Non-polluting Automobiles
IEEE transactions on Vehicular TechnologY, VOL 43, No 4, Nov 94
G.J.Hoolboom, P.Eng., Consultant (formerly with Westinghouse Canada)
B.Szabados, IEEE SM, P.Eng., McMaster University
ABSTRACT - The advantages/disadvantages of energy storage devices, which can provide nonpolluting automobile systems are discussed. Four types of storage devices are identified:
- electro-chemical (batteries)
- hydrogen
- electro-mechanical (flywheels)
- Molten salt heat storage
A high speed flywheel with a small permanent magnet motor/generator has more advantages
than any of the other systems and might become a real competitor to the internal combustion
engine. A flywheel/motor/generator system for automobiles becomes now practical, because of
the technological advances in materials, bearings and solid state control circuits. The motor of
choice is the squirrel cage induction motor, specially designed for automobile applications. Some
of the special design considerations are: high speed (up to 16000 rpm), high break-down torque,
83% coil pitch, stranded stator copper wire, and special cooling arrangements. The preferred
controller for the induction motor is a forced commutated cyclo-converter, which transforms a
variable voltage/variable frequency source into a controlled variable-voltage/variable-frequency
supply. A modulation strategy of the cyclo-converter elements will be selected to maintain a
unity input displacement factor (power factor) under all conditions of loads voltages and
frequencies. The system is similar to that of the existing automobile, if only one motor is used:
Master controller - Controller - motor - gears (fixed) - differential-wheels. In case of two
motors, the mechanical differential is replaced by an electric one: Master controller - Controller
- motor - gears (fixed) - wheel. A four wheel drive vehicle is obtained when four motors with
their own controllers are used.
I. INTRODUCTION
T
he interest in non-polluting automobiles increased significantly when the California Air Resources
Board (CARB) mandated that 2 percent of all vehicles lighter than 1700 kg (3750 lbs), sold by
each manufacturer in the state in 1998, be zero- emission vehicles [ 3 ]. Furthermore the CARB rules
require that the proportion of zero-emission vehicles increase to 5 % by 2001 and to 10% by 2003.
The rules also require reduced emissions from conventional vehicles.
The term "non-polluting" is only relative : while the electric automobiles do not pollute, the
original source of the energy (electricity) does cause pollution during its generation. However the
centrally generated pollutants can be controlled (and reduced) much easier than the distributed
pollution sources of individual vehicles.
A vehicle of 2200 kg requires approximately 78 kWh of stored energy and a power of 94 kW, to
have a range of 200-250 km and a reasonable acceleration of 10-96 km/h in 10 sec. (see Appendix).
A lighter vehicle requires less stored energy and power, however the new energy storage devices and
drive systems will add more weight, offsetting any weight reductions of existing vehicles.
This paper analyzes the present energy storage options in view of using them on an electric moving
vehicle (EV). The actuator motor technology and power controller technology is then considered.
Finally, based on this analysis, an optimum system configuration is proposed.
II. ENERGY STORAGE TECHNOLOGY
A- Electrochemical devices
Research and development to find and improve batteries as a energy source for Electric Vehicles
has been active since 1945 and some progress has been made. Presently only lead-acid is the
commercially economically viable battery technology suitable for Electric Vehicles and it is far from
ideal. EV's have a limited range and relative poor performance because of the weight and volume of
the battery. Furthermore the life of the battery is rather limited.
Electrochemical devices must meet the following constraints for EV applications:
- high specific energy density (Wh/kg and Wh/l)
- high discharge rate or specific power (W/kg)
- high number of deep discharge cycles (life cycles)
These constraints are mutually exclusive using presently available materials. A battery can be
designed with a high specific energy density and a low discharge rate. If the discharge rate is
increased, the specific energy density is reduced. Furthermore a high discharge rate will reduce the
number of discharge cycles (life cycles).
