2008-01-0461
Comparison of Powertrain Configuration for Plug-in HEVs from
a Fuel Economy Perspective
Vincent Freyermuth
Argonne National Laboratory
Eric Fallas, Aymeric Rousseau
Argonne National Laboratory
Copyright © 2007 SAE International
ABSTRACT
With the success of hybrid electric vehicles (HEVs) and
the still uncertain long-term solution for vehicle
transportation, Plug-in Hybrid Electric Vehicles (PHEV)
appear to be a viable short-term solution and are of
increasing interest to car manufacturers. Like HEVs,
PHEVs offer two power sources that are able to
independently propel the vehicle. They also offer
additional electrical energy onboard. In addition to
choices about the size of components for PHEVs,
choices about powertrain configuration must be made. In
this paper, we consider three potential architectures for
PHEVs for 10- and 40-mi All Electric Range (AER) and
define the components and their respective sizes to
meet the same set of performance requirements. The
vehicle and component efficiencies in electric-only and
charge-sustaining modes will be assessed.
INTRODUCTION
For the past couple of years, the U.S. Department of
Energy (DOE) has invested considerable research and
development effort into Plug-in Hybrid Electric Vehicle
(PHEV) technology because of the potential fuel
displacement offered by the technology. The PHEV
R&D Plan [1], driven by the desire to reduce
dependence on foreign oil by diversifying the fuel
sources of automobiles, describes the different activities
required to achieve the goals. DOE will use Argonne
National Laboratory’s (ANL’s) PSAT to guide its analysis
activities, stating that “ANL's Powertrain Systems
Analysis Toolkit (PSAT) will be used to design and
evaluate a series of PHEVs with various 'primary electric'
ranges, considering all-electric and charge-depleting
strategies.”
Argonne designed PSAT [2, 3] to serve as a single tool
that can be used to meet the requirements of automotive
engineering throughout the development process, from
modeling to control. Because of time and cost
constraints, designers cannot build and test each of the
many possible powertrain configurations for advanced
vehicles. PSAT, a forward-looking model, offers the
ability to quickly compare several powertrain
configurations.
When designing a vehicle for a specific application, the
goal is to select the powertrain configuration that
maximizes the fuel displaced and yet minimizes the
sizes of components. In this study, three vehicle
powertrain configurations are sized to achieve similar
performance for two All Electric Range (AER)
approaches. The component sizes and the fuel economy
of each option are compared.
VEHICLE CONFIGURATIONS
Three separate families of powertrain configurations
exist for advanced vehicle configurations:
1. Series
2. Parallel
3. Power Split
For each option, several hundreds of combinations are
possible, including the number of electric machines, their
location, and type of transmission. In this study, one
configuration from each family was selected.
The series engine configuration is often considered to be
closer to a pure electric vehicle when compared to a
parallel configuration. In this case, the vehicle is
propelled solely from the electrical energy. Engine speed
is completely decoupled from the wheel axles, and its
operation is independent of vehicle operations. As a
result, the engine can be operated consistently in a very
high efficiency area. The configuration selected includes
a single gear ratio before the transmission, which is a
configuration similar to that in the GM Volt [4].
In a parallel configuration, both the electric machine and
the engine can be used to directly propel the vehicle.
The configuration selected is a pre-transmission parallel
hybrid, which is similar to the one used by
DaimlerChrysler for the PHEV Sprinter [5]. The electric
machine is located in between the clutch and the multigear transmission.
The power split configuration uses a planetary gear set
to transmit power from the engine to the wheel axles,
which is a configuration similar to that used in the Toyota
Prius [6]. The power split system is the most commonly
used system in currently available hybrid vehicles. The
split system allows the engine speed to be decoupled (to
some extent) from vehicle speed. On one hand, the
power from the engine can flow mechanically to the
wheel axle via the ring of the planetary system. On the
other hand, the engine power can also flow through the
generator, producing electricity that will feed the motor to
propel the wheels. Hence, the power split system allows
both “parallel like” and “series engine like” operations to
be combined.
engine to remain off throughout the cycle, regardless of
the torque request from the driver.
Vehicle mass is calculated by adding the mass of each
component to the mass of the glider. The mass of each
component is defined on the basis of its specific power
density.
To maintain an acceptable battery voltage (around
200 V), the algorithm will change the battery capacity
rather than the number of cells to meet the AER
requirements. To do so, a scaling algorithm [7] has been
developed to properly design the battery for each
specific application.
