National Renewable Energy Laboratory 

Innovation for Our Energy Future 

A national laboratory of the U.S. Department of Energy

Office of Energy Efficiency & Renewable Energy

NREL is operated by Midwest Research Institute ● Battelle Contract No. DE-AC36-99-GO10337 

King County Metro Transit: 

Allison Hybrid Electric Transit 

Bus Laboratory Testing 

R.R. Hayes, A. Williams, J. Ireland, 

and K. Walkowicz 

Technical Report 

NREL/TP-540-39996 

September 2006 
King County Metro Transit: 

Allison Hybrid Electric Transit 

Bus Laboratory Testing 

R.R. Hayes, A. Williams, J. Ireland, 

and K. Walkowicz 

Prepared under Task No. FC06.3000 

Technical Report 

NREL/TP-540-39996 

September 2006 

National Renewable Energy Laboratory

1617 Cole Boulevard, Golden, Colorado 80401-3393 

303-275-3000 • www.nrel.gov 

Operated for the U.S. Department of Energy 

Office of Energy Efficiency and Renewable Energy 

by Midwest Research Institute • Battelle 

Contract No. DE-AC36-99-GO10337
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Table of Contents 

Acknowledgments.......................................................................................................................... iv 

Abstract........................................................................................................................................... 1 

Lab Description and Methods..................................................................................................... 1 

Emissions Measurement.............................................................................................................. 4 

Project Specific Setup and Methods............................................................................................ 5 

Test Description and Results .......................................................................................................... 8 

Driving Style Impact on Hybrid Vehicle Testing ....................................................................... 8 

Appendix A. Test Cell Instrumentation........................................................................................ 13 

Appendix B. Test Vehicle Information......................................................................................... 15 

Appendix C. Test Fuels................................................................................................................. 16 

Appendix D: Road Load ............................................................................................................... 17 

Appendix D: Road Load ............................................................................................................... 17 

Appendix E: Test Cycles .............................................................................................................. 18 

Appendix F: Driving Style............................................................................................................ 21 

Appendix G: Tabulated Test Results............................................................................................ 22 

iii
Acknowledgments 

Special thanks to the following test participants who helped complete this project. 

U.S. Department of Energy 

Susan Rogers 

Lee Slezak 

National Renewable Energy Laboratory 

Stuart Black 

Thomas McDaniel 

Chris Tennant 

KC Metro 

Jim Boon 

Todd Gibbs 

George Stites 

GM Allison 

Peter Chiang 

Paul Nama 

Wes Hamilton 

Steve Huseman

Peter Chiang 

Caterpillar 

Dave Bradshaw 

NC Power 

Bill Hofer 

Craig Johnson 

iv
Abstract 

The National Renewable Energy Laboratory’s (NREL) ReFUEL facility conducted chassis 

dynamometer testing of two 60-foot articulated transit buses, one conventional and one hybrid. 

The testing period for the baseline vehicle was May 2, 2005 through May 17, 2005, while the 

testing period for the hybrid vehicle was June 27, 2005 through July 1, 2005. Both test vehicles 

were 2004 New Flyer buses powered by Caterpillar C9 8.8L engines, with the hybrid vehicle 

incorporating a GM-Allison advanced hybrid electric drivetrain. Both vehicles also incorporated 

an oxidizing diesel particulate filter. The fuel economy and emissions benefits of the hybrid 

vehicle were evaluated over four driving cycles; Central Business District (CBD), Orange 

County (OCTA), Manhattan (MAN), and a custom test cycle developed from in-use data of the 

King County Metro (KCM) fleet operation. The hybrid vehicle demonstrated the greatest 

improvement in fuel economy in the low speed, heavy stop-and-go driving conditions of the 

MAN test cycle (74.6%), followed by the OCTA (50.6%), CBD (48.3%), and KCM (30.3%). 

Emission trends were similar to fuel economy improvement trends. The hybrid showed 

reductions in NOx emissions over the MAN cycle (38.7%), the OCTA (28.6%), CBD (26.6%), 

and KCM (17.8%). In order to evaluate the effects of additional engine and vehicle loading due 

to air conditioning and grade, select cycles were repeated with and without these added loads. 

Vehicle exhaust emissions, fuel consumption, and state of charge of the energy storage system 

were measured for repeated test conditions. The remainder of this document includes the 

experimental setup, test procedures, and results from vehicle testing performed at the NREL 

ReFUEL laboratory. 

Lab Description and Methods 

The vehicles were tested at the ReFUEL laboratory, which is operated by NREL and located in 

Denver, Colorado. The lab includes a heavy-duty vehicle (chassis) test cell and an engine 

dynamometer test cell with emissions measurement capability. Researchers perform regulated 

emissions measurements using procedures consistent with the Code of Federal Regulations 

(CFR) title 40, section 86, subpart N. Extensive data acquisition and combustion analysis 

equipment can be used to relate the effects of different fuel properties and engine settings to 

performance and emissions. Other laboratory capabilities include: systems for sampling and 

analyzing unregulated emissions, on-site fuel storage and fuel blending equipment, high-speed 

data acquisition hardware and software to support in-cylinder measurements, and fuel ignition 

quality testing. Instrumentation and sensors at the laboratory are maintained with National 

Institute of Standards and Technology (NIST)-traceable calibration. Test procedures, 

calibrations, and measurement accuracies are maintained to meet requirements outlined in the 

current CFR title 40, section 86, subpart N. 

Chassis Dynamometer 

The ReFUEL Chassis Dynamometer is installed in the main high-bay area of the laboratory. The 

roll-up door to the high bay is 14 ft. x 14 ft., high enough to accept all highway-ready vehicles 

without modification. The dynamometer is installed in a pit below ground level, so that the only 

exposed part of the dynamometer is the top of the 40 in. diameter rolls. Two sets of rolls are 

installed, so that twin-axle tractors can be tested. The distance between the rolls can be varied 

1 
between 42 in. and 56 in. The dynamometer will accommodate vehicles with a wheelbase 

between 89 in. and 293 in. The dynamometer can simulate up to 80,000 lb vehicles at speeds up 

to 60 mph. 

