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  #1  
Old 09-28-2021, 03:22 PM
thiggins thiggins is offline
 
Join Date: Mar 2020
Location: Saluda,NC
Posts: 46
Default Exceeded recommended RPM with cold oil

In the POH for the legacy RV-12 the POH mentions a minimum oil temp of 120 degrees along with a max RPM.

Example : “Taxi RPM – 1800–2500 RPM until oil temp reaches
120° F (50° C)”

Why is this? What specific damage can occur?

Why? : A buddy of mine was taxiing with oil temp around 100 degrees and got bogged down in grass…He knew better, but advanced the throttle well beyond the RPM recommendation. He says the engine now makes unpleasant noises he’s never heard before, and is running rough. It has around 570 hours on it.

What could have been compromised? (We fear big $$$)

Who could help him diagnose it? (We need a Rotax expert. )

Thanks
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  #2  
Old 09-28-2021, 03:35 PM
Bob Y Bob Y is offline
 
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Location: Piedmont, SC
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Default

If nobody replies here, you might post the question on the Rotax Owners Forum.

https://www.rotax-owner.com/en/

You do need to subscribe to post questions.
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  #3  
Old 09-28-2021, 04:22 PM
NinerBikes NinerBikes is offline
 
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Location: Granada Hills
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The Rotax uses a dry sump tank and oiling system. There is rubber hose between the sump tank and the oil pump, which creates suction.

If the oil viscosity is too high from the oil being not up to operating temperature, and the engine rpms and oil pump demand is too great, the rubber hose, due to low vacuum pressure from the oil pump can collapse inwards and starve the oil pump, and drop the oil pressure to 0, thereby damaging the bearings and bearing surfaces of the engine, causing seizures, galling etc.

That was a big no no that your friend performed. It's possible he just destroyed the engine.
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  #4  
Old 09-28-2021, 05:36 PM
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Piper J3 Piper J3 is offline
 
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Just one slight correction for above post. Oil hoses are teflon lined and are very rigid. I doubt that lines would collapse with vacuum. Also, oil tank is elevated above inlet port of oil pump so positive pressure is assured.
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  #5  
Old 09-28-2021, 05:40 PM
AndrewR AndrewR is offline
 
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Location: Ballarat, VIC
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Default

Quote:
Originally Posted by NinerBikes View Post
If the oil viscosity is too high from the oil being not up to operating temperature, and the engine rpms and oil pump demand is too great, the rubber hose, due to low vacuum pressure from the oil pump can collapse inwards and starve the oil pump
The oil suction hoses should be rated for suction, i.e. they should not collapse if the demand is too high.

I think this scenario is unlikely at 100F. While the manual says normal temperature is >120F, in reality I expect there are Rotax engines going to full power at <100F every day, and you would be hearing about more failures if it was a common issue.

It probably depends on the design of the oil system though - i.e. length of the lines, resistance through the oil cooler etc.

Is there oil pressure logging or alarm? That would be the best indication.
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  #6  
Old 09-28-2021, 06:00 PM
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emsvitil emsvitil is offline
 
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It's possible the noises were there before...........

It's just that he really started listening because he ran the rpm up and is being ultra sensitive to any noise he didn't think was there before.
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  #7  
Old 09-28-2021, 06:53 PM
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rvbuilder2002 rvbuilder2002 is offline
 
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Location: Hubbard Oregon
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Everyone has their own opinion based on logic. But the reason already given is the engineering reason given by Rotax for why the limitation exists. Whether it’s collapse of a hose or in ability to suck the oil at a high enough rate from the tank or pick your own idea… But that is the official reason. If there hadn’t been instances of problems in the past I wouldn’t imagine that they would’ve set the limitation.
Now the limitation could just be because Rotax doesn’t know what any given airframe has for oil hoses. It was common in the past that some were just plain rubber hose with clamps on the hose barb city Chand. So maybe it is a limitation that is not realistic for some of the current engine installations but Rotax has never removed a limitation.

EDIT
The second part of the reason for the limit (that I forgot when I initial wrote the post) is that because the crank case pressure is used exclusively to push oil back to the tank, if the rpm gets too high before the viscosity is low enough, the pump can suck oil from the tank faster than crank case pressure can return it to the tank. If this goes on for too long, the oil pick-up can suck air which would result in oil pressure loss.
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Last edited by rvbuilder2002 : 09-28-2021 at 11:17 PM.
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  #8  
Old 09-28-2021, 07:03 PM
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Piper J3 Piper J3 is offline
 
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I routinely run my engine at 2500-2800 RPM at startup. Engine runs smooth at this speed while warming up from a cold start, and at this speed, the engine is not making much power. Smooth engine run is important for gearbox longevity as has been discussed in many threads. The 912 uses a pressed-together roller bearing crankshaft so positive oil lubrication is not super important at engine start.
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Bought Flying RV-12 #120058 Oct 2015 with 48TT - Hobbs now 750

LSRM-A Certificate 2016 for RV-12 N633CM
Special Thanks... EJ Trucks - USN Crew Chief A-4 Skyhawk
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  #9  
Old 09-28-2021, 07:21 PM
kkmarshall kkmarshall is offline
 
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The 912 has roller rod bearings but the mains are plain bearings. So short duration oil starvation is a big concern. If it in fact happened.
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  #10  
Old 09-28-2021, 08:30 PM
R100RS R100RS is offline
 
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I am not recommending that anybody do anything with their 912 that is contrary to Rotax recommendations. I have a few observations.

First, 100 degrees F is not "cold oil." 20 degrees F oil is cold oil. IIRC, after 2 minutes of operation, engine RPM can be increased to 2500. So, an unheated engine in a cold climate can go to 2500 RPM regardless of how low the oil temperature might be. Seems like a bigger risk than running the engine above 2500 with 100 degree F oil. (I don't remember if the 912 has a lower bound temperature restriction on starting).

Second, sorry, but Rotax recommends things that don't always make sense. Like allowing use of several high quality motorcycle engine oils for many years and then requiring only their special blend.

Third, It would have to be one flimsy hose to collapse when drawing 100 degree F oil from the tank. It's possible, I guess, but pretty odd. It's not like there is some small orifice that prevents the free flow of oil from the tank.

Regarding noises from the engine... that could be caused by all kinds of things either related or not related to exceeding 2500 RPM. BTW, by how much did he exceed 2500 RPM? Big difference between going to 3000 RPM and going to 5200 RPM (much higher seems unlikely with a fixed pitch propeller).

l own two 912uls engines with a total time of about 1400 hours. My personal opinion is that is it very unlikely that running a 912 with 100 degree F motor oil above 2500 RPM wrecked the engine.

Last edited by R100RS : 09-28-2021 at 08:33 PM.
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