Vehicle Assumptions
Motor Power
VEHICLE DESCRIPTION AND COMPONENT
SIZING
Battery Power
The selected vehicle class represents a midsize sedan.
The main characteristics are defined in Table 1.
Table 1: Main Vehicle Characteristics
Glider mass
Frontal area
Coefficient of drag
Wheel radius
Tire rolling resistance
990 kg
2
2.1 m
0.29
0.317 m
0.008
The components of the different vehicles were sized to
meet the following vehicle performance standards:
•
•
•
0–60 mph < 9 s
Gradeability of 6% at 65 mph
Maximum speed > 100 mph
To quickly size the component models of the powertrain,
an automated sizing process was developed. A flow
chart illustrating the sizing process logic is shown in
Figure 1. Although engine power is the only variable for
conventional vehicles, PHEVs have two variables:
engine power and electric power. In our case, the engine
is sized to meet the gradeability requirements.
To meet the AER requirements, the battery power is
sized to follow the Urban Driving Dynamometer
Schedule (UDDS) driving cycle while in all-electric mode.
We also ensure that the vehicle can capture the entire
energy from regenerative braking during decelerations
on the UDDS. Finally, battery energy is sized to achieve
the required AER of the vehicle. The AER is defined as
the distance the vehicle can travel on the UDDS until the
first engine start. Note that a specific control algorithm is
used to simulate the AER. This algorithm forces the
Engine Power
Battery Energy
No
Convergence
Yes
Figure 1: Process for Sizing PHEV Components
The main characteristics of the sized vehicles are
described in Tables 2 and 3. Note that engine power is
similar for the parallel and power split configurations and
significantly higher for the series configuration. This
difference is explained by the inefficiencies associated
with the additional components (both generator and
electric machine) included in the series configuration.
Because the electric machine is the only component
used in the series to propel the vehicle, its power is also
significantly higher than that in the other configurations.
However, because no multi-gear transmission is
considered and the component-specific powers
represent 2015 technologies, the overall difference in
vehicle mass among all of the configurations is minimal.
Note that while the series configuration is heavier for the
10-mi (16-km) AER case, the power-split configuration
offers the largest mass for the 40-mi (64-km) AER case.
Finally, the PHEV will operate in electric-only mode at
higher vehicle speed in comparison with regular hybrids.
The architecture therefore needs to be able to start the
Table 2: Component Size – 10-mi AER case
Battery capacity (A•h)
Total vehicle mass
(kg)
Split
74
Series
109
48
62
90
NA
63
106
58
52
55
18
21
18
1675
1667
1700
Pretrans
parallel
79
Split
77
Series
114
50
71
95
NA
65
111
61
64
58
71
69
71
1764
1800
1794
VEHICLE CONTROL STRATEGY ALGORITHMS
PHEV vehicle operations can be divided into two modes,
as shown in Figure 2:
•
Charge Sustaining (CS)
90
30
Pretrans
parallel
76
Table 3: Component Size – 40 mi AER case
Parameter
Engine power (kW)
Propulsion motor
power (kW)
Generator power
(kW)
Battery power (kW)
Battery capacity
(A•h)
Total vehicle mass
(kg)
Charge Depleting (CD)
Charge depleting (CD): When the battery state of
charge (SOC) is high, the vehicle operates under a
so-called blended strategy. Both battery and engine
can be used. Engine use increases as SOC
decreases. The engine tends to be used in heavy
acceleration as well, even though the SOC is high.
For fuel economy purposes, this blended strategy is
defined for a battery going from full charge to a self-
Distance
Figure 2: Control Strategy SOC Behavior
SERIES CONFIGURATION - Because the engine is
completely decoupled from the vehicle operation,
numerous control strategies can be chosen. In this
study, the engine “on” logic is based on battery SOC. As
shown in Figure 3, the engine turns ON when a lower
SOC limit is reached (e.g., 0.25) and will stay on until the
battery gets recharged to its high limit (e.g., 0.3) if the
power request remains positive. If a braking event
occurs, the engine is allowed to shut down and will
restart when the lower SOC limit is reached again. When
the engine is ON, it operates close to its best efficiency
point, unless a component saturates (for instance, the
battery could reach its maximum charging capability).
SOC behavior over a UDDS - 10AER series case
SOC (normalized behavior) in % and engine on flag
Parameter
Engine power (kW)
Propulsion motor
power (kW)
Generator power
(kW)
Battery power (kW)
•
sustained SOC, which is typically from 90% to 30%
SOC.
