[f-AA] Ed Kollin on Oil
Mark Zimmermann
The Chemistry of Valve Sticking, Sludge and Deposit Formation in Aircraft Engines.
Over the years, much has been said and many articles have been written about valve sticking in piston aircraft engines. The proposed “solutions” have included mouse milk curatives, changes in operating procedures, Service Bulletins and hardware changes from manufacturers, and yet the elusive explanation continues to be a topic of discussion for both pilots and mechanics. Most everyone, including the flying public, the manufacturers (Lycoming and Continental), and the oil companies, widely (and wildly) misunderstand the genesis of this problem. As a “Fuels and Lubes” formulation chemist, I feel I can shed some light on this complex problem.
Operational Considerations
Valve sticking, most common in Lycoming engines, first manifests itself when starting a cold engine. The sticking valve prevents the cylinder from producing power, resulting in the characteristic shake called “Morning Sickness”. As the cylinder warms, the valve is most often released and the engine acts normally, right up to the point where the valve jams solid in the guide, causing at least a bent pushrod and pushrod tube, or worse a damaged camshaft. “Morning Sickness” is the engine talking to you, and you should listen and have it tended to sooner than later. Lycoming has issued a service bulletin, SB388, for both determining the amount of deposits in the valve guides and then how to clean them.
The conventional wisdom is that “lead” from the fuel circulates in the oil and collects in the valve guides where it combines with some “coked” oil and causes the deposits. Adding Tricresylphosphate (TCP) to the fuel to scavenge the lead is often proposed as a solution. Alternatively, use of auto fuel is sometimes hailed as the solution because it eliminates the lead from the engine and thus the problem. Neither of these “solutions” actually addresses the real problem, which as it turns out is fuel related deposits.
Aircraft Engines (and Oils) are “Different”
Aircraft oil primarily needs to deal with contamination from blow-by over its short life of 25-50 hours, constituting a major difference between automotive oil, which has to contend with thermal and oxidative degradation over its much longer life of 100-300 hours. It is this contamination with FUEL entering the crankcase via blow-by that leads to 99+% of the deposit formation in air-cooled piston aircraft engines.
Because they are air-cooled, aircraft engines have large clearances engineered into them to deal with wide operating temperature differentials of more than 400°F. The clearances allow the aluminum pistons to expand in their steel cylinders without binding. The piston/cylinder clearance and ring gaps directly affect the amount of blow-by gas that blasts into the crankcase during the combustion events. See figures 1 and 2. [Figures removed to reduce file size]
Figure 1
Notice the very large clearance between the aluminum piston and the steel cylinder when cold. Aluminum expands much more with temperature than steel. The piston rings take up the clearance. Automobile engines are designed with much less clearance because temperatures are much more tightly controlled due to water cooling.
[Figure removed to reduce file size]
Figure 2. There are three or four rings on an aircraft piston. The top two compression rings minimize pressure losses and keep the combustion gasses where they belong. The rings expand and contract as well as rotate in the ring grooves as the piston moves up and down in the cylinder. These motions are critical to keeping the ring grooves free of deposits as they form.
The third ring is the oil control ring. It determines the thickness of the oil film on the cylinder and the oil consumption of the engine.
The bottom ring is an oil “scraper” ring. It keeps excess oil off the cylinder walls and aids the oil control ring.
[Figure removed to reduce file size]
Blow-by gas contaminants enter the oil during the latter stages of the compression stroke and beginning stages of the combustion stroke. The high-pressure gasses in the combustion chamber, reaching over 800psi, “blow-by” the piston rings and enter the crankcase. See figure 3. This renegade gas is a complex mixture of raw fuel (bad), reactive partially combusted fuel (REALLY BAD), and completely combusted fuel, which is a mixture of exhaust gases including CO2, a lot of water, 1 micron particles of lead bromide (see below) as well as a great deal of heat.
Figure 3. [Figure removed to reduce file size]
A summary of the total blow-by, containing raw fuel, partially combusted fuel, and fully combusted fuel, entering the crankcase during the compression and power stroke where combustion chamber pressures are highest.
Tetraethyl lead (TEL) boosts the octane of avgas. To minimize lead oxide deposits from forming when TEL containing fuel is burned, leaded fuel contains scavengers that react with the lead to form new compounds other than lead oxide. Without scavengers, lead oxide would coat the combustion chamber and short out the spark plugs. Avgas contains ethylene dibromide, which forms lead bromide when the fuel is burned. Lead bromide remains in the exhaust gas phase longer than lead oxide and allows the lead to exit the engine out the exhaust pipe. Lead Bromide is formed at moderate and high combustion temperatures and pressures. Small, soft lead bromide particles flood into the crankcase with the blow-by fuel.
