Constant air/fuel ratio control system for EPU/IPU combustor

A fuel and air control system (10, 100) for a power unit which produces output power from a turbine which rotates in response to pressurized gas provided by a combustor (12) which combusts fuel which is injected into the combustor by a primary fuel injector (14) to produce the pressurized gas cooled by fuel injected into the combustor by a secondary fuel injector (16) in accordance with the invention includes a fuel control valve (30) coupled to a source of fuel which controls a combined fuel flow through a fuel flow path (32) from the control valve to the injectors in response to a fuel flow control signal; an air control valve (44) coupled to an air source which controls a mass flow of air through an airflow path to the combustor in response to an airflow control signal; and a controller (34), providing the control signals, for causing the valves to produce flows resulting in a constant air/fuel ratio in the combustor during combustion when the fuel flow and air flow are varied under control of the controller.

CROSS REFERENCE TO RELATED APPLICATION 
Reference is made to U.S. patent application Ser. No. 450,671 which was 
filed on Dec. 14, 1989 and is assigned to the Assignee of the present 
invention. Ser. No. 450,671 is incorporated herein by reference in its 
entirety. 
DESCRIPTION 
1. Technical Field 
The present invention relates to controls for systems for generating 
auxiliary or emergency power on airframes. 
2. Background Art 
Emergency power units (EPU) are utilized by airframes for generating 
emergency hydraulic and/or electrical power when the jet propulsion 
engines have flamed out for maintaining control of the airframe at 
altitudes where the air breathing auxiliary power unit (APU) will not 
operate. An APU is used for generating emergency power and power on the 
ground such as for starting. EPUs are typically used in high performance 
aircraft at altitudes above 40,000 feet where insufficient air exists to 
generate sufficient power with an air breathing APU. An integrated power 
unit (IPU) is an integrated system which performs the functions of an EPU 
and an APU. An EPU and an IPU utilize a combustor which decomposes a 
stored fuel and gas to generate a gas stream which powers a rotor of a 
turbine which drives one or more power generating units for generating the 
aforementioned emergency power to maintain control of the airframe during 
flameout or starting of the propulsion engines. 
The Assignee of the present invention manufactures combustors utilized in 
EPUs and IPUs which have a control valve regulating the flow of fuel to a 
primary injector and an airflow control which maintains a stoichiometric 
air/fuel ratio in a primary combustion zone and a control valve regulating 
the flow of fuel to a secondary fuel injector which injects fuel to cool 
the resultant combustion product gas stream prior to impingement on the 
turbine rotor to a temperature which will not damage the turbine rotor. 
The speed of a EPU turbine is controlled in systems currently manufactured 
by the Assignee of the present invention with separate primary and 
secondary fuel servo systems. Airflow is calculated by measuring pressure 
and temperature of air flowing into a venturi which is inputted to the 
combustor of the turbine. Primary fuel flow is controlled by the primary 
fuel servo based upon the calculated mass flow and the desired 
stoichiometric fuel ratio in the primary combustion zone of a constant 
13.25. The secondary fuel flow to the combustor is calculated to have an 
air/fuel ratio equal to a constant 2.30. Two separate fuel servo systems 
were utilized based upon the assumption that a separate temperature 
control loop was required for the turbine inlet temperature. Additionally, 
the secondary fuel injector is an atomizing injector which has limitations 
regarding the maximum fuel flow which may flow without destroying the 
atomized spray pattern. Two separate servos permitted maximum and minimum 
fuel flow limits to be applied to the secondary injector without affecting 
the primary fuel flow air/fuel ratio. 
U.S. patent application Ser. No. 450,671, assigned to the Assignee, 
discloses an airframe power unit in which the flow of fuel to the primary 
and secondary injectors is controlled by separate servo systems. Ser. No. 
450,671 includes a fuel control system which operates in the same way as 
the aforementioned control of turbine speed of EPUs manufactured by the 
Assignee of the present invention. 
The prior art fuel flow control systems for power units manufactured by the 
Assignee of the present invention require expensive metering valves as a 
consequence of separate servo controlled valves being present in the 
primary and secondary fuel circuits. The dynamic range of the primary fuel 
injector has a ratio of maximum flow rate to minimum flow rate of 20:1. A 
valve had to be used having a large turndown ratio as a consequence of the 
power unit requiring precise control of fuel metering throughout the 
dynamic range of flow from the minimum flow rate to the maximum flow rate 
to the primary combustor. Valves having a large turndown ratio are more 
expensive and have substantial hysteresis which decreases accuracy in the 
fuel control. 
