Compared with automotive engines, our piston aircraft engines are positively filthy. They have huge pistons and cylinders (to produce enough power while turning so slowly) and extremely sloppy clearances (because they’re air-cooled and have horrible temperature regulation).
As a result, the amount of combustion chamber leakage “blow-by” into the crankcase oil is very large compared to small, tight, water-cooled car engines. Blow-by in aircraft engines is the root cause of deposit formation. And deposit formation can cause (cool side) stuck valves as well as stuck rings and sludge problems.
Figure 1 shows a cutaway of a Lycoming cylinder. Note the large clearance between the piston, on the left, and the cylinder, on the right, at room temperature. This clearance decreases as the engine heats up, however, it is always much greater than that found in a water-cooled engine. And greater clearance means greater amounts of blow-by contamination.
Blow-by consists of raw fuel, partially-combusted (reactive) fuel, known as ‘Varnish Precursors’ (VPs), steam (water), lead bromide particles and a lot of heat. Most engines introduce one to several ounces of blow-by into the oil every hour of operation. The vast majority of blow-by goes out the crankcase vent (breather), however small amounts of VPs and water remain in the oil to cause us all kinds of grief.
Virtually all the deposits in the oil-wetted areas of the engine are caused by these blow-by VPs. Everyone is familiar with VPs. They are what you get when you buy a can of varnish. When applied from the can to a wood or metal surface, VPs react with each other in a process called polymerization, leaving the familiar ‘varnish’ film. VPs act the same way in an engine and looking inside most used engines, you can observe the formation of clear ‘varnish’ films over most of the internal parts. These films darken with heat exposure becoming amber in colour (see Figure 2).
Varnish films formed on the hottest parts such as the pistons or valve guides eventually degrade to hard black carbon deposits (see Figure 3).
Lines of Defense…
How well do aircraft oils deal with this filth?
Apparently, not well as we still find copious amounts of deposits in our engines when we service or rebuild them.
The first line of defense in the cleanliness war is the Ashless Dispersant (AD). It is considered a ‘keep clean’ additive as it does not clean up existing deposits. ADs work because they are attracted to the VPs and several AD molecules, preventing their polymerization into varnish films. The ‘encapsulated’ VPs exit the engine when the oil is dumped at oil change time.
It is a common belief that the dispersant holds the lead bromide particles in suspension, but this is incorrect. The only things the dispersants bind to are VPs.
Ashless Dispersant makes up 3% of the oil. So there is a limited number of dispersant molecules to keep the engine clean. The base stock, which makes up 90-95% of the oil, helps the dispersant by holding some VPs in solution. When both the dispersant is consumed and the basestock is saturated with VPs, deposit films quickly start forming.
During the ‘break-in’ period (the first 10-50 hours from overhaul), most manufacturers and engine builders recommend using a non-dispersant pool, commonly referred to as ‘mineral oil.' Some think the dispersant interferes with the break-in process (it doesn’t) and others want a thin film of varnish to form over the internal parts of the engine for rust protection.
Lycoming turbocharged engine owners must use AD oil for the life of the engine from break-in to TBO. This is because the manufacturer realized that any early deposits lead to future problems. Following break-in, everyone is instructed to use AD oil.
Another problem caused by varnish formation is sludge formation. Sticky varnish films capture lead bromide particles. (Recall that burning leaded fuel with ethylene dibromide lead scavenger forming lead bromide particles that end up in the oil). The lead bromide particles aren’t a problem until they are captured in a varnish deposit, making it substantially thicker.
A little varnish, acting as glue, combines with countless lead bromide particles to form a dense heavy sludge. We see examples of this sludge problem throughout the engine as it settles in the low flow areas like the sump, constant-speed prop hub, and inside the crankshaft. Lead sludge has the consistency of room temperature butter.
When these carbon/sludge deposits build up in the exhaust valve guide, they cause (cool side) valve sticking, (see Figure 4).
How do you Prevent/Minimize Valve Sticking? There are some simple answers to this question...
Hot Side: Run your engine hard, at least occasionally, 65-75% ROP or LOP enough to just stay out of the GAMI 'Red Box' (www.advancedpilot.com/redbox.html) and Cylinder Head Temperatures (CHTs) below 400°F. These will maximize combustion temperatures and minimize lead deposit build-up on your exhaust valve stems. The ideal range for CHTs is from 325°F-380°F. A lower CHT is fine if you are at a higher power setting.
Cool Side: Change your oil frequently enough to prevent varnish build-up. For most aircraft, this is from 25 hours to 35 hours (or every 3-4 months). If you fly a lot, you can stretch it. If you have a big motor (L-540 or C-550) with a 6-8 quart sump, don't stretch it. The ideal oil temperature range is from 180°F-220°F.
You should also lean very aggressively when on the ground, as it reduces the amount of fuel entering the oil. Lean until the engine barely runs smoothly. This will keep your oil and engine cleaner and you will not be able to run to full power risking a take-off with a leaned mixture.
I will mention that the oil additive, Camguard, contains unique deposit control additives. These additives work in conjunction with the ashless dispersant to keep engines deposit and sludge free and it is the only product to offer this technology.
You and your mechanic should expand the use of a borescope to include looking for hot side valve deposit buildup as well as signs of cool side crankcase varnish deposits. Keeping an eye on these deposit buildups, and modifying operation accordingly is the best way of avoiding some costly problems.
By Edward Kollin, Technical Director, Aircraft Specialties Lubricants
Aircraft Specialties Lubricants
2860 N Sheridan Rd., Tulsa, OK 74115
CamGuard - Protection in the Air & on the Ground