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Oil, your reciprocating engine, and how it fits together.


Anticept

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I'd like to do an overview of what is contained in aviation oil, and explain why we use what we use. I will also explain some of the gotchas of oil, and why it breaks down, as well as a basic engine primer. This is a general overview, not necessarily geared towards any one particular make of engine.

My sources are from A&P school, from material created by the Aeroshell company and the BP Research Laboratories.

First, a couple primers:

Primer for plain bearings: Also known as bushings, they are used almost everywhere where there are high compression forces experienced. There is not a lot of use for ball or roller bearings in engines because ball bearings have very little surface contact and poor loading characteristics, and roller bearings are only effective in the direction in which they roll, and rapidly deteriorate if any force is applied other than parallel to their rolling direction. There are exceptions such as in radials and their deep groove ball bearings, but that's not relevant. Plain bearings also have a plated material called "Babbitt Metal", a sacrificial layer that also helps retain oil.

With that said, bearing surfaces consist of peaks and valleys. If two bearing surfaces are allowed to contact, we end up with these peaks and valleys chipping away at each other, and rapid deterioration of the mating surfaces. See here. These peaks and valleys are considered a good thing though! As a minor function, they prevent cold welding (two perfectly matched metallic surfaces can bond with little heat or pressure), but what is a major and important function is they also hold oil in place!

Primer for cylinder barrels: Barrels are an extremely hostile environment. Pistons are machined in a way that they are just ever so slightly smaller than the barrels, but this tiny gap allows "blow by", which is gasses leading by the piston and into the crankcase. To help prevent this, we use rings that are compressed around the piston. The compression rings are used to negate this blow by as much as possible as they expand to fill any gaps, and the oil ring (and sometimes supplemented by a scraper ring further down the piston) are used to pick up excess oil and direct it into the center of the piston and back to the crank case. Without the oil and compression rings, your oil and fuel consumption will skyrocket, and you will blow a good portion of your oil right out of the exhaust.

To promote oil retainment in a cylinder barrel walls, we do something called "honing". Honing comes in many forms, but the one that people are most familiar with is "crosshatching". Without this crosshatching, it becomes much more difficult for oil to do it's job, as the rings will scrape too much of the oil away, and increase wear.

Now, oil serves two primary functions in engines: Cooling and Lubrication.

Lubrication

We are all too familiar requirement of lubrication in high performance applications. Lubricants act as barriers between moving parts. It is next to impossible to machine a part with a perfectly smooth surface without tremendous cost. You wouldn't want this anyways, because metals exhibit the ability to cold weld.

Oil reduces friction, thereby reducing heat, and it also acts as a liquid bearing, filling in the spaces between parts and helping to distribute the load. It is this properly that allows plain bearings to function so well, but it is also the reason that loss of oil pressure results in catastrophic failure within minutes. Without oil, only a fraction of the mating surfaces in plain bearings contact, creating high loads in small surface areas. Metallic particles begin to form as the surface breaks away, which in turn further scores up the bearing surfaces and accelerates this process very rapidly.

Oil works better as the speed between the surfaces increases. It's sort of like hydroplaning tires. At slow speeds and high loads, the bearing surfaces can displace the oil and contact, just like you can drive through water puddles at slow speed and still have traction. However, at high speeds, the oil lubricates better, just like tires skip across the top of water, resulting in hydroplaning. In fact, in super high speed industrial applications, they don't even use oil. They use pneumatic cushioning. Interference fit bushings (think big rod hammered into a small hole) are used, and then high pressure air is pumped into the bushing, and the high rotation speed of the rod causes it to ride on a cushion of air with no contact of the mating surfaces.

