Control Operation

LOGIC ACTIONS

Control operation for starting to core-idle is similar for the LM2500 and LM6000. There are two phases.

The first phase, which applies to the majority of the start, until the core approaches idle speed, is performed with the airflow control disabled and no eighth stage or compressor discharge bleed. The bulk Tflame schedules, during this initial phase have no effect. Instead a start fuel control calculates upper and lower bulk (or total) fuel flow limits (WFMAX = WFMAXSI and WFLBO = WFLBOSI respectively) based on independent max. and min. equivalence ratio schedules. These schedules were originally intended to correspond to combustor thermal stress and lean blowout limits but were ultimately adjusted during the LM6000 development engine testing to provide reliable starts (blowout-free) with acceptable combustor acoustic levels. Note that for the LM6000, the VBV’s throughout a start, are scheduled just as for their non DLE counterparts, i.e fully open (100%) once LP rotor speed reaches 1250 rpm. As far as the overall fuel control is concerned, during the first this initial start phase, in addition to the upper and lower fuel flow limits, two other fuel flow regulators/limiters can come into play, i.e. a core speed acceleration rate regulator and a max. WF/PS3 accel schedule limit. The WF/PS3 accel schedule limit exists in both the LM2500 and LM6000 controls, and is based on their non DLE counterparts, but in general is encountered only on the LM2500. The schedules when originally developed for the non DLE engines were intended to provide compressor stall protection. The WF/PS3 accel schedule limit is “merged” with the start fuel control upper limit WFMAXSI through a Min select to form the final WFMAX upper limit. By virtue of the fuel control priority selection logic, the WFMAX upper limit will always override the WFLBO lower limit, which means that it is possible for the WF/PS3 accel schedule limit to override both the upper and lower start fuel control fuel flow limits. A leaking or badly calibrated PS3 pressure transducer, resulting in a low sensed pressure, could in turn result in the WF/PS3 accel schedule inadvertently lowering the final fuel flow and producing a hung or aborted (flame out) start. Note that although the start fuel control upper and lower fuel flow limits are, like the “idle-and-above” Tflame algorithm limits, a function of T3 and PS3, because of accuracy concerns in the start region the T3 and PS3 are from internal model estimates, rather than the sensors. Therefore, although errors in sensed PS3 will affect the WF/PS3 max. fuel flow limit, errors in neither PS3 nor T3 sensed values will affect the start fuel control upper and lower fuel flow limits. LHV is an input to the start fuel control and so errors in LHV will affect the start fuel control upper and lower limits!

When the second phase of the start to core-idle is entered the complete airflow control/bulk flame temperature control strategy is enabled, and the upper and lower fuel flow limits come from the Tflame algorithm as opposed to the start fuel control. Transition from the first phase to the second phase is strictly a function of core speed. At a specific core speed (N25SEL = N25SIATV = N25SI + N25SIJA = 6300 rpm for the LM6000 and NGGSEL=NGGSI =4900rpm for the LM2500) the airflow control is enabled and as the core approaches that same specific speed the fuel flow upper and lower limits transition from the start fuel control limits to the Tflame algorithm limits. This occurs over the core speed range of 6200 to 6300 rpm for the LM6000 and 4800 to 4900 rpm for the LM2500.

Other DLE-specific control actions occur during the first phase of the start. When the IGNITE mode of the start is entered, in addition to the opening of the shutoff valves and the energizing of the ignitor, in order to ignite the fuel, the outer staging valve(s) that supplies fuel to the three combustor cups alongside the energized ignitor(s) is opened. At this point all of the inner staging valves are closed. The outer staging valve(s) is open for the complete ten seconds of the IGNITE mode. Note that both the LM2500 and LM6000 have provision for two ignitor locations. The staging control logic will open staging valve #22 and/or #9 depending upon whether ignition demands IGN1DMD and/or IGN2DMD are set during the IGNITE mode.

When either or both of these staging valves are open during the IGNITE mode, the outer ring fuel flow is determined just as it is for operation above idle in AB or ABC mode, i.e. as described earlier, the outer fuel flow WFOREFABC is calculated in the Tflame algorithm based on a scheduled ring flame temperature TFLOREF. This fuel flow represents the outer fuel flow per staging valve and, depending upon the number of “ignition” outer staging valves open (one or two), is translated into a total outer ring fuel flow demand (WFOREF). Being that the outer fuel flow is derived from the Tflame algorithm, it will be influenced by errors in any Tflame inputs, in particular PS3 and T3. So, during the IGNITE mode, a bulk or total fuel flow is demanded (WF36DMD) , and from this is subtracted the outer ring fuel demand (WFOREF) to give a resultant pilot ring fuel flow demand (WFPREF). The inner ring is not fueled during the IGNITE mode, only the pilot ring and three or six of the outer ring combustor cups (one or two of the outer staging valves). As the start progresses, part of the inner combustor ring can also be fueled. The logic that determines this is fairly straightforward and functions as follows: If the demanded fuel flow (WF36DMD), under the influence of the core speed accel rate regulator, in attempting to track the core speed accel rate schedule, is forced onto the start fuel control upper limit (WFMAXSI) for more than three seconds then the combustor

