Abstract:
An ignition unit for a turbojet engine, including: an electrical power supply, a single control channel to receive a control signal from a computer, a main sparkplug ignition channel to energize at least one main sparkplug of a main combustion chamber, and an afterburner sparkplug ignition channel to energize at least one afterburner sparkplug of an afterburner chamber. The ignition unit is configured, in response to pulsed controls on the single control channel, to selectively activate the main sparkplug ignition channel or the afterburner sparkplug ignition channel.

Description:
GENERAL TECHNICAL FIELD 
       [0001]    The present invention relates to the aeronautical field, and deals more specifically with an ignition unit for a turbojet engine for igniting a main combustion chamber and for igniting afterburning. 
       BACKGROUND OF THE INVENTION 
       [0002]    Turbojet engines for aircraft are currently equipped with an afterburner device. An afterburner device comprises means for injecting fuel in proximity to flame-holder members and at least one afterburning ignition spark plug located in an afterburning ignition area. Under afterburning conditions, additional fuel is injected in order to obtain an increase in thrust. 
         [0003]    Such a turbojet engine thus includes two ignition means: main spark plugs for igniting the main combustion chamber, and afterburning spark plugs for igniting afterburning. 
         [0004]    For reasons of simplicity, bulkiness and costs, a single ignition unit simultaneously controls the ignition of the main combustion chamber as well as the afterburning ignition. In this way, independently of whether activation of the ignition of the main chamber or the afterburning is desired, the main spark plugs and the afterburning spark plugs are simultaneously energized. The main spark plugs fire even when only the firing of the afterburning spark plugs is desired. 
         [0005]    Now, during the operation of the turbojet engine, a main spark plug is subject to very high pressure, of the order of 30 bars, in altitude. Under these conditions, the firing of a main spark plug implies a rapid degradation of the latter. The lifetime of a main spark plug may be considerably reduced by this, causing additional maintenance costs and reducing the availability of the aircraft as well as the reliability of their performances. 
         [0006]    It is therefore desirable to only energize a main spark plug when the latter should be sought, i.e. upon starting or restarting the turbojet engine. 
         [0007]    Complete segregation between the routes for igniting a main spark plug and the routes for igniting an afterburning spark plug is capable of meeting this requirement, such that the energization of a main spark plug is independent of the energization of an afterburning spark plug. 
         [0008]    However, the application of two ignition units, one dedicated to the main plugs and the other to the afterburning spark plugs, represents an overcost, increased bulkiness and a mass increase which may not be acceptable. Further, the requirement of an additional computer output for the independent controls may be incompatible with the existing systems. 
         [0009]    Further, replacement of an ignition unit may not be concomitant with a possible adaptation of other elements of the ignition system, and notably with the adaptation of the control signal sent by the computer for controlling the ignition. Thus, it is necessary that the modifications brought to the ignition system may be applied independently of each other. 
       SUMMARY OF THE INVENTION 
       [0010]    The object of the present invention is notably to provide a solution allowing segregation of the energization of the main spark plugs and of the afterburning spark plugs, which is compatible with the existing elements of an ignition system without any increase in mass and in the bulkiness of the ignition system. 
         [0011]    For this purpose, according to invention, an ignition unit is proposed for a turbojet engine, said unit comprising:
       an electric power supply,   a single control channel for receiving a control signal from a computer,   a channel for igniting a main spark plug in order to energize at least one main spark plug of a main combustion chamber,   a channel for igniting an afterburning spark plug in order to energize at least one afterburning spark plug of an afterburning chamber,       
 
         [0016]    said unit being capable, in response to pulsed commands on said single control channel, of selectively activating the main spark plug ignition channel or the afterburning spark plug ignition channel. 