Table I shows a number of promissing EV battery technologies which are being pursued in
several laboratories [ 4, 6 ]:
Table I
Wh/kg
Wh/l
W/kg
Lifecycles
Lithium/monosulfide
60-200
120-300
50-70
150-200
Zinc/Bromide
70-100
50-75
30-50
200-350
Nickel/Zinc
60-100
120-300
80-120 100-150
Nickel/Iron
50-90
100-150
80-120 500-1100
Nickel/metal hydride
50-90
150-300 100-140
80-500
Sodium/ Sulfur
70-200
100-150 100-400 300-900
Lead acid
30-60
30-60
20-80
300-500
The lead acid battery is included for comparison and it is clear that all of the new technologies
show promise for considerable improvement. However significant work has to be done, before
commercial batteries are available for EV's. The difficulty of obtaining high values for all those
parameters, has lead to some suggestions that EV's might best be powered by a pair of batteries. The
main unit would be optimized for range (specific energy) and another unit for acceleration and hill
climbing (specific power). This second unit would be recharged from the range unit, during stops or
less demanding driving.
Other parameters, which must be met for the new EV batteries, are shown in Table II [ 4 ]:
TABLE II
Power density (W/l)
Life (years)
Life (cycles)
Ultimate cost ($/kWh)
Recharge time (hours)
250 - 600
5 - 10
600 -1000
100 - 150
36
TABLE III
Wh/kg* (* excluding the container.)
Hydrogen 39,400
Gasoline 12,880
B- Hydrogen
Hydrogen seems to be an ideal non-polluting fuel for motor vehicles : it burns cleanly, leaving just
plain H2O as a result of the oxidation process. Hydrogen is by far the most abundant element in the
universe and it is attractive as a fuel because it has the highest energy density per unit of weight of any
fuel [ 16, 24 ], see Table III.
The present methods of storing hydrogen are suitable and safe for the industrial uses, but are
unacceptable for moving vehicles. For example, hydrogen stored as a compressed gas calls for large
and heavy containing vessels. At a typical pressure of 136 atmosphere, hydrogen gas in a steel
container weighs about 30 times more than the equivalent amount of gasoline. The same container
takes up to 25 times more space than a gasoline tank (holding the equivalent amount of gasoline).
Liquid hydrogen would improve the specific energy density, but it is extremely cold (it boils at 20
degrees Kelvin or -253 degrees Celsius) and it is highly volatile if, it is spilled.
Another way of storing hydrogen is as a metal hydride [ 24 ]. Most metals will form metal
hydrides. In many cases the reaction is simple and direct, consisting of bringing gaseous hydrogen (H2)
in contact with the metal (M) : M + H2 ® MH2 . This reaction is reversible and its direction is
determined by the pressure of the hydrogen gas and the temperature. If the pressure is above a certain
level, the reaction proceeds to the right to form the metal hydride.
The primary reason metal hydrides have been proposed for the storage of hydrogen is that they
← Indeed it is possible to pack more hydrogen into a metal
accommodate a high density of hydrogen.
hydride than into the same volume of liquid hydrogen. Table IV shows the specific densities of a
number of hydrides, as compared with pure hydrogen (liquid and compressed gas) and gasoline:
TABLE IV
Wh/kg*
Wh/l*
- Magnesium Hydride
2758
3978
- Magnesium-Nickel Hydride
1245
3191
- Vanadium Hydride
815
3751
- Iron-Titanium Hydride
689
3782
- Lanthanium-Penta Nickel Hydride
539
3506
- Liquid Hydrogen (LH2)
39,400
2758
- Gaseous Hydrogen (GH2) @ 100 atm. 39,400
365
- Gasoline
12,880
9688
(*Excl. the container)
Metal hydrides are inherently safe and release their hydrogen when heat is applied. This heat can be
obtained from the waste heat of the engine, a standard internal combustion engine. The chemical
reaction is bi-directional and the system will hold up under many cycles of charging and discharging.
