United States
.Environmental Protection
Agency
Air and Radiation
EPA420-R-98-006
August 1998
&EPA
Evaluation of a Toyota
Prius Hybrid System
(THS)
> Printed on Recycled Paper
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EPA 420-R-98-006
Technical Report
Evaluation of a
Toyota Prius Hybrid System (THS)
by
Karl H. Hellman
Maria R. Feralta
Gregory K. Piotrowski
August 1998
NOTICE
Technical reports do not necessarily represent final EPA
decisions or positions. They are intended to present technical
analysis of issues using data which are currently available. The
purpose in the release of such reports is to facilitate the
exchange of technical information and to inform the public of
technical developments which may form the basis for a final EPA
decision, position, or regulatory action.
United States Environmental Protection Agency
Office of Air and Radiation
Office of Mobile Sources
Advanced Technology Support Division
Technology Development and Support Group
2565 Plymouth Road
Ann Arbor, MI 48105 '
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Contents
Page Number
Contents ii
Summary iv
I. Introduction 1
II. Description of Test Vehicle/Powerplants
A. Vehicle 3
1 i
B. Internal Combustion Engine . „ .... 5
C. Electrical Propulsion System .......... 6
D. Vehicle Operating Strategy . . .... 6
III. Test Facilities and Analytical Methods ........ 7
IV. Prius Test Issues „ 8
V. Test Procedure 14
VI. Discussion of Test Results
A. Performance (0-60 mph Acceleration) Testing . . 18
B. Testing Over Consecutive HFET Sequences .... 23
C. Combined City/Highway Emissions/Fuel Economy
Testing 26
D. Testing Over Designated Sequence #1 30
E. Hot Start FTP Following Battery Discharge ... 32
F. Testing Over SC03 Driving Schedule 36
G. Testing Over US06 Driving Cycle 39
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H. Testing Over Select Vehicle Steady-State
Speed Modes . . . . 41
! '
I. Fuel Economy Issues • •
1. Adjustments for Net Charge/Discharge
of Battery Pack 44
2. "Running" Fuel Economy 49
3. Comparison to Current Model Vehicles ... 52
4. What About the 66-MPG Value Associated
With This Car? 53
VII. Highlights from Testing 56
VIII. Acknowledgments 58
IX. References ' . .- 59
Appendix A - Test Vehicle Specifications
Appendix B - Internal Combustion Engine Specifications
Appendix C - Test Fuel Properties
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SUMMARY
A Toyota Hybrid System (THS) vehicle was evaluated by EPA
over a variety of testing modes and conditions. The THS,
provided by the Toyota Motor Corporation, is a hybrid electric
vehicle powered by an internal combustion engine and an
alternating current synchronous motor. The hybrid system allows
the engine to operate in a lower speed, higher torque, and higher
efficiency mode, and enables regenerative braking recharging of
the vehicle battery pack. The THS is described by Toyota as a
charge sustaining hybrid, meaning that the vehicle power
management system maintains the charge in the battery pack by
recharging with the on-board engine. The Prius THS tested here
is a Japan-market production vehicle, originally designed for the
Japanese vehicle market. The THS would be classified as a
subcompact-class vehicle under the United States classification,
and the curb weight, with a full 50-liter fuel tank, is 2783
pounds.
Zero-60 mile-per-hour (mph) acceleration testing, as a
measure of performance, was conducted. Acceleration time
depended upon the state of charge of the battery pack. With a
battery pack charged in a normal manner contemplated by the
designers, 0-60 mph acceleration times slightly in excess of 14
seconds were measured. With a battery discharged well below the
normal level of charge contemplated by the Toyota designers, 0-60
mph acceleration time increased to slightly over 19 seconds.
One important evaluation sequence consisted of two
consecutive tests over the Federal Urban Dynamometer Driving
Schedule (hereafter, "4-bag FTP"), followed immediately by a
single test over the Highway Fuel Economy Test (HFET) cycle.
This cycle is explained in Section V, Test Procedure, and is
hereafter referred to as the "Number 2" test sequence.
Emission levels of hydrocarbons, carbon monoxide, and oxides
of nitrogen over the 4-Bag FTP were well below current Federal
light-duty vehicle emission standards. A city/highway composite
fuel economy, using traditional "55/45" weighting may be
described with the Number 2 test sequence above, if the measured
HFET conducted immediately following the "4-bag FTP" is used for
the highway fuel economy number. A 55/45 composite fuel economy
calculated in this manner using the data given in this report was
IV
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48.6 mpg. A variation of this test sequence is to follow the 4-
bag FTP with two HFET cycles, the first, a:preconditioning drive,
and the second, a measured HFET (hereafter, the "Number 1" test
sequence, see Section V, Test Procedure). The average
city/highway (55/45) composite fuel economy calculated with data
from this report over the Number 1 procedure was 49.8 mpg.
Fuel economy could be affected by the net charge/discharge
of the vehicle battery pack over the measured cycle, but the
results presented in the summary above were not adjusted for this
occurrence. All of the tests here were conducted with California
Phase-II fuel (see Appendix C) , at Toyota's request, and
represent calculated volumetric fuel economies unadjusted in the
manner currently used for gasoline-fueled certification vehicles.
I.
Introduction
The oil price shock experienced by the global marketplace in
the mid-1970's had a profound effect on automobile design, toward
models of increasing fuel efficiency. This, experience was
reinforced by the adoption in the United States of Federal
Average Fuel Economy Standards, which provided requirements for
automobile manufacturer sales fleet fuel economy. Federal and
state clean-air regulations also required automakers to respond
with automobiles that emitted lower levels.of tailpipe
hydrocarbons, carbon monoxide, and oxides of nitrogen.
Recently, concern has been raised in the scientific
community and the public at large about climate change caused by
the increased use of fossil fuels. In the United States, the use
of fossil fuels for transportation purposes is a major source of
carbon dioxide (C02) emissions. C02 is recognized by most of the
scientific community as a "greenhouse effect" gas, promoting
global warming. Improvements in new vehicle fuel economy have
been leveling off in the United States, consistent with the fact
that the fuel economy standards have remained constant. In
addition, as domestic crude oil stocks become economically less
profitable to extract, U.S. petroleum needs are being satisfied
by relatively less expensive foreign source oil. This foreign
source oil is a major concern with respect to a United States
foreign trade deficit with the rest of the world.
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Government and industry have responded to these
environmental and trade concerns with a Presidential initiative,
the "Partnership for a New Generation of Vehicles" (PNGV). PNGV
is a joint Federal government/private industry effort which has
as one goal the design and production of prototype vehicles with
three times the fuel economy of 1993 baseline vehicles. These
prototypes must be similar in load carrying capacity and
performance to "baseline" sedan vehicles, such as the 1993
Chrysler Concorde, Chevrolet Lumina, or Ford Taurus vehicles.
They must also meet Federal safety and emission standards in
their year of introduction. Foreign automakers are also
researching and designing automobiles that increase fuel
efficiency and yet reduce regulated emissions to levels that are
below those of comparable vehicles today.[1]
One near-term option for a relatively fuel efficient vehicle
is a so-called "hybrid" powerplant vehicle. Hybrids typically
make use of two powerplants, for example, a "conventional"
internal combustion engine and an "unconventional" powerplant
such as an electric motor system or flywheel energy storage
system. The "unconventional" powerplants drive the vehicle
during design-selected driving modes and may be designed to
regenerate braking energy normally dissipated as heat. The
"conventional" internal combustion engine may supplement, or
recharge, the unconventional powerplant, and the engine may
funnel its motive force either to the drive wheels directly
through a common system with the unconventional powerplant, or
possibly through a combination of both. Internal combustion
engine displacement, output, "unconventional" powerplant
characteristics, modes of operation, etc. are determined by the
system designers to optimize the characteristics of the
powerplants considered.
Toyota Motor Corporation recently announced the development
of the Toyota Hybrid System (THS).[2] This hybrid vehicle makes
use of an internal combustion engine, optimized for efficiency, a
high-power battery, and an electric motor/generator (see vehicle
description below). This vehicle has been in production in Japan
since December 1997.
EPA made a request to Toyota Motor Corporation for a Toyota
Hybrid System for evaluation at EPA's National Vehicle and Fuel
Emissions Laboratory. Toyota responded favorably to this request
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on February 3, 1998[3] and supplied EPA with a Prius hybrid
vehicle on April 23, 1998. According to Toyota, the vehicle
loaned to EPA by Toyota is representative of the Japanese
production configuration and was not designed to meet the U.S.
emission standards. This vehicle was evaluated over several
different driving cycles and test conditions, and the results
from this testing are presented and discussed in this report.
II. Description of Test Velhicle/Powerplants
A. Vehicle
The THS uses a 4-door Toyota Prius vehicle, with a curb
weight of 2783 Ibs with 100-percent fuel fill (Figures 1, 2, and
3, below). At Toyota's request, the vehicle was tested at 3000-
Ib test weight. A complete description of the test vehicle is
provided in Appendix A.
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Figure 1
Toyota Prius Hybrid Electric Vehicle
Figure 2
Toyota Prius THS - View of Vehicle Trunk
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Figure 3
Toyota Prius THS - Engine Compartment
B. Internal Combustion Engine
According to Toyota, the gasoline engine employed in this
vehicle is specific to the THS and is not used currently with any
other Toyota vehicle. The '.basic design is a 16-valve, 1.5-liter
displacement, in-line 4-cylinder engine, with the head and block
both constructed of aluminum. The combustion chamber is a
compact pentroof design, employing a slanted squish area. The
engine is mounted transversely, with a slight cant to the rear
(Figure 3). An underfloor catalyst is placed in close proximity
to the exhaust manifold. :
The engine employs Toyota Variable Valve Timing with
Intelligence (WT-i). This system varies valve timing through
the means of electronic control of an oil control valve, which in
turn controls oil pressure to hydraulically actuate the WT
system pulley. The range of timing employed by the WT-i in the
THS is 40°.
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The engine upper speed limit is 4000 rpm, and it operates
through a generally narrow speed range. The operating cycle
employed is a modified Atkinson cycle, with an expansion ratio of
13.5:1. Effective compression ratio is about 9:1, and late
intake valve closure is employed. The engine produces 58
horsepower at 4000 rpm. Details are given in Appendix B, with
further details available from the Toyota Motor Corporation.
C. Electrical Propulsion System
The electrical drive system employs a permanent magnet
electric motor, a generator, an inverter, and a nickel/metal-
hydride battery pack. The motor itself acts as a generator
during deceleration or braking, to convert the kinetic energy
usually dissipated as heat in the friction brakes into
electricity to recharge the battery. Detailed specifications are
available from the Toyota Motor Corporation.
D. Vehicle Operating Strategy
The THS utilizes a unique power "splitting" device that
permits part of the internal combustion engine output to be
applied to the wheels directly, while a portion of the output is
applied to the generator, to the electric motor, and then to the
wheels. Because of this "split," the Prius powertrain functions
in a slightly different manner than a "conventional" parallel
hybrid powertrain might be expected to function. A brief
description of the THS powertrain function over typical driving
modes is provided below.
The driving modes and strategies described below are
summarized from the Toyota promotional literature.[1] EPA has not
verified whether the vehicle drives in the manner prescribed
here. This information is presented for informative purposes
only.
Star tup /Iiiqht Load: Toyota claims that the nickel hydride
battery, providing power through the motor, runs the vehicle at
startup and at light load.
Normal Driving; During "normal driving," the power splitting
planetary geartrain is adjusted to divide the engine's output
into two paths, one to drive the wheels directly and the second
to drive the generator to produce electricity for the motor
and/or charge the battery. The motor then can provide additional
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driving force to the wheels. The electronic control unit
controls the distribution of power to the two separate driving
force paths for maximum efficiency.
Deceleration/Braking: During deceleration, the inertia of
the vehicle turns the motor, which then acts as a generator to
recharge the battery.
Battery Charging: The electronic control unit signals the
charging unit to maintain a near constant charge in the battery
pack.
Full-Throttle Acceleration: During full-throttle
acceleration or under heavy load, the motor is assisted by the
battery so engine power and battery power are both used to drive
the vehicle.
III. Test Facilities and Analytical Methods
EPA testing was conducted on dynamometer #2 of Site #1,
Light-Duty Vehicle Test Site, at the EPA National Vehicle and
Fuel Emissions Laboratory. The dynamometer used was a Horiba
Electric Dynamometer, Model CDC-900, incorporating a single 48-
inch roll, and capable of continuous power absorption from 0-125
hp at 65-mph conditions. The power exchange unit is an AC
induction motor/generator. A Philco-Ford constant volume
sampler, with blower set at 350 cfm was used. Hydrocarbon
emissions were measured with a Beckman Model 400 flame ionization
detector. Methane emissions were measured with a Bendix 8205
methane analyzer. CO and CO2 were measured with Horiba AIA-23
infrared analyzers. NOx emissions were measured by
chemiluminescent method with a Beckman 941-A analyzer.
