In recent weeks during these twilight years of oil, this board has seen a number of threads about different kinds of engines, the fuels used as a source of power, and the different types of oil used as lubricant.
It may be tempting to believe, especially for those who may have had experience of assembling car engines, that all engines are the same, and need the same oils and same treatment. Nothing could be further from the truth.
It has been posted on this board that a new engine needs to be treated very gently – no more than half or even a quarter throttle opening, for around 1000 miles. Certainly, such treatment may have benefited cars with legacy engines, whose technology was rooted in the 1960s, or earlier – cars like England’s famous Morris Minor, which had to be driven at no more than 30mph for the first 500 miles. But does the same philosophy hold true with every type of internal combustion engine? I think not.
I remember reading, many years ago, an article in an aviation magazine entitled “Don’t Baby That Engine”, which I will quote from memory. The author, a gentleman by the name of Alan Bramson who served as a pilot in WW2, proffered that whilst the baby treatment might be ideal for 1960s car engines, it would be the kiss of death to an aero engine of the type used in small private aircraft.
Why’s that then? Well, the engines are certainly different. A typical aero engine like the 150hp Lycoming O-320 engine in something like the Piper Warrior PA28-151 is air cooled, and instead of having a cylinder block, the cylinders are mounted over the pistons and then bolted to the crank case. These engines are designed to operate at lower speeds than gasoline powered car engines, and for many the “red line” speed is a mere 2700rpm.
When the pistons/cylinders are fully bedded in from new, the cylinder bores will have a slight ovoid shape, owing to the small sideways forces of the pistons caused by the crank mechanism during normal running. There’s no way for a workshop to machine the cylinders in this way, so the initial fit is deliberately tight, with the intention that the pistons will bed in naturally. To facilitate this bedding in process, a criss-cross pattern is cut in the cylinder walls, its purpose being to retain oil to provide adequate lubrication of the cylinders during this bedding in process. The idea is that as the engine beds in, and the cylinders are worn to the ovoid cross section I mentioned earlier, the criss-cross pattern will be worn away, and the pistons will make a tight fit in the cylinders.
The crucial thing during the running process of an aero engine like the Lycoming O-320 is that it must be worked hard! Minimum power setting should be 75% in normal operation!! The power setting has to be high, otherwise the pressure in the cylinders will not be sufficient to force the piston rings against the cylinder wall, the criss-cross pattern will not be worn down and the piston will not make a tight fit in the cylinder. Instead, the engine oil will burn on and form a glaze on the cylinder walls, leading to poor compression and excessive oil consumption.
An aircraft like the Piper Warrior PA28-151 has a normal engine operating speed of around 2350rpm for 75% power. Those owner/operators who thought they knew more about engines than the engine manufacturer themselves, and even cast aspersions on the Pilot’s Operating Handbook were probably the ones Bramson was talking about when he spoke of those who decided to “baby” their aero engines by operating them at 2000rpm, or even 1800rpm during the crucial breaking in period. These were the scenarios presented in the Bramson article. The likely outcome of such treatment, as Bramson went on to describe, is that the pistons/cylinders will not have bedded in properly because of the oil glaze which has been allowed to form on the cylinder walls. If the compression is below acceptable limits, then the only answer would be to strip down the engine and rebuild it.
