SAE TECHNICAL
`PAPER SERIES
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`950493
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`
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`The Effects of APU Characteristics on the
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`Design of Hybrid Control Strategies
`for Hybrid Electric Vehicles
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`Catherine Anderson and Erin Pettit
`AeroVironment
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`Reprinted from: Design Innovations in
`Electric and Hybrid Electric Vehicles
`(SP-1 089)
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`Q
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`The Engineering Society
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`#2ForAdvancing Mobility
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`International Congress and Exposmon
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`Detroit, Michigan
`Land Sea Air and Spacew
`Februarv27-March2.1995
`INTERNATIONAL
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`400 Commonwealth Drive, Warrendale, PA 1 5096-0001 U.S.A. Tel: (412)776-4841 Fax:(41 2)776-5760
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`Page 1 of 9
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`FORD 1219
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`Page 1 of 9
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`FORD 1219
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`FORD 1219
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`The Effects of APU Characteristics on the Design of
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`Hybrid Control Strategies for Hybrid Electric Vehicles
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`Catherine Anderson and Erin Pettit
`AeroVironment
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`950493
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`ABSTRACT
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`A hybrid control strategy is an algorithm that determines
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`when and at what power level to run a hybrid electric vehicle's
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`auxiliary power unit (APU) as a function of the power demand
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`at the wheels, the state of charge of the battery, and the current
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`power level of the APU. The design of this strategy influences
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`the efficiency of the overall system. The strategy must
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`balance the flow of power between the APU, the battery, and
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`the motor, with the intent of maximizing the average fuel
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`economy without overstressing the battery and curtailing its
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`The development of a system’s powertrain components
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`and the design of an optimum control strategy for that system
`should be concurrent to allow tradeoffs to be made while the
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`designs are still fluid. An efficient optimization process must
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`involve all aspects of the system, including costs, from the
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`beginning.
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`In this paper, we explore the methodology behind the
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`design of a hybrid control strategy. We also discuss the APU
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`and battery design characteristics that are crucial to the
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`strategy design, focusing on the interdependence of these
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`design characteristics within the entire system. Finally, we
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`propose a process for the development of an optimized hybrid
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`powertrain and the corresponding control algorithm.
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`INTRODUCTION
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`A "hybrid" vehicle usually refers to one that incorporates
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`a minimum of two independent power sources to supply the
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`drivetrain. One of the primary advantages of this dual power
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`supply system is it allows flexibility in power distribution
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`between sources. This versatility enables greater optimization
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`of the vehicle powertrain to meet the required performance of
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`the system.
`In order to profit from such system flexibility, one
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`must integrate into the system an intelligent control strategy
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`that uses each component to the overall system's best
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`advantage.
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`A hybrid control strategy is an algorithm that determines
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`how power in a hybrid powertrain should be distributed as a
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`function of the vehicle parameters (power demand, battery
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`state of char e SOC), component temperatures, etc.) and of
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`Page 3 of 9
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`component characteristics. One must develop this strategy
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`carefully as part of the vehicle design process from the
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`beginning. While the strategy determines the best operating
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`points for the components, the range of available component
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`characteristics provides the limits within which the strategy
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`must operate.
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`This paper explores the iterative process of concurrent
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`powertrain component and control strategy design with an
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`emphasis on optimizing the system as a whole. We focus
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`primarily on the auxiliary power unit and the characteristics of
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`the powertrain components that drive the strategy design.
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`HYBRID VEHICLE CONCEPT
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`Hybrid vehicles can be divided into two main categories:
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`parallel, in which both systems are used to mechanically drive
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`the wheels; and series, where the power supply systems are
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`coupled directly to a power bus which then transfers power to
`the wheels.
