The Chemistry of Valve Sticking and sludge, stuck rings and varnish. Operational Considerations. Aircraft Engines (and Oils) are Different

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1 The Chemistry of Valve Sticking and sludge, stuck rings and varnish. 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 form 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 hours, constituting a major difference between automotive oil, which has to contend with thermal and oxidative degradation over its much longer life of hours. It is this contamination with FUEL entering the crankcase via blow-by that leads to 99+% of the deposit formation in aircooled 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.

2 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.

3 Compression rings Oil control ring Scraper ring Oil return/supply holes for oil control ring 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. 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 CO 2, a lot of water, 1 micron particles of lead bromide (see below) as well as a great deal of heat.

4 Bad Really Bad Bad Fully combusted fuel, water & heat Partially combusted fuel & very reactive free radicals Raw fuel Figure 3. 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 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 (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.

5 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. See figure 4. Remove light gray Butane Representative hydrocarbon fuel molecule Butane represents gasoline and is made up of only carbon and hydrogen. Fully combusted fuel leaves us with CO2, water, and work energy to turn the propeller. Butanol One, of many, partial oxidation products of butane Butanoic acid Another partial oxidation product of butane Figure 4.

6 The second molecular degradation pathway is dehydrogenation, where two hydrogen atoms are stripped from the hydrocarbon fuel molecule and the molecule responds by forming a double bond between carbon atoms(c=c) to keep the molecule intact. Double bond containing compounds, known as olefins, are quite unstable, especially at elevated temperatures. Dehydrogenation allows for polymerization (combining two or more molecules through the unstable double bonds) and growth of the deposit. See Figure 5. Butene (an olefin) Dehydrogenation of butane (note the C=C) Figure 5. In many cases, both of these reactions occur on the same molecule from the beginning. 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 like) film. When enough of these oxygen containing molecules collect, the oil takes on an actual 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. attracts the attention of the ashless dispersant (AD). Dispersants have basic, polyamine head groups with long hydrocarbon tails for oil solubility. The (basic) head groups are strongly attracted to the acidic group(s) of the deposit precursor molecules. 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 or wears metals. Also, they do not clean-up existing deposits.

7 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 hours!), the precursors are free to stick to the hot metal surfaces where they react with any dehydrogenated 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 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 6. 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.

8 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.

9 Figure 8 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 easily and preferentially oxidized molecules by the free radicals HOWEVER they 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 hydrocarbon 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.

10 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 (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 blowby 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.

11 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 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.