CHASSIS TESTING VERSUS POWERTRAIN TESTING FOR 

HEAVY-DUTY VEHICLE GREENHOUSE GAS EMISSION 

MEASUREMENTS

Joseph Anthony, Jayant Sarlashkar,

Southwest Research Institute

Houshun Zhang, James Sanchez, Matthew Spears, Angela 

Cullen, US EPA
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Introduction

• Heavy-duty, on-highway market vehicles are powered by

engines that have been emissions-certified as an individual

component.

• EPA/NHTSA Greenhouse Gas Emission and Fuel Efficiency

regulations require evaluation of the entire vehicle.

• EPA/NHTSA are researching methods to validate fuel

efficiency results from vehicle simulation models.

• This presentation is a comparison of two methods used for

model validation.

g/hp-hr g/ton-mile
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Chassis Dynamometer Facility Compared To 

Powertrain Test Facility 

Cons

•Large facility required for full vehicle

•Climate control for entire test cell

•Road speed fan

Pros

•Convenient vehicle evaluation

•Does not require OEM support to 

operate a powertrain in a vehicle

•Correct packaging and heat transfer
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Powertrain Test Facility Compared To Chassis 

Dynamometer Facility 

Cons

•Questions about representative 

packaging and heat transfer

•Requires engine and transmission 

control integration

•Difficult test systems that deliver/ 

absorb power through the wheels, 

i.e. hybrid with wheel hub motors.

Pros

•Uses existing engine test cell 

equipment

•Easier to simulate different vehicle 

configurations

•Easier to control
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Key Differences Between Chassis Testing and 

Powertrain Testing 

Engine Trans Driveline

Wheels

Torque

Speed

T

1

Physical 

Inertia, friction 

Dyno-Set 

simulated inertia, 

road load

T

2

Cut

Engine Trans Driveline

Wheels

Torque

Speed

T

1

Physical 

Inertia, friction 

Simulated 

inertia, road load

T

2

Cut

Chassis dynamometer includes, 

Powertrain testing does not 

• Both simulate the same static inertia and road load forces

• Chassis dynamometer testing requires the “Dyno-Set” coefficients to be 

determined to account of the portion of the vehicle road load that is present 

by having the vehicle on the chassis dynamometer.
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Powertrain Test Cell Operation

Pedal Position

(Torque Command)

Vehicle

Transmission

Energy Storage 

(electric, hydraulic, flywheel)

Software Models

Dyno 

Motor

Speed 

Control

Driveshaft 

Torque/Speed 

Feedback

Driveshaft

Speed

Command

Time - Speed Trace of HD UDDS 

(Heavy-duty Urban Dynamometer Driving Schedule)

0

10

20

30

40

50

60

70

0 200 400 600 800 1000

Time, sec. Speed, mph

Drive Cycle 

(Vehicle Speed vs. 

Time)

Driver
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Powertrain Test Cell Implementation

Pedal Position

(Torque Command)

Dyno 

Motor

Speed 

Control

Driveshaft 

Torque/Speed 

Feedback

Driveshaft

Speed

Command

dSPACE

MABX I

Pedal,

Driveshaft

Speed

Target

Cycle Speed

via 

CAN
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Powertrain Test Cell Facility Used for this Work

445 kW DC 

motor used to 

control output 

shaft speed

Friction 

brake used 

to hold dyno 

at zero 

speed 

Inline torque 

meter 

Eddy current motor NOT 

used for this application

Powertrain
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Close-up View Friction Brake Required to Hold 

Zero Speed

DRAFT
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Uncertainty interval of plus-minus two standard errors

10

Average fuel economy over each drive cycle for 

the powertrain cell tests compared to the 

chassis dynamometer tests

1.5% 2.2%

3.8% 2.4%

6.0%

3.0%

0

0

0

0

0

0

0

4

6

8

10

12

14

16

18

55 MPH 65 MPH ARB 

Transient

WHVC CILCC Parcel

Fuel Economy, MPG

Chassis

Powertrain

% Diff

Steady-state driveshaft 

power match

The primary source of difference is 

described in the following slides

DRAFT
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Different Driving Behaviors Produce Different 

Fuel Economy Results

0

10

20

30

40

50

60

70

80

400 450 500 550 600 650 Accelerator Pedal Position [%]

Time [seconds]

Chassis

Powertrain

The accelerator pedal position overshoots more during the powertrain 

test than the chassis test, then the pedal drops to a lower value.

DRAFT
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Different Driving Behaviors Produce Different 

Fuel Economy Results- Cont’d

0

1

2

3

4

5

6

400 450 500 550 600 650 Transmission Gear Number

Time [seconds]

Chassis

Powertrain

The lower pedal value during the powertrain test causes 

the transmission to upshift, reducing the engine speed.

DRAFT
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Different Driving Behaviors Produce Different 

Fuel Economy Results- Cont’d

0

500

1000

1500

2000

2500

400 450 500 550 600 650 Engine Speed [rpm]

Time [seconds]

Chassis

Powertrain

The engine speed is lower when the transmission upshifts, which affects 

fuel economy.

DRAFT
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Replaced torque-based controller with a speed-based controller 

Replaced human driver with software model used in powertrain test cell

14

Recent Chassis Development For Improved 

Correlation
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The results from our studies at SwRI:

• The chassis and powertrain fuel economy results are statistically 

different, but the standard error is quite small.

• Powertrain testing can appropriately represent the overall operation of 

the vehicle.

• Test result variation is approximately the same for each method. 

15

Summary (1)
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The following contribute to the difference in fuel economy results 

between the chassis and powertrain testing:

• Driver Behavior affects transmission shifting , resulting in significant 

fuel economy differences

• Aftertreatment thermal management differences between chassis and 

powertrain testing 

• Dyno-to-dyno control and response differences

• Heat exchanger behavior

• Modeling of driveline characteristics 

16

Summary (2)