An engine that uses 15 gallons of fuel per hour can put up to 0.1 gallons of raw and partially combusted fuel into the crankcase per hour. This volume of fuel is accompanied by a lesser amount of water (burning of a gallon of avgas produces about a gallon of water, most of which goes out the exhaust). Most of these fuel components and water in the crankcase are volatile and evaporate at engine temperatures and exit the engine through the crankcase breather. It is the partially combusted fuel components known as “free radicals” that cause all of the deposit problems.
Combustion is the reaction of the hydrocarbon fuel with oxygen, leading to the formation of carbon dioxide, water, and the energy with which to fly. Combustion (rapid oxidation) of hydrocarbons in most of the cylinder proceeds via very reactive free radicals (OH, Peroxide, Hydroperoxide and Alkoxy). Free radicals rip off of the hydrogens, and then attack the carbon backbone.
However, some of the fuel trapped near the cylinder wall and edge of the piston, where the combustion has not started (raw fuel) or just begun (partially reacted) is pushed past the piston rings as blow-by. This blow-by fuel leaves us with a reactive blend of fuel components and hot free radicals in the sump.
There are many degradation pathways involved in the combustion (oxidation) of hydrocarbons, but this discussion will greatly simplify this and focus on two of the most relevant to this discussion.
The first molecular degradation pathway is the addition of an oxygen atom or atoms to the hydrocarbon. Partial oxidation of “hydrocarbon fuel molecules” gives a wide range of new molecules, most of which go on to cause deposit problems in the crankcase.
The addition of oxygen in the partially combusted (burned) fuel molecules makes them heavier and less volatile. These partially oxidized species are referred to as deposit precursors. They are reactive, do not readily evaporate from the oil, and the addition of oxygen makes the molecules acidic. It is this acidity that makes them “polar” (having electrical or magnetic polarity) and thus attracted to metal surfaces like a magnet. Furthermore, once stuck to the hot metal surface, the molecules continue to decompose and polymerize with other reactive molecules into a resinous deposit (varnish) film.
When enough of these oxygen containing molecules collect, the oil takes on an additional sticky character. This stickiness is responsible for the phenomenon of increasing oil consumption over time. If the first quart of makeup oil is required after 15 hours, a second quart may be needed after only 8 addition hours as “sticky” oil walks past the rings into the combustion chamber and is burned.
Ashless Dispersants
The acidic nature of the deposit precursors causes it to be strongly attracted to the Ashless Dispersant (AD) additive. The dispersant binds to the deposit precursor molecules and holds them in suspension/solution, away from metal surfaces, until the oil is drained. Dispersants are referred to as keep clean additives. Dispersants only bind to acids as they are formed and do not react with dirt, lead bromide particles or wears metals. Also, they do not clean-up existing deposits.
In automotive oils, dispersants work in conjunction with metallic detergents. But we cannot use these metallic components in aviation oils. There are two types of detergents used in auto oils. The neutral detergents, which clean and prevent deposits and the over based detergents, which are sources of alkalinity in the form of calcium carbonate (TUMS, Rolaids). The calcium carbonate neutralizes acids as there they are formed to prevent pitting corrosion and rust. This differs from the action of dispersants, which surround the organic acid, to keep them from forming deposits.
When all the dispersant is consumed by surrounding the acidic deposit precursors (and this can happen in as little as 15-20 hours!), the precursors are free to stick to the hot metal surfaces where they react with any dehydrogenated (unsaturated and unstable double bond) hydrocarbon molecules and form thin deposit films.
This deposit formation pathway is equivalent to the chemistry found in the can of varnish you buy at the paint store. The oxygen(s) in the varnish molecule draws it to the surface where multiple (unsaturated or double bond containing) molecules then polymerize (crosslink) with each other through a series of free radical reactions.
The thin film coating referred to as lacquer starts out clear and is common to see on cooler engine components such as gears and crankshafts. Additional material and heat thicken the deposit into an amber varnish coating such as seen on the piston skirts, connecting rods and cases. See figure 6. [Figure removed to reduce file size]
Engine “painted” with varnish at 800 hours
Time and heat further degrade the deposit, first into a soft material, then into the hard, black carbon deposits seen on the hottest engine parts, such as the underside of the pistons and the exhaust valve guides (over 450 degrees F). See figure 7. [Figure removed to reduce file size]
Figure 7.
Carbon deposit on the underside of the piston. Deposits interfere with the heat transfer through the piston, causing piston temperatures to rise, which cause deposit rates to increase - a vicious cycle.