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`SERIES SYSTEM - The philosophy behind a series
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`hybrid vehicle lies in its combination of a primary and a
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`secondary energy conversion. In the primary conversion, an
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`APU converts a highly transportable, stable Chemical fuel to
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`mechanical energy (or directly to electrical energy in certain
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`cases) and, subsequently, to electrical energy. The most
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`frequently considered APUs for hybrid systems include
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`various internal and external combustion engines and fuel
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`cells. This primary conversion device can be decoupled from
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`the wheel power demand (unlike the engine in a conventional
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`car) as a Load Leveling Device (LLD), which acts as an
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`energy buffer, is included in the system. This LLD alternately
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`stores energy (either directly from the primary conversion at
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`low wheel power requirements or from the kinetic energy of
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`the decelerating vehicle) and provides the propulsion motor
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`with energy when the demand exceeds the APU power output.
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`Some LLDs that have been proposed for use in hybrid vehicles
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`include batteries, supercapacitors, hydraulic and/or pneumatic
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`storage devices, and flywheels.
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`The secondary conversion, occurring in the inverter and
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`motor, transforms the electrical energy from either source into
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`the mechanical energy that drives the vehicle. Figure l is a
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`schematic of the energy flow within the vehicle.
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`FORD 1219
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`8
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`Page 3 of 9
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`FORD 1219
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`Since all the power sources and sinks are directly coupled
`by a DC power bus. control of the entire system can be
`achieved by simply commanding the APU output. The
`accessory and wheel loads pull required power off the bus
`with the LLD supplying the balance of power in the system.
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`Alternator
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`Figure 1: Series hybrid vehicle component configuration.
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`ICE Vehicle
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`IncreasingAPUPower
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`Power Assist
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`Range Extender
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`Pure Electric
`Vehicle
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`Increasing Battery Storage ——-)
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`Figure 2: Comparison Chart of Power Assist and Range
`Extender Series Hybrid Vehicles
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`In addition, the series hybrid design may fall into one of
`two categories: "power assist" or "range extender" (see figure
`2). A power assist hybrid uses the LLD to manage the power
`output from the APU to maximize efficiency and emissions in
`the APU. The usable storage capacity of the LLD is quite
`small (on the order of 1-5 kWh), and the APU must be capable
`of providing the maximum sustained power the vehicle is
`expected to need, with the LLD providing peak powers and
`transients. A range extender hybrid uses a very small APU
`with a substantial LLD such that the vehicle will perform
`similarly to a pure electric vehicle. with the additional small
`“m“ M..-“ "imply extending the range. Since the APU for
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`a range extender is small compared with the power demand. it
`is most often run at its maximum power level, and hybrid
`control strategies are fairly simple.
`The primary disadvantage of the series hybrid system in
`most cases is the extra inefficiencies included in converting
`the mechanical power output from the APU into electrical
`power and then back into mechanical power. Often. however,
`the increased flexibility of the system offers more optimized
`components that overcome this disadvantage.
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`PARALLEL SYSTEM- In a parallel hybrid vehicle,
`there is a direct mechanical connection between the APU and
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`the wheels through a transmission. As shown in figure 3, the
`electric propulsion system may either drive the same set of
`wheels as the APU through the transmission (Option 1), or
`drive the other set of wheels directly (Option 2).
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`Figure 3: Parallel hybrid vehicle component
`configuration.
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`The main advantages of the parallel configuration over the
`series is that the power from the engine is used directly by the
`drivetrain with no alternator or inverter losses. However,
`because the APU is directly coupled to the wheels. the APU
`speed is determined by the vehicle speed and the transmission
`ratio. This direct coupling limits the flexibility of hybrid
`strategy design, and (without a novel clutched transmission)
`forces the APU to idle when the vehicle is at rest.
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`A parallel hybrid does have an efficiency advantage when
`the vehicle spends a majority of its driving time at a
`substantial cruise, but a vehicle that is operated on a "city
`driving" profile will lose this transmission efficiency
`advantage to inefficiencies in the APU engine.
`In addition, if
`the front and rear axles of the vehicle are driven by different
`power sources, the vehicle may exhibit changes in handling
`characteristics as the power distribution between the sources is
`adjusted.
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`The thought processes presented in this paper are
`sufficiently general that they can be applied to any type of
`vehicle. To fully explore the flexibility allowed by the hybrid
`system, we focus on the design of a strata
`”
`"
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`versatile layout; the power assist hybrid. For simplicity, we
`use the example of a generalized IC engine and Pb-Acid
`battery for the APU and battery, respectively, as a focus for
`the discussion.
