Method and system for starting an aircraft turboengine

Abstract

The invention relates to a method for starting an aircraft turboshaft engine, said turboshaft engine comprising a combustion chamber, a compressor shaft on which a compressor wheel is mounted to feed compressed air to said combustion chamber, at least one starter connected to said shaft so as to provide it with a specified starting torque for driving it in rotation. The method comprises accelerating the compressor shaft during a first start-up phase, then stabilizing the rotational speed of the compressor shaft during a second start-up phase. During acceleration of the compressor shaft, the rotational speed of the shaft is regulated such that the acceleration of the shaft remains substantially constant.

Claims

1. Method for starting an aircraft turboshaft engine, said turboshaft engine comprising a combustion chamber, a compressor shaft on which a compressor wheel is mounted to feed compressed air to said combustion chamber, at least one starter connected to said compressor shaft so as to provide the compressor shaft with a specified starting torque for driving the compressor shaft in rotation, said method comprising: accelerating the compressor shaft during a first start-up phase (P1); then stabilising the rotational speed of the compressor shaft during a second start-up phase (P2) so as to allow fuel to be injected into the combustion chamber and to allow the fuel to be ignited, the rotational speed of the compressor shaft being regulated during acceleration of the compressor shaft during the first start-up phase (P1) and rotational speed of the compressor shaft being stabilised during the second start-up phase (P2) such that the acceleration of the compressor shaft remains substantially constant, wherein said accelerating the compressor shaft during a first start-up phase (P1) includes obtaining an acceleration value over an interval of time; calculating the difference between the obtained acceleration value and a reference acceleration value; comparing the calculated difference with a predetermined acceleration threshold; and determining a speed reference or torque reference from the calculated difference if said acceleration threshold is exceeded.

2. Method according to claim 1, wherein during the stabilisation of the rotational speed of the compressor shaft during the second start-up phase (P2), the rotational speed of the compressor shaft is kept between 10% and 15% of a nominal rotational speed of the shaft.

3. Method according to claim 1, further comprising detecting ignition is carried out when the temperature of the gases output from the turboshaft engine exceeds a predetermined threshold.

4. Method according to claim 1, further comprising, after ignition of the fuel, accelerating the shaft, which allows the compressor wheel to be accelerated, said acceleration being regulated by the torque reference.

5. Method according to claim 1, wherein the rotational speed is regulated periodically.

6. Method according to claim 1, wherein said obtaining an acceleration value over an interval of time is carried out from two consecutive measurements of the rotational speed of the compressor shaft.

7. System for implementing a method for starting up an aircraft turboshaft engine, said system comprising a turboshaft engine and a turboshaft engine management computer, the turboshaft engine comprising a combustion chamber, a compressor shaft on which a compressor wheel is mounted to feed compressed air to said combustion chamber, at least one starter connected to said compressor shaft so as to provide the compressor shaft with a specified starting torque for driving the compressor shaft in rotation, said starter configured to accelerate the compressor shaft during a first start-up phase in response to information received from the engine management computer, wherein said engine management computer is configured to accelerate the compressor shaft during a first start-up phase including obtaining an acceleration value over an interval of time; calculate the difference between the obtained acceleration value and a reference acceleration value; compare the calculated difference with a predetermined acceleration threshold; and determine a speed reference or torque reference from the calculated difference if said acceleration threshold is exceeded, and and said starter is configured to stabilise the rotational speed of the compressor shaft during a second start-up phase in response to information received from the engine management computer, so as to allow fuel to be injected into the combustion chamber and to allow the fuel to be ignited, the engine management computer being configured to regulate the rotational speed of the compressor shaft such that the acceleration of the compressor shaft remains substantially constant.

8. System according to claim 7, wherein the starter is electric and comprises an electronic circuit for controlling the torque provided to the shaft.

