GB2307950A

GB2307950A - Fuel control for turbojet engine

Info

Publication number: GB2307950A
Application number: GB9625548A
Authority: GB
Inventors: Peter Wittmann
Original assignee: MTU Motoren und Turbinen Union Muenchen GmbH
Current assignee: MTU Aero Engines GmbH
Priority date: 1995-12-09
Filing date: 1996-12-09
Publication date: 1997-06-11
Anticipated expiration: 2016-12-09
Also published as: DE19545987A1; DE19545987C2; GB9625548D0; FR2742188A1; GB2307950B; FR2742188B1

Description

2307950 f ' Limiting excessive speeds in turbo-jet engines The invention

relates to a device for protecting an engine from racing or excess speeds by controlling the fuel supply, comprising a main metering valve, whose valve setting is adjustable and whose pressure difference between inlet and outlet is kept constant by a differential pressure controller arranged parallel to the main metering valve, and a safety feedback valve which, independently of the main metering valve, on excess speeding of the engine, feeds the fuel back to the fuel tank before it enters the main metering valve, in such a way that a maximum permissible engine speed is kept constant.

Protection from excess speeds during engine operation is carried out, in normal circumstances, via regulatory loops and control units which act upon the main metering valve, and is described in patent DE-38 30 805 C2. Protection from excess speeds in the case of unforeseen malfunctions of the main metering valve with which the fuel for the engine is regulated has until now been ensured, independently of the main metering valve and its control and regulatory loops, by a safety feedback valve which, on opening, guides the fuel via a bypass in the fuel line to the main metering valve back to the fuel reservoir even before the fuel reaches the main metering valve, partially or completely in dependence on the overshoot behaviour of the engine. The quantity of fuel branched off by means of the bypass is controlled by the safety feedback valve in such a way that the speed of the engine is stabilised and fixed at a given maximum speed value. This engine condition is referred to henceforth as emergency running. Intervention by means of the pilot's gas throttle has up until now only been possible at the main metering valve, or its control or regulatory loops, which means that, in the event of a malfunction of the main metering valve and after activation of the independently acting safety feedback valve, the turbine is driven constantly at the maximum permissible speed in the emergency running condition, and the possibility of intervention is limited to switching off the engine. Disadvantageously therefore up until now, for each parameter to be monitored, such as low pressure turbine speed or high pressure turbine speed, only a predetermined constant maximum permissible mechanical rate of revolution has been available to which the safety feedback valve regulates the speeds. The disadvantage in such prior art systems is that no possibility for modulation or intervention by the pilot exists after switching over to the safety feedback valve with the engine in emergency running in a stationary condition becoming overheated. Hence partial secondary damage cannot be excluded, since the only fall-back position is a limitation of the maximum permissible mechanical speed.

Hitherto switching over to and activation of the safety feedback valve has involved considerable dangers for the engine. Since the activation of the safety feedback valve has up until now depended on an overshoot of the maximum permissible speed, as described above, on switching over, a considerable overshoot of the maximum permissible speed occurs until the fuel is regulated back to the actual required desired fuel value. The maximum permissible speed must therefore be set so that under no circumstances will the maximum rotational disc speed (beyond which disintegration of a disc can occur) be exceeded during the overshoot. Under certain circumstances this can mean that in uncritical ranges the engine cannot be driven up to its limits.

Particularly serious overheating can occur in is present protection systems with the predetermined fixed maximum speed values if, at the time of a malfunction of the main metering valve, the engine is being operated at a predetermined low operational speed, since in this case the engine is accelerated to the maximum permissible speed by the safety feedback valve in a relatively short time regardless of overheating.

A further risk of engine instability with present protection systems with an independent safety feedback valve occurs when, for example, only a partial function of the main metering valve system fails, such as the constant maintenance of the differential pressure by the main metering valve. With breakdowns of this type the main metering valve still functions, but with a steeper characteristic curve, for which the control and regulatory loops of the main metering valve are not designed, so that pump effects, unstable overshoots and overheating can arise, which can lead to damage to or destruction of the engine.

A further disadvantage of the prior art is that, up until now, the safety feedback valve has been controlled by a logical control element corresponding to a PI-controller. A PI-controller has the disadvantage that the integrating section, which at the end forms the necessary desired fuel value, must be set up using a desired/actual deviation (overshoot), i.e. the size of the overshoot is dependent on the current fuel consumption. Furthermore the transient behaviour of a PI-controller is characterised by several overand undershoots. In particular undershooting brings the danger of the engine cutting out.

An object of the present invention is to provide a device for protecting an engine from excess speeds and temperatures which overcomes the above disadvantages of the prior art and maintains the controllability of the engine despite malfunctions of individual components at least within limits. Furthermore it is an object of the invention to reduce the previously common long reaction times before the protection from excess speeds and temperatures operates.

