I have a history of high oil consumption in my IO360-A1B6, with relatively fresh Chrome cylinders (about 170 hours). My oil consumption has been as high as one hour per quart. You advised some things for me to check.
I did the compression checks. 80 lbs exactly on all 4. Borescope did not reveal anything noteworthy. Channeling looked fine, no evidence of broken rings that I saw. My old friend, an A&P-IA ,also looked. He’s thinking, like you, to drain the oil (Exxon Elite 20/50), and also (like you) said to put in Phillips 20/50. I have a case on order.
I took it up to 6500′ or so, established the higher power settings, and leaned it the way you suggested. The only problem was I had moderate turbulence, with squirrelly winds making it difficult to keep the power on in descent and keep the wings on, too. For the first time, I dipped into the bottom of the yellow for a moment. Despite a few bumps, control was smooth, strong and harmonious.
After an hour I landed, let the oil drain back to the case for a while, and found that it only used maybe a half a cup. It was previously using a quart per hour, when I first wrote to you.
So Mike, please, if you have an opportunity, explain to me why low MP, and/or not running it harder, would or could cause a glazed cylinder. I would imagine the opposite.
It sounds like you have caught this problem in time. Switch to the 20-50 Phillips oil, and run it hard. Read on.
Air-cooled aircraft engines have huge cylinder bores and wide internal clearances, compared to auto engines. This is because of the large differences in coefficient of expansion between the core materials in the engine. Case in point, an aluminum case, a huge (5″ or so) aluminum piston, coupled with steel cylinders. There are large clearances that the hydraulic lifters have to take up, as the case expands away from the camshaft. The aluminum pistons have to be fitted very loose (relatively speaking) in the steel cylinder bores. Otherwise, when the engine heats up, they would get too tight and seize up. In fact, the normal assembly clearances in many parts of an aircraft engine are the equivalent of the clearances in a completely worn out auto engine.
In part, this means three primary things have to exist, in order for the engine to achieve designed performance:
– A lot of oil has to get used to help seal loose clearances. This means high oil consumption, compared to what we are used to in cars. And if oil consumption is low, as in engines you hear of that get 18-20 hours per quart, I can guarantee that the cylinders in those engines will not make it to TBO. Case in point: most Continental large-displacement engines. They have a TBO of 1,500 hours (instead of 2,000), and very few of their cylinders make it past 800 hours. But they get 15 hours to a quart of oil, and their owners brag about it. I happen to think that cylinder work costs far more than oil.
– The oil control rings (scraper rings) are designed to leave a measured amount of oil on the walls of the cylinder. This oil film lubricates the piston and rings as they rise up in the bore on the compression stroke. The design relies on the scraping effect of the higher-up compression rings to remove any excess oil, as the piston travels down on the power stroke. There is a difference in the oil film thickness during the different engine strokes. On the exhaust and intake strokes, there is a thicker film, as cylinder pressures are low (see below). This helps lube the cylinder assembly, without excessive consumption, as there is no combustion during these strokes (nor on the compression stroke). While the film is thicker, most of the excess still gets scraped back down under the rings. As it thickens, the oil layer just sits there, doing its job, until it is thick enough to get scraped thinner by the rings.
– The piston rings cannot be made to fit tightly in either the piston or the cylinder; and the end gaps are large. If the rings were made large enough to tightly seal in a hot piston and cylinder, the ends would butt and they would seize, when the engine cools off (as in a long descent). Since they can’t be made tight enough by design, they rely almost completely on gas pressure to seal under operating conditions. As the piston rises in the bore on the compression stroke, a slim gap exists above the rings, in the ring grooves of the piston. Because the ring is expanded outward toward the cylinder, a slim gap also exists behind the ring (between ring and the back of the piston groove). The ring is snug against the bottom of the groove, due to compression, and oil helps seal that joint. To sum up the dynamics, the slim gap above and behind the rings allows the combustion gas pressures to get behind the rings. This forces the rings out against the cylinder walls. In turn, this greatly increases the scraping force on the oil film, and greatly thins out the layer. This happens just in time, and prevents the combustion process from burning it off instead (during the power stroke). If it doesn’t get burned off the cylinder walls, two things are prevented; excess oil consumption, and glazed cylinder walls.
This dynamic is exacerbated by chrome cylinders, because they are so ‘slick’. The channeling is the only place that can actually hold oil; there are almost no microscopic scratches in the chrome that can hold oil. At the same time, the hardness of the chrome makes it very difficult for the rings to achieve a perfect ‘wear-in’ or break-in. So you have a case where the oil can’t ‘hide’ in the scratches (only in the channeling), and an imperfect match of ring and cylinder, which allows too much oil to remain exposed to the combustion flame. When you add low combustion pressures to the mix, which don’t enable the rings to go more tightly against the bore, you get very high oil consumption, incomplete break-in, and ultimately glazed cylinders.
None of these engines should ever be operated below 65% power for extended periods. For example, I personally advocate and practice well-planned descents at cruise power, which begin far enough out to enable 200-300 FPM descents. This is also highly efficient, as it allows you to regain some of the energy lost in the climb, and gives you the trip times of a faster plane. If descents are planned to begin 20-60 miles out, depending on altitude, you can carry cruise power to within a few miles of the pattern. That way the low power operation period is minimized. These engines should cruise at 70-75%. I personally don’t think that any engine with chrome cylinders should ever be operated below 75% for extended periods. These are not car engines, that spend most of their lives at 30% power or below; and which would die an early death if operated at 100% for as long as an hour or two. Your engine was certified with a power run of 100 hours or more at 100% power. This was real 100% power, too; not the installed output of an engine being constrained by the airframe manufacturer’s induction and exhaust system. Your Lycoming is certified to operate at 100% for an unlimited time (unlike most Continentals). There is nothing unreasonable about routinely operating it at 70-75%. The people who think they need to ‘baby’ these engines, to make them last longer, simply do not understand the physics underlying their design. They need frequent flights, high power settings, and regular oil changes.
I hope you know enough about the internal design and operation of this type of engine, to understand this explanation. I don’t know of a shorter way to explain it, other than to just say ‘trust me; run it hard’; and I didn’t think that was what you were after.