Engine Oil Design

Homebuilt Aircraft & Kit Plane Forum

Help Support Homebuilt Aircraft & Kit Plane Forum:

Skippydiesel

Well-Known Member
Joined
Dec 15, 2021
Messages
66
Dear Skippy, those (clearly, synthetic) oils are entirely inappropriate to this engine when using AVGAS. In fact, they are deadly. Only the base mineral oil is able to suspend the lead compounds in cylinder blowby, synthetics drop it out of suspension in ways that block oil galleries. The typical oil used is a 100 or 120 monograde ashless dispersant, corresponding to SAE 50 and SAE 60. Detergent oils are completely out of the question. In point of fact, they are OBSOLETE in aviation oils mainly due to the hazards they create. Multigrade aviation oils are available too - for a number of reasons, they are not as popular as you might think they should be. In certain parts of this engine, oil can reach rather high temperatures (notably in the rocker box on the exhaust valve side), resulting in oxidation and sludge formation, but there is also the sludge produced from blowby lead products to consider. These high temperatures can result in degradation of the viscosity modifiers used to maintain viscosity at higher temperatures in multi-grade oils - dependent on individual installations and operations, in many cases it never happens at all. Gasoline oil dilution and/or pre-heat allows mono-grade oils to be used in cold environments. Please do consider whether or not your automotive tribology knowledge actually crosses over to aviation engines - in many cases, it is most inappriopriate.
Hi Paul - You make a persuasive argument.
I never suggested synthetics should be used in aviation, only mentioned this technology to illustrate how far lubricant engineering has come, since my youth/involvement in engines (of the earth bound type) starting in my teens mid 1960's.
As for the adoption of multi grade oils in aviation - I cant help feel that the ultra conservative approach, adopted by many in this world, leans strongly toward hostility to any change. Mainstream oil companies do not invest in technologies that are unlikely to work but there can be no easy prediction of market acceptance.
Of course if you are wedded, to what is now a very old technology (the air-cooled direct drive aircraft engine) compared with the ground based liquid cooled automotive world , you will tend towards lubricant/fuel technology of the same era.
I live in & fly in Australia, many years ago did spend two winters in Canada, so I do have a little understanding of the challenges that extremely cold (to me) climates bring to oil & fuels - we used to change to lighter (less viscus) multi grades in winter - now you can probably run the same full/semi synthetic multigrade oil all year round.
I understand that many lower powered LyCons can run perfectly satisfactorily on ULP, so the issue of lead contaminations of oil, is a choice the owner/maintainer makes for these engines and I would agree that managing this pollutant must require some extreme strategies - the question surely is why impose this on your hip pocket?
I have no experience of the Mercedes diesel aviation engines ( I do have a life long passion for diesel engines) but as they are derived from a car engine, I suspect they run on automotive (or very similar) multi grade oils.
To me this comes down to the acceptance or not of the latest (suitable) technology. I have a fairly jaded view of what I see a the grim reluctance of a large sector of the aviation (internal combustion engine) world, to move toward promising alternatives.
 

Skippydiesel

Well-Known Member
Joined
Dec 15, 2021
Messages
66
You can be a doubter, but doubt has little influence on the aviation world's reality. Lycoming and Continental aviation oils, are mostly single-weight, and even the multiweight oils are being run in engines that were designed to run single-weight. Lyc oils are typically 50 weight. Consider a positive displacement oil pump (they all are) trying to push 100% of the oil through a cooler with the temp of the oil somewhere below freezing temps. Stuff can break.

Try googling 'vernatherm for Lycoming' for more info.
I don't doubt your statement but I would question; why on earth owner/mechanics are sticking with such ancient oil technology, when more efficient alternatives exist.

I suggest you read the debate, on this Forum, "Engine Oil design" - very informative.
 

rv7charlie

Well-Known Member
Supporting Member
Joined
Nov 17, 2014
Messages
3,288
Location
Pocahontas MS
I suggest that I was the 1st person to respond, in that specific thread you mention. I again ask, what's your experience with 'traditional' aircraft engines and their operation? I would think that if you had experience with them, you'd have some understanding of 'why owner/mechanics are sticking with such ancient oil technology'. A 9 series Rotax is *not* the same animal as a 'traditional' a/c engine, nor is it subject to the same rules/regulations/real world operating limitations.
 