         [0017]    The invention is advantageously but optionally completed by the following features, taken alone or in any of their technically possible combination:
       the unit further comprises a control module for decoding the control signal in order to determine which ignition channel has to be activated;   selective activation of the main spark plug ignition channel or of the afterburning spark plug ignition channel depends on the pulse durations of the control signal;   the duration of a pulse of the control signal for controlling the activation of an ignition channel depends on the energy required for energizing the spark plug corresponding to said ignition channel;   the signal has a time-out after each pulse of the control signal, said time-out corresponding to a signal level different from the signal level of the pulse and being followed by a continuous command at a signal level corresponding to the signal level of the pulse;   the unit comprises an electric power supply channel common to the main spark plug ignition channel and to the afterburning spark plug ignition channel, said common electric power supply channel comprising a single capacitive block intended to be brought to a voltage depending on the ignition channel to be activated;   each of the ignition channels includes a thyristor connected to the single capacitive block, said thyristor being controlled depending on the state of the common electric power supply channel and of the pulse control;   the unit is adapted for beginning the charging of the capacitive block as soon as a pulse is detected and before determining the ignition channel to be activated;   the unit is capable, in response to a control voltage step on said single control channel, of alternately activating the main spark plug ignition channel and the afterburning spark plug ignition channel.       
 
         [0026]    The invention also deals with an ignition system for a turbojet engine, comprising:
       an ignition unit according to the invention,   a main spark plug of a main combustion chamber which may be energized through the main spark plug ignition channel of said unit,   an afterburning spark plug which may be energized through the afterburning spark plug ignition channel of said unit,   a computer connected to the control channel of said unit for sending a control signal to said unit.       
 
     
    
     
       SHORT DESCRIPTION OF THE FIGURES 
         [0031]    Other features, objects and advantages of the invention will become apparent from the description which follows, which is purely illustrative and non-limiting, and which should be read with reference to the appended drawings wherein, 
           [0032]      FIG. 1  is a diagram illustrating an ignition unit from the state of the art; 
           [0033]      FIG. 2  is a diagram illustrating an ignition unit according to a possible embodiment of the invention; 
           [0034]      FIG. 3  is a time diagram illustrating the behavior of the ignition unit of the state of the art in response to a control signal of the state of the art; 
           [0035]      FIG. 4  is a time diagram illustrating the behavior of the ignition unit according to a possible embodiment of the invention in response to a control signal of the state of the art; 
           [0036]      FIG. 5  is a time diagram illustrating the behavior of the ignition unit according to a possible embodiment of the invention and of the ignition unit of the state of the art in response to a control signal according to a possible embodiment of the invention within the scope of the energization of a main spark plug; 
           [0037]      FIG. 6  is a time diagram illustrating the behavior of the ignition unit according to a possible embodiment of the invention and of the ignition unit of the state of the art in response to a control signal according to a possible embodiment of the invention within the scope of the energization of an afterburning spark plug. 
       
    
    
     DETAILED DESCRIPTION 
       [0038]      FIG. 1  is a diagram illustrating an ignition unit  100  of the state of the art and showing its most important components. Such an ignition unit  100  has an electric power supply  101  and a single control channel  102  for receiving a control signal from a computer. The control channel  102  is here materialized with two physical lines for producing redundancy improving the reliability of the control of the ignition unit  100 . 
         [0039]    An electric processing stage  103  applies electric filtering of the power supply and of the control signal, and includes a protective device against overvoltages, in order to protect the unit against possible alterations of these inputs which may damage the ignition unit  100  or deteriorate its operation. 
         [0040]    The ignition unit  100  also includes a control module  104  which receives the control signal after that it has passed in transit through the control channel  102  and has been processed by the electric processing stage  103 . This control module  104  determines the activation of the channels for igniting spark plugs from the control signal, and controls the electric conversion stage  105  so that the latter converts the received power supply of 28 volts into 2,800 volts. 