This resinous binder also captures lead bromide particles floating in the oil. The lead particles are not a problem until they “fill” a varnish deposit, adding to its thickness. It is a common belief that the dispersant holds the lead particles in suspension, but this is incorrect. They are not acidic and thus are invisible to the dispersant molecules. The vast majority of the lead bromide particles are also about one micron in diameter or about 1000 times too large for a dispersant molecule to bind with and hold in suspension.
The lead particles go on to combine with deposit precursor goo, forming a dense heavy sludge which settles in the low flow areas like the sump, prop hub and inside the crankshaft. Lead sludge has the consistency of butter. See Figure 8. Oil changes performed when the oil is hot maximizes removal of both lead particles and deposit precursors.
Figure 8 [Figure removed to reduce file size]
Lead sludge accumulation over the life of the engine. The sludge inside the crankshaft is deposited by the centrifugal force of the spinning crankshaft and held together by deposit precursor” goo”.
Antioxidants
The dispersants do a good job of neutralizing and solubilizing the deposit precursors to keep the engine clean, but other powerful weapons for cleanliness are antioxidant additives, which help by preventing or interrupting the oxidation process.
Antioxidants are molecules that are easily and preferentially oxidized by the free radicals, essentially capturing the radicals into a more stable and less harmful combined molecule. And the antioxidants remain soluble in the oil once they are oxidized, a crucial property. They take the reactive hit to protect the hydrocarbons in the sump, including both the engine oil and the very sensitive blow-by fuel. The antioxidant concentration in crankcase oil is typically determined by observing the thermal stress the oil will see i.e. temperature and time. However, the added volume of unburned fuel from the blow-by is not adequately considered in determining the necessary antioxidant concentration in aircraft oils. Commonly used antioxidants in aviation oil include hindered phenols and diphenylamines.
The point where the antioxidants are depleted and the dispersant is overwhelmed with deposit precursors is the starting bell where all the problems begin. The hottest components, the undersides of the pistons and the exhaust valve guides, are affected first. Deposits form and build in the exhaust valve guides. The lead bromide particles exacerbate the problem by adding to the deposit thickness.
There is an important equilibrium between deposit formation and deposit removal in the guide because of the mechanical motion of the valve. The valve motion is good on the one hand because it removes some deposits from the guide, but bad on the other hand because deposits can be pushed deep into the guide exacerbating valve sticking. Valve sticking is more commonly seen in Lycoming engines while valve guide deposits causing wear is more common in big bore Continentals. Additionally, ring groove deposits causing piston ring sticking and cylinder wear is also a common problem for big Continentals.
Fuel Additives/Alternate Fuels
This brings us to the use of TCP and autofuel. Tricresyl phosphate (TCP) is often added to the fuel of lower power engines for reducing lead fouling of spark plugs. It acts as a supplemental lead scavenger to help the ethylene dibromide. Ethylene dibromide works well in higher horsepower engines, but at the lower power often seen in low compression engines, lead oxide deposits can plate out onto spark plugs and short them out. TCP forms lead phosphate at these lower temperatures that is less electrically conductive than lead oxide. So, if you have a spark plug lead fouling problem, TCP can help. However, TCP in the fuel does nothing for the fuel derived deposit problems, and thus nothing for valve sticking.
Unleaded autofuel use is fairly widespread and it would be utilized more because it is cheaper. However, alcohol free autofuel is not widely available at airports. Autofuel does not contain lead, so smaller lower powered engines that use it do not suffer from plug fouling. However, autofuel contains olefins as a cheap way to boost octane. If you remember, olefins are reactive, double bond containing molecules (unsaturated or dehydrogenated) that can readily polymerize, or oxidize, into varnish films as previously discussed. It is the olefins in autofuel that make it stink, limit shelf life, and cause it to form gum and varnish deposits in fuel systems.
Many people have great success with autofuel. However, many engine shops claim that autofuel causes premature cylinder problems, including valve sticking and piston top and ring land erosion, so they will not warranty engines that use it.
If you use autofuel, you should use it exclusively. Equally important, mixing unleaded autogas with avgas provides the engine with the reactants to maximize deposits by providing olefins from the autofuel, and lead particles from the avgas.
Deposit Control Conclusions
The best ways to prevent deposits in piston aircraft engines are 1) Minimize blow-by fuel from entering the crankcase through aggressive leaning 2) Frequent oil changes to remove contaminants in the oil 3) Use of deposit control additives such as Camguard.
Camguard Technology
One of the attributes of Camguard is its ability to stop deposit formation. This keeps new engines clean and allows older engines to slowly clean themselves through normal component motions i.e., rings and valves cleaning ring grooves and valve guides. See figure 9 and 10.
Figure 9. Exhaust valve guide showing zero deposits and zero wear.
