “hybrid-
electric” but some of these buses do not have any batteries at all, much like an electric trolley, and
therefore cannot recover energy during braking. The best example of an engine dominant hybrid
would be a vehicle that adds the ability to capture regenerative braking energy but has little if any
pure electric range such as the Honda Insight. Electric vehicles with range extending APUs such
as the AVS Capstone turbine hybrid are considered battery dominant. On the other end of the
spectrum is the Toyota Prius that utilizes batteries for load leveling, regenerative braking and
some minimal level of electric only range. Most of the hybrid-electric transit buses on the street
today are charge sustaining and are considered engine dominant even though they possess an
electric range of nearly 10 miles, quite a distance for a 40-foot transit bus.
An advantage to an engine-dominant hybrid is that the APU provides most of the energy
immediately to the drive motors thus eliminating the energy losses inherent in the energy storage
system. In a battery-dominant hybrid, just the opposite occurs, and the drive motors get most of
the energy from the energy storage system. The advantage of this system is that load following is
minimized for the engine, allowing the zone of torque and speed operation of the engine to be
more closely defined.
The issue of engine dominance vs. battery dominance in the emission testing sense is important
because charge sustaining, engine dominant hybrid-electric vehicles derive all of their power
from the onboard APU while battery dominant vehicles derive most of their power from the
utility grid.
A dual mode hybrid (Toyota Prius) is designed so that it embodies both series and parallel hybrid
operation characteristics. The engine and two motor/generators are integrated with a geartrain to
form a sophisticated continuously variable transmission. This is a more complex type of hybrid
in terms of design and management. As with series and parallel hybrids, the dual-mode hybrid

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can be either engine or battery dominant. Currently there are no transit buses of this design in
service in the United States.
2.1.3 Batteries and Regenerative Braking
Regenerative braking allows the kinetic acceleration energy to be recovered to recharge the
batteries during vehicle deceleration. During acceleration, a certain amount of energy is required
to bring the vehicle mass up to speed. However, when a vehicle equipped with a conventional
bus comes to a stop, that energy is dissipated through the service brakes as heat, essentially
wasted energy. When a hybrid bus decelerates, the drive motor torque is reversed and the
resistance of an electromagnetic field creates electrical energy that is cycled back to recharge the
batteries. During peak demands some power is provided by energy stored during regeneration, so
that the demand on the engine is lessened. This reduces fuel consumption and emissions. Also,
when power is demanded rapidly, the hybrid system need not demand instantaneous high power
output from the engine, but may instead raise engine power slowly, relying on power available in
the batteries for good vehicle pedal response. The reduction of instantaneous demand on the
engine, or “smoothing”, can have a strong effect in reducing diesel engine PM production.
Not all of a vehicle’s kinetic energy can be captured by the batteries, however. Generally
speaking, a vehicle can and usually does stop much more quickly than it can accelerate and the
hybrid-electric energy storage system has limited ability to accept the energy quickly. This is
because acceleration is limited by the power available from the drive system while braking is
limited by the traction of the
tires. Figure 2.1 shows a
representation of the energy
required for a vehicle to
accelerate to a set speed and
cruise, and finally brake to a
stop. This illustration, taken
from a single cycle element
of the Central Business

District chassis testing cycle,
shows that the deceleration
takes place about twice as
fast as the acceleration.
During rapid deceleration
like this, the ability to
recover the kinetic energy
through the regenerative
braking system is limited by
the amount of energy the drive system (drive motor, controller, and batteries) can accept. Usually
the system and battery pack are designed for the peak power demand during acceleration. If the
battery pack cannot accept all the energy during deceleration, the service brakes are engaged.
The energy is dissipated as heat. Devices such as ultracapacitors, that can accept high charge
rates, are likely to emerge as an energy-saving feature in future hybrid designs.
Hybrid brake system configuration also affects regenerative braking system efficiency. Since
regenerative braking uses the drive motors in reverse, and most current hybrid-electric designs are
rear wheel drive, the braking energy that passes through the rear wheels is all that can be
captured. A significant portion of braking for any vehicle occurs at the front brakes and the only
way to capture this braking energy is to put drive motors on the front wheels as well (or brake
entirely with the rear wheels, which may result in unsafe handling). Other losses occur in the
Figure 2.1: Vehicle Energy Requirements
-300
-200
-100
0
100
200
300
Horsepower (HP)
-5

