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crashed_05

TOT spike?

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So, you introduce fuel at 15% N1 and get a jump in TOT. You continue cranking the starter and get a secondary spike in TOT at around 30% N1. What causes this second spike in TOT? I'm guessing its the closing of the compressor bleed valve.

 

Anyone have any input on this?

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I always thought it was the second stage nozzles kicking in.

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So, you introduce fuel at 15% N1 and get a jump in TOT. You continue cranking the starter and get a secondary spike in TOT at around 30% N1. What causes this second spike in TOT? I'm guessing its the closing of the compressor bleed valve.

 

The source of that second peak has to do with the internal operation of the fuel control unit. The fuel control unit controls, schedules, and meters fuel flow to the engine along its full range of operation. The second peak occurs around the Speed Enrichment Cycle. Speed Enrichment provides the engine acceleration (additional fuel volume and pressure) needed to prevent engine stagnation after initial light off. The increase fuel pressure will also determine when the single-line duplex fuel nozzle will open to secondary fuel. Bottom line there's more fuel flowing. As a side note, the bleed valve normally remains open during the entire start cycle.

 

There are two fuel control adjustments that maybe used to change these starting characteristics, Start Derichment and Start/Acceleration. They correct lean (slow) starts or rich (hot) start conditions.

 

 

OPERATION OF THE BENDIX FUEL CONTROL SYSTEM

 

(Bendix operational guide form 15-621B)

 

2-19. STARTING THE ENGINE.

 

a. The engine will be cranked with theDP-N1/N2 fuel control throttle in the cutoff position. At approximately 15 to 20 percent N1, the twist grip handle on the pilot's collective pitch lever will be moved to "start" (idle), which will move the DP-Nl/N2 fuel control throttle lever to the 30 degree position. The engine should light off and begin to accelerate. Acceleration will be proportional to the rise in compressor discharge pressure (Pc) until speed enrichment occurs at approximately 47 percent N1. When N1 reaches approximately 63 percent RPM, the DP-Nl/N2 fuel control will start to govern and maintain idle RPM.

 

b. The DP-N1/N2 fuel control has a start deriehment assembly, which incorporates a bellows for sensing Pc pressure. During the initial stages of starting, when Pc is low, the bellows will expose a hole from the Py portion of the bellows chamber. The exposed hole will bleed off Py pressure from the bellows assembly to atmospheric pressure and move the metering valve toward the minimum flow stop. As Pc pressure increases the derichment bellows will elongate and close this bleed-off hole. This will allow the py pressure at the head end of the bellows assembly to build up as it did in the earlier controls and move the metering valve away from the minimum flow stop. With the start derichment device, low turbine outlet temperatures can be maintained immediately after light off while the basic acceleration schedule can be richened so there will be no tendency toward stagnation before idle RPM is attained.

 

 

 

Bendix Fuel Control

Edited by iChris
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No need to spend $11K when iChris answered it so informatively! A RR250 Series Ground School will teach a pilot about this as will most A&Ps that are asked. Mike

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Another reason not to make your mech mad. Other than keeping your ship flying you have an great amount of knowledge available right in your own hanger.

 

"Keep Mech happy, save $11,000" That could be a bumper sticker. Ha!

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Ask yourself at what point during the start process does the acceleration bleed valve close? Think about it. What does this do to gas flow, and temperature?

Edited by avbug

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Ask yourself at what point during the start process does the acceleration bleed valve close? Think about it. What does this do to gas flow, and temperature?

 

The bleed valve on a RR250 series is open during the entire start cycle (unloads the compressor for faster spool up). On most RR250 series engines the bleed valve modulates (advancing towards closed or retarding towards open) and is closed at approximately 94%NG or higher. As previous documented by iChris, the start fuel enrichment is what keeps the acceleration going and causes the temp. increase between approx. 28 and 32% NG.

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I always thought about it like this. No spike, wet start. No second spike, hung start. Big first or second spike, hot start.

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