This object is solved by using a computer to detect a malfunction of the main metering valve and, in order to maintain the controllability of the engine, to apply at least part of the functions of the main metering valve to the safety feedback valve.

For this purpose, monitoring of the engine speeds by means of sensors is available, as previously, for detecting a malfunction. When partial functions of the main metering valve, in particular the control of the running speed of the engine, are imposed on the safety feedback valve via a computer which recognises malfunctions, the hitherto extremely risky transfer from the main metering valve to the safety feedback valve can be made more reliable. The step of imposing partial functions on the safety feedback system on transferring control and regulation of the fuel supply from the main metering valve ensures that the engine remains controllable, at least within limits, instabilities are suppressed, temporary excess temperatures with the risk of burning out the engine are reduced and stationary excess temperatures are avoided.

In a preferred embodiment of the invention the computer includes a PIDcontroller, for which a delay or deceleration control loop having the highest priority, with respect to the signals relating to the speed, forms an input signal, and which gives as its output signal a desired fuel value for the safety feedback valve. In this way the usual priority list, used up until now, for the main metering valve, as given in DE-38 30 805 C2, with which the data relating to speed are processed with the highest priority, is is turned on its head. This has the advantage that, after an overshoot and the initiation of the control of the safety feedback valve, the overshoot in the speed is stabilised or delayed with the highest priority and guided back to a desired value without any undershoot and without a multiple overshoot.

In a further preferred embodiment of the device the computer has a PIDcontroller, which can be the same PIC-controller as just described, to which is connected, as an input signal, a pilot function for the desired fuel value which is calculated by the computer from the operational data of the turbine and is weighted by a suitable factor. This action, which cannot be realised with the previously used hydromechanical or electricalanalogue control and regulatory loops, has the advantage that the amount of the fuel supply is coarsely set by the pilot function with a safety margin given by the factor and only a remaining minimal difference from the operational speed needs to be finely adjusted or compensated for by the safety feedback valve.

In a further preferred embodiment of the device the desired valve setting value of the main metering valve is subjected to a predetermined lower limit, the limit being calculated by means of the computer from the actual fuel consumption and the operational data of the turbine and weighted with a reduction factor. if this lower limit is passed, switch- over occurs and the further control of the fuel supply is carried out via the safety feedback valve using the computer. The lower limit of the set point of the main metering valve advantageously means that the main metering valve, in the case of failure of the differential pressure controller, is prevented from continuing further with a steeper characteristic curve and from driving the engine into instabilities which can no longer be 1 20, controlled. The regulation and control in the region of the fuel supply that can no longer be controlled by the main metering valve is taken over automatically by the safety feedback valve.

Preferably in a further embodiment of the present invention an operating threshold value for a monitoring speed NO, i.e. the upper limit of the allowed engine speed range, is not fixed but is matched by means of a follow-up logic to the respective operationallyconditioned speed with a minimal difference. In this variation of the device, in contrast to the concept of the device up until now, no fixed maximum speed is predetermined, rather a speed differential for the initiation and takeover by the safety feedback valve is defined, which is applied to the respective operational speeds with the aid of the computer.

For this purpose the operating threshold value for the monitoring speed N. is matched in a network by means of a maximum value node (MAX), a minimum value node (MIN) and a feedback of the most recent comparison value of the maximum value node (MAX), to a desired speed value (N..,,), presettable by the pilot, and the actual speed (Ni.t) of the engine, wherein in the minimum value node (MIN) the actual speed is compared with the last fed-back comparison value, and the smaller of the two values in the maximum value node (MAX) is compared with the desired speed value (N.. ,,) and the larger of the two values forms a new comparison value, so that a follow-up desired activation value for the monitoring speed (N.) is always available.

The detection of malfunctions is, with embodiments of the invention, not dependent on the presence of additional monitoring sensors, for example of the main metering valve, though such sensors can be present. Rather the device in accordance with the invention preferably detects a malfunction through a speed is overshoot over a followed-up operating threshold value. This has the advantage that the monitoring sensors can be maintained as previously and malfunctions can be detected independently of the engine reaction and countermeasures can be introduced.

Malfunctions of the engine and thus speed overshoots are mainly caused by failure of the differential pressure controller or by failure of the valve adjuster. With the device in accordance with the invention these malfunctions are brought under control by the safety feedback valve with the computer connected upstream, in such a way that the temporarily occurring maximum overshoot and excess temperature is minimised, the engine does not experience stationary overheating, does not fall into unpredictable instabilities and remains controllable.

