TECHNICAL FIELD
The invention relates to a method for driving a motor
vehicle and to a drive system for a motor vehicle, for the
exclusively electro-motorised movement of which at least
one electric motor is indirectly or directly connected to a
drive shaft of the motor vehicle or at least the drive hub
of a wheel of the motor vehicle. The at least one electric
motor is supplied with electrical energy from an energy
storage unit, the energy storage unit in turn being
supplied with electrical charge current from a generator,
which is driven by a gas engine.
STATE OF THE ART
Electro-mobility is of increasing interest in
industrialised countries, but its actual distribution and
use, in particular in electric vehicles, is far less than
expected and wished for in industry and politics. The
reason for this, on the one hand, is the short operating
range of electric-motor-driven motor vehicles and, on the
other, the up-to-now excessive purchasing cost. The user’s
high expectations with regard to comfort and range are
forcing the technological development to optimise the
degree of efficiency of all components involved in the
electric drive chain, which means a similar increase in
manufacturing cost. As an example, highly-efficient
synchronous electric motors are used for the direct drive
of the wheels, which are supplied with power from lithium
ion batteries with particularly high energy density.
In addition, apart from the current known operating range
problem, exclusively electrically driven vehicles are also
suffering from an operating problem, which has its origins
in the excessively long charging times of the electric
energy storage unit which the vehicle carries along, and in
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the charging-point infrastructure which up to now has not
been sufficiently widespread.
In order to avoid or reduce the above-mentioned problems
motor vehicles with hybrid drive variants are on offer, in
which drive systems are used which are based on both
electric motors and combustion engines. In particular in
cases, where both a combustion engine and at least one
electric motor are provided for directly driving the motor
vehicle, gear and clutch units, which are constructionally
complex and expensive with regard to manufacture, are
required, a fact which, apart from a considerable
battery/accumulator weight, is ultimately reflected in the
overall weight of the motor vehicle.
On the other hand, hybrid drive concepts for electrically
driven motor vehicles are known, for the movement of which
at least one electric motor connected to a drive shaft is
used, and for the energy supply of which a rechargeable
battery / accumulator is present. For the purposes of an
electrical energy supply extending beyond the charge
capacity of the battery an additionally provided combustion
engine, e.g. in the form of a Diesel or petrol engine, is
used, which is connected to a generator for power
generation, which is used for charging the battery / the
accumulator. A drive concept of this kind is explained in
detail in the publication DE 41 21 386 A1, where, via a
motor vehicle trailer coupling of a motor vehicle driven
exclusively with the aid of an electric motor, an energysupplying
trailer, which can be coupled and decoupled is
available, which comprises a combustion engine as drive for
a generator. The electrical energy which can be generated
with the aid of the generator driven by the combustion
engine is used for charging the vehicle battery, thereby
allowing the operating range achievable with the electric
motor vehicle to be increased by exclusively using the
total charge stored in the vehicle battery.
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A similar drive concept with an energy-supplying trailer
for electric cars has been disclosed in the publication DE
94 04 746.4 U1, where a petrol engine is provided for
driving the generator sitting on the energy-supplying
trailer. A further comparable power aggregate on a trailer
for electric motor vehicles is known from the DE 37 32 869
A1.
Apart from using Diesel or petrol engines for driving the
generator for power generation, the publication De 10 2009
045 979 A1 proposes to use a gas turbine, which is operated
with a mixture of fresh air and a solid, liquid and/or
gaseous fuel, thus enabling it to operate with optimised
efficiency and with fewer emissions compared to
conventional Diesel or petrol engines. Comparable hybrid
drive systems with a combination consisting of gas-turbine
and generator as electric power source for charging the
battery /accumulator unit necessary for driving the
electric motor have been disclosed in the publications DE
10 2009 000 530 A1, DE 10 2005 035 313 A1 and US
2001/0017532 A1.
Apart from Diesel, petrol and gas-turbine drives for
driving a generator for power generation and charging an
accumulator / a battery as a power supply for a motor
vehicle driven exclusively with the aid of at least one
electric motor, it is also known to use a gas engine driven
by liquefied petroleum gas (LPG) or compressed natural gas
(CNG), thereby achieving lower exhaust gas emissions
compared to Diesel and petrol engines. Such hybrid electric
cars combined with a gas engine are described, for example,
in the publications DE 10 2008 051 324 A1, DE 10 2010 028
312 A1, DE 195 09 625 A1 and DE 10 2009 027 294 A1.
The publication DE 2007 004 172 A1 discloses an electric
vehicle the electric motor of which is supplied with
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electrical energy by constantly discharging a battery
carried along. A generator driven by a combustion engine
and also carried along is able to recharge the battery,
when the motor vehicle is parked, i.e. when it is at a
standstill. Only in case of an emergency, is a so-called
emergency drive mode realised by activating an operating
device in the vehicle, which allows the vehicle to be
driven without traction battery in a restricted drive mode.