Charge sustaining (CS): Once the battery is down to
30%, the vehicle operates in CS mode, which is
similar to a regular hybrid vehicle.
SOC (%)
engine at high vehicle speed. In the series configuration
(where the engine is completely decoupled from the
vehicle speed) and in the parallel configuration (where
the engine can be decoupled via the clutch), starting the
engine is not an issue. In the power split configuration,
the generator is used to start the engine. Because all of
those elements are linked to the wheels via the planetary
gear system, one needs to make sure that the generator
(the speed of which increases linearly with vehicle speed
when the engine is off) still has enough available torque
— even at high speed — to start the engine in a timely
fashion.
Engine ON
Battery SOC
0.3
0.25
0.2
0
50
100
150
Time (s)
200
250
300
Figure 3: Series Engine SOC Behavior
PARALLEL AND POWER SPLIT CONFIGURATIONS The vehicle control philosophies behind both of those
configurations are similar. The first critical part of the
control strategy logic is related to the engine ON/OFF
logic. As Figure 4 shows, the engine ON logic is based
on three main parameters:
1. The requested power is above a threshold.
2. The battery SOC is lower than a threshold.
3. The electric motor cannot provide the requested
wheel torque.
In addition to these parameters, further logic is included
to ensure proper drive quality by maintaining the engine
ON or OFF for a certain duration. To avoid unintended
engine ON events resulting from spikes in power
demand, the requested power has to be above the
threshold for a predefined duration. The engine OFF
logic condition is similar to that of the engine ON
condition. Both power thresholds used to start or turn off
the engine and to determine the minimum duration of
each event have been selected as input parameters of
the optimization problem.
To regulate the battery SOC, especially during the CD
mode, the power demand that is used to determine the
engine ON/OFF logic is the sum of the requested power
at the wheel plus additional power that depends on
battery SOC. This power can be positive or negative,
depending on the value of the current SOC compared to
the target.
Figure 6: SOC Comparison between Each
Configuration on UDDS – PHEV 10 Case
Figure 4: Simplified Engine ON/OFF Logic
Figure 5 shows the different parameters used to define
the additional power to regulate the SOC in greater
detail. The SOC target has been set when the vehicle is
considered entering the charge-sustaining mode (30%
SOC). The parameters “ess_percent_pwr_discharged”
and “ess_percent_pwr_charged” are percentages used
to control the depleting rate and the CS operating
window, respectively.
CONTROL COMPARISON - Even if the philosophical
control strategy is similar for some powertrain
configurations, the implementation varies to take
advantage of the vehicle’s properties. Figure 6 shows
the evolution of the battery SOC on the UDDS, starting
from a charged battery at 90%, for the PHEV
10 vehicles.
Note that the series configuration discharges
the fastest, followed by the parallel and the
configurations. The differences are related
phenomena, including component operating
vehicle mass, and control parameters.
Figure 5: Example of Additional Power to Regulate
SOC
the battery
power-split
to several
conditions,
To compare the different powertrain configurations as
fairly as possible, we tried to maintain the consistency of
the controls as much as possible. However, note that
the results obtained depend on the control choices
made.
FUEL ECONOMY RESULTS
Because several approaches are still considered to
calculate the fuel economy of PHEVs, we will use the
fuel consumed on 15 successive drive cycles to
compare the different configurations. Such long
distances ensure that in all cases, the final battery SOC
is approximately 30%. Table 4 shows the fuel economy
results for each powertrain configuration and AER
considered. In addition to the UDDS, the HWFET
(Highway Fuel Economy Drive Cycle) was also
considered.
Table 4: PHEV Fuel Economy Results
Pre-trans parallel –
10AER
Series – 10AER
Split – 10AER
Pre-trans parallel –
40AER
Series – 40AER
Split – 40AER
UDDS
MPG
(L/100 km)
HWFET
MPG
(L/100 km)
53/4.4
51.4/4.57
46.6/5
60.4/3.9
43.4/5.42
50.9/4.62
66.4/3.54
60/3.92
64.6/3.64
78.9/2.98
51.1/4.6
59.1/4
URBAN DRIVING - The split configuration provides the
best fuel economy in urban driving. In the 10AER case,
the parallel configuration outperforms the series
configuration. The higher efficiency of the power transfer
from engine to wheels benefits the parallel case.