Victor Bravo

Well-Known Member
Supporting Member
Joined
Jul 30, 2014
Messages
11,129
Location
KWHP, Los Angeles CA, USA
Skippy, the old antique design "traditional" aircraft engines have operating clearances and dimensional tolerances that would look like the Grand Canyon to a modern automtive engine designer.

I recall Klaus Savier at Lightspeed Engineering once told me that the piston clearance on a Porsche car engine was less than one tenth of a typical Continental 100HP airplane engine.

Our cylinders go out of round, some of them have "choke" or tapered bores, and the metallurgy between pistons and cylinders is not nearly as matched as they do in car engines. The expansion and contraction of parts, and the commensurate changes in clearance, would make one of those automotive designers s**t in their pants.

That is one reason we need much thicker and more "cushion-y" oil.
 
Last edited:

TFF

Well-Known Member
Joined
Apr 28, 2010
Messages
17,658
Location
Memphis, TN
It’s not ancient oil design. The oil is quite bespoke. Air cooled engines need their needs just like water cooled. Or half water cooled. Your engine does not do better or worse. It just exists. Rotax doesn’t cover the US needs in aviation. It just has a segment. It won’t ever fill in like it does in Europe. It’s a political engine as much as an airplane engine. I would say if it came down to Rotax and gliders only in the US, everyone would trade in their planes for AckAck cannons.
 

Bob H

Member
Joined
Jun 9, 2022
Messages
23
Terrific information Bob - I have been flying Rotax 912ULS for about 12 years - so far great reliability, economy, smooth, quiet (relatively) and performance.
To minimise any need to use AvGas, on an away trip, I carry two 20L collapsible fuel bladders, so that I can get ULP off field - system works well and there is usually a friendly business/private individual who will lend or transport me to the nearest reliable source of 95-98 RON.
For personal piece of mind, I do a 50 hr oil change (no filter) using AiroShell Sport plus 4 - no science behind this but I do hope I am enhancing my engines longevity.
I live in California and we can not get any ethanol free gas so I have to use our 91 octane mogas with no problems in the 912 S. Have flown into Death Valley at - 280' making really low density altitude with no issues. I use auto fuel hoses that are formulated to withstand the ethanol and no problems. And after 18 yrs of flying the 912, no issues with carb rubber parts either.
 

Dan Thomas

Well-Known Member
Joined
Sep 17, 2008
Messages
7,349
You can be a doubter, but doubt has little influence on the aviation world's reality. Lycoming and Continental aviation oils, are mostly single-weight, and even the multiweight oils are being run in engines that were designed to run single-weight. Lyc oils are typically 50 weight.
Here in Canada, and I suspect in Alaska, multigrades are by far the preferred oils. Ambient temperature ranges are wide, and multigrades make sure that the engine pump can suck the stuff up out of the sump when cold, and still keep the moving stuff apart when hot.

1660780293555.png

This graph shows the viscosity behavior of an auto oil. Aircraft multigrades do the same thing. The oil doesn't get thicker as it heats up. It just thins less than a single-grade.

Here are Lycoming's oil recommendations:

1660780491065.png
https://www.lycoming.com/sites/default/files/SI1014N Lubricating Oil Recommendations.pdf
And Continental's:

1660780595282.png
From http://pceonline.com/wp-content/uploads/2017/04/M-0standardpractice2017-01-15.pdf
See the numerous multigrade recommendations.
 

Bob H

Member
Joined
Jun 9, 2022
Messages
23
The Rotax 912 has aluminum cylinders and alum pistons for a matched CTE. The cyl walls are nicosil ceramic coated which is like diamond in hardness so there is no cyl wall wear. Because of the matched expansion/contraction of pistons and cyls, the clearance is around 1 mil on dia.
This tight clearance almost eliminates oil consumption. I go over 30 hrs before needing a quart of oil. And because the engine is water-cooled, there is no shock cooling issue. And because the cyl heads are operating around 220F, the valves never get burned. They last like in a car.
You never have to do a top end job on the 912. Or a cylinder replacement.
 

