Engine Lubrication, Part V
Lubrication is needed to overcome friction caused by surfaces sliding or rolling over each other. No matter how polished or closely machined a surface is, on a microscopic level there are always small jagged edges or uneven spots. If these surface irregularities come into contact they may break off or even seize and become attached together.
With further movement these tiny parts will break and float around in the oil and eventually may cause damage if the oil is not filtered or when oil filters not changed at the proper interval.
In an engine, be it a turbine, diesel or spark ignited, oil is used to lubricate all moving parts so that durability and reliability is assured for many thousands of hours of trouble free service life.
For you as the pilot/owner it is therefore important to have a basic understanding of how oil acts in a engine during its time in service.
Types of Lubrication
The amount of friction between two metal parts depends on a number of factors:
- Temperature, either ambient and in the engine itself has an effect on friction
- Surface finish, the better the surface is machined or polish the lower the coefficient friction the surfaces have
- Load, the heavier the load on a surface the more friction there is
- Speed of movement, the increase of speed of sliding surface will increase the friction
- Nature of movement, sliding or rolling motion have different friction characteristics
- Type of lubricant, the type of oil and its characteristics also have an effect on friction (viscosity)
If we want to reduce friction we need to change or remove the factors which may have an adverse effect on the surfaces in motion. There are a number of ways to achieve just that.
In case of sliding friction, you can use a rolling element like a ball or even needle bearing elements. The use of sacrificial surfaces can be used too, think of lead/copper journal bearings. And last but not least, the changing of viscosity, different or improved additives or even changing from oil to grease can reduce friction to a large extend.
In the small area where the sliding or rolling surfaces are lubricated we find three modes of lubrication regimes:
- Boundary lubrication
- Hydrodynamic lubrication, HDL
- Elastohydrodynamic lubrication, EHL
Below you will find a short discussion of each of these.
This occurs when an engine is started, at low speed or even in high load conditions. At this time the two moving (rolling or sliding) surfaces may come into real contact and damage could be the result. Some specialist say that 70% of all wear in an engine occurs in this regime. This is why engine stop/start systems do more harm than good eventually.
To make sure that no damage is done during these regimes, you must use a lubricant which is formulated with antiwear or even extreme pressure additives. These additives react with the surfaces in contact due to the high pressure and temperature and form a chemical film on those surfaces. This film is then sacrificed as the surfaces come into contact so that the film wears off and not the metal surface.
By increasing the viscosity of the lubricant, by increasing its thickness, boundary friction can be minimized in some situations. Although care must be taken not to increase viscosity too much as the internal friction of the lubricant increases too and can give rise to higher temperatures and higher fuel consumption at normal operating conditions.
Hydrodynamic lubrication, HDL
This is when a full film of oil has separated an engine shaft (crank or camshaft) from its support and no contact exists between the parts. The oil is keeping the shaft and bearing apart by its viscosity. Also, during hydrodynamic lubrication there is no friction except in the lubricant itself, where molecular structures shear during operation.
HDL requires that the machined surfaces have a high degree of geometric conformity and relatively low pressure. This situation can be found between rotating crank or camshafts and the journal or sleeve bearings.
Once the engine is at operating temperature and the shafts are at normal engine speeds it should be possible to remain in hydrodynamic regime forever so that friction is at minimum. This is the regime we want for our engines.
Elastohydrodynamic lubrication, EHL
This type of lubrication occurs where surfaces have a low degree of conformity combined with high contact pressures as found in gear drives (Rotax) and rolling bearing elements (wheel bearings). The lubricants are caught by the moving surfaces and under high pressure the viscosity increase to such a high level that it forms a semi-solid film keeping the two moving surfaces separated.
And as long as these conditions do not change, the metal surfaces will not come into contact. In fact these surfaces may actually deform long before the semi-solid oil or grease film breaks, due to this remarkable property of the lubricant.
As you can see, the main job of engine oil is to keep moving parts separated, transfer forces generated by the pistons rotating the propeller, removing heat. Its a dirty job in a high pressure, high temperature environment and oil does that job perfectly!