1. Field of the Invention
The subject invention is directed to control systems for gas turbine engines, and more particularly, to power turbine overspeed limiters for turboshaft engines used in helicopters.
2. Background of the Related Art
Overspeed limiters for gas turbine engines are known in the prior art. The function of an overspeed limiter is to prevent the occurrence of excessive turbine speeds by limiting fuel flow to the engine. Examples of prior art overspeed limiters are described in U.S. Pat. No. 4,302,931 to White et al. and U.S. Pat. No. 4578,945 to Peck et al., the disclosures of which are herein incorporated by reference in their entireties.
Turboshaft engines installed on helicopters require a unique power turbine overspeed limiter. It needs to protect the engine against a loss of load and potential power turbine overspeed, and preclude false overspeed limiter trips. False limiter trips significantly reduce engine power, and can occur when a pilot is flying the helicopter aggressively.
In the past, these requirements have been addressed by providing overspeed limiters that do not completely shutdown the engine when an overspeed condition is detected, but instead trip to a minimum fuel flow condition. Other prior art systems shutdown the engine when an overspeed condition is detected, and then restart the engine using an auto-relight feature.
In modem low inertia gas turbines, minimum fuel flow systems are not capable of containing loss-of-load power turbine overspeeds at high altitude, because the horsepower requirements are too significant. Auto-relight systems are not guaranteed to restart an engine under all operating conditions, especially when the primary engine control system has failed and the engine is operating with a simple manual backup control.
Accordingly, there is a need in the art for an overspeed limiter for a turboshaft engine that is capable of effectively handling power turbine overspeeds at both high and low altitude.
The subject invention is directed to a control system for a turboshaft engine utilized in a helicopter which includes means for providing minimum fuel flow to the engine when an overspeed condition is detected at a relatively low altitude (e.g., below 10,000 feet), and means for shutting off fuel flow to the engine, and thus shutting down the engine, when an overspeed, loss of load condition is detected at a relatively high altitude (e.g. above 10,000 feet), provided that the overspeed, loss of load condition is detected along two different engine speed signal paths, including a derivative path and a non-derivative path.
The subject invention is also directed to a control system for a turboshaft engine which provides a desired minimum fuel flow to the engine when an overspeed condition is detected and when the engine is operating in a first operating range and shuts off fuel flow to the engine when an overspeed condition is detected and the engine is operating in a second operating range. It is presently envisioned that an overspeed condition is detected based on at least one of, for example, a power turbine speed signal, a gas generator speed signal and main rotor speed signal.
Preferably, in the first operating range the engine is operating at a relatively low altitude and in the second operating range the engine is operating at a relatively high altitude. In a representative system, the relatively low altitude includes 0 to 10,000 feet above sea level and the relatively high altitude exceeds 10,000 feet above sea level, for example.
In a preferred embodiment, an overspeed condition is detected along two different control logic paths, a first logic path which includes derivative and non- derivative control logic and a second logic path which includes non-derivative logic (e.g. proportional logic).
It is envisioned that the control system further includes a hardware latch having reset logic associated therewith. Additionally, the control system can include software test interfaces for testing the performance of the control system.
In a representative embodiment, a minimum fuel flow is provided to the engine when an overspeed condition is detected through a first solenoid valve and the fuel flow to the engine is shut off when an overspeed condition is detected by a second solenoid valve.
In preferred embodiments the control system is a dual channel system having an interchannel communication means for ensuring that a fault in one channel of the system will not shut down the control system or impact the systems ability to limit engine overspeed.
The present invention is also directed to a control system for a turboshaft engine which includes a mechanism for shutting off fuel flow to the engine when an overspeed condition is detected, wherein the overspeed condition is detected by first and second control logic paths. It is envisioned that the first control logic path includes a derivative path and a non-derivative path and the second control logic path includes a non-derivative path. Preferably, the control system includes a device for disabling the fuel shut-off mechanism when the engine is operating below an altitude of 10,000 feet.
The present disclosure is also directed to a control system for a turboshaft engine which includes a mechanism for providing a minimum fuel flow to the engine when an overspeed condition is detected in a first operating range; and a mechanism for shutting off fuel flow to the engine when an overspeed condition is detected in a second operating range. It is preferred that the overspeed condition is detected along two different engine speed signal paths, including a derivative path and a non-derivative path.
Preferably, in the first operating range the engine is operating at a relatively low altitude and in the second operating range the engine is operating at a relatively high altitude. In a representative system, the relatively low altitude includes 0 to 10,000 feet above sea level and the relatively high altitude exceeds 10,000 feet above sea level, for example.
In a preferred embodiment, an overspeed condition is detected along two different control logic paths, a first logic path which includes derivative and non-derivative control logic and a second logic path which includes non-derivative logic (e.g. proportional logic).
It is envisioned that the control system further includes a hardware latch having reset logic associated therewith. Additionally, the control system can include software test interfaces for testing the performance of the control system.
In a representative embodiment, a minimum fuel flow is provided to the engine when an overspeed condition is detected through a first solenoid valve and the fuel flow to the engine is shut off when an overspeed condition is detected by a second solenoid valve.