Camguard Certification engine 300 HP Lycoming IO-540 in aerobatic use for 540 hours. Oil temperatures up to 375˚F and cylinder head temperatures up to 475˚F
[Finger removed to reduce file size]
Figure 10. The piston on the left is typical of the pistons from the Camguard Certification engine at 540 hours. On the right is a 500 hour piston from an engine not using Camguard.
[Finger removed to reduce file size]
Richard Murray
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http://mail.westmont.edu/mailman/listinfo/aeronca
Mark Zimmermann
Richard Murray
Ian Harvie
dream engine with about 450 hours on it. Use mogas most of the time,
100LL only when I can't get mogas.
Ian
On 11/02/2016 7:58 AM, Richard Murray wrote:
> Thanks. I've have two Lycomings in as many years produce a stuck valve
> which after running for a bit seems the free up until the engine sits
> for an extended period.
>
>
> On Wed, Feb 10, 2016 at 3:25 PM, Mark Zimmermann
> <fmarkzi...@gmail.com <mailto:fmarkzi...@gmail.com>> wrote:
>
> This is something you'd have to discuss with Ed. He's very
> approachable but I'm dumb as a box of rocks on this subject. I
> know that he's commented on MMO and what it chemically is. I've
> seen pictures of engines that have used his camguard vs those that
> did not and there is no comparison. I do have another of his
> articles and will see if I can post it.
>
> On Feb 10, 2016, at 2:01 PM, Richard Murray <murra...@gmail.com
> <mailto:murra...@gmail.com>> wrote:
>
>> Very interesting article Mark.
>>
>> Given your knowledge of this subject, why do Lycoming engines
>> seem more receptive to sticking valves than Continental?
>>
>> Further can MMO (besides repelling elephants from fruit trees)
>> minimize the occurrence of sticking valves?
>>
>> Richard n OH hoping the snow melts in Tennessee by the weekend
>>
>>
>>
>> On Wed, Feb 10, 2016 at 2:33 PM, Mark Zimmermann
>> <fmarkzi...@gmail.com <mailto:fmarkzi...@gmail.com>> wrote:
>>
>> The Chemistry of Valve Sticking, Sludge and Deposit Formation
>> in Aircraft Engines.
>>
>> Over the years, much has been said and many articles have
>> been written about valve sticking in piston aircraft
>> engines.The proposed “solutions” have included mouse milk
>> lead is often proposed as a solution.Alternatively, use of
>> auto fuel is sometimes hailed as the solution because it
>> eliminates the lead from the engine and thus the problem.
>> Neither of these “solutions” actually addresses the real
>> problem, which as it turns out is fuel related deposits.
>>
>> Aircraft Engines (and Oils) are “Different”
>>
>> Aircraft oil primarily needs to deal with contamination from
>> blow-by over its short life of 25-50 hours, constituting a
>> major difference between automotive oil, which has to contend
>> with thermal and oxidative degradation over its much longer
>> life of 100-300 hours. It is this contamination with FUEL
>> entering the crankcase via blow-by that leads to 99+% of the
>> deposit formation in air-cooled piston aircraft engines.
>>
>> Because they are air-cooled, aircraft engines have large
>> clearances engineered into them to deal with wide operating
>> temperature differentials of more than 400°F.The clearances
>> allow the aluminum pistons to expand in their steel cylinders
>> without binding. The piston/cylinder clearance and ring gaps
>> directly affect the amount of blow-by gas that blasts into
>> the crankcase during the combustion events. See figures 1 and
>> 2.[Figures removed to reduce file size]
>>
>> Figure 1
>>
>> Notice the very large clearance between the aluminum piston
>> and the steel cylinder when cold. Aluminum expands much more
>> with temperature than steel. The piston rings take up the
>> clearance.Automobile engines are designed with much less
>> engine through the crankcase breather.It is the partially
>> additive.The dispersant binds to the deposit precursor
>> molecules and holds them in suspension/solution, away from
>> metal surfaces, until the oil is drained. Dispersants are
>> referred to as keep clean additives. Dispersants only bind to
>> acids as they are formed and do not react with dirt, lead
>> bromide particles or wears metals. Also, they do not clean-up
>> existing deposits.
>>
>> In automotive oils, dispersants work in conjunction with
>> metallic detergents.But we cannot use these metallic
>> components in aviation oils.There are two types of detergents
>> used in auto oils.The neutral detergents, which clean and
>> prevent deposits and the over based detergents, which are
>> sources of alkalinity in the form of calcium carbonate (TUMS,
>> Rolaids). The calcium carbonate neutralizes acids as there
>> they are formed to prevent pitting corrosion and rust. This
>> differs from the action of dispersants, which surround the
>> organic acid, to keep them from forming deposits.
