Fuel injection system

A gas turbine engine combustor has a plurality of circumferentially fuel injectors. Two lower nozzles are pilot nozzles and are of the fuel pressure atomized type, and the majority of nozzles are of the air atomized type. A pressurizing valve maintains high fuel pressure to the pilot nozzles at all operating conditions. The fuel/air ratio for the pilot nozzles is optimized facilitating start up and increased stability during transient conditions.

Technical Field 
The invention relates to gas turbine engines and in particular to fuel 
injection systems therefore. 
In gas turbine engines combustors receive air from the compressor and 
deliver hot gas to a turbine. A plurality of fuel injection nozzles supply 
fuel for combustion within the combustor. 
It is desirable to ignite the fuel quickly and smoothly even under adverse 
conditions, without producing a surge. This includes starting of the 
engine at low temperatures and high altitudes. It is also important to 
maintain combustion during transient operation. 
Improved performance requirements and restrictions on smoke emission have 
led to designs with improved atomization and mixing and where the fuel/air 
ratio is lean at the front end. An ignition envelope exists with the 
fuel/air ratio related to the local velocity, pressure and temperature of 
the gases with ignition only being possible within this envelope. The lean 
front end or the low local fuel/air ratio has increased the difficulty of 
keeping within this ignition fuel/air ratio envelope. 
There are also advantages to the use of relatively low volatile fuel since 
such fuel may be handled with less hazard. This also increases the 
difficulty of obtaining and maintaining ignition. 
Conventional pilot injectors exist which are turned on for ignition and off 
thereafter. This involves a time delay which can be particularly critical 
if the pilot is required during transient operation as well as purge 
requirements when the pilot is shut off. This also introduces control 
complications and concommitant expense. 
A combination fuel injection system with airblast nozzles provides fuel to 
the nozzle tip at relatively low pressure because air energy is used to 
atomize the fuel in lieu of fuel pressure drop. When operating under 
adverse conditions such as high altitude where air energy and temperature 
are low (low temperature results in high fuel viscosity) it is difficult 
to atomize fuel. Further, during light-off and deceleration conditions the 
local fuel/air ratio is low. This invention addresses both of these 
problems by providing pressure atomizing pilots which utilize fuel 
pressure drop for atomization and redistribution of fuel to ensure an 
adequate fuel/air ratio at light-off and during low power deceleration 
conditions. 
It is further important that all nozzles be properly atomized at all times 
to avoid puddling and/or pulsating combustion. 
DISCLOSURE OF THE INVENTION 
A fuel injection system has a plurality of circumferentially spaced fuel 
injectors arranged in a generally conventional manner. The majority of 
these fuel injectors are the conventional air atomized nozzle type. A 
minority, perhaps two, of these injections are of the pressure atomized 
nozzle type, these being the pilot nozzles. Each pilot has an igniter 
adjacent thereto. 
The fuel pump supplies fuel to the engine fuel control which in turn 
supplies metered fuel to the air atomized nozzles through a variable 
restriction (pressurizing) valve which is capable of increasing the 
upstream pressure by flow restriction. The pressure atomized nozzles are 
connected upstream of this valve so that they receive high pressure fuel, 
which is required for good atomization, during all operating conditions. 
The function of the pressurizing valve is to increase pressure to the pilot 
nozzles and to effect fuel distribution in favor of the pilot nozzles 
during light-off and low power operations. This introduces fuel at the 
location of the pilot nozzles rapidly at high pressure so that a properly 
rich fuel/air ratio with well atomized fuel is maintained for easy 
ignitability. 
As fuel flow is increased, the variable restriction allows the fuel flow to 
increase significantly to the air atomized nozzles. Ignition time is 
decreased because of the relatively small volume which must be filled 
before fuel is ignited through the pilot nozzles. Furthermore, as fuel is 
introduced into the main nozzles it starts flowing first through the lower 
nozzles because of the static head component of the pressure drop. This 
permits the main nozzles to light-up sequentially from bottom to top 
producing a smooth light-off. Since air atomization is used there is good 
atomization even at the low fuel flows. 
The pilot nozzles are maintained in operation at all times. Accordingly, 
during a transient such as rapid deceleration the pilots are already 
operating. The decrease in fuel flow requested by the main fuel control 
decreases the pressure with the variable restriction valve automatically 
closing thereby maintaining the full required atomization pressure for the 
pilot nozzles. Accordingly, stability during the transients is enhanced.

