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914 wastegate operation

in 912 / 914 Technical Questions 8 years ago
Hi to all.

As I'm not familiar with the 914 or turbocharged engines whatsoever,I would like to ask a couple of things. I can understand the operation (fundamentally) of the wastegate, but what I do not know is when it is open or closed during engine operation. I can understand that are many variables effecting it's operation, but honestly I can't actually understand when it opens or closes. E.g. During take off with 115% setting on throttle, is it fully closed until it starts over boosting or it is open? During cruise with 5000rpm and 31 Hg, is it open or closed?
One more thing. If the bowden cable that links the servo to the wastegate, is not properly adjusted and at it's "fully closed" position the wastegate is leaking, what the results will be (performanwise)?
Thank you all and any information is much appreciated!

Spiros

  • Re: 914 wastegate operation

    by » 8 years ago

    -The 914 is a "altitude" or boosted engine; the manifold pressure goes over atmospheric pressure. As opposed to a "normalized" engine that tries to maintain sea-level manifold pressure.
    -When the wastegate is closed the max amount of exhaust pressure routed into the turbo.
    -When the wastegate is open the exhaust pressure is allowed to escape into the muffler.
    -Even with the w/g fully open you will get some boost.
    -the wastegate will be closed at idle. Strange but true: The TCU is trying to reach its target MAP and the w/g will be closed until it achieves this target.
    -When the throttle is set to 115% the wastegate is fully closed until 40 InHg is reached.
    If the cable is not adjusted correctly: too loose - the wastegate cannot close and you will lose manifold pressure.
    If the servo/cable is not setup correctly (throttle closed/Master ON/allow servo to cycle/Master OFF/adjust w/g to fully closed position) then it is possible the servo cannot move and the warning light will flash.
    I am sure some others will chime in
    Thank you said by: Spiros Economou
      
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  • Re: 914 wastegate operation

    by » 7 years ago

    Hi don't want to highjack the post. I have a 914T in a Bushbaby Explorer (Kitfox) plane. The performance has been satisfactory, but always battled to get a good boost pressure on takeoff. I initially suspected it was my prop set up. I changes to a variable pitch prop, performance improved but was not fantastic. Had a takeoff and could not even get boost. Started doing some investigations and found that the Bowden cable had a lot of play in it. From what I can remember there has always been some play in the cable. After setting it as per the manual (Heavy) no place in the waste gate then 1mm extra the performance was fantastic. Spoke to the AP that built the plane and he said they set up the cable with play to limit the boost? I am sure after 200 hours the cable has stretched and that is why the boost has been dropping.

    The concern I now have is that I am boosting at 42 at WOT on take-off. How would I limit this to the 40 recommended by Rotax.

    Look forward to your response.
      
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  • Re: 914 wastegate operation

    by » 7 years ago

    Hi Sean

    The cable must be tightened according to Rotax specs. Having it loose, you loose pressure as there is a constant leak of gases when there shouldn't be e.g. When you need full boost. You cannot somehow restrict the maximum boost pressure, by slackening the cable. If you believe that the engine overshoots the target value, then the TCU is a place to start. You have to "see" what the unit sees , when in full boost, with the TLR software. If your reading is correct, then there are other things you have to look at.
      
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  • Re: 914 wastegate operation

    by » 7 years ago

    Thanks makes sense. Will get the software loaded and check. Could you point me in the direction where the specification is for the USB to serial converter. I remember seeing it on the blog but could not find it. I have loaded the sw on a windows 10 64 bit laptop but am having trouble with the serial converter. When I switch it to com 1 the sw give me an error.
      
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  • Re: 914 wastegate operation

    by » 7 years ago

    http://www.rotax-owner.com/en/videos-topmenu/rotax-minute/435-tcutips

    This is the video explaining everything regarding the software. Unfortunately I cannot help you as I still have an old laptop for this case, running XP with a built in serial port. But beware, when going full boost a slight overshoot of the target pressure can occur. After that, pressure is stabilized within the operation limits.
      
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