0
5
10
15
20
25
Speed (mph)
HP Speed
Speed
HP
Acceleration
Deceleration
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batteries and other components of the electrical systems as well as the mechanical losses
including the differential.
2.2 Engine Operation and Control
A typical diesel internal combustion engine is a mechanical power device that converts chemical
heat and expansion energy into mechanical rotational energy via a crankshaft. In a conventional
vehicle this rotational power is directed to the vehicle wheels via a transmission and a differential
so that the proper wheel speeds can be obtained. The transmission and differential are geared
such that at low vehicle speeds the system has a significant amount of torque multiplication (as
much as 100x). As vehicle speeds increase the transmission is shifted up (to a lower numerical
gear ratio) resulting in lower torque multiplication such that in final drive the vehicle engine is
operating at about three to twelve times wheel speed depending upon vehicle design parameters.
The important point in conventional mechanical drive is that the engine is connected to the
wheels of the vehicle so that engine speed is dependent upon vehicle speed under constrained
transmission conditions. The highway portion of the FTP cycle consists of significant engine
operation at high speed while load varies typical of a vehicle operating essentially in top gear and
cruising at highway speeds. Also, the FTP embodies the “gear-bound” technology typical of the

1970s. The varying engine load is then induced by hills such that engine load increases but
engine speed tracks with the relatively constant speed of the vehicle. The end result of this type
of vehicle configuration is that the engine can operate over nearly all of its operating range for
both speed and torque. Engine operation is generally vehicle dependent as well as duty cycle
dependent (see Chapter 3 for a discussion of duty cycles).
A series hybrid-electric vehicle essentially consists of an electric vehicle where all of the power is
provided to the wheels by the electric drive motors and power can be derived exclusively from
the batteries if necessary. In a hybrid vehicle, the engine is used to generate electrical power
from a liquid or gaseous fuel that is stored on board the vehicle. While the APU may consist of a
fuel cell, which produces electric power directly, most of the hybrid vehicles today have either a
turbine or a piston engine, which is producing rotational mechanical power. To generate
electricity the engine or turbine is connected to a generator. Because the main electric system in a
hybrid-electric vehicle is isolated, the frequency of the power (60 cycles for ground power) does
not apply.
There are several benefits to a series hybrid-electric layout that are a direct result of having the
engine de-coupled from the wheels. The generator can be sized so that the engine is never
required to produce maximum torque and as a result avoids the typical engine operating zone
with relatively high particulate emissions, but still maintains the ability to vary speed. Even in a
load following application the engine responds to vehicle power demands instead of only torque
demands as in a conventional vehicle.
Compare a series hybrid-electric vehicle in which all power must be provided by the APU to a
conventional vehicle on the Central Business District chassis testing cycle. During acceleration
the conventional vehicle’s engine speed increases at near maximum torque and then shifts gears.
Engine speed again increases at maximum torque until 20 mph is achieved. At 20 mph the engine
speed tracks vehicle speed (based on the overall gear ratio of the vehicle) and engine torque falls
to a relatively small value necessary to overcome road load. In the hybrid-electric vehicle the
total available power from the drive motors limits the demand for acceleration. Because the
engine in not connected to the wheels, the engine can ramp up to the engine speed necessary to
produce that power and stay there. The end result is that while the engine in a hybrid-electric
vehicle varies over a substantial speed range, the torque for each speed is relatively constant and

is below maximum available engine torque thereby allowing the engine to ramp up evenly while
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minimizing PM emissions. In addition to avoiding the high PM zone (upper left and center
quadrants) of the torque speed map, hybrid-electric APUs avoid much of the upper right and
lower right quadrants of high NOx emissions and low engine efficiency. The lower right region
(high engine speed low torque) of low engine efficiency is avoided as it is physically impossible
for the engine to operate there because it is coupled to a generator and control system that will
cause the engine speed to decrease with reduced power output.
By de-coupling the engine from the vehicle wheels, and using the batteries to provide
supplemental power, you now have the ability to level the electrical demand from the APU
(referred to as load leveling). That is during relatively short high power accelerations the APU is
assisted by the vehicle’s batteries in supplying power to the drive motors. Much of this
supplemental energy provided by the batteries will be recovered via regenerative braking when
the vehicle comes to a stop. As a result of this load leveling, the only time the engine in a hybrid
electric vehicle would venture into the high speed, high torque, maximum power upper right
quadrant of the torque speed map would be during extended hill climbing or battery pack failure.
The end result is an engine that in theory operates in a defined operating range with reduced
transient speed and torque changes.
The overall efficiency of a hybrid drive system is determined by a combination of factors not the
least of which is the efficiency of the engine itself. While steady state engine operation may
allow the engine to be tuned for peak efficiency, much of this energy will have to pass through
the batteries, which are only about 80 percent efficient. APU load following is generally used to
avoid the inefficiency of the batteries as long as the engine and generator efficiency for its
operating range is within 20 percent of optimum. Other reasons include wanting to preserve the
life of the batteries or maximizing regenerative braking energy recovery by removing the flow of
energy to the batteries from the APU during braking.
The engine in the hybrid-electric vehicle should in practice be far more steady state than a
conventional vehicle because the engine operating points are generally closer together and over a
smaller range.

