IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS, VOL 44, NO 1, FEBRUARY 1997
`
`19
`
`Propulsion System Design of
`Electric and Hybrid Vehicles
`
`Mehrdad Ehsani, Fellow, IEEE, Khwaja M. Rahman, Student Member, IEEE, and Hamid A. Toliyat, Member, IEEE
`
`Abstract—There is a growing interest in electric and hybrid-
`electric vehicles due to environmental concerns. Recent efforts
`are directed toward developing an improved propulsion system
`for electric and hybrid-electric vehicles applications. This paper
`is aimed at developing the system design philosophies of electric
`and hybrid vehicle propulsion systems. The vehicles’ dynamics
`are studied in an attempt to find an optimal torque-speed profile
`for the electric propulsion system. This study reveals that the
`vehicles’ operational constraints, such as initial acceleration and
`grade, can be met with minimum power rating if the power train
`can be operated mostly in the constant power region. Several
`examples are presented to demonstrate the importance of the
`constant power operation. Operation of several candidate motors
`in the constant power region are also examined. Their behaviors
`are compared and conclusions are made.
`
`Index Terms— Electric vehicle, hybrid electric vehicle, motor
`drives, road vehicle electric propulsion, road vehicle propulsion.
`
`I. INTRODUCTION
`
`Environmental Protection Agency (EPA), conventional ICE
`vehicles currently contribute 40%–50% of ozone, 80%–90%
`of carbon monoxide, and 50%–60% of air toxins found in
`urban areas [1]. Besides air pollution, the other main objection
`regarding ICE automobiles is their extremely low efficiency
`use of fossil fuel. Hence, the problem associated with ICE
`automobiles is threefold: environmental, economical, as well
`as political. These concerns have forced governments all
`over the world to consider alternative vehicle concepts. The
`California Air Resource Board (CARB) is among the few that
`acted first through the declaration of the Clear Air Act of
`September 1990. This act requires that 52% of all vehicles sold
`in that state be either low-emission vehicles (LEV’s)—48%,
`ultralow-emission vehicles (ULEV’s)—2%, or zero-emission
`vehicles (ZEV’s)—2%, by 1998 [2]. Similar measures are
`being considered in other states and nations as well.
`EV’s and hybrid-electric vehicles (HEV’s) offer the most
`promising solutions to reduce vehicular emissions. EV’s con-
`stitute the only commonly known group of automobiles that
`qualify as ZEV’s. These vehicles use an electric motor for
`propulsion and batteries as electrical-energy storage devices.
`Although there have been significant advancements in motors,
`power electronics, microelectronics, and microprocessor con-
`trol of motor drives, the advancement in battery technology has
`been relatively sluggish. Hence, the handicap of short range
`associated with EV’s still remains. Given these technology
`limitations, the HEV seems to be the viable alternative to the
`ICE automobile at the present. HEV’s qualify as ULEV’s and
`do not suffer from the range limitations imposed by EV’s.
`These vehicles combine more than one energy source to propel
`the automobile. In heat engine/battery hybrid systems, the
`mechanical power available from the heat engine is combined
`with the electrical energy stored in a battery to propel the
`vehicle. These systems also require an electric drivetrain to
`convert electrical energy into mechanical energy, just like the
`EV. Hybrid-electric systems can be broadly classified as series
`or parallel hybrid systems [3].
`In series hybrid systems, all the torque required to propel
`the vehicle is provided by an electric motor. On the other
`hand, in parallel hybrid systems the torque obtained from the
`heat engine is mechanically coupled to the torque produced
`by an electric motor [3]. In the EV, the electric motor behaves
`exactly in the same manner as in a series hybrid. Therefore,
`the torque and power requirements of the electric motor are
`roughly equal for an EV and series hybrid, while they are
`lower for a parallel hybrid.