Cooling

Just as vital is the need for an engine to stay cool. Ever wonder why your oil lines are so big? Well, what many non mechanics don't understand, is that oil isn't something that has a slow trickle through your engine. Not even close. It looks like someone bottled up hurricane katrina, shook it up, smacked it around a bit so that it's carrying an attitude of a pissed off bull in a rodeo, and then set it loose in your crank case. Waterfalls of oil are slung from the crankshaft journals, oil shoots out of spray nozzles aimed at the camshaft, the cylinder heads containing the valves are drenched in oil, and cylinder barrels are flooded by all the oil being thrown about. Words can't describe how much oil an engine slings. All this oil not only helps to greatly extend the life of the engine, but it also cools the engine! In fact, pistons NEED lots of oil, or they will overheat as that is one of the only ways that a piston can be cooled!


What is oil made of?

There's quite a few hurdles to get over that makes oil function well. When oil manufacturers say they put billions into researching oil properties, they aren't kidding. Oil is an incredible feat of engineering, because not only does it need to fulfil the primary functions of lubricants in an engine, but it also needs to be able to survive the hostile environment.

So, what goes into oil?

The base material, mineral oil or synthetic?

Mineral oil is a by-product of cracking crude oil, and is produced in very large quantities. This is the founding block of natural oil (synthetics are derived differently), and contains a lot of desirable properties, but by itself, is not a good oil in combustion engines, so we have to put in additives.

Synthetic oil seeks to mimic these properties of mineral oil, but without the drawbacks. Recent developments in synthetics have almost completely achieved that goal, but there's still one major drawback to it in aviation, as explained in the section "The synthetic hiccup". Turbine engines LOVE synthetic oil, as it also plays nice with kerosene-like fuels like jet fuel.

Modern aviation oil used in air-cooled engines tends to either be mineral oil based, or a mixture of the two, as a way to try and get the good properties of both, and negate the bad.


Oil Grades

Goes without saying, but I am going to anyways: the number one enemy of engines and oil, is high heat, or extreme cold. Wait? Extreme cold? What?

Well, there's a little thing about oil, called viscosity. If an oil has a low viscosity, then it flows freely, but it is harder for it to stay where it needs to be to do it's job. If it has high viscosity, then it will stick to surfaces, but it increases fluidic friction, and is harder to pump through the engine. Well, oil viscosity is affected by temperature. The lower the temperature, the thicker the oil.

So what can we do about it? Well, we can control oil temperature! The closer we hold oil to a desired operation temperature range, the better we can formulate the oil to best cope with those conditions. However, this isn't always the case. So we come to viscosity index.

In pure mineral oil with no additives, the oil will only perform well in a temperature range. It is too thick when cold. Multigrade oils are those in which we create blends so that the viscosity remains stable at multiple temperatures. We assign that oil a viscosity index. That's why we give two different ratings to some oils, such as "15w-30". It means that at low temps, it acts like a weight 15 oil, and at higher temps, it acts like 30. Generally we assign these values at the water freezing and water boiling points. The higher the weight of an oil, the thicker it is. So how the flipping heck do we get oil to act like that?

Well, natural oil doesn't really exhibit this property. But, some synthetics do. So, in aviation, if the engine allows it, we can use oils that we blend both mineral oil and synthetic oil, and call it semi-synthetic (I'll explain why we don't use full synthetic in air cooled engines in a moment). Think of the synthetic molecules like a little spring. When they get hot, they expand, increasing their resistance to flow, and at cold temperatures, they shrink back down.

The synthetic hiccup

Lead is the problem child of aviation fuel, but necessary for most air aspirated and high compression engines. We put it in fuel in the form of Tetraethyllead, which is used to stabilize the fuel and allow us to compress it more without causing it to explode spontaneously. However, it's also very toxic, so we don't really want it floating around our atmosphere. So, how is lead related to OIL?

Well, synthetic oil has a problem with lead. The lead residue settles out of the synthetic oil, and will build up in really bad places. Natural oil holds the lead in suspension, which is very important to prevent it from gumming up pumps and hoses, and carries it to the filter. But, we want synthetic properties in our oil too, because it lasts much longer in our engine and extends life! So, that is why some engines use semi-synthetic, it's a compromise between the two, and allows us to have properties of multi-grade oil.