The chassis dynamometer is composed of three major components: the rolls – which are in direct 

contact with the vehicle tires during testing, the direct current (DC) electric motor (380 hp 

absorbing / 360 hp motoring) dynamometer, and the flywheels. 

The rolls are the means by which power is absorbed from the vehicle. The rolls are attached to 

gearboxes that increase the speed of the central shaft by a factor of 5. The flywheels, mounted on 

the back of the dynamometer, provide a mechanical simulation of the vehicle inertia. 

The electric motor is mounted on trunnion bearings, and is used to measure the shaft torque from 

the rolls. The energy absorption capability of the dynamometer is used to apply the “road load,” 

which is a summation of the aerodynamic drag and friction losses that the vehicle experiences in 

use, as a function of speed. The road load may be determined experimentally if data are 

available, or estimated from standard equations. The electric dynamometer is also used to adjust 

the simulated inertia, either higher or lower than the 31,000 lb base dynamometer inertia, as the 

test plan requires. The inertia simulation range of the chassis dynamometer is 8,000 – 80,000 lbs. 

The electric motor may also be used to simulate grades and provide braking assist during 

decelerations. 

The test vehicle is secured with the drive axles over the rolls. A driver’s aid monitor in the cab is 

used to guide the vehicle operator in driving the test trace. A large fan may be used to cool the 

vehicle radiator during testing. The chassis dynamometer is supported by 72 channels of data 

acquisition, in addition to the emissions measurement, fuel metering, and combustion analysis 

subsystems. 

The dynamometer is capable of simulating vehicle inertia and road load during drive cycle 

testing. With the vehicle jacked up off the rolls, an automated dynamometer warm-up procedure 

is performed daily, prior to testing, to ensure that parasitic losses in the dynamometer and 

gearboxes have stabilized at the appropriate level to provide repeatable loading. An unloaded 

coast down procedure is also conducted to confirm that inertia and road load are being simulated 

by the dynamometer control system accurately. Between test runs a loaded coast down procedure 

is performed to further ensure stability of vehicle and dynamometer parasitic losses and accurate 

road load simulation during testing. 

2 
Figure 1. Chassis Dynamometer Schematic 

Fuel Storage and Blending 

Buildings designed specifically for safely storing and handling fuels are installed at the ReFUEL 

facility. The fuel storage shed is 8 ft. x 26 ft. and holds up to 48 drums (55-gal each). Features 

include heating/cooling, secondary containment to 25% of capacity, continuous ventilation, 

explosion-proof wiring/lighting, and a dry chemical fire suppression system. 

The fuel blending can be performed on a gravimetric or 

volumetric basis, with capability for both large (L/kg) 

and small (cc/g) scale measurements. A fuel line inside 

a sealed conduit delivers the fuel from the supply drum 

to the fuel metering/conditioning system inside the 

ReFUEL laboratory, eliminating the need for bulk fuel 

storage inside the laboratory. Another fuel line in the 

same conduit delivers waste fuel back to the fuel 

blending shed for storage (waste fuel is generated only 

when a fuel changeover requires a flush of the system). 

Fuel Metering and Conditioning 

The fuel metering and conditioning system, Pierburg 

Model PII 514, (shown in Fig. 2) supports test work for 

both the engine and the chassis dynamometers. The meter measures volumetric flow to an 

accuracy of +/- 0.5% of the reading, with a reproducibility of 0.2%. An in-line sensor measures 

the density at an accuracy of +/- 0.001 g/cc, allowing an accurate mass measurement over the test 

cycle even if the density of the fuel blend is not known prior to testing. 

Figure 2. 

Pierburg Fuel Metering System 

Air Handling and Conditioning 

Dilution air and the air supplied to the test engine or vehicle for combustion are derived from a 

common source, a roof-mounted system that conditions the temperature of the air and humidifies 

as needed to meet desired specifications. This air is passed through a HEPA filter, in accordance 

with the (2007) CFR specifications, to eliminate background particulate matter as a source of 

uncertainty in particulate measurements. 

3 
Emissions Measurement

The ReFUEL laboratory’s emissions 

measurement system supports both the engine 

and chassis dynamometers. It is based on the 

full-scale exhaust dilution tunnel method with a 

constant volume sampling (CVS) system for 

mass flow measurement. The system is designed 

to comply with the requirements of the 2007 

CFR, title 40, part 86, subpart N. Exhaust from 

the engine or vehicle flows through insulated 

piping to the full-scale 18 in. diameter stainless 

steel dilution tunnel. A static mixer ensures thorough mixing of exhaust with conditioned, 

filtered, dilution air prior to sampling of the dilute exhaust stream to measure gaseous and 

particulate emissions. 

Figure 3. Venturi Nozzles 

A system with three Venturi nozzles is employed to maximize the flexibility of the emissions 

measurement system. Featuring 500 cfm, 1000 cfm, and 1500 cfm Venturi nozzles and gas-tight 

valves, the system flow can be varied from 500 cfm to 3000 cfm flow rates in 500 cfm 

increments. This allows the dilution level to be tailored to the engine size being tested (whether 

on the engine stand or in a vehicle), maximizing the accuracy of the emissions measurement 

equipment. 

The gaseous emissions bench is a Pierburg model AMA-2000. It features continuous analyzers 

for total hydrocarbons (HC), oxides of nitrogen (NOx), carbon monoxide (CO), carbon dioxide 

(CO2), oxygen (O2). The system features auto-ranging, automated calibration, zero check, and 

span check features as well as integrating functions for calculating cycle emissions. It 

communicates with the ReFUEL data acquisition systems through a serial interface. There are 

two heated sample trains for gaseous emissions measurement: one for HC, and another for the 

other gaseous emissions. NOx and HC measurements 

are performed on a wet basis, while CO, CO2 and O2, 

are done on a dry basis. Sample probes are located in 

the same plane in the dilution tunnel. 

