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Journal of Traffic and Transportation Engineering 4 (2016) 49-60 doi: 10.17265/2328-2142/2016.01.006 Connectivity-Enhanced Route Selection and Adaptive Control for the Chevrolet Volt Jeffrey Gonder 1 , Eric Wood 1 and Sai Rajagopalan 2 1. National Renewable Energy Laboratory, Golden CO 80401, USA 2. General Motors R&D, Warren MI 48090, USA Abstract: The National Renewable Energy Laboratory and General Motors evaluated connectivity-enabled efficiency enhancements for the Chevrolet Volt. A high-level model was developed to predict vehicle fuel and electricity consumption based on driving characteristics and vehicle state inputs. These techniques were leveraged to optimize energy efficiency via green routing and intelligent control mode scheduling, which were evaluated using prospective driving routes between tens of thousands of real-world origin/destination pairs. The overall energy savings potential of green routing and intelligent mode scheduling was estimated at 5% and 3%, respectively. These represent substantial opportunities considering that they only require software adjustments to implement. Key words: ITS (intelligent transportation systems), green routing, vehicle drive cycles/profiles, route-based/adaptive powertrain control, vehicle telematics/navigation, vehicle energy use prediction. 1. Introduction Energy security, fuel cost and air quality concerns have led to increased powertrain electrification in new vehicles. At the same time, the ubiquitous availability of advanced vehicle telematics systems, such as OnStar, has made real-time information on driving routes, traffic and road topology readily accessible. Together, these trends offer the potential for increased powertrain efficiency, particularly in vehicles with both a traction battery and a combustion engine. Such vehicles can leverage route-specific information to anticipate road loads and schedule power flows in the most efficient manner possible. Significant research exists in the literature exploring pathways for increasingly connected vehicles to optimize modern transportation systems. Some example applications include: routing algorithms that leverage digital maps to make drive cycle predictions with the goals of minimizing travel time, energy use and/or Corresponding author: Jeffrey Gonder, senior engineer, supervisor, research fields: real-world travel, optimal vehicle design and control, impact of drive cycle and intelligent transportation technologies on fuel efficiency. emissions [1-4]; vehicle speed advisory programs targeted at encouraging efficient driving habits, such as gradual accelerations and intermediate travel speeds [5-8]; predictive powertrain controls that employ drive cycle predictions for optimal energy management in hybrid electric and plug-in hybrid electric vehicles [9-19]. As a contribution to this growing field of research, the NREL (National Renewable Energy Laboratory) and General Motors, in collaboration with the U.S. Department of Energy, evaluated connectivity-enhanced route selection and adaptive control techniques to further increase energy efficiency in the Chevrolet Volt platform. The project included both simulation and chassis dynamometer testing to develop energy prediction algorithms applied to the Volt over multiple real-world driving profiles. The algorithms were used to implement and evaluate green routing and adaptive intelligent control mode scheduling for the Volt over predicted travel routes. D DAVID PUBLISHING

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Page 1: Connectivity-Enhanced Route Selection and … · Connectivity-Enhanced Route Selection and Adaptive Control for the Chevrolet Volt ... s were sub the 0.1-mile characterist ed to the

Journal of Traffic and Transportation Engineering 4 (2016) 49-60 doi: 10.17265/2328-2142/2016.01.006

Connectivity-Enhanced Route Selection and Adaptive

Control for the Chevrolet Volt

Jeffrey Gonder1, Eric Wood1 and Sai Rajagopalan2

1. National Renewable Energy Laboratory, Golden CO 80401, USA

2. General Motors R&D, Warren MI 48090, USA

Abstract: The National Renewable Energy Laboratory and General Motors evaluated connectivity-enabled efficiency enhancements for the Chevrolet Volt. A high-level model was developed to predict vehicle fuel and electricity consumption based on driving characteristics and vehicle state inputs. These techniques were leveraged to optimize energy efficiency via green routing and intelligent control mode scheduling, which were evaluated using prospective driving routes between tens of thousands of real-world origin/destination pairs. The overall energy savings potential of green routing and intelligent mode scheduling was estimated at 5% and 3%, respectively. These represent substantial opportunities considering that they only require software adjustments to implement. Key words: ITS (intelligent transportation systems), green routing, vehicle drive cycles/profiles, route-based/adaptive powertrain control, vehicle telematics/navigation, vehicle energy use prediction.

1. Introduction

Energy security, fuel cost and air quality concerns

have led to increased powertrain electrification in new

vehicles. At the same time, the ubiquitous availability

of advanced vehicle telematics systems, such as OnStar,

has made real-time information on driving routes,

traffic and road topology readily accessible. Together,

these trends offer the potential for increased powertrain

efficiency, particularly in vehicles with both a traction

battery and a combustion engine. Such vehicles can

leverage route-specific information to anticipate road

loads and schedule power flows in the most efficient

manner possible.