BEST MODE FOR CARRYING OUT THE INVENTION 
FIG. 1 illustrates a plurality of circumferentially spaced fuel injectors 
for injecting fuel into the combustor with the majority of these being 
main nozzles 10 of the air atomized type. Such nozzles as illustrated in 
U.S. Pat. No. 3,713,588 are characterized by the fact that atomization of 
the fuel is obtained from the energy of the air passing through the 
nozzle, so that good atomization of the fuel is obtained even at low fuel 
flows. 
Two of the injectors are pilot nozzles 12 and 14 located at a lower 
elevation with respect to the other injectors. Fuel supply line 16 
receives fuel from the fuel pump 18 through engine fuel control 20 and is 
connected directly to the pressurizing valve 22. A fuel supply line 24 is 
located substantially upstream of the main variable restriction action of 
the pressurizing valve 22 and delivers fuel directly to pilot nozzles 12 
and 14. A fuel supply line 26 carries fuel to the airblast nozzles 10 
being taken substantially downstream of the variable restriction action of 
pressurizing valve 22. 
These pilot nozzles 12 and 14 are of the pressure atomizing type where 
atomization of the fuel is obtained from the pressure drop of the fuel 
itself. These usually include a swirling chamber whereby the fuel swirls 
at high velocity, prior to injection through an orifice, with the energy 
for the atomization coming from the pressure drop of the fuel itself. This 
type nozzle is characterized by the fact that atomization is sensitive to 
fuel pressure drop, but is relatively insensitive to air velocities. 
FIG. 2 is a rough schematic flow diagram of an extreme case of the control 
arrangement. The preferred arrangement including minimum flow to the main 
nozzles and some modulation of the pilot flow is described in connection 
with the pressurizing valve description. Fuel pump 18 draws fuel from fuel 
tank 28 delivering it through supply line 30. Engine fuel control 20 
regulates the flow into the fuel distribution lines 16. A pressurizing 
valve 22 is used to regulate the pressure and flow in the fuel 
distribution line 24 and 26 to the pressure atomizing and airblast 
nozzles, respectively. 
It can be seen that pressurizing valve 22 can be operated to maintain the 
desired high pressure of the fuel passing to the pilot nozzles 12 and 14. 
Additional flow passing through pressurizing valve 22 will fill 
distribution line 26. The lower main nozzles 10 will first start flowing. 
For that reason it is preferred that the pilot nozzles 12 and 14 be located 
at a lower elevation to ignite the adjacent main nozzles will be the first 
ones to receive flow. These pilot nozzles, however, while being near the 
bottom should not be directly on the bottom. Such a bottom location could 
result in misfiring of the igniter should excessive fuel puddle at that 
location. 
FIG. 3 illustrates a section through the combustor 40 with a pilot nozzle 
12 located therein and an associated igniter 34. Air swirl vanes 42 are 
located in a conventional manner to swirl the air for air-fuel mixing, 
although atomization of the fuel does not depend on this airflow. The air 
atomized type nozzles are arranged in a similar manner, but without the 
igniters, at the other locations. 
As the gas turbine engine load is increased, pressure in line 16 is further 
increased with valve 22 approaching the full open position. The fuel flow 
through the pilot nozzles 12 and 14 increases slightly because of the 
pressure increase, while the fuel flow through the main nozzles increases 
substantially. 
It is desirable to provide a distribution of fuel around the circumference 
which results in a uniform temperature without either a hot streak or a 
cold streak. A hot streak produces over temperature at the turbine beyond 
that which may be tolerated. A cold streak on the other hand results in 
increased temperature at other locations if the same average temperature 
is maintained. 
The primary and dilution air downstream of the pilot nozzles can be 
tailored to reduce the potential for a hot or cold streak. This air 
tailoring in conjunction with good atomization and a circumferential 
mixing produced by the high energy air blast nozzles produces sufficient 
mixing to avoid deterioration in combustor or turbine life because of 
temperature streaks. 