`0278–0046/97$10.00 ª
`
`THE CONCEPT of the electric vehicle (EV) was con-
`
`ceived in the middle of the previous century. After the
`introduction of the internal combustion engine (ICE), EV’s
`remained in existence side by side with the ICE for several
`years. The energy density of gasoline is far more than what
`the electrochemical battery could offer. Despite this fact, the
`EV continued to exist, especially in urban areas due to its
`self-starting capability. However, soon after the introduction
`of the electric starter for ICE’s early this century, despite
`being energy-efficient and nonpolluting, the EV lost the battle
`completely to the ICE due to its limited range and inferior
`performance. Since then,
`the ICE has evolved,
`improved
`in design, and received widespread acceptance and respect.
`Although this essentially is the case, EV interest never perished
`completely and whenever there has been any crisis regarding
`the operation of ICE automobiles, we have seen a renewed
`interest in the EV. The early air quality concerns in the 1960’s
`and the energy crisis in the 1970’s have brought EV’s back
`to the street again. However, the most recent environmental
`awareness and energy concerns have imposed, for the first
`time since its introduction, a serious threat to the use of ICE
`automobiles.
`The ICE automobile at the present is a major source of
`urban pollution. According to figures released by the U.S.
`
`Manuscript received February 26, 1996; revised April 17, 1996.
`The authors are with the Texas Applied Power Electronics Center, Depart-
`ment of Electrical Engineering, Texas A&M University, College Station, TX
`77843-3128 USA.
`Publisher Item Identifier S 0278-0046(97)00069-5.
`
`1997 IEEE
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`IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS, VOL. 44, NO. 1, FEBRUARY 1997
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`This paper presents the EV and HEV propulsion system
`design philosophies. The paper is organized as follows. Section
`II describes the design constraints and the variables for EV and
`HEV systems. Design philosophies of EV and HEV propulsion
`systems are presented in Sections III and IV, respectively. Sec-
`tion V examines several of the most commonly used motors
`for EV and HEV system design. Section VI presents some test
`data from the Texas A&M University, College Station, hybrid
`vehicle. Summary and conclusions are presented in Section
`VII.
`
`II. SPECIFICATIONS OF EV AND
`HEV PROPULSION SYSTEM DESIGN
`
`;
`
`A. System Design Constraints
`Vehicle operation consists of three main segments. These
`are: 1) the initial acceleration; 2) cruising at vehicle rated
`speed; and 3) cruising at the maximum speed. These three
`operations provide the basic design constraints for the EV
`and HEV drivetrain. A drivetrain capable of meeting these
`constraints will function adequately in the other operational
`regimes. Refinements to these basic design constraints are
`necessary for an actual commercial product, but those are
`beyond the scope of this paper. The objective here is to meet
`these constraints with minimum power. The variables defining
`the above design constraints are:
`;
`1) vehicle rated velocity,
`2) specified time to attain this velocity,
`;
`3) vehicle maximum velocity,
`4) vehicle mass and other physical dimensions.
`1) Initial Acceleration: The initial acceleration force takes
`, in some
`the vehicle from standstill to its rated velocity,
`seconds. This force is supplied entirely by
`specified time,
`the electric power train in an EV or series HEV. In a parallel
`HEV, the acceleration force is supplied by the electric power
`train in combination with the ICE power train.
`: The electric mo-
`2) Cruising at Rated Vehicle Speed
`tor provides the necessary propulsion force at rated vehicle
`speed in the EV and series HEV. On the other hand, the ICE of
`the parallel HEV should be capable of delivering enough force,
`without any help from the electric power train, to overcome
`road load and cruise at the rated vehicle speed on a grade of at
`least 3%. In addition, there should be a margin of about 10%
`power to charge the batterypack.
`maximum
`3) Cruising at Maximum Vehicle Speed: The
`cruising force is provided by the electric motor in the EV
`and series HEV. In the parallel HEV, the electric motor and
`ICE should work in combination to provide the required force
`to sustain the vehicle at its maximum velocity.
`
`B. System Design Variables
`The main component of the EV is its electrical power train.