The amazing properties of synthetic oil is why, if you know you will absolutely never ever use any kind of leaded fuel in your 912 engine, you should use full synthetic per the rotax manuals. Do not run full synthetic oil if you are going to use leaded fuel!

Ashless Dispersant

There's another problem that we have to deal with in an engine: carbon from burnt fuel. Most of the carbon is blown out of the exhaust, which is why exhaust pipes look brownish and black, but what about the blow-by? Some of it leaks by the rings of the piston and gets blown into the crankcase. Why does this matter? Well, this carbon is just like the lead deposits. It likes to get together for a little party in your engine, and soon enough it will develop classic sign of an engine cared for by an "oil conspiracist", sludge. To break up the party, we add an "ashless dispersant" to the oil, which serves to coat the carbon particles and keeps them from sticking together.

Why is it called "ashless"? Well, synthetic oils, such as in automotive oils, contain anti-wear additives designed to coat engine parts in a sacrificial layer. It's not a problem in auto engines and the 912 because of their design (extremely low oil consumption due to tight tolerances) but it is a major problem in air cooled engines. Air cooled engines burn oil due to the loose tolerances, and it leaves metallic ash residue, which gathers on the piston rings. Not good; it eats the cylinder walls up. When these additives started making their way into oils, this started the major divergence of automotive and aircraft oil. Ashless dispersant is old tech mineral oil with the ashless dispersant added, and it's what works in air cooled engines. Rotax engines LOVE synthetic and semi-synthetic oils for those anti-wear additives, and this is why you should not run ashless dispersant oil in a 912.

Anti-corrosion

We also add anti-corrosion additives to oil, which helps to protect parts from oxidizing (rusting and corroding). This is an important additive for long life!

Anti-oxidizer

What? Didn't we just say we have an anti-corrosion agent? Yes, the anti-corrosion additive protects engine parts, but what protects the oil? The anti-oxidizer! As the oil is slung around, air gets mixed in with the oil. This leads to a couple of problems, one of which deals with oxygen. You see, oxygen (O2) is one of the most corrosive substances in the world. If we didn't need it to breathe, we would be better off without it. Not only can it deteriorate metal, but it can displace atoms and break bonds in oil, breaking it down.

In fact, oxidation of oil is the single largest reason that we have to do oil changes in reciprocating engines, but yet we don't really have to in turbines. Because of all that oil slinging and elevated temperatures, we are mixing in a lot of air, accelerating the breakdown of our oil, and there is only so much an anti-oxidizer can do.

Anti-Foaming

All that oil slinging by the crankshaft also creates another problem: foaming. Foaming oil is not much better than no oil at all, as pumps will cavitate and rapidly break, and provides very little lubrication and cooling properties. Therefore, we add an anti-foaming agent to discourage this.

With that said, I want to take a sidebar and discuss a bit more about oil foaming. Unless you are a chemical engineer and understand oil, DO NOT CLEAN ENGINE PARTS WITH DETERGENTS. Engine parts are semi-porous, and these detergents can be absorbed by the metals, and released when they heat up. They will attack the oil and additives, and one of the telltale signs is oil foaming.

One of the oil foaming symptoms is a sudden, large, rapid drop of oil pressure with no changes to throttle, RPM, or flight conditions. Even if oil pressure is still in the green, a rapid drop should be an immediate red flag. It could just be a simple sensor problem, or you could be leaking/low on oil, or it could be foaming.

Antacid agent

Wait what? ACID?

Yep! Guess what one of the problems with gasoline is? Combustion byproducts create acids when it contacts moisture that is present in sitting oil. It's not really a problem... unless it sits around! Remember that blowby problem? Engines which run too rich will have some of those combustion byproducts carried in the unburned fuel right by the rings and into the oil. Engine cases are vented, and they undergo an effect where the engine "breathes" as the barometric pressure changes, carrying moisture into the engine. Antacid additives coat the engine parts with a barrier that neutralizes and repels acidic compounds, but it doesn't last forever. Where oxidation is a problem on parts exposed to air and moisture, engines with a mixture problem will eventually overpower the antacid and start corroding parts that sit in your oil.