In this case the generator driven by a combustion engine is
as an exception activated while driving and the electric
power made available via the generator is available
directly, i.e. by bypassing the traction battery.
The publication WO 2013/000534 A1 describes a serially
constructed hybrid motor vehicle comprising an electrical
energy store, the charge state of which can be increased
with the aid of a known range extender comprising a
combustion engine with a generator.
The technical teaching revealed in the publication relates
to an economic power control of the combustion engine of
the range extender with the aid of the generator. The
combustion engine is controlled by the number of
revolutions, the revolutions being controlled in such a way
that the load torque of the generator is controlled.
Control of the load torque of the generator in turn is
effected based on the control of the charge current, with
which the generator charges the energy store carried along.
The publication DE 699 27 341 T2 describes a hybrid vehicle
which provides for a combustion engine driving a generator.
The generator is connected both to the battery and to a
second generator for driving the driving wheels. The
technical teaching disclosed in this publication is
concerned with a control logic specifically tailored to
individual components of the hybrid vehicle, with which the
operation of at least the combustion engine and of the
first generator shall be realised in dependence of a
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plurality of value comparisons between correction and
required-actual values.
SUMMARY OF THE INVENTION
It is the requirement of the invention to further develop a
method for driving a motor vehicle and to a respective
drive system for a motor vehicle for the exclusively
electro-motorised movement of which at least one electric
motor is indirectly or directly connected to a drive shaft
of the motor vehicle or at least the drive hub of a wheel
of the motor vehicle, the at least one electric motor being
supplied with electrical energy from an energy storage
unit, which in turn is supplied with an electrical charge
current from a generator driven by a gas engine, in such a
way that on the one hand the operating range problem
affecting electrically driven motor vehicles is alleviated
/ solved and on the other hand, low-cost components shall
be reverted to. In particular, contrary to the predominant
development credo of assembling highly optimised individual
components in order to realise an electric vehicle
optimised with regard to operating range and efficiency,
the requirement consists in reverting to tried and tested
technologies and to use these in such a way as to solve the
operating range problem, but at a much more convenient
realisation effort.
The solution to the requirement on which the invention is
based, is cited in claim 1. A drive configured according to
the solution for a vehicle driven by an electric motor is
the subject of claim 9. The features representing the
solution concept in an advantageous manner are the subject
of the sub-claims as well as of the description below.
The idea on which the invention is based goes back to the
serial hybrid drive principle, where an exclusively
electrically driven motor vehicle provides at least one
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electric motor, which is indirectly or directly connected
with the drive shaft of the motor vehicle or at least with
a drive hub of a wheel of the motor vehicle and is supplied
with electrical energy from an energy storage unit, wherein
the energy storage unit is charged via a generator driven
by a gas engine. The solution concept, however, turns away
from the prevailing development and operating strategy of
generic hybrid vehicles in that the energy storage unit is
deliberately not configured so as to be optimised with
regard to its energy density and maximum charge capacity.
Rather the maximum charge capacity of the energy storage
unit is dimensioned such that the electrical energy stored
in it is just enough for coping with a minimum operating
range for a mode of operation based exclusively on an
electric motor as drive, i.e. without charging the energy
storage unit by means of the gas-engine-generator unit
carried along. The expressly selected very limited energy
storage capacity of the energy storage unit is chosen to at
least ensure that a minimum operating range of approx. 20
to 50 km is ensured. The deciding factor for determining
the operating range is instead the fuel carried along for
combustion in the gas engine by which the generator is
driven, which ensures that the energy storage unit is
charged.
The drive concept according to the solution in the field of
electromotive drive technology represents a change of
paradigm, which is clearly directed against the generally
declared development aims in the field of electrical drive
systems / electrical hybrid drive systems, which concern an
optimisation of the degree of efficiency of each individual
component. There is no doubt that such technically
optimised systems lead to high development cost which
ultimately means very high purchasing cost for the consumer
so that such highly modern electric-motor-driven vehicles
remain reserved for a limited circle of interested parties
who are in possession of the necessary funds.
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The drive concept according to the solution is directed
precisely against this technical paradigm which is
characterised by a constant shift in technical capacity
limits, claiming instead to intelligently solve the central
range problem existing with electric cars on the basis of
existing tried and tested technologies, but with low-cost
components, which is ultimately reflected in a
significantly reduced purchasing price, in order to in the
end distinctly maximise acceptance of electric-motor-driven
vehicles.