EPA was provided by Toyota with a Hioki Model 3167 AC/DC
Clamp-on Power Hi-Tester meter for use in quantifying the
charging/discharging of the vehicle battery pack over a test
mode. This meter provided an integrated measure of current flow
and time to determine the net charge/discharge of the battery
through the vehicle power management system during a test mode.
This measure was given in amp-hours and is presented as "net"
amp-hours in the data below (charge versus discharge). The sign
convention used here is negative for net charging of the battery
pack by the engine, and positive for net ,discharge of the battery
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through the motor to the wheels. Toyota provided EPA later in
the evaluation with a Toyota Techno S2000 engine-scanning tool
which could calculate an approximation to the state of charge of
the battery pack (see discussion in IV, below). Toyota stated to
EPA that the S2000 meter provides an index of SOC used internally
by THS. Some testing was conducted utilizing SOC approximations
before and after test modes.
IV. Priua Teat Issues
The Toyota Prius hybrid (THS) system is currently offered
for sale to the general public in Japan but is not sold in the
United States. The California Air Resources Board has published
a draft test procedure for emissions certification of new hybrid
electric vehicles in the state of California. EPA has not
promulgated test procedures for Federal emissions and fuel
economy testing of hybrid vehicles.
Because of the nature of the hybrid electric powerplant, the
power management system and hence, the emissions and fuel economy
profile, of a hybrid vehicle may be considerably different than a
vehicle equipped with a single internal combustion engine.
Testing issues, because of these powerplant/management
differences, may be raised in the interest of test-to-test
variation, fairness, data measurement, significance, and safety.
Some of these issues are discussed below.
The following terminology is used in this report to describe
the battery conditions of the test vehicle:
State of Charge (SOC): If a battery's maximum charge
capacity can be expressed in amp-hours, then the state of charge
(SOC) is the percentage of that maximum amp-hour value remaining
in the battery. SOC depends on many variables, and determining
SOC is quite difficult. At present, there is not a universally
accepted method or instrument for determining SOC from batteries.
Since one of the goals of testing hybrids is to either (1)
have the state of charge of the energy storage system be the same
at the beginning and the end of the test or (2) be able to adjust
the results for any difference, measuring the energy storage
system's status—the battery's state of charge in this case—is a
critical test procedure issue.
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(NA-h) ; Net amp-hours represent the integrated
result of the current into arid out of the battery over a given
time period. In order to measure NA-h, EPA utilized a meter
provided by Toyota, a Hioki 3167 meter. Values determined using
this meter are reported as net amp-hours in the test and the
tables in this report.
Charge Index (CD : This index is used by the control system
of the Toyota THS system, 'it is a calculated value which
represents an approximation to the SOC of the battery. This
index was measured by use of a scan tool, S2000, provided to EPA
by Toyota. '
It can be seen that both NA-h and CI are indicators of the
battery's status and thus may be related to the battery's SOC.
Using Net Amp-Hours as a Surrogate for State of Charge
When Net amp-hours are used to characterize the status of
the battery, consideration must be given to the voltage
associated with discharging and charging the battery.
When current is being drawn from a battery, the voltage
drops from the no-load (open circuit) voltage. Figure 4 shows
discharging battery voltage versus percent- depth of discharge
for a nickel metal hydride battery—the same type of battery as
the one in the Prius test vehicle. The data are from GM Ovonic.
The voltage at 90-amp discharge is less than the voltage at lower
currents.
When a battery is being charged, the voltage generally
exceeds the open circuit voltage. Therefore, it is possible
that, for equal amp-hours in and out of the battery, the input
and output power may not be the same. Therefore, using a
criteria of Net amp-hours = zero may be slightly conservative in
ensuring that the'battery's status is the same before and after a
test or a portion of a test.
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Figure 4
Discharging Battery Voltage Versus Level of Discharge
Nickel-Metal Hydride Battery
NIMH EV Battery Voltage at 20°C
2O 4O «O 80
Percent Depth of Discharge
A 90 Amp • 45 Amp • 30 Amp
100
Maintenance Mode for Dynamometer Driving
It was necessary to invoke a "maintenance" mode of operation
to permit dynamometer testing of the Prius THS. The Prius THS is
equipped with a traction control system which would be activated
when a significant difference between the front drive wheel and
the rear wheel speeds is noticed. To allow testing on the EPA
single-roll dynamometer, this "maintenance" mode was invoked by a
series of accelerator and transmission position changes.
According to Toyota, when the vehicle is in "maintenance mode"
and when the gear shift selector is placed in the "drive"
position, the traction control mode is disabled, and remaining
vehicle operations are the same as the actual road driving mode.
EPA did not perform any over-the-road tests of the Prius vehicle.
Manufacturer-Supplied Metering Equipment: Toyota supplied
EPA with a directional current measurement (Hioki 3167) meter to
determine net battery-pack charging/discharging over a driving
mode or period of time.
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Toyota also supplied EPA with a meter' that Toyota claimed
could be used to provide an internally used index of state of
charge. (See Test Facilities section above.) Any equipment used
to measure important parameters probably should be evaluated by
EPA and/or by another independent group such as NIST with respect
to performance, accuracy, and reliability. Although EPA made use
of this manufacturer-supplied equipment during this evaluation in
the interest of timeliness, EPA did not perform a separate
evaluation of either instrument.
Effect of Preconditioning on Hybrid Electric Vehicles
The mpg of a test vehicle may be affected by the
preconditioning, i.e., how the vehicle was operated prior to the
test results being measured. Most prior operation considerations
in the past have been related to vehicle thermal effects,
primarily the degree to which the engine and emission control
system in the vehicle have warmed up. For example, a cold-start
test phase and a hot-start test phase are included in the
standard emission test sequence.
In addition, current test protocols typically provide for
operating a vehicle the day before the test on a specified urban
driving cycle prior to the pre cold-start soak period.
Since the Prius is equipped with a gasoline-fueled spark
engine, it was expected that there would be some sensitivity of
the results to preconditioning based on engine and emission
thermal characteristics. What was not known before the test
program was run was the degree to which the emissions and fuel
economy results of the Prius would be affected by its hybrid
propulsion system, especially by the status of its battery.
Preconditioning Cars Equipped with Big Batteries: When a
battery isn't being used, it loses charge.', Conventional vehicles
do have starting, lighting, and ignition (SLI) batteries, but
their self-discharge has never been an issue for emission and
fuel economy testing as far as preconditioning goes.
For some cars equipped with a hybrid electric propulsion
system utilizing a battery big enough to provide traction power,
the preconditioning issue may need to be re-examined. Figure 5
shows a .battery self-discharge curve for a. nickel-metal hydride
battery. For a 20°C temperature, this battery loses about 1.3
percent of its charge per day. It should be noted that the data
are from a nickel-metal hydride battery that is not the same as
the nickel-metal hydride battery in the Toyota Prius.
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Figure 5
Nickel-Metal Hydride Battery Self Discharge
NIMH EV Battory Charg« Retention
1OO
8 10
Stand HUM, Day*
A 40°C • 20°C • 0°C
If one thinks of the battery as a fuel tank, a 1.3-percent
loss per day is like a conventional vehicle with a 20-gallon fuel
tank leaking a little more than a quart of fuel per day. The
maximum power capacity of the THS battery back is 1.8kW-hr,
however, which Toyota claims is equivalent to only 0.238 gallons
of gasoline. In this example, therefore, a 1.3-percent discharge
per day would be equivalent to about 0.003 gallons of gasoline
per day.
Two Wheels on the Dvno versus Four Wheels on the Road; This
issue has always been of concern to EPA since vehicles have been
tested on chassis dynamometers. For hybrids with regenerative
braking, the issue is more complicated. Consider the example of
a vehicle like the Prius. It has front-wheel drive, and the
front wheels are used for regenerative braking. Conventional
friction brakes provide the braking for the rear wheels. In over
the road driving, the percent of power consumed by the brakes is
split between the front brakes and the rear brakes. According to
Toyota, for the THS Prius, for normal braking conditions over LA-
4 and HFET test schedules, the braking distribution is 90-95
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percent to the drive axle. As a hypothetical example, say the
front brakes provide 75 percent of the braking and the rear
brakes the balance, 25 percent. When the vehicle is put on a
single-roll chassis dynamometer, the front wheels drive, and the
rear wheels do not move at all. If the vehicle is equipped with
an antilock braking system and/or a traction control system, the
system must be defeated, which was done for this testing.
On the dynamometer, the vehicle's front wheels provide 100
percent of the traction—like they do on the road—and 1QO
percent of the braking—which is not what happens on the road.
.The potential problem with hybrid vehicle testing is with
the representativeness of the regenerative;braking on the
dynamometer. Consider the previous example of 75%/25% road
braking. If the front-wheel regenerative braking takes some
fraction of the braking energy over the road, for example, say it
is one-third, then a comparison of the over-the-road to chassis
dynamometer braking energy distribution is shown in .Table 1.
Given the example in Table 1, the amount of energy available
for recharging on the dynamometer may be too high compared to
what actually happens on the road, and it could be conjectured
that the resulting mpg from the chassis dynamometer will be
inappropriately high. : '•
Table 1
Hypothetical Hybrid Vehicle
Braking Type
Front Wheels
Regeneration
Front Wheels
Dissipative
Rear Wheels
Dissipative
Total
Braking Energy
Over the Road
25%
50%
25%
100%
Chassis Dynamometer
33%
67%
'. 0%
100%
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Resolving this potential problem is beyond the scope of this
report. Utilization of a chassis dynamometer configuration which
exercises all the vehicle's wheels or special programming of a
single-roll electric dynamometer's power absorption
characteristics are two concepts that might be worthwhile to
investigate.
Safety Considerations: Any hybrid vehicle will have an
energy storage system that stores energy that can be rapidly
discharged and recharged. When test vehicles are brought into a
laboratory such as EPA's for testing, modifications are made to
them to allow efficient testing, so some work is done on the test
cars by test personnel. For hybrid vehicles, it is likely that,
in the future, developers of hybrid propulsion systems should
include information about possible hazards for people that work
on the cars so accidents do not occur. For example, the Prius
vehicle's battery pack is rated at 288 volts—a voltage high
enough to be treated with care as it exceeds the voltage value
usually considered to be potentially lethal.
V.
Test Procedure
The testing described here was not conducted 'for purposes of
emissions certification or the determination of official fuel
economy values. The goal of this test program was to
characterize the emissions, fuel economy, and (limited)
performance of the THS over a variety of driving cycles and
conditions of interest to EPA. In addition, there is no current
Federal test procedure for the emissions certification of hybrid
electric vehicles. EPA therefore tested the vehicle in a manner
that would collect data of interest to a number of different
audience groups.
Two test sequences, used in the THS evaluation here, are
described in Figures 6 and 7. Figure 6 describes a 4-bag FTP
test, preceded by a preconditioning drive over the LA-4 sequence
and followed by a warm-up and measured Highway Fuel Economy Test
(HFET) sequence. This entire test sequence is hereafter referred
to as "Test Sequence 1" in this report. The second test sequence
described in Figure, 7 is hereafter referred to as "Test Sequence
2." Test Sequence 2 differs from Sequence 1 by designating the
final vehicle drive on the day preceding a Sequence 2 test day as
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the preconditioning, regardless of the cycle driven, and by the
4-bag FTP being followed by a single, measured HFET. For all
testing described as Sequence 1 or Sequence 2, below, the
preconditioning drive between the second LA-4 and the HFET
sequences was not conducted because the delay in all cases was
less than 10 minutes. Tests were also conducted over the US06
and SC03 driving schedules; these tests are described in detail
in separate subsections of Discussion of Test Results, below.
Performance testing, limited to 0-60 'mph acceleration
testing on the chassis dynamometer, was conducted. The mode of
deceleration after the acceleration affected the charging of the
battery. "Hard" and "medium" braking, determined by length of
braking time from 62-63 mph to stopped conditions, with the
gearshift selector in drive, caused the regenerative braking
system to recharge the battery pack. "Coasting down" the
vehicle, from 62-63 mph to 5 mph, braking thereafter to stop,
recharged the vehicle during deceleration to a lesser degree than
braking during the entire deceleration. .Engaging the "neutral"
position on the gear-shift selector, rather than the "drive"
position, disengaged the engine and regenerative braking systems
from the battery-charging circuit. This action was taken to
discharge the battery; Toyota cautions against engaging the
neutral position during actual driving for safety reasons. The
resulting state of charge of the battery, following such testing,
influenced successive acceleration tests. This testing is
described further in the Discussion of Test Results, below.