Therefore, aircraft engines of all types should not only be run regularly so that parts stay coated in oil, but they need to be ran for a time too. Getting oil above 190 degrees and running it for a while promotes purging moisture and these acidic compounds, if present.

 

Anti-wear additive

 

Anti-wear additives used to be common in oil. The most common (and relevant) agent is ZDDP. This additive would coat metallic surfaces in a sacrificial layer, protecting the metal from direct physical contact and vastly decreasing wear. Unfortunately, ZDDP byproducts damages catalytic converters, so it's use in common oil has all but ceased. This is why Rotax recommends many types of motorcycle oils; since motorcycles don't have catalytic converters, and also undergo a lot more stress, ZDDP is still in high use.

 

Anti-wear additives are most important for the rotax gearbox. No gear meshes perfectly, there are small gaps which cause Gear Backlash.  Some of the oils Rotax recommends do not have much of the ZDDP additive, and as a consequence, they are often rated as acceptable, but least preferred in their charts.

 

For those of us that use Aeroshell Sport 4, it's a good oil. It's not as good as a couple of the high end oils recommended by rotax, but it's very flexible and has a good amount of ZDDP, so it is quite good to use everyday.

 

Other miscellaneous additives and agents

There's a lot of other additives in oil as well, such as cleaning agents to break up deposits that might have already settled in the engine, to the anti-wear additives briefly touched on (EDIT 4/26/14, I added the anti-wear section). I don't know them all, and even if I did, it would fill an encyclopedia. I listed the big ones contained in most oils, there's plenty of references out there about oil and it's evolution.


Hope you all learned something. Feel free to criticize, submit your own research and sources, and ask questions. I'm not a chemist so I only know what I have been told in our oil and lubrication lectures, and what is found in the BP and Aeroshell material.

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Decalin is a lead scavenger. If anything, it would promote the buildup of lead in the oil, only to settle somewhere else in your engine *removed this part since it's speculative*. The only option for full synthetic is not to deal with lead in the first place.

 

Oil Sport PLUS 4 is a semi-synthetic. You want to go full synthetic if you absolutely are not using any leaded fuel.

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I was told to only use a full synthetic if you will burn ZERO 100LL, otherwise run a semi like Aeroshell Sport Plus 4.

 

As said in both my posts. Not just zero 100LL, but zero leaded fuel period. When lead drops out and forms deposits, the only way to break that stuff up is with a chisel or a lot of heat.

 

If you absolutely know you will not burn leaded fuel, then use full synthetic, it's better for the 912 engine.

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Use either a full or semi synthetic for auto fuel and only a semi synthetic for 100LL. If you run a tank of 100LL on a trip (usually mixed wit auto fuel) and you happen to have full synthetic oil in that one tank of fuel isn't an issue. It is the regular use of 100LL that cause issues. The easiest way to remove lead or carbon build up is to remove the parts and use a soda blaster. It is just baking soda in a bead blaster. It will clean any part spic and span and will not cause any wear on the metal parts, but will remove all grease, lead and carbon build up. I have used it and it works very well.

Using 91 Oct. will always put you ahead over 100LL in the long run, but the occasional use when we go cross country shouldn't make you loose any sleep.

 

p.s.

100LL will cause acid in the engine when it cools and sucks in humidified air and the lead and water come into contact. It will etch the hardest metals in the engine usually around the valves.

 

The Aero Shell Sport Plus 4 is a 10-40W semi synthetic that is made in England specifically for the Rotax. It is more or less motorcycle oil. Other oils Like Mobile or Shell has MFG's in the US and Europe and formulas can and are different from country to country under different MFG sites.

