HYBRID PROPULSION SYSTEM FOR A MULTI-ENGINE AIRCRAFT
Technical field
The invention lies in the field of free turbine
5 engines, as are commonly to be found on helicopters.
It should be recalled that a free-turbine engine
(sometimes referred to as a gas turbine (GT)) comprises a
power turbine or "free'' turbine that, in a helicopter,
drives its rotors via an overrunning clutch (freewheel)
10 and a-main gearbox (MGB), together with a gas generator
constituted mainly by a compressor, a combustion chamber,
and a high pressure turbine.
Stepdown gearing of an "accessory gearbox" serves to
connect the shaft of the gas generator to an electrical
15 machine (ELM) constituted by a stator~nd a rotor and
capable of operating equally well as a motor (starter) or
as a generator. In motor mode, the electrical machine is
powered by a source of electrical energy, and it develops
torque for driving the gas generator of the turbine
20 engine in rotation, in particular for the purposes of
starting it and of putting it in a standby mode, thus
providing assistance to the gas generator. In generator
mode, the electrical machine is driven in rotation by the
gas generator so as to take off mechanical power
25 therefrom, which power is then converted into ele6trical
power for powering a direct current (DC) low voltage
onboard network (OBN) of the aircraft in flight. The OBN
is generally connected to a low voltage electricity
storage device, e.g. a 28 volt (V) storage battery.
30 The invention relates more particularly to a hybrid
propulsion system for a multi-engine aircraft, in
particular a twin-engine or three-engine aircraft, i.e. a
system having at least one engine that can be put on
standby during a stage of flight referred to as an
35 "economic stage of flight", while one or more other
engines are kept active.
2
State of the art
When an aircraft having two turbine engines is in a
cruising flight situation, Documents FR 2 967 132 and
FR 2 967 133 propose putting one of the two engines into
5 a standby mode so as to desynchronize its free turbine
from the MGB while simultaneously increasing the power
from the other engine, thereby making it possible to
reduce the overall fuel consumption of the system.
That invention thus lies in particular in the
10 context of reducing the consumption of a helicopter
having at least two engines, in which, during economic
cruising flight, i.e. in a stage of flight characterized
by a relatively low power command on each engine thus
giving rise to very high specific consumption (SC), one
15 of the engines is put on standby so· tha·t the other engine
operates at high power and as a result benefits from much
lower specific consumption.
Several variants of that standby mode have been
proposed.
20 In a standby mode referred to as "ordinary idle",
the combustion chamber is alight and the shaft of the gas
generator is rotating at a speed lying in the range 60%
to 80% of its nominal speed.
In a first variant, referred to as ''sUper-idle", the
25 gas generator of the desynchronized gas generator can be
regulated on an idle mode at low speed in which the shaft
of the gas generator rotates at a speed lying in the
range 20% to 60% of its nominal speed.
In a second variant, referred to as "assisted super-
30 idle", the gas generator of the gas turbine that is
desynchronized from the MGB may also be regulated on a an
idle mode that is slow, with assistance drive torque then
being applied to the gas generator by means of the
electrical machine and the accessory gearbox.
35 In a third variant, the combustion chamber of the
turbine engine may be completely shut down, and it is
then proposed to maintain the gas generator in rotation
3
at a speed suitable for facilitating re-lighting at the
end of the stage of cruising flight. The appropriate
range of speeds may be referred to as a preferred
ignition window. This mode of operation, referred to as
5 "turning" mode involves providing prolonged assistance to
the gas generator. The shaft of the gas generator, with
mechanical assistance, rotates at a speed lying in the
range 5% to 20% of its nominal speed.
In these modes of operation, which are likely to be
10 maintained throughout the duration of cruising.flight,
the power transmitted to the MGB by the engine on standby
is generally zero, and as a general rule it is not
possible to take power from its gas generator.
In the above-mentioned variants, it is necessary to
15 be capable of reactivating the desynchronized engine :
quickly, in particular in an emergency situation, e.g. in
the event of another engine failing, if there are three
or more engines in all, or in the event of the other
engine failing if there are two engines. In particular,
20 that is why the gas generator is maintained in rotation
at a speed suitable for facilitating re-lighting in the
system where the combustion chamber is shut down.