The particulate matter sample control bench is 

managed by the ReFUEL data acquisition system 

through a serial connection. It maintains a desired 

sample flow rate through the particulate matter (PM) 

filters in proportion to the overall CVS flow, in 

accordance with the CFR. Stainless steel filter 

holders, designed to the 2007 CFR requirements, 

house 47 mm diameter Teflon membrane filters 

through which the dilute exhaust sample flows. The 

PM sampling system is capable of drawing a sample 

directly from the large full-scale dilution tunnel or 

using secondary dilution to achieve desired 

Figure 4. Pierburg Emissions Bench 

4 
temperature, flow, and concentration characteristics. A cyclone separator, as described in the 

CFR requirements, is employed to mitigate tunnel PM artifacts. 

A dedicated clean room/environmental chamber is installed inside the ReFUEL facility. It is a 

Class 1000 clean room with precise control over the temperature and humidity (+/- 1°C for 

temperature and dew point). This room is used for all filter handling, conditioning, and weighing. 

The microbalance for weighing PM filters features a readability of 0.1 μg (a CFR requirement) 

and features a barcode reader for filter identification and tracking, and a computer interface for 

data acquisition. The microbalance is installed on a specially designed table to eliminate 

variation in the measurement due to vibration. The microbalance manufacturer (Sartorius) was 

consulted on the design of the clean room to ensure that the room air flow would be compatible 

with the microbalance. 

Figure 5. Class 1000 Clean Room, Filter Housing, and Microbalance 

Project Specific Setup and Methods 

The test vehicles were installed on the chassis dynamometer as shown in Figure 6. A process and 

instrumentation diagram of each vehicle test 

setup is included as Figures 11 and 12 in 

Appendix A, which contains detailed 

information regarding sensor description and 

placement (which were used to ensure accurate 

operation of vehicle and test equipment). 

Additional data from the engine control unit 

were also recorded for quality assurance using a 

pc-based acquisition system connected via serial 

interface. 

Test Vehicles 

The baseline, conventional test vehicle was a 

2004 New Flyer bus powered by the Caterpillar 

C9 8.8L 330 hp diesel engine. The hybrid test 

vehicle, which was based on the same 2004 New 

Flyer platform also incorporated the Cat C9 

engine along with Allison’s EP50 hybrid electric drive train (parallel hybrid). Both vehicle 

Figure 6. Chassis Cell with Test Vehicle 

5 
descriptions are shown in Appendix B, Table 8. The hybrid electric drive system uses two 

induction motors, each with a continuous power rating of 100 hp and a peak power rating of 200 

hp. The nickel metal hydride battery pack has a nominal voltage of 650 V. Conditioned cooled 

air was supplied to the hybrid vehicle battery pack for controlled cooling and ventilation in an 

attempt to more closely simulate operating conditions in King County, WA. Each test vehicle 

was equipped with an Englehard diesel particulate filter (DPF). 

Test Fuel 

The same DPF was used on each vehicle during testing (see Appendix B, Table 8 for 

description). All testing was performed with low sulfur diesel (BP15). The fuel supplied to the 

engine of each test vehicle was continuously conditioned and metered. Fuel analysis information 

is also included in Appendix C. 

Air and Exhaust 

Intake air was conditioned and supplied to each test vehicle by the ReFUEL system with 

continuous recorded measurements of ambient pressure, inlet restriction, humidity, and 

temperature of the inlet air (as described in Figure 11 and 12). Approximately 44 ft. of 6 in. 

diameter, insulated, stainless steel tubing connected the test vehicle exhaust pipe to the dilution 

tunnel, with recorded temperatures measured at the outlet of the vehicle exhaust pipe, at the 

entrance to the dilution tunnel, and at the plane of the emissions sampling probes. Exhaust 

pressures were also measured to ensure back pressures did not exceed those specified by the 

manufacturer. Typical peak exhaust back pressures resulting from the emissions sampling 

systems were 4 in. H20. 

Vehicle Simulation 

The simulated vehicle inertia test weight for the test vehicle was calculated as half of the loaded 

vehicle weight, equaling 49,200 lb. for the conventional and 50,500 lb. for the hybrid. The 

vehicle loss coefficients are shown in Table 1. Road load coefficients for the baseline vehicle 

were derived from track coast down data provided by Allison. Two sets of track coast down data 

were provided by Allison for the hybrid bus; each resulting in different curves. These data sets 

were obtained at different times under different conditions with two different hybrid buses, one 

being the test vehicle. The coast down data of the other hybrid bus was of better quality than the 

test vehicle track coast down data, so this was used as the basis for generating road load curves 

for the test hybrid bus. 

Table 1. Road Load Coefficients

Coefficient Conventional Bus Hybrid Bus 

A 545 lb. 545 lb. 

B 4.1967 lb./mph 1.3778 lb./mph 

C 0.06262 lb./mph2 0.09100 lb./mph2

Road load coefficients for the hybrid vehicle were thus derived based on a combination of track 

coast down data provided by Allison and coast down data measured on the dynamometer at 

ReFUEL. The coefficients used during the dynamometer testing for the hybrid bus match very 

well with the track coast down at speeds above 15 mph, and match well with the behavior during 

6 
the dynamometer measured coast downs 

below 15 mph. Plots of the vehicle road load 

and coast down curves used for the simulation 

are shown in Appendix D. 

The appropriate chassis dynamometer load 

settings were then derived to simulate the 

calculated road load coefficients for the test 

vehicle so that the load forces simulated on 

the chassis dynamometer rolls during testing 

would best match the target road load curves. 

State Of Charge Considerations 

The Society of Automotive Engineers (SAE) 

Recommended Practice J2711 is a 

recommended protocol for measuring fuel 

economy and emissions of hybrid-electric and conventional heavy-duty vehicles. The 

recommended practice provides a description of state of charge (SOC) correction for chargesustaining HEVs. 

Figure 7. Current Clamp 

 

Using the nominal battery pack voltage of 650 V and a continuously recorded measurement of 

DC current (Figure 7) into and out of the battery pack the net energy change (NEC) was 

calculated for each test run. Note that the percentage change in state of charge on all cycle runs 

was less than 1%, thus there was no need to correct data per SAE J2711. 

 

Drive Cycles 

With the assistance of KCM technical staff, in-use duty cycle data were logged during actual bus 

operation to capture representative speed-vs-time-vs-grade on some of KCM’s typical routes. 