 

Read the article below. It is a little older and is setup by Amsoil, but has some very good info on oils and compares different oil;s and why we need a motorcycle oil. I have published this a few times over the years. time to drag it out of mothballs again.

 

 

Oil Comparison.pdf

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Anticept, thanks for posting the info on oil.  I burn 100% MoGas and use Amsoil or Mobil 4T 10W-40 full synthetic motorcycle oil based on the info found in Roger's post he references here and put in the forum a few years ago.  Just took off my gearbox for rebuild and it was whistle clean with NO traces of lead inside.  My oil reservoir is also clean with no sludge when I clean this at oil changes.

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Decalin is a lead scavenger. If anything, it would promote the buildup of lead in the oil, only to settle somewhere else in your engine. The only option for full synthetic is not to deal with lead in the first place.

 

Oil Sport PLUS 4 is a semi-synthetic. You want to go full synthetic if you absolutely are not using any leaded fuel.

I assume Decalin is like TCP, where it substitutes another chemical in the fuel to react with the lead in the fuel during combustion. The P is phosphate, if I remember. The lead phosphate has a lower melting point than the lead bromate (or whatever it is that forms without TCP), which is lower than the normal exhaust temperature and exits the engine in the exhaust. If Decalin is similar, then it should reduce the lead buildup in the oil since most will be leaving.

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p.s.

100LL will cause acid in the engine when it cools and sucks in humidified air and the lead and water come into contact. It will etch the hardest metals in the engine usually around the valves.

 

I don't believe lead and water form an acid. Acids form from other byproducts of combustion and they form in gasoline other than 100LL, also. That is why new oil is very basic - to counter the acids formed between oil changes.

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  • 2 months later...

Hi Glenn! I didn't notice your post before. Decalin is NOT TCP.

 

As I understand, the decalin is to prevent the lead from building on the valves and plugs in Rotax engines. In continental and lycoming, the higher temperatures in chambers and heads, as well as design considerations, allows the lead to be burned away naturally (and even provides a little extra lubrication!), whereas the colder head temperatures in Rotax makes this process more difficult. You may be right, use of decalin might decrease the amount of lead overall, but my point still stands, you shouldn't use full synthetic with leaded fuel.

 

General note: I have added the Anti-Wear section on ZDDP towards the end of my post

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A little copy and paste from another site.

 

Just at little info on Decalin.

 

Decalin RunUp contains two types of additive, one to reduce the negative effects of tetraethyl lead in aviation fuel and the other to improve the combustion efficiency, reduce combustion deposits and clean the fuel delivery system in aircraft.  Users can expect the same level of engine cleanliness when using this fuel and Decalin RunUp as they saw with 100LL.

Some white deposits may be seen on the plugs and in the exhaust pipe during use; this is normal with RunUp since the lead in the fuel is being converted to non conductive lead phosphate. When no additive is used, lead oxide is formed. It is lead oxide which causes plugs to misfire due to its partial conductivity shorting out the plug. In addition, lead oxide tends to form in large clumps or clinkers whereas the lead phosphate is crumbly and if any is left behind in the combustion chamber it deposits in even layers.

  • Scavenges Lead in aviation fuel after combustion to prevent lead oxide buildup on valves but still allows the lead to perform the anti-knock function prior to combustion.
  • It is an excellent additive for auto conversions, where it reduces buildup on oxygen sensors and plugs. If you have to use 100LL, then this stuff is for you.
  • Prevents valve seat erosion from valve seat micro welding. It is equivalent to TCP (for experimental only).
  • Does not contain volatile solvents so it is safe to ship and safe in the cockpit. You can carry it with you for out-of-town airports.
  • Easy to use graduated measuring and dispensing reservoir built into the bottle. No messy syringes!
  • Tested and stable down to 0 degrees F.

For the 912is guys:

Make sure you remove and clean your oxygen sensor every 20 hours to remove these lead phosphate deposits when using Decalin RunUp.
The dosage of 1/2 ounce per ten gallons of fuel is a minimum. This can be increased to a maximum of 2 ounces per gallon for problem engines.