`However, in the HEV the propulsion system is a combination
`of the electric motor and ICE. The electric propulsion design
`variables are:
`
`Page 2 of 9
`
`1) electric motor power rating;
`2) motor rated speed;
`3) motor maximum speed;
`4) the extent of constant power speed range beyond the
`rated speed;
`5) gear ratio between motor shaft and the wheel shaft
`(transmission).
`Designing an optimal torque-speed profile for the ICE is
`beyond the scope of this paper. However, assuming a typical
`ICE torque-speed profile, we will specify the required ICE
`power by our design procedure. Therefore, the design variables
`for the mechanical propulsion system are:
`1) ICE size;
`2) gear ratio between ICE and the wheel shaft.
`As mentioned earlier, the main design objective is to find
`the minimum drive weight, volume, and cost that will meet the
`design constraints with minimum power. EV system design is
`addressed first. The HEV system design is then presented as
`a modification of the EV system design.
`
`C. Road Load Characteristics
`consists of rolling resistance
`The road load
`, and climbing resistance
`[4]:
`aerodynamic drag
`
`,
`
`(1)
`
`The rolling resistance
`on the road:
`
`is caused by the tire deformation
`
`(2)
`
`where
`is the tire rolling resistance coefficient. It increases
`with vehicle velocity and also during vehicle turning ma-
`, and
`is the
`neuvers. Vehicle mass is represented by
`gravitational acceleration constant.
`, is the viscous resistance of air acting
`Aerodynamic drag,
`upon the vehicle:
`
`(3)
`
`where
`is the aerodynamic drag
`is the air density,
`is the vehicle frontal area,
`is the vehicle speed,
`coefficient,
`is the head-wind velocity.
`and
`with positive operational sign)
`The climbing resistance (
`with negative operational sign)
`and the downgrade force (
`is given by
`
`(4)
`
`where
`is the grade angle.
`A typical road load characteristic as a function of vehicle
`speed is shown in Fig. 1. The following assumptions are made
`in the plot:
`1) velocity independent rolling resistance;
`2) zero head-wind velocity;
`3) level ground.
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`
`Fig. 1. Typical road load characteristics as a function of vehicle speed.
`
`Fig. 2. Torque-speed diagram of an electrical motor in terms of tractive force
`and vehicular speed with gear size as the parameter.
`
`These assumptions will be used in the analysis presented
`in the following sections, unless otherwise specified. These
`assumptions do not change the general trend of the solution
`and can be easily relaxed.
`available from the propulsion system is
`The motive force
`. The net
`partially consumed in overcoming the road load,
`, accelerates the vehicle (or decelerates when
`force,
`exceeds
`). The acceleration is given by
`
`(5)
`
`where
`is the rotational inertia coefficient to compensate for
`the apparent increase in the vehicle’s mass due to the onboard
`rotating mass.
`
`III. EV SYSTEM DESIGN
`The main component of the EV drivetrain is its electric
`motor. The electric motor in its normal mode of operation can
`provide constant-rated torque up to its base or rated speed.
`At this speed, the motor reaches its rated power limit. The
`operation beyond the base speed up to the maximum speed
`is limited to this constant power region. The range of the
`constant power operation depends primarily on the particular
`motor type and its control strategy. However, some electric
`motors digress from the constant power operation, beyond
`certain speed, and enter the natural mode before reaching
`the maximum speed. The maximum available torque in the
`natural mode of operation decreases inversely with the square
`of the speed. This range of operation is neglected in the
`analysis presented in this section, unless otherwise specified.
`It is assumed that the electric motor operates in the constant
`power region beyond the base speed and up to the maximum
`speed. Nevertheless, for some extremely high-speed motors
`the natural mode of operation is an appreciable part of its
`total torque-speed profile. Inclusion of this natural mode for
`such motors may result in a reduction of the total power
`requirement. Of course, power electronic controls allow the
`motor to operate at any point in the torque-speed plane, below
`the envelope defined by the mentioned limits. However, it
`
`Page 3 of 9
`
`is the profile of this envelope that is important in the motor
`drive selection and design.