NREL derived a custom drive cycle (the KCM cycle) based on this data, to best simulate the real 

world duty cycle during testing. It consisted of four distinct modes to demonstrate highway and 

city (variable speeds and grades) driving. In addition, the overall KCM cycle was performed both 

with and without simulated grade and air conditioning to assess the effects of these conditions on 

performance. Testing was also performed using standard transit bus test cycles including the 

CBD, MAN, and OCTA. To ensure that the NEC of the charge sustaining Energy Storage 

System (ESS) was >1% of the total cycle energy, the shorter drive cycles were lengthened. The 

MAN cycle was doubled in length, giving a total cycle time of 2178 seconds and the CBD was 

tripled, for a total time of 1722 seconds. Plots of the CBD, MAN, OCTA, and custom KCM test 

cycles along with tabulated cycle statistics are shown in Appendix E. Additional test cycles were 

performed using both vehicles with the air conditioning system turned on throughout the test run 

over both CBD and KCM drive cycles. The SAE J2711 procedure outlines a specified tolerance 

for how closely the actual measured test speed versus time data matches the target test cycle 

trace. All test runs were validated based upon this procedure. 

7 
Test Description and Results 

Initially, on each test day the chassis dynamometer was run through a standard automated warmup procedure to ensure that dynamometer parasitics had stabilized. Periodic unloaded and loaded 

coast downs were also performed to ensure that inertia and road load were being simulated 

correctly according to the set inputs. 

Each test vehicle was operated over repeated hot-start runs for each of the four drive cycles. For 

the purposes of this report, a valid hot-start run is defined as a test run performed following a 

previous similar run, separated by a soak time of 20 - 30 minutes. In order to evaluate the effects 

of additional auxiliary loads the CBD and KCM cycles were repeated with and without air 

conditioning. To evaluate the effects of grade the KCM cycle was repeated with and without 

grade simulation. 

Two different drivers were used during baseline, conventional vehicle testing to evaluate driverto-driver variability. Hot start CBD and KCM cycles were repeated three times with each driver. 

A statistical analysis showed no significant difference of NOx emissions or fuel economy 

between the two drivers at the 95% confidence level. All hybrid vehicle data presented in this 

report was collected with a single driver, with the exception of a triplicate CBD data set. 

Plots of the averaged data for each test vehicle on each drive cycle are shown for fuel economy, 

fuel consumption, and NOx emissions in Figures 8 through 10, with error bars representing the 

95% confidence interval of the means. Detailed tabulated data from each test run are also 

included as Appendix G. 

Driving Style Impact on Hybrid Vehicle Testing 

According to the recommended practices of SAE J2711, adherence to the drive cycle is 

confirmed by a regression analysis comparing actual speeds with target speeds. If the resulting 

trend line when plotting actual versus target speed varies from unity by more than 10% or an R2

of less than 0.8 then the test run is considered invalid. Over the course of testing the hybrid 

vehicle on several CBD cycles, it was noticed that different driving styles, each producing valid 

results according to the recommended practice of SAE J2711, could potentially produce 

measurable differences in fuel economy. When operating the hybrid vehicle the drivers indicated 

that they perceived a higher sensitivity in the vehicle’s throttle response due to the powertrain’s 

high torque and regenerative braking capability when trying to maintain steady cruise speed (20 

mph) repetitively during the CBD test cycle. 

Upon review of the data, it was noticed that dithering of the throttle during cruise segments of 

the CBD cycle was correlated with higher integrated cycle energy than compared to the baseline. 

A triplicate of hot start CBD runs was completed with each driving style (with relatively more 

and less acceleration overshoot and throttle dither). The data, which indicates the difference in 

total cycle energy (approx 10% between styles) and fuel economy is tabulated in Appendix G. 

The difference in average hybrid bus fuel economy on the CBD test cycle between the two sets 

of runs, is approximately 4%-5%. Figure 19, Appendix F, shows a snap shot of two such cycles 

compared to the CBD target driver’s trace that best illustrate the extremes. 

8 
As the drivers became more accustomed to the throttle response of the hybrid and consciously 

matched the CBD trace more steadily, less throttle dither was observed. All of the averaged data 

comparing hybrid CBD results to the conventional bus, plotted in Figures 8-10, are based on the 

latter test runs with less throttle dither. Conscious driver effort was not made to reduce throttle 

dither during CBD runs with the conventional bus. While formal driver style studies were not 

within the scope of this project and different styles were not rigorously tested, these results 

suggest that the hybrid vehicle may have a higher sensitivity to driving style on fuel economy 

performance than the baseline vehicle, especially over a test cycle dominated by repeated steady 

target cruise speeds. 

Fuel Economy 

The average measured fuel economy and fuel consumption over each test cycle for both vehicles 

are plotted in Figures 8 and 9. The same results are also shown in Table 2 for both vehicles, 

along with absolute improvement in miles per gallon of the hybrid bus versus the conventional 

bus. 

Percentage improvements in fuel consumption and fuel economy for the hybrid versus the 

conventional bus over each test cycle are summarized in Table 2. A breakdown of fuel 

consumption and economy for the composite KCM cycle is summarized in Table 3. The hybrid 

vehicle demonstrated the highest percentage improvement in fuel economy (mpg basis) over the 

MAN driving cycle (74.6%), followed by the OCTA (50.6%), CBD (48.3%), and KCM (30.3%) 

test cycles. The benefits of the hybrid powertrain are most pronounced during lower speed, stopand-go driving. 