>>
>> When all the dispersant is consumed by surrounding the acidic
>> deposit precursors (and this can happen in as little as 15-20
>> hours!), the precursors are free to stick to the hot metal
>> surfaces where they react with any dehydrogenated
>> (unsaturatedand unstable double bond) hydrocarbon molecules
>> and form thin deposit films.
>>
>> This deposit formation pathway is equivalent to the chemistry
>> found in the can of varnish you buy at the paint store. The
>> oxygen(s) in the varnish molecule draws it to the surface
>> where multiple (unsaturated or double bond containing)
>> molecules then polymerize (crosslink) with each other through
>> a series of free radical reactions.
>>
>> The thin film coating referred to as lacquer starts out clear
>> and is common to see on cooler engine components such as
>> gears and crankshafts.Additional material and heat thicken
>> the deposit into an amber varnish coating such as seen on the
>> piston skirts, connecting rods and cases. See figure 6.
>> [Figure removed to reduce file size]
>>
>> Engine “painted” with varnish at 800 hours
>>
>> Time and heat further degrade the deposit, first into a soft
>> material, then into the hard, black carbon deposits seen on
>> the hottest engine parts, such as the underside of the
>> pistons and the exhaust valve guides (over 450 degrees F).
>> See figure 7.[Figure removed to reduce file size]
>>
>> Figure 7.
>>
>> Carbon deposit on the underside of the piston. Deposits
>> interfere with the heat transfer through the piston, causing
>> piston temperatures to rise, which cause deposit rates to
>> increase - a vicious cycle.
>>
>> This resinous binder also captures lead bromide particles
>> floating in the oil.The lead particles are not a problem
>> until they “fill” a varnish deposit, adding to its
>> thickness.It is a common belief that the dispersant holds the
>> i.e. temperature and time.However, the added volume of
>> unburned fuel from the blow-by is not adequately considered
>> in determining the necessary antioxidant concentration in
>> aircraft oils.Commonly used antioxidants in aviation oil
>> include hindered phenols and diphenylamines.
>>
>> The point where the antioxidants are depleted and the
>> dispersant is overwhelmed with deposit precursors is the
>> starting bell where all the problems begin. The hottest
>> components, the undersides of the pistons and the exhaust
>> valve guides, are affected first.Deposits form and build in
>> the exhaust valve guides. The lead bromide particles
>> exacerbate the problem by adding to the deposit thickness.
>>
>> There is an important equilibrium between deposit formation
>> and deposit removal in the guide because of the mechanical
>> motion of the valve.The valve motion is good on the one hand
>> because it removes some deposits from the guide, but bad on
>> the other hand because deposits can be pushed deep into the
>> guide exacerbating valve sticking. Valve sticking is more
>> commonly seen in Lycoming engines while valve guide deposits
>> causing wear is more common in big bore Continentals.
>> Additionally, ring groove deposits causing piston ring
>> sticking and cylinder wear is also a common problem for big
>> Continentals.
>>
>> Fuel Additives/Alternate Fuels
>>
>> This brings us to the use of TCP and autofuel. Tricresyl
>> phosphate (TCP) is often added to the fuel of lower power
>> engines for reducing lead fouling of spark plugs. It acts as
>> a supplemental lead scavenger to help the ethylene dibromide.
>> Ethylene dibromide works well in higher horsepower engines,
>> but at the lower power often seen in low compression engines,
>> lead oxide deposits can plate out onto spark plugs and short
>> them out. TCP forms lead phosphate at these lower
>> temperatures that is less electrically conductive than lead
>> oxide.So, if you have a spark plug lead fouling problem, TCP
>> Aer...@westmont.edu <mailto:Aer...@westmont.edu>
>> http://mail.westmont.edu/mailman/listinfo/aeronca
>>
>>
>> _______________________________________________
>> Aeronca mailing list
>> Aer...@westmont.edu <mailto:Aer...@westmont.edu>
>> http://mail.westmont.edu/mailman/listinfo/aeronca
>
> _______________________________________________
> Aeronca mailing list
> Aer...@westmont.edu <mailto:Aer...@westmont.edu>
> http://mail.westmont.edu/mailman/listinfo/aeronca
>
>
>
>
> _______________________________________________
> Aeronca mailing list
> Aer...@westmont.edu
> http://mail.westmont.edu/mailman/listinfo/aeronca
>
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Don Harvie
-------- Original message --------
From: Mark Zimmermann <fmarkzi...@gmail.com>
Date: 11/02/2016 6:33 am (GMT+10:00)
To: aer...@westmont.edu
Subject: [f-AA] Ed Kollin on Oil
The Chemistry of Valve Sticking, Sludge and Deposit Formation in Aircraft Engines.