Significant research exists in the literature exploring

pathways for increasingly connected vehicles to

optimize modern transportation systems. Some

example applications include:

routing algorithms that leverage digital maps to

make drive cycle predictions with the goals of

minimizing travel time, energy use and/or

Corresponding author: Jeffrey Gonder, senior engineer, supervisor, research fields: real-world travel, optimal vehicle design and control, impact of drive cycle and intelligent transportation technologies on fuel efficiency.

emissions [1-4];

vehicle speed advisory programs targeted at

encouraging efficient driving habits, such as gradual

accelerations and intermediate travel speeds [5-8];

predictive powertrain controls that employ drive

cycle predictions for optimal energy management in

hybrid electric and plug-in hybrid electric

vehicles [9-19].

As a contribution to this growing field of research,

the NREL (National Renewable Energy Laboratory)

and General Motors, in collaboration with the U.S.

Department of Energy, evaluated

connectivity-enhanced route selection and adaptive

control techniques to further increase energy efficiency

in the Chevrolet Volt platform. The project included

both simulation and chassis dynamometer testing to

develop energy prediction algorithms applied to the

Volt over multiple real-world driving profiles. The

algorithms were used to implement and evaluate green

routing and adaptive intelligent control mode

scheduling for the Volt over predicted travel routes.

D DAVID PUBLISHING

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50

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52

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Connectivity-Enhanced Route Selection and Adaptive Control for the Chevrolet Volt

53

Fig. 5 Discretized correlation between cycle characteristics, SOC, and electric consumption rate generated from detailed simulation results when the Volt engine was on.

Fig. 6 Engine-on fuel rate estimates correlate with the difference or “error” between the hybrid (engine-on) electric rate and the all-electric (engine-off) electric rate estimates.

target SOC around the middle of the figure) and the

product of the average speed and acceleration

characteristics of the nanotrip. Consistent results

within this space for nanotrips with similar

characteristics again enable discretization of the data

into a look-up map as illustrated by Fig. 5. The

engine-on fuel rate relationship was found to correlate

well with the difference between the engine-on and

engine-off electric rate estimates (as provided by the

cycle-characteristic-based look-up maps in

Figs. 4 and 5). Fig. 6 illustrates the resulting correlation

established as compared to the actual results from the

detailed simulation.

The described look-up maps/correlations define

everything needed to predict energy use (fuel and

electricity consumption) from speed and acceleration

cycle characteristics. The predictions can be further

refined by establishing similar correlations for

additional cycle segment characteristics, such as road

grade. Fig. 7 illustrates the translation model (trained

by the detailed simulation and test data over cycles

with different baseline (Wh/mi) demands run at different

300

200

100

0

−100

−200

−300

Relative state of charge

Veh

icle

spe

ed ×

veh

icle

acc

eler

atio

n (m

ph·m

ph/s

)

Electric rate (W

h/mi)

Electric rate estimation error (hybrid-all-electric)

Fue

l rat

e (g

al/m

i)

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Connectivity-Enhanced Route Selection and Adaptive Control for the Chevrolet Volt

54

Fig. 7 Grade-based electric rate translation model created to adjust the zero-grade look-up maps described in Figs. 4-6 to account for driving segment grade.

Fig. 8 The grade translation model agrees well with individual electric rate adjustments observed from simulation and test data over similarly characterized nanotrips at different road grades.

grades) to estimate electric rate impact as a function of

road grade for similarly characterized nanotrips. As

shown in Fig. 8, the grade-based translation model

shows a very good ability to predict the grade-adjusted

electric consumption rates based only on the

zero-grade electric rate as an input.

As a point of validation, the energy prediction maps

(trained on simulation data) were evaluated against test

data from an instrumented Chevrolet Volt run on a

vehicle dynamometer over a comprehensive range of

driving conditions including combinations of

speed/acceleration characteristics, positive and negative

Veh

icle

ele

ctri

c ra

te (

Wh/

mi)

−6 −4 −2 0 2 4 6 Road grade

Sim

ulat

ed e

lect

ric

rate

(W

h/m

i)

Predicted electric rate (Wh/mi)

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Connectivity-Enhanced Route Selection and Adaptive Control for the Chevrolet Volt

55

(a)

(b)

Fig. 9 Distributions when comparing energy estimation maps to dynamometer test data: (a) modeled electric; (b) fuel rate errors. Red dashed line represented zero error.

road grades, control modes (CD/CS), and battery SOCs.

Fig. 9 provides a snapshot of these comparisons by

showing distributions of model error (energy

prediction map minus measured test data) for both

electric and fuel use rates. Model error distributions

can be seen to center around zero (represented by the

red dashed lines). Further evaluation of model error

will be an important part of future work, to include

assessing fuel and electricity consumption impacts

when the driving type predictions turn out to be

incorrect.

3.3 Green Routing

Evaluation of the connectivity-enabled green routing

Nan

otri

p co

unt

Electric rate error (prediction test) (Wh/mi)

Fuel rate error (prediction test) (gal/mi)

30

25

20

15

10

5

0

Nan

otri

p co

unt

300

250

200

150

100

50

0

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56

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Connectivity-Enhanced Route Selection and Adaptive Control for the Chevrolet Volt

57

monetary value of time spent in the vehicle. The top

line in the plot corresponds to total percent increased

travel time, and the bottom line corresponds to total

percent energy (and resulting energy cost) reductions

provided by the “greenest” routes.