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Figure 6
Test Sequence Number 1
Preconditioning drive
LAM 1 cycle
(SOC stabilised)
Vehicle soak
4 Bags
Cold start exhaust test
Cold transient
Stabilized
Hot start exhaust test
Hot transient
Stabilized
Preconditioning drive
I
Highway fuel economy test.
1 st. cycle : Warmup
2nd. cycle : Emission sampling
12~36 hours
Measure net change in Ampere-h's
4 10 minutes
Measure net change in Ampere-h's
>3 hours
13 hours
Measure net change in Ampere-h's
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Figure 7
Test Sequence Number 2
4 Bags
Vehicle soak
Cold start exhaust test
Cold transient
Stabilized
Hot start exhaust test
Hot transient
Stabilized
Preconditioning; drive
I
Highway fuel economy test
1st. cycle : Emission sampling
12"36 hours
Measure net change in Ampere-h's
I 10 minutes
Measure net change in Ampere-h's'
>3 hours
hours
Measure net change in Ampere-h's
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Tests were conducted at 10, 20, 30, 40, and 50 mph "steady-
state" conditions. With a vehicle operated with an internal-
combustion-engine-only powerplant, the vehicle is normally driven
at the desired vehicle speed conditions until certain parameters,
such as coolant temperature, engine oil temperature, etc. reach
relatively steady-state conditions prior to emissions sampling.
Because of the nature of the THS powerplants (the variance of the
operation of the charging system and engine operation possibly
related to state of battery charge) , it was uncertain to EPA when
"steady-state" conditions might be reached. It is important to
note, therefore, that "steady-state" as referred to in this
section, does not reflect battery condition. We conducted these
"steady-state" tests in a manner consistent with steady-state
testing of a conventional powerplant-only vehicle. The test
results are referred to as steady-state test results, but the
battery state of charge, particularly at 10-mph conditions, may
materially influence the test results. The vehicle was operated
at 10-mph speed conditions for a length of time (approximately 5
minutes) immediately following a driving preparation over the LA-
4 cycle. Emissions sampling at 10-mph conditions was then
conducted over a 10-minute sample interval. The vehicle was then
driven to the next highest (20 mph) speed test point, driven for
a period of time, and then sampled. This procedure was used for
all steady-state sample points. This testing is described in a
separate section below.
Testing was also conducted over the Hybrid Vehicle Testing
Procedure in SAE J1711 (draft), and the (draft) California Air
Resources Board Hybrid Electric Vehicle Test Procedure.[4, 5]
This testing was conducted for purposes of familiarization only
and not to simulate a certification process. The data from this
testing is presented in separate test summaries apart from this
report.
VI. Discussion of Test Results
A. Performance (0-60 mph Acceleration) Testing
The Prius THS was first tested at the EPA National Vehicle
and Fuel Emissions Laboratory on March 24, 1998. The vehicle was
driven over the LA-4 (Urban Dynamometer Driving Schedule) cycle
in order to familiarize the driver with the operation of the
vehicle. This LA-4 cycle was followed immediately by driving the
Highway Fuel Economy Test Cycle, again for the purpose of driver
familiarization.
18
-------
Upon completion of this familiarization performance testing,
involving acceleration tests from 0 miles/hour (mph) (vehicle
stop) to 60 mph was conducted. This testing was conducted, not
only to obtain 0-60 mph test data but to determine the
sensitivity of the vehicle's performance under hard acceleration
conditions to the state of charge of the battery. (Per the
discussion of vehicle driving strategies above, the THS
electronic control unit requires electric:motor power in addition
to the mechanical power of the internal combustion engine at wide
open throttle for maximum power at the drive wheels.)
Table 2 presents the results of 0-60 mph testing conducted
on March 24, 1998. The testing was conducted by putting the foot
accelerator pedal to the floor at start and releasing the
accelerator at 62-65 mph conditions; 0-60 mph times were
determined by analysis of the driver's trace of the acceleration
"Braking conditions" with 20 or 30 seconds refers to the time of'
braking, from start immediately following release of the
accelerator, to 0 mph (vehicle stop). Twenty (20) seconds
indicates 20 seconds of total deceleration time (braking during
deceleration), much "harder" braking than 'the "30 seconds"
conditions. This difference in braking conditions was done to
determine the effect of regenerative braking on battery charging.
"Coast/D" refers to coasting down the vehicle in Drive shift
selector position. The vehicle was allowed to coast from the
release of the accelerator pedal at 62-65 mph conditions to 5
mph, at which time moderate braking was applied to bring the
vehicle to a stop. "Net Charge" refers to the net
charging/discharging measured with the Hidki meter which occurred
during the accel/decel cycle. (See Table 2 for conventions.)
During the three initial trials at "20 second" (harder)
braking conditions, there was a net discharge from the battery
over each accel/decel cycle. 0-60 mph times remained relatively
constant over these three tests. Net battery discharge was much
less during the fourth accel/decel, and the 0-60 mph time
appeared to be affected by the three previous battery pack
discharges. The 0-60 mph time rose considerably, to 17.3 seconds
on the fifth accel/decel, and there was a slight net charge to
the battery over this cycle. This charging appeared to influence
performance, as the 0-60 mph time recorded on the sixth test was
similar to those noted during the initial three accelerations.
19
-------
Table 2
Toyota Prius HEV
0-60 mph Acceleration Testing, 3/24/SI8
Test Number
1
2
3
4
5
6
7
8
9
10
11
12
Braking
Conditions
20 seconds
20 seconds
20 seconds
30 seconds
30 seconds
30 seconds
Coast/D
Coast/D
Coast/D
20 seconds
20 seconds
20 seconds
0-60 mph
Time (sec)
14.4
14.2
14.2
15.1
17.3
14.2
14.9
18.9
20.1
22.1
19.2
17.3
Net Charge
(Amp-hr s ) *
0.232
0.327
0.325
0/033
-0.073
0.055
0.319
0.170
0.055
-0.254
-0.182
-0.078
* Negative Net Charge refers to net charging of the
cycle, while positive Net Charge refers to a net
battery over tne
battery discharge.
The effect of regenerative braking on change in charging was
noted during the coastdown in drive mode. In the "maintenance
mode" used during testing of the THS, the engine can charge the
battery during decelerations, when the gear shift selector is
placed in "drive," independently from the charging done through
regenerative braking. During the first "coastdown in drive"
(seventh accel/decel cycle) a substantial net discharge from the
battery was noted. The effect of eliminating regenerative
braking during the deceleration phase can be noted in the next
three successive tests, with 0-60 mph times rising to a high of
22.1 seconds in test number 10. At this point, braking (hard)
during deceleration was reintroduced, and 0-60 mph times began to
fall, as net charging of the battery occurred during the
accel/decel phase. The effect of reintroduction of regenerative
braking led to a 0-60 mph time of 17.3 seconds after two cycles.
20
-------
Table 3 refers to 0-60 mph testing conducted on April 6,
1998. "Coast/N"1 refers to coasting down from the end of the
acceleration by putting the gear shift selector into "neutral"
position, coasting down to 5-mph conditions, and braking to a
full stop thereafter. This mode of operation is not recommended
by Toyota but was performed to simulate extreme conditions of
battery discharge.
Table 3
Toyota Prius HEV
0-60 mph Testing, 4/6/98
Test Number
1
2
3
4
5
6
7
8
9
10
11
12
13
14
Braking
Conditions
20 seconds
20 seconds
20 seconds
30 seconds
30 seconds
30 seconds
Coast/N
Coast/N
Coast/N
Coast/N
Coast/N
Coast/N
Coast/N
Coast/N
0-60 mph
(sec)
14.1
14.1
14.2
14.1 ;
14.1
13.9
14.0 '
15.3
18.0 ;
18.9
19.1
19.4
19.6
19.5
Net Charge
( Amp-hr s ) *
.314
.302
.297
.305
.303
.299
.298
.222
.117
.065
.054
.040
.042
.039
* "Net Charge" here refers to the net charge (positive values meaning
net battery discharge and negative values meaning charging of the
battery pack) over the acceleration phase only, not to include the
deceleration.
21
-------
The "hard" versus "medium" braking did not seem to influence
the 0-60 mph acceleration times noted in this testing. Coasting
down in neutral prevented regenerative braking and engine
recharge from charging the battery during the deceleration, as
"neutral" position decoupled both the engine and regenerative
braking from the charging circuit. Coasting down in neutral was
done to quickly discharge the vehicle battery pack; Toyota
recommends against engaging the neutral position during actual
driving for safety reasons. The net discharge of the battery
gradually lessened as these tests were repeated. Figure 8
presents the decreasing net discharge from the battery per
acceleration graphed against 0-60 mph acceleration time. A 0-60
mph acceleration time in excess of 19 seconds is indicated when
the discharge of the battery during acceleration is minimized,
i.e., engine-only operation. Therefore, it seems that the
acceleration time of this vehicle is influenced by the state of
charge of the battery and ranges from about 14 seconds to about
20 seconds, depending on the state of charge.
Figure 8
0-60 mph Acceleration Time versus Battery Discharge
Net Charge (Amp-hr)
0.3-
.25-
0.2-
.15-
0.1 -
0.05-
f(x) = -4
RA2 = 9
58261 9E
955203E
1
•2*x+9.£
-1
1 —
\
\
33507E-
\
\
1
\
^
10 12 14 16
0-60 mph Acceleration (sec)
18 20
22
-------
B.
Testing Over Consecutive HPET Sequences
On two separate occasions, EPA conducted tests over
successive HFET cycles with the THS. This testing was conducted
to determine whether a relationship between battery
charge/discharge over the, HFET and measured fuel economy could be
determined. The first sequence consisted' of five consecutive
HFET s; data from this test is presented below in Table 4.
Table 4
Toyota Prius THS
Five Consecutive Tests Over HFET Cycle
Test
No.
1
2
3
4
5
FID
HC
g/mi
0.01
0.01
0.02
0.02
0.03
NMHCE
g/mi
0.01
0.01
0.01
0.02
0.02
CO
g/mi
0.4
0.3
0.3
0.3
0.3
NOX
g/mi
0.05
0.06
0.07
0.05
0.06
C02
g/mi
183
163
162
163
161
Fuel
Economy
MPG
47.00
52.80
53.02
52. 67
53.30
Net
Charge
Amp-hrs*
-0.871
-0.114
0.049
-0.099
-0.063
* Negative values indicate net charging of the battery pack over the test
cycle; positive values indicate net discharge of the battery pack.
Prior to the first HFET in Table 4, a number of 0-60 mph
acceleration tests were conducted (See A, above). The battery
discharging which occurred during these acceleration tests
influenced the first test substantially. The following tests
recorded fuel economies in excess of 52 mpg for relatively
similar battery charge/discharge levels. The highest HFET cycle
fuel economy recorded here, 53.30 mpg, occurred with a slight net
battery charging over the test cycle.
23
-------
Figure 9, below, is a simultaneous plot of fuel economy
versus net battery charging/discharging measured in net amp-
hours. The straight line least squares method relationship shown
here gives a zero net battery charge/discharge value of 53.32
mpg.
Figure 9
Consecutive Tests Over The HFET Cycle
Fuel Economy versus Net Battery Charge/Discharge
(Data From Table 4)
Fuel Economy [MPG]
54-
53
52
51
50~
49
48"
*± 1
46-
A*-
}
f(x
" R~2
V"
>s
) - 7
= 0.