         [0041]    The electric conversion stage  105  for this purpose includes a switching power supply  106  of the flyback converter type and a transformer  107  for raising the voltage to the intended level. A rectifier  108  then rectifies the current for charging the capacitors  111 ,  121  of each of the channels  110 , 120  for igniting spark plugs. 
         [0042]    A spark-gap  109  allows discharging of the capacitors  111 ,  121  when the desired voltage threshold is attained, both at a channel for igniting the main spark plugs  110  and at a channel for igniting the afterburning spark plugs  120 . The discharge current of the capacitors  111 ,  121  is then transmitted to the induction coils of the output stages  112 ,  122  and causes firing of the ignition spark plugs to which energy is transmitted through means  113 ,  123  for connections with the latter. The output stage  112 ,  122  notably allows control of the duration of the spark of the plugs with which it is associated. 
         [0043]      FIG. 3  illustrates the charging of the capacitors  111 ,  121  of the ignition unit  100  of the state of the art in response to a control signal of the state of the art. The upper curve  30  illustrates the time-dependent change in the control signals. The middle curve  31  illustrates the charging energy of the capacitor  111  of the main spark plug ignition channel  110 , and the lower curve  32  illustrates the charging energy of the capacitors  121  of the afterburning spark plug ignition channel  120 . The scale is arbitrary. 
         [0044]    A voltage step or front of the control signal causes activation of the channels for igniting the main spark plugs  110  and of the channels for igniting the afterburning spark plugs  120 . As explained earlier, there is no segregation between the ignition spark plugs. The main spark plugs therefore fire at the same time as the afterburning spark plugs, as shown by the simultaneity of the drops in the energy stored by the capacitors  111 , 121  on the charging energy curves  31 ,  32  of the latter. 
         [0045]    In order to overcome the drawbacks discussed earlier, according to the invention, an ignition unit for a turbojet engine is proposed, which is capable, in response to pulse commands on a single control channel, of selectively activating the main spark plug ignition channel or the afterburning spark plug ignition channel.  FIG. 2  illustrates a possible embodiment of such an ignition unit and the description which follows will be made in reference to this embodiment illustrated by  FIG. 2 . 
         [0046]    The ignition unit  200  has an electric power supply  201  and a single control channel  202  for receiving a control signal from a computer. The control channel  202  is here materialized by two physical lines for producing redundancy improving the reliability of the control of the unit  200 . 
         [0047]    An electric processing stage  203  applies electric filtering to the power supply and to the control signal, and includes a protective device against overvoltages, in order to protect the unit against possible alteration of these inputs which may damage the ignition unit  200  or deteriorate its operation. 
         [0048]    The ignition unit  200  includes a main spark plug ignition channel  210  for energizing at least one main spark plug of a main combustion chamber and a channel for igniting an afterburning spark plug  220  in order to energize at least one afterburning spark plug of an afterburning chamber. 
         [0049]    The ignition unit  200  also includes a control module  204  which receives the control signal after it has passed in transit through the control channel  202  and has been processed by the electric processing stage  203 . This control module  204  decodes the control signal in order to determine the ignition channel which has to be activated. 
         [0050]    The ignition unit  200  also comprises an electric power supply channel  205  common to the main spark plug ignition channel  210  and to the afterburning spark plug ignition channel  220 , said common electric power supply channel  205  comprising a single capacitive block  209  intended to be brought to a given voltage depending on the ignition channel to be activated. 
         [0051]    Thus, unlike the control unit  100  of the state of the art illustrated by  FIG. 1 , the control unit  200  according to the possible embodiment of the invention has optimized volume in that it has a single capacitive block  209  for both ignition channels  210 ,  220 , whereas the control unit  100  of the state of the art illustrated by  FIG. 1  provides a capacitor for each ignition channel  110 ,  120 . 
         [0052]    In order to charge the single capacitive block  209  to a desired voltage, the common electric power supply channel  205  includes a switching power supply module  206  of the flyback converter type and a transformer  207  for raising the voltage. A rectifier  208  then rectifies the current for charging the capacitive block  209 . 