2.3 Emission Implications
By using a smaller engine in a hybrid-electric vehicle and by electronically controlling the engine
operating points, emission savings are realized. Assuming a diesel powered hybrid-electric
vehicle, the issues surrounding minimizing hydrocarbon (HC) and carbon monoxide (CO)
emissions are essentially taken care of due to the fact that already low HC and CO emissions are
further reduced by using add-on controls such as an oxidizing catalyst. The real tradeoff in
optimizing an engine is between NOx and PM emissions.
Presently the challenge in designing and calibrating diesel engines lies in simultaneously meeting
PM and NOx requirements. A modern diesel engine, if optimized solely for efficiency, will yield
about 15 to 20 grams of NOx per indicated horsepower-hour of work. This happens over a broad
range of speeds and loads where the indicated power represents work done at the piston, and
includes both the brake (output) power and the friction losses in the engine. This yield is
consistent because NOx formation requires both the presence of high temperatures and oxygen,
and these are both available in the high temperature zones during the heterogeneous combustion
in the cylinder. Production of PM is more closely allied to the air to fuel ratio in the engine.
Diesel engines, unless far over-fueled, operate in a lean condition and are generally un-throttled
with only the fuel flow varying. Although some PM may arise from unburned fuel at very light
loads, steady state PM generally increases exponentially with load, and it is a smoke limit (and
hence fuel limit) that determines the maximum rated power of most engines (PM is therefore
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readily reduced by de-rating the engine). Over the last decade PM has also been reduced through
much improved injection system design and improved in-cylinder charge motion.
NOx emissions have historically been reduced by retarding the injection of the fuel with respect
to the piston phasing. In this way, tailpipe NOx emissions levels can be reduced by a factor of up
to three before the loss of engine efficiency due to late injection becomes unacceptable and before
thermal management of engine components becomes problematic. Unfortunately, retarded timing
causes high PM production because there is less time for the fuel to burn and because the average
temperatures during combustion are lower. The timing issue is therefore often termed a “NOx -
PM tradeoff”. Present day engines operate with retarded timing for NOx, and high air to fuel

ratios with high-pressure injection to reduce PM.
In calibrating an electronically controlled engine, the manufacturer has the ability to configure
injection timing at any operating point independently of the timing at other points, which was not
the case with earlier mechanical injection and which was not traditionally anticipated by federal
regulations. Faced with the objective of meeting an emissions certification standard while
meeting the need for good fuel efficiency, it is likely that the calibration engineer will favor
retarded timing to reduce NOx in those areas most heavily visited by the certification test. It is
therefore essential from a regulatory perspective that the certification represents sufficiently well
the real in-use speed and torque ranges of the engine, else emissions inventory will become de-
coupled from regulation. It is for this reason that the present document seeks to identify the real
world operating zones of hybrid vehicle engines.
Internal combustion engines as well as turbines are limited in power by the amount of combustion
air they can aspirate. In most cases more than enough fuel can be supplied if necessary.
Assuming that the spray and duration characteristics of diesel fuel injection are respectable the
formation of particulate will be minimized, as long as there is sufficient excess air. In a diesel
engine sufficient excess air is far lean of stoichiometric as are heavy-duty natural gas engines.
Light duty CNG, propane and gasoline engines are typically stoichiometric allowing the use of
three way catalysts for control of NOx. The amount of air moving through an internal
combustion engine depends upon its displacement, speed (rpm) and whether it is fitted with a
turbocharger. At low engine speed (idle for instance) very little air flowrate is available to the
engine and as a result power output is low as well. As engine speed increases more combustion
air is available and more fuel can be injected increasing engine power. If minimal additional fuel
is injected engine rpm will increase moderately with minimal PM emissions. Generally speaking,
faster rates of change in engine speed require that larger amounts of fuel be injected during the
transient phase, which may result in excess PM emissions. As a result, rapid changes in engine
speed at high load would likely result in a PM event while a rapid change in engine rpm at
relatively light load (revving the engine in neutral) would not result in a PM event.
Transient operation of a turbocharged diesel involves even more subtle fueling management. The
following simplistic argument illustrates the issue. At the one extreme, if a sudden demand for
high power is met simply by allowing full fueling, that power demand is met quickly. However,