\
\
.1335:
975
V
\
x + 5:
\
N
3.32
^
\
0.2 0 -0.2 -0.4 -0.6 -0.8 -1
Net Charging [Amp-hrs]
O = MPG at Zero Net Charging
Table 5, below, presents the results of testing over 12
consecutively conducted HFET's with the THS. A problem with the
programming of the Hioki 3167 meter prevented the reading of
battery charge/discharge over the first four HFET.'s here. The
test results from the first test described in Table 5 were
influenced by the deep discharge of the vehicle battery pack
prior to the commencement of testing. The remaining tests had
fuel economy and net charge/discharge values not dissimilar to
those recorded with the latter four tests referred to in Table 4
24
-------
Table 5
Toyota Prius THS
Twelve Consecutively Conducted Tests Over HFET Cvcle
Test
No,
1
2
3
4
5
6
7
8
9
10
11
12
FID
HC
g/mi
0.13
0.04
0.04
0.04
0.04
0.04
0.05
0.04
0.04
0.04
0.04
0.04
NMHCE
g/mi
0.12
0.03
0.04
0.04
0.04
0.03
0.04
0.03
0.03
0.04
0.03
0.03
CO
g/mi
0.7
0.3
0.3
0.3
0.3
0.3
0.3
0.3
0.3
0.3
0.3
0.3
NOX
g/mi
0.07
0.07
0.06
0.05
0.05
0.06
0.08
0.06
0.06
0.06
0.06
0.06
C02
g/mi
186
159
159
160
157
160
165
159
160:
160
159
160
Fuel
Economy
MFC
46.08
53.92
54.01
53.82
54.65
53.80
52.27
54.17
53.84
53.85
54.25
53.65
Net
Charge*
Amp-hrs
NA
NA
NA
NA
0.055
-0.016
-0.034
0.018
-0.023
-0.024
-0.029
0.064
'Negative values relate to net battery-pack charging over the test cycle;
positive values relate to net battery-pack discharge over the cycle.
Figure 10 is. a least squares linear relationship between net
charge/discharge and fuel economy for the last eight tests in
Table 5. Greater scatter in the data appears to be indicated,
but this is a function somewhat of the narrow range of charge/
discharge conditions experienced in the latter eight tests. The
"zero net charge/discharge" fuel economy indicated by Figure 10
for this data is 53.80 mpg, somewhat similar to the 53.32 mpg
referred to in Figure 9.
25
-------
Figure 10
Consecutive Tests Over the HFET Cycle
Fuel Economy versus Net Battery Charge/Discharge
(Data From Table 5)
Fuel Economy [MPG]
54.5
54
53.5
53
1.5
52
51.5-'
51-
f(x) = 7.467x + 53.80
R*2 = 0.175
•f-
•f-
0.08 0.06 0.04 0.02 0 -0.02-0/04
Net Charging [Amp-hrs]
O = MPG at Zero Net Charging
C. Combined City/Highway Emissions/Fuel Economy Testing
The Prius THS was evaluated over two test conventions
utilizing a modified Federal test procedure (FTP) and Highway
Fuel Economy Test (HFET) cycle sequence. (See Test Procedure,
Section V, above). Test Sequence No. 1 consists of an LA-4
vehicle conditioning drive, followed the next day with a 4-Bag
FTP (cold start and hot start Urban Dynamometer Driving Schedule
tests), and two HFET cycles (conditioning and measured). Test
Sequence No. 2 eliminated the preconditioning HFET and the need
to precondition over the LA-4 during the preview test day. All
testing was conducted using California Phase II Reformulated
Gasoline (see Appendix C) at the request of Toyota. Dynamometer
coefficients and suggested vehicle test weight was supplied by
Toyota, with details provided in Appendix A.
26
-------
Table 6, below, presents the results of testing over the 4-
Bag FTP cycles. Integrated, cumulative battery charging,
measured immediately before and after each test, is also'
provided. The difference between these data points, or net
charging/discharge of the,battery pack over the test cycle, is
also presented. The tests conducted on April 3, April 23 and 24,
1998, were conducted to the Test Sequence No. 1 procedure,
utilizing a conditioning HFET as well as a "measured" HFET,
following the 4-Bag FTP. EPA measured emissions during both
HFET's therefore enabling the first HFET to be used to simulate
the Test Sequence No. 2 cycle, referred to above. HFET results
using the Test Sequence No. 2 procedure are given in the
following table, Table 7.
Table 6
Toyota Prius THS
"4-Bag FTP" Test Cycle
Test
Date
03/25/98
03/27/98
03/28/98
03/31/87
04/01/98
04/02/98
04/03/98
04/23/98
04/24/98
FID
HC
g/mi
0.06
0.08
0.07
0.07
0.06
0.06
0.06
0.06
0.06
NMHC
E
g/mi
0.06
0.07
0.06
0.06
0.06
0.05
0.06
0.05
0.05
CO
g/mi
0.5
0.4
0.5
0.6
0.4
0.4
0.4
0.4
0.4
NOX
g/mi
0.04
0.05
0.05
0.05
0.05
0.06
0.05
0.07
0.05
CO2
g/mi
178
175
181
181
173
17,2
181
182
174
Fuel
Economy
MP6
48.24
49.07
47 .43
47.40
49.65
49.94
47.48
47.21
49.36
Net
Charge*
Amp-hrs
NA
0.181
0.203
-0.344
0.166
0.184
-0.285
-0.132
-0.038
Negative values relate to
positive values relate to
net battery-pack charging over the test cycle;
net battery-pack discharge over the cycle.
27
-------
Table 7
Toyota Prius THS
HFET Test Results
First HFET After the Four-Bag Test Used
Test
Date
03/25/98
03/27/98
03/28/98
03/31/98
04/01/98
04/02/98
04/03/98
04/23/98
04/24/98
FID
EC
g/mi
0.03
0.04
0.05
0.04
0.04
0.04
0.04
0.01
0.02
NMHCE
g/mi
0.03
0.03
0.04
0.04
0.04
0.04
0.04
0.01
0.02
CO
g/mi
0.3
0.3
0.4
0.3
0.3
0.4
0.3
0.2
0.2
NOX
g/mi
0.06
0.05
0.04
0.05
0.04
0.04
0.06
0.06
0.04
C02
g/mi
176
173
176
174
174
175
170
180
111
Fuel
Economy
MPG
48.76
49.71
48.75
49.48
49.41
49.03
48.65
47.96
48.62
Net
Charge*
Amp'hrs
-0.462
NA
NA
-0.370
-0.409
-0.412
-0.488
-0.370
-0.388
Negative values relate to
positive values relate to
net battery-pack charging over the test cycle;
net battery-pack discharge over the cycle.
Emissions of hydrocarbons, carbon monoxide, and oxides of
nitrogen over the a 4-Bag FTP exhibit little test-to-test
variability. These emission levels are low with respect to
current model year Federal certification standards, particularly
the levels of oxides of nitrogen. It should be noted that some
of these tests have positive, or net battery discharge, levels
over the entire 4-Bag FTP. A net battery discharge may indicate
that more work was done by the battery/motor system to drive the
vehicle over the test cycle than for a similar test with net
battery charging (from the internal combustion engine). For
similar work at the drive wheels, a greater share or proportion
of the work done by the battery/motor implies less work done by
the internal combustion engine, with a net discharge of the
battery pack. Some of the tests involve net charging, and others
had net battery discharge noted over the 4-Bag FTP. For the
charge/discharge levels noted here, there is not enough
sensitivity exhibited to state conclusively a relation between
28
-------
emissions and charge/discharge. This is an area where further
testing may be helpful. Fuel economy varied over a range of
slightly less than 3 mpg, from 47.21 mpg to a high of 49.94 mpg.
A discussion of calculated fuel economy versus charging over the
test cycle is given later. '
Little variation in emissions over the HFET following the 4-
Bag FTP is exhibited in Table 7. The two.tests with the lowest
net charging of the battery noted here (-0.370 amp-hrs) had some
of the highest and lowest calculated fuel economies over the
HFET, 49.48 and 47.976 mpg, respectively.
Composite, or "55/45" fuel economy values are given below in
Table 8. The second column presents composite emissions
calculated in the manner normally 'used by'EPA to calculate
composite city/highway fuel economy. Slightly higher fuel
economies appear to be associated with tests that had less total
battery recharging noted over the entire test.
Table 8
Toyota Prius THS
Composite City/Highway Fuel Economy
Test Sequence No. 2
Test Date
03/25/98
03/27/98
03/28/98
03/31/98
04/01/98
04/02/98
04/03/98
04/23/98
04/24/98
Composite Calculated
Fuel Economy
MPG ;
48.47
49.36 '•
48.02
48.31
49.54
49.53
48.00
47.54 ;
49.02
Net Charge*
Amp-hrs
NA
NA
NA
-0.714
-0.243
-0.228
-0.772
-0.502
-0.426
Negative values relate to net battery-pack charging over the test cycle/-
positive values relate to net battery-pack discharge over the cycle.
29
-------
D. Testing Over Designated Test Sequence No. 1
The test sequence designated as No. 1 consisted of the 4-Bag
FTP followed by two tests over the HFET cycle, a conditioning
test and the "measured" test. Sequence No. 1 was evaluated four
times, but EPA sampled both HFET's in order to facilitate a
comparison between the sequences designated "1" and "2."
The 4-Bag FTP test results were presented earlier in Table
6, in the section discussing the Sequence No. 2 "test cycle," as
the 4-Bag FTP's over Sequence No. 2 are inclusive of those over
Sequence No. 1. This data is given here in Table 9, to
facilitate a later comparison. HFET results from both the No. 2
and No. 1 sequences are given simultaneously in Table 10 for
comparison.
Table 9
Toyota Prius THS
Tests Over 4-Bag FTP
Test Sequence No . 1
Test Date
04/03/98
04/17/98
04/23/98
FID
HC
g/mi
0.06
0.05
0.06
0.06
NMHCE
g/mi
0.06
0.04
0.05
0.05
CO
g/mi
0.4
0.4
0.4
0.4
NOX
g/mi
0.05
0.05
0.07
0.05
CO2
g/mi
181
177
182
174
FE
MP6
47.48
48.57
47.21
49.36
Net
Charge*
Amp-hrs
-0.285
-0.252
-0.132
-0.038
w NeCjcLui. Ve Vo-iUfcio .LCXCL ue ^.^ n*= i- *-'«*-*-'— *• j c- — *-•- y ^
positive values relate to net battery-pack discharge over the cycle.
We monitored manifold vacuum as an indicator on engine
"on/off" condition during the LA-4. (Toyota has stated that the
engine shuts off during decelerations and at idle.)
Approximately 27-31 engine starts were noted over the cold start
LA-4 cycle.
30
-------
Table 10
Toyota Prius TKS
HFET Results — Test Sequence No. 1
Test
Date
04/03/98
04/17/98
04/23/98
04/24/98
FID
HC*
g/mi
0.04/
0.04
NA/
0.01
0.01/
0.01
0.02/
0.02
NMHCE
g/mi
0.04/
0.04
NA/
0.01
0.01/
0.01
0.02/
0.02
CO
g/mi
0.3/
0.3
NA/
0.2
0.2/
0.2
0.2/
0.1
NOX
g/»i
0.05/
0.06
NA/
0.04
0.06/
0.05
0.04/
0.05
C02
g/mi
177 /
164
NA/
166
is o/
167
177 /
165
FE
g/mi
48. 65/
52.60
NA/
51.88
47. 96/
51.45
48. 62/
52.04
Net**
Charge
Amp-hrs
-0.488/
-0.080
-0.444/
-0.052
-0.370/
-0.014
-0.338/
-0.097
* The first figure in each column relates to test results over the first
• HFET following the 4-Bag FTP (Test Sequence No. 2) test; the second
figure in each column relates to test results over the second HFET
following the 4-Bag FTP (Test Sequence No. 1).
** Negative values relate to:net battery-pack charging over the cycle;
positive values relate to.net battery-pack discharge over the cycle.
The emission results, with the exception of CO2, are
relatively similar for the sequence No. 2 HFET compared to Test
Sequence No. 1. The vehicle battery pack was charged
significantly more by the internal combustion engine over the
first HFET. CO2 emissions increased, and hence fuel economy
decreased over the first HFET because of the additional charging
work done by the engine. ]
The difference in HFET fuel economy is approximately seven-
percent higher for the average of the second HFET versus the
first HFET.
These differences in highway fuel economy will carry over to
calculated composite (city and highway) fuel economy values.
Table 11 below presents calculated composite fuel economies for
the four tests above conducted with both Test Sequences No. 1 and
No. 2 procedures.
31
-------
Table 11
Toyota Prius THS
Composite HPG - Test Sequences No . 2 and No . 1 Procedures
Test Date
04/03/98
04/17/98
04/23/98
04/24/98
City MPG
47.48
48.57
47.21
49.36
Highway MPG*
48.65/52.60
NA/51.88
47.96/51.45
48.62/52.04
Composite MPG
48,00/49.66
NA/50.01
47.54/49.03
49.02/50.53
* The first figure in the column refers to test results over the first
HFET following the 4-Bag FTP test (Test Sequence No. 2 procedure); the
second figure in each column refers to test results over the second
HFET following the 4-Bag FTP (Test Sequence No. 1 procedure).
The average composite fuel economy for the three tests
conducted with Test Sequence No. 2 was about 48.2 mpg,
approximately three-percent lower than the 49.8 mpg average over
the same tests with Test Sequence No. 1 procedure. This
difference is probably due in large part to the additional
charging of the THS battery pack during the first HFET conducted
following the 4-Bag FTP.
