Ring step area lubrication and low/no oil consumption

[Blake Sobiloff]

In an article about the longevity of cylinders in aircraft applications the author claims that engines that use little to no oil aren't letting enough oil into the ring step area of the cylinder, causing a significant reduction in cylinder longevity.

(In case you aren't familiar with aviation-speak, "TBO" means time between overhaul. Many general aviation engines go between 1200 and 2000 hours of operation between overhauls, and most OCIs are 50 hours of operation.)

The author asserts that unless your engine is using a quart of oil every 20 hours you aren't getting good lubrication of the ring step area. To put this is automotive terms, you'd need to be using a quart every 1,000 miles or so.

Now, I've never had a car that guzzled oil that quickly. In fact, my current cars use no oil during 7,500 mile OCIs. One has 45,000 miles on it and the other has 92,000 miles on it. So, from my experience in the automotive world this author's theory sounds like bunk.

Is there something special about aircraft applications that would make this author's theory valid?

 

[moribundman]

Sounds like my Audi has an airplane engine!

But seriously, I'm inclined to believe that some oil consumption is better for engine longevity than no noticeable oil consumption over many thousand of miles.

 

[tom slick]

the small amount of recip engines I've been around (I'm a turbine guy) they are old technology. IMHO automotive type engines are higer tech. I don't know if that plays into this equation.

 

[bulwnkl]

I think the guy's talking out of somewhere other than his mouth, but then again I wouldn't know...

 

[JDP]

I answered a question a while back, on another forum, about oil consumption by presenting a calculation based on constant RPM operation.

At 1800 RPM @ 60 mph with aconsumption of 1 qt/1000 miles, it works out to roughly

0.032 oz/mile
0.000017777778 OZ/rev
0.000002962963 oz/rev/cyl
0.000005925926 oz/power stroke

for a 6 cylinder engine - that is if my spreadsheet is as wonderfull as it should be

Of course there are more variables in actual operation than that but it does give a rough idea.

I've always said that if it is "using oil", at least I know oil is getting to where it is needed. (not that spending less on oil and maintenance isn't better and all that too.....

Considering that the break point for warrenty on an oil consumption complaint is somewhere between 4-600 miles, maybe minimal oil consumption levels are a bragging rights kinda thing as well as something to do with the nuisance factor involved with checking and adding, more than some kinda defect to be fixed - unless of course there is blue smoke coming from the tail pipe or oil leaking on the ground.

JDP

 

[forcedtalon]

In an airplane engine the RPMs are always real high above 4-5k so I can see oil usage there. Also Airplane engnines are clearacned diffrently since there under heaver loads and higher RPM consitantly.

 

[Hirev]

That is a real old theory I remember reading about it in real old books from the 50s in automotive courses I took in the 70s. I do not think this theory would apply to modern water cooled passenger cars and truck motors.

The problem here from what I see is we are talking about apples and oranges or Water cooled vs air cooled.

It could apply to air cooled motors. But to tell you the truth I have never seen a air cooled motor that did not use at least some oil do mostly from the greater piston to bore fitment required in an air cooled application. The technology in the article is shall we say very dated compared to most modern automotive engines.

 

[Blake Sobiloff]

forcedtalon, most general aviation engines run at 2700 RPM or less. I wouldn't consider that high RPM operation. Maybe you're familiar with one of the kit planes powered by a Mazda rotary engine, which does turn two or three times faster, but they're a very, very small minority.

Hirev, I thought the same thing at first, but then I realized that a modern air cooled motorcycle engine (like most Harley Davidsons) doesn't necessarily burn oil, either. So maybe that's not a significant difference?

I mean, air cooled or water cooled, how does the ring step area get oiled? There has to be some lubrication of that area, regardless of engine design. Do the rings drag a bit of oil along with them on the way to TDC?

 

[tom slick]

another issue that may affect oil consumption is most light aircraft recips are horizontally opposed. most automotive engines are "V" or vertical inlines.