Maintaining the gas generator in rotation in the
preferred ignition window ("turning" mode) and providing
25 prolonged assistance to the gas generator regulated to
idle ("assisted super-idle" mode) require relatively
little power, with the advantage of the system lying in
it being used over a long duration of flight.
Proposals are made in Documents FR 2 967 132 and
30 FR 2 967 133, among other solutions, to make use of an
electric starter powered by a starter/generator connected
to the gas generator of the other engine, or of a
generator driven directly or indirectly by the free
turbine of the other engine.
35 For an emergency restart beginning from a low speed
situation in which the combustion chamber is shut down,
it is necessary to apply high power to the shaft of the
4
gas generator because of the large inertia of the
rotating assemblies and because of the opposing torque
from the compressor of the engine. This power needs to
be delivered for a duration that is short, of the order
5 of a few seconds, in order to guarantee that the engine
starts quickly.
In Document FR 2 967 133, it is suggested, among
other solutions, to make use of a source of electrical
energy, in particular a supercapacitor, in order to power
10 an electrical machine that supplies a burst of as-sistance
to the gas generator.
In Document EP 2 581 586, proposals are also made to
use two supercapacitors (which are electricity storage
members), each of which is charged respectively by an
15 '. e,lectricity generat:or driven by the g.a:s:· gene·rat·or·'' a£· ··one
of the two engines, and each of which serves to provide a
burst of power for starting the other engine when it is a
shut-down state.
In this context, the present invention has in
20 particular the object of providing practical technical
means for performing the "rapid reactivation" function on
board an aircraft having at least two engines beginning
from an economic mode of the turbine, by using, instead
of the conventional electric starter, an electrical power
25 system powered either by the onboard network or else by a
specific electrical power network and making it possible
to perform the following different modes of operation:
starting the gas turbine engine on the ground;
economic mode, in which one engine is in standby
30 mode, which is a mode that is economical in terms of
energy and in which mechanical power is not delivered to
the rotor of the aircraft;
· normal reactivation in flight of the engine that
was previously in economic mode, constituting reliable
35 starting from standby mode, without significant time
constraint; and
5
5
· rapid reactivation in flight of the engine that
was previously in economic mode, which constitutes
emergency starting, making it possible in a minimum
length of time to bring the engine up to power from
standby mode, i.e. to take the engine quickly from
standby mode in order to reach "nominal" power in which
the engine supplies mechanical power to the main gearbox.
Leaving standby mode in an emergency involves
lighting the combustion chamber and driving the shaft of
10 the gas generator up to a speed lying in the range 80% to
105% in a period of time that is shorter than 10 seconds
15
20
(s) after issuing the command to leave standby mode.
Leaving standby normally involves lighting the
combustion chamber and driving the shaft of the gas
generator up to ca speed lyJnCJ in .the- range 80%.-· to· -106'% in
a period of time lying in the range 10 s to 1 minute
(min) after issuing the command to leave standby mode.
A turbine engine suitable for operating in a standby
mode is referred to as a hybrid turbine engine.
Hybridizing propulsion systems makes it possible to
increase their efficiency. In contrast, the weight of
present electrical power components makes them difficult
to use for applications on board an aircraft.
It is therefore necessary to devise and develop an
25 architecture that is pared-down to the minimum in order
to propose a propulsion system that is capable of flying
in economic cruising mode, in which the power needed for
flight is delivered by a minimum number of engines, while
the others are in standby mode, while nevertheless
30 enabling an engine to exit standby mode efficiently
whether by normal reactivation or by rapid reactivation.
For questions of reliability, it is also necessary
to be able to carry out regular testing of the
reactivation system and to satisfy all operating safety
35 requirements and certification requirements for
propulsion systems.
i:
6
The architectures for hybrid aircraft propulsion
systems that have been proposed in the past are complex
and involve large amounts of onboard weight, or they do
not make it possible to carry out tests on equipment for
5 providing rapid reactivation, or they do not satisfy the
necessary reliability and availability requirements.
Summary of the invention
In order to remedy the above-mentioned drawbacks, in
10 accordance v1ith the .. invention, there is provided a hybrid
propulsion system for a multi-engine aircraft, the system
comprising a plurality of free-turbine engines each
having a gas generator, and including at least one first
engine, referred to as a "hybrid" engine, that is
sui table f:er opera.t:i .. 11q in ;::\·F::.r
respectively in isolated manner and in alternation with
the other of said first and second electronic power
modules, each of the first and second electrical machines
with normal reactivation power or starting power (Pdem).