In preferred embodiments the control system is a dual channel system having an interchannel communication means for ensuring that a fault in one channel of the system will not shut down the control system or impact the systems ability to limit engine overspeed.
The present disclosure is also directed to a method for limiting turboshaft engine overspeed that includes measuring at least one engine speed parameter; sensing the altitude at which the engine is operating and determining whether an overspeed condition exists based on the measured speed parameter. The disclosed method uses a control system which includes a first logic path that has derivative and non-derivative control logic and a second logic path that has non-derivative logic. The inventive method also includes the steps of providing a desired minimum fuel flow to the engine when an overspeed condition is detected by the engine control system when the engine is operating in a first altitude range; and shutting off fuel flow to the engine when an overspeed condition is detected by the engine control system and the engine is operating in a second operating range.
These and other aspects of the subject invention will become more readily apparent to those having ordinary skill in the art from the following detailed description of the invention taken in conjunction with the drawings.
So that those having ordinary skill in the art will better understand how to make and use the overspeed limiter system of the subject invention, embodiments thereof will be described below with reference to the drawings wherein:
a is a cross-sectional view of a portion of the shaft for the power turbine having three speed probes positioned adjacent thereto for detecting a shaft speed signal;
The subject invention overcomes the disadvantages of prior art overspeed limiters used on turboshaft engines in helicopters by preferably providing both a minimum flow overspeed system for low altitude operation (e.g., below 10,000 feet) and a fuel shutoff system for high altitude (e.g., above 10,000 feet) overspeed/loss-of-load conditions.
The system of the subject invention is uniquely implemented in analog hardware and in the software of the primary engine control unit (ECU) to satisfy all certification standards for: a) the probability of loss of overspeed protection; b) the probability of a false overspeed trip; and c) the probability of a single failure causing an engine overspeed condition and loss of overspeed protection at the same time.
Referring now to the drawings, wherein like reference numerals identify similar aspects or features of the subject invention, there is illustrated in
A pressurizing and leak tight shutoff valve 26 is in fluid communication with fuel metering valve 18. It delivers fuel to the engine by way of a dual coil true minimum flow overspeed solenoid valve 28, which is normally de-energized. A fuel regulator valve 30 also receives filtered fuel from filter 16. Regulator valve 30 communicates with the jet inducer 12 and also communicates with a dual coil overspeed and fuel shutoff solenoid valve 32 that is normally de-energized. Regulator valve 30 is also in fluid communication with fuel metering valve 18. The fuel shutoff solenoid valve 32 is also in fluid communication with the pressurizing shutoff valve 26. As explained in below, the overspeed limiter system of the subject invention controls the operation of the two dual coil solenoid valves 28 and 32. During normal operating conditions, both solenoid valves 28 and 32 are de-energized.
Referring now to
As illustrated, two NP engine signals (i.e., power turbine speed signals) are brought into each of the two engine control unit (ECU) channels (Channel A and Channel B). As shown, two Analog Overspeed Limiter Lanes of protection are provided for each channel. The logic for these lanes of overspeed protection have been identified by reference numerals 41a/43a for Channel A and reference numerals 41b/43b for Channel B. Each ECU channel includes a respective shutdown coil (32a, 32b) of fuel shutoff solenoid 32.
Channel A, Analog Overspeed Limiter Lane 1, identified by reference number 41a, includes a derivative path and it receives the engine speed signal NPeng3. Channel A, Analog Overspeed Limiter Lane 2, identified by reference number 43a, includes a non-derivative path (e.g. proportional logic) and it receives the engine speed signal NPEng2. Channel B, Analog Overspeed Limiter Lane 1, identified by reference number 41b, includes a derivative path and it receives the engine speed signal NPEng2. Channel B, Analog Overspeed Limiter Lane 2, identified by reference number 43b, includes a non-derivative path (e.g., proportional logic) and it receives the engine speed signal NPEng1.
The hardware voting scheme of
NPEng1+NPEng2=derivative path+non-derivative path
NPEng2+NPEng3=derivative path+non-derivative path
NPEng1+NPEng3=derivative path+non-derivative path
This voting scheme ensures that under all valid votes, one derivative and one non-derivative engine speed signal path lane detects an overspeed condition. It should be noted that the disclosed control system provides high side and low side drivers to each of solenoids 32a and 32b. As shown, Lane #1 of Channels A and B control the high side drivers for solenoids 32a and 32b and Lane #2 of Channels A and B controls the low side drivers for each of these solenoids.
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Although the subject invention has been described with respect to preferred embodiments, those skilled in the art will readily appreciate that changes and modifications may be made thereto without departing from the spirit and scope of the subject invention as defined by the appended claims.
This application claims priority to U.S. Provisional Patent Application Ser. No. 60/576,779, filed Jun. 3, 2004, the disclosure of which is herein incorporated by reference in its entirety.
Filing Document | Filing Date | Country | Kind | 371c Date |
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PCT/US05/19699 | 6/3/2005 | WO | 12/8/2006 |
Number | Date | Country | |
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60576779 | Jun 2004 | US |