E. Hot Start FTP Following Battery Discharge
A single 4-Bag FTP test was conducted with the THS following
the deep discharge of the battery pack. This test was conducted
to determine the effect on emissions and fuel economy of an
excursion or incident which would cause the battery pack to
discharge to a very low state of charge. Until the battery pack
charged to a "normal" level, this discharge would cause the
vehicle power management system to task the THS internal
combustion engine heavily, to include vehicle propulsion and
battery pack charging, over a given driving cycle.
EPA had determined that a demanding acceleration from stop,
followed by coasting down the vehicle with the gear selector in
"N" position, would discharge the battery. (A wide open throttle
acceleration from stop causes the THS power management system to
invoke battery-pack operation of the electric motor. Changing
the gear shift selector from "D" position to "N" disengages the
engine/battery and regenerative braking recharging systems in the
dynamometer driving mode used.) It should be noted that Toyota
cautions against using the neutral "gear shift" position during
actual driving for safety reasons.
32
-------
A 0-60 mph wide open throttle acceleration from stop was
performed on the vehicle, the driver removing his foot from the
accelerator and engaging the "N" gear selector mode when the
vehicle reached approximately 61 mph. The vehicle was allowed to
coast to 5-mph conditions in "N" position, light braking was then
used to bring the vehicle to full stop. The change in battery-
pack charging/discharging during this acceleration was recorded
(net discharge of battery pack occurred.), as well as 0-60 mph
acceleration time, by means of a stopwatch. This "preparation"
of the vehicle/battery pack was repeated until simultaneously, 1)
0-60 mph acceleration times stabilized, and 2) net battery
discharge during acceleration decreased to nearly zero. At this
point, EPA was satisfied that the THS power management system and
battery pack had caused the pack to discharge to the lowest
charge level possible with this preparation.
Table 12 presents Bag 1 (first 505 seconds of the LA04
cycle) emissions. The effect of deeper battery discharge is
noticeable in higher hydrocarbon and NOx emissions, and lower
fuel economy compared with the "normally" charged (conditioned
over -the LA-4 cycle the previous afternoon) battery pack test
data. The large net charge indicated substantial battery
charging during this mode, with respect to the "normally" charged
battery tests.
Table 12
Toyota Prius Hybrid Vehicle
I A- 4 Test Cycle
Test Following Battery-Pack Discharge
Cold Start "Bag 1" Emissions/FE
Test
Conditions
/Date**
Discharge
03/31/98
04/01/98
04/02/98
FID
HC
g/mi
0.41
0.25
0.22
0.20
NMHCE
g/mi
0.40
0.23
0.21
0.19
CO
g/mi
1.2
1.4
1.2
0.8
NOX
g/mi
0.3
0.1
0.2
0.1
CO2
g/mi
307
254
235
228
FE
MPG
27.9
33.6
36.9
37.5
Net
Charge*
Amp-hrs
-1.577
-0.522
-0.154
-0.164
Negative values relate to net battery-pack charging over the test cycle;
positive values relate to net battery-pack discharge over the cycle.
Dated tests denote "normally" charged battery pack prior to test.
33
-------
Tables 13-15 present Bag 2 through Bag 4 data for the same
tests. Emission levels and even fuel economy for the "deeply
discharged" battery pack test do not differ greatly from the data
added for comparison purposes. This may indicate relatively
rapid battery recharging to manufacturer desired state of charge
level following a substantial battery pack drain.
Table 13
Toyota Prius Hybrid Vehicle
LA- 4 Test Cycle
Test Following Battery-Pack Discharge
Stabilized "Bag 2" Emissions/FE
Test
Conditions
/Date**
Discharge
03/31/98
04/01/98
04/02/98
FID
HC
g/mi
0.01
0.01
0.01
0.01
NMHCE
g/mi
0.01
0.01
0.01
0.004
CO
g/mi
0.1
0.2
0.1
0.2
NOX
g/mi
0.05
0.03
0.04
0.04
C02
g/mi
130
141
133
129
FE
MPG
66.3
61.1
65.0
66.7
Net
Charge*
Amp-hrsi
-0.506
-0.332
-0.458
-0.433
+ Negative values relate to net battery-pack charging over the test cycle;
positive values relate to net battery-pack discharge over the cycle.
** Dated tests denote "normally" charged battery pack prior to test.
34
-------
Table 14
Toyota Prius Hybrid Vehicle
LA- 4 Test Cycle
Test Following Battery-Pack Discharge
Test
Conditions/
Date**
Discharge
03/31/98
04/01/98
04/02/98
Hot Start "Bag 3" Emissions/FE
FID
HC
g/mi
0.04
0.05
0.04
0.05
NMECE
g/mi
0.03
0.04
0.03
0.04
CO
g/mi
0.3
0.6
0.5
0.6
NOX
g/mi
0.08
0.06
0.06
0.07
C02
g/mi
191
198
197
192
FE
MPG
45.0
43.3
43.7
44.6
Net
Charge*
Amp-hrs
-0.273
-0.278
-0.347
-0.236
* Negative values relate to net battery-pack charging over the test cycle;
positive values relate to net battery-pack discharge over the cycle.
** Dated tests denote "normally" charged battery pack prior to test.
Table 15
Toyota Prius Hybrid Vehicle
LA- 4 Test Cycle
Test Following Battery-Pack Discharge
Stabilized "Bag 4" Emissions/FE
Test
Conditions
/Date**
Discharge
03/31/98
04/01/98
04/02/98
FID
HC
g/mi
0.01
0.01
0.01
0.01
NMHCE
g/mi
0.01
0.01
0.004
0.002
CO
g/mi
0.2
0.2
0.1
0.2
NO,
g/mi
0.04
0.03
0.04
0.04
C02
g/mi
136
146
139
146
FE
MPG
63.5
59.1
61.9
59.1
Net
Charge*
Amp-hrs
-0.284
-0.119
-0.208
-0.151
Negative values relate to net battery-pack charging over the test cycle;
positive values relate to net battery-pack discharge over the cycle.
Dated tests denote "normally" charged battery'pack prior to test.
35
-------
Table 16 represents the total effect on fuel economy and
emissions over the 4-bag FTP of the. deep battery-pack discharge
prior to testing. Total weighted hydrocarbons and NOx emission
levels exceed those of the "normally preconditioned" battery-pack
testing, though CO levels are still arguably as low as those from
the comparison tests. Battery charging over the entire FTP cycle
is much greater for the deeply discharged battery-pack test, but
this is influenced by the high degree of recharging in Bag 1.
Fuel economy is approximately 5 percent below the simple average
of the "normally preconditioned" battery tests presented here.
Table 16
Toyota Prius Hybrid Vehicle
LA-4 Test Cycle
Test Following Battery-Pack Discharge
Composite Emissions/Fuel Economy
Test
Conditions
/Date**
Discharge
03/31/98
04/01/98
04/02/98
FID
HC
g/mi
0.10
0.07
0.06
0.06
NMHCE
g/mi
0.10
0.06
0.06
0.05
CO
g/mi
0.4
0.6
0.4
0.4
NOX
g/mi
0.11
0.05
0.05
0.06
C02
g/mi
185
181
173
172
•
FE
MFC
44.41
47 .40
49.65
49.94
Net
Charge*
Amp-hrs
-1.060
-0.344
-0.166
-0.184
* Negative values relate to net battery-pack charging over the test cycle;
positive values relate to net battery-pack discharge over the cycle.
** Dated tests denote "normally" charged battery pack prior to test.
F. Testing Over SC03 Driving Schedule
The SC03 test cycle is designed to measure emissions during
ambient conditions when air conditioning use is likely. The test
cell was conditioned to 95°F and 100 grains water/lb. of dry air
Test procedures for this 594-second test are given in the Code of
Federal Regulations.[6] Note that the ambient conditions
simulated are those which are conducive to formation of ozone in
the atmosphere.
36
-------
The EPA testing plan called for a sequence of four tests
over the SC03 on the day of the evaluation, a preparation (no
emissions measurement), followed by three consecutive tests with
emissions measured. A 10-minute soak separated each SC03 test.
The "conditioning" and the first two SC03 (measured) tests
were conducted without incident. Approximately 400 seconds into
the third measured SC03 te;st, a fuel line hose in the top of the
fuel tank (in the fuel sending unit assembly) became detached
from its connection to the fuel sending unit. Fuel bubbled out
of the tank through this opening, leaking over the tank and
underside of the vehicle onto the floor of the test cell. The
odor of fuel in the vehicle was immediately noted by the vehicle
driver, and the test was halted. The fuel leak and the spilled
fuel were addressed. A plastic clip which attached the -fuel line
to the top of the fuel sending unit had been previously removed
by EPA to facilitate the drain of fuel from the vehicle tank
through this opening. This clip had been replaced after the"
fuel drain was conducted (when the vehicle was initially accepted
by EPA). The driver noted that this clip had "popped off" the
hose/sending-unit fitting when the leak occurred.
The fuel line was replaced on the sending unit, and the
attaching clip reinstalled. The vehicle was soaked again to test
conditions, and the test sequence begun again with the "prep"
SC03. During this test, the fuel line and the retainer clip
disconnected from the fuel-sending unit. A fuel leak from .the
tank, similar to the above, then developed. The fuel leak was
attended to, and the testing halted to better determine the, cause
of the fuel-fitting failuires.
Toyota Technical Center representatives consulted with EPA
to analyze the cause of the fuel-fitting failure. It was
suggested by Toyota that the fitting retainer clip may have been
designed to be used or "stretched" only once; therefore, re-use
of this clip may not have been advisable. Toyota supplied EPA
with another retainer clip (new), and this clip was used to
secure the fuel line to the sending unit. Toyota and EPA jointly
decided to suspend further testing over the SC03 cycle in the
interest of Toyota making a more detailed determination at a
later date of the cause of the failure. The original, "reused"
retainer clip was returned to Toyota for analysis.
37
-------
The issue of preparing hybrid vehicles for testing at
vehicle laboratories is one that must be addressed|in greater
detail by EPA and the vehicle manufacturers in the future (see
Testing Issues, above). Additional documentation will most
certainly have to be supplied by the manufacturers concerning
alternative modes of operation (if any) of the powerplants,
detailed start and prep instructions, safety information
concerning operation of the battery and electric motor, safety
during off-vehicle charging (if necessary), etc.
The data from the first two tests over the SC03 test cycle
(conducted without incident), together with charging information
over the first (conditioning) SC03 cycle excursion, is presented
below. This data is presented with limited comment; no
comparatory data is given here. The calculated fuel economy
figures are similar for both tests, and the net battery charges
for both "measured" tests were similar in magnitude and sign
(slight battery charge provided by engine).
The "conditioning" (no emissions measured) drive had a
higher net charge to the battery than the following two tests
over the SC03 cycle. During the afternoon prior to the day of
the SC03 cycle testing, a number of 0-60 mph acceleration tests
had been conducted. The vehicle was coasted down in the "drive"
mode (no regenerative braking) during the decelerations following
the 0-60 mph accelerations. Therefore, the state of charge of
the hybrid electric battery pack may have been reduced prior to
the conduct of the SC03 testing, helping to explain the
relatively higher amount of charging noted during the "prep" SC03
driving cycle.
38
-------
Table 17
Toyota
Testing Over SCO 3 ("Air
Test
No.
1
2
3
FID
HC
g/mi
N/A
0.04
0.08
NMHCE
g/mi
N/A
0.04
0.06
CO
g/mi
N/A
0.4
0.6
Prius HEV
Conditioner") Test Sequence
NO,
g/mi
N/A
0.02
0.06
C02
g/mi
N/A
258
256
Net
Charge*
Amp-hrs
-0.407
-0.65
-0.51
FE
MPG
N/A
33.36
33.57
Negative values relate to net battery-pack charging over the test cycle;
positive values relate to net battery-pack discharge over the cycle.
6. Testing Over the US06 Test Cycle
The Federal US06 Driving Schedule is a high speed,
"aggressive" schedule, involving some high accelerations. These
driving modes may cause some vehicles to operate in an "open
loop," or full power mode (with respect to air/fuel setpoints on
a 3-way catalyst equipped ;vehicle.)
Six consecutive emission tests with the THS over the US06
cycle were conducted on a single day. The THS had been tested
over the 4-bag FTP, followed by three tests over the HFET cycle,
prior to the US06 testing. The vehicle was driven and tested
over the US06 cycle three times in succession. Testing was then
halted, and a number of 0-60 mph accelerations, followed by shift
into neutral and braking to stop, were conducted. This action
put the vehicle battery pack into a state of deeper discharge
characterized by the appeairance of a warning indicator on the
vehicle video dashboard. With the vehicle battery in this state
of discharge, the THS was tested over the US06 cycle again.
