High viscosity of aeronautical piston engines lubricating oil

[greybeard]

When I frequented the local flying club many years ago, I heard technicians say that they used "100 viscosity" lubricating oil for the piston engines of small touring planes. At the time, common car engine oils had grades of 10W/40 or so. I've always wondered why airplanes needed such high viscosity, guessing it was the same for WWII era.

 

[Engineman]

When I frequented the local flying club many years ago, I heard technicians say that they used "100 viscosity" lubricating oil for the piston engines of small touring planes. At the time, common car engine oils had grades of 10W/40 or so. I've always wondered why airplanes needed such high viscosity, guessing it was the same for WWII era.

Very roughly, Motor car engine oil viscosities are half the quoted aviation grade. So, a car 40 is equal to an aviation 80.

Eng

 

[Engineman]

Very roughly, Motor car engine oil viscosities are half the quoted aviation grade. So, a car 40 is equal to an aviation 80.

Eng

Overall, there are many considerations for Piston Aero-engine oil suitability. Certainly, manufacturer specification is critical and the requirements of specific engines vary a lot.
Beware of the simple looking codes, you need to read up on the details if you run an aero-engine! For instance, Aero oils are often defined by the "weight" or "grade", like an 80 or 100.
This number is relative to viscosity, but you will notice that is appears very different to the SAE car oil 40 or 50, because it is measured differently. Additionally, they are different!
How about an aero 100 or W100 for instance? The 100 would be a "straight" oil, the W100 would be an oil with dispersant properties and the difference between these two could cause engine damage if used incorrectly.
So, how do you think using your example of 10W/40 in a mid life aero-engine designed for straight 80 would effect?

Eng

 

[greybeard]

So, how do you think using your example of 10W/40 in a mid life aero-engine designed for straight 80 would effect?

Frankly, so far I'm a bit confused . Mainly because I miss a criterion for aeronautical oil. On Wikipedia, I read that there's a standard for automotive, the SAE J300. According to it the 10W/40 equals a centipoise viscosity of 7000 at -25°C and between 12.5 and 16.3 centistokes at 100°C. But what about aviation piston engine oils?

 

[Engineman]

Frankly, so far I'm a bit confused . Mainly because I miss a criterion for aeronautical oil. On Wikipedia, I read that there's a standard for automotive, the SAE J300. According to it the 10W/40 equals a centipoise viscosity of 7000 at -25°C and between 12.5 and 16.3 centistokes at 100°C. But what about aviation piston engine oils?

Hmmm, not sure why you have gone from a general question about motor oil and Av oil to a specific point about testing standards? Well, the Av oils have their own standards and you will have to do a bit of research if you want to compare specific viscosities. You will possibly find viscosities quoted on tech data from different manufacturers. However, the SAE testing standards themselves are available to buy as I believe that they are copyright.

Eng

 

[MIflyer]

Aviation motor oils design for air cooled engines must be manufactured to avoid the creation of metallic ash, which can cause serious problems with the piston rings. Over 30 years ago Mobil tried an aviation version of their Mobil 1 synthetic automotive oil for aircraft piston air cooled engines and ended up paying for a lot of repairs.

So no, sticking an automobile 10W40 oil in an aircraft air cooled piston engine would not be a good idea.

 

[pbehn]

Engine oils and lubrication starts off as a simple topic and rapidly enters branches of science that make your head burst. I was briefly involved in melting points softening points flash points and viscosities of fuels and coatings. It is another world of science, like what compressive load can the molecular bonds of an oil withstand?

 

[SaparotRob]

.. It is another world of science, like what compressive load can the molecular bonds of an oil withstand?

How drunk is it?

 

[pbehn]

How drunk is it?

Yup, I started to have thoughts like that. I concluded that producing oils needs the oil companies and engine designers to know a lot of clever stuff and left it at that.

 

[MIflyer]

When I was at OC-ALC we had a problem with sintered bearing (kind of a porous sleeve made from compressing powdered metal) was not doing the job. Turned out that the manufacturer had coated it with an anti-corrosion oil that was preventing the oil that was supposed to impregnate it from getting down into the nooks and crannies. The answer was to boil the things in the proper oil for a while.

Also while at OC-ALC I was involved in considering use of a silicone oil with a viscosity of 20,000 poise. It was intended for use in sonar transducers and had the consistancy
of soft Jello. Trying to stick something through it was like poking a sheet of very tough Saran Wrap; it would stretch for just about forever. If you got some on your hands it would be there for a while. We finally found a much better substance for calibrating high temperature sensors.

The present approach of using "0W20" weight oil in automobile engines seems unwise to me, but what do I know? Is a 0 weight oil even there?

 

[pbehn]

When I was at OC-ALC we had a problem with sintered bearing (kind of a porous sleeve made from compressing powdered metal) was not doing the job. Turned out that the manufacturer had coated it with an anti-corrosion oil that was preventing the oil that was supposed to impregnate it from getting down into the nooks and crannies. The answer was to boil the things in the proper oil for a while.

That is just LOL funny. Imagine the time and money involved in developing a sintered bearing that does just what you want it to as regards lubricant flow and load bearing? Then someone has an idea to make things better.

 

[special ed]

Eastman Kodak used sintered bronze bearings with an attached metal clip which held a piece of one inch square by 1/8 inch thick felt in contact with the bearing. Six month maintenance on the equipment meant a couple drops of 30W oil on the felt. These bearings seemed to last forever if kept lubricated, as the heat of the shaft running in them pulled the oil to the shaft surface. The only problems happened if seldom lubed or a new guy thought he just discovered WD-40 as a "better" lubricant. Two weeks after the new guy finished, every bronze bushing need to be replaced as the alcohol in the WD-40 reversed the oil flow and drove out the oil impregnated within the bronze. Newer equipment by the 1970s did away with felt as it appears the bearing composition changed and now direct oil drops were applied at the shaft/bearing surface. Bearings were now often changed due to wear.