A number of experimental vehicles has been built, using metal hydrides as a fuel carrier for hydrogenpowered automobiles. In automotive applications weight is a critical factor and metal hydrides suffer
from a considerable weight penalty compared with gasoline. However they can be quite competitive
with the electric battery. A disadvantage of hydrogen as a fuel is that no infra-structure for the distribution of hydrogen exists. A large investment is required to establish a series of "hydrogen"-stations
around the country. It probably would be more economical to swap complete metal hydrides blocks
which are recharged in a few central facilities.
C- Flywheels
The use of flywheels for storing energy is not a new concept. More than 25 years ago, the Oerlikon
Engineering Co. of Zurich, Switzerland made the first passenger bus, solely powered by a flywheel.
This 64 inch flywheel weighed 3300 lbs and was running at 3000 rpm in a hydrogen atmosphere to
reduce windage losses. The 70 passenger bus had a limited range (approx. half a mile) and at each bus
stop, a pantograph was raised to electric lines and the flywheel was recharged for two minutes. The
"wireless" trolley bus system operated until 1959, when the last of the three dozen Oerlikon busses
was retired [21].
Oerlikon also developed a number of small, flywheel driven locomotives, for use in steelworks,
mines and railyards.
The kinetic energy stored in a flywheel is given by the equation :
E = 1/2 *J * w2
where
J = moment of inertia, and w = angular rotating speed
This energy is proportional to the rotational speed squared and by increasing this speed one can
store more energy. However there is a limit : the stress in the material goes also up with the speed
squared and once the maximum tensile strength is exceeded, the flywheel disintegrates.
The shape of the flywheel is important. It should be designed in such a way that the stress of the
material is the same throughout. The crossection of an optimized disk flywheel is shown in figure 1
and this type of design can store substantially more energy. All of the older types of flywheel have
been made of materials equally strong in all directions, however some materials have significantly
higher tensile strengths in one
particular direction : piano wire,
carbon fibres, glass fibres,
Kevlar fibres, etc. This has lead
to a new type of flywheel
design, where the high stress
fibres are embedded in a matrix
Figure 1: Crossection of optimized disk
of plastic (epoxy). Those fibres
are aligned in such a way that
the centrifugal force acts along
the fibre in the direction of its
high tensile strength( figure 2 )
[5, 6, 14, 18, 21]. A number of
the bars (displaced a few
degrees) can be combined to
Figure 2: Bar design of laminated flywheel
form an optimized disk.
The specific energy density (Wh/kg) of a flywheel is proportional to the ratio of tensile strength/
specific density [5, 6, 19 ]. A number of materials show promise as shown in Table V.
TABLE V
Design
Specific
Ratio:
Theoretical
Practical
Tens.Strength
Density
Tens.Str./Dens.
Energy Dens.
Energy Dens.
2
3
N/m
kg/m
Nm/kg
Wh/kg
Wh/kg
Carbon 750
1,550
483,100
515
57.5
E-glass
250
1,900
131,600
140
14
S-glass
350
1,900
184,200
196
19.6
Kevlar
1000
1,400
714,300
762
76.2
Maraging steel 900
8,000
112,500
240
24
The estimated practical energy density is significantly lower than the theoretical value, because of
the additional weight of the bearings, motor/generator, shaft and containement vessel.
The flywheel rotates in a vacuum to reduce windage losses. This vacuum, will be checked in a
service station regularly. Magnetic bearings are the only suitable type operating at high speed in a
vacuum with low losses [ 10 ]..To extract energy from the flywheel, an electrical generator is used.
This generator must be able to withstand high rotating speeds in a very difficult cooling medium
(vacuum).
Three types of electrical machines can be used for this purpose :
1. Permanent magnet motor/generator.
2. Claw pole type motor/generator.
3. Inductor type motor/generator.
All three types have brushless rotors, which can be designed for high speed operation. The
permanent magnet motor/generator has no field winding and the voltage will therefore increase with
speed. The advantage of this type of machine is that the efficiency is significantly higher than the other
two types. The claw pole motor/gerator has a rotor consisting of two claw like pole pieces and a nonrotating field winding around a flux carrying rod. The inductor type motor/generator [11] has a field
winding in the stator and a rotor with large slots, causing the magnetic flux to pulsate. The pulsating
flux generates a voltage in the AC stator winding.