Battery discharging in the same manner as above was again
conducted, the US06 test repeated, and the discharge/test
sequence repeated a third time, for a total of three tests in
this mode. This "discharged battery" testing was conducted to
determine the effect on emissions and fuel economy of driving the
THS Prius over a demanding driving schedule with the battery
package greatly discharged. Toyota notes that this condition of
battery discharge would not be contemplated by their designers
under normal driving conditions. Results from this testing are
presented below in Table 18.
39
-------
The CO2 emission levels are an indication that the vehicle
engine attempted to recharge the battery pack following the
deeper discharge. Calculated fuel economy therefore decreased,
approximately 11 percent from the level of the "normally" charged
battery US06 tests. Emission levels, however, changed little as
a result, of the deeper battery discharge (a slight increase in CO
emissions was noted).
In general, the vehicle driver noted a degradation in
driving performance over the US06 following the battery deep
discharge (compared to the "normally" charged battery). For
example, the driver was not able to follow the driver's trace
over one early, particularly demanding, acceleration during each
of the deep discharge battery US06 tests. No effort is made here
to quantify the THS performance or relate battery charge to the
ability to follow the trace and time following start of test. It
is noted, however, that performance over the US06 was related to
the state of charge of the battery, for a "deeper" discharge of
the battery pack prior to the start of the test.
Table 18
Toyota Prius HEV
Consecutive Tests Over US06 Schedule
Battery
Mode
« j\" * *
"A"
"A"
« g "*••*•*
"B"
"B"
HC
g/mi
0.03
0.03
0.03
0.03
0.03
0.04
NMHCE
g/mi
0.03
o.:3
0.03
0.03
0.03
0.03
CO
g/mi
0.5
0.6
0.6
0.7
0.9
0.7
C02
g/mi
232
235
234
267
251
260
N0«
g/mi
0.09
0.09
0.07
0.09
0.07
0.09
Fuel
Economy
MPG
37.02
36.64
36.64
32.19
32.90
33.05
Net
Charge*
Amp-hrs
0.265
0.058
0.302
-0.854
-0.554
-0.405
tNCMCLwJUVC? VCtJ-Ll^O i w J»d n^ ^ w liv-. i- *^ i_* <_ w *— j- jf £^ «>-•** _.._— -j — -- --j -_ - _ — _ _ _ _ _^
positive values relate to net battery-pack discharge over the cycle.
** "A" denotes normally charged battery pack prior to first US06, then
followed by two additional US06 tests in succession.
*** »B// denotes deep discharge of battery pack, evidenced by dashboard
warning indicator, prior to test over US06 cycle.
40
-------
H.
Testing Over Select Vehicle Steady-State Speed Modes
The THS was tested over several steady-state vehicle
operating modes for emissions and fuel economy. The hybrid
powerplant system must also be considered in the discussion of
the results from this testing, however.
With a conventional vehicle, a desired vehicle speed is
driven with a set of dynamometer load coefficients. Parameters
such as coolant temperature, oil temperature, emissions, etc are
monitored until the tester is satisfied that steady-state
operating conditions are present, and then emissions sampling is
begun. With a hybrid electric vehicle, like the THS, the state
of battery-pack charge may determine the apportionment of engine
and motor operation of the drive wheels, battery recharging
during vehicle operation, etc. Therefore, the operation of the
internal combustion engine and electrical drive system might
change during "steady-state" testing. It is important to note,
therefore, that "steady-state" as referred to in this section,
does not reflect battery condition. We conducted these "steady-
state" tests in a manner consistent with steady-state testing of
a conventional powerplant-only vehicle. The rest results are
referred to as steady-state test results here, but the battery
state of charge, particularly at 10-mph conditions, may
materially influence the test results.
We conducted testing over the 4-bag FTP, HFET, and US06
cycles prior to the steady-state testing referred to below. With
the engine warm, the vehicle was driven to 10-mph conditions and
this mode was driven for approximately three minutes prior to ten
minutes of emission sampling. The vehicle was then driven to 20-
mph conditions, a brief stabilization period was allowed, and
emissions sampling was conducted for another 10-minute period.
This procedure was repeated for 30, 40, and 50 mph conditions.
The results from this testing are presented in Table 19, below.
41
-------
Table 19
Toyota Prius Hybrid Vehicle
Steady-State Emissions Testing
Test Mode
10 mph
20 mph
30 mph
40 mph
50 mph
FID
EC
g/mi
0.01
0.03
0.01
0.01
0.01
NMHCE
g/mi
0.00
0.02
0.01
0.01
0.00
CO
g/mi
0.1
0.1
0.1
0.2
0.2
C02
g/mi
103
140
134
137
156
NOX
g/mi
Neg.
0.01
0.01
0.00
0.00
E-E
MPG
83.71
61.66
64.39
63.06
55.18
Net
Charge*
Amp*hris
0.205
-0.235
-0.268
0.027
0.241
* Negative values relate to net battery-pack charging over the test cycle;
positive values relate to net battery-pack discharge over the cycle.
The THS operated in a battery-only mode for a portion of the
10-mph steady-state test, and a net discharge from the battery
was noted. Net charging of the battery occurred during the
following 20- and 30-mph modes. C02 emissions per mile, and
hence, fuel economies, were relatively similar for the 20-, 30-,
and 40-mph steady-states, regardless of the amount of battery-
pack charging done by the internal combustion engine. Fifty-mph
steady-state emission levels approximate those from HFET schedule
tests conducted with the THS, and net battery discharging was
noted over this 10-minute test. NOx emissions were low during
all of this steady-state testing. Fuel economy test results are
presented in this section with the caveat that these figures
involve conventional vehicle steady-state test procedures, rather
than transient cycle operation.
The steady-state mpg values generated can be compared to the
mpg values generated during cyclic testing by plotting both
values versus average speed. The results presented in Figure 11
show the relationship for the Toyota Prius.
In order of average speed, the cyclic data points included
here are average fuel economy figures for the New York City
Cycle, Bag 2 of the FTP, the cold start LA-4 (1370 seconds), the
SC03 cycle, Bag 1 of the FTP, the US06 cycle, and the HFET cycle.
It should be noted that the high fuel economy recorded for the
second point (Bag 2 of the FTP) may have been influenced by
42
-------
substantial battery discharge during this segment. Though they
are similar in average speed, the HFET and US06 cycles have very
different fuel economies associated with them due to the
"aggressive" accel/decel nature of the US06 versus the "steadier"
HFET cycle.
In general, however, the steady-state fuel economy figures
are substantially higher than fuel economies from averages over
cyclic driving. This is generally due to the changes in inertia
inherent to cyclic driving, deeper charging/discharging of the
battery with cyclic driving, and the inability to recover 100
percent of braking energy with the regenerative braking system.
The greatest variance is perhaps indicated at low speed
conditions where the start/stop New York Cycle average fuel
economy is compared to 10-mph steady-state conditions. (These
steady-state conditions are affected by the substantial battery
discharge; hence, a lower operational mode for the internal
combustion engine.) Between 20 and 40 mph,:the steady-state fuel
economy is roughly double that obtained as an average over
certain transient cycles.
Figure 11
"Steady-State/Cyclic" MPG vs. MPH
Miles per Gallon
100
Cyclic MPG
S. S. MPG
20 30 40
Miles per Hour
50
*Cyclic mpg data points here are New York City Cycle, Bag 2 of FTP, cold-start
LA-4, SC03 cycle, Bag 1 of FTP, US06 cycle, and HFET cycle, in that order.
43
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X. Fuel Economy Issues
1. Adjustments for Net Charge/Discharge of Battery Pack
Hybrid electric vehicles must be equipped with an energy
storage system capable of storing energy to drive an electric
motor. This system, a battery pack in the case of the THS, may
be recharged off-vehicle from a power grid, through regenerative
braking energy recovery, or through a recharging circuit onboard
the vehicle. The THS battery pack is normally recharged by a
combination of regenerative braking and the recharging circuit
using the onboard internal combustion engine (see Vehicle
Operating Strategy, above). •
• , i
Over a vehicle operating mode such as the 4-Bag FTP, the
battery pack of the charge-sustaining THS may charge and
discharge a number of times, according to the operating strategy.
EPA recorded the net charge/discharge during each testing mode
(see Test Procedures, above). A net charge/discharge of zero
over a test mode may indicate little or no change in the state of
charge of the battery pack. (This may depend upon the state of
charge when the charging/discharging occurred, however.) A net
discharge of the battery over a test mode may indicate less
demand placed on the internal combustion engine powerplant during
that mode compared to a "zero" charge/discharge. If fuel economy
is calculated in a "conventional" manner for a charge-sustaining
hybrid such as the THS, (using emissions data from the internal
combustion engine), a net battery discharge will overstate fuel
economy. Conversely, a net battery charge during a test cycle
may understate conventionally calculated fuel economy.
Toyota suggested a factor to adjust fuel economy for a net
charge/discharge over a test cycle.[3] This method involves the
generation of a multiplier, described by Toyota as a "k" factor,
to apply against unadjusted fuel economy. The "k" factor would
relate fuel economy to charging/discharging over a test cycle,
and its application would seek to adjust fuel economy to a "zero"
net charge/discharge level. EPA plotted net charge/discharge
over several test modes against unadjusted fuel economy to
determine relationships between battery charging and fuel economy
for the THS.
Figure 12 relates" fuel economy to net charge/discharge for
several tests conducted over the 4-Bag FTP. A straight line
curve fit using regression analysis was used to generate the
trend line described here.
44
1 „;,:,;
-------
Figure 12
4-Bag FTP Fuel Economy vs. Net Battery Charge/Discharge
50
AQ «;
49-
48 5
48-
47 5
47-
1
• ;
•
*
X
1(x) = 2.728981 E+0-x + 4.854834E+1
RA2 = 3.220969E-1
\
•
\
L^
B
•
\
'•
X
• •
0.3 0.2 0.1 0 -0.1 -0.2 -0.3- -0.4
For the range of charge/discharge here, the fuel economy
data exhibit a high degree of scatter. A y-intercept of about
48.55 mpg for zero net charge over the 4-Bag FTP is described. A
simple average of the fuel economy data points plotted (from
Table 6)^is 48.45 mpg. Therefore, if a zero net battery
charge/discharge fuel economy description was required, on the
basis of the data presented here, the answer might be close to a
simple average of the data.
This data might be"used to generate a factor to adjust the
fuel economies of individual tests which have non-zero net
battery-pack charge/discharge recorded. Table 20 adjusts the
fuel economies of the "unadjusted" data in Figure 12. The
adjustment factor is the slope of the line in Figure 12, or 2.729
mpg (net amp-hours). Fuel economies with net battery charging
noted over the cycle are adjusted upward, while those with net
discharges are adjusted downward.
45
-------
Table 20
Toyota Prius THS
Fuel Economies Adjusted for Net Battery Charge/Discharge
4 -Bag FTP Cycle
Test Date
3/27/98
3/28/98
3/31/98
4/1/98
4/2/98
4/3/98
4/17/98
4/23/98
4/24/98
4 -Bag
FTP Cycle
MPG
49.07
47.43
47.40
49.65
49.94
47.48
48.57
47.21
49.36
Net Charge/
Discharge*
Amp-hrs
0.191
0.203
-0.344
0.166
0.184
-0.285
-0.252
-0.132
-0.038
"Adjusted"
Fuel Economy
MPG
48.54
46.88
48.34
49.20
49.44
48.26
49.26
47.57
49.47
* Negative values relate to net battery-pack charging over the test cycle;
positive values relate to net battery-pack discharge over the cycle.
The magnitude of the adjustments that would be made by this
method range from 0.55 mpg (subtractive) to 0.94 mpg (additive).
The largest adjustment amounts to a slightly greater than two-
percent boost in fuel economy.
Tests conducted over the HFET cycle following a 4-Bag FTP
(Test Sequence No. 2) were characterized by substantial battery-
pack recharging during the cycle. No HFET immediately following
an FTP test exhibited net battery-pack discharging.
i
HFET fuel economy for the HFET conducted immediately
following a 4-Bag FTP are graphed below in Figure 13, versus net
battery pack charging in amp-hours. The trend line here is
constructed in the same manner as described earlier for Figure 12
46
-------
Figure 13
HFET Fuel Economy vs. Net Battery Charge/Discharge
(Test Sequence No. 2)
Fuel Economy [MPG]
50
49.5
47.5
f(x) = 1.997x + 49.78
= 0.028
48.5--
0 -0.1 ;-0.2 -0.3 -0.4 -0.5
Delta Amp-hrs (Negative is Charging)
!