Over the years, much has been said and many articles have been written about valve sticking in piston aircraft engines. The proposed “solutions” have included mouse milk curatives, changes in operating procedures, Service Bulletins and hardware changes from manufacturers, and yet the elusive explanation continues to be a topic of discussion for both pilots and mechanics. Most everyone, including the flying public, the manufacturers (Lycoming and Continental), and the oil companies, widely (and wildly) misunderstand the genesis of this problem. As a “Fuels and Lubes” formulation chemist, I feel I can shed some light on this complex problem.
Operational Considerations
Valve sticking, most common in Lycoming engines, first manifests itself when starting a cold engine. The sticking valve prevents the cylinder from producing power, resulting in the characteristic shake called “Morning Sickness”. As the cylinder warms, the valve is most often released and the engine acts normally, right up to the point where the valve jams solid in the guide, causing at least a bent pushrod and pushrod tube, or worse a damaged camshaft. “Morning Sickness” is the engine talking to you, and you should listen and have it tended to sooner than later. Lycoming has issued a service bulletin, SB388, for both determining the amount of deposits in the valve guides and then how to clean them.
The conventional wisdom is that “lead” from the fuel circulates in the oil and collects in the valve guides where it combines with some “coked” oil and causes the deposits. Adding Tricresylphosphate (TCP) to the fuel to scavenge the lead is often proposed as a solution. Alternatively, use of auto fuel is sometimes hailed as the solution because it eliminates the lead from the engine and thus the problem. Neither of these “solutions” actually addresses the real problem, which as it turns out is fuel related deposits.
Aircraft Engines (and Oils) are “Different”
Aircraft oil primarily needs to deal with contamination from blow-by over its short life of 25-50 hours, constituting a major difference between automotive oil, which has to contend with thermal and oxidative degradation over its much longer life of 100-300 hours. It is this contamination with FUEL entering the crankcase via blow-by that leads to 99+% of the deposit formation in air-cooled piston aircraft engines.
Because they are air-cooled, aircraft engines have large clearances engineered into them to deal with wide operating temperature differentials of more than 400°F. The clearances allow the aluminum pistons to expand in their steel cylinders without binding. The piston/cylinder clearance and ring gaps directly affect the amount of blow-by gas that blasts into the crankcase during the combustion events. See figures 1 and 2. [Figures removed to reduce file size]
Figure 1
Notice the very large clearance between the aluminum piston and the steel cylinder when cold. Aluminum expands much more with temperature than steel. The piston rings take up the clearance. Automobile engines are designed with much less clearance because temperatures are much more tightly controlled due to water cooling.
[Figure removed to reduce file size]
Figure 2. There are three or four rings on an aircraft piston. The top two compression rings minimize pressure losses and keep the combustion gasses where they belong. The rings expand and contract as well as rotate in the ring grooves as the piston moves up and down in the cylinder. These motions are critical to keeping the ring grooves free of deposits as they form.
The third ring is the oil control ring. It determines the thickness of the oil film on the cylinder and the oil consumption of the engine.
The bottom ring is an oil “scraper” ring. It keeps excess oil off the cylinder walls and aids the oil control ring.
[Figure removed to reduce file size]
Blow-by gas contaminants enter the oil during the latter stages of the compression stroke and beginning stages of the combustion stroke. The high-pressure gasses in the combustion chamber, reaching over 800psi, “blow-by” the piston rings and enter the crankcase. See figure 3. This renegade gas is a complex mixture of raw fuel (bad), reactive partially combusted fuel (REALLY BAD), and completely combusted fuel, which is a mixture of exhaust gases including CO2, a lot of water, 1 micron particles of lead bromide (see below) as well as a great deal of heat.
Figure 3. [Figure removed to reduce file size]
A summary of the total blow-by, containing raw fuel, partially combusted fuel, and fully combusted fuel, entering the crankcase during the compression and power stroke where combustion chamber pressures are highest.
Tetraethyl lead (TEL) boosts the octane of avgas. To minimize lead oxide deposits from forming when TEL containing fuel is burned, leaded fuel contains scavengers that react with the lead to form new compounds other than lead oxide. Without scavengers, lead oxide would coat the combustion chamber and short out the spark plugs. Avgas contains ethylene dibromide, which forms lead bromide when the fuel is burned. Lead bromide remains in the exhaust gas phase longer than lead oxide and allows the lead to exit the engine out the exhaust pipe. Lead Bromide is formed at moderate and high combustion temperatures and pressures. Small, soft lead bromide particles flood into the crankcase with the blow-by fuel.