The electrical machine of choice for this application is the permanent magnet motor/generator,
because of its high efficiency. The stator winding has to be cooled by using a liquid, circulating
through the hollow conductors. The stator core is cooled by conduction, through the stator winding and
through the vessel wall.
A matter of concern in any system intended for use in automobiles is the safety factor. A number of
accidental events can occur in a flywheel based system :
- Loss of vacuum: The temperature of the air, the flywheel and casing will increase and the speed
of the flywheel will decrease rapidly.
- Overspeed: The diameter of the flywheel will expand and it will rub against the casing. The
resulting friction will cause the flywheel to slow down. The temperature of the flywheel and the
casing will increase, but no physical damage will occur.
- Damage to the casing: This will result in a loss of vacuum and/or the flywheel will hit the casing.
The stored energy will be dissipated in the disintegration of the flywheel , but will be contained
within the containment vessel.
A typical flywheel/motor/generator combination will have a top design speed of 30,000 to 50,000
rpm, and a minimum design speed of 7,500 to 12,500 rpm,. This 4:1 range would utilize about 94 % of
III. MOTOR TECHNOLOGY
The conventional internal combustion engine, with only minor modifications, can be used with
hydrogen as fuel. However the efficiency of the engine is very low (approx. 35 %), which makes this
an unattractive option, considering the high cost of the hydrogen fuel. Using a high efficiency fuel cell
to convert the hydrogen into electricity and using
an electric motor, will improve the overall
systems efficiency and will result in a more
practical system.
All other fuel systems will provide electrical
power with the exception of the molten salt heat
storage system, where the Stirling engine is used.
The electrical motor is therefore the motor of
choice. The squirel cage induction motor is the
prefered machine for heavy duty, high speed
Figure 3: Flywheel with electric
transportation applications under a variety of
Torque, Current(p.u.)
1
Motoring
Current
Torque
rotor frequency
Speed(p.u)
1
2
3
4
Braking
Torque
Current
-1
Figure 4: Torque/Current-speed requirements of a traction system
ambient conditions, because of its rugged, brushless rotor. Automobile applications require the motor
to operate under full field and constant torque conditions from zero to base speed and under constant
power (weak field) conditions from base speed to top speed (figure 5).
To reduce weight a high speed motor and fixed gears should be used. Typical speeds are base
speed of 4,000 rpm, and top speed of 16,000 rpm.
The power converter technology to be used, can either be pulse width modulation techniques or
direct AC to AC conversion with cyclo-converter techniques. In both cases the converter would
generate a high harmonic content for which the motor must be designed. Furthermore, the induction
motor must operate in an adverse environment characterized by ambient temperatures between -30 C
and 45 C, and splashing of water, snow and dirt will occur. Furthermore corrosive atmosphere and
vibration and shocks would be experienced.
Electrically, special designs have to be considered to
overcome some of the inherent weaknesses of the induction
motor:
A. Weak field operation.
The breakdown torque decreases approximately with
the square of the speed ( TBD = To/n2 ), but the required
motor torque in traction applications decreases linearly
with speed ( TR = 1/n). The induction motor can typically
only be utilized up to 80 % of the breakdown torque, so the
maximum speed obtainable will be: nmax = .8 * To
If a maximum speed of 4 per unit is required, the motor
must be designed with a breakdown torque of: TBD = 4/.8
= 5, which means an overdesigned motor by 1.4 - 2.5 times.
Figure 5: Braking circuit
B. Braking torque.
The required braking torque in traction applications is much larger than the motoring torque. To
obtain a higher braking torque in the "weak field" region of operation, a special braking circuit could
be used (figure 6). This circuit consists of three braking resistors in series with the motor windings
and its starpoint. Three thyristors ( or transistors) are connected in parallel with the resistors and the
effective resistance is therefore determined by the firing angle of the thyristors.