O = MPG Zero Net-Charging
A great deal of -scatter in this data is apparent, though the
range of net battery charging is only 0.370 amp-hours through
0.489 amp-hours. The extrapolated zero net charge/discharge fuel
economy value is 49.78 mpg. An adjustment of each individual
test in the manner described above would have the effect of
raising the reported fuel economy for each data point given here,
as each point involved net battery-pack charging. Table 21,
below, lists the adjustments to the individual HFET cycle tests
(Test Sequence No. 2 procedure) suggested by this method.
47
-------
HFET
Fuel E
Test: Date
3/25/98
3/31/98
4/1/98
4/2/98
4/3/98
4/23/98
4/24/98
Tal
Toyota
Tests, Sequenc
conomy Adjuste<
HFET
MPG
48.75
49.48
49.41
49.03
48.65
47.96
49.36
3le 21
Prius THS
e No. 2 Test Pr
i for Net Baittei
Net Battery
Charge*
Amp-hrs
-0.462
-0.370
-0.409
-0.412
-0.488
-0.370
-0.388
ocedure
cy Charge
"Adjusted"
Fuel Economy
MPG
49.67
50.22
50.22
49.85
49.62
48.70
50.13
* Negative values relate to net battery-pack charging over the test cycle;
positive values relate to net battery-pack discharge over the cycle.
Figure 14, below, presents HFET mpg for the second HFET
following a 4-Bag FTP test (Test Sequence No. 1, see Test
Procedures, above) versus net battery pack charging/ discharging,
Figure 14
HFET Fuel Economy vs. Net Battery Charge/Discharge
(Test Sequence No. 1)
b-3~
D2 . b ,
51 . 5
bl
50"
49 "i
— .—
.-— -•
^^
^•^
1
— -^*
1
^^^
f(x) = -10.075*x +• 51.380
R^2 = 5.9061E-1
' ' • ' i • ' ' ' i ' ' ' '
Net Change in Amp-hrs
48
-------
Thls data appears to!show an unexpected relationship between
battery-pack charging and fuel economy, i.e., lower fuel economy
with lower battery charging. No reason is given here for this
unexpected relationship, other than general test variability An
adjustment factor based on this relationship would have the
effect of lowering reported fuel economy values for tests with
net battery charging exhibited. Further testing would have to be
conducted to explain this relationship.
Toyota suggested that EPA should combine the data in Figures
13 and 14 (HFET data) in order to determine a fuel economy
adjustment factor or net zero charging fuel economy value for
HFET testing. This was suggested because .of the clustering of
data around two net charging values for the HFET in these
figures. Figure 13 refers to the first HFET following the 4-Bag
FTP (Test Sequence No. 2) while Figure 14 refers to the second
HFET after the 4-Bag FTP (Test Sequence No. 1). These tests,
both using the HFET cycle, are nevertheless dissimilar because of
the difference in time sequence when they :are conducted. EPA
believes it may be misleading to include both sets of data in a
graph to determine an adjustment factor because of this time
difference.
2.
"Running" Fuel Economy
Another way to express fuel economy is actual miles driven
versus actual gallons consumed. This method is illustrated in
Table 22, where miles driven and gallons of fuel consumed over
the 4-Bag FTP and EPA-suggested HFET, immediately following the
FTP, are presented. Fuel economy calculated in this manner has
the effect of eliminating the conventional' cold/hot weighting
factors. A "composite" fuel economy based on dividing total
miles driven versus gallons of fuel consumed is given for each
test. A running average of fuel economy calculated in this
manner for these tests is also given. The "running average" fuel
economy calculated in this manner slightly exceeds 48 mpg.
49
-------
Table 22
Toyota Prius THS
"Running" Fuel Economy - Actual Miles/Actual Gallons
Test Sequence No. 2 FTP/HFET Cycle
Test
Date
03/25/98
03/27/98
03/28/98
03/31/98
04/01/98
04/02/98
04/03/98
04/23/98
04/24/98
Miles
25.189
25.161
25.028
25.041
25.012
25.041
25.041
25.055
25.109
Gallons
0.5241
0.5124
0.5254
0.5228
0.5072
0.5061
0.5255
0.5310
0.5156
Test
Average
MPG
48.06
49.10
47.64
47.90
49.31
49.47
47.65
47.18
48.70
Cumulative
Miles
25.1.89
50.350
75.378
100.419
125.431
150.472
175.513
200.568
225.677
Cumulative
Gallons
0.5241
1.0365
1.5619
2.0847
2.5919
j.0980
3.6235
4.1545
4.6701
Cumulative
MPG
48.06
48.58
48.26
48.17
48.39
48.57
48.06
48.58
48.26
Table 23 depicts "conventionally" calculated mpg for Test
Sequence ISIo. 2 versus "composite" fuel economy using the
conventional city/highway weighting of actual miles driven/fuel
consumed for the city/highway tests. The conventionally
calculated composite city/highway fuel economies only slightly
exceed the composite values calculated using actual miles/gallons
for the city and highway values.
This calculation also eliminates the traditional cold/hot
city test weighting factors but calculates composite mpg with the
traditional 45/55 weighting factors.
50
-------
Table 23
Toyota Prius THS
Composite City/Highway Fuel Economy
Conventionally Calculated vs. Actual Miles/Gallons
Test
Date
03/25/98
03/27/98
03/28/98
03/31/98
04/01/98
04/02/98
04/03/98
04/23/98
04/24/98
"Conventionally
"Calculated
Composite MPG
48.47
49.36
48.02
48.31
49.54
49.53
48.00
47.54
49.02
Total MPG
City *
47.59
48.68
46.89
46.86
49.25
49.80
47.00
46.67
48.74
Total MPG
Highway *
48.75
49.71
48.75
49,48
49.48
49.41
48.65
47r96
48.62
Composite
Miles/Gallons
MPG
47.80
48.64
47.49
47.44
48.86
49.24
47.63
47.38
48.55
Actual miles vs. actual gallons
Table 24
Overall Fuel Economy Summary
Test
Procedure
Test Sequence
No. 2
Test Sequence
No. 1
Miles/Gallons
Adjusted for
State of Charge
No
Yes
No
No
MPG
48.6
49.1
49.8
48.3
From
Table 8
Figures 12/13
Table 11
Table 22
Table 24 presents a summary of some of the many ways that
fuel economy was analyzed in this report, showing that for
several approaches, the overall mpg for the Prius is between 4!
and 50 mpg.
51
-------
3. Comparison of the Fuel Economy and Emissions of the
Prius to Other Vehicles
Since there are other vehicles that get about the same fuel
economy that the Prius does, it is of interest to compare the
emissions of those other high fuel economy production vehicles to
that measured from the Prius. All emissions data used for the
comparison are low mileage data.
Table 25
High Fuel Economy Vehicles
Comparison of Emission
Vehicle
Suzuki
Metro
VW New
Beetle
VW
Passat
VW
Jetta
Toyota
Prius
Toyota
Corolla
Car
Class
Sub-
compact
Sub-
compact
Mid-size
Compact
Sub-
compact
Compact
5=^=
Test
Wt
2125
3125
3375
3125
3000
2750
HC
0.04
0.02
0.22
0.10
0.06
0.18
CO
0.3
0.1
0.5
0.4
0.5
1.2
=====
NOX
0.04
0.69
0.62
0.59
0.05
0.12
Com-
posite
MPG
54.5
51.6
50.4
50.9
48.6
36.5
Trans-
mission
M5
M5
M.5
M5
AT(THS)
M5
The low mileage emissions for the high fuel economy cars are
not so far from each other except for NOx. The higher NOx values
in the table are all associated with the VW vehicles, all of
which are Diesels, and they are more than an order of magnitude
higher in NOx than the Prius.
52
-------
The fuel economy of the Prius can also be compared to a
vehicle of the same weight class and the same volume-based size
class. Figures 15 and 16 show how the Prius compares to model
year 1998 vehicles in the 3000-lb weight class and the subcompact
car class, respectively.
The Prius is 64-percent better in fuel economy than the
average 3000-lb car, and 66-percent better in fuel economy tl
the average subcompact car.
The roughly 65-percent improvement in fuel economy shown by
the Toyota Prius, compared to the average of other vehicles, is a
comparison in which either weight class or car class was held
constant. It is very likely that the Toyota Prius has slower 0-
60 mph acceleration performance than the average vehicles in the
two classes, since the average 0-60 time for vehicles in the
3000-lb weight class is estimated to be 10.5 seconds and for the
subcompact car class, 10.2 seconds. The 0-60 time measured for
the Prius on the chassis dynamometer is 14.2 seconds using the
average of the six tests with the 20-second braking protocol.
If one accounts for the performance difference between the
Toyota Prius and the average 3000-lb car and the average
subcompact car, the fuel economy advantage of the Toyota Prius is
reduced to a 45-percent increase.
4. What About the 66-mpg Value Associated With This Car?
When the Prius vehicle was introduced as a commercial
product in Japan, the fuel economy when tested over the Japanese
test procedure also became available. Indeed, the brochure on
the Prius, provided to EPA as part of the background information
on the vehicle, quoted fuel efficiency on the Japanese 10-15 city
driving mode of 28.0 km/liter. Converting to mpg:
28.0km
mile
l.6Q9km
3.7S5liters
gallon
= 65.9MPG
or
about 66 mpg. This 66-mpg value is even cited in Toyota's press
release of December 11, 1997—without the caveat that the results
were obtained on a test cycle substantially different from the
one used in the U.S. to measure and report fuel economy. Others,
for example Business Week. December 15, 1997, page 108, picked
up this and other information and also reported the 66-mpg value.
53
-------
Figure 15
Distribution of MY98 55/45 MPG
3000 Pound Inertia Weight Class
Number of Vehicles
15
J Conventional Vehicles
0 5 10 15 20 25 30 35 40 45 50 55 60
55/45 Unadjusted MPG
Figure 16
Distribution of MY98 55/45 MPG
EPA Subcompact Class
Number of Vehicles
15
Conventional Vehicles
B Gasoline — Diesel
0 5 10 15 20 25 30 35 40 45 50 55 60
55/45 Unadjusted MPG
54
-------
The necessary clarification needed is to remember that fuel
economy results always need to be referenced to the specific test
method by which they were derived. This has not been an issue in
the U.S. for more than 25 years, since the Federal Trade
Commission ruled that only EPA fuel economy values could be used
for advertising. So when people in the U.S. now see fuel-economy
values cited, it is logical to assume that they think it is a
U.S. test-based value. The 66 mpg for the Prius is not. It is a
value determined on the Japanese 10-15 mode procedure which has
historically yielded higher fuel economy values than the official
U.S. test. ;
The fuel economy results reported in this report are
consistent with the driving cycles and calculation procedures
used to determine fuel economy in the U.S.
EPA tested the Prius vehicle on California Phase-II
gasoline, at Toyota's request. Current Federal vehicle
certification uses a correction factor that takes into account
fuel properties to adjust calculated volumetric fuel economy when
Phase-II fuel is used. The fuel economies presented in this
report are unadjusted. To express these fuel economies in the
adjusted manner currently used in the certification process, each
fuel economy value would have to be increased by about one
percent.
The most current properties of the test fuel indicate a net
heat of combustion, by ASTM D3338, of 18518 BTU/lb. The specific
gravity of the fuel, 0.7402, indicates a value of 114,430
BTU/gallon, approximately 0.26 percent higher than the baseline
fuel. On the basis of the BTU content of the fuel alone, this
would mean that the fuel economies presented in this report would
have to be adjusted downward, multiplying the reported figures by
a factor of 0.9974.
55
-------
VII. Hiahlicrhts from Testing
1. The THS was evaluated over two test sequences involving
the Federal Urban Dynamometer Driving Schedule and the Highway
Fuel Economy Test. These tests are described in Figure 6 and 7,
Test Procedures, and the Discussion of Test Results, above.
Test Sequence No. 1 involved a 4-Bag FTP followed
immediately by a conditioning HFET and a second, measured HFET.
Measured emissions of hydrocarbons, carbon monoxide, and oxides
of nitrogen were well below the current Federal standards for
gasoline-fueled light-duty vehicles. The average composite
city/highway fuel economy measured over Test Sequence No. 2 was
48.6. The average composite city/highway fuel economy measured
over Test Sequence No. 1 was 49.8. Both of these figures are
uncorrected for state of charge or battery-pack
charging/discharging over the test cycles.