An engine that uses 15 gallons of fuel per hour can put up to 0.1 gallons of raw and partially combusted fuel into the crankcase per hour. This volume of fuel is accompanied by a lesser amount of water (burning of a gallon of avgas produces about a gallon of water, most of which goes out the exhaust). Most of these fuel components and water in the crankcase are volatile and evaporate at engine temperatures and exit the engine through the crankcase breather. It is the partially combusted fuel components known as “free radicals” that cause all of the deposit problems.
Combustion is the reaction of the hydrocarbon fuel with oxygen, leading to the formation of carbon dioxide, water, and the energy with which to fly. Combustion (rapid oxidation) of hydrocarbons in most of the cylinder proceeds via very reactive free radicals (OH, Peroxide, Hydroperoxide and Alkoxy). Free radicals rip off of the hydrogens, and then attack the carbon backbone.
However, some of the fuel trapped near the cylinder wall and edge of the piston, where the combustion has not started (raw fuel) or just begun (partially reacted) is pushed past the piston rings as blow-by. This blow-by fuel leaves us with a reactive blend of fuel components and hot free radicals in the sump.
There are many degradation pathways involved in the combustion (oxidation) of hydrocarbons, but this discussion will greatly simplify this and focus on two of the most relevant to this discussion.
The first molecular degradation pathway is the addition of an oxygen atom or atoms to the hydrocarbon. Partial oxidation of “hydrocarbon fuel molecules” gives a wide range of new molecules, most of which go on to cause deposit problems in the crankcase.
The addition of oxygen in the partially combusted (burned) fuel molecules makes them heavier and less volatile. These partially oxidized species are referred to as deposit precursors. They are reactive, do not readily evaporate from the oil, and the addition of oxygen makes the molecules acidic. It is this acidity that makes them “polar” (having electrical or magnetic polarity) and thus attracted to metal surfaces like a magnet. Furthermore, once stuck to the hot metal surface, the molecules continue to decompose and polymerize with other reactive molecules into a resinous deposit (varnish) film.
When enough of these oxygen containing molecules collect, the oil takes on an additional sticky character. This stickiness is responsible for the phenomenon of increasing oil consumption over time. If the first quart of makeup oil is required after 15 hours, a second quart may be needed after only 8 addition hours as “sticky” oil walks past the rings into the combustion chamber and is burned.
Ashless Dispersants
The acidic nature of the deposit precursors causes it to be strongly attracted to the Ashless Dispersant (AD) additive. The dispersant binds to the deposit precursor molecules and holds them in suspension/solution, away from metal surfaces, until the oil is drained. Dispersants are referred to as keep clean additives. Dispersants only bind to acids as they are formed and do not react with dirt, lead bromide particles or wears metals. Also, they do not clean-up existing deposits.
In automotive oils, dispersants work in conjunction with metallic detergents. But we cannot use these metallic components in aviation oils. There are two types of detergents used in auto oils. The neutral detergents, which clean and prevent deposits and the over based detergents, which are sources of alkalinity in the form of calcium carbonate (TUMS, Rolaids). The calcium carbonate neutralizes acids as there they are formed to prevent pitting corrosion and rust. This differs from the action of dispersants, which surround the organic acid, to keep them from forming deposits.
When all the dispersant is consumed by surrounding the acidic deposit precursors (and this can happen in as little as 15-20 hours!), the precursors are free to stick to the hot metal surfaces where they react with any dehydrogenated (unsaturated and unstable double bond) hydrocarbon molecules and form thin deposit films.
This deposit formation pathway is equivalent to the chemistry found in the can of varnish you buy at the paint store. The oxygen(s) in the varnish molecule draws it to the surface where multiple (unsaturated or double bond containing) molecules then polymerize (crosslink) with each other through a series of free radical reactions.
The thin film coating referred to as lacquer starts out clear and is common to see on cooler engine components such as gears and crankshafts. Additional material and heat thicken the deposit into an amber varnish coating such as seen on the piston skirts, connecting rods and cases. See figure 6. [Figure removed to reduce file size]
Engine “painted” with varnish at 800 hours
Time and heat further degrade the deposit, first into a soft material, then into the hard, black carbon deposits seen on the hottest engine parts, such as the underside of the pistons and the exhaust valve guides (over 450 degrees F). See figure 7. [Figure removed to reduce file size]
Figure 7.
Carbon deposit on the underside of the piston. Deposits interfere with the heat transfer through the piston, causing piston temperatures to rise, which cause deposit rates to increase - a vicious cycle.
This resinous binder also captures lead bromide particles floating in the oil. The lead particles are not a problem until they “fill” a varnish deposit, adding to its thickness. It is a common belief that the dispersant holds the lead particles in suspension, but this is incorrect. They are not acidic and thus are invisible to the dispersant molecules. The vast majority of the lead bromide particles are also about one micron in diameter or about 1000 times too large for a dispersant molecule to bind with and hold in suspension.