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`HYBRID CONTROL STRATEGIES
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`There are two distinct extremes in the spectrum of control
`strategies. One is a system that uses a "thermostat" algorithm
`to command the APU (i.e. the APU is turned on to a constant
`power level when the SOC of the LLD is below a certain
`lower threshold, and off when the SOC exceeds an upper
`threshold). In this mode. the LLD must accommodate all the
`transient power requirements. Although the APU may be
`operating at its most efficient point. the losses in the LLD
`from excessive cycling may surpass the savings from an
`optimized APU. For the example wheel power curve shown in
`figure 4, figure 5 shows the corresponding APU and LLD
`power requirements generated by a thermostat mode.
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`Federal Urban Cycle
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`Wheel Power
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`60
`A 40
`E 20
`I.
`0 '
`g-zoO
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`n- as
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`1500 1700 1900 2100 2300 2500 2700 2900 3100 3300 3500
`Time(s)
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`Figure 4: The p0wer required at the wheels for a segment
`of the federal urban drive (LA4).
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`The other extreme commands the APU to follow the
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`actual wheel power whenever possible (similar to a
`conventional automobile). Using this strategy, the LLD
`cycling will diminish. and the losses associated with charge
`and discharge will be minimized. The APU, however, must
`then operate over its entire range of power levels and perform
`fast power transients. both of which can adversely affect
`engine efficiency and emissions characteristics. Figure 6
`shows the APU and LLD power requirements generated by
`this "following" mode for the same wheel power curve shown
`in figure 4. It should be noted that this is the mode a parallel
`hybrid vehicle always uses.
`For most of the APUs and LLDs under consideration,
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`neither of these strategies would be the optimum strategy. The
`ideal hybrid conu'ol strategy is one that minimizes the
`combined inefficiencies of both the APU and the LLD while
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`meeting the desired performance and the emission limits (as
`well as any other specific system characteristics that are being
`used as measures of design merit). The optimum strategy is
`highly dependent on the characteristics of the powertrain
`components and the planned use of the vehicle. Unfortunately
`as one attempts to optimize a system, the characteristics of the
`components begin to conflict, driving the strategy in different
`directions.
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`Page 5 of 9
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`Constant APU Mode
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`APU Power
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`1500 1700 [9(1) 2100 2300 2500 2700 2900 3100 3300 3500
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`[JD Power
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`0888
`Power(kW) 8
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`8‘5
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`1500 1700 1900 2100 2300 2500 2700 2900 3100 3300 3500
`Tune(s)
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`Figure 5: The APU and LLD power outputs that satisfy
`the wheel requirements using a constant APU thermostat
`strategy.
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`APU follower Mode
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`APU Power
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`o888
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`(kW) r'oo as
`Power
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`1500 1700 1900 2100 2300 2500 2700 2900 3100 3300 3500
`Time(s)
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`Figure 6: The distribution of power for a load following
`APU.
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`COMPONENT CHARACTERISTICS AND DESIGN
`TRADE-OFFS
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`LLDs - The LLD (in this case a battery) must be the most
`accommodative element in the powertrain. When there is a
`large power demand or production from the wheels (as during
`hard acceleration or braking), it must supply or accept the
`power required. In a hybrid application, the battery pack
`generally hm lower capacity than it would have for a pure
`electric vehicles (particularly for a power assist hybrid where
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`battery life are available, some qualitative statements can be
`made:
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`1. A lead acid battery will degrade more (per a
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`throughput kWh) if cycled deeply (cycled through a
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`wide range of SOCs) than shallowly. The long term
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`effects of microcycling (cycling over a small range of
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`SOCs) are not fully known.
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`2. A battery will last longer if it has lower energy
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`throughput.
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`3. Hard cycling (high power cycling), even hard
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`microcycling, will shorten the life.