In another aspect of the invention, each of the
first and second electronic power modules is adapted to
be capable of receiving power respectively from the first
or the second electrical energy storage member for
powering respectively and simultaneously with the other
25 of said first and second electronic power modules, each
of the first and second electrical machines with half
rapid reactivation power (Prr/2).
In yet another aspect of the invention, each of the
first and second electronic power modules is adapted to
30 be capable of receiving power from said specific
electrical power supply network in order to power
respectively and simultaneously with the other of said
first and second electronic power modules the first and
second electrical machines either with half normal
35 reactivation power or half starting power (Pdem/2), or
else with half standby power (Pv/2).
,._,
8
In a variant, each of the first and second
electronic power modules is adapted to be capable of
receiving power respectively from the first or the second
electrical energy storage member in order to power
5 respectively and simultaneously with the other of said
first and second electronic power modules, the first and
the second electrical machines either with half normal
reactivation power or starting power (Pdem/2), or else
with half standby power (Pv/2).
10
15
In yet. another aspect of the invention; ·each af .. the
first and second electronic power modules is adapted ·to
be capable of receiving power from said specific
electrical power supply network. in .order to power
respectively in isolated manner and in alternation with
·the o:thcr, o·I.- sG..id firs·t and second· ·e,l·ect-roni-c·:pow.erer.;."':
modules the first and second electrical machines either
with normal reactivation power or starting power (Pdem),
or else with standby power (Pv).
In yet another aspect of the invention, each of the
20 first and second electronic power modules is adapted to
be capable of receiving power from said specific
electrical power supply network or respectively from the
first or the second electrical energy storage member in
order to power respectively in isolated manner and in
25 alternation with the other of said first and second
electronic power modules, or in simultaneous manner, the
first and second electrical machines with variable power
(Pvar) less than or equal to half the total power (Prr)
needed for rapid reactivation of said hybrid engine, in
30 order to be able to carry out power tests periodically.
In a particular embodiment, the first and second
electrical energy storage members comprise two storage
members that are physically dissociated.
In another possible embodiment, the first and second
35 electrical energy storage members comprise two storage
members that are distinct but physically grouped
together.
9
The invention also provides a multi-engine aircraft
including a hybrid propulsion system as mentioned above.
The aircraft may be a helicopter.
5 Brief description of the figures
10.
15
Other characteristics and advantages of the
invention appear from the detailed description of
particular embodiments of the invention given with
reference to the accompanying drawings, in which:
· Figure 1 is. a diagram of a hybrid a-rchitecture. of
a propulsion system for a turbine engine having two
controlling electric powertrains in a firsL embodiment of
the invention;
· Figure 2 is a diagram of a hybrid architecture of
:Y fYr:opu1·s:~~orv,::.>:y:st~..:.;m for a turbine engine~ havin'~g~ .t.:woi·
controlling electric powertrains in a second embodiment
of the invention;
Figure 3 is a diagram showing the operation of the
Figure 1 hybrid architecture in standby mode with a
20 single active controlling electric powertrain;
Figure 4 is a diagram showing the operation of the
Figure 1 hybrid architecture in standby mode with two
active controlling electric powertrain;
Figure 5 is a diagram showing the operation of the
25 Figure 1 hybrid architecture in normal reactivation or
starting mode with a single active controlling electric
powertrain powered by an onboard network;
Figure 6 is a diagram showing the operation of the
Figure 1 hybrid architecture in a normal reactivation or
30 starting mode with a single active controlling electric
powertrain powered by an electrical energy storage
member;
Figure 7 is a diagram showing the operation of the
Figure 1 hybrid architecture in normal reactivation or
35 starting mode with two active controlling electric
powertrains powered by the onboard network;
5
10
10
Figure 8 is a diagram showing the operation of the
Figure 1 hybrid architecture in rapid reactivation mode
with two active controlling electric powertrains powered
by electrical energy storage members; and
Figure 9 is a diagram showing the operation of the
Figure 1 hybrid architecture in a mode for carrying out
variable power tests with two active controlling electric
powertrains powered by the onboard network and by
electrical energy storage members.