According to the solution the gas engine is therefore
operated during movement of the motor vehicle in such a way
that an average power demand which can be allocated to the
electric motor corresponds to an average power output,
which can be allocated to the gas engine, so that a charge
state which can be allocated to the energy storage unit
does not change or merely changes within a tolerancecovered
charge state range. This means that the average
power withdrawn by the electric motor from the energy
storage unit for movement of the motor vehicle, is
indirectly supplied by the gas engine carried along, which
is preferably continuously operated while the motor vehicle
is in operation. The energy storage unit serves merely as
an energy buffer which is continuously charged with the aid
of the gas engine generator unit due to the power
generation, whilst the at least one electric motor serving
as drive for the motor vehicle obtains electrical energy
from the energy storage unit. Of course, the electric
charge drawn from the energy storage unit by the at least
one electric motor may mathematically not exactly
correspond to the electric charge supplied by the generator
of the energy storage unit, instead, the charge state of
the energy storage unit fluctuates, due to the continuous
discharging and charging operations occurring due to motor
vehicle operation, within a tolerance range of approx. ±30%
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about a charge state which can be allocated to the energy
storage unit.
In one exemplary embodiment it would be feasible to
configure the energy storage unit merely as a kind of
suitably dimensioned electric capacitor, the charge
capacity of which is small compared to heavy-weight
accumulators. Due to the capacitor being uninterruptedly
charged during motor vehicle operation and the discharging
at the same magnitude by the at least one electric motor it
is not necessary to store a charge quantity within the
energy storage unit which exceeds the mean energy demand of
the electric motor. All the same the charge capacity of the
energy storage unit should always be chosen such that it is
possible for the at least one electric motor to draw an
increased amount of energy at short notice, which may be
caused e.g. by a quick acceleration process such as an
overtaking operation.
In order to compensate for a short-term increased discharge
of the energy storage unit due to a short-term increased
power demand of the at least one electric motor, the gas
engine which is preferably operated at a discrete specified
number of revolutions (speed) and optimised with regard to
efficiency and exhaust gas, must be operated within a
tolerance-covered speed range.
Alternatively it is possible to operate the gas engine at
varying discrete speeds, at which the gas engine is
likewise operated, optimised as regards efficiency and
emissions/exhaust gases. If operation-dependent driving
situations occur, at which an increased prolonged energy
consumption occurs on the part of the at least one electric
motor, e.g. if the motor vehicle has to overcome prolonged
climbs, the gas engine is operated at a discrete specified
increased speed, due to which the energy storage unit, also
adapted to the increased power demand of the at least one
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electric motor, is supplied with an increased charge
current.
The operating concept according to the solution for driving
a motor vehicle, for the exclusively electro-motorised
movement of which at least one electric motor is indirectly
or directly connected with the drive shaft of the motor
vehicle, utilises, as operating-range-determining energy
source, not the maximum charge capacity of the energy
storage unit carried along, but the quantity of the fuel
carried along, which serves to drive the gas engine and
thus the generator connected therewith to generate power.
In a preferred embodiment the charge capacity of the energy
storage unit as well as the gas engine and the amount of
fuel carried along for operating the gas engine are chosen,
adjusted in relation to one another, in such a way that the
electrical energy share obtained by exclusive combustion of
the fuel within the gas engine and the driving of the
generator connected therewith, is at least 60%, preferably
at least 70% up to a maximum of 90% of the maximum
operating range achievable with the motor vehicle. That
means that the electrical energy share stored in a fully
charged energy storage unit contributes between a mere 10%
up and a maximum of 40% to the maximum operating range of
the drive system configured according to the solution.
Moreover an operation would be feasible, where the energy
storage unit is completely discharged, which is the case
when the maximum operating range of the motor vehicle has
been reached. If in this case it would be possible only to
fill the fuel tank, i.e. the gas tank, but not the
electrical energy storage unit, the maximum operating range
of the motor vehicle in this case would depend exclusively
on the technically usable energy content of the fuel
quantity, i.e. the share of the fuel in the maximum
achievable range would be 100%.
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Even if the operating concept according to the solution
deviates from the dogma of utilising electrical energy
exclusively in terms of a zero-emission vehicle, the drive
concept according to the solution avoids weight-dependent
and charge-capacity-dependent range problems and in
addition permits a low-cost realisation of electric motor
vehicles using conventional technologies such as e.g. the
use of conventional and low-cost accumulators, preferably
based on a nickel metal hybrid, an alkali-manganese, a zinc
chloride or a zinc carbon battery.