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`Charge
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`Discharge
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`(Ohms)
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`InternalResistance
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`lllllll
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`ll1l
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`0
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`0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
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`State of Charge (%)
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`1
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`Figure 7: The charge and discharge internal resistances
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`versus the state of charge of a battery.
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`Figure 8 shows the difference between the SOCs of the
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`thermostat and follower extremes discussed above (see figures
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`3-5) over multiple repetitions of the federal urban driving
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`cycle. In the thermostat mode,
`the APU power output is
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`greater than the average power for the cycle causing the state
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`of charge to continue to increase until it reaches a defined
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`maximum state of charge (in this case 80%) requiring the APU
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`to turn off. The follower mode, on the other hand, provides
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`only a slight constant increase in SOC due to the battery's
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`absorption of regenerative energy during the cycle.
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`The deep cycled battery might only last half as long as the one
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`kept within a tight SOC window. However, the costs of
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`replacing the battery versus the cost of building an APU
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`capable of fast transient response(that can protect the battery)
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`must be weighed.
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`APUs - Because the APU is decoupled from the
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`drivetrain, there is greater flexibility in its design. The design
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`need not be performance driven as in conventional IC engines,
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`but can be focused on other characteristics, such as emissions,
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`that may be more important for the specific vehicle being
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`designed. Most importantly, however, the APU characteristics
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`must be chosen to complement the LLD requirements; thus,
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`the need for a working strategy throughout the design process.
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`Characteristics crucial to the design include maximum power
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`output, transient capabilities, fuel efficiency, emissions
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`characteristics, engine noise vibration harshness (NVH), and
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`service life.
`—‘
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`FORD 1219
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`the APU is of considerable size). To maintain the same
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`performance, therefore, the power density must be greater. In
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`addition, the state of charge of the battery can be significantly
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`affected during a short acceleration or deceleration so that the
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`small-scale charge/discharge period (or "microcycling”) that
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`the pack sees is a more significant percentage of its capacity.
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`Differing control strategies can place varying demands on the
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`cycling of the battery. Using the thermostat APU strategy, the
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`battery would be required to cycle at the frequency of the
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`wheel power demand, while the follower APU strategy would
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`only require the battery to cycle when the wheel power
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`demand exceeds the APU power capability.
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`There are several characteristics of the battery that one
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`must keep in mind when trying to quantify tradeoffs between
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`the battery and the rest of the system:
`the charge/discharge
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`efficiency, the total capacity of the pack, the transient
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`capabilities, and, the hardest to determine, the life of the
`batteries.
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`ChargeZDischarge Efficiencies - A battery is most
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`efficient within a range of SOCs that minimizes its charge and
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`discharge resistances. In figure 7, one can see the general
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`shape of a Pb-Acid battery's internal resistance versus state of
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`charge curves for charging and discharging the battery. A
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`balance point must be chosen on these curves to minimize
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`resistive losses, yet still leave room for power peaks (both
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`motoring and regenerative braking) at the wheels. This tends
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`to push the strategy design to keep the SOC within the 50-70%
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`region for minimum losses in both charge and discharge. This
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`leaves enough capacity to handle an extended period of battery
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`discharge (such as during a long hill climb) and enough
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`"headroom" to absorb a long period of charging such as that
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`which occurs during a long downhill. If the SOC is not
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`maintained within the 50-70% region, the performance may be
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`compromised. This diminished performance may take the
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`form of lost regenerative energy or limited power output
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`during accelerations.
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`Capacity - The capacity of the pack is comparatively easy
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`to measure, and the effects of the change of capacity on the
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`strategy are fairly intuitive. (It should be noted that the
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`capacity at one rate of discharge is different from the capacity
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`at another rate, and therefore the definition of "capacity" is
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`subject to discussion.) In general, the larger the battery pack
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`capacity, the more the vehicle can be run like an pure electric
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`vehicle with the APU providing supplemental power. With a
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`large capacity, it is' easier to achieve the power required for
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`standard driving, and the pack does not have to be so rigidly
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`constrained to a small window of states of charge. A small
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`pack, however, must be used almost exclusively as a short-
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`terrn energy buffer without significant energy storage.