Figure 8. Average Fuel Economy

9 
Figure 9. Average Fuel Consumption 

Table 2. Measured Fuel Economy 

 Manhattan OCTA CBD CBD w/AC KCM 

KCM no 

grade 

KCM no 

grade w/ 

AC 

Conventional Bus 

Fuel Economy (mpg) 1.46 2.15 2.19 1.92 2.90 3.03 2.76 

Hybrid Bus 

Fuel Economy (mpg) 2.56 3.24 3.15* 3.25 2.61 3.78 3.98 3.45 

Fuel Economy 

Increase with Hybrid 

Bus (mpg) 1.09 1.09 0.96* 1.06 0.69 0.88 0.95 0.69 

*Earlier runs, with more throttle dither and higher total cycle energy 

Table 3. Measured Fuel Economy KCM Composite Breakdown 

 

KCM 

(I5) 

KCM 

 (Rte 174)

KCM 

(Rte 120)

KCM 

(Rte. 106) 

Conventional Bus 

Fuel Economy (mpg) 4.33 2.35 2.30 2.09 

Hybrid Bus 

Fuel Economy (mpg) 4.82 3.17 3.36 2.86 

Fuel Economy 

Increase with Hybrid 

Bus (mpg) 0.49 0.82 1.06 0.77 

10 
Table 4. Hybrid Vehicle Percentage Fuel Economy and Consumption Improvement 

 Manhattan OCTA CBD 

CBD w/ 

AC KCM 

KCM no 

grade 

KCM no 

grade w/ 

AC 

Fuel Economy 

 (mpg % 

improvement) 74.6% 50.6% 43.8%* 48.3% 35.9% 30.3% 31.4% 25.0% 

Fuel Consumption 

(gallon/mile % 

improvement) 42.9% 33.7% 30.5%* 32.8% 26.7% 23.4% 24.2% 20.2% 

*Earlier runs, with more throttle dither and higher total cycle energy 

Table 5. Hybrid Vehicle Percentage Fuel 

Economy and Consumption Improvement KCM Composite Breakdown

 

KCM 

(I5) 

KCM 

(Rte 174)

KCM 

(Rte 120)

KCM 

(Rte 106) 

Fuel Economy 

(mpg % improvement) 11.4% 34.6% 45.8% 37.2% 

Fuel Consumption 

(gallon/mile % improvement) 10.3% 25.8% 31.5% 27.2% 

Percentage differences in measured fuel economy due to grade and auxiliary loading from air 

conditioner operation are shown in Table 6. Auxiliary load testing indicates that the hybrid 

vehicle experienced a slightly larger penalty in fuel economy as a result of air conditioning when 

compared to the conventional bus. The percentage change in fuel economy due to simulated 

grade during the KCM cycle was similar for both vehicles. 

Table 6. Fuel Economy Penalties from AC and Grade 

CBD KCM 

 Conventional Hybrid Conventional Hybrid 

Air Conditioning 

(mpg % penalty) 12.3% 19.7% 8.9% 13.3% 

Grade 

(mpg % penalty) - - 4.3% 5.0% 

Emissions 

Average NOx emissions from both vehicles over each cycle are plotted in Figure 10. Similar in 

trend to fuel economy improvements, the hybrid vehicle showed highest percentage reduction in 

NOx emissions over the MAN driving cycle (38.7%), followed by the OCTA (28.6%), CBD 

(26.6%), and KCM (17.8%) cycles. Other measured emissions, including CO, THC, and PM 

were generally very low for both vehicles as a result of the exhaust aftertreatment (DPF) system. 

Most data sets showed a statistically significant reduction in CO, THC ,and PM emissions for the 

hybrid vehicle. Tabulated results for each vehicle are shown in Appendix G. Average percentage 

reductions in emissions for each driving cycle are tabulated in Table 7. Due to either relatively 

high variability in the data over small datasets or small differences in the mean, some data sets 

did not exhibit statistically significant differences in emissions between vehicles at the 95% 

confidence level, using the Student’s t-test and pooled variances. Significant variability was seen 

when comparing CBD PM datasets between driving styles. 

11 
Table 7. Hybrid Vehicle Emissions Percentage Reductions 

 Manhattan OCTA CBD KCM 

NOx (gallon/mile, % reduction) 38.7% 28.6% 26.4%* 26.6% 17.8% 

PM (gallon/mile, % reduction) 92.6% 50.8% 88.2%* 97.1% ns 

CO (gallon/mile, % reduction) ns** 32.0% 22.0%* 48.0% 59.5% 

THC (gallon/mile, % reduction) ns ns 66.7%* 75.2% 56.3% 

*Earlier runs, with more throttle dither and higher total cycle 

 **ns = not statistically significant at 95% confidence or not enough data to determine

Figure 10. NOx emissions 

12 
Appendix A. Test Cell Instrumentation 

Figure 11. Baseline Vehicle Process and Instrumentation 

Diagram 

13 
Figure 12. Hybrid Vehicle Process and Instrumentation Diagram 

14 
Appendix B. Test Vehicle Information 

Table 8. Detailed Vehicle Information

Vehicle Description Baseline Hybrid 

Model Year Jun-04 Jul-04 

Make New Flyer New Flyer 

Odometer 25,188 94,871 

Vin 5FYD2UW024U026978 5FYH2UW004U026690 

Model D60LF DE60LF 

Inertia Weight Class 66,790 66,790 

Test Weight 49,200 50,500 

Engine Baseline Hybrid 

Manufacturer Cat Cat 

Date of Man. Sep-05 N/A 

Displacement 8.8 L 8.8 L 

Serial Number MTB01121 MTB01232 

Model C9 C9 

Power 330 330 

Torque 1149 1149 

Fuel BP15 BP15 

ECM # 1045006KA 17546257IE 

Personality # 1593231-01 2601435-00 

Aftertreatment Device CAT DOC/DPF CAT DOC/DPF 

Chassis Baseline Hybrid 

Axle Ratio N/A N/A 

Transmission Type Allison Allison 

Transmission Model B500 Auto Hybrid EP 

Trans. Model # N/A N/A 

Brake Type Front Air Drum Air Drum 

Brake Type Rear Air Drum Air Drum 

Wheelbase N/A N/A 

Tires Baseline Hybrid 

Front 

Size 305/70 R225 305/70 R225 

Make Firestone Firestone 

Tread Depth 0.710/0.700 in 0.710/0.700 in 

Pressure D/P 130/130 psig 

Center 

Size 305/70 R225 305/70 R225 

Make Firestone Firestone 

Tread Depth Out D/P 0.685/0.675 in 0.685/0.675 in 

 Inside D/P 0.725/0.705 in 0.725/0.705 in 

Pressure Out D/P 135/130 psig 135 psig nominal 

 Inside D/P N/A N/A 

Rear 

Size 305/70 R225 305/70 R225 

Make Firestone (siped) Firestone (siped) 