A power flow versus frequency for operating under motoring and braking conditions is shown in
figure 7. The seemingly odd shape of the "regenerated power" curve is a direct result of the varying
phase angles of the currents and voltages. A disadvantage of this circuit is the generation of harmonics
in the motor current, which would increase the motor losses.
Another possibility to improve the braking torque of the induction motor in the "weak field" region
of operation, is to use the circuit shown in figure 4. During motoring, the switch "2" is in the "on" state
and both windings are connected in series. Note that the windings are displaced by thirty electrical
degrees, which improves the waveform of the airgap flux, reducing the motor losses. During braking,
the switches "1" are both in the "on" state and both windings are connected in parallel : the motor flux
and its torque are doubled.
C. Other special design considerations.
Power (p.u.)
Motoring
1
Speed (p.u.)
1
Braking
-1
2
3
Regenerating
region
Dissipating
region
Figure 6: Power flow
Figure 7: Series/parallel connec-
4
Efficiency
Optimum V/f
Constant V/f
Load
Figure 8: Efficiency improvement with optimal slip control
motoring , 200% load
motoring , 100% load
Voltage (p.u.)
zero load
generating , 100% load
generating , 200% load
1
1
Frequency (p.u.)
Figure 9: Voltage/frequency relationship at different load levels.
- General: Low primary resistance, low primary leakage reactance and low secondary leakage
reactance to provide a high break-down torque.
- Stator: Thinner laminations and high Si-content iron to reduce iron losses. Coil pitch of 83 % to
reduce the 5th & 7th harmonic. Stranded copper wire to reduce eddy current losses.
- Rotor: No deep bar rotor to reduce skin-effect losses. Stainless steel retaining rings shrunk over
the copper end-rings to contain centrifugal forces. Smooth rotor surface and elimination of the
blower to reduce windage losses.
- Cooling: Oil spray over the stator winding end-turns with the oil circulating through an external
cooler.
IV. CONTROLLER TECHNOLOGY
A- Inverters
When batteries or hydrogen (+ fuel cell) are used as the primary fuel, the resulting electrical power
is available in the form of DC power. An inverter is required to convert the DC into AC power,
suitable for driving an induction motor. The pulse width modulated (PWM) inverter is the prefered
controller, because of its flexibility and relatively low harmonic output distorsion. To reduce
Source
Source
Load
Load
Figure 10: 3-pulse forced
commutated cyclo-converter
Figure 11: 6-pulse forced commutated
cyclo-converter
Source
Load
Figure 12: Matrix converter
Figure 13: Battery fueled vehicle
switching losses at the relatively high switching frequencies, transistors are used for the power
switches. For low power applications, the MOSFET transistor can be used. For higher power the
Insulated Gate Bipolar Transistor (IGBT) is a good candidate [ 3 ]. The IGBT is currently available in
voltages up to 600 V and currents up to 750A. Using a microprocessor (microcontroller) to control the
PWM inverter, provides the flexibility to incorporate optimizing strategies.
In conventional drive systems, the induction motor is driven under constant V/f. There is only one
torque-speed curve which meets a load torque curve at a given reference speed. Thus the slip is fixed.
However if the induction motor is driven under variable V/f, there are many torque-speed curves
which meet a load curve at a given reference speed. Therefore each slip is different. The efficiency of
the motor is dependent upon the slip and therefore must have an optimum value : the maximum
efficiency slip. Such a strategy can easily be implemented with the microprocessor, resulting in
significant efficiency improvements at reduced loads as depicted in figure 8 [7 ].
At very low speeds, the V/f curve of induction motors is non-linear and load dependent (see
fig.10). It is clear that any calculations based on simplifying assumptions will lead to severe under- or
over-excitation of the motor, resulting in either torque loss or additional motor losses. One solution is
the direct measurement and feedback of the airgap flux in the motor. However this has the
disadvantage of specially designed motors, more complex wiring and the risk of a broken wire
carrying the feedback signal under operating conditions. A more elegant method is the derivation of the
airgap flux level from terminal measurements of currents & voltages, in phase and magnitude [12].