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the number of cups from thirty plot cups to thirty pilot cups plus eight inner cups, and the upper and lower fuel flow limits will increase accordingly. This allows WF36DMD to increase and thereby “speed-up” the start. The inner ring fuel flow is determined just like the outer ring, i.e. in the same manner as it is for above-idle operation in BC/2 (LM6000 only), BC or ABC mode.

In summary:

All three rings can be fueled during a start with total fuel flow being determined by the core rate regulator and limited by Tflame max. and min. fuel flow limits. Errors in fuel properties (SG, fuel temp. and Cp/Cv) affect mass flow metering accuracy and errors in LHV affect max. and min. fuel flow limits - problems with either of these can hang-up a start or prevent a lite-off completely - it’s important to recognize that a 5 % error in fuel flow can mean approximately a 150 deg F error in Tflame! Remember that max fuel flow has an overriding WF/PS3 limit and that PS3 sensor calibration or leaks resulting in low PS3 can cause the WF/PS3 accel limit to hang-up a start. Also remember that PS3 and T3 sensors do not affect bulk Tflame limits in the initial phase of the start before the airflow/Tflame control is enabled, but they do affect the outer ignition fuel flow/Tflame and the inner fuel flow/Tflame if staging to BC/2 occurs. The outer ignition fuel flow is essential for lite-off to occur - the appropriate staging valve must be opened, i.e. the one that fuels the cups alongside the energized ignitor. At a core speed of 4900 rpm for the LM2500 and 6300 rpm for the LM6000 the airflow/Tflame control is enabled and the fuel control fuel flow/Tflame limits come from the Tflame algorithm. At this point the bulk Tflame min. and max. schedules become effective. Fuel metering or fuel property errors can at this transition point result in a blowout!

Typical start characteristics for both the LM2500 and LM6000 are shown in Fig 4.1 and 4.2 respectively.

0 400 800 1200 1600

0 20 40 60 80 100 120 140

Time - sec

WF36DMD-pph

0 2000 4000 6000 8000

0 20 40 60 80 100 120 140

Time - sec

NGGSEL-rpm

WF36DMD

NGGSEL

0 500 1000 1500 2000 2500 3000

0 20 40 60 80 100 120 140

Time - sec

NPTSEL-rpm

Figure 4.1a Typical LM2500 DLE start characteristics

0 200 400 600 800 1000

0 20 40 60 80 100 120 140

Time - sec

T54SEL-degF.

0 2 4 6 8 10

0 20 40 60 80 100 120 140

Time - sec

BRNDMD

T54

BRNDMD NPTSEL

GE Proprietary Information - Technical Export License TSU/OTS March 27th, 1997

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1 2 3 4

0 20 40 60 80 100 120 140

Time - sec

DWB36PCT-%

Figure 4.1b Typical LM2500 DLE start characteristics

0 1 2 3 4 5

0 20 40 60 80 100 120 140

Time - sec

PX36SEL-psipeaktopeak

Figure 4.1c Typical LM2500 DLE start characteristics PX36SEL DWB36PCT

N25SEL

0 2000 4000 6000 8000

0 20 40 60 80 100 120

Time - sec

N25SEL-rpm

WF36DMD

0 500 1000 1500 2000

0 20 40 60 80 100 120

Time - sec

WF36DMD-pph

Figure 4.2a Typical LM6000 DLE start characteristics

GE Proprietary Information - Technical Export License TSU/OTS March 27th, 1997

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0 200 400 600 800 1000 1200

0 20 40 60 80 100 120

Time - sec

T48SEL-degF.

BRNDMD

0 2 4 6 8 10

0 20 40 60 80 100 120

Time - sec

BRNDMD

N2ROTOR

0 500 1000 1500 2000 2500

0 20 40 60 80 100 120

Time - Sec

N2ROTOR-rpm

Figure 4.2b Typical LM6000 DLE start characteristics

DWB36PCT

0 20 40 60 80 100

0 20 40 60 80 100 120

Time - sec

DWB36PCT-%

PX36SEL

0 1 2 3 4