One of the most commonly asked questions about piston aircraft engines is “why didn’t my engine reach full TBO?”
There are many answers to this question, but I think the number one answer is cold starts. I do not mean just winter operation, but that initial start-up, especially after sitting for an extended period even in the middle of summer.
The two main concerns are the cam and lifter and the pistons.
The problem with cam and lifters is that the lifters are mounted horizontally in the engine so that the lifter face is vertical. During storage or idle time, the oil tends to run off the face of the lifter, leaving it dry for that initial start-up.
That first few degrees of rotation with little or no lubrication and the high load from the valve springs can cause spalling which, at first, is just small scratches on the cam and lifter face. Once these scratches first appear, the unit loading on the remainder of the contact interface goes up so that all subsequent starts rapidly increase the wear process.
To compound this process, if your aircraft is located in a humid climate, the wear surfaces will tend to get a slight rust coating over a fairly short time. Then on that initial startup, the rust “rouge” will act as a lapping compound to further accelerate the wear rate.
So, what should you do?
One possibility is a pre-oiler to lube the parts prior to that initial start.
My friend Paul McBride did a nice column on that awhile back. In Lycoming engines, oil does not reach the lifter face when pre-oiling, so is not of too much benefit during that initial start-up.
They do have pre-oiler systems that place a small tube over the cam that will direct oil to the cam prior to start up. These systems can only be installed when the engine is disassembled, like at overhaul.
The normal pre-oiler system does work better on a Continental engine because oil will drip out of the crankshaft bearings and hit the camshaft, which is located below the crankshaft.
So, what else can you do to protect the cam and lifters?
The best thing is to fly your aircraft regularly. And the flight needs to be long enough to get the oil temperature up to at least 180°F to drive off the moisture that can build up from condensation.
Many pilots go out to the airport and start their engines and run them for a few minutes to coat all the parts. The problem here is that they hit the primer a bunch of times to get it started. This puts raw fuel into the crankcase.
The sulfur in the fuel combines with the moisture in the crankcase to form acids, which greatly increase the rusting and the subsequent wear of the cam and lifters. So, don’t do that.
If you start your engine, it should be run long enough to evaporate the moisture. If your plane is going to sit for an extended period of time, I recommend adding a quart or so of the Phillips preservative oil.
The problem with pistons
The second area of wear after a cold start is the pistons.
Because aluminum pistons expand more with increased temperature than the iron bores, they are assembled with excess clearance. As the engine warms up to normal operating temperature, the piston to bore clearance is reduced to the proper amount.
However, if your engine is operated at a high load before the clearances normalize, excessive piston movement hitting the cylinder wall, which is called slap, can cause excessive wear on piston skirts and cylinder bores.
In your automobile, you can start up and drive away slowly. But in an aircraft, you go to full power during takeoff. If your engine is not warmed up properly, the high load can cause problems.
How long should you idle your engine before takeoff?
Most pilots remember to warm up their engines when it is 20°F, but do not worry about it on a nice 70° day.
But piston expansion is so much per degree. At 20° it will go up, say, 280°, assuming a 300° cylinder wall temperature. But at 70° it will still increase 230°. I usually recommend idling your engine until the oil temp goes up 30° to 50°, especially if you have a short taxi.
So how critical is warm-up?
A few years ago, I was in a discussion with a manager from a nuclear power plant. His problem was the Atomic Energy Commission required that every month they test out the standby diesel generator sets. One of the requirements was that the engines be started and come to full power within 30 seconds. These engines ran on non-zinc containing oils.
Even though they were in a heated building with heaters to heat the oil to operating temperatures, they were having engine failures after only two to three years. Meanwhile the same units that were tested with a longer warm-up time were lasting indefinitely.