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
HYBRID TRANSIT BUS CERTIFICATION
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

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.
HYBRID TRANSIT BUS CERTIFICATION
Agreement No.: NAVC0599-AVP009903 09/15/00 iv
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.
HYBRID TRANSIT BUS CERTIFICATION
Agreement No.: NAVC0599-AVP009903 09/15/00 v
Executive Summary
The purpose of this report is to help facilitate the introduction of hybrid-electric drive technology

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.
HYBRID TRANSIT BUS CERTIFICATION
Agreement No.: NAVC0599-AVP009903 09/15/00 vi
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

6.2 Future Research Needs
Appendix A: Attendance List for NAVC Hybrid Transit Bus Certification
Workgroup Meetings
HYBRID TRANSIT BUS CERTIFICATION
Agreement No.: NAVC0599-AVP009903 09/15/00 viii
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
HYBRID TRANSIT BUS CERTIFICATION
Agreement No.: NAVC0599-AVP009903 09/15/00 ix
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

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.

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

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.
HYBRID TRANSIT BUS CERTIFICATION
Agreement No.: NAVC0599-AVP009903 09/15/00 3
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

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.
HYBRID TRANSIT BUS CERTIFICATION
Agreement No.: NAVC0599-AVP009903 09/15/00 4
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

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
HYBRID TRANSIT BUS CERTIFICATION
Agreement No.: NAVC0599-AVP009903 09/15/00 5
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

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)
HYBRID TRANSIT BUS CERTIFICATION
Agreement No.: NAVC0599-AVP009903 09/15/00 6
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

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

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

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

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

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

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
HYBRID TRANSIT BUS CERTIFICATION
Agreement No.: NAVC0599-AVP009903 09/15/00 13
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

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
HYBRID TRANSIT BUS CERTIFICATION
Agreement No.: NAVC0599-AVP009903 09/15/00 14
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

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