`In order to free up the motor speed from the vehicle speed,
`for design optimization, gearing between the motor shaft and
`the drive shaft is required. In our design, we will make the
`following assumptions.
`1) Single gear ratio transmission operation—power elec-
`tronic control allows instantaneous matching of the
`available motor torque with the required vehicle torque,
`at any speed; therefore, multiple gearing in order to
`match the motor torque-speed to the vehicle torque-
`speed is no longer a necessity;
`2) Ideal loss free gear—without loss of generality, the gear
`losses can be incorporated at the end of analysis.
`The gear ratio and size will depend on the maximum motor
`speed, maximum vehicle speed, and the wheel radius. Higher
`maximum motor speed, relative to vehicle speed, means a
`higher gear ratio and a larger gear size. The selection criterion
`for the maximum motor speed will be further discussed later.
`The torque-speed diagram of a typical motor is drawn in
`Fig. 2, but in terms of tractive force and vehicular speed for
`different gear ratios. Notice the electric motor base speed and
`maximum speed, in terms of the vehicle speed, depend on the
`gear ratio. A design methodology based on the three regions
`of operation will now be presented.
`
`A. Initial Acceleration
`The force-velocity profile of a typical motor is redrawn in
`is the electric motor rated speed,
`Fig. 3. In this figure,
`is the vehicle rated speed, and
`is the vehicle maximum
`speed. The motor maximum speed must correspond to this
`, after the gear ratio transformation. The figure also shows
`(the dashed curve) the force-velocity profile of the motor in the
`natural mode. This mode of operation, however, is neglected
`unless otherwise specified.
`. For
`The range of operation for initial acceleration is –
`now, we will focus our attention only on this interval. For
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`IEE TRANSACTIONS ON INDUSTRIAL FLECI'RONICS, VOL. 44, NO. 1, FEBRUARY 1997
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`
`
`
`
`MotorPlatedPower
`
`(kW)
`
` 0
`
`20
`
`60
`40
`Veh’cle Speed (rrph)
`
`80
`
`100
`
`0
`
`10
`
`40
`30
`20
`Motor Rated Speed Vrm (mph)
`
`50
`
`60
`
`Fig. 3. Typical torque-speed profile of electric motor in terms of tractive
`force and vehicular weed
`
`Fig. 4. Acceleration power requirement as a fimction ofmotor rated speed
`CWWresistanoelessase, dashedcurve—inthepresenceofroad
`load
`
`maximum acceleration, the motor operates in constant rated
`
`force (torque), Emmi = P," /1,v,.,,,, up to the motor rated
`speed,
`’U,.,,,_. and in constant power, FL. = Pm/v, at speeds
`beyond the base speed, up to the vehicle rated speed,
`'u,.,_..
`Here, Pm is the motor rated power. We assume 1'”. > mm.
`The wisdom of this assumption will become clear shortly. The
`difiermtial equation describing the performance of the system
`is given by (5) and is repeated here for convenience:
`
`(1
`
`_ d1) _ F— Fu'
`_ dt _ km -m,
`
`F is the motive force available from the propulsion system
`and Frr‘ is the nmning resistance (road load). The boundary
`conditions are:
`
`at t = 0, vehicle velocity ’u = 0.
`
`at t = tf, vehicle velocity 1.! = Ur”.
`To gain insight, we will solve (5) under the most simplifying
`assumptions.
`
`l) The vehicle is on a level ground.
`2) The rolling resistance is zero.
`3) Aerodynamic drag is zero.
`
`These assumptions will be relaxed later for a more realistic
`solution. The above assumptions will result in a closed-form
`solution for the motor rated power Pm. The insight gained
`from the closed-form solution is also valid for the more
`
`practical design involving rrmning resistances.
`With these simplifying assumptions, the governing difier-
`ential equation reduces to
`
`a. — — = —
`_ (11!
