What's new
Van's Air Force

Don't miss anything! Register now for full access to the definitive RV support community.

Exceeded recommended RPM with cold oil

thiggins

Well Known Member
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
 
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.
 
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.
 
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.
 
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.
 
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.
 
Last edited:
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.
 
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.
 
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:
120F seems to be a common number for a lot of rotating equipment oil.An explanation I have heard is viscosity at lower temperatures causes reduced flow through the orifices that feed the bearings.
 
My observation is that if oil temp is 100°F, oil pressure is down in the 60 lb range so oil viscosity is in the working range or close to it. Oil RPM above 2500 RPM is in the yellow or caution range up to about 4000 RPM. If Rotax didn't want me to operate the engine above 2500 until oil temp was 120°F they'd have put a red line in the RPM gauge, not yellow, is how I operate it.
 
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.

I edited my initial post with additional info above that I had forgotten to add initially. This is the most important reason for the limitation.
 
The Shell Sport 40 oil is rated as a 10W-40 oil. This means it is 10 viscosity at “0” degrees Fahrenheit and 40 viscosity (thicker) at 212 Fahrenheit. The oil is changing viscosity, getting thicker, as the engine warms up. Doesn’t this resolve what is being discussed above?
 
A 10 Winter motor oil at 0F has a flow rate of 150 to 160 centistokes, thick and slow.

A 40 weight motor oil at 190 to 220F has a flow rate of 13 to 14.8 centistokes. thin and fast




What is the centistoke range of that motor oil at 110F ? 70 ? 75? 80? centistokes?
 
10w-40 means the oil has the viscosity that 10w wt. oil would have at 0 deg. not that its viscosity is 10w.
 
Logs

Does Anyone know of a good Rotax person near Western NC/Asheville , NC?

Is getting a BUDS connector and software worth doing?
Would it show oil pressure in the logs ?
Where do we get a BUDS connector anyway?

Thanks for all the input/information, very helpful.
 
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.

Interesting explanation. Is it mentioned in ROTAX documentation?

Since blow-by around the rings is where the crankcase pressure comes from, crankcase pressure should increase with engine RPM, just as the output from the engine-driven oil pump increases. After all, even without combustion pressure driving the pistons (and creating blow-by) in a running engine, there is enough pressure in the crankcase to push all the oil out of the crankcase when burping a cold engine.

At 100 degrees F, it is hard to imagine such a phenomenon (oil accumulation in the crankcase) occurring at any RPM.
 
After all, even without combustion pressure driving the pistons (and creating blow-by) in a running engine, there is enough pressure in the crankcase to push all the oil out of the crankcase when burping a cold engine.

Actually there’s not. At least not enough to push all of it back to the tank. A very common cause of people overfilling the oil tank is when checking the oil when it’s cold, particularly when it’s very cold, and then adding oil based on what the dipstick reading is. When the oil is cold it’s still leaves a very thick coating on the interior of the engine case even after an air bubble has been pushed up through the hose back to the oil tank, causing the classic burp.

No, this info is not in the documentation but it is taught by some instructors in the maintenance classes.
 
Last edited:
Does Anyone know of a good Rotax person near Western NC/Asheville , NC?

Is getting a BUDS connector and software worth doing?
Would it show oil pressure in the logs ?
Where do we get a BUDS connector anyway?

Thanks for all the input/information, very helpful.

You will be able to get engine and flight data logs from your Dynon or Garmin system. (oil pressure and much more)

BUDS might be necessary to find problems with the ECU, ignition and fuel injection systems, but not always. And, it usually requires an experienced Rotax AMT to read the data.
 
This is what I've read some where, it's because the oil filter has a by pass system in it in case of a clogged filter the unfiltered oil will now go into the engine, to prevent this the oil temp has to ne at least 120 F to be thin enough NOT to cause the by pass to open...I sincerely believe that no serious harm was done to the engine as long as the occurance did not happen right after starting and there was no warm up period or was done on an extremely cold day...call Lockwood Aviation, they are the resident experts on the Rotax.
 
There are threethings that I think your friend should check first in the engine.

1. the oil pressure at idle and at standard operating temperature should be within range, lower would indicate (in most cases) bearing wear etc.

2. if pressure is good then a differential compression test should be run

3. send away an oil sample to get an understanding of particles in the oil.

Those tests will give a basic understanding of engine health
 
This is what I've read some where, it's because the oil filter has a by pass system in it in case of a clogged filter the unfiltered oil will now go into the engine, to prevent this the oil temp has to ne at least 120 F to be thin enough NOT to cause the by pass to open...I sincerely believe that no serious harm was done to the engine as long as the occurance did not happen right after starting and there was no warm up period or was done on an extremely cold day...call Lockwood Aviation, they are the resident experts on the Rotax.

I took the Rotax maintenance and service courses with Lockwood some years back and I can confirm the limitation is to prevent bypassing the filter when the oil is thicker whilst cold. It is mentioned somewhere in Rotax's documentation somewhere but I can't remember where.
 
I took the Rotax maintenance and service courses with Lockwood some years back and I can confirm the limitation is to prevent bypassing the filter when the oil is thicker whilst cold. It is mentioned somewhere in Rotax's documentation somewhere but I can't remember where.

I took the Lockwood IRMT class as well and can confirm that this this the reason they give.
 
All this talk about the oil bypassing the filter when cold… Where is the harm in that? So, the oil bypasses the filter for a few minutes to improve circulation when oil is cold and engine is warming up. The oil is clean and filtered from previous flight. When the engine is warm the oil flows through the filter again. Sounds pretty good to me.

Consider old Continental A65-8 with just a course oil screen and no filter...:eek:
 
I’ll take a stab at that, Jim. The oil pump is a positive displacement pump so flow rate through the pump is constant for a given RPM. If the bypass valve comes open that bypass flow is not going through the engine. While it may not be starving the bearings, it is definitely less flow than the design is intended to provide at that RPM.
 
The internal oil filter bypass valve is designed to prevent damage to the spin-on filter itself. If oil flow through the filter element is restricted the bypass valve opens to prevent over-pressurization of the sheet metal filter housing and possible rupture. Oil is still flowing through the spin-on filter cartridge at the same rate only not through the filter media itself. Once the oil pump pressurizes the oil galleries in the engine it doesn’t matter if the oil filter element is being bypassed – oil is still flowing at the same volume to all lubrication points.

The oil pressure, typically 50 psig at cruise and normal operating temperature, is an indication of how well the sleave bearings are able to retain oil. Worn sleave bearings will flow more oil and thus pressure will be lower.
 
Follow up -

I was away on vacation so didn't get involved in the discussion much.....

I will add my opinion that I doubt that one engine start over speed event did any damage that would cause it to start making unusual noises.

If is did, engines would regularly be being damaged because people do it all the time.

If there are new noises that aren't normal, I would be looking for a different cause.
 
He was able to speak with a Rotax expert, who had him do some testing, pulling the engine through,… We think it’s ok. The rough running turned out to be some fouling of the front plugs we believe. All good.

Thanks for all the replies and info.

Tim
 
Jim,

Duh! I smacked myself in the forehead after that one. You’re right: the bypass path is around the filter, not back to the tank.

Rich
 
Back
Top