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`Transient Capabilities - A battery can change pOWer levels
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`almost instantaneously, unlike the APU which is limited by its
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`mechanical inertia. When the APU cannot respond quickly
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`enough to fluctuations in power demand, the battery must
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`make up the difference. The battery must be able to sustain
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`output at a peak power during these transients until the APUs
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`power output reaches the commanded power.
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`L_ife - Unfortunately, most available data on battery life is
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`of limited applicability to hybrid systems. The complexities of
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`the reactions within batteries make it almost impossible to
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`predict battery life except as a questionable extrapolation of
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`filthough few quantitative predictions of
`Page 6 of 9
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`68
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`Page 6 of 9
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`FORD 1219
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`

`

`Thermostat Made
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`I; 80E60’40
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`@100
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`E 20
`g 0
`"’
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`1500
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`2500
`Time (5)
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`3500
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`Follower Mode
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`@100
`I; 80
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`§ 0m
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`3°60_
`“ ‘°_3 20
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`1500
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`2500
`Time (s)
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`3500
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`Figure 8: The state of charge during repetitions of the
`federal urban driving cycle with a constant APU at 20 kW
`in the thermostat mode (see figures 2,3) and in the
`follower mode(see figure 4).
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`WW - The maximum power output of
`the APU will affect strategy design choices in a similar
`manner to the capacity of the battery. With a high power
`capability, one may design the strategy to operate more or less
`like a conventional car engine in a power following mode,
`whereas a low power capability will force the strategy to run
`the engine at its highest power level so that it can keep up with
`current demands and store extra energy for periods of high
`demand.
`
`W - Mechanically, the transient
`capabilities of an engine are limited by the inertia involved in
`increasing or decreasing the engine speed. Although slower
`transients are desirable for reducing emissions, slow transients
`can curtail the life of the battery or potentially harm the
`engine. For example, slow transients can be a serious problem
`during a transition from a hard acceleration to a hard braking.
`If the APU has been commanded by the control strategy to
`supply a high power during an acceleration, and suddenly full
`regenerative braking is required, the LLB may not be able to
`accept the total power coming to it, unless the APU can reduce
`its power quickly. This limitation will cause a loss of much of
`the regenerative energy available. In an extreme case, the
`APU may be unloaded by an over-voltage condition, leading
`to potential overspeed. The APU control strategy must be
`robust, such that no combination of driver actions will result in
`damage to any drivetrain component.
`
`Eye] Effigjency - The fuel efficiency of an APU generally
`varies as a function of the power level. The specific fuel
`consumption (SFC) of an engine is typically best at middle
`power levels and worst at the low and high power extremes.
`The APU operating strategy that will maximize fuel efficiency
`is one that runs the APU primarily in the range of powers over
`which the SFC is best (often termed the engine's "sweet spot").
`The ratin of the highest power level to the lowest power level
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`Page 7 of 9
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`69
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`used in the strategy is called the turn down ratio. The
`narrower this sweet spot is, and, thus, the smaller the most
`efficient turn down ratio is, the more the fuel efficiency
`requirements constrain the strategy toward a thermostat mode
`(see figure 9, a series of SFC graphs showing varying sweet
`spots). Increasing the range of high efficiency and thus the
`turn down ratio and the ability of the strategy to follow drive
`power more closely (therefore relieving some stress on the
`battery) can increase the complexity and cost of the engine or
`lower the peak fuel efficiency. Tradeoffs must be made
`between engine complexity, cost, fuel efficiency, and battery
`lifetime. For example, if a long battery lifetime is the most
`important aspect of the system, then a large sweet spot is
`desired, possibly sacrificing engine simplicity, efficiency, or
`low engine emissions. In a situation where the average power
`is fairly constant a smaller sweet spot may be the most
`efficient solution.
`
`Emisigns, - Frequently, one of the principle aims of a
`hybrid vehicle is to reduce vehicle emissions to ULEV (Ultra
`Low Emission Vehicle) levels. Consequently, APU emissions
`are very important for system success. In general, emissions
`are minimized when a stoichiometric air to fuel ratio is
`maintained by a closed loop feedback system (using an oxygen
`sensor for feedback). In some operating regimes, such as
`engine starts and transients, the stoichiometric ratio is very
`difficult to maintain resulting in an increase in emissions.
