We all take for granted that when we turn-the-key & start our engines, that we can go about our business & do what we've got to do. This is a good thing, but it's worth knowing what actually happens from that moment, & why keeping your oil in good condition is worth its weight in gold ! Your cars designers ( & Oil Companies ) have had to overcome a lot of hurdles to get things to the stage of us not giving things a second thought ! However, if your a little curious as to what does go in your engines lube-oil-system, keep reading...
PRINCIPALS OF LUBRICATION
Lube-oil is expected to perform the following functions & provide the following abilities for our engines.
*Lubrication
*Cooling
*Sealing
*Cleaning ( detergence & dispersant )
*Have self-preservation
*Noise reduction
Mechanical-friction is caused in an engine due to the sliding or rotating motion of engine components. Friction is dependent on the load & the surface finish of the 2 components that are in relative motion. All metal surfaces have surface "asperities", - ( peaks ). When 2 surfaces slide relative to each-other, the peaks of these asperities strike each-other causing resistance-to-motion. The peaks shear-off in the process & collect as wear-debris. The friction generates heat & causes the wear of the engines components. In extreme cases the heat generated causes the peaks of the surface asperities to fuse together ( weld ) causing the seizure of engine components.
The main function of your lube-oil is to successfully keep 2-surfaces apart preventing their surface-asperities from comming into contact with each-other, while allowing a smooth relative motion. The lube-oil must also carry away the heat generated by friction. In order to successfully create this film of lubricant between 2-surfaces, the following conditions must be satisfied...
*There must be copious amount of oil.
*An optimum clearance between the 2-surfaces must exist.
*Leakage past the clearance-space must be controlled - ( losses supplemented ).
*The relative-speed between the 2-surfaces must be "high".
*The 2-surfaces must have a good surface-finish.
*The temperature of the surfaces must be controlled by allowing a calculated amount of oil to leak.
*The lubricating-oil must have good / correct viscosity ( stability of pump-ability in-between a required temperature-range ) & oiliness properties ( the ability to adhere to a surface & lubricate ).
Fluid-film-lubrication can be achieved hydro-dynamically or hydro-statically.
HYDRO-DYNAMIC lubrication relies on high-relative-motion between 2-surfaces. An oil-wedge is formed that keeps the surfaces apart. The relative motion causes the oil to be drawn in-between the surfaces due to the phenomenon called "Viscous-drag". The oil is dragged-in faster than it is squeezed-out, & this creates the pressure under a shaft, lifting it up, suspended & held by the oil. Your engines oil-pump, provides the constