Poor Starting 912iS
in 912iS Technical Questions
6 years ago
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by Malcolm Huddart » 6 years ago
Some more information, the UK Rotax Agent has been at BRP-Rotax Austria this week
speaking to the engineers regarding our poor cold starting probelm.
They have devised a test to check if it is the sprag clutch.1. Spin the engine up on the Starter motor without the Fuel or Fuel Pump-on.2. Monitor the RPM Gauge - if it doesn't get above 700 rpm the Sprag Clutch has failed.The ECU won't initiate a Start Routine until the ECU see's >700-750 rpm.The Start Routine consists of Firing the Spark Plugs after >700rpm, whether it injectsfuel and only fires certainly spark plugs before >700rpm they don't know the exact sequence.They are going to perform a test at BRP Austria today to see what the effects of lowcranking speed.We did the test on our engine and it was struggling to get to 230 rpm. I am a bit suspiciousthat our "Stock Flight EMU" is mis-reading. I wasn't present at this test so I can't get any moredetails. However, before completely condemning the sparg clutch, I want to do some moretests with an Optical Tacho to see what speed the Prop is spinning at.I just wonder if its worth others trying this test and noting the RPM on Start-up
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by Malcolm Huddart » 6 years ago
I did some measurements on the Starter speeds. I used an external Prop Tachometer plus the BUDS Software to verify the engine speed. They tied up with the EMU Tachometer, although sometimes the EMU Tachometer wouldn't work.The numbers were:-Engine Cold (first start of the day). 240 rpmEngine Cold + External Jump Battery. 480 rpmEngine Cold with Plugs out (no compression). 240 rpm (I didn't try it with external battery)* - Engine Speeds (Prop Speeds corrected by Gearbox ratio 2.43:1)Tried starting it with the external battery, it fired on rear two cylinders ran for a few seconds then stopped, it took several more goes at starting it,where it ran at 400-600 rpm then after 15-20 seconds it kicks into life and runs up normally to 2000 rpm. It continues to work for the rest of the day,starting each time normally.I measured the Voltage on Starter Motor, it dropped to 10v on cranking (I didn't get the chance to measure it with the External Battery).Starter solenoid Ok - no Voltage drop across it while cranking (this is one item that was never changed when engine was replaced).I think it is worth changing the Starter Motor before tearing the engine to bits.I would have thought if the Sprag clutch was slipping, removing the Plugs should having reduced the load (compression) and it should have spun faster.The sage conitues.......Before pulling the engine to pieces to change the sprag clutch which I can't get my head around the logic of doing this, If it is slipping why doesn't the cranking speed increase when the plugs and removed ?I am either going to connect a high current cable from a large battery to the Starter motor and note the Cranking speed. In the meantime I will try and obtain another Starter Motor to try.
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by Jim Flock » 6 years ago
The Sprag Clutch has been mentioned a few times. Could someone explain to the group what it is and how it works on the 912iS?
Aviation Real Estate Specialist & iRMT
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by RotaxOwner Admin » 6 years ago
Jim Flock wrote:
The Sprag Clutch has been mentioned a few times. Could someone explain to the group what it is and how it works on the 912iS?
It is sometimes referred to as the "slipper" clutch and it does just that - it is designed to 'break free' at a certain torque value, to protect your engine in the event of a prop strike, for example.
Have a look through the 912iS Heavy maintenance Manual - section 72-20-00, that will give you a good picture of what the sprag clutch is made up of... ( see https://www.rotax-owner.com/en/support-topmenu/engine-manuals#912-i-series-engines )
Thank you said by: Jim Flock
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by Jim Flock » 6 years ago
Thanks for the explanation. What is the Sprag's involvement in the start process?
Aviation Real Estate Specialist & iRMT
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