HIGHWAY DRIVING - The split and parallel
configurations provide similar best fuel economy in
highway driving and both outperform the series
configuration. The series configuration suffers from dual
power conversion — from mechanical (engine) to
electrical (generator) and back to mechanical (electric
machine). The parallel configuration performs better
under the conditions of urban driving than under the
conditions of highway driving because higher driving
speeds require lower battery use. Engine efficiency in
the parallel case is lower than that in the split case.
However, the parallel case does not incur losses due to
the power recirculation that occurs in the split case, and
these losses tend to be higher as vehicle speed
increases.
As shown in Table 5, engine efficiency is higher for the
series configuration than for the other configurations. In
this case, the engine is completely decoupled from the
wheel and, therefore, can be operated at its best
efficiency point. In the split case, the extra degree of
freedom provided by the gearbox enables the engine
and vehicle speeds to be decoupled, which allows
engine efficiency to remain high. The parallel
configuration provides the lowest engine efficiency
because engine speed is directly linked to the wheel via
the fixed-ratio gearbox. As a consequence, its operation
at best engine efficiency is more difficult.
Data in Table 5 also show that engine efficiency
depends on the driving conditions for the parallel case,
unlike the other configurations. The results also
demonstrate that the parallel configuration tends to
perform better in under highway driving than under city
conditions.
UDDS
%
27.7
34.1
32.6
27.5
34.2
32.5
Parameter
Pre-trans parallel – 10AER
Series – 10AER
Split – 10AER
Pre-trans parallel – 40AER
Series – 40AER
Split – 40AER
HWFET
%
29.2
34.6
32.9
29
34.3
32.8
Table 6 presents both charge-sustaining fuel economy
(referred to as the fuel path) and the electrical
consumption during EV mode (referred to as the
electrical path). Figure 7 is a graphical representation of
the information on the 40 AER vehicles presented in
Table 6. The electric-only and CS modes are described
in more detail in the following paragraph.
Table 6: Electrical versus Fuel Path
Electrical
Path vs.
Fuel Path
Pre-trans
parallel –
10AER
Series –
10AER
Split –
10AER
Pre-trans
parallel –
40AER
Series –
40AER
Split –
40AER
Electrical Consumption (wh/km)
Fuel economy
Table 5: Engine Average Efficiency
Electrical
Path
UDDS
Wh/km
Fuel Path –
CS UDDS
MPG
(L/100 km)
Fuel Path –
CS HWFET
MPG /
(L/100 km)
134
45.7/5.14
46.9/5
133
42.2/5.57
40.6/5.79
131
43.3/5.43
46.9/5
140
42.9/5.48
45.3/5.2
138
41.3/5.69
39.3/5.98
137
51.0/4.6
45.1/5.2
140
120
parallel 40AER
series 40AER
split 40AER
100
80
60
40
20
0
0
1
2
3
4
5
Fuel Economy (l/100km)
Figure 7: Electrical vs. Fuel Path – 40AER case
6
ELECTRICAL PATH - Electrical path efficiency is
practically identical for all three configurations. Table 7
shows an overview of the overall efficiency of the main
components for the 40AER case. The 10AER case was
not reported here as results and trends are similar to
those of the 40AER case.
Table 7: Component Average Efficiency on UDDS
Parameter
Electric Machine (%)
Transmission (%)
Final drive (%)
Single gear (%)
Battery (%)
Pre-trans
Parallel 40AER
85.8
94.1
97.5
NA
95
Series
40AER
83.4
NA
97.5
97.5
95
Split
40AER
83.6
96.6
97.5
NA
95
The split and series cases are similar configurations
when operated in EV mode. The electric machine is
directly linked to the wheels in both cases through a
fixed gear and a final drive. Minor spin losses occur in
the planetary system, but these losses are not enough to
put the split system at a disadvantage when compared
to the series configuration.
The parallel configuration, even though overall efficiency
is identical to that of the other cases, shows a different
partition of the losses. Transmission efficiency in this
configuration is approximately 2% lower than that in the
power split configuration. However, because of the
presence of the transmission between the electric
machine and the final drive, the electric machine can be
operated more efficiently than the series or power split
configurations, as shown in Figures 8 and 9. The electric
machine efficiency in this case is approximately 2%
higher than that in the split case, cancelling out the extra
losses occurring in the transmission.