The lead particles go on to combine with deposit precursor goo, forming a dense heavy sludge which settles in the low flow areas like the sump, prop hub and inside the crankshaft. Lead sludge has the consistency of butter. See Figure 8. Oil changes performed when the oil is hot maximizes removal of both lead particles and deposit precursors.
Figure 8 [Figure removed to reduce file size]
Lead sludge accumulation over the life of the engine. The sludge inside the crankshaft is deposited by the centrifugal force of the spinning crankshaft and held together by deposit precursor” goo”.
Antioxidants
The dispersants do a good job of neutralizing and solubilizing the deposit precursors to keep the engine clean, but other powerful weapons for cleanliness are antioxidant additives, which help by preventing or interrupting the oxidation process.
Antioxidants are molecules that are easily and preferentially oxidized by the free radicals, essentially capturing the radicals into a more stable and less harmful combined molecule. And the antioxidants remain soluble in the oil once they are oxidized, a crucial property. They take the reactive hit to protect the hydrocarbons in the sump, including both the engine oil and the very sensitive blow-by fuel. The antioxidant concentration in crankcase oil is typically determined by observing the thermal stress the oil will see i.e. temperature and time. However, the added volume of unburned fuel from the blow-by is not adequately considered in determining the necessary antioxidant concentration in aircraft oils. Commonly used antioxidants in aviation oil include hindered phenols and diphenylamines.
The point where the antioxidants are depleted and the dispersant is overwhelmed with deposit precursors is the starting bell where all the problems begin. The hottest components, the undersides of the pistons and the exhaust valve guides, are affected first. Deposits form and build in the exhaust valve guides. The lead bromide particles exacerbate the problem by adding to the deposit thickness.
There is an important equilibrium between deposit formation and deposit removal in the guide because of the mechanical motion of the valve. The valve motion is good on the one hand because it removes some deposits from the guide, but bad on the other hand because deposits can be pushed deep into the guide exacerbating valve sticking. Valve sticking is more commonly seen in Lycoming engines while valve guide deposits causing wear is more common in big bore Continentals. Additionally, ring groove deposits causing piston ring sticking and cylinder wear is also a common problem for big Continentals.
Fuel Additives/Alternate Fuels
This brings us to the use of TCP and autofuel. Tricresyl phosphate (TCP) is often added to the fuel of lower power engines for reducing lead fouling of spark plugs. It acts as a supplemental lead scavenger to help the ethylene dibromide. Ethylene dibromide works well in higher horsepower engines, but at the lower power often seen in low compression engines, lead oxide deposits can plate out onto spark plugs and short them out. TCP forms lead phosphate at these lower temperatures that is less electrically conductive than lead oxide. So, if you have a spark plug lead fouling problem, TCP can help. However, TCP in the fuel does nothing for the fuel derived deposit problems, and thus nothing for valve sticking.
Mark Zimmermann
I am fascinated with Ed's writings and would love to hear a seminar by him. I did, once upon a time, hear a webinar with him and it was very interesting.
Sorry,
MZ
> On Feb 10, 2016, at 6:59 PM, Don Harvie <donh...@yahoo.com.au> wrote:
>
> Thank you Mark,
>
> A fascinating article. Could you please post the source so I can check out the pictures too.
>
> Don H
>
>
>
Cy Galley
One thing that one should be aware of is that Lycoming has a SB about checking valve guide clearance with "wobble Test". I don't have the details in front of me but one could look it up or ask Lycoming.
Cy Galley - Chairman of Emergency Aircraft Repair
A service project of EAA Chapter 75 since 1963
Don Harvie
-------- Original message --------
From: Mark Zimmermann <fmarkzi...@gmail.com>
Date: 11/02/2016 12:06 pm (GMT+10:00)
To: aer...@westmont.edu
Subject: Re: [f-AA] Ed Kollin on Oil
I am fascinated with Ed's writings and would love to hear a seminar by him. I did, once upon a time, hear a webinar with him and it was very interesting.
Sorry,
MZ
> On Feb 10, 2016, at 6:59 PM, Don Harvie <donh...@yahoo.com.au> wrote:
>
>
> A fascinating article. Could you please post the source so I can check out the pictures too.
>
> Don H
>
>
>
psa...@graffitis.com
Here is a pdf copy of it
http://www.shortwingpipers.org/forum/attachment.php?attachmentid=6881&d=1409152357
Paul
From: "Don Harvie" <donh...@yahoo.com.au>
Sent: Wednesday, February 10, 2016 9:48 PM