         [0053]    Each of the ignition channels  210 ,  220  includes a thyristor  211 ,  221  connected to the single capacitive block  209 , said thyristor  211 ,  221  being controlled depending on the state of the common electric power supply channel  205  and on the pulse command decoded with the control module  204 . The switching power supply module  206  manages the charging of the transformer  207  for switching control to the thyristor  211 ,  221 . 
         [0054]    The thyristor  211 ,  221  of the ignition channel  210 ,  220  to be activated is made to be conducting when the desired charging energy of the capacitive block  209  is attained. The discharge current of the capacitive block is then transmitted to the induction coils of the output stages  212 ,  222  and causes firing of the ignition spark plug to which the energy is transmitted through means  213 ,  223  for connections with the latter. 
         [0055]    The ignition unit  200  according to invention thus gives the possibility of having two distinct ignition channels  210 ,  220  in spite of a single common electric power supply channel  205 . It therefore allows an appreciable gain in weight and in volume, as well as in components, having a favorable impact on the unit costs of an ignition unit. 
         [0056]    In a preferred embodiment, the selective activation of the main spark plug ignition channel  210  or of the afterburning spark plug ignition channel  220  depends on the durations of the pulses of the control signal. 
         [0057]      FIG. 5  is a time diagram on which is illustrated the time-dependent change  50  of the control signal according to a possible embodiment of the invention within the scope of the energization of a main spark plug. 
         [0058]    A main spark plug ignition pulse  51  with a determined duration, for example 40 ms, during which the signal assumes a high level, followed by a time-out  52  of 40 ms during which the signal assumes a low level, tells the control module  204  that it is the main spark plug ignition channel  213  which should be activated in order to energize the main spark plug so that the latter fires. 
         [0059]    The middle curve  55  illustrates the charging energy of the capacitive block  209  in response to the control signal illustrated by curve  50 . The pulse  51  and the time-out  52  are followed by a continuous control  53  at a signal level corresponding to the pulse signal level  51 . The charging of the capacitive block  209  begins with the continuous control  53 , after the control module  204  has decoded the control signal. 
         [0060]    The response time to the first firing is formed by the duration of the pulse  51  i.e. 40 ms, the duration of the time-out  52 , i.e. 40 ms, and the charging time during the application of the continuous control  53 . The characteristics of the common electric power supply channel  205  are selected so that the charging time of the capacitive block  209  is reduced relatively to the charging time of the state of the art, so that the response time to the first firing remain substantially the same as in the state of the art, i.e. 180 ms. The response times of the following firings are preferably substantially the same, i.e. 180 ms. The firing sequence of said at least one main spark plug is maintained as long as lasts the continuous control  53 . 
         [0061]    The lower curve  56  illustrates the charging energy of the capacitor  111  of the main spark plug ignition channel  110  in an ignition unit  100  of the state of the art, in response to the control signal illustrated by curve  50 . The charging of the capacitor  111  begins as soon as the pulse  51  occurs since the ignition unit of the state of the art interprets said pulse as the voltage step of the control signal in the state of the art. First charging  57  then begins right from the beginning of the pulse  51 , and the charging is then interrupted at a stable level  58  during the time-out  52  before resuming in a second phase  59  when the continuous control  53  is applied. The following firings are similar to those of the state of the art since the then applied continuous control  53  is similar to the voltage step of the control signal of the state of the art. 
         [0062]    Thus, the pulse control gives the possibility of controlling the ignition unit  100  of the state of the art within the scope of the energization of the main spark plugs, with simply introduction of the delay, here 40 ms, due to the time-out  52  in the control signal. Consequently, the response time to the first firing is 220 ms while the period of the following firings remains 180 ms during the continuous control  53  since there is then no time-out. 
         [0063]      FIG. 6  is a time diagram on which is illustrated the time-dependent change  60  of the control signal according to a possible embodiment of the invention within the scope of the energization of an afterburning spark plug. 