DISCLOSURE OF INVENTION 
The present invention is a fuel and air control system and a method of 
controlling a power unit in which a constant air/fuel ratio is provided in 
a combustor having a primary fuel injector which injects fuel into the 
combustor to produce a pressurized gas which is cooled by fuel injected 
into the combustor by a secondary fuel injector in which a fuel control 
valve which is coupled to a source of fuel controls a combined fuel flow 
through a fuel flow path from the control valve to the injectors in 
response to a fuel flow control signal. The invention eliminates the 
separate primary and secondary fuel control servo systems contained in the 
prior art. The elimination of separate primary and secondary fuel control 
servo systems has advantages which are that the overall control system is 
simplified, control is accomplished with fewer independent variables with 
a resultant higher reliability and the expense of the fuel control valve 
is reduced as a consequence of the single valve having a lower turndown 
ratio than that which was required in the prior art primary fuel flow 
circuit. Furthermore, the present invention eliminates the use of a 
venturi for calculating mass airflow required to produce a constant 
air/fuel ratio. The venturi contained in the prior art system which is 
disclosed in Ser. No. 450,671 causes an additional pressure drop from the 
air pressure source. 
In a preferred embodiment of the present invention, the airflow control 
valve and the fuel flow control valve are integrated into a single valve 
in which airflow to the combustor and the combined rate of fuel flow to 
the primary and secondary injectors is controlled by a single signal from 
a controller. The signal controls a metering device which meters a rate of 
airflow and fuel flow by moving the metering device a distance which is a 
function of the power level produced by the power unit which is commanded 
by the controller. The single valve includes a mechanism for providing 
temperature compensation to change the distance which only the air 
metering device moves as a function of temperature of the air flowing 
through the valve. 
Preferably, the air supplied to the air control valve is provided at a 
constant pressure from the output of a pressure regulator coupled to a 
higher pressure source of air. Elimination of the venturi in the prior art 
which was used to calculate the mass flow of air for control in the 
present invention permits a lower air pressure source to be used which in 
an EPU is a stored bottle of air which results in lesser weight as a 
consequence of the air bottle pressure being lowered with a savings in the 
weight of air stored and the thickness of the walls of the air bottle. 
A fuel and air control system for a power unit which provides output power 
from a turbine which rotates in response to pressurized gas provided by a 
combustor which combusts fuel which is injected into the combustor by a 
primary fuel injector to produce the pressurized combustion gases cooled 
by fuel injected into the combustor by a secondary fuel injector in 
accordance with the invention includes a single fuel control valve coupled 
to a source of fuel which controls a combined fuel flow through a fuel 
flow path from the control valve to the injectors in response to a fuel 
flow control signal; an air control valve coupled to an air source which 
controls a mass flow of air through an airflow path to the combustor in 
response to an airflow control signal; and a controller, providing the 
control signals, for causing the valves to produce flows resulting in a 
constant air/fuel ratio in the combustor during combustion when the fuel 
flow is varied under the control of the controller. The mass flow of air 
through the airflow path to the combustor is not dependent upon pressure 
in the combustor. 
The invention further includes an air valve position sensor providing a 
position signal to the controller specifying a position of the air control 
valve; a temperature sensor coupled to the airflow path for providing a 
temperature signal to the controller specifying air temperature within the 
airflow path; and wherein the controller, in response to the position and 
temperature signals, calculates a mass flow of air in the airflow path and 
generates the fuel flow control and airflow control signals as a function 
of the calculated mass flow and an output power level commanded by the 
controller. The output power level is commanded by the controller by 
varying the air flow and the fuel mass flow rate is commanded to vary in 
dependence upon measured air flow of a calculated air flow calculated from 
a measured air pressure and measured air temperature while maintaining a 
constant primary and overall air/fuel ratio. 
An air pressure regulator is coupled to a source of pressurized air for 
providing air at a constant pressure to the air control valve. 
In a preferred embodiment of the present invention, the fuel control valve 
and air control valve are formed into a single valve which varies fuel 
flow and airflow in response to a single control signal from the 
controller while maintaining a constant air/fuel ratio. The single valve 
comprises a valve which meters a rate of airflow and fuel flow through the 
single valve by moving a metering device a distance which is a function of 
a power level commanded by the controller. The single valve includes a 
mechanism for providing temperature compensation to change the distance 
which the metering device moves in only controlling airflow as a function 
of temperature of air flowing through the valve. 
The power unit may be an emergency power unit within an airframe or an 
integrated power unit within an airframe. 
A method of controlling a power unit which provides output power from a 
turbine which rotates in response to pressurized gas provided by a 
combustor which combusts fuel which is injected into the combustor by a 
primary fuel injector to produce the pressurized gas cooled by fuel 
injected into the combustor by a secondary fuel injector in accordance 
with the invention includes controlling a combined fuel flow to the 
injectors in response to a fuel flow control signal; controlling a mass 
flow of air to the combustor in response to an airflow control signal; and 
generating the control signals to produce a constant air/fuel ratio in the 
combustor. The mass flow of air through the airflow path to the combustor 
is not dependent upon pressure in the combustor. The mass flow of air is 
calculated in response to a sensed temperature of air flowing to the 
combustor and a sensed position of an air control valve controlling the 
mass flow of air; and the signals are a function of the calculated mass 
flow and specified output power level. The output power level s commanded 
by varying the air flow and the fuel mass flow rate is commanded to vary 
in dependence upon calculated air flow calculated from a measured air 
pressure and measured air temperature while maintaining a constant primary 
and overall air/fuel ratio.

