Power plants – Combustion products used as motive fluid
Reexamination Certificate
1999-07-08
2001-02-06
Thorpe, Timothy S. (Department: 3746)
Power plants
Combustion products used as motive fluid
C060S039190
Reexamination Certificate
active
06182438
ABSTRACT:
TECHNICAL FIELD
The present invention relates generally to a method and apparatus for starting a gas turbine engine, and, more particularly, to an improved method and apparatus for first simulating the anticipated running conditions of the engine during a calibration mode, and for subsequently operating the engine during an operational mode.
BACKGROUND ART
Small gas turbine engines, such as those used as auxiliary power units (“APU”), are normally controlled by metering the output of a fixed displacement fuel pump that is driven at a constant fraction of engine speed through a geared transmission. The metered flow rate is typically controlled through an engine speed control loop to achieve the desired engine power setting. In such systems, excess fuel pump output is typically bypassed across a relief valve, and constitutes a significant inefficiency.
During engine start up, it is necessary to control the mass flow rate of fuel from the engine-driven pump without the benefit of an engine speed control loop, since prior to engine light-off, fuel delivery rate has no influence on engine speed. This typically requires ancillary elements, such as a pressure regulator, a metering valve and some type of flow-measuring instrument. All of these elements add significant cost to the engine control system. Moreover, mass flow rate is a function of temperature, a variable that can quickly change. For example, a commercial jetliner may quickly move from one location, at which the tarmac temperature may exceed, say, 100° F., to another location at which the ambient temperature is less, say, than 32° F. It is common to operate the APU while the aircraft is being serviced at the gate. However, the mass flow rate of fuel necessary to start the APU will have to be adjusted for differences in the ambient temperature.
Accordingly, it would be generally desirable to provide an improved method and apparatus for starting a gas turbine engine that would omit the need for these elements. By omitting these, the overall cost of the engine control system can be reduced. Additionally, if such a simplified system were also to permit fuel pump speed to be controlled independently from that of the engine, then improved power efficiency during normal engine operation would also be realized.
DISCLOSURE OF THE INVENTION
The improved method and apparatus disclosed herein contemplates initially calibrating the fuel flow vs. motor speed characteristics when operated with the pump discharge directed through an orifice sized to accurately simulate the pressure/flow characteristics of the engine primary combustor nozzles. Various pump speeds are commanded by a pump controller and the resulting orifice pressure drops are recorded, along with fuel temperature. This data is then processed by the pump controller to establish a relationship between pump speed and mass flow rate.
The calibration orifice is then switched out of the pump flow circuit, and the flow is redirected to the engine combustor nozzles so that the pump controller can set the desired mass flow rate based on the calibration data. After engine light-off, the engine controller transitions to its normal operating mode, in which pump speed is modulated in response to an outer control loop that is closed on turbine speed. This system thus economically compensates for variations in pump performance with temperature and wear.
With parenthetical reference to the corresponding parts, portions or surfaces of the disclosed embodiment, merely for purposes of illustrations and not by way of limitation, the present invention broadly provides an improved method and apparatus for starting a gas turbine engine.
In one aspect, the invention provides an improved method of starting a gas turbine engine (
17
) having a variable-speed motor-driven pump (
13
) operatively arranged to supply fuel from a source to the engine through a combustor nozzle (
19
) when the engine is operated normally. The improved method comprises the steps of: providing a calibration orifice (
16
) sized to simulate the pressure-flow characteristics of the nozzle; initially directing the flow from the pump through the orifice; operating the pump at preselected speeds; measuring the temperature of the flow at each of the speeds; measuring the pressure drops of the flow across the orifice at each of the speeds; calculating the fuel mass flow rate as a function of pump speed and temperature; selectively directing flow through the combustor nozzle rather than the orifice; and controlling the speed of the pump to produce a desired mass flow rate for starting the engine.
In another aspect, the invention provides improved apparatus (
10
) for defining the mass flow rate output of a motor-driven variable-speed fuel pump (
13
) as a function of pump speed during start up of a gas turbine engine (
17
). The improved apparatus comprises: at least one combustor nozzle (
19
); a calibration orifice (
16
) sized to simulate the pressure/flow characteristics of the nozzle; a valve (
15
) for selectively directing the flow to either the nozzle or the orifice; a controller (
12
) for operating the pump at various speeds; a temperature sensor (
14
) for measuring the temperature of the pump output flow; a pressure sensor (
18
) for measuring the pressure drop of the pump output flow across the orifice at different speeds; and means (
12
) for calculating the mass flow rate of the pump output flow as a function of pump speed; whereby the pump may be controlled to produce a mass flow rate.
Accordingly, the general object of the invention is to provide an improved method of, and apparatus for, starting a gas turbine engine.
Another object is to provide an improved method and apparatus for starting a gas turbine engine in which the mass flow rate of fuel delivered by a pump is determined as a function of pump speed and fuel temperature, such that approximately-correct quantities of fuel may be delivered to the engine during start up.
These and other objects and advantages will become apparent from the foregoing and ongoing written specification, the drawings, and the appended claims.
REFERENCES:
patent: 5305596 (1994-04-01), Lampe et al.
patent: 5369948 (1994-12-01), Vertens et al.
patent: 5966925 (1999-10-01), Torikai et al.
Gartenberg Ehud
Moog Inc.
Phillips, Lytle, Hitchcock, Blaine & Huber LLP
Thorpe Timothy S.
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