 

[pbehn]

Eastman Kodak used sintered bronze bearings with an attached metal clip which held a piece of one inch square by 1/8 inch thick felt in contact with the bearing. Six month maintenance on the equipment meant a couple drops of 30W oil on the felt. These bearings seemed to last forever if kept lubricated, as the heat of the shaft running in them pulled the oil to the shaft surface. The only problems happened if seldom lubed or a new guy thought he just discovered WD-40 as a "better" lubricant. Two weeks after the new guy finished, every bronze bushing need to be replaced as the alcohol in the WD-40 reversed the oil flow and drove out the oil impregnated within the bronze. Newer equipment by the 1970s did away with felt as it appears the bearing composition changed and now direct oil drops were applied at the shaft/bearing surface. Bearings were now often changed due to wear.

As I said a couple of posts up, there is a lot of very clever stuff involved, some of it being the opposite of "common sense" which isnt sense or common.

 

[Engineman]

As I said a couple of posts up, there is a lot of very clever stuff involved, some of it being the opposite of "common sense" which isnt sense or common.

Yes, some good points here. Indeed, lubrication is basically simple but, in detail or specific applications it is very complicated. For consumer products like cars, motorbikes and aeroplanes, the manufacturers specifications are a good guide, essential information for the unknowing owner/operator, and there are good specifications for the owner/operator to refer and adhere to. It is essential that owner/operators adhere to the lubricant SPECIFICATION, not the marketing hype. Problems arise in everyday automotive lubricant sales where marketing hype can mislead consumers. Reading the marketing blurb on the bottle would make you think every oil is perfect for almost everything. Also, the tales of "my old mate" and his car that did 100,000 miles on the cheapest possible oil etc, is no guide whatsoever that old or low spec products are suitable for higher tech applications. A particular problem for the unknowing is whether using modern spec lubricants in an old machine is worthwhile? The answer here can be Yes, Maybe or No. The old machine will likely not come with guidance for modern lubricant specifications and so, old machine owner/operators, you may need to learn more about some of the requirements of lubrication and lubricants!

Eng

 

[MIflyer]

thought he just discovered WD-40 as a "better" lubricant

WD-40 was not designed as a lubricant but was created for Water Displacement on the Atlas ICBM. You could not paint the tank on the Atlas because it was made of thin stainless steel and since it was a balloon, tended to flex and pop the paint off, assuming the cryogenic temperatures on the LOX tank even left any. Our converted Atlas ICBM space boosters typically had all the paint gone off the LOX tank, leading to a truncated national star insignia which was painted along the LOX and RP-1 tank interface. The original producer was the Rocket Chemical Company of San Diego. As a water displacing substance, WD-40 contains DMSO, a cleaning chemical, and so will clean out other lubricants or other substances already there, including water.

 

[greybeard]

Sorry guys, I think there is a gigantic misunderstanding: I see we slipped from a simple question of mine to rocket science. Probably my fault since English is not my native language.
So far I believe first answer by Engineman is clear enough for me: unit of measure of aviation oil viscosity is about half of automotive one, so the values are twice. In addition, I was just wondering which is the standard for viscosity of aeronautical oils, like the SAE J300 is for automotive. Nothing more. Thanks anyway to all who kindly replied.

 

[Engineman]

Sorry guys, I think there is a gigantic misunderstanding: I see we slipped from a simple question of mine to rocket science. Probably my fault since English is not my native language.
So far I believe first answer by Engineman is clear enough for me: unit of measure of aviation oil viscosity is about half of automotive one, so the values are twice. In addition, I was just wondering which is the standard for viscosity of aeronautical oils, like the SAE J300 is for automotive. Nothing more. Thanks anyway to all who kindly replied.

Hi Greybeard, There are many different standards and specifications for lubricants, but possibly the SAE J1966 and J 1899 might cover your interest. As I said, many specific standards are only available to purchase, but you can usually find product specifications with details online. The standards are produced by many institutions and government bodies. They include SAE, ASTM, JASO, ACEA and API to name but a few. Military standards are held by many different countries.
Cheers

Eng

 

[special ed]

WD-40 was not designed as a lubricant but was created for Water Displacement on the Atlas ICBM. You could not paint the tank on the Atlas because it was made of thin stainless steel and since it was a balloon, tended to flex and pop the paint off, assuming the cryogenic temperatures on the LOX tank even left any. Our converted Atlas ICBM space boosters typically had all the paint gone off the LOX tank, leading to a truncated national star insignia which was painted along the LOX and RP-1 tank interface. The original producer was the Rocket Chemical Company of San Diego. As a water displacing substance, WD-40 contains DMSO, a cleaning chemical, and so will clean out other lubricants or other substances already there, including water.

The DMSO is why people used it for a topical arthritis pain remedy.

 

[emu27]

Used to fly the S-76 where Dextron oil was a permitted lubricant for the main gearbox, we found after a period of operation with high OAT's the pressure would drop below limits. Investigation found the oil still passed specs and the opinion was the long chain molecules were being chopped into shorter chains by the action of the oil passing through the oil jets. A change to Mobil Jet fixed all issues. The reason for using the Dextron was because it was one third the price of Mobil Jet. Dextron was originally developed as a automotive automatic gearbox oil. As one advertisement says, "Oils ain't oils".


View: https://www.youtube.com/watch?v=dcDbqQOcZps

 

[Simon Thomas]

The ashless dispersant oil specification is SAE specification J-1899.
There is another SAE spec for non-additive oil, but I don't want to look it up and you don't want to use it.