9. Aircraft comprising: a turboshaft engine; and a system comprising a turboshaft engine management computer, wherein the turboshaft engine comprises: a combustion chamber; a compressor shaft on which a compressor wheel is mounted to feed compressed air to said combustion chamber; and at least one starter connected to said compressor shaft so as to provide the compressor shaft with a specified starting torque for driving the compressor shaft in rotation, said starter configured to accelerate the compressor shaft during a first start-up phase (P1), and configured to stabilise the rotational speed of the compressor shaft during a second start-up phase (P2), so as to allow fuel to be injected into the combustion chamber and to allow the fuel to be ignited, wherein the turboshaft engine and said turboshaft engine management computer are configured to: accelerate the compressor shaft during the first start-up phase (P1), wherein acceleration of the compressor shaft includes obtaining an acceleration value over an interval of time; calculating the difference between the obtained acceleration value and a reference acceleration value; comparing the calculated difference with a predetermined acceleration threshold; and determining a speed reference or torque reference from the calculated difference if said acceleration threshold is exceeded, said starter is configured to then stabilise the rotational speed of the compressor shaft during a second start-up phase (P2) so as to allow fuel to be injected into the combustion chamber and to allow the fuel to be ignited, the rotational speed of the compressor shaft being regulated during acceleration of the compressor shaft during the first start-up phase (P1) and the rotational speed of the compressor shaft being stabilised during the second start-up phase (P2) such that the acceleration of the shaft remains substantially constant.

10. Aircraft comprising; a turboshaft engine; and a system comprising a turboshaft engine management computer, wherein the turboshaft engine comprises: a combustion chamber; a compressor shaft on which a compressor wheel is mounted to feed compressed air to said combustion chamber; and at least one starter connected to said compressor shaft so as to provide the compressor shaft with a specified starting torque for driving the compressor shaft in rotation, wherein the turboshaft engine and said turboshaft engine management computer are configured to: accelerate the compressor shaft during a first start-up phase (P1), wherein acceleration of the compressor shaft includes obtaining an acceleration value over an interval of time; calculating the difference between the obtained acceleration value and a reference acceleration value; comparing the calculated difference with a predetermined acceleration threshold; and determining a speed reference or torque reference from the calculated difference if said acceleration threshold is exceeded, the turboshaft engine and said turboshaft engine management computer configured to then stabilise the rotational speed of the compressor shaft during a second start-up phase (P2) so as to allow fuel to be injected into the combustion chamber and to allow the fuel to be ignited, the rotational speed of the compressor shaft being regulated during acceleration of the compressor shaft during the first start-up phase (P1) and the rotational speed of the compressor shaft being stabilised during the second start-up phase (P2) such that the acceleration of the shaft remains substantially constant.

Description

(1) Other features and advantages of the invention will become apparent upon reading the following description, with reference to the accompanying drawings given by way of non-limiting example and in which identical reference signs are given to similar elements. In the drawings:

(2) FIG. 1 schematically shows the system according to the invention,

(3) FIG. 2 shows the method according to the invention which is implemented by the system,

(4) FIG. 3 is a graph showing the three start-up phases of a turboshaft engine of an aircraft equipped with the system according to the invention,

(5) FIG. 4 is a graph of the rotational speed of the compressor shaft of the system according to the invention.

DESCRIPTION OF THE SYSTEM ACCORDING TO THE INVENTION

(6) The system 10 for starting up an aircraft turboshaft engine according to the invention, shown in FIG. 1, comprises a turboshaft engine 100 and means 200 for managing said turboshaft engine 100.

(7) The turboshaft engine 100 comprises a combustion chamber 120, a compressor shaft 140 on which a compressor wheel 160 is mounted to feed compressed air to said combustion chamber 120 and an electric starter 180 connected to said shaft 140 by a transmission relay box 170, so as to provide the shaft 140 with a specified starting torque for driving it in rotation.

(8) The compressor shaft 140 is arranged coaxially with the compressor wheel 160 and the combustion chamber 120 along an axis X.

(9) The transmission relay box 170, which may for example be in the form of several pinion stages, allows transmission of the torque provided by the starter to the compressor shaft 140. It is well understood that the starter 180 could be mounted directly on the compressor shaft 140 without this limiting the scope of the present invention.