Tread Depth Out D/P 0.355/0.542 in 0.355/0.542 in 

 Inside D/P 0.375/0.465 in 0.375/0.465 in 

Pressure Out D/P 130/125 psig 135 psig nominal 

 Inside D/P 90/125 psig N/A 

15 
Appendix C. Test Fuels 

Table 9. BP15 Test Fuel Analysis 

Property ASTM Test Method 

Low Sulfur Diesel 

(BP15) 

Cetane Number D613 51.1 

Cloud Point, °C D2500 -12 

Density, kg/m3 D4052 837 

Kinematic Viscosity, 40 °C, CSt D445 2.5 

Carbon, mass % D5291 86.04 

Hydrogen, mass % D5291 13.48 

Sulfur, ppm D5453 13 

Flash Point, °C D93 63.5 

Aromatics, vol% D1319 29 

Heat of Combustion, MJ/kg D240 45.7 

16 
Appendix D: Road Load

Figure 13. Road Load Curves

Figure 14. Coast Down Curves 

17 
Appendix E: Test Cycles 

Figure 15. Manhattan x2 Test Cycle

Figure 16. Orange County Test Cycle

18 
Figure 17. CBD x 3 Test Cycle 

Figure 18. King County Metro Test Cycle

19 
Table 10. Cycle Statistics

Test Cycle CBDx3 Manhattanx2 Orange Co KCM 

Total Time (sec) 1722 2178 1909 1964 

Time at Idle (%) 20.0 36.1 21.3 18.9 

Average Cycle Speed (mph) 12.58 6.83 12.33 23.42 

Average Speed while driving (mph) 15.73 10.68 15.67 28.87 

Maximum Speed (mph) 20.0 25.3 40.6 60.0 

Total Distance (miles) 6.02 4.13 6.54 12.78 

Number of Stops (stops/mile) 6.98 9.68 4.74 1.88 

Average Acceleration Rate (ft/sec2) 2.66 1.77 1.48 1.61 

Maximum Acceleration Rate (ft/sec2) 3.37 6.75 5.95 14.18 

Table 11. KCM Composite Breakdown - Cycle Statistics 

Test Cycle I-5 Rte 174 Rte 120 Rte 106 

Total Time (sec) 499 460 724 278 

Time at Idle (%) 10.02 18.04 23.07 25.54 

Average Cycle Speed (mph) 41.85 19.61 16.39 14.98 

Average Speed while driving (mph) 46.5 23.92 21.29 20.1 

Maximum Speed (mph) 60 46.75 38.32 31.22 

Total Distance (miles) 5.81 2.51 3.3 1.16 

Number of Stops (stops/mile) 0.17 2.79 3.64 3.44 

Average Acceleration Rate (ft/sec2) 0.96 1.64 1.75 1.76 

Maximum Acceleration Rate (ft/sec2) 1.65 5.58 14.18 6.06 

20 
Appendix F: Driving Style

Figure 19. CBD Driving Styles 

21 
Appendix G: Tabulated Test Results 

Table 12. Baseline Vehicle Data 

Date Run Cycle Driver CO2 NOx THC CO PM 

Fuel 

Consumption

Fuel 

Economy Distance 

Energy From 

Vehicle 

 g/mile g/mile g/mile g/mile g/mile g/mile mpg miles hp-s 

5/9/2005 554 OCTA John 4584 18.95 0.03 2.20 0.047 1470 2.15 6.50 83259 

5/9/2005 555 OCTA John 4603 18.98 0.03 1.92 0.056 1476 2.14 6.50 84110 

5/9/2005 556 OCTA John 4550 18.81 0.03 2.75 0.046 1461 2.16 6.47 82701 

avg 4579 18.91 0.03 2.29 0.050 1469 2.15 6.49 83357 

std dev 26.94 0.09 0.00 0.42 0.01 7.67 0.01 0.02 709.56 

cov 0.59 0.47 5.75 18.40 11.41 0.52 0.52 0.27 0.85 

 

5/6/2005 544 Manhattan Tom 6662 29.33 0.03 2.39 0.038 2147 1.47 4.15 65735 

5/6/2005 545 Manhattan Tom 6716 29.72 0.05 3.93 0.039 2160 1.46 4.13 64747 

5/6/2005 546 Manhattan Tom 6763 29.70 0.03 3.06 0.038 2173 1.45 4.10 65987 

avg 6714 29.58 0.04 3.13 0.038 2160 1.46 4.13 65490 

std dev 50.49 0.22 0.01 0.77 0.00 13.04 0.01 0.03 655.39 

cov 0.75 0.73 30.58 24.61 1.42 0.60 0.65 0.61 1.00 

 

5/4/2005 536 CBD Tom 4586 19.56 0.16 1.27 0.112 1450 2.18 6.01 67754 

5/4/2005 537 CBD Tom 4599 19.58 0.10 1.56 0.115 1449 2.18 6.02 68591 

5/4/2005 538 CBD Tom 4581 19.52 0.10 1.49 0.105 1439 2.20 5.99 68084 

5/4/2005 539 CBD John 4607 19.74 0.10 2.22 0.093 1451 2.18 5.92 66723 

5/4/2005 540 CBD John 4569 20.01 0.14 1.89 0.098 1441 2.20 5.97 67919 

5/4/2005 541 CBD John 4580 19.62 0.11 2.16 0.081 1442 2.19 6.00 67649 

avg 4587 19.67 0.12 1.77 0.101 1445 2.19 5.98 67787 

std dev 13.88 0.18 0.02 0.39 0.01 5.32 0.01 0.04 617.13 

cov 0.30 0.92 20.33 21.86 12.76 0.37 0.35 0.60 0.91 

 

5/16/2005 582 CBD w/ AC John 5227 21.26 0.03 0.91 0.071 1660 1.91 5.97 70180 

5/16/2005 583 CBD w/ AC John 5153 21.09 0.03 1.03 0.067 1637 1.93 6.02 70342 

5/16/2005 584 CBD w/ AC John 5181 21.25 0.02 1.03 0.085 1644 1.92 5.98 69689 

avg 5187 21.20 0.03 0.99 0.074 1647 1.92 5.99 70070 

std dev 37.10 0.09 0.01 0.07 0.01 11.65 0.01 0.03 340.03 

cov 0.72 0.44 27.51 6.73 12.59 0.71 0.66 0.44 0.49 

 