Detailed description
The hybrid propulsion system for a multi-engine
aircraft of the invention comprises a plurality of freeturbine
engines each equipped with a gas generator, among
:15 .-r~- ·.,-Jhich·, e.11gine.s•·- Gt.--·.cleas L a first eh:g ~:rr.c·1 :-- ;o.-c:, -1-ry.br±d:. ~e:rTg i:n,~!
is suitable for operating in at least .. one standby mode
during stabilized flight of the aircraft, while other
engines of the plurality of engines are operating alone
during the stabilized flight.
20 Figures 1 to 9 show the hybrid turbine engine 1 on
its own together with the controlling electric
powertrains of this hybrid engine, while the other
engines in use may be conventional. Nevertheless, it is
also possible on a single aircraft to make use of a
25 plurality of hybrid engines analogous to the hybrid
engine 1 described with reference to the accompanying
drawings. The invention can thus apply to all of the
engines of an aircraft of multi-engine architecture.
With reference to Figure 1, it can be seen that the
30 hybrid engine 1 is associated with first and second
identical electric powertrains, each comprising a
respective electrical machine 2, 3 capable of operating
as a starter and as a generator, which machine is itself
connected to a respective electronic power module 4, 5,
35 itself selectively connected to a specific electrical
power supply network 8, such as an onboard network, and
.-~- ,j
11
to at least one electrical energy storage member,
respectively 6, 7.
Each of the electric powertrains is adapted to
deliver a maximum power that is not less than half the
5 total power Prr needed for rapid reactivation of the
hybrid engine l.
Figure l shows the first and second electrical
energy storage members 6, 7, which comprise two storage
members that are physically dissociated, each of which is
10 capable of delivering at least half of the power and of
the total energy needed for rapid.reactivation of the
engine l, or each of which is capable of delivering the
power necessary for normal reacti:vat.ion of the engine l.
Nevertheless, as shown in Figure 2, the first and
15·· ,. -··seco-r1d- electric-al_ eilt2r']y stCJLage nrenro·~r:s·-~mi:ty-' :compri-se·~: t\~o
distinct storage members 66, 67 that are isolated from
each other, but that are physically grouped together in a
single physical entity 60, with each storage member
constituting half of this entity.
20 The storage members 6, 7 or 66, 67, also referred to
for short as "stores", may be electrochemical or
electrostatic in nature.
Each of the first and second electric powertrains is
adapted to be capable of delivering selectively to the
25 hybrid engine l either normal reactivation power or
starting power Pdem, or else standby power Pv, or else
half-standby power Pv/2, or else half-rapid reactivation
power Prr/2.
Normal reactivation power or starting power is
30 generally about 20% of the total rapid reactivation power
Prr.
Standby power is generally about 3% to 5% of the
total rapid reactivation power Prr.
Each dedicated electronic power module 4, 5 is
35 capable of powering the corresponding electrical machine
2, 3 for a limited time with at least half of the power
needed for rapid reactivation, i.e. Prr/2, or with the
12
power needed for normal reactivation Pdem (which also
corresponds to starting power) .
Each dedicated electronic power module 4; 5 is
itself supplied with energy either by the corresponding
5 store 6, 66; 7, 67, or by the onboard network 8 of the
aircraft, or by both together. It should be observed
that the power available from the onboard network 8 is, a
priori, limited since the onboard network 8 also needs to
supply the electrical power needed for all of the onboard
10 systems.
Each dedicated electronic power module 4, 5 is also
capable of continuously powering the corresponding
electrical machine 2, 3 for its use in the standby mode
of the engine 1, and it is also adapted to control the
-_ 1_5. ·.corrGsponding .elect:L·i..::a.J_ machine 2ri·_ .. .J~·,'far- :.the·· .re:liab"l,e:.
starting procedure or for the normal reactivation
procedure.
Each of the electrical machines 2, 3 is adapted to
deliver at least half of the power needed for rapid
20 reactivation, and the power needed for normal
reactivation.
Furthermore, each electrical machine 2, 3 that
drives the gas generator of a hybrid engine 1 is capable
of maintaining that engine continuously in standby mode,
25 of starting the engine 1, and of performing normal
reactivation.
The engine 1 has an accessory gearbox suitable for
receiving both electrical machines 2, 3, in addition to
the standard equipment needed for proper operation of the
30 engine 1.
