Reliable over-temperature control circuit

A turbine engine over-temperature shut-off control circuit includes a series of separate timing circuits and output control signal generation gates. A corresponding set of temperature set-point triggering comparator circuits apply energization signals to their associated timing circuits and gates, to define a temperature vs. time envelope, which turns the turbine off when the turbine has been at a predetermined dangerously high temperature for more than a predetermined length of time.

FIELD OF THE INVENTION 
This invention relates to safety, shut-off control circuits for turbine 
engines. 
BACKGROUND OF THE INVENTION 
Gas turbine engines are becoming more widely used, for example, in 
fire-fighting and other equipment where the public may be in physical 
proximity to the engine. Accordingly, the public liability factor has 
focused increased attention on the rare catastrophic failures of turbine 
engines, usually resulting from overheating, and on over-temperature 
protective circuits to prevent such accidents. 
The manufacturers of gas turbine engines have determined the maximum time 
periods that the engines can withstand over-temperatures at certain 
specified temperature levels. Over-temperature control circuits which have 
been proposed heretofore, have included derivative temperature monitoring 
circuits which utilized thermocouple signals. These circuits involved 
charge derived signals and were very susceptible to electromagnetic 
interference (EMI). Accordingly, they were not well suited to engine 
environments. 
It is therefore a principal object of the invention to provide a simple and 
more reliable turbine over-temperature protection circuit, and one which 
is compatible with present-day technology. 
SUMMARY OF THE INVENTION 
In accordance with the present invention, a turbine engine over-temperature 
shut-off control circuit includes a series of separate timing circuits and 
output control signal generation gates connected to receive output signals 
from the timers. A corresponding set of temperature set-point triggering 
comparator circuits receives an input signal indicating the turbine 
temperature, and they each apply energization signals to their associated 
timing circuits and enabling signals to the gates. This circuitry serves 
to define a temperature vs. time envelope, which turns the turbine off 
when the turbine has been at a predetermined dangerously high temperature 
for more than a predetermined length of time. 
Advantages of the circuit include the use of standard solid state chips 
which are highly resistant to EMI, and which have low cost and a high 
degree of reliability. Further, it has been determined that this type of 
digitized circuitry provides a more reliable and effective shut-off 
circuit than the charge controlled integrating circuits previously 
employed. 
Other objects, features, and advantages of the present invention will 
become apparent from a consideration of the following detailed 
description, and from the accompanying drawings.