Besides there is already a comprehensive and widespread
infrastructure with regard to fuel for gas engines,
preferably in the form of liquid petroleum gas (LPG) or
compressed natural gas (CNG).
Apart from the possibility of charging the energy store
during movement of the motor vehicle via the described gas
engine generator operation as well as due to energy
recovery during braking operations, where the reverse
electro-dynamic principle is used, it is also possible, for
a respective fuel reserve, to charge the motor vehicle when
at standstill with the aid of the gas engine and the
generator connected therewith. Thus the motor vehicle can
be operated independently even of possible electrical
charge stations, in particular in areas, in which there is
no infrastructure for the charging of purely electrically
driven vehicles or does not exist to a sufficient extent.
Realisation of the drive system configured according to the
solution for a motor vehicle with at least one electric
motor which is indirectly or directly connected with a
drive shaft of the motor vehicle, with an electrical energy
storage unit which for electrical energy supply is
electrically connected to the electric motor, as well as a
gas engine in operative connection with a generator for
electric power generation, the generator being electrically
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connected to the electrical energy storage unit for the
supply of charge current, is achieved by using a monovalent
gas engine which is preferably driven with LPG or CNG fuel,
wherein the amount of fuel carried along has an energy
content substantially determining the maximum operating
range of the motor vehicle. The electrical energy storage
unit on the other hand comprises a charge capacity which
does not determine the operating range for the motor
vehicle. Or a quasi-monovalent gas engine may be used,
which apart from purely burning LPG or CNG can if required
also be operated with petrol.
The use of a gas engine in contrast to conventional petrol
engines permits significantly lower emission values
especially immediately upon a cold start, because gas
engines are more efficient compared to conventional
combustion engines due to the higher compression.
Furthermore it is possible to increase efficiency of gas
engines by operating the gas engine at an optimal operating
point, i.e. at a discretely specified speed. Moreover gas
engines, as compared to conventional petrol or Diesel fuel
driven combustion engines, have the advantage that
refilling is possible in principle by connecting to a gas
connection in buildings with gas connection.
SHORT DESCRIPTION OF THE INVENTION
The invention will now be explained without restriction of
the general inventive concept by way of an exemplary
embodiment with reference to the only drawing, which
schematically depicts all components for an electric motordriven
vehicle according to the solution.
WAYS OF REALISING THE INVENTION, COMMERCIAL APPLICABILITY
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The only figure shows the drive concept according to the
invention based on a purely electric-motor-driven motor
vehicle 1, where the at least one electric motor 2 required
for driving which serves to drive the drive shaft 3, is
connected to an electrical energy store 4, which is
electrically charged with the aid of a generator 5 driven
by a monovalent or bivalent gas engine 6. The fuel 7 for
operating the gas engine 6 is stored in a suitable fuel
tank 8 carried along by the motor vehicle 1. The mode of
operation according to the solution is based on an
approximation of the average power demand of the electric
motor 2 and the average power output of the gas engine 6.
In this way it can be ensured that the charge state of the
energy storage unit 4 carried along does not or not
essentially change while the motor vehicle 1 is travelling.
Only by this mode of operation is it possible to configure
the electrical energy storage unit 4, not as an energy
source determining the operating range as is the case with
all previously known solutions, but rather the energy
storage unit 4 serves merely as a buffer unit or
intermediate storage unit for the electrical energy
generated with the aid of the gas engine 6 and the
downstream generator 5. In a theoretical extreme case the
electrical energy storage unit 4 could be configured in the
form of a mere capacitor, the charge capacity of which is
limited depending on the system and which merely serves to
pass the electrical energy stored intermediately in the
capacitor onto the electric motor 2. Typically the energy
storage unit 4 may be configured as a nickel metal hybrid,
an alkali-manganese, a zinc chloride or a zinc carbon
battery.
Due to the completely different mode of operation of the
energy storage unit 4 as compared to charge-capacityoptimised,
highly modern battery systems, there are no
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stringent requirements to be met with regard to charge
capacity characteristic, thereby allowing the drive concept
according to the solution to use conventional, in
particular low-cost energy storage units. Moreover, the
method according to the solution permits the operation of a
gas engine 6 for at least one optimised working point so
that despite a conventional drive technology highest
economic requirements as well as ecological requirements
are met. The maximum operating range of the electric-motordriven
motor vehicle 1 achievable with the drive concept
according to the solution is determined essentially by the
fuel 7 carried along, e.g. liquid petroleum gas (LPG) or
compressed natural gas (CNG) for operating the gas engine
6. In this way operating ranges are achieved which
correspond to the operating ranges of conventionally driven
motor vehicles 1, so that there are no “range” resentments
for the final customer as is the case with electric cars of
latest design.