the turbocharger still takes finite time (several seconds) to achieve operating speed and relies on
the increased exhaust flow from the added fueling to achieve this speed. During this finite time
the engine would be heavily fueled but would not have full airflow, and black smoking would
result.
At the other extreme, the transient could be followed in a quasi steady-state fashion. At the
demand for high power, an incrementally small additional quantity of fuel could be added, and
the turbocharger speed would rise incrementally, increasing the airflow incrementally, and
allowing another increment of fuel to be added. If these increments are sufficiently small, no
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additional PM would arise in the transient, but response time of increased power would be
unacceptably slow. Present day transient management strategies lie between these two extremes.
Since series hybrid vehicles do not require rapid power deployment from the engine, a far less
aggressive transient strategy can be adopted, and the production of additional PM above and
beyond the steady state level can be significantly reduced. The end result is that a large portion
of the PM emissions from diesel engines happens as a result of transient engine operation both in
speed and power. This is borne out in emission test information where PM emission rates for a
diesel engine on the FTP exhibit roughly twice the PM emissions of the same engine on the Euro
III steady-state test cycle.
We believe there is sufficient evidence that NOx emissions are primarily a result of peak
combustion temperatures and residence time and that the engine is generally unaffected by
transient vs. steady state operation. The similarity in NOx emission rates of engines on both the
FTP and the Euro III 13 mode test indicates that engine manufacturers are largely able to tune
NOx emissions to the required standard and that NOx emissions are largely similar at all but peak
power load points. Even though the operating points of the FTP and the Euro III tests are
substantially different the emission rates for these tests are similar.
In summary, hybrid powertrains can offer lower emissions than conventional powertrains for the
following reasons:
• The recapture of energy during regenerative braking means that the cycle-averaged
power demand on the engine is reduced, leading to lower fuel usage and hence lower

emissions.
• The availability of electrical power from the batteries to satisfy rapid transient power
demands means that sudden power demands on the engine are less necessary. The
reduction in engine transient severity (“smoothing”) leads to lower PM and CO
emissions.
• The obviation of the need for the engine to operate at all torques and speeds in its range
means that the engine/alternator combination can be configured to favor the most
efficient operating zones of the engine. Reduction in fuel usage implies lowered
emissions.
• The smaller size of a hybrid engine in an application reduces the overhead of engine
pumping and frictional losses, and reduces fuel consumption and hence emissions.
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3.0 Existing Engine Test Cycles
As reported in Chapter 1, the NAVC Hybrid Transit Bus Certification Workgroup chose to
pursue a near-term solution for certifying hybrid buses using an engine rather than vehicle test.
In choosing an appropriate, alternate engine test cycle, the Workgroup reviewed the existing FTP
transient cycle test currently used to certify bus engines as well as a number of existing steady-
state engine cycles that might match well to in-use hybrid engine operation. Chapter 3 describes
the salient features of each of the transient and steady-state cycles. These cycles are then
compared to actual hybrid bus engine operation in Chapter 4.
3.1 The FTP Transient
Cycle
Typically, engines for heavy
transit buses are certified on
the FTP transient test cycle.
The FTP transient cycle
consists of four phases that
mimic different types of
driving conditions from the

CAPE-21 database that was
derived from a variety of
heavy-duty vehicles operating
in Los Angeles and New York
during the early 1970s. Figure
3.1 shows the FTP transient
cycle varies both engine speed
and torque over the course of
the test. These conditions are
simulated to consider traffic in and around cities on both surface roads and highways. The first
portion of the cycle is a New York Non Freeway (NYNF) phase that is meant to represent light
urban traffic volume but with frequent starts and stops. The second phase, the Los Angeles Non
Freeway (LANF), is meant to represent high volume, relatively free-flowing urban traffic (i.e.,
few starts and stops). The third segment is the Los Angeles Freeway (LAFY) portion which is
meant to represent crowded highway traffic. The final phase is a repetition of the NYNF
segment. The FTP transient cycle can generally be described as a cold start test followed by a hot
start test. A cold start is classified as starting the engine and test cycle after the engine has sat
overnight and has cooled down to cell temperature. Overall, the FTP transient cycle consists of a
wide variety of speeds to simulate operating the engine in a vehicle on several different kinds of
duty cycles, and also frequently varies the engine load to provide for few instances of stabilized,
sustained operating conditions.
Although the FTP target torques may suggest high motoring efforts, diesel engines offer little
motoring resistance and are simply operated in “closed rack” at maximum possible negative
torque during these sections of the FTP. Such operation is permitted in the subsequent test
verification procedure.
7

7
Motoring in the FTP is necessary to reduce engine speeds according to the rates prescribed in the FTP.
There is actually very little motoring in the FTP. It would be more accurate to call the negative torque

“engine braking”. The original reason for not including negative torque in the report figures is that no work
Figure 3.1: The FTP Transient Cycle
FTP Engine Speed and Torque
-120
-80
-40
0
40
80
120
0 200 400 600 800 1000 1200
Time (Seconds)
Percent Speed
-40
0
40
80
120
160
200
240
280
Percent Torque
rpm torque