 

[Hirev]

quote:

---

I mean, air cooled or water cooled, how does the ring step area get oiled? There has to be some lubrication of that area, regardless of engine design. Do the rings drag a bit of oil along with them on the way to TDC?

---

Yes I agree that the rings do drag a little oil along with them to TDC. I have always seen a little oil film at the top of ring travel on motors recently ran before pulling them apart. The oil ring(s) only eliminate excessive amounts. The 1st and 2nd rings still get a limited amount of oil off the cylinder wall.

How much oil is dragged to TDC. And how much oil is necessary for the application. Is the key here I think?

Piston design, piston to cylinder wall clearance, ring design, ring material, ring coating in some cases, cylinder wall finish, and Cylinder wall material all play a role.

The part I found that was so old school to me is chrome plated and Iron rings. Out side of dirt track motors and desert racers I don't believe anyone uses chrome plated rings in light duty gas motors nor have they since before my time. And Iron rings are usually only seen in cheap rebuild kits this day and age as far as automotive motors.

The US auto manufactures generally used a ductile iron based Moly faced ring facing even in the 60s. As opposed to a plain iron ring. While Japanese prefer a steel based gas nitrided ring facing. Latest trend in US auto manufactures is now also going to a steel based moly faced ring.

Tom Slick also brings up a good point about horizontally opposed motors.

 

[goodvibes]

If the rings and cylinders weren't getting enough oil towards the top to begin with wouldn't the wear accelorate to where oil would then not be conpletely cleaned off the walls on the down stroke and the problem take care of itself? I don't get it. Your engine is in jepordy because it isn't using enough oil doesn't make sense to me. If it doesn't begin to use oil, especially after some time, than the rings are sealing well and not showing wear(enough lubrication).

 

[Eddie]

Old designed aircraft engines typically are overhauled at ~2000 hours I believe. Automobile engines used in light planes i.e. VW air cooled & Subaru water cooled typically are checked for overhaul needs at ~ 10,000 hours. There is lettle comparison between theold designed aircraft engine and the modern auto engine.

 

[Oldmoparguy1]

Don't forget that aircraft recips are run at very high power levels. It is normal for the engine to run at 100% power for takeoff and climb for several minutes, then throttled back to 75% power and run at that setting for hours at a time. Try that with your (any make/model) car.

 

[mechtech]

If increased wear was the result of low oil consumption, things would even out as the wear would lead to increased consumption.
High flash point oils that stick to the cyl walls help oil consumption a lot.

 

[Eddie]

The Cessnas I use to fly usuall took off at full power using ~2,800 rpm and throttle back to ~ 2,300 cruise. Gee I run my Forester for all day on vacations at 3000-3400 rpm. Aircraft engines are low speed engines (auto engine conversions with gear box excluded) Typical old aircraft engine crank clearence is so high that 40-60 wt oil is the norm. Just no way to compare the two IMO.

 

[JohnBrowning]

I have never had a Toyota that used any more then 1/2 a quart over 3000 mile OCI with dino oil and no oil over 6500 mile OCI with synthetic. Dad always used 10W30 and 10W40 as winter oils and 20W50 as warm weather oils. So the fact that cheap dino 5W30 never saw the inside of anything we ever owned might play a role. Now it is 10W30.5W40 and 15W50 in Mobil-1 used in all of Mom and Dads vechiles and most of mine as well.

 

[bulwnkl]

Goodvibes & mechtech: Exactly. This guy has concocted a long-winded, nice-sounding (as long as you don't think about it) explanation for his preferred, totally-wrong pet hypothesis.

 

[Eddie]

Right on bulwnki:

 

[buster]

I've also been told by a well respected auto engineer that some oil consumption is beneficial and that engines that use some oil do last longer. For the reasons above and also to lubricate the valve guides better. Some engines are designed to use some oil, Hondas for example (some not all Hondas). My 2001 Corolla that used 1qt per 3k miles ran like new with 180k on it.

 

[reyjay1]

Maybe for those that don't use oil this is the good argument for UCL.