2. Limited performance testing (0-60 mph acceleration
testing) was conducted. With a normally charged battery pack, an
average 0-60 mph time of slightly over 14 seconds was noted.
This time increased to about 19.5 seconds with the battery pack
discharged well below what Toyota considers a normal state of
charge.
! j
3. A number of tests over the HFET cycle in succession
were conducted. The average fuel economy over the HFET,
following ^conditioning drives" over several HFET cycles, was
about =3.8 mpg. This fuel economy figure ignores state of
battery charge or charging/discharging of the battery over the
tests.
4. Several tests in succession over the US06 driving cycle
were conducted. Some of these tests were conducted with the
battery pack in a state of deeper than normal discharge prior to
commencement of testing. A relationship between the state of
charge of the battery pack and the ability of the THS to follow
precisely the aggressive driving trace of the US06 was noted.
5. An attempt to relate net battery charging/discharging
over a test cycle or driving mode was made by EPA. Battery state
of charge, or in the absence of that measurement, net battery
charge/discharge over a driving cycle is important if fuel
economy and emissions are calculated in a conventional manner,
56
-------
examining emissions from the engine only from a charge-sustaining
hybrid electric vehicle. Measured fuel economy over Test
Sequence No. 2 versus net battery charging/discharging over these
modes was presented in Figures 12 and 13. A relationship between
these two parameters using linear least squares regression was
established. The predicted value of 4-Bag FTP or "city" fuel
economy relating to a zero net charge/discharge over this test
cycle was 48.5 mpg. The predicted fuel economy for the HFET
(Test Sequence No. 2) immediately following a 4-Bag FTP relating
to a zero net charge/discharge of the battery pack was 49.8 mpg.
57
-------
VIII.Acknowledgments
:.
The authors gratefully acknowledge the Toyota Motor
Corporation and the Toyota Technical Center, U.S.A., for
providing the THS and technical support for this evaluation.
The authors acknowledge the efforts of Robert Moss, Testing
Services Division (TSD), EPA, who drove the vehicle over all
testing referred to here. Carl Paulina, TSD, served as test
coordinator for TSD, and William Courtois, TSD, processed the
test results and programmed the necessary special driver's
traces.
The following individuals served as emissions analyzer
operators during the THS evaluation: Darlene Curtis, Philip
Conde, Scott Wilson, Jeff Cieslak, all of TSD. Special
recognition is also given to Peter Forgacs, TSD, for outstanding
and timely electronics support.
i
i
The authors also acknowledge Robert Heavenrich, David Swain,
Lillian Johnson, and Robert Kelly of the Advanced Technology
Support Division (ATSD), EPA, who provided comparison fuel
economy data, provided battery characteristics, typed this
manuscript, and provided electronics support, respectively.
58
-------
IX. References
1• Toyota Electric and Hybrid Vehicles. Toyota Motor
Corporation International Communications Dept., Tokyo, Japan,
December 1997.
2. "Toyota Hybrid System," 1997 Press Information, Toyota
Motor Corporation, International Public Affairs Division, Toyota
City, Japan, May 1997.
3. Letter, Matsubara, A., Toyota Motor Corporation, to Oge,
M., U.S. EPA, February 3, 1998.
4. "Recommended Practice For Measuring The Exhaust
Emissions and Fuel Economy of Hybrid-Electric Vehicles," Society
of Automotive Engineers (SAE) Procedure J1711 (Draft), February
4, 1998. ;
5. "Proposed California Air Resources Board Hybrid Electric
Vehicle Test Procedures," Draft, California Air Resources Board,
El Monte, CA, April 3, 1998.
6. "Exhaust Emission Test Procedure for SC03 Emissions," 40
CFR, Part 86, Section 160-00, dtd July 1, 1997.
59
-------
Appendix A
Test: Vehicle Specifications
Toyota Hybrid System (THS)
Vehicle Type
Seating Capacity
Interior Volume
Vehicle Class
Vehicle Length
Vehicle Height
Vehicle Width
Wheelbase
Tire Pressure
Curb Weight
Vehicle Inertia
Test Weight
Dynamometer
Set Coefficients
(Parasitic Drag
or Windage)
Toyota Prius 4-Door Sedan
5
98.6 ft3
Subcompact
4.275 m
1.490 m
1.695 m
2.550 m
32 psi
2783 Ibs (w/full fuel tank)
3000 Ibs
A = 7.12 Ibs
B = -0.0971 Ibs/mph
C = 0.02078 Ibs/mph2
60
-------
Appendix B
THS Internal Combustion Engine Specifications
Cylinders
Displacement
Bore
Stroke
Mechanical Compression
Ratio
Expansion Ratio
Valve Train
Combustion Chamber
Ignition
Fuel System
Maximum Engine Speed
Power
1.5 liters
75 mm
84.7 mm
9.0:1
13.5:1
4-Valve, Dual Overhead Cam
incorporates Toyota Variable Valve
Timing with Intelligence (VVT-i)
Pentroof Design
Toyota Direct Ignition
Electronic Sequential Port Fuel
Injection
4000 rpm
Maximum 43 kW at 4000 rpm
Features: The engine uses all-aluminum head and block
construction. The engine incorporates modified Atkinson cycle
operation, with late intake-valve closure, to enable a lower
effective compression ratio, when compared to the expansion
ratio. Toyota claims a "thinner" crankshaft, lower tensile
strength piston rings, and reduced valve spring loads for this
engine compared to a "comparable" 1.5-liter, 4-valve engine.
These economies are enabled as a result of the reduced engine
speed (maximum 4000 rpm) arid output of this engine compared to
similar engines, it is claimed by Toyota. Detailed
specifications are available from the Toyota Motor Corporation.
61
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Appendix C
California Phase II Test Fuel Specifications, Page 1
Time In: 00:00 Time Out: 00:00
19'Mar'98 NVFEL Fuel Analysis Report
CA Phase II 2/23/98
Facility Name: UE EPA NVFEL Fuels Group Facility Type: In house
Owner: US EPA Phone: (734) 741-7881
2565 Plymouth Road
Ann Arbor, MI 48105 Washtenaw County
Inspector: NST Inspection Date : 2/23/98
Samples Type: Test Fuel
Inspection information logged in by PAB on 2/23/98.
CA Pha« 11 2/23/98 FTAG: 6960 Comments:
Test Test Method
Code
62 Vapor Pressure by Appendix E Method 3
62 Vapor Pressure by Appendix E Method 3
692 Degrees API
691 Specific Gravity 60 Degrees F
69 Density @ 60 deg F
101 D 86 Initial Boiling Point
D 86 Initial Boiling Point
D 86 Initial Boiling Point
10 Percent
10 Percent
10 Percent
50 Percent
CA Phase II 2/23/98 Page I of 2
101
101
no
no
no
ISO
ISO
ISO
190
190
190
200
200
200
201
201
201
202
202
202
203
203
203
65
65
SO Percent
50 Percent
90 Percent
90 Percent
90 Percent
End Point
End Point
End Point
Residue
Residue
Residue
Total Recovery
Total Recovery
Total Recovery
Loss
Loss
Loss
Percent Evaporated at 200 Degrees F
Percent Evaporated at 200 Degrees F
Results Units
6.92 PS1A
6.92 PSIA
59.64 Degrees API
0.73954 g/mL @ 60 F
0.74027 g/cm-03 @ 60 deg F
98.3 Degrees F
97.59 Degrees F
99.69 Degrees F
141.39 Degrees F
141.69 Degrees F
1 39.3 Degrees F
208.8 Degrees F
207.5 Degrees F
209.5 Degrees F
300.29 Degrees F
300.1 Degrees F
298.39 Degrees F
377.7 Degrees F
391.79 Degrees F
390.39 Degrees F
1 mL
0.8 mL
1.29mL
98.59 mL
98.3 mL
97.5 mL
0.6 mL
1.2mL
0.69 mL
46 Volume Percent
45 Volume Percent
: |
Possible
Violation?
No
No
No
No
No
No
No
No
No
No
No
No
No
No
No
No
No
No
No
No
No
No
No
No
No
No
No
No
No
No
No
!
Analyst
PAB
PAB
PAB
PAB
PAB
PAB
PAB
PAB
PAB
PAB
PAB
PAB
PAB
PAB
PAB
PAB
PAB
PAB
PAB
PAB
PAB
PAB
PAB
PAB
PAB
PAB
PAB
PAB
PAB
PAB
PAB
Leaked: No
Analysis Date
2/23/98
2/23/98
2/23/98
2/23/98
2/23/98
2/23/98
2/23/98
2/23/98
2/23/98
2/23/98
2/23/98
2/23/98
2/23/98
2/23/^8
2/23/98
2/23/98
2/23/98
2/23/98
2/23/98
2/23/98
2/23/98
2/23/98
2/23/98
2/23/98
2/23/98
2/23/98
2/23/98
2/23/98
2/23/98
2/23/98
2/23/98
62
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Appendix C
California Phase II Test Fuel Specifications, Page 2
I9.Mar.98
65 Percent Evaporated at 200 Degrees F
66 Percent Evaporated at 300 Degrees F
66 Percent Evaporated at 300 Degrees F
66 Percent Evaporated at 300 Degrees F
49 Oleflns in Petro. Prod, by ASTM D 1319-93
46 Aromatics in Petro. Prod, by ASTM 1319-93
541 Methanol by MSD (Screen)
541 Methanol by MSD (Screen)
551 MTBE by MSD (Screen)
551 MTBE by MSD (Screen)
561 ETBE by MSD (Screen)
561 ETBE by MSD (Screen)
571 TAME by MSD (Screen)
571 TAME by MSD (Screen)
592 Volume Percent Oxygenates by MSD (Screen)
592 Volume Percent Oxygenates by MSD (Screen)
591 Weight Percent Oxygen by MSD (Screen)
591 Weight Percent Oxygen by MSD (Screen)
63 Benzene in Gasoline by MSD D5769
63 Benzene in Gasoline by MSD D5769
48 Aromatics in Gasoline by MSD D5769
48 Aromatics in Gasoline by MSD D5769
64 Benzene in Gasoline by ASTM D 3606
543 Methanol by OFID
543 Methanol by OFID
532 Ethanol by OFID
532 Ethanol by OFID
531 Ethanol by MSD (Screen)
531 Ethanol by MSD (Screen)
585 t-Butanol by OFID
585 t-Butanol by OFID
57 TAME by OFID
57 TAME by OFID
593 Volume Percent Oxygenates by OFID
593 Volume Percent Oxygenates by OFID
59 Weight Percent Oxygen by OFID
59 Weight Percent Oxygen by OFID
421 Sulfur in Gasoline by ASTM D 2622
Fuel Analysis Report
45.29 Volume Percent
89.9 Volume Percent
90 Volume Percent
90.19 Volume Percent
6.77 Volume Percent
23.7 Volume Percent
0 Volume Percent
0 Volume Percent
11.6 Volume Percent
11.8 Volume Percent
0 Volume Percent
0 Volume Percent
0 Volume Percent
0 Volume Percent
11.8 Volume Percent
II .6 Volume Percent
2.11 Weight Percent
2.16 Weight Percent
0.727 Volume Percent
0.73 Volume Percent
24.26 Volume Percent
24.75 Volume Percent
0.7179 Volume Percent
0 Volume Percent
0 Volume Percent
0 Volume Percent
0 Volume Percent
0 Volume Percent
0 Volume Percent
0 Volume Percent
0 Volume Percent
0.2 Volume Percent
0.16 Volume Percent
10.91 Volume Percent
11.06 Volume Percent
2.02 Weight Percent
1.99 Weight Percent
13 Parts Per Million
A Phase
No
No
No
No
No
No
No
No
No
No
No
No
No
No
No
No
No
No
No
No
No
No
No
No
No
No
No
No
No
No
No
No
No
No
No
No
No
No
II 2/23/98
PAB
PAB
PAB
PAB
NST
NST
EL
EL
EL
EL
EL
EL
EL
EL
EL
EL
EL
EL
EL
EL
EL
EL
TW
YTS
YTS
YTS/TW
YTS/TW
EL
EL
YTS/TW
YTS/TW
YTS/TW
YTS/TW
YTS/TW
YTS/TW
YTS/TW
YTS/TW
AJA
Page 2 of 2
2/23/98
2/23/98
2/23/98
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3/16/98
3/16/98
2/25/98
2/2S/98
2/2S/98
2/25/98
2/25/98
2/25/98
2/25/98
2/25/98
2/25/98
2/25/98
2/25/98
2/25/98
2/25/98
2/25/98
2/25/98
2/25/98
2/27/98
2/25/98
2/25/98
2/25/98
2/25/98
2/25/98
2/25/98
2/25/98
2/25/98
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