5/10/2005 558 KCM Tom 3513 14.64 0.04 0.41 0.118 1112 2.84 12.64 150810 

5/10/2005 559 KCM Tom 3462 14.52 0.06 0.55 0.095 1092 2.90 12.66 148390 

5/10/2005 560 KCM Tom 3447 14.65 0.06 0.64 0.091 1090 2.90 12.68 150350 

5/10/2005 561 KCM John 3412 14.68 0.04 0.81 0.141 1081 2.92 12.76 150980 

5/10/2005 562 KCM John 3440 15.02 0.04 0.71 0.113 1091 2.90 12.67 150750 

5/10/2005 563 KCM John 3401 14.90 0.04 0.84 0.088 1081 2.92 12.70 149290 

avg 3446 14.74 0.04 0.66 0.108 1091 2.90 12.69 150095 

std dev 40.00 0.19 0.01 0.16 0.02 11.10 0.03 0.04 1033.78 

cov 1.16 1.28 21.23 24.78 18.88 1.02 0.99 0.33 0.69 

 

5/11/2005 567 KCM w/o Grade Tom 3279 13.55 0.03 0.44 0.043 1041 3.04 12.78 132430 

5/11/2005 568 KCM w/o Grade Tom 3293 13.34 0.02 0.39 nm 1046 3.03 12.79 134080 

5/11/2005 569 KCM w/o Grade Tom 3281 13.26 0.01 0.48 0.032 1042 3.04 12.80 135140 

5/17/2005 593 KCM w/o Grade John 3297 13.90 0.02 0.69 0.021 1042 3.03 12.82 136310 

5/17/2005 594 KCM w/o Grade John 3330 13.93 0.02 0.56 0.030 1053 3.00 12.82 134260 

avg 3296 13.60 0.02 0.51 0.031 1045 3.03 12.80 134444 

std dev 20.24 0.31 0.01 0.12 0.01 4.91 0.02 0.02 1430.81 

cov 0.61 2.29 31.13 23.52 28.72 0.47 0.56 0.14 1.06 

 

5/16/2005 588 KCM w/o Grade w/ AC John 3660 15.31 0.04 0.57 0.062 1164 2.72 12.72 130540 

5/16/2005 589 KCM w/o Grade w/ AC John 3590 15.40 0.04 0.36 0.072 1145 2.76 12.76 133270 

5/17/2005 591 KCM w/o Grade w/ AC John 3586 15.10 0.02 0.40 0.051 1133 2.79 12.75 133880 

5/17/2005 592 KCM w/o Grade w/ AC John 3590 15.13 0.02 0.56 0.041 1134 2.79 12.76 132100 

5/13/2005 573 KCM w/o Grade w/ AC Tom 3647 15.57 0.03 0.10 0.078 1155 2.74 12.68 130950 

avg 3615 15.30 0.03 0.40 0.061 1146 2.76 12.73 132148 

std dev 35.90 0.20 0.01 0.19 0.02 13.45 0.03 0.03 1438.91 

cov 0.99 1.28 25.23 48.08 24.80 1.17 1.15 0.27 1.09 

22 
Table 13. Hybrid Vehicle Data

Date Run Cycle Driver CO2 NOx THC CO PM 

Fuel 

Consumption

Fuel 

Economy 

ESS vs Cycle 

Energy NEC Distance 

Energy From 

Vehicle 

 g/mile g/mile g/mile g/mile g/mile g/mile mpg % amp-hr miles hp-s 

6/27/2005 652 OCTA John 3015 13.34 0.04 1.48 0.021 979 3.22 -0.25 -0.30 6.56 86920 

6/27/2005 653 OCTA John 2989 13.48 0.03 1.69 0.024 970 3.26 -0.30 -0.35 6.55 85670 

6/27/2005 654 OCTA John 3001 13.70 0.03 1.49 0.029 973 3.24 0.22 0.25 6.53 84910 

avg 3001 13.51 0.03 1.55 0.024 974 3.24 -0.11 -0.13 6.55 85833 

std dev 12.66 0.18 0.00 0.12 0.00 4.72 0.02 0.29 0.34 0.02 1014.91 

cov 0.42 1.33 7.27 7.70 16.63 0.48 0.48 -259.22 -256.34 0.23 1.18 

%diff vs conv -34.5% -28.6% 8.5% -32.0% -50.8% -33.7% 50.6% 0.9% 3.0% 

 

6/27/2005 655 Manhattan John 3718 17.89 0.07 3.47 0.002 1215 2.60 -0.61 -0.56 4.14 66170 

6/27/2005 656 Manhattan John 3790 18.18 0.07 3.07 0.002 1240 2.54 -0.47 -0.44 4.17 67780 

6/27/2005 657 Manhattan John 3806 18.30 0.01 1.88 0.005 1248 2.53 -0.48 -0.45 4.14 67140 

avg 3771 18.12 0.05 2.81 0.003 1234 2.56 -0.52 -0.49 4.15 67030 

std dev 46.83 0.21 0.04 0.83 0.00 17.13 0.04 0.08 0.07 0.02 810.62 

cov 1.24 1.17 69.64 29.44 52.31 1.39 1.44 -15.39 -13.79 0.42 1.21 

%diff vs conv -43.8% -38.7% 29.4% -10.2% -92.6% -42.9% 74.6% 0.6% 2.4% 

 

6/30/2005 681 CBD John 3038 14.52 0.01 nm 0.003 984 3.20 -0.26 -0.29 6.08 69100 

6/30/2005 682 CBD John 2958 14.37 0.04 1.08 0.004 960 3.28 -0.79 -0.84 6.08 68790 

7/1/2005 686 CBD John 2978 14.43 0.04 0.75 0.002 968 3.26 -0.57 -0.62 6.08 69120 

avg 2991 14.44 0.03 0.92 0.003 971 3.25 -0.54 -0.58 6.08 69003 

std dev 41.93 0.08 0.02 0.23 0.00 11.95 0.04 0.26 0.28 0.00 185.02 

cov 1.40 0.53 59.62 25.32 33.64 1.23 1.26 -48.72 -48.02 0.00 0.27 

%diff vs conv -34.8% -26.6% -75.2% -48.0% -97.1% -32.8% 48.3% 1.6% 1.8% 

 