(10) The starter 180 also comprises means 182 for accelerating the compressor shaft 140 in order to accelerate the rotational speed of the compressor shaft 140 during a first start-up phase P1 and during a third start-up phase P3, with reference to FIG. 3.

(11) The starter 180 also comprises means 184 for stabilising the rotational speed of the compressor shaft 140 during a second start-up phase P2, with reference to FIG. 3, so as to allow fuel to be injected into the combustion chamber 120 and to allow said fuel to be ignited.

(12) The acceleration means 182 and/or the stabilisation means 184 are controlled by an electronic circuit (not shown) so as to manage the level of torque provided to the shaft 140 by the starter 180. Such an electronic circuit is configured to receive speed references and/or torque references such that the starter 180 provides the shaft 140 with a torque allowing said speed to be reached or to be equal to said torque.

(13) The means 200 for managing the turboshaft engine 100 may be in the form of a computer, for example of the FADEC-type (full authority digital engine control) engine computer type, connected to the electronic circuit for controlling the starter 180 by one or more communication links 300, for example one or more digital buses, allowing references, for example speed references and/or torque references, to be sent via the management means 200 to the electronic circuit for controlling the starter 180.

(14) The means 200 for managing the turboshaft engine 100 allow, by sending references to the starter 180, the rotational speed of the shaft 140 to be regulated such that its acceleration remains substantially constant during the first start-up phase P1, whatever the environmental conditions of the turboshaft engine 100, such as temperature and pressure conditions.

(15) Implementation of the Method According to the Invention

(16) The method according to the invention is shown in FIG. 2 and described with reference to FIG. 3.

(17) When a turboshaft engine needs to be started up, on the ground or in flight, an activation command is sent to the electric starter 180, in a step E0, so that it provides torque to the shaft 140 via the transmission relay 170.

(18) The compressor shaft 140 is then accelerated, during a step E1, by the starter 180 during a first start-up phase P1.

(19) During this first start-up phase P1, the rotational speed of the shaft 140 is regulated so that the acceleration of the shaft 140 remains substantially constant during the first phase P1.

(20) More specifically, once the compressor shaft 140 is set in rotation by the starter 180, the speed N of the shaft 140 is measured periodically, for example every 100 ms, by a sensor (not shown).

(21) This measurement is periodically communicated to the management means 200, for example via the communication link 300. The management means 200 thus calculate an acceleration value over a time interval [t1, t2] from two consecutive speed measurements n1 and n2, measured at times t1 and t2 respectively, shown in FIG. 4, according to the formula:

(22) $A\left[t1,t2\right]=\frac{n2-n1}{t2-t1}$

(23) The acceleration A.sub.MES measured over the time interval [t1, t2] is thus compared with a reference acceleration A.sub.REF corresponding to a constant predetermined acceleration value, for example from environmental conditions of the turboshaft engine 100 determined by past experience, for which there are correspondence tables which associate determined ambient conditions with a reference acceleration A.sub.REF. Of course, the reference value A.sub.REF may be different for each start-up phase.

(24) As regards the first start-up phase, the reference acceleration A.sub.REF may be determined such that it lasts for a duration that is long enough for it not to have to accelerate too suddenly and exceed the start-up window, but that is short enough to start up the turboshaft engine rapidly, for example between three and four seconds. Therefore, preferably, the reference acceleration A.sub.REF is at least equal to 2.5% of the nominal speed per second, such that the duration of the first phase is less than 4 seconds, and is less than 25% of the nominal speed per second, such that it is possible to stabilise the rotational speed of the shaft within the ignition window.