With reference to Figures 3 to 9 there follows a
description of the various modes of operation of the
architecture of the invention. In these figures,
elements of the architecture that are not active are
35 drawn in dashed lines, while elements of the architecture
that are active are drawn in normal manner with
continuous lines.
5
10
15
13
Figures 3 and 4 show how the standby mode of the
engine 1 can be implemented with the two electric
powertrains in two different embodiments, in which energy
is always taken from the onboard network 8.
As shown in Figure 3, the power Pv needed for
standby mode, which represents about 3% to 5% of the
total available power Prr, can be delivered in
alternation by the two electric powertrains on different
missions.
Figure 3- shows the electric powertrain including the
first electrical machine 2 and the first electronic povJer
module 4 powered by the onboard network 8 as being active
while the second electrical machine 3, the second
electronic power module 5, and the stores 6 and 7 are not
roles should be interchanged so that it is the second
electrical machine 3 and the second electronic power
module 5 powered by the onboard network 8 that are
active, while the first electrical machine 2, the first
20 electronic power module 4, and the stores 6 and 7 are not
involved.
Figure 4 shows an embodiment in which, in standby
---mode of the engine 1, both electric -powertrains are
active simultaneously, but each delivers a power of only
25 Pv/2 equal to half the power Pv needed for standby mode,
i.e. of the order of 1% to 3% of the total power Prr.
The first and second electrical machines 2 and 3 and the
first and second electronic power mo&1les 4 and 5 are
thus active simultaneously, both drawing power from the
30 onboard network 8, while the stores 6 and 7 are not
involved.
35
Figures 5 to 7 show how the normal reactivation mode
or starting mode of the engine 1 can be performed by the
two electric powertrains in three different embodiments.
In the first embodiment shown in Figure 5, the
energy corresponding to normal reactivation or mechanical
power Pdem, which is typically of the order of 20% of the
14
total power Prr needed for rapid reactivation, is taken
from the onboard network 8 and only one electric
powertrain is used.
Figure 5 shows the electric powertrain comprising
5 the first electrical machine 2 and the first electronic
power module 4 powered by the onboard network 8 as being
active, while the second electrical machine 3, the second
electronic power module 5, and the stores 6 and 7 are not
involved. In a following mission of the aircraft, the
10 roles should be. interchanged so that it is the second
electrical machine 3 and the second electronic power·
module 5 powered by the onboard network 8 that are
active, while.the first electrical machine 2. the first
electronic power module 4, and the.stores 6 and 7 are not
15
The embodiment of Figure 6 is analogous to the
embodiment of Figure 5 insofar as only one electric
powertrain is used, however the energy corresponding to
normal reactivation or mechanical power Pdem, which is
20 typically of the order of 20% of the total power Prr
needed for rapid reactivation, is taken not from the
onboard network 8, but from a store.
In Figure 6, the electric powertrain comprising the
first electrical machine 2 and the first electronic power
25 module 4 powered by the store 6 is shown as beir1g active,
while the second electrical machine 3, the second
electronic power module 5, the store 7, and the onboard
network 8 are not involved in this operation. In a
following mission of the aircraft, the roles should be
30 interchanged so that it is the second electrical machine
3 and the second electronic power module 5 powered by the
store 7 that are active, while the first electrical
machine 2, the first electronic power module 4, the store
6, and the onboard network 8 are not involved.
35 Naturally, when the embodiment of Figure 2 is used,
the store 66 and the store 67 perform the same roles as
the stores 6 and 7, respectively.
15
Figure 7 shows an embodiment in which, in normal
reactivation or starting mode of the engine 1, both
electric powertrains are active simultaneously, but with
each delivering power of orily Pdem/2 equal to half of the
5 power Pdem needed for standby mode, i.e. typically of the
order of 20% of the total power Prr. The first and
second electrical machines 2 and 3 and the first and
second electronic power modules 4 and 5 are thus active
simultaneously.
1Q Figure 7 shows. connections indicating that.energy is
taken by the first and second electronic power modules 4
and 5 from the onboard network 8, while the stores 6 and
7 are not involved.