DETAILED DESCRIPTION 
Referring more particularly to the drawings, FIG. 1 is a block diagram 
showing a turbine engine 12 with fuel control arrangements 14, which are 
operated by a signal from the over-temperature control circuit 16 to shut 
off the engine. 
FIG. 2 is a plot 18 of temperature vs. time showing an over-temperature vs. 
time plot for a particular turbine engine. Considering a particular 
temperature point TP-8 on the plot 18, it indicates that the engine may 
not be operated for more than six seconds at a temperature equal to or 
above 843 degrees Centigrade. Similarly, the points TP-2 and TP-5 indicate 
that the turbine engine may not be operated for more than one second at a 
temperature of 927 degrees Centigrade or above, or at a temperature of 780 
degrees Centigrade or above for more than 15 seconds. The two end points 
TP-2 and TP-5 as well as the central point TP-8 were provided by design 
specifications; and the Z-shaped configuration of characteristic plot 18 
was developed as compatible with the horizontally asymptotic nature of the 
curve at the temperature level of TP-5. 
Consideration will now be given to the digitization circuit of FIG. 3, and 
we will then return to the operation of the circuit in connection with the 
characteristic of FIG. 2. 
In FIG. 3 a series of comparator circuits 21 through 28 are provided, with 
each having one input connected to the temperature indication voltage lead 
30, and the other input lead connected via set-point potentiometers 31 
through 38 respectively, to a reference voltage 40. Each of the comparator 
circuits 21 through 28 will trigger to an ON state when the temperature 
input signal from lead 30 exceeds the reference potential provided by the 
set-point potentiometers. 
The output from comparator circuit 21 is supplied both to the timing 
circuit 41 and to the "NAND" logic circuit 51. Accordingly, when the 
temperature signal from the input lead 30 exceeds the reference voltage 
provided by potentiometer 31, a signal will be applied both to a timing 
circuit 41 and to the "NAND" circuit 51. A NAND circuit requires inputs at 
both of its two input circuits in order to provide an output signal. 
Incidentally, a "NAND" circuit is similar to an "AND" circuit, with the 
exception that it has an inverted output signal. If the temperature stays 
above the triggering level, for the duration required to provide an output 
signal from timing circuit 41, both of the input leads to the NAND circuit 
51 are energized, and an output signal will be transmitted through the 
isolation diode 61 to the output lead 70 to shut down the turbine engine. 
Of course, if the temperature drops below the triggering level for the 
comparator 21 before an output signal is provided from the timer 41, one 
of the inputs to the NAND circuit 51 is withdrawn, and no output signal 
occurs. 
The remaining timing circuits 42 through 48, NAND circuits 52 through 58, 
and their associated output diodes 62 through 68 form similar logic 
circuit arrangements for the other temperature set points as shown in FIG. 
2. Of course, each of the potentiometer and timing circuits is 
individually adjustable to define the desired envelope of the type shown 
in FIG. 2, which may of course vary for different turbine engines. 
Now, one dangerous time for the overheating of turbine engines is the 
start-up interval. Cooling is relatively low, and particularly if there 
has been a prior shutdown without a sufficient cooling-off period, the 
engine may overheat. Also, if the associated storage battery which may 
initially drive the compressor is weak or discharged, there by be 
insufficient initial cooling. In practice, the temperature may initially 
rise rapidly by as much as 300 degrees Centigrade per second. Accordingly, 
with reference to the circuit of FIG. 3 there may be a complex pattern of 
successive energizations of the comparators associated with successive 
temperature check points, with various of the timing circuits being in 
different stages of operation. 
For simplicity and for ease of understanding, however, in returning to a 
consideration of FIG. 2, the two straight lines 82 and 84 will be 
considered on the basis of an initial temperature of 920 degrees C. when 
the circuit is put into operation, and a declining temperature as 
indicated by the downward slope of lines 82 and 84. Initially, under these 
extremely hypothetical circumstances, all of the lower temperature check 
points would be energized, and the associated comparator-triggering 
circuits and timing circuits would be operative. 
Considering the more rapidly dropping temperature characteristic 84, first, 
it may be noted that, after about one-half second the temperature will 
have dropped to point 86 below 913 degrees C., the approximate temperature 
of TP-1. Accordingly, one input to the NAND gate 53 will be disabled, thus 
precluding any output from this circuit. Similarly, as time passes, before 
the timing circuits produce an output signal, the temperature has dropped 
below the critical level, so that one of the necessary inputs to each NAND 
circuit is missing, and the engine shut-off circuit is not actuated. 
Incidentally, it may be noted that the temperature points as shown in FIG. 
2 do not run sequentially from TP-1 at the highest temperature down to 
TP-8 at the lower temperature. Instead, they have been selected somewhat 
arbitrarily as they were actually constructed. However, in FIG. 3, for 
convenience, the circuits have been designated from top to bottom in order 
of their temperature set points, and they accordingly correspond to the 
sequence of the temperature points shown in FIG. 2 from the highest to the 
lowest temperature. 
Now, referring to the slower temperature drop-off illustrated by plot 82, 
the timing circuits associated with the first four higher temperature 
points do not permit the energization of the associated NAND gates. 
However, in connection with TP-8, involving the comparator 25 and the 
timer circuit 45, it may be noted that point 88 indicates a temperature of 
approximately 860 degrees C., as compared with the triggering point of 
approximately 843 degrees for comparator 25 associated with TP-8. 
Accordingly, both of the two inputs to the NAND circuit 55 will be 
energized after six seconds, and the shutoff control signal provided to 
lead 70 will be operative to turn off the turbine engine. 
Incidentially, it is again emphasized that the two straight line 
characteristics 82 and 84 are merely exemplary and have been considered in 
order to give a qualitative feel for the operation of the circuit. In 
practice, as noted above, there may be an initial rapid build-up of 
temperature for one reason or another, and then, as the compressor starts 
to operate at a more efficient level, the temperature will drop rapidly to 
more acceptable levels. 
It is again noted that, in connection with prior types of circuits which 
have been proposed, true integration circuits have been used with 
arrangements for weighting the integrated charge in accordance with the 
jet engine temperature, as well as the time of the over-temperature 
interval. However, these circuits tended to become overly complex, and as 
noted above, were subject to malfunction as a result of EMI. On the other 
hand, the present digital circuitry using standard logic circuits, avoids 
the sensitivity problems, and provides a high degree of reliability. Also, 
in the event of possible failure of one of the circuits, the other 
circuits provide a further measure of reliability by turning the turbine 
engine off after a slightly longer period of time. Incidentally, it may be 
noted that the logic circuits as shown in FIG. 3 may be implemented by 
standard C-MOS circuits. The timers 41 through 48 may be implemented by 
EXAR Part No. 559, and the comparators and NAND gates may be implemented 
by any standard C-MOS circuitry. 
In closing, it is to be understood that the foregoing detailed description 
and the drawings relate to one illustrative circuit for implementing the 
present invention. It could also be implemented through the use of other 
electronic circuits having equivalent logic functions, and including 
additional logic circuits, amplifiers, inverters, or signal regenerators 
to accomplish these functions. Accordingly, the present invention is not 
limited to that precisely as shown and described hereinabove.