For a better understanding of the invention embodiments of it will now be described, by way of example, with reference to the accompanying drawings, in which:

Fig. 1 shows a schematic plan of a device in accordance with the invention with a computer controlling a safety feedback valve; 2 shows input signal connections for a PID-controller in the computer for setting a desired fuel value for the safety feedback valve; Fig. 3 shows a logical network for following-up a monitoring or activation value for the engine speed; and Fig. 4 shows an example of the variation in speed with follow-up of the operating threshold value for the engine speed in accordance with the logical network according to Figure 3.

Fig Fig. 1 shows a schematic plan of the device in accordance with the invention with a computer 15, which controls a safety feedback valve 12. In this plan fuel-carrying lines 2, 3, 6, 11, 13, 17, 19 and 20 are shown by double lines and the main direction of flow of the fuel inside the lines is shown with an arrow. Signal lines 8, 10, 14 and 16 are shown as single lines.

The fuel from the fuel reservoir 1 is fed via a supply line 2 to a fuel pump 4, which provides for a corresponding overpressure in the overpressure pipe 17. The overpressure pipe 17 feeds the fuel to a main metering valve 7. This main metering valve has a valve adjustment device 18 and a differential pressure controller 5. The differential pressure controller 5, which is arranged in the fuel line parallel to the main valve, or rather to its valve adjustment device 18, measures, via lines 3 and 19, the pressure difference before and after the valve adjustment device 18 and maintains a constant pressure difference with the aid of the return pipe 6 to the fuel reservoir 1. As a result of the stabilised pressure difference in the main metering valve 7, a reproducible fuel flow is achieved with the aid of the valve adjustment device 18. The valve adjustment is controlled via the signal line 8 by a control and regulatory apparatus 9, which for its part receives operating condition parameters via the signal lines 10 from the engine (not shown) and compares them with preset desired operating values.

via the signal lines 16 the computer receives signals which denote the operational condition of the engine, particularly speed values of the engine shafts, such as the high pressure turbine speed and/or the low pressure turbine speed. With the aid of data of this type the computer detects malfunctions of the main metering valve, particularly if an overshoot of the 0 speed over a predetermined desired value occurs. In this case, in accordance with the invention, in order to maintain the controllability of the engine the safety feedback valve 12 is made to perform at least part of the functions of the main metering valve 7. To this end the safety feedback valve 12 opens and partially allows the fuel to flow back via the fuel return line 13 into the fuel reservoir 1.

The desired fuel value, which is to be kept to by the safety feedback valve 12, is controlled by a PIDcontroller 21, whose input signal connections are shown in Figure 2. In contrast to the main metering valve 7 and the solutions known up until now, via the node V. with the input values for a maximum deceleration Vmax and an actual acceleration Bi, and via the maximum node MAX, as shown in Figure 2, the deceleration control loop receives the highest priority as compared to the parameters relating to speed, such as maximum speed N,,,, actual speed Ni,,t, desired speed N. ,.11 or corresponding acceleration values, such as B,,.,, and Bi., which are connected via the nodes V2, V3 and V, and are supplied as inputs to the minimum node MIN and thence to the maximum node MAX. With this input signal combination for the PID of the computer 15 it is ensured that a speed overshoot is eliminated with enough delay or deceleration to suppress an overshoot or a build-up of vibrations.

Figure 2 shows, in addition, that the PIDcontroller 21 is acted upon in the computer by a pilot function. To this end the necessary fuel consumption is calculated from empirical data and functions, as well as measured actual values, such as low pressure turbine speed and/or high pressure turbine speed, dynamic pressure at the input and temperature at the input, and is fed to a proportional control element 22 as a signal T,, j, which multiplies or weights this is value, for example, by a factor of 1.1 to 1.3 and outputs it as a pilot signal T... to the PID-controller 21 in the computer 15 in the case of an activation of the safety feedback valve 5. Consequently the PIDcontroller need only adjust or compensate for the remaining fuel difference of 10 to 30% up to the desired fuel value T.,.11, corresponding to a desired speed value.

Figure 3 shows a logical network for following up or in-service adjustment of the monitoring or activation value N. for the engine speed at which switching-over to the control of the safety feedback valve 12 via the computer 15 occurs.

In the prior art this monitoring or activation value is at or slightly below the maximum permissible speed N,,.. of the engine, which is fixed at a level which avoids serious damage to the engine. In Figure 3 a logical network in accordance with the invention is proposed that is incorporated into the computer 15 and ensures a relatively early takeover of control by the safety feedback valve 12 without using engine-specific data. The great advantage of following-up is that no enginespecific data need be used and therefore no costly engine test runs are necessary in order to determine and confirm engine data. The monitoring and activation value Na is higher by a speed difference AN than the desired speed value N.,.11 or the actual speed value Nj.,t, depending on which value is the greater, this being decided by the MAX-combination node in Figure 3. In order to prevent an overrun of the N.-value in the case of a defect- induced overrun of the engine, the last calculated value is compared with the actual speed value in the MIN-node of Figure 3 by the feedback 31, so that the erroneous overrun of the engine is detected and N. is frozen.

