Most airplane owners know at a high level that breaking in an engine is an important activity. What most cannot explain is the chain reaction of outcomes that affect Pilots and Owners if break-in does not occur properly. One likely scenario includes the following events:
- “Glazing of cylinders” can occur within the first 2 hours of flight time
- Rings do not seat properly
- Oil consumption increases in excess of 25% of normal operating guidelines
- Pilots experience decrease in engine power
- Significant maintenance is required to fix the engine including the removal of the cylinders to re-hone the barrels and replace the piston rings.
- Downtime of plane incurs opportunity costs for commercial applications
- Overall value of plane is impacted at time of sale because of maintenance records
That being said, the mechanics of the process is helpful to know and it starts with the fundamental of how important piston ring seating is to proper engine break-in. Your understanding of the factors involved in the break-in process will aid in correctly operating the engine during this important time. Piston ring seating means the rings and the cylinder wall must wear-in together to provide an effective seal for the combustion chamber and to keep combustion gas blow-by and oil consumption to a minimum.
Initial Ring Seating
During ring seating, the basic purpose is to establish metal-to-metal contact between the piston ring face and the cylinder barrel. In order for this process to take place, the rings must breach the lubricating film of oil on the cylinder wall. As the rings begin to seat, the ring-to-cylinder wall surface area increases and it becomes harder for the rings to breach the protective film of oil. You can aid in this process by keeping the manifold pressures at high levels during the early stages of break-in. The higher manifold pressure, as controlled by throttle position, force the piston rings to expand against the cylinder walls, breaching the protective film of oil and allowing the slight wear that we are trying to achieve.
During initial break-in it is not uncommon to have cylinder head temperatures above the normal range. This elevated temperature is an indication that initial ring seating is taking place. As the rings begin to seat to the cylinder walls, the temperatures will drop. This usually occurs over a period of 10 to 20 minutes. Cylinder head temperatures can remain slightly elevated for several more hours until complete ring seating has been accomplished. Ideally, the rings should seat within the first 10 to 15 hours of engine operation as evidenced by stabilized oil consumption and decreased cylinder head temperatures.
Since this engine is either new or rebuilt, it has “tighter” running clearances than the engine you just retired. It stands to reason that cylinder head temperatures and oil temperatures can run slightly higher. While hot oil runs thinner and aids in ring seating, it is important that you do not let either temperature red line.