LIST OF REFERENCE SYMBOLS
1 motor vehicle
2 electric motor
3 drive hub
4 energy storage unit
5 generator
6 gas engine
7 fuel
8 fuel tank
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We Claim:
1. A method for driving a motor vehicle (1), for the
solely electro-motorised movement of which at least
one electric motor (2) is indirectly or directly
connected to a drive shaft (3) or drive hub of the
motor vehicle (1), which is supplied from an energy
storage unit (4) with electrical energy, which is
supplied with electrical charge current from a
generator (5) driven by a gas engine (6),
characterised in that the gas engine (6) during
movement of the motor vehicle (1) is operated in such
a way that an average power demand which can be
allocated to the electric motor (2) corresponds to an
average power output which can be allocated to the gas
engine (6), so that a charge state which can be
allocated to the energy storage unit (4) does not
change or changes merely within a tolerance-covered
charge state range.
2. The method according to claim 1, characterised in that
the gas engine (6) is operated at a constant speed or
at a speed within the tolerance-covered speed range,
at which, at a so-called speed operating point, the
gas engine (6), in operation, is optimised with regard
to efficiency and/or emissions.
3. The method according to claim 1, characterised in that
the gas engine (6) is operated at varying discrete
speeds, so-called speed operating points, or about one
of these speed operating points within a tolerancecovered
speed range such that when a power demand of
the electric motor (2) exceeds the average power
demand, the speed of the gas engine (6) is increased
step-wise or in a switchable manner or in terms of a
tolerance-covered speed range, and in that the gas
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engine (6), in operation, is optimised with regard to
efficiency and/or emissions.
4. The method according to claim 2 or 3, characterised in
that the gas engine (6), starting from the at least
one speed operating point, is operated with a
temporally limited, variable-speed booster function,
making it possible for a power demand of the electric
motor (2) to exceed the average power demand.
5. The method according to one of claims 1 to 4,
characterised in that the gas engine (6) is operated
with a fuel in the form of liquid petroleum gas (LPG)
or compressed natural gas (CNG).
6. The method according to one of claims 1 to 5,
characterised in that a maximum charge capacity which
can be allocated to the energy storage unit (4) and a
maximum quantity of fuel (7) carried along in the
motor vehicle (1) are chosen such that a maximum
operating range achievable with the motor vehicle (1),
for a share in the operating range, is achieved by
burning the fuel (7) exclusively within the gas engine
(6) and conversion by means of the generator (5) into
electrical energy for driving the motor vehicle (1),
and in that the share is at least 60%, preferably at
least 70%, especially preferably at least 90%.
7. The method according to one of claims 1 to 6,
characterised in that the tolerance-covered charge
state range is max. ±30% of the charge state which can
be allocated to the energy storage unit.
8. The method according to one of claims 1 to 7,
characterised in that the electrical energy storage
unit (4) is charged during standstill of the motor
vehicle (1) by operating the gas engine (6).
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9. A drive system for a motor vehicle (1) with at least
one electric motor (2) indirectly or directly
connected to a drive shaft (3) or drive hub of the
motor vehicle (1), an electrical energy storage unit
(4) electrically connected with the electric motor (2)
for the supply of electrical energy, and a gas engine
(6), which for electric power generation is
operatively connected with a generator (5), which is
electrically connected to the electrical energy
storage unit (4) for the supply of charge current,
characterised in that the gas engine (6) is a mono- or
bivalent gas engine, and the electrical energy storage
unit (4) has a charge capacity which is not rangedetermining
for the motor vehicle (1).
10. The drive system according to claim 9, characterised
in that the maximum charge capacity of the energy
storage unit (4) and a quantity of fuel (7) carried
along in the motor vehicle (1) are chosen such that a
maximum operating range achievable with the motor
vehicle (1), for a share of the operating range, can
be achieved by burning the fuel (7) exclusively within
the gas engine (6) and conversion by means of the
generator (5) into to electrical energy for driving
the motor vehicle (1), and in that the share is at
least 60%, preferably at least 70%, especially
preferred at least 90%.
11. The drive system according to claim 9 or 10,
characterised in that the electrical energy storage
unit (4) is a low-cost accumulator, preferably in the
form of a nickel metal hybrid, alkali-manganese, zinc
chloride or zinc carbon battery.
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12. The drive system according to one of claims 9 to 11,
characterised in that the gas engine (6) comprises a
fuel tank (8) for liquid petroleum gas (LPG) or
compressed natural gas (CNG).