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In another thread, two key steps of starting a turbofan (ignition and fuel introduction) are given as follows:

  • With increased N2 rotation, ignition will be turned on. [...]
  • With further increase in N2 rotation, fuel flow will be introduced.

For some engines I saw that ignition is typically turned on at about 15% rotation, and fuel is introduced at about 30%.

I understand that the value for fuel introduction (therefore light off) is given by the need to further power the engine as early as possible, but not too early or the system will be disrupted. However I don't understand why there is a precise moment for ignition. My questions are:

  • Between 15% and 30% rotation, ignition will be on without any fuel. Why doesn't ignition instead occur just before fuel introduction?
  • Conversely, why wouldn't ignition occur earlier? Is there a physical requirement of the compressor turning at a certain speed before ignition can be on?

Thank you for your answers.

mennecybean
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Traditionally the fuel is introduced by the pilot at a specified minimum RPM, but this is a function controlled by the pilot-eye-hand brain interface (only on the newer pushbutton start FADEC engines is this function fully automated) and we all know how imperfect this interface is.

With a traditional system, you need to allow some fudge factor in case the pilot adds fuel early, and also if there are system faults it gives a bit of time for the fault to be detected and displayed. So most start systems start the igniters running as soon as the starter is on (in the pre-FADEC days it wasn't too hard to ruin an engine, which is why captains traditionally handle starting).

The RPM at which fuel is to be turned on is actually bit sooner than really necessary, to let the engine help the starter bring it up to self-sustaining RPM. The minimum RPM is there because you need a minimum mass flow going to the burner can to control the flame and therefore the temperatures (these engines are "air cooled" after all and the majority of the mass flow is not actually being burned, but being used to contain the flame boundaries - some flame does make it back toward the turbine during start, as you see when the ITT spikes, but it has to be minimized, and the more mass flow the better).

You can delay introducing fuel well past the minimum RPM, and you will actually see a lower ITT peak on a non-FADEC engine by doing so (because, as I said above, more mass flow provides more flame control), but this really adds to wear and tear on the starter.

When I was in the Regional Airline business supporting CRJs we observed that a lot of captains made it a practice to delay fuel introduction quite late (like waiting to 25-30% instead of 20% for the GE CF-34) because they noticed that the ITT peak was lower when fuel was delayed. However this had to be balanced against shorter starter life and we advised against getting carried away with this because the marginally lower starting ITT peak didn't appear to actually improve engine life, while shortening starter life significantly.

On the other hand, it's important to know that it isn't essential to get the fuel on right now when the RPM is at the fuel introduction value and you can do it at a relaxed pace. If the nominal value is 20%, putting fuel on at 23 or 25% does no harm, so take it easy (pilots in training (including me) would fumble with the thrust lever lockout thinking they had to get the fuel on exactly at the right time).

John K
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    Thank you. My question was actually about ignition, but your explanations for fuel introduction are very informative as well. – mennecybean Feb 20 '20 at 17:05
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It can be tough to generalize because engines are different, but in the turbines I have flown the igniters are energized immediately by the push to crank button. Fuel is typically introduced right around 15% RPM. Too early and you risk a hot start due to low airflow, but delaying will generally not hurt anything, you are just using up time in your start cycle.

I’m not sure where you got your 15-30% figure, but 30% RPM off ground power or battery alone seems very high.

To your question “why doesn’t ignition occur just before fuel introduction?” it does. “Just before” is a relative and subjective term, it doesn’t take very long to spin up to 15%.

To your question “is there a physical requirement for the compressor to be turning at a certain speed before ignition can be on?” No. Per my comments above, they will usually start firing as soon as the start sequence is engaged. There isn’t a separate button for igniters, just the start button.

Again, this is based only on my personal experience with several different turbine engines.

Michael Hall
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  • Thank you. I got the numbers from this video, mentioning ignition at 2:50 ("the engine core needs to spin at 14% of its maximum speed") and showing it at 5:30, and for fuel introduction it is at 3:17 and 5:36. It is on an ERJ, seems to be with FADEC. – mennecybean Feb 20 '20 at 17:04
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It depends on the jet engine model that is being started. On some Honeywell models, the pilot will bring the thrust levers to idle thrust when there is at least 10% N2 rotation and positive rotation on the N1. At this point, the ignition comes on and fuel is introduced to start the engine.

wbeard52
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