`F
`dt
`m.
`
`(assuming km : l).
`
`This difl'erential equation is solved with the previous bound-
`ary conditions and the force-speed profile of Fig. 3. The
`difiermtial equation is integrated Within the acceleration in-
`
`terval of 0 to v”. in 0 to t; s, in order to get a closed-form
`solution for the rated power Pm:
`
`The left-hand side integral is broken into two parts: the 0—v,.,,,
`constant force operation and the «v,.,,,~L',.L_ constant power
`operation:
`
`(iv
`v”
`"m (h)
`m. — +171 — = t
`
`/
`
`Pm
`
`1.2”"
`
`/ m a f
`
`1;
`
`.
`
`7
`
`‘ )
`
`Now solving for Pm, we get
`772.
`.
`(8)
`Pm : E (vi-7‘7" + ”EL-l
`For minimum motor power, differentiating Pm with respect
`to um, and setting it to zero gives
`
`7er = 0
`
`(9)
`
`This establishes a theoretical
`
`limit for minimum motor
`
`the electric motor operates entirely
`power. For 11,-," = O,
`in the constant power region. Therefore, if the motor is
`performing Oil)”. in tf seconds in constant power alone, the
`power requirement is minimum. 0n the other hand, if the
`motor operates in the constant torque (force) region during
`
`the entire 0 to t; period, we will have v,.,,, = 11”.. In this
`case, (8) shows that the power requirement is twice that of
`constant power operation. The solid line curve of Fig. 4 shows
`an example of the motor power requirements between these
`two extremes. Of course, entire operation in constant power
`regime is not practically realizable. However, this theoretical
`discussion demonstrates that longer constant power range of
`operation will lower the motor power.
`Having discussed the simplified resistanceless case, we now
`solve the more realistic case involving the running resistance.
`The vehicle differential (5) can be solved under the same
`boundary conditions as before with the presence of the running
`resistance Fw. In this case, a closed-form solution is feasible.
`However, the result is a transcendental equation involving
`rated motor power 1’", , rated motor velocity mm, rated vehicle
`
`n“. d .
`
`m f l =
`
`0
`
`F
`
`If
`
`0
`
`dt.
`
`(6)
`
`velocity 'l-‘,.,/., acceleration time ti, and all the other system
`constants, e.g., vehicle mass m, rolling resistance coeflicient f,
`
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`23
`
`it
`
`is desired to obtain
`
`for the
`
`, etc. The resulting equation
`aerodynamic drag coefficient
`for a specific motor rated
`can be solved numerically for
`, using any standard root-seeking method such as
`velocity
`the secant method [5].
`Let us assume that
`following case:
`• 0–26.82 m/s (0–60 mi/h) in 10 s;
`• vehicle mass of 1450 kg;
`• rolling resistance coefficient of 0.013;
`• aerodynamic drag coefficient of 0.29;
`• wheel radius of 0.2794 m (11 in);
`• level ground;
`• zero head-wind velocity.
`A plot of the resulting motor rated power versus motor rated
`speed, in terms of vehicle speed, is shown in Fig. 4 (the dashed
`curve).
`Examination of Fig. 4 (the dashed curve) results in the
`following conclusions:
`curve shows the same general
`1) rated power versus
`trend of the resistanceless case;
`2) rated motor power requirement is minimum for contin-
`;
`uous constant power operation
`3) rated motor power is roughly twice that of continuous
`constant power operation for constant force (torque)
`;
`operation
`4) rated motor power remains close to its minimum up to
`about 20 mi/h of rated motor speed and then grows
`rapidly.
`
`B. Cruising at Rated Vehicle Velocity
`A power train capable of accelerating the vehicle to the
`, will always have sufficient cruising power
`rated velocity,
`at this speed. Hence, the constraint of cruising at rated vehicle
`speed is automatically met for the case of the EV. Of course,
`cruising range is another issue related to the battery design
`which is outside the scope of this paper. However, minimizing
`the power of the drive will help the battery size.