Figure 9: Electric Machine Operating Conditions on
Parallel 10AER – Density = f (time)
FUEL PATH - From highest to lowest, fuel economy in
the CS mode is as follows: power split, parallel, and
series. Engine fuel consumption maps are proportional
to maximum engine power. Power split and parallel
configurations have similar engine sizes. The series
configuration requires a significantly bigger engine to
meet the performance requirement. The series is
therefore at a disadvantage from the start. In addition,
the power goes through two electric machines,
increasing the amount of losses and, hence, the power
required by the engine. This impacts the series
configuration even more in highway driving.
The parallel configuration suffers from the losses in the
transmission and its inability to operate the engine at its
best operating point, as shown in Figure 10. This
configuration is also not capable of regenerating all of
the braking energy from the wheels during downshifting
events.
Figure 10: Engine Operating Conditions on Parallel
10AER – Density = f (time)
Figure 8: Electric Machine Operating Conditions on
Series Engine 10AER – Density = f (time)
The power split allows the engine to be operated close
to its most efficient point without the engine sending all
of its power through both electric machines, as shown in
Figure 11. High engine efficiency and the ability to send
mechanical power directly to the wheel allow this
configuration to provide the best CS fuel economy.
its behalf, a paid-up nonexclusive, irrevocable worldwide
license in said article to reproduce, prepare derivative
works, distribute copies to the public, and perform
publicly and display publicly, by or on behalf of the
Government.
REFERENCES
Figure 11: Engine Operating Conditions on Power
Split 10AER – Density = f (time)
CONCLUSIONS
Hybrid vehicles offer a compromise between
conventional and purely electric vehicles. Depending on
the degree of hybridization, hybrids become more or less
close to one of the two extremes. Several powertrain
configurations, including series, pre-transmission
parallel, and power split, were compared with respect to
component sizes and fuel economy for PHEV
applications.
Although both the power split and series configurations
require two electric machines and an engine, the series
configuration, as expected, requires significantly higher
component power as a result of the many component
efficiencies between the engine and the wheel.
In terms of efficiency, all of the configurations achieve
similar characteristics when operated in electric mode.
Both series and power split configurations do not use a
multi-gear transmission, but the parallel configuration
makes up for losses by operating the electric machine at
higher efficiency points. In CD mode, the power split
provides the best fuel economy as a result of its dual
path of power from the engine to the wheel.
On the basis of the thermal and electrical consumption
analysis, series configurations appear to be an
appropriate choice for vehicles designed to provide long
AER because of their simplicity in terms of control and
their ability to operate in electric-only mode at high
vehicle speed. The power-split configurations appear to
be a valid choice for vehicles based on a CD approach.
ACKNOWLEDGMENTS
This work was supported by DOE’s FreedomCAR and
Vehicle Technology Office under the direction of Lee
Slezak. The submitted manuscript has been created by
UChicago Argonne, LLC, Operator of Argonne National
Laboratory (“Argonne”). Argonne, a U.S. Department of
Energy Office of Science laboratory, is operated under
Contract
No.
DE-AC02-06CH11357.
The
U.S. Government retains for itself, and others acting on
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http://www1.eere.energy.gov/vehiclesandfuels/pdfs/
program/phev_rd_plan_02-28-07.pdf
2. Argonne National Laboratory, PSAT (Powertrain
Systems Analysis Toolkit), http://www.transportation.
anl.gov/.
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“Feasibility of
Reusable Vehicle Modeling:
Application to Hybrid Vehicles,” SAE paper 2004-011618, SAE World Congress, Detroit, March 2004.
4. General Motors Corporation, http://www.gm-volt.com
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Market
Transformation
Challenge:
the
DaimlerChrysler/EPRI Sprinter Van PHEV Program,”
EVS21, April 2005.
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M., “Integrating Data, Performing Quality Assurance,
and Validating the Vehicle Model for the 2004 Prius
Using PSAT,” SAE paper 2006-01-0667, SAE World
Congress, Detroit, April 2006.
7. Sharer, P.; Rousseau, A.; Pagerit, S.; and Nelson,
P., “Midsize and SUV Vehicle Simulation Results for
Plug-in HEV Component Requirements,” SAE paper
2007-01-0295, SAE World Congress, Detroit, April
2007.
8. Carlson, R., et al., “Testing and Analysis of Three
Plug-in Hybrid Electric Vehicles,” SAE paper 200701-0283, SAE World Congress, Detroit, April 2007.
CONTACT
Aymeric Rousseau
Center for Transportation Research
(630) 252-7261
arousseau@anl.gov