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Maybe no messy syringes, but the handy measuring bottle it comes in is perhaps not as strong as it should be.

 

Mine sprung a leak, though fortunately I had it in a plastic refrigerator tray so I recovered most of it.

 

I think I've heard of at least one other case of a leaky bottle.

 

Coincidentally, I threw 1/4 oz into my Sky Arrow today when I put 5 gals of 100LL in at Knoxville Island Home today. Pretty rare occurrence unless I'm on a cross-country.

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A little copy and paste from another site.

Just at little info on Decalin.

Decalin RunUp contains two types of additive, one to reduce the negative effects of tetraethyl lead in aviation fuel and the other to improve the combustion efficiency, reduce combustion deposits and clean the fuel delivery system in aircraft. Users can expect the same level of engine cleanliness when using this fuel and Decalin RunUp as they saw with 100LL.

Some white deposits may be seen on the plugs and in the exhaust pipe during use; this is normal with RunUp since the lead in the fuel is being converted to non conductive lead phosphate. When no additive is used, lead oxide is formed. It is lead oxide which causes plugs to misfire due to its partial conductivity shorting out the plug. In addition, lead oxide tends to form in large clumps or clinkers whereas the lead phosphate is crumbly and if any is left behind in the combustion chamber it deposits in even layers.

  • Scavenges Lead in aviation fuel after combustion to prevent lead oxide buildup on valves but still allows the lead to perform the anti-knock function prior to combustion.
  • It is an excellent additive for auto conversions, where it reduces buildup on oxygen sensors and plugs. If you have to use 100LL, then this stuff is for you.
  • Prevents valve seat erosion from valve seat micro welding. It is equivalent to TCP (for experimental only).
  • Does not contain volatile solvents so it is safe to ship and safe in the cockpit. You can carry it with you for out-of-town airports.
  • Easy to use graduated measuring and dispensing reservoir built into the bottle. No messy syringes!
  • Tested and stable down to 0 degrees F.

For the 912is guys:

Make sure you remove and clean your oxygen sensor every 20 hours to remove these lead phosphate deposits when using Decalin RunUp.

The dosage of 1/2 ounce per ten gallons of fuel is a minimum. This can be increased to a maximum of 2 ounces per gallon for problem engines.

Just to add to this discussion:

 

This is decalin: https://en.wikipedia.org/wiki/Decalin

 

This is TCP: https://en.wikipedia.org/wiki/1,2,3-Trichloropropane EDIT: sorry THIS is tcp (thanks glennm for the link correction): http://en.m.wikipedia.org/wiki/Tricresyl_phosphate

 

Completely different chemicals, but achieve similar goals. TCP is corrosive though, and should be avoided like the plague.

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Just to add to this discussion:

 

This is decalin: https://en.wikipedia.org/wiki/Decalin

 

This is TCP: https://en.wikipedia.org/wiki/1,2,3-Trichloropropane

 

Completely different chemicals, but achieve similar goals. TCP is corrosive though, and should be avoided like the plague.

 

No. It is tricresyl phosphate not propane. It would not convert the lead to lead phosphate if it was propane.

 

http://en.m.wikipedia.org/wiki/Tricresyl_phosphate

 

Being an organophosphate, I would think it would be phased out with all the organophosphate herbicides or maybe being a neurotoxin has something to do with it. LOL.

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Next time I will look at what I linked instead of just copying the wiki result! I searched "tcp chemical," very briefly looked, and half assedly grabbed the top link :P. Tis the problem when using an IPad to pull research up!

 

EDIT: i would like to point out, I am not a chemist. I only dabble and let the professionals do the chemical analysis, and I try to find multiple sources to corroborate. I try to research and draw conclusions from multiple reputable sources, but sometimes I get a source wrong, or it's hard to find good information on something.

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