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I have read somewhere regarding turbofan engine 2 spool that says

  1. "LP or HP turbine will drive the LP or HP compressor"

  2. "During engine starting, the starter will drive the HP compressor via the gearbox"

So I'm confused on whether the turbine or starter drives the compressor. Is it only during starting the starter will drive the HP compressor and after that turbine will take over?

Thanks

sherline lin
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    Your question is how (or if) the turbine takes over, hopefully the linked post, "How is bleed air used to start a jet engine?" answers that, if not please edit the question to clarify what is unclear. –  Feb 07 '19 at 16:07
  • @ymb1 - OP: "I'm confused on whether the turbine or starter drives the compressor", so isn't your proposal for a duplicate far fetched? The OP is asking about the mechanical link between the accessory box and the N2 spool, as answered by John K. – mins Feb 07 '19 at 17:32
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    @mins - I'm not sure what good would an extended discussion do. Anyway, on the contrary no, the post I proposed, unlike the other one or the answer, explains the disconnect mechanism of the starter. OP's confusion is confusing (as evident by the different interpretations), hence my proposal to revise the question if OP is still confused, as is standard procedure here. –  Feb 07 '19 at 17:37

1 Answers1

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The starter drives the entire HP spool (HP compressor, interconnecting shaft, and HP turbine). The gearbox in question is the accessory gearbox that runs the various pumps and is driven off the HP spool shaft, which by happy coincidence is on the outside of the LP shaft. The starter is mounted on that gearbox and is able to back drive the HP spool through the gear train for engine starting.

The LP spool, which is the fan and LP turbine with its shaft running down the center of the HP spool, is just going along for the ride during the start.

John K
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  • Question 1 "The LP spool, which is the fan and LP turbine with its shaft running down the center of the HP spool, is just going along for the ride during the start. " - does it mean that the LP shaft is connected to the HP shaft? So once HP shaft turns, LP shaft will turns too?

    Question 2 So once starter cuts off, everything will run on its own?

    Thanks

     – sherline lin Feb 07 '19 at 14:24
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    The LP and HP shafts have no mechanical connection. But the air displaced by the LP compressor will start turning the HP compressor. – Hobbes Feb 07 '19 at 16:07
  • That's what I meant by "going along for the ride". The LP shaft has a set of bearings that suspend it within the HP shaft and it is free to spin independently. Replace the fan at the front end with a gearbox and propeller, and you have a free-turbine turboprop. In a TP, the HP rotor (comp/shaft/turbine) and the burner can between them is called the "gas generator". On a turbofan it's called the "core". The starter system winds up the core to start the engine. The fan only starts to turn when the airflow through the engine starts spinning its turbine. – John K Feb 07 '19 at 17:03
  • So during starting, the HP spool will be turn by the starter then the air displaced by the HP compressor will start turning the LP spool? – sherline lin Feb 08 '19 at 01:11
  • To the extent that there is enough air moving past the LP turbine blades to make the disc start to spin, yes. Depending on the engine, you might see the fan start to rotate slowly on the flow past the LP turbine being created by the starter alone, but not really start to move until light off. – John K Feb 08 '19 at 01:37
  • So starter is the one who is doing all the job "start turning", then why in some books i read they say turbine helps to turn the conpressor? – sherline lin Feb 08 '19 at 12:46
  • The turbine may start to produce a bit of torque to offset the load of the compressor, to the extent that the flow through the cold engine being generated by the compressor applies a bit of torque at the turbine, but this is fairly minor. Doesn't start to generate any significant torque until there is flame in the burner can. Even after lightup, with hot gases now going through the turbine, the flow has to increase to a certain level before the turbine can produce enough torque to support the compressor's load without the help of the starter. – John K Feb 08 '19 at 13:44