This method is also called "vector control" and is used to control torque seperately from flux, as with
the DC motor.
B- Converters
When flywheels are used as the prime source of energy, the available electrical power is AC, with
a variable frequency and variable voltage. The required power for the induction motor is also AC,
with a different adjustable frequency and voltage. The desired converter is therefore a direct AC-AC
converter and the logical choice would be the cyclo-converter. The classical cyclo-converter is the
naturally commutated cyclo-converter [22]. However this type of converter has a limited output
frequency (approx. 50 % of the input freq.) and a lagging power factor depending upon the load power
factor. This lagging input power factor has a demagnitizing effect on the permanent magnet generator of
the power source, which is very undesirable.
Recharging Circuit
Auxilliaries
Auxa
il re
is
Figure 14: Flywheel driven
vehicle
Recharging Circuit
Another family of direct AC-AC
converters are the forced commutated
cyclo-converters. Depending upon
Figure 15: Four wheel hybrid system, using a
the modulation strategy, different
flywheel and a battery.
members of the family can be
obtained [ 8 ].
- Unrestricted frequency changer
- Slow switching frequency changer
- Controllable displacement factor frequency changer
- Unity displacement factor frequency changer
The input displacement factor of the last two types can be varied or kept at unity, without the use of
passive reactive components, and independently of a changing lagging power factor load. The reactive
and extrabasal (harmonic) input VA remains practically zero under all balanced load conditions. The
extrabasal current components circulate between the internal elements of the cyclo-converter,
bypassing the input source [8].
The power circuit of a 3-pulse and a 6-pulse forced commutated cyclo-converter is shown in
figures 11 and 12 respectively. The 6-pulse converter has lower harmonics in the output, but requires
double the number of power switches and the induction motor must have non-connected stator
windings. A different control algoritm will result in a matrix converter (figure 13), which is
topologically identical to the 3-pulse forced commutated cyclo-converter (figure 11). However the
matrix converter has better spectral characteristics [9] and higher available fundamental output than
the 3-pulse converter, for symmetrical PWM modulation. A disadvantage of the matrix converter is
that it includes subharmonic components for both input and output sides; however their amplitudes are
small.
In summary the cyclo-converter is an direct AC-AC converter, whereby the output waveform is
constructed from input segments. The forced commutated cyclo-converter has an additional degree of
freedom in its modulation strategy and this can be used to maintain a unity input displacement factor.
The advantages of this type of cyclo-converter are:
- unity input displacement factor
- bidirectional power flow
- Relatively low switching losses
- High overall system efficiency, because of the single stage power conversion.
- Low harmonic distortion in the output, specially with a high ratio of input/output frequency.
- Equal load sharing of the power switches, even at very low output frequencies.
V. SYSTEM TECHNOLOGY
A schematic of a battery fueled vehicle with a single motor is shown in figure 10. The induction
motor has two stator windings, each connected to its own inverter. Note that those inverters are rated
for only half the full motor rating. The inverters can be connected in series or parallel, depending upon
the relays "1" or "2". During motoring, the series connection is used (as shown in figure 10), while
during regenerative braking the parallel connection is used (doubling the field and thereby the torque).
Instead of mechanical relays, power transistors may be used for the switches. If two motors are used
in the vehicle, each motor will have only one winding and one inverter. Again the two inverters may
be connected in series or parallel.
A practical hydrogen fueled vehicle will use a fuel cell to generate electrical power. The prefered
storage medium is a metal hydride, from which the hydrogen can be released by application of heat.
The released hydrogen gas is stored in a "collection" vessel, from which a controlled amount of gas
will flow through the fuel cell. The hydrogen is "burnt" in the fuel cell, resulting in electrical power
and water. The DC power is converted into AC, by means of an inverter (controller), to drive an
induction motor.