Dan Thomas

Well-Known Member
Joined
Sep 17, 2008
Messages
7,349
But would you disable the Vernatherm in a Lyc?

Nope. It's there to get the oil temp up sooner.

The pressure is on the oil cooler anyway, whether the Vernatherm is open or closed. Cold oil can cause higher pressure in the whole system, and oil coolers have been known to get bulged by that.
 

Dan Thomas

Well-Known Member
Joined
Sep 17, 2008
Messages
7,349
My understanding is that the Vernatherm on a Lyc allows oil to bypass the cooler when it's cold. Otherwise, the positive displacement pump would almost certainly rupture the cooler in really cold weather.

Lyc oil schematic
This one makes it a bit clearer:

1660793704807.png
The "oil cooler bypass valve" is the vernatherm. Note that the cooler is under full pressure whether that valve is open or closed. When the valve heats up, it closes the bypass and the oil has to go through the cooler. I doesn't open any line to the cooler, since they're both open anyway.

The verntherm is also spring-loaded to release if the oil in the cooler is too stiff.
 

Skippydiesel

Well-Known Member
Joined
Dec 15, 2021
Messages
66
What a great debate - love it! So many differing opinions but just the one topic - crankcase oil.
 

Skippydiesel

Well-Known Member
Joined
Dec 15, 2021
Messages
66
I suggest that I was the 1st person to respond, in that specific thread you mention. I again ask, what's your experience with 'traditional' aircraft engines and their operation? I would think that if you had experience with them, you'd have some understanding of 'why owner/mechanics are sticking with such ancient oil technology'. A 9 series Rotax is *not* the same animal as a 'traditional' a/c engine, nor is it subject to the same rules/regulations/real world operating limitations.
Well I do have my PPL - trained in & flew mainly Cessna's (from straight tails to CSR) for a few years, before discovering Recreational Aviation Australia (RAA) and the delights of Rotax 912ULS in an ATEC Zephyr. RAA allow you to do all your own maintenance, in non commercial use aircraft, subject to passing a proficiency test. Now have a, yet to fly, Sonex/Rotax 912ULS (registered RAA). In preparation for the Sonex, recently did my tail wheel endorsement in a Citabria.
So very little hands on experience with LyCons BUT have to say flying behind a LyCon is great, flying behind a Rotax 912 ULS is positivity stunning (the modern airframe helps too).
Sure I understand that old technology engines, require old technology (like) oils - what I don't understand is why you hang on like grim death to the old technologies - still producing.
Seems to me, that many of the respondence in the "Engine Oil Design" thread seem to like multi grade, ash dispersant (not quite sure what this means) aviation oils.
Nostalgia is one thing (I like old diesel Mercedes cars) but being hostile/excluding/blocking technologies that are likely improve power to weight, lower fuel consumptions, increase reliability and reduce emissions is just beyond my understanding.
True the Rotax 9's are not "traditional a/c engines" - my opinion, clearly the better future, especially with the development of fuel injected variants.
 

BJC

Well-Known Member
Supporting Member
Joined
Oct 7, 2013
Messages
15,619
Location
97FL, Florida, USA
So many differing opinions but just the one topic - crankcase oil.
That is one of the problems with internet forums, including HBA. To benefit from this one, the reader must learn to distinguish fact from opinion. As Orion frequently wrote, “What one believes is irrelevant in science.”
True the Rotax 9's are not "traditional a/c engines" - my opinion, clearly the better future, especially with the development of fuel injected variants.
It may not be “traditional”, but, even though I don’t like it, it has dominated the sport aviation world market for decades.


BJC
 

Skippydiesel

Well-Known Member
Joined
Dec 15, 2021
Messages
66
It may not be “traditional”, but, even though I don’t like it, it has dominated the sport aviation world market for decades.


BJC
Oh perceptive BJC , I would encourage you to ask; why is this so?

It may only be a small part of aviation - the very same Rotax 9's come in identical Certified variants. At only 80 -115 hp they probably have few applications (there is a Tecnam- P006T 140 knots/4 seats) - these engines are proper aircraft engines that can not be dismissed as being just for sport aviation.
 
Last edited:
Top