`During a cold-start, the engine must run rich to achieve
`sufficient vaporization of the fuel. Rich running results in
`high hydrocarbons (HC) and carbon monoxide (CO)
`emissions, but low nitrogen oxides (NOx) emissions. A hot-
`start has many of the same problems as a cold start, but the
`time duration before the engine and catalytic converter warm
`up is much shorter. A hybrid strategy which minimizes engine
`cycling will minimize start—related emissions, but that may
`require that the engine have a higher tum-down ratio.
`Transients present an emissions problem that is largely
`related to the speed of the transient. The closed loop feedback
`system that maintains the stoichiometric air fuel ratio is
`sufficient during quasivsteady state modes, however, it can
`only react as fast as the 02 levels can be sensed. If the
`transient is too fast, the engine may run rich, increasing CO
`and HC emissions, or lean, increasing NOx emissions. Some
`of this effect can be reduced using a hybrid strategy that only
`allows slow transients, but this places greater strain on the
`LLD.
`
`As a series hybrid vehicle decouples both the speed and
`the power of the APU from the speed and power requirement
`at the wheels, this extra degree of freedom can also be used to
`reduce emissions. For a given required power output, there
`are many combinations of speed and torque that could be used
`to provide that power. If the engine is run in a low speed, high
`load state, the fuel efficiency, noise, and hydrocarbon
`emissions are all improved. At high loads, however, the NOx
`emissions are high and traditional NOx reducing measures
`such as Exhaust Gas Recirculation (EGR) are more difficult.
`Choosing this optimum engine operating point as a function of
`power is an important design consideration but it is not
`necessarily part of the hybrid strategy design.
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`MW - Engine noise is not
`much of an issue as far as the performance of a drivetrain is
`concerned, but to avoid customer distress, it must be
`considered as an influencing factor on a hybrid strategy. For
`example, a strategy that has the engine On at full power while
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`FORD 1219
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`8
`8
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`Page 7 of 9
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`FORD 1219
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`

`

`—|
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`all declarations up to 0.25 g, then feather in the rear brakes to
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`prevent skidding and instability during hard decelerations.
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`The amount of energy available from regenerative braking
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`influences the fuel economy greatly, especially in heavy stop
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`and go traffic. The efficiency of the regenerative braking
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`depends on the resistance of the battery to charging and,
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`therefore, on the state of charge of the battery. This, once
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`again, creates conflicting optimization factors, for the APU
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`cannot be run at its most efficient point if it is desired that the
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`battery stay within a certain range of SOCs. The APU must be
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`run within its limited high efficiency range and the battery
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`must be maintained around a state of charge that has minimum
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`charge and discharge resistances.
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`COST - Inasmuch as hybrid powertrains must compete
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`against conventional powertrains for cost and performance, the
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`overall success of the powertrain is extremely dependent on
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`cost. More expensive components may increase the
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`capabilities and the life, but if that makes the system
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`unsaleable, the improvements are useless. In the end, every
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`tradeoff that is made in the powertrain system must be done
`with cost in mind.
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`DEVELOPMENT OF A WORKING STRATEGY
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`The design of hybrid systems must begin with the overall
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`system constraints. Depending on the type of vehicle (for
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`example, a large passenger car or a delivery truck),
`there are
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`weight and volume limits. These limits include both the
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`general physical dimensions of the car as well as the physical
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`characteristics of the powertrain. These limits and the power
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`requirements of the system provide an initial basis for
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`choosing the relative sizes of the APU and the LLD. System
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`power electronics are more efficient if constrained to within a
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`fairly narrow bus voltage range. This additional limiting
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`factor must also be considered in the component design.
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`These constraints, along with the control strategy provide the
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`system wide link between components.
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`All of the design factors discussed above influence the
`characteristics of the final vehicle; therefore, it is crucial to
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