To facilitate interpretation, consider two example

vertical slices on this plot. In the first example, the

points intersecting the y-axis represent the extreme

scenario with no value of time penalty counted against

increased time requirements by the green routes. The

aggregate results represented by this scenario could

realize a 12.3% reduction in energy use and cost, but a

14.4% increase in travel time. The second example

considers the vertical slice at a time value of $35/h. For

this scenario, the qualifying alternative green routes

could decrease overall energy use and cost by 1.0%

with a negligible increase in travel time.

3.4 Control Mode Scheduling

The intelligent CD vs. CS mode scheduling

evaluation involved similar large-scale analysis,

specifically, of over 100,000 potential routes identified

from the TSDC O/D database. The evaluation required

first adding an extra analysis layer to compare the

default mode schedule (CD followed by CS) as

compared to the optimal (least fuel consuming) mode

schedule. As a simplified overview, the methodology

included in the added layer begins by assuming that all

driving could be accomplished in CD mode (initially

ignoring energy limits of the vehicle battery). It then

incrementally substitutes driving segments from CD to

CS operation, until the final trip SOC equals that from

the default CD followed by CS operation. Trip

segments are prioritized for substituting from CD to CS

control based on minimizing the cost/benefit ratio of

doing so, where the cost is defined as the increased fuel

use incurred by the substitution, and the benefit is

defined as the decreased electric depletion rate.

The top plot in Fig. 12 shows an example of a

nominal vs. optimal battery SOC depletion profile

generated by the methodology described above. It

should be noted that when evaluating this project’s

high-level CD vs. CS mode scheduling opportunity,

each trip considered was assumed to begin at an initial

Fig. 12 Nominal (solid blue line) vs. optimal (dashed red line) battery SOC and fuel use profiles for one example route.

0 5 10 15 20 25 Distance (mi)

0 5 10 15 20 25 Distance (mi)

Bat

tery

SO

C

Tot

al f

uel (

gal)

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Fig. 13 Efficiency improvements from control mode scheduling optimization evaluated across over 100,000 representative driving routes.

SOC such that 50% or more of the trip was completed

in CS mode. The bottom plot in Fig. 12 shows the

corresponding fuel consumption profile for the

nominal vs. optimal control mode scheduling over the

example profile. In this example, fuel consumption was

reduced 25% by scheduling CS operation during

highway driving segments in the first half of the route

and saving CD operation for the city driving

anticipated at the end of the route.

Fig. 13 shows the scatter of fuel savings opportunity

from optimal mode scheduling applied to over 100,000

trips. These results indicated that very large percent

fuel savings results predominantly occur at short

driving distances, which may be an artifact of the

reduced starting SOC approach applied for these

shorter trips. However, a number of trips (even longer

than 30~40 miles in length) realize fuel savings on the

order of 10%, and the average fuel savings across all

trips exceeds 3%.

4. Conclusions

Under this project, NREL collaborated with General

Motors to evaluate connectivity-enabled efficiency

enhancements for the Chevrolet Volt. Project

accomplishments included developing and

demonstrating the ability to estimate drive cycle

characteristics over anticipated driving routes. The

project team further developed a high-level model to

predict vehicle fuel and electricity consumption based

on driving characteristic and vehicle state inputs. The

team combined and leveraged these techniques in

pursuit of energy efficiency optimization via green

routing and intelligent control mode scheduling.

The green routing and intelligent control mode

scheduling enhancements were evaluated using

prospective driving routes between tens of thousands

of real-world O/D pairs. Considering the aggregate

green routing benefit multiplied by the fraction of O/D

pairs where the default (fastest) route consumed more

fuel, the overall green routing fuel savings opportunity

could approach 5% (assuming a low value of passenger

time). The average efficiency benefit from intelligent

high-level scheduling of CS vs. CD control showed a

similar magnitude—a little over 3% potential fuel

savings on trips that require some mix of CS and CD

operation. An 8% fuel savings (when taken as additive

benefits) represents a substantial opportunity

especially considering that only software adjustments

20

18

16

14

12

10

8

6

4

2

0

0 20 40 60 80 100 120 140 160 180 200 Trip distance (mi)

Perc

enta

ge o

f ef

fici

ency

incr

ease

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are required to realize this efficiency gain.

Future work efforts could include adding

route-based optimization to lower-level controls (in

addition to the high-level CD vs. CS control mode

scheduling), formally incorporating real-time

traffic/congestion information to further improve cycle

metric predictions, and evaluating result sensitivity to

various conditions and erroneous predictions.

Additional options could include implementing,

refining and more robustly testing both green routing

and adaptive control approaches in a development

vehicle, and/or considering additional vehicle

platforms/powertrains.

Acknowledgments

This study was jointly supported by the U.S.

Department of Energy’s Vehicle Technologies Office

and General Motors. The authors would specifically

like to thank David Anderson of the U.S. Department

of Energy and Chen Fang Chang of General Motors for

their guidance and support.

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