It can be seen that the pilot nozzles 12 and 14 remain in operation at all 
times. In the event of a rapid deceleration the fuel control valve 20 
rapidly decreases fuel flow with pressure dropping in line 16. The 
pressurizing valve 22 automatically adjusts thereby providing an increased 
percentage flow through the pilot nozzles 12 and 14. This continues to 
maintain a good fuel/air ratio at these locations to retain ignition 
stability during such a transient. 
During start-up of the gas turbine engine the fuel pump is started against 
the closed fuel control 20. As the fuel control is opened the pressure in 
line 16 increases with a majority of the fuel passing through pilot 
nozzles 12 and 14. Igniters 32 and 34 are located adjacent to the pilot 
nozzles with electrical energy being supplied for ignition purposes. 
Adequate fuel is supplied through the pilot nozzles to produce a 
relatively high fuel/air ratio which is well within the ignitability 
envelope. Since the fuel distribution line 24 volume is small relative to 
the distribution system 26, ignition delay is substantially reduced. For 
instance, tests have shown that compared to a conventional ignition time 
delay of 12 seconds, the delay has been reduced to 2 seconds. 
It is possible to carry out the control scheme either in the extreme form 
or in a modified form by the use of conventional sensing and feedback 
controls. The pressurizing valve 22 as illustrated in FIG. 4 offers a 
simplier reliable and light weight apparatus for carrying out the desired 
control of the system. It also incorporates several improvements over the 
earlier described extreme case. 
The pilot nozzles have already been designed for the optimum fuel/air ratio 
at the low flow condition. Continued operation with only the pilot nozzles 
operating not only produces a poor temperature distribution but is also 
unnecessary since the fuel flow requirements are never so low that they 
are satisfied by the pilot nozzle requirement alone. Accordingly, a 
minimum flow bypass 72 to the main nozzles is incorporated. 
As pressure is increased beyond the minimum flow requirements, the flow to 
the pilots would tend to increase. Since airflow has not been 
substantially increased at this time and since they already are designed 
for the optimum fuel/air ratio, this would result in local hot spots in 
the event of continued operation at this condition. Accordingly, the fuel 
flow to the pilot nozzles is dipped slightly before it is again increased. 
The body 50 of the pressurizing valve encloses a floating piston 52. 
Biasing spring 54 urges the piston toward the inlet against shock absorber 
56, spring 58 and filter 60. The spring 54 is enclosed within chamber 62 
which is fluidly connected by conduit 64 to the main fuel pressure in line 
26. This exposes the downstream of piston 52 to a pressure signal which is 
substantially representative of the pressure existing within the 
combustor, noting that the fuel pressure drop through the air atomized 
nozzles is nominal. 
The inlet pressure from line 16 operates on the upstream side of the piston 
with the pressure differential between the inlet fuel pressure and the 
combustor pressure thereby operating to set the position of piston 52. 
As illustrated, the piston is in its minimum pressure and minimum flow 
condition illustrating a flow of fuel through opening 68 which meters the 
pilot flow passing it through outlet chamber 70 into the pilot line 24. 
This device provides the desired flow to the pilots during initial 
condition. The bypass conduit 72 passes fuel at this minimum flow 
condition through the main outlet 74 into the main fuel distribution 
bypass 26. This bypass is sized to provide the difference between the 
pilot flow required and the minimum flow requirements of the engine. 
As fuel pressure is increased under the influence of the engine fuel 
control 20, the piston 52 moves away from the inlet side. The recess 76 of 
the piston moves to uncover port 78 thereby increasing the fuel flow to 
the main nozzles. As opening 68 of the pilot line moves and starts to 
close off the characterized slot 80 moves into position over port 82. 
Therefore, after a slight dip in the pilot fuel flow caused by the 
throttling action, the flow is increased through the action of 
characterized slot 80. 
It can therefore be seen that the pressurizing valve 22 carries out the 
desirable function of maintaining the desirable fuel/air ratio to the 
pilot while modifying the particular amounts in accordance with the 
particular combustor and engine design. 
Accordingly, the rapid starting pilots then ignite the adjacent airblast 
nozzles which in turn ignite the remainder of the airblast nozzles as a 
static head is overcome and the manifold is completely filled. This 
results in smooth ignition because fuel is ignited as it is introduced to 
each nozzle and not allowed to puddle during manifold filling. It is much 
easier to ignite poorly atomized fuel with a pilot torch than directly 
with an electrical igniter.