914 surging at altitude

by Jonathan Devine in General Tech Discussion 11 years ago

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Re: 914 surging at altitude

by Roger Lee » 11 years ago

Roger Lee

126 5.14k 4.58k

Hi Mike,

You engineer types are out of my league. :)
I would need years to catch up.
I don't know about tapping into the engine system, but you could certainly add an additional sensor.

Roger Lee
LSRM-A & Rotax Instructor & Rotax IRC
Tucson, AZ Ryan Airfield (KRYN)
520-349-7056 Cell

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Re: 914 surging at altitude

by Mike Goodrich » 11 years ago

Mike Goodrich

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Roger
Us engineer types :) often wonder and worry :( about things that more practical guys like you B) know we don't have to worry about ;)
I have two small thermocouples that I'm going to try to stick up the airbox drains. That way I get the temperature at each end and because air is always flowing out, I know I'm measuring air temperature and not metal temperature. :whistle:
Mike G

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Re: 914 surging at altitude

by Jonathan Devine » 11 years ago

Jonathan Devine

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I have been struggling to download TCU data on an old laptop as it crashes. New laptop is the 64/32 bit incompatibility. As soon as I get a bit of free time and high enough ceiling I will stream and run the TCU data directly out on a climb, and hopefully this will not crash it. This will give me clear indication of extent of surging, as well as the airbox temperature at the time. I will check the throttle settings at the same time. Not sure that the airbox temp will help as boost pressure remains constant. The different altitudes reached before surging is noted may be a function of throttle setting rather than absolute altitude.

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Re: 914 surging at altitude

by Mike Goodrich » 11 years ago

Mike Goodrich

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Jonathan
What software/hardware are you using to download TCU data, I'm interested in a copy.

If the airbox temperature increases while the engine revs remain the same this will increase airbox/boost pressure and the TCU will open the wastegate to slow the turbo and bring the pressure back down to the pressure programmed into the TCU for the throttle setting you have. You probably won't see this happen on the boost gauge because the boost will remain more or less constant. Since the turbo has now slowed down it is making less pressure and less temperature so the pressure starts to fall and the TCU closes the wastegate to get the pressure back. Again you see nothing because the boost pressure remains the same but during this the engine looses and gains power because the mass flow of air to the engine has fallen and then risen again.
I doubt if this is your problem because I don't think (maybe wrong) that these temperature changes would happen quick enough to create the 1 cycle per second you describe and I would imagine that this would be more likely to happen at lower altitudes where the ambient air is warmer and hence the airbox temperature is higher.
Also thinking more about it if you were in the 108 to 110% throttle position range that is problematic I imagine that there would be some fluctuations in boost pressure.

I've been trying to find more data on the Turbo Rotax use for the 914. It is a Garratt turbo but does anyone know the model number or have a copy of the performance maps or even the dimensions of the compressor and turbine wheels? This may help me to understand Jonathan's problem and would help me to work out a simple method to check turbo performance and efficiency.
If anyone has or knows about a wrecked 914 turbo I'm interested, I'd like to be able to dismantle one before I start on mine.
mike G

Thank you said by: Jonathan Devine

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Re: 914 surging at altitude

by Jonathan Devine » 11 years ago

Jonathan Devine

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I am suspecting that there might be a minor induction leak that becomes problematic as ambient pressure drops. Will keep looking, but get the data anyway. The program is DIV-914 from the tech support documents menu on www.flyrotax.com Not straightforward to search - I just scrolled through all documents as the searches didn't work. Was difficult to get running - required lots of time and a 16 year old son, and still downloaded several minutes of idle engine as opposed to several hours of useful operating data!

Thank you said by: Mike Goodrich

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914 surging at altitude

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