A schematic of a flywheel driven vehicle is shown in figure 14. The variable speed of the flywheel
is causing a variable frequency and variable voltage output of the permanent magnet generators. The
output of these generators is converted into a controllable AC power which is driving the induction
motors. A master controller (not shown) will regulate the appropriate frequency difference between
the two motors when the vehicle makes a turn, which is similar to the mechanical differential function.
The "recharging circuit" is not on board of the automobile, but is located at home or at a charging
station, where the car can be "plugged in". Auxiliaries are kept alive through a small battery, which in
turn is fed from a rectifier.
A hybrid system consisting of a flywheel and a battery is shown in figure 15 [10,11]. The flywheel
will provide the energy during the demanding parts of the operating cycle, while the battery will
"recharge" the flywheel during the slow speed and stand-still parts of the driving cycle.
Note:
The term "hybrid" is sometimes used to mean the combination of a small conventional
internal combustion engine, driving a generator, a battery and a flywheel. One or more electric motors
would be used, each with their own controller [2,13,20]. Such a system does not qualify under the
term " non-polluting" and is therefore excluded from any consideration in this paper.
VI. CONCLUSIONS
The recently passed "zero emission requirements" law in California, is stimulating the interest in
and development of non-polluting automobiles. The first types of vehicles will probably be special
purpose cars, with a limited range and performance. Furthermore a number of commercial vehicles
(busses, delivery vans, maintenance vehicles, etc.) and/or rapid transit trains [15], will be introduced
as test beds for the new technology.
In this paper we have reviewed the advantages/disadvantages of a number of non-polluting energy
storage devices :
- batteries
- hydrogen
- flywheel
- molten salt heat storage
None of these devices have all the ideal characteristics for automobile applications.
We have attempted to chose from the surveyed technologies, one optimal system. It would consist
of a high speed flywheel rotating in vacuum, direct connected permanent magnet motor/generator, a
power converter and a high speed squirrel cage induction motor. The power converter would be a
forced commutation cyclo-converter which would provide the transfornation from variable voltage/
variable frequency service to a controlled variable voltage/variable frequency supply to the motor,
with high efficiency and no need for storage elements.
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B. Giera et al., "Hybridantrief mit Gyro-Komponente für wirtschaftliche und dynamische
Betriebsweise", Elektrotechnische Zeitschift, Ausgabe A, 94, Jahrgang Heft 11, S.653-660,
Nov. 1973.
A.P.Armagnac, "Super flywheel to power zero-emission car", Popular Science, pp.41-43
August 1970.
G.J.Hoolboom, "A polyphase, all solid state cyclo-converter", International Electrical and
Electronics Conference, Toronto, Paper 63-1040, October 1963.
Marks' Standard Handbook for Mechanical Engineers, pp 11-3 to 11-5, Eight edition.
J.J.Reilly and G.D. Sandrock, "Hydrogen Storage in Metal Hydrides", Scientific American.
APPENDIX
Energy and Power requirements for a practical automobile.
A. Vehicle performance specifications.
The amount of strored energy and maximum power extraction depends upon the vehicle
performance specifications, the conversion efficiency and the efficiencies of the components in the
drive train.
The performance specifications should be somewhat similar to those of existing automobiles, in
order to obtain acceptance in the market place [23].
Typical specs are :
Mass Vehicle (incl.fuel) 1600
Passengers
400
Luggage
200
Total 2200
Range
kg
kg
kg
kg
250 km at a constant speed of 48 km/h, or :
200 km at a constant speed of 88 km/h, or :
10 km, 10% slope at a constant speed of 48 km/h.
Speed
Maximum 120 km/h.
Acceleration
From 0-96 km/h in 10 sec.
Deceleration
From 96-0 km/h in 7 sec.
Rapid recharge Fully charged in less than 40 min.
Slow recharge Fully charged in 8 hours.