`
`C. Cruising at Maximum Vehicle Velocity
`The power requirement to cruise at maximum vehicle speed
`can be obtained as
`
`(10)
`
`this
`Since aerodynamic drag dominates at high speeds,
`power requirement
`increases with the cube of maximum
`vehicle velocity. If this vehicle power requirement is greater
`,
`than the motor power calculated previously
`will define the motor power rating. However,
`then
`will dominate
`, since modern vehicles
`in general,
`are required to exhibit a high-acceleration performance. As
`mentioned before, some extremely high-speed motors usually
`have three distinct modes of operation. The initial constant
`torque operation, followed by a range of constant power
`operation, then to the maximum speed in natural mode (see
`Fig. 3). For such a motor it may be advantageous to use
`
`Page 5 of 9
`
`TABLE I
`EV POWER REQUIREMENT AS A FUNCTION OF CONSTANT POWER RANGE
`
`the entire constant power range for initial acceleration of the
`vehicle. The operation beyond that would be in the natural
`mode. This would allow a longer constant power operation
`in the initial acceleration. Consequently, the motor power
`requirement will be lower. This scheme will work provided
`the motor has adequate torque in natural mode to meet the
`constraints at the maximum vehicle speed. Otherwise, some
`part of the constant power operation has to be used for the
`vehicle operation beyond the rated vehicle speed.
`Natural mode of motor operation is not the preferred mode
`beyond the rated vehicle speed. Unfortunately, no control
`algorithm presently exists to operate some high-speed motors
`entirely in constant power beyond their base speed. However,
`the natural mode, if included, can lower the overall power
`requirement. The speed at which the electric motor can enter
`the natural mode and still meet the power requirement at
`maximum vehicle speed is obtained from
`
`(11)
`
`Note that the initial acceleration power is also a function of
`(extended constant power range). Hence,
`and
`have
`to be solved iteratively. Also, the gear ratio between the drive
`shaft and the motor shaft is to be determined by matching
`with the motor speed at which it enters the natural mode. More
`discussion about the natural mode of operation appears in
`Section VI. The rest of the analysis is done assuming constant
`power operation beyond the base speed up to the maximum
`speed.
`The importance of extending the constant power speed
`range can be better understood by comparing the required
`motor power for different constant power speed ranges (as
`a multiple of its base speed). Table I shows an example of
`power requirement for several constant power ranges for the
`following case:
`1) maximum motor speed is 10 000 r/m;
`2) maximum vehicle speed is 44.7 m/s (100 mi/h);
`3) other system variables and constants are the same as the
`previous example.
`Here, the required gear ratio to match the maximum motor
`speed to the maximum vehicle speed for a wheel radius of
`0.2794 m (11 in) is 1:6.55. The results of Table I suggest an
`extended range of 4–6 times the base motor speed in order to
`significantly lower the motor power requirement.
`Finally, we examine the effect of maximum motor speed
`and the extended constant power range on the overall system
`performance. In the context of EV/HEV design, we classify
`motors with maximum speeds of less than 6000 r/m as
`low-speed motors, those with speeds of 6000–10 000 r/m as
`medium-speed motors, and those with speeds of 10 000 r/m
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`Fig. 5. Rated motor shaft torque as a function of maximum motor speed.
`
`Fig. 6. Drive shaft torque as a function of extended constant power range.
`
`and beyond as high-speed motors. The power requirement is
`not a function of the motor maximum speed. Motor maximum
`speed only affects the gear size. However, maximum speed
`has a pronounced effect on the rated torque of the motor. An
`example of this is illustrated in the surface plot of Fig. 5. Low-
`speed motors with extended constant power speed range have a
`much higher rated shaft torque. Consequently, they need more
`iron to support this higher flux and torque. Furthermore, higher
`torque is associated with higher motor and power electronics
`currents. This will also impact the power converter silicon
`size and conduction losses. Extended speed range, however,
`is necessary for initial acceleration as well as for cruising
`intervals of operation. Therefore, the rated motor shaft torque
`can only be reduced through picking a high-speed motor.