(25) With reference to FIG. 4, the shaft has an acceleration which corresponds to the reference acceleration A.sub.REF previously at time t1 and subsequently at time t3. Using the method according to the invention, when the difference between the measured acceleration A.sub.MES over the time interval [t1, t2] and the reference value A.sub.REF is greater than a predetermined threshold, for example 5% of the reference value A.sub.REF, that is to say when the acceleration is not substantially constant and equal to the predetermined acceleration reference value A.sub.REF, the management means 200 determine a rotational speed reference V.sub.CONS for the shaft 140 and send this to the circuit for controlling the starter 180, thereby allowing this difference to be corrected. Such a reference aims to allow the acceleration of the shaft 140 to be regulated by the starter, that is to say that said acceleration converges, at the earliest, for example from the following time interval [t2, t3], towards the predetermined reference value A.sub.REF.

(26) Therefore, with reference to FIG. 4, the speed reference V.sub.CONS sent to the starter 180 by the management means 200 indicates a reduction in the rotational speed of the shaft 140 such that the acceleration of the shaft over the time interval [t2, t3] is less than the reference acceleration A.sub.REF such that it converges towards said reference value A.sub.REF, the shaft thus reaching the speed n3 at the time t3.

(27) The management means 200 send the speed reference V.sub.CONS to the circuit for controlling the starter 180, for example at the time t2 or slightly thereafter, such that the starter 180, via its control circuit, rapidly adapts the torque provided to the shaft 140 and therefore the rotational speed of the shaft 140 from the received speed reference V.sub.CONS.

(28) The method according to the invention therefore allows the acceleration value of the compressor shaft 140 to be made to converge towards a reference value A.sub.REF when they are significantly different, in particular over the duration of the first start-up phase P1.

(29) When a predetermined threshold for the rotational speed of the shaft 140 is reached, for example within the ignition window, the second start-up phase P2 begins. A stabilisation step E2 allows the rotational speed of the shaft 140 to be stabilised within the ignition window over a duration that is long enough to allow fuel to be injected into the combustion chamber 120 and to allow said fuel to be completely ignited. Preferably, this stabilisation step E2 is carried out by setting the reference acceleration value A.sub.REF to zero such that speed is constant and is kept for example between 10% and 15% of the nominal speed of the turboshaft engine.

(30) The method according to the invention may also comprise a step E3 of detecting the ignition of the fuel, during which step the temperature of the flow of gas output from the turboshaft engine is measured and said measurement is compared with a reference temperature value indicating the ignition of the combustion chamber.

(31) Once the ignition is detected, a latency step E4, for example for 0.5 seconds, allows it to be ensured that the ignition has taken place correctly and in a sustainable manner, before a third start-up phase of the turboshaft engine begins during which the rotational speed of the shaft 140 increases.

(32) Therefore, in a step E5 of accelerating the shaft 140, the compressor wheel 160 is accelerated, for example up to its maximum speed, during the start-up phase P3, such that the turboshaft engine can reach an aircraft flight mode.

(33) Preferably, this step E5 includes continuous stabilisation of the torque provided by the starter to the compressor shaft during the third phase P3. In a manner similar to that in step E2, the management means 200 allow the torque to be regulated by sending torque references C.sub.CONS to the starter.

(34) The resistive torque of the turboshaft engine depends on several factors, in particular on the rotational speed of the shaft and on the thermal behaviour of the turboshaft engine. However, since the difference between the torque provided by the starter and the resistive torque of turboshaft engine is proportional to the acceleration of the shaft, the acceleration varies depending on the resistive torque of the turboshaft engine, at a constant turboshaft-engine reference torque C.sub.CONS.

(35) Such continuous control of the torque provided to the compressor shaft 140 by the starter 180 allows the starter 180 to be prevented from providing, during this phase, too high a torque, which would risk leading to the combustion chamber of the turboshaft engine 100 being extinguished, as explained above.

(36) Finally, the starter 180 can be stopped during the third phase P3, for example at the start thereof.

(37) Therefore, the method and the system according to the invention allow the rotational speed of the shaft to be continuously controlled, in particular during the first start-up phase, such that the acceleration of the shaft remains substantially constant whatever the environmental conditions of the turboshaft engine (low temperature, low pressure, etc.).