         [0064]    An afterburning spark plug ignition pulse  61  of a determined duration, in this case 80 ms, during which the signal assumes a high level, followed by a time-out  62  of 40 ms during which the signal assumes a lower level, tells the control module  204  that the afterburning spark plug ignition channel  223  is the one which has to be activated in order to energize the afterburning spark plug so that its firing causes afterburning ignition. 
         [0065]    The middle curve  65  illustrates the charging energy of the capacitive block  209  in response to the control signal illustrated by the curve  60 . The pulse  61  and time-out  62  are followed by a continuous control  63  at a signal level corresponding to the pulse signal level  61 . The charging of the capacitive block  209  begins with the continuous control  63 , after the control module  204  has decoded the control signal. 
         [0066]    The response time to the first firing of the afterburning spark plug is formed by the duration of the pulse  61 , i.e. 80 ms, the duration of the time-out  62 , i.e. 40 ms, and the charging time during the application of the continuous control  63 . The characteristics of the common electric power supply channel  205  are selected so that the time for charging the capacitive block  209  is reduced relatively to the charging time of the state of the art, so that the response time to the first firing remains substantially the same as in the state of the art i.e. 180 ms. The response time to the following firings are preferably substantially the same, i.e. 180 ms. The firing sequence of said at least one main spark plug is maintained as long as lasts the continuous control  63 . 
         [0067]    The lower curve  66  illustrates the charging energy of the capacitors  121  of the ignition channel  120  of the afterburning spark plug in an ignition unit  100  of the state of the art, in response to the control signal illustrated by curve  60 . The charging of the capacitors  121  begins as soon as the pulse  61  since the ignition unit of the state of the art interprets said pulse as an applied voltage step of the controlled signal in the state of the art. First charging  67  therefore begins from the beginning of the pulse  61 , since the charging is interrupted at a stable level  68  during time-out  62  before resuming in a second phase  69  when the continuous control  63  is applied until the firing. The following firings are similar to those of the state of the art since the continuous control  63  which is then applied, is similar to the voltage step of the control signal of the state of the art. 
         [0068]    Thus, the pulse control gives the possibility of controlling the ignition unit  100  of the state of the art within the scope of energization of afterburning spark plugs, with simply introduction of a delay of 40 ms due to the time-out  62  of the control signal. Consequently, the response time to the first firing is 220 ms while the period of the following firings remains 180 ms. 
         [0069]    The compatibility of the pulsed controls is complete with the existing systems. The result of this is that the control signal may be adapted for having pulsed controls before setting into place an ignition unit  200  according to invention. Further, the pulsed controls do not require any additional control channel and are compatible with the existing computers, as well as with the connections of these computers with the control units. 
         [0070]    The pulsed commands have the advantage of being easily identifiable with the ignition unit according to the invention. Also, selecting the ignition by the pulse duration is also easy to process with the ignition unit according to the invention. Indeed, the logic of such a control is simple and does not require any complex detection circuit. 
         [0071]    Further, the pulse command may easily be developed for integrating other functionalities. Notably, preheating of the spark plugs may be set into place by means of a dedicated pulse width, with a pulse width corresponding to the preheating of a main spark plug and another width corresponding to the preheating of the afterburning spark plug. 
         [0072]    The wear of the spark plugs may also be monitored by injecting different energy levels into the spark plugs. In this case, a specific pulse command is sent to the unit  200  by the control signal in order to inject a corresponding energy level. The determination of the energy level below which said spark plug does not fire, allows determination of the wear of the spark plug, in order to be able to possibly proceed with preventive replacement. 