`This would, however, affect the gear ratio. A good design
`is the result of a tradeoff between maximum motor speed and
`the gear size. However, this tends to be more in favor of
`selecting a medium- or high-speed motor. For an extremely
`high-speed motor, a sophisticated gear arrangement might be
`necessary for speed reduction. Planetary gear arrangement [6]
`could be the choice that is compact but allows high-speed
`reduction. Extended constant power range, on the other hand,
`will increase drive shaft torque and stress on the gear, as can
`be seen in Fig. 6. Hence, another design trade-off is involved
`between the gear stress and the extended constant power
`
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`
`Fig. 7. Family of ICE force-velocity profile with ICE size as the parameter.
`
`range. It can be seen from the results of Table I that, after
`a certain point, there is not any appreciable power reduction
`with extended constant power range. Any further extension
`of constant power range beyond this point will only adversely
`affect the gearing and drive shaft appreciably, without reducing
`the power requirement. This will set the upper limit of the
`extended range of the constant power operation.
`Overall, the EV drive system design philosophy can be
`summarized as follows.
`1) Power requirement for acceleration decreases as the
`range of constant power operation increases, i.e., more
`specifically, as the ratio of the vehicle rated speed to
`motor rated speed increases.
`2) The gear ratio between the electric motor and the drive
`shaft is determined by the motor and vehicle maximum
`speeds.
`3) Power requirement for cruising at the maximum vehicle
`speed is obtained directly from the road resistance at
`maximum speed. In general, this power requirement will
`be lower than the initial acceleration power requirement.
`4) High-speed motors would be more favorable for EV
`application, in general.
`
`IV. HEV SYSTEM DESIGN
`In the series hybrid vehicle the electrical system design
`is identical to that of the EV. The ICE size is specified for
`keeping the batteries charged. The parallel HEV system design
`philosophy, however, requires an extension of the EV system
`design philosophy. The gear ratio (single gear) between the
`ICE and the wheel shaft can be obtained by matching the
`maximum speed of the ICE to the maximum speed of the
`wheel shaft. Following the same procedure as the EV system
`design, the variables for HEV system design are calculated to
`fulfill the design constraints, as shown below.
`
`A. Cruising at Rated Vehicle Velocity
`The ICE size is determined mainly by the vehicle cruising
`power requirement at
`its rated velocity. Fig. 7 shows an
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`EHSANI et al.: DESIGN OF ELECTRIC AND HYBRID VEHICLES
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`25
`
`TABLE II
`MOTOR DATA
`
`TABLE III
`RATED POWER AND CONVERTER VA REQUIREMENTS FOR
`THE MOTORS OF TABLE II FOR A TYPICAL EV APPLICATION
`
`V. ELECTRIC PROPULSION SYSTEMS
`FOR EV AND HEV DESIGN
`An electric propulsion system is comprised of three main
`elements: power electronic converter, motor, and its controller.
`This section is devoted to examining several of the most
`commonly used motors and their control for EV and HEV
`propulsion. The importance of extended speed range under
`motor constant power operation in an EV and HEV was
`established in the previous sections. This mode of operation
`is referred to as field weakening, from its origins in dc motor
`drives. A detailed study of several commonly used motors for
`EV and HEV propulsion application is presented in [7]. In this
`section, we present a design example of several motors for the
`constant power operation. This example will help clarify the
`capabilities of these motors for vehicle applications.
`EV Data:
`• vehicle rated speed of 26.82 m/s (60 mi/h);
`• required acceleration of 26.82 m/s in 10 s;
`• vehicle maximum speed of 44.7 m/s (100 mi/h);
`• vehicle mass of 1450 kg;
`• rolling resistance coefficient of 0.013;
`• aerodynamic drag coefficient of 0.29;
`• frontal area of 2.13 m ;
`• wheel radius of 0.2794 m (11 in);
`• level ground;
`• zero head wind.