BEST MODE FOR CARRYING OUT THE INVENTION 
FIG. 1 illustrates a first embodiment 10 of the present invention. The 
present invention is preferably utilized as a fuel and air control system 
for a power unit which may be, but is not limited to a EPU or IPU utilized 
in an airframe as discussed above with respect to the prior art. The power 
unit includes a combustor 12 which has a primary combustion zone in which 
a primary fuel injector 14 injects fuel to produce stoichiometric 
combustion at a constant air/fuel ratio of 13.25. The combustor 12 also 
contains a secondary fuel injector 16 which injects fuel to cool the 
combustion gases in accordance with known operation of combustors 
manufactured by the Assignee as stated above to produce a ratio of 2:30. A 
conventional igniter 18 provides a spark discharge to ignite combustion 
within the combustor 12. A temperature sensor 20 senses the combustion 
temperature within the combustor 12 which is used as a control parameter 
for the operation of the combustor to prevent the gas stream 22 from 
reaching a temperature sufficiently high to damage the rotor of the 
turbine (not illustrated) which produces the output power from the power 
unit. A sensor 24 senses the turbine inlet temperature for purposes of 
system control in accordance with the prior art. Air atomization valve 26 
controls the primary fuel atomization air injected into the combustor 
through input 27 as a function of sensed pressure sensed by pressure port 
28 in the combustor in accordance with the prior art. The aforementioned 
elements of the combustor are conventional. Air injector 29 injects air 
into the combustor to maintain stoichiometric combustion. 
A single fuel servo and .DELTA.P valve 30 controls a combined fuel flow 
through a fuel flow path 32 from the fuel servo and .DELTA.P valve to the 
injectors 14 and 16 in response to a fuel control signal applied from 
controller 34. The fuel flow path 32 includes a primary fuel path 34 and a 
secondary fuel path 36 which respectively are coupled at a bifurcation 38 
within the fuel flow path. The ratio of the cross-sectional area in the 
primary fuel path 34 and in the secondary fuel path 36 determines the 
relative flow rates from the bifurcation point 38. The fuel servo and 
.DELTA.P valve 30 replaces the individual primary and secondary fuel servo 
valves contained in the prior art with the advantages being an increased 
turndown ratio in the fuel servo and .DELTA.P valve than that realized in 
the primary fuel servo in the prior art as a consequence of the overall 
ratio of maximum to minimum fuel flow rates in the system being less than 
the overall ratio of maximum to minimum fuel flow in the primary fuel 
circuit in the prior art. Further the elimination of the secondary fuel 
flow servo valve saves weight, increases reliability and simplifies the 
overall control. Fuel shutoff/purge valve 40 is controlled by the 
controller 34 to selectively connect fuel to the fuel flow path 32 or high 
pressure air from the air source through purge line 42 to the fuel flow 
path to permit blowing out of the primary and secondary fuel circuits 34 
and 36. Air control valve 44 has a throttle member (not illustrated) which 
varies the airflow outputted from pressure regulator 46 which produces a 
constant output air pressure in dependence upon the power level commanded 
by the controller 34. The rate of fuel flow commanded by the fuel servo 
and .DELTA.P valve 30 is the control parameter for controlling a constant 
overall air/fuel ratio under the control of the controller 34. The 
controller 34 may receive an external command (not illustrated) for 
controlling the output power or may control the output power in accordance 
with a control program contained within the controller 34. The air control 
valve 44 contains a position sensor for providing a signal to the 
controller 34 which specifies the position of the air control valve to 
provide closed loop control. Temperature sensor 48 provides the controller 
34 with the temperature of the air within the airflow path 50 between the 
air control valve 44 and the combustor 12. The mass flow of air to the 
combustor is calculated by the equation: 
##EQU1## 
wherein C.sub.1 is a coefficient dependent upon the oxidant being used; 
C.sub.3 is an orifice coefficient which is a function of the feedback from 
the position signal from the air control valve 44; P.sub.REG is the 
regulator pressure in pounds per square inch absolute which is outputted 
by the pressure regulator 46 and T.sub.REG is the air temperature in 
degrees Rankin sensed by the sensor 48. The controller outputs an airflow 
control signal to the air control valve 44 which is the independent 
variable controlling the power output of the power unit. The sensed 
temperature and air control valve position is utilized by the controller 
34 to calculate the mass airflow flowing to the combustor 12 in accordance 
with the aforementioned equation. The controller 34 outputs a fuel flow 
control signal to the fuel servo and .DELTA.P valve 30 which is dependent 
upon the commanded air flow to command a position of the throttling 
mechanism within the fuel control valve to produce a constant air/fuel 
ratio in the combustor with the primary air/fuel ratio being a 13.25:1 
stoichiometric ratio and the secondary fuel ratio being a 2:30:1 ratio to 
cool the pressurized gas 22 from the combustor to a temperature at which 
the turbine rotor will not be damaged. 
