Powertrain vs. Chassis Dyno and 

their Variability Discussion

Southwest Research Institute

10-Dec-2014

1
2

Introduction

•Comparison of powertrain testing and chassis

testing

•Sources of chassis test result variation

–Driver Behavior

–Thermal Management

–Roll friction uncertainty

•Chassis test cell configuration improvements

to mitigate sources of test results variability

0

5

10

15
3

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
4

Powertrain Test Facility Compared To 

Chassis Dynamometer Facility 

Cons

•Questions about representative 

packaging and heat transfer

•Requires engine and transmission 

control integration

•Difficult to include systems that 

may require power through the 

wheels, i.e. regenerative braking.

Pros

•Uses existing engine test cell 

equipment

•Easier to simulate different vehicle 

configurations

•Easier to control
5

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 some inertia and road load forces
6

Powertrain Test Cell Operation

Pedal Position

(Torque Command)

Vehicle

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
7

Conventional HD Chassis Implementation

Pedal Position

(Torque Command)

Dyno 

Motor

Vehicle Speed 

Feedback

Vehicle

Speed

Command

Target

Cycle Speed

Vehicle

Software Model

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)

Roll Torque 

Measurement
Driver Behavior

8
Two Examples of Driver Behavior 

Influencing Fuel Economy

• Transmission gear selection

• Thermal management 

9
Uncertainty interval of plus-minus two standard errors.

10

Average Fuel Economy of 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 error is 

described in the following slides
11

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.
12

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.
13

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.
Thermal Management 

Variability

14
Impact of Thermal Management on Fuel 

Economy for -15% Crr

• Occurs from ~180 sec to 

~320 sec

• Duration of thermal 

management modes 

categorized as,

– Critical

– Moderate

– Minimal

• Fuel economy reduced 

by 1.8% with each 

successive degree of 

thermal management 

15

0

1 0

2 0

3 0

4 0

5 0

6 0

7 0

8 0

9 0

100

150 200 250 300 350 400 Fuell Rate (L/hr)

Fuel Rate for -15% Crr Condition

5.54 mpg (Run 940)

5.313 mpg (Run 944)

5.25 mpg (Run 951)

5.39 mpg (Run 952)

5.29 mpg (Run 957)

Runs WithActive Thermal 

Managment
Thermal Management for EPA Optimized Case

16

0

1 0

2 0

3 0

4 0

5 0

6 0

7 0

8 0

9 0

100

150 200 250 300 350 400 Fueling Rate (L/hr)

Fueling Rate

5.54 mpg (Run 940)

5.29 mpg (Run 957)

5.53 mpg (Run 943) - Optimized Case

5.51 mpg (Run 956) - Optimized Case
Sources of Variation of Thermal 

Management

• Most of the variation is due to variable soak 

times. 

• When soak times are longer than 20 minutes, 

the thermal management time is typically 

longer.

• Driver behavior contributes to thermal 

management variation. If the driver is 

aggressive, the engine produces more heat in 

the exhaust and less thermal management at 

idle is required.

17
HD Chassis Dynamometer 

Tire Friction

18
Experiments Conducted on HDC with Box 

Truck

10-Dec-2014 19

Three different experiments were conducted 

with the box truck installed on the HDC

Day 1 – Constant speed with and without the driveshaft installed at 45 

mph, 55 mph, and 65 mph

Day 2 – Repeat of Day 1, but at speeds from 5 mph to 65 mph in 5 mph 

increments

Day 3 – Recorded tire pressure and temperature cold, then warmed up 

the tires by motoring at 45 mph with driveshaft installed and 

engine running
Friction measured at Constant Speed on 

Different Days

10-Dec-2014 20

y = -0.0059x2 + 5.7961x + 439.33

R2 = 0.9816

y = 0.0267x2 + 2.3887x + 449.37

R2 = 0.9918

300

400

500

600

700

800

900

0 10 20 30 40 50 60 70 Dynamometer Roll Force [N] 

Vehicle Speed [mph]

First run with DS

First run w/o DS

Second run with DS

Second run w/o DS

From two different runs on 

different days, the friction 

force varies up to 3% of 

the total road load force at 

constant speed.
500

550

600

650

700

750

800

850

900

950

1000

00:00 05:46 11:31 17:17 23:02 28:48 34:34 Dynamometer Roll Force [N] 

Time [Min:Sec]

With driveshaft

Friction as a Function of Tire Temperature 

and Pressure at 45 MPH

10-Dec-2014 21

~270 N after 30 min. from cold

Average Tire Temperature Change = 9.7 degC

Average Tire Pressure Change = 45 kPa
Test Result Variation 

Comparison of Different HDC 

Configurations

22
23

Conventional HD Chassis Implementation

Pedal Position

(Torque Command)

Dyno 

Motor

Vehicle Speed 

Feedback

Vehicle

Speed

Command

Target

Cycle Speed

Vehicle

Software Model

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)

Roll Torque 

Measurement
24

Experimental HD Chassis Implementation

Pedal Position

(Torque Command)

Dyno 

Motor

Vehicle Speed 

Feedback

Vehicle

Speed

Command

Target

Cycle Speed

Vehicle

Software Model

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)

Driveshaft 

Torque 

Measurement

Driver

Software Model

Eliminates Friction 

Estimation 

Uncertainty

Provides consistent, 

predictable driver 

behavior
HDC Test Results Variation For Various 

Vehicles and Varying HDC Configurations

10-Dec-2014 25

Kernel Density: HDC Coef of Variation by Vehicle

cov=sigma/mu

Density

0

100

200

300

0.00 0.01 0.02 0.03

F650

Refuse

T270

T700

High test weight, using driveshaft 

torque meter to control roll speed 

& powertrain driver model

Test weight closest to dyno 

roll inertia, human driver

Medium test weight, human 

driver 

Highest test weight, human 

driver 
• From our studies at SwRI:

• 1. Driver characteristics RANK 1 (most important)

• 2. Aftertreatment thermal management differences between 

chassis and powertrain testing 

• 3. Dyno-to-dyno control and response differences

• 4. Heat exchanger behavior

• 5. Modeling of driveline characteristics

• We are continuing to refine our test procedures to 

improve accuracy and precision.

26

Summary