Nevertheless, in a variant, in the embodiment of
.15·:·" :·rFi~Jure, ·7, -·.vdle-:;_:>2 L·otb_ \:!l<:~(:t.ric pO\:J\3-rt-.ra,ins are act.Lve
simultaneously, the first and second electronic power
modules 4 and 5 could take energy corresponding to Pdem/2
from the stores 6 and 7 respectively (or 66 and 67 if the
embodiment of Figure 2 is being used) and not from the
20 onboard network 8.
Figure 8 shows an embodiment in which, in rapid
reactivation mode of the engine 1, both electric
powertrains are active simultaneously in simultaneous and
coordinated operation, but each delivers power of only
25 Prr/2 equal to half the total power Prr needed for rapid
reactivation mode. The first and second electrical
machines 2 and 3 and the first and second electronic
power modules 4 and 5 are thus active simultaneously.
In the embodiment of Figure 8, energy is taken by
30 the first and second electronic power modules 4 and 5
firstly from the stores 6 and 7 (or 66 and 67 for the
embodiment of Figure 2), in equal shares for power of the
order of Prr/2. Nevertheless, additional power, where
necessary, may be taken by the first and second
35 electronic power modules 4 and 5 from the onboard network
8.
16
Figure 9 shows a configuration of the architecture
of Figure 1 in which a test is carried out by applying
varying power Pvar, where Pvar can vary between almost
zero power and power equal to half of the total power
5 Prr, for each of the complete electric powertrains in
order to guarantee proper operation and performance for
the system.
The test is preferably carried out each time the
propulsion system of the aircraft is started on the
10 ground, but it can also be carried out :i-n flight, 'should
that be necessary.
The energy needed for testing proper operation may
be supplied by the onboard network & or by the energy
storage members 6, 7 or 66, 67, as required:
-· .. The tcr:;ts nw:{ b·~; -p\.~Lformed irrl:.·:a-lct:.-<::;rn,at,icanr tnr_r;(.~t.r·:
simultaneously on both electric powertrains.
By way of example, Figure 9 shows the situation in
which all of the branches of all of the electric
powertrains are tested simultaneously with variable power
20 Pvar that is thus delivered by the stores 6 and 7 and by
the onboard network 8 to each of the electronic power
modules 4 and 5.
The present invention provides various advantages
over existing solutions, and in particular it makes it
25 following possible:
30
· a spot reactivation test on every other mission
for each electric powertrain by means of the starting
procedure before each mission and alternating the use of
the electric powertrains;
· a permanent test of the operation of the electric
powertrain by means of the standby mode, which makes use
of the electric powertrain(s) and which causes the
electrical machines to rotate permanently while economic
mode is in use;
35 · segregation between the electric powertrains is
provided in particular for the energy storage portion by
making use of two identical stores 6 and 7 that are
---- ----,--,?·'
'
5
17
physically dissociated and each suitable for storing half
of the maximum required energy (Prr/2), or by using a
single store 60 grouping together two identical stores 66
and 67 each suitable for storing half of the maximum
required energy (Prr/2), these two identical stores 66
and 67 being in a single physical unit but being isolated
from each other;
· redundancy for normal reactivation mode by having
two independent electric powertrains;
10 redundancy for the power suppli.es :insofar as
normal reactivation can be obtaine-d either· f.rom a store
6, 7 or 66, 67 or from the onboard network 8, depending
on the availability of these sources; and
minimized and optimized dimensioning of the two
powertrains to be added together .in order to obtain the
power needed for rapid reactivation (see Figure 8).
In general, the invention is not limited to the
embodiments described, but extends to any variant within
20 the ambit of the scope of the accompanying claims.
i,
'
I
I
ii

CLAIMS
1. A hybrid propulsion system for a multi-engine
aircraft, the system comprising a plurality of freeturbine
engines each having a gas generator, and
5 including at least one first engine (1), referred to as a
"hybrid" engine, that is suitable for operating in at
least one standby mode during stabilized flight of the
aircraft, while other engines of said plurality of
engines operate alone during such stabilized flight, the
10 hybrid engine .(1) being associated with at least one
first electric powertrain comprising a first electrical
machine (2) capable of operating as a starter and as a
generator, itself connected to a first electronic power
module (4), itself selectively connected to a specific
cle.c::Jc.rica·1 p.O'\'{~~·:l:.·'. ~'Jupply netv1ork . (:8 . .)··, -: suoh ·:as:.·