6/30/2005 680 CBD w/ AC John 3735 17.84 0.01 0.84 0.101 1211 2.60 -0.69 -0.93 6.07 70420 

6/30/2005 672 CBD w/ AC John 3724 17.36 0.03 0.52 0.042 1204 2.62 -0.63 -0.84 6.03 69270 

avg 3730 17.60 0.02 0.68 0.072 1207 2.61 -0.66 -0.89 6.05 69845 

std dev 7.50 0.34 0.01 0.23 0.04 5.06 0.01 0.04 0.06 0.03 813.17 

cov 0.20 1.92 47.59 33.17 57.42 0.42 0.42 -6.14 -7.07 0.47 1.16 

%diff vs conv -28.1% -17.0% -26.7% -31.1% -3.7% -26.7% 35.8% 1.0% -0.3% 

 

6/28/2005 659 KCM John 2614 12.03 0.02 0.26 0.239 835 3.78 -0.10 -0.20 12.79 153270 

6/28/2005 660 KCM John 2624 12.18 0.02 0.31 0.135 839 3.76 0.63 1.21 12.78 152900 

6/28/2005 661 KCM John 2604 12.11 0.02 0.24 nm 832 3.79 0.17 0.32 12.79 152190 

avg 2614 12.11 0.02 0.27 0.187 836 3.78 0.23 0.45 12.79 152787 

std dev 10.27 0.07 0.00 0.03 0.07 3.54 0.02 0.37 0.71 0.01 548.84 

cov 0.39 0.61 19.26 12.61 39.10 0.42 0.42 159.88 160.15 0.05 0.36 

%diff vs conv -24.1% -17.8% -56.3% -59.5% 73.7% -23.4% 30.3% 0.8% 1.8% 

 

6/28/2005 662 KCM w/o Grade John 2506 11.57 0.01 0.10 0.048 803 3.93 0.08 0.14 12.83 137080 

6/30/2005 677 KCM w/o Grade John 2472 11.18 0.03 0.17 0.015 786 4.01 -0.78 -1.43 12.82 134990 

6/30/2005 679 KCM w/o Grade John 2472 11.15 0.02 0.27 0.036 788 4.00 -0.54 -1.00 12.80 135070 

avg 2483 11.30 0.02 0.18 0.033 792 3.98 -0.41 -0.76 12.82 135713 

std dev 19.67 0.24 0.01 0.09 0.02 9.25 0.04 0.44 0.81 0.02 1184.25 

cov 0.79 2.09 61.88 48.47 50.11 1.17 1.09 -106.09 -106.35 0.12 0.87 

%diff vs conv -24.7% -16.9% -0.0% -64.7% 6.5% -24.2% 31.4% 0.0% 0.9% 

 

6/29/2005 668 KCM w/o Grade w/ AC John nm nm nm nm nm 924 3.41 -0.92 -2.00 12.82 141150 

6/29/2005 670 KCM w/o Grade w/ AC John 2854 13.74 0.01 0.43 0.062 907 3.47 -0.36 -0.77 12.77 134250 

6/29/2005 671 KCM w/o Grade w/ AC John 2874 13.78 0.01 0.34 0.051 913 3.45 -0.33 -0.70 12.79 135280 

avg 2864 13.76 0.01 0.39 0.056 914 3.45 -0.54 -1.16 12.79 136893 

std dev 14.21 0.03 0.00 0.06 0.01 8.75 0.03 0.33 0.73 0.03 3722.18 

cov 0.50 0.23 3.36 16.65 13.41 0.96 0.89 -61.49 -63.04 0.20 2.72 

%diff vs conv -20.8% -10.1% -66.7% -2.5% -8.2% -20.2% 25.0% 0.0% 3.6% 

6/1/2005 629 CBD* Tom 3082 14.49 0.02 1.21 nm 1007 3.14 0.40 0.45 6.03 75630 

6/1/2005 630 CBD* Tom 3088 14.54 0.04 1.34 nm 1008 3.14 0.81 0.90 6.03 76020 

6/1/2005 631 CBD* Tom 3045 14.38 0.04 1.59 nm 998 3.17 0.84 0.94 6.06 75360 

avg 3072 14.47 0.04 1.38 0.0119 1004 3.15 0.68 0.76 6.04 75670 

std dev 23.41 0.08 0.01 0.19 0.006 5.58 0.02 0.25 0.27 0.02 331.82 

cov 0.76 0.56 31.71 14.01 0.5 0.56 0.54 35.98 35.71 0.29 0.44 

%diff vs conv -33.0% -26.4% -66.7% -22.0% -88.2% -30.5% 43.8% 1.0% 11.6% 

Note - Runs 629 through 631 are with driving style 1 (more throttle dither) 

23 
F1147-E(12/2004) 

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September 2006 

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5a. CONTRACT NUMBER

DE-AC36-99-GO10337 

5b. GRANT NUMBER

4. TITLE AND SUBTITLE

King County Metro Transit: Allison Hybrid Electric Transit Bus 

Laboratory Testing 

5c. PROGRAM ELEMENT NUMBER

5d. PROJECT NUMBER

NREL/TP-540-39996 

5e. TASK NUMBER

FC06.3000 

6. AUTHOR(S)

R.R. Hayes, A. Williams, J. Ireland, and K. Walkowicz 

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National Renewable Energy Laboratory 

1617 Cole Blvd. 

Golden, CO 80401-3393 

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NREL/TP-540-39996 

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13. SUPPLEMENTARY NOTES

14. ABSTRACT (Maximum 200 Words)

The National Renewable Energy Laboratory’s (NREL) ReFUEL facility conducted chassis dynamometer testing of 

two 60-foot articulated transit buses, one conventional and one hybrid. This document includes the experimental 

setup, test procedures, and results from vehicle testing performed at the NREL ReFUEL laboratory. 

15. SUBJECT TERMS

ReFUEL; chassis dynomometer; bus; hybrid; King County 

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