Aux. PowerAirconditioning/heating
3 kW
Windshield wipers
.1 kW
Lights
1 kW
Radio/Hifi eqpt.
.2 kW
Power windows
.2 kW
Motor cooling pump & fan
1.5 kW
Total
6 kW
Efficiencies
Motors
Gearboxes
Controllers (inverters)
Controlled rectifier/converter
Flywheel generator
Batteries
95
99
97
98
95
92
%
%
%
%
%
%
B. Energy requirements.
An evaluation of the energy requirements for each of the three range specifications will be made :
a. No gradient, constant speed of 48 km/h and a distance of 250 km.
Required traction force [23] :
1.1 * 100 * 4.448 = 500 N
Total distance traveled :
250 * 1000 = 250,000 m
Total time to travel 250 km :
(250/48) * 3600 = 18,750 sec.
Aux. power requirements :
6 * 1000 *18,750 = 112,500,000 Joules (watt.sec.)
Energy used during the trip :
112,500,000 + 500 * 250,000 / (.95 *. 99 * .97 * .98 * .95) =
112,500,000 + 147,173,561 = 259,673,561 J = 72.13 kWh.
b. No gradient, constant speed of 88 km/h and a distance of 200 km.
Required traction force [23] :
1.1 * 200 * 4.448 = 980 N.
Total distance traveled :
200 * 1000 = 200,000 m.
Total time to travel 200 km :
(200/88) * 3600 = 8,182 sec.
Aux. power requirements :
6 * 1000 * 8,182 = 49,092,000 Joules
Energy used during the trip :
49,092,000 + 980 * 200,000/(.95 * .99 * .97 * .98 * .95) =
49,092,000 + 230,768,143 = 279,860,143 J. = 77.74 kWh.
c. Gradient of 10%, constant speed of 48 km/h and a distance of 10 km.
Required traction force [23] :
1.1 * 564 * 4.448 = 2800 N.
Total distance traveled :
10 * 1000
= 10,000 m.
Total time to travel 10 km :
(10/48) * 3600 = 750 sec.
Aux. power requirements :
6 * 1000 * 750 = 4,500,000 Joules
Energy used during the trip :
4,500,000 + 2800 * 10,000/(.95 * .99 * .97 * .98 * .95) =
4,500,000 + 32,966,878 = 37,466,878 J. = 10.4 kWh.
A minimum energy storage of 78 kWh is required to give the automobile a range of 200-250 km.
This is based on a total vehicle weight of 2200 kg ; and is significantly less if a lighter vehicle is
used.
C. Power requirements.
The power requirements depend upon the max. acceleration/deceleration rates of the vehicle
and the allowable time to recharge the batteries, the metal hydrides or the flywheel.
a. Acceleration (0-96 km/h in 10 sec.)
Acceleration rate :
96,000/(3600 * 10) = 2.67 m/s2
Acceleration force :
2200 * 2.67
= 5,867 N.
Total distance traveled :
.5 * 2.67 * 10 * 10 = 133.5 m.
Total transient energy required :
5,867 * 133.5/(.95 * .99 * .97 * .98 * .95) = 922,183 Joules
Power required :
922,183/10
= 92.2 kW
b. Deceleraration ( 96-0 km/h in 10 sec.)
Deceleration rate :
96,000/( 3600 * 10) = 2.67 m/s2
Deceleration force :
2200 * 2.67
= 5,867 N.
Total distance traveled :
.5 * 2.67 * 10 * 10 = 133.5 m.
Total transient energy returned to battery or flywheel :
5,867 * 133.5 /(.95 * .99 * .97 * .98 * .95) = 922,183 Joules.
Power required :
922,183/ 10
= 92.2 kW.
c. Slow recharge ( 78 kWh in 8 hours).
Power required :
78/8
= 9.75 kW.
d. Rapid recharge (78 kWh in 50 minutes).
Power required :
78/(50/60)
= 93.6 kW.
A minimum power of 94 kW is required to meet the specified acceleration/deceleration rates.