The single fuel servo and .DELTA.P valve 30 has a smaller turndown ratio 
than the primary fuel servo system of the prior art. Assuming a dynamic 
range of 5:100 pounds per hour primary fuel rate and a 50:400 pounds per 
hour secondary fuel rate, the present invention will achieve control with 
a minimum flow rate of 55 pounds per hour and a maximum flow rate of 500 
pounds per hour. The prior art turndown ratio of 20:1 in the primary fuel 
path requires a more expensive valve and is more difficult to control than 
the approximate 10:1 turndown ratio achieved by the single fuel servo and 
.DELTA.P valve 30. 
FIG. 2 illustrates a second embodiment 100 of the present invention. Like 
reference numerals identify like parts in FIGS. 1 and 2. The embodiment 
100 of FIG. 2 differs principally from the embodiment 1 of FIG. 1 in that 
a single valve 102 which is controlled by a single independent variable 
from the controller 104 jointly controls the air and fuel flow while 
maintaining a constant air/fuel ratio for the primary and secondary 
injectors 14 and 16. The single valve 102 comprises a valve which meters a 
rate of airflow and fuel flow through a pair of orifices by moving a 
metering device a distance which is a function of a power level commanded 
by the controller. A suitable structure for implementing the valve 102 is 
described below with reference to FIG. 3. Additionally, temperature 
compensation may be achieved by changing the distance which the metering 
device moves only in metering airflow as a function of temperature for 
regulating only the airflow rate. The embodiment 100 eliminates the 
calculation of the mass flow rate as a consequence of the air/fuel valve 
102 metering the flow of fuel and air to the combustor for variable power 
levels mechanically to maintain the constant air/fuel ratio. Feedback on 
the line 106 to the controller 104 is optional for the purpose of 
monitoring the flow rate actually occurring. The single control signal 
which regulates both the air and fuel flow through the valve 102 is 
coupled to the valve from the controller by line 108. 
FIG. 3 illustrates a schematic of a suitable valve for implementing the 
function of valve 102 in FIG. 2. The valve 102 is a modification of a 
commercially available bipropellant valve manufactured by Moog for 
regulating the flow of propellants to a combustor. Alternatively, the 
valve may be a modification of a valve manufactured by South Bend 
Controls, Inc. of South Bend, Ind. As a consequence of the blow down of 
pressurized air from an air pressure storage (not illustrated) through the 
pressure regulator 46 in FIGS. 1 and 2, the temperature in airflow path 50 
may vary. Temperature compensation is desirable in the airflow metering 
path for the valve to compensate for variation in temperature in the 
airflow. As illustrated, a servo control (not illustrated) is attached to 
metering device 200 which has metering surfaces 202 and 204 disposed in 
the orifices 206 and 208 respectively coupled to the fuel and airflow 
sources for controlling the air and fuel flows. As the signal on line 108 
varies from controller 104, the position of the metering surfaces 202 and 
204 is varied as a function of the signal applied to the servo control. As 
a result of the metering surfaces 202 and 204 moving relative to the 
orifices 206 and 20 through which fuel and air flows, the flow through 
those orifices is varied as a function of the signal applied to the valve 
102 from the controller 104. It should be understood that the valve 102 
may be implemented with different types of metering devices than 
illustrated while practicing the invention with the only requirement being 
that the metering device maintains a constant air/fuel ratio in the 
combustor 12. A temperature compensation device 210 is preferably included 
in the valve 102 which varies the flow rate through the air control 
orifice as a function of the temperature in the airflow path 50. The 
temperature compensation device 210 may be in the form of a stack of one 
or more bimetallic washers, such as a washer 210, which expand and 
contract as a function of temperature of the air flowing through the valve 
102 which permits the metering surface 204 to be moved with respect to the 
air control orifice 208 without significantly moving with respect to the 
fuel control orifice 206 to provide correction for variation in 
temperature caused by blow down from the air pressure regulator 46. 
While the invention has been described in terms of its preferred 
embodiments, it should be understood that numerous modifications may be 
made thereto without departing from the spirit and scope of the invention 
as defined in the appended claims. It is intended that all such 
modifications fall within the scope of the appended claims.