It is understood that to keep competitive and protect their investment in equipment, enterprises in the aircraft industry require lubrication solutions of the highest quality at the best possible prices. While specialist lubricants for the sector are designed to ensure that system components can move freely and efficiently, they must also tick multiple boxes.
Reducing wear and tear from friction and excessive heat in aviation engines, they are also required to clean and cool engines while acting as a sealant against contamination and corrosive forces. Rust protection can be especially important when fleets are used infrequently, as unused aircraft are at high risk from corrosive elements during spells of downtime.
In this article, we’ll take you on an in-depth examination of the most important principles of aircraft engine lubrication. Read on to find out more.
Getting to grips with the principles of aircraft engine lubrication
Like many other forms of lubrication, the prime role of every aircraft engine lubricant is to effectively reduce any friction occurring between moving components. As oils and other liquid lubricants can easily be circulated, they are universally deployed to keep aircraft engines in top condition. The theory behind fluid lubrication is based upon the separation of component surfaces ensuing that no metal-on-metal contact ever takes place. Providing that the oil film stays intact, metallic friction is exchanged for internal fluid friction within the lubricant.
In optimum conditions, unwanted wear and friction and restricted to an absolute minimum. As a rule, oil is typically pumped throughout the aircraft engine reaching all areas within that require lubricating. Overcoming the friction of all moving components of the engine naturally consumes energy and results in unwanted heat. However, reducing friction while an engine operates can effectively increase its potential power output overall. Aircraft engines are commonly subjected to many kinds of friction.
Lubricating the aircraft engine
The first quality most people consider when it comes to a lubricant is, naturally, its lubricating properties. Effective lubrication always requires an oil film that is both strong and thick enough between moving components to keep wear and friction to an absolute minimum.
Oil properties may include hydrodynamic, dynamic, mixed film or boundary forms.
Mixed-film or boundary lubrication is used in the aircraft engine’s upper cylinder area within its outer boundary. This point is the most remote area of the engine requiring lubrication, as the oil rings take most of the oil film from the cylinder walls before it ever ascends to the upper cylinder. There must always be a residual amount of lubricant in the upper cylinder so that the engine is protected on start up.
If an engine is left idle, some lifters may be pressed and loaded with maximum spring pressure. In such cases, most of the lubricant has been squeezed from the junction. As a result, when the aircraft engine starts, it takes time to get oil to where it is required. Therefore, a good boundary oil film is necessary.
Dynamic lubrication comes from the pressure generated by the oil pump. This pressure delivers a sufficient flow of oil to the lubricating system. Hydrodynamic lubrication offers a smooth surface for moving parts to operate on and prevents direct contact occurring between moving parts.
In hydrodynamic lubrication, contact pressure is kept lower and spread over a greater surface area. As a result, a constant supply of oil is necessary between the components for hydro-dynamic lubrication.
Lubricants must clean aircraft engines
All oils designed for the aviation industry can clean. This concept commonly conjures the removal of varnish, sludge and other accumulated deposits on the plugs in the dedicated oil pan, or within the screen. There are additional benefits to this process, however, for aircraft operators. When lubricants keep an aircraft engine clean, they also clean the ring belt area to ensure that the combustion process is kept under better control. When these rings can move freely, an airplane engine will operate at higher efficiencies and has enhanced ring seal. It also produces far less blow-by and will consume less oil.
A ring belt that remains dirty will restrain the fluid movement of the engine rings within their grooves and, as a result, they cannot seal. Resultantly, pressure is often created between the cylinder wall and the ring face, leading to scuffing, scarring and undue wear.
How aviation lubrication keeps engines cooler
Air-cooled airplane engines depend on the oil that they use for cooling more than automotive engines cooled by water. As a rule, automotive oil commonly accounts for around 40 per cent of the cooling capacity required, but in aviation engines, oils must be relied upon to carry off a far greater percentage of heat from the engine.
Oil acts as a heat-transfer medium and flows via the crankcase and dedicated oil coolers, and as it flows, it dissipates the heat from all moving components, continually cooling piston rings and engine bearings.
Without the cylinder wall being cooled by this film of oil film, the rings would have a poor heat transfer path. As a result, issues like melting, scarring and galling can occur. Engine oil also cools the entire valve train and its valve springs.
Understanding friction types
Friction can be defined as the action of one surface or object rubbing against another. As one surface slides over the other surface, the result is sliding friction, which is commonly found during plain bearings use. Despite their smooth appearance to the naked eye, metal surfaces are never completely flat and possess microscopic defects known as asperities which are just out from the surface. As a result, when surfaces come into close contact during motion, friction occurs.
Rolling friction is another friction type. This friction occurs when a sphere or roller rolls over other surfaces, for instance with ball and roller bearings, referred to sometimes as antifriction bearings. Friction created during rolling friction is always less than the amount created when sliding friction occurs and this bearing utilises an inner race and outer race with balls, (or sometimes steel spheres), which roll between the races or moving parts as required.
Wiping friction is another friction type and happens in between gear teeth. This type of friction involves a wide range of pressure and the loads being applied to gears can often be extreme. As a result, the lubricant must have the capacity to withstand heavy loads.
The main functions of aircraft engine oil
Along with efficiently reducing unwanted friction, the lubricating oil film also acts as a protective cushion in between metal components. This cushioning effect is especially important for many parts like reciprocating engine crankshafts as well as connecting rods, which are both subject to extreme shock-loading. When a piston is forced down during a power stroke, it will apply loads between the connecting rod bearing and the engine’s crankshaft journal. As a result, the load-bearing properties of the oil must stop the lubricating film from being forcibly squeezed out and leading to metal-on-metal contact within the bearing. Additionally, as lubricant circulates throughout the engine, it also absorbs heat from the cylinder walls and pistons. Specifically in reciprocating engines, these parts are heavily dependent on the lubricating oil to act as a coolant.
Lubricating oil cooling can often account for close to 50% of total engine cooling. For this reason, it is regarded as an excellent medium for transferring unwanted heat from the aircraft engine to the onboard oil cooler. The engine oil also helps by forming a seal dividing the cylinder wall and the piston which prevents leakage of gases emitted from the internal combustion chamber.
Engine oils also clean the engine by mitigating abrasive wear by collecting foreign particles and taking them to the filter where the can be removed. An additive in the oil called a dispersant, holds the unwanted particles in suspension and enables the filter to effectively trap them as the lubricant passes through the oil filter. The oil also inhibits corrosion of the engine’s interior by leaving a protective coating on parts when the aircraft engine is shut down. Among many others, this is a key reason why an engine should never be shut down for extended periods of time. The oil coating that prevents corrosion will break down over time, allowing rust and corrosion to creep in.
The aircraft engine’s lubricating oil is its life blood, and it is vital for the engine to perform its job and to extend the length in between overhauls. Understanding these key principles is the foundation of effective aircraft engine lubrication.
Oil as a sealant in aircraft engines
Finally, oil creates a seal between cylinder walls and the rings, but also assists by sealing the rubber and synthetic seals and gasketed areas for the engine’s crankshaft. When the oil flows around those areas, it works to retain the seal. For this reason, aviation oil must always involve a formulation that is fully compatible with an engine’s seal materials to extend the active service life of seals.
When the correct aviation engine lubricant is selected for a specific make and model of engine, all these principles will be sufficiently satisfied. As a result, aircraft operators can enjoy maximum performance from a well-protected engine.