`The motor data are shown in Table II. The motor data
`chosen are for the commercially available samples of these
`motors. Clearly, more specific motors can be designed for
`vehicle applications, but such data were not available for this
`paper. Based on the vehicle data, the power requirement to
`cruise at the maximum speed is 41 kW. The motor power for
`acceleration and converter volt–ampere (VA) requirement for
`each motor are shown in Table III.
`The extended constant power range available from the
`induction motor clearly makes it highly favorable for vehicle
`
`Fig. 8. Acceleration power requirement as a function of vehicle speed at
`which ICE is added.
`
`example of the road load characteristics on a 3% grade, with
`other constants remaining the same as before. The figure
`also shows a series of force-velocity curves of the ICE
`(the throttle wide open) with the piston displacement as the
`variable. The correct ICE size can be determined from the
`intersection of the road load curve with the ICE force-velocity
`profile at rated velocity, plus allowing a 10% margin for the
`batterypack recharging. The exact amount of margin needed is
`the subject of a more complicated analysis involving vehicle
`driving cycles, battery size, charge/discharge characteristics,
`etc.
`
`B. Initial Acceleration
`The rated power to be delivered by the electric motor is
`reduced in the case of the parallel HEV due to the mechanical
`power available from the ICE. An example of the effect of ICE
`torque blending on the rated power requirement of the electric
`motor during initial acceleration is shown in Fig. 8. The figure
`shows four different extended speed range operations of the
`electric motor. The abscissa is the vehicle speed at which the
`ICE torque is added. In all of the four cases, the ICE with
`its low starting torque contributes little up to about the vehicle
`speed of 20 mi/h. Therefore, this low-speed and low-efficiency
`operation of the ICE may be avoided without significantly
`increasing the electric motor power requirement. It is important
`to note that an extended constant power operation of the
`electric motor is still a necessity to keep the power requirement
`low (Fig. 8).
`
`C. Cruising at Maximum Velocity
`At maximum vehicle velocity the power requirement is
`. This power is supplied by a combination of the
`ICE and the electric motor. Once the ICE size is determined,
`the required electric motor power can be uniquely identified.
`As mentioned before, this power, in general, would be less
`than the power requirement for the initial acceleration.
`
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`26
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`IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS, VOL 44, NO 1, FEBRUARY 1997
`
`TABLE IV
`COMPARISON OF GENERAL MOTORS EV, IMPACT, AND OUR DESIGNED EV
`
`application. On the other hand, the limited constant power
`range of the BLDC motor makes it appear inferior to the
`induction motor, despite its high power factor and high ef-
`ficiency. The extremely high speed operation of the SRM and
`its relatively longer constant power range helps it to overcome
`some of the difficulty associated with its lower power factor
`operation. Furthermore, the SRM converter is simpler and
`easier to control.
`
`VI. TEST DATA
`In this section, an EV and an HEV prototype are discussed.
`The actual design specifications of these vehicles are compared
`with our theoretical design of these same vehicles, based on the
`ideas presented in this paper. The EV is the General Motors
`Corporation IMPACT car and the HEV is the Texas A&M
`University ELPH car.
`
`A. General Motors EV IMPACT
`General Motors announced the first version of its EV,
`IMPACT, in January 1990. Over the years, there have been
`several modifications of the IMPACT. The following are the
`most recent specifications of the IMPACT. We have included
`only those features which are pertinent to this study.
`Performance:
`1) 0–26.82 m/s (0–60 mi/h) acceleration in 8.5 s;
`2) top speed of 35.76 m/s (80 mi/h).
`Dimensions:
`1) frontal area 2.2578 m ;
`2) drag coefficient 0.19;
`3) curb weight 1347.17 kg.
`Design Features:
`1) 102.16-kW three-phase induction motor;
`2) IGBT power inverter module—102 kW;
`3) high-speed rated 205/50 R15 tires.