Figure 4 shows, for a typical example, the variation of the speed using follow-up of the monitoring value N. f or the engine speed in accordance with the logical network according to Figure 3. The time axis is shown as t and the speed axis as N. The desired value N.,.11 and the actual value Ni., are at the start, with t=O, in this example the same, i.e. they start from a stationary or balanced operational condition. At the time point t,, the desired speed is reduced by the pilot's intervention. The actual speed N,., does not follow this jump in the desired value, but rather decreases linearly, for example. Even before the lower desired value given by the pilot is reached, a malfunction might arise at the time point t, for example, that allows the actual speed Ni.t to rise in an uncontrolled fashion, so that the corresponding Ni.tF increases. At the moment of breakdown the monitoring speed N. remains at the value NCIF which was last calculated as a result of the logical connections according to Figure 3. If the actual speed continues to increase unchanged, then the safety feed back valve 12 takes over, at the latest at time point t2 when the actual speed reaches N,,,, again, all control functions for controlling the fuel for the engine with the aid of the computer 15 and regulates the fuel downwards so that the actual speed decreases in the direction of arrow A until it reaches the desired value N.,,,11. A renewed controlled rise in the speed at the time point t4 is also possible with the device in accordance with the invention by means of the safety feedback valve 12.

As a comparison the diagram of Figure 4 after the malfunction at ti shows with a dashed line the variation of the acceleration of the engine in the direction of arrow B as it would proceed with known safety measures, so that a switching-over to the safety feedback valve can only occur at t3, an overshoot being unavoidable and the engine being maintained at the maximum monitoring speed N,,,.e, until the pilot switches the engine off completely or the main metering valve 7 unexpectedly resumes its function.

is

Claims

1. A device for protecting a turbo jet engine from excess speeds regulating the fuel supply, comprising:

a main metering valve, whose valve setting is adjustable; a differential pressure controller arranged parallel to the main metering valve and adapted to keep its pressure differential between inlet and outlet constant; and a safety feedback valve which is adapted, independently of the main metering valve, on excess speeding of the engine to feed the fuel back to the fuel tank before it enters the main metering valve, in such a way that a maximum permissible engine speed is maintained constant; characterised in that the device further includes a computer adapted to detect a malfunction of the main metering valve and, in order to maintain the controllability of the engine, to carry out at least some of the functions of the main metering valve by controlling the safety feedback valve.

2. A device according to claim 1, in which the computer includes a PIDcontroller having as an input signal a deceleration control loop with the highest priority with respect to the signals relating to speed, and setting as an output signal a desired fuel value for the safety feedback valve.

3. A device according to claim 1 or 2, in which the computer has a PIDcontroller to which is connected as an input signal a pilot function for the desired fuel value, which is calculated by the computer from the operational data of the engine and weighted with a factor.

4. A device according to any preceding claim, in which the computer is adapted to calculate a lower limit of the desired value for the valve setting of the main metering valve from the actual fuel consumption and from the operational data of the engine and to weight it with a reduction factor, and to carry out the further control of the fuel supply via the safety feedback valve with the computer when this lower limit is passed.

5. A device according to any preceding claim, and further including a follow-up logic element for matching an operating threshold value for a monitoring speed (%) to the respective operationally-conditioned speed with a minimum difference (AN).

6. A device according to any preceding claim, in which an operating threshold value for a monitoring speed is matched by means of a maximum value node, a minimum value node and a feedback of the most recent comparison value of the maximum value node (MAX) to a desired speed value (Nsoll), presettable by the pilot, and the actual speed (Nist) of the engine, wherein the actual speed (Nist) is compared in the minimum value node (MIN) with the fed-back last comparison value and the smaller of the two values in the maximum value node (MAX) is compared with the desired speed value (Nsoll) and the greater of the two values forms a new comparative value, so that a followed-up desired activation value for the monitoring speed is always available.

7. A device according to any of claims 1 to 5, in which a speed overshoot above a followed-up operating threshold value is used to indicate the said malfunction.

8. A protection device substantially as described herein with reference to any of the accompanying drawings.

9. A method for protecting a turbo jet unit from excess speeds by means of a system for regulating the -is- fuel supply, the system comprising a main metering valve, whose valve setting is adjustable and whose pressure differential between inlet and outlet is kept constant by a differential pressure controller arranged parallel to the main metering valve, and a safety feedback valve which, independently of the main metering valve, on excess speeding of the engine feeds the fuel back to the fuel tank before it enters the main metering valve, in such a way that a maximum permissible engine speed is maintained constant; the method being characterised in that a malfunction of the main metering valve is detected by a computer and, in order to maintain the controllability of the engine, functions of the main metering valve are imposed by the computer on the safety feedback valve.

is