         [0073]    Preferably, the duration of a pulse of the control signal for controlling the activation of an ignition channel depends on the energy required for energization of the correspondent spark plug at said ignition channel. The duration of the pulse of the control signal may thus be selective, all the shorter since the charging energy of the capacitive block  209  required for energization of a spark plug is significant. For example, in the case when the energization of an afterburning spark plug requires 0.5 J and the energization of the main spark plug requires 2.2 J, the duration of a pulse of the control signal for activating the afterburning spark plug ignition channel is greater than the duration of a pulse of the control signal for activating the main spark plug ignition channel. Thus, the charging of the capacitive block  209  starts all the faster since the required energy is high. 
         [0074]    The ignition unit  200  according to the invention is compatible with the control signal in the form of a voltage step, of the system of the state of the art.  FIG. 4  is a time diagram illustrating the behavior of the ignition unit  200  according to a possible embodiment of the invention in response to a control signal of the state of the art. The control signal has a voltage step  40  for controlling the firing of the main spark plugs and of the afterburning spark plugs. The lower curve  41  illustrates the charging of the capacitive block  209  of the ignition unit  200 . 
         [0075]    In the presence of a voltage step in the controlled signal, not preceded with a pulse, the ignition channel of the afterburning spark plugs  220  and the ignition channel  210  of the main spark plugs are activated alternately, causing in alternation firing of the afterburning spark plugs and of the main spark plugs, respectively. 
         [0076]    However, the presence of a voltage step is only determined after the longest duration of the two pulse durations respectively corresponding to the pulse controlling the activation of the ignition channel of the main spark plugs and of the afterburning spark plugs, by the absence of a falling front and of a time-out which would determine the presence of a pulse. With the exemplified durations, a duration of 120 ms is reached corresponding to the 80 ms pulse followed by the 40 ms time-out controlling the ignition of the afterburning spark plugs. 
         [0077]    When the control module  204  is in presence of a voltage step and has determined the absence of pulses at the end of a predetermined duration, the control module determines that the main spark plugs and the afterburning spark plugs should fire alternately. 
         [0078]    For this purpose, the charging of the capacitive block  209  has in this case a first charging phase  42  at the end of which firing of the afterburning spark plugs occur, followed by a second charging phase  43  at the end of which firing of the main spark plugs occurs, followed by a third charging phase  44  at the end of which firing of the afterburning spark plugs occurs followed by a fourth charging phase  45  at the end of which firing of the main plugs occurs, etc. The firing alternation of the main spark plugs and of the afterburning spark plugs continues as long as lasts the voltage step of the control signal. 
         [0079]    Preferably, the duration between two consecutive firings of a spark plug is the same during the firing sequence, and preferably the same durations separate two consecutive firings of the ignition spark plugs and two consecutive firings of the afterburning spark plugs. 
         [0080]    In the example shown, the consecutive firings of the main spark plugs are spaced apart by 180 ms and the consecutive firings of the afterburning spark plugs are also spaced apart by 180 ms. 
         [0081]    Thus, the ignition unit according to invention may be controlled by means of a control signal of the state of the art. Therefore, the ignition unit according to the invention may be set into place before adapting the control signal. 
         [0082]    The response times of the ignition unit according to the invention may be improved, both in the case of a control signal with pulses and with the control signal of the state of the art only having one voltage step, by beginning the charging of the capacitive block  209  as soon as the control signal passes to a signal level corresponding to a pulse or a voltage step. Indeed, since the capacitive block  209  is unique, the indication by the control signal that ignition is desired necessarily causes the requirement of charging the capacitive block  209 , both in the case when the control signal indicates that only one of the ignition channels should be activated and in the case when both ignition channels have to be activated alternately. 
         [0083]    Typically, an ignition system for a turbojet engine according to invention will comprise:
       an ignition unit  200  as described above,   a main spark plug of a main combustion chamber which may be energized through the main spark plug ignition channel  210  of said unit,   an afterburning spark plug which may be energized through the afterburning spark plug ignition channel  220  of said unit,   a computer connected to the control channel  202  of said unit for sending a control signal to said unit.