Ultracapacitor Assisted Powertrains: Modeling, Control, Sizing, and the Impact on Fuel Economy

TLDR: A model predictive control strategy is created for power management which achieves better fuel economy than the rule-based approach and indicates a potential for 5% to 15% improvement in fuel economy in city driving with the proposed mild hybrid powertrain.

Abstract: This paper considers modeling and energy management control problems for an automotive powertrain augmented with an ultracapacitor and an induction motor. The ultracapacitor-supplied motor assists the engine during periods of high power demand. The ultracapacitor may be recharged via regeneration during braking and by the engine during periods of low power demand. A reduced-order model and a detailed simulation model of the powertrain are created for control design and evaluation of fuel economy, respectively. A heuristic rule-based controller is used for testing the impact of different component combinations on fuel economy. After a suitable combination of engine, motor, and ultracapacitor sizes has been determined, a model predictive control strategy is created for power management which achieves better fuel economy than the rule-based approach. Various component sizing and control strategies tested consistently indicate a potential for 5% to 15% improvement in fuel economy in city driving with the proposed mild hybrid powertrain. This order of improvement to fuel economy was confirmed by deterministic dynamic programming which finds the best possible fuel economy.

Figures

Figure 3.8: Contour plot which shows the fuel efficiency of the 160kw engine as a function of engine torque and shaft speed. The fuel efficiency is defined as the power (kW) over the fuel consumption rate (g/sec).

Figure 1.1: The 48 volt BMOD0140 Maxwell ultracapacitor module with capacitance of 140F (Dimensions 41.6×19×16cm, Mass≈ 13.6 kg). While the maximum total energy stored is a mere 161kJ, this energy can be released in just a few seconds generating considerable power boost to a vehicle. The maximum power for this product is 4800W per unit mass or almost 65kW instantaneous maximum power. Newer products listed on Maxwell website [1] have even higher power densities.

Table 2.1: Values of constant parameters used in full-order model

Table 3.5: Comparison of the fuel economy increase achieved by various engine, motor, and ultracapacitor size combinations, run over the UDDS cycle with no road grade.

Figure 3.9: Contour plot which shows the fuel efficiency of the 120kw engine as a function of engine torque and shaft speed. The fuel efficiency is defined as the power (kW) over the fuel consumption rate (g/sec).

Table 2.2: Descriptions of variable parameters used in full-order model

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ULTRACAPACITOR ASSISTED POWERTRAINS: MODELING, CONTROL,
SIZING, AND THE IMPACT ON FUEL ECONOMY
A Thesis
Presented to
the Graduate School of
Clemson University
In Partial Fulfillment
of the Requirements for the Degree
Master of Science
Mechanical Engineering
by
Dean Rotenberg
December 2008
Accepted by:
Dr. Ardalan Vahidi, Committee Chair
Dr. Nader Jalili
Dr. John Wagner

Abstract
This thesis investigates possible fuel economy gains attainable by a combination of high-power
density ultracapacitors (also called supercapacitors) and an induction motor integrated into a conventional
vehicle powertrain for power assistance.
Periods of quick acceleration require a much higher power output from an automobile than what
is encountered under more typical driving conditions. A simple kinetic energy calculation can show that
accelerating a 2000 kg vehicle (roughly the size of a Ford Explorer SUV) from 0 to 60 mph in 10 seconds
requires almost 70 kW of power, in addition to the power needed to overcome road and air drag forces.
Situations such as these consume a disproportionately high amount of fuel, and have a negative impact on
the fuel economy of the vehicle. In conventional powertrains, the engine is typically sized much larger than
is needed for steady-state operation, in order to meet these spikes in power demand. A larger engine is more
expensive to manufacture and to operate. Such rapid transients in power may be better handled by the use
of high power density ultracapacitors which represent the latest trend in electrostatic energy storage systems.
While the total energy an ultracapacitor can store is typically ten times less than a battery of the same size,
the ultracapacitor is capable of releasing or storing energy roughly ten times faster. The potential of this
relatively new technology to assist the combustion engine during brief demand spikes, and to capture kinetic
energy through regenerative braking, is the subject of this study.
A mild parallel hybrid powertrain is considered in which an ultracapacitor-supplied motor assists the
engine during periods of high power demand, and the ultracapacitor may be recharged by the engine during
periods of low demand, and through regenerative braking. A detailed simulation model of the powertrain is
created to evaluate the fuel economy of the vehicle. The fuel economy gains are strongly dependent on how
well the power split decision is made, that is the decision of how to distribute the power demand between
the engine and the electric motor at each instant in time. To this end two forms of implementable control
are designed to determine the power split between the engine and motor. A rule-based controller, which can
ii

be quickly tuned and implemented, is applied for more exploratory simulations. Simplicity and expedience
in both tuning and implementation make this method useful for testing the impact of different component
combinations on fuel economy. After a suitable combination of engine, motor, and ultracapacitor sizes has
been determined, an optimization-based power management strategy is created which shows a better overall
performance. Various component sizing and control strategies tested consistently indicate a potential for 10
to 15 percent improvement in fuel economy in city driving with the proposed mild hybrid powertrain. This
order of improvement to fuel economy was confirmed by deterministic dynamic programming (DDP) which
finds the best possible fuel economy.
iii

Acknowledgments
I would like to thank Dr. Ardalan Vahidi for his help in the completion of my written works, for
valuable knowledge and advice on the subject of my study, and for making my stay here at Clemson possible
through research funding over the last two years. My graduate education would not have been able to progress
without his help.
I would also like to acknowledge Dr. Ilya Kolmonovsky for valuable input to the formulation of the
proposed hybrid powertrain, and for the use of deterministic dynamic programming to assess the full poten-
tial of the proposed configuration, and to compare the performance of the implemented power management
strategies.
Special appreciation goes to Dr. John Wagner and Dr. Nader Jalili for taking the time to evaluate
my thesis and defense.
iv

Frequently asked questions

Q1. What contributions have the authors mentioned in the paper "Ultracapacitor assisted powertrains: modeling, control, sizing, and the impact on fuel economy" ?

The potential of this relatively new technology to assist the combustion engine during brief demand spikes, and to capture kinetic energy through regenerative braking, is the subject of this study. Various component sizing and control strategies tested consistently indicate a potential for 10 to 15 percent improvement in fuel economy in city driving with the proposed mild hybrid powertrain. 

Q2. What is the purpose of this thesis?

A mild parallel hybrid powertrain is considered in which an ultracapacitor-supplied motor assists theengine during periods of high power demand, and the ultracapacitor may be recharged by the engine during periods of low demand, and through regenerative braking. 

Q3. What is the main idea of the thesis?

The fuel economy gains are strongly dependent on how well the power split decision is made, that is the decision of how to distribute the power demand between the engine and the electric motor at each instant in time. 

Q4. How much energy can an ultracapacitor store?

While the total energy an ultracapacitor can store is typically ten times less than a battery of the same size, the ultracapacitor is capable of releasing or storing energy roughly ten times faster. 

Q5. How much power does an ultracapacitor require to accelerate a vehicle?

A simple kinetic energy calculation can show that accelerating a 2000 kg vehicle (roughly the size of a Ford Explorer SUV) from 0 to 60 mph in 10 seconds requires almost 70 kW of power, in addition to the power needed to overcome road and air drag forces. 

Q6. How does the proposed hybrid powertrain perform in city driving?

Various component sizing and control strategies tested consistently indicate a potential for 10 to 15 percent improvement in fuel economy in city driving with the proposed mild hybrid powertrain. 

Q7. What is the purpose of the study?

After a suitable combination of engine, motor, and ultracapacitor sizes has been determined, an optimization-based power management strategy is created which shows a better overall performance. 

Q8. Who is the author of this article?

I would also like to acknowledge Dr. Ilya Kolmonovsky for valuable input to the formulation of theproposed hybrid powertrain, and for the use of deterministic dynamic programming to assess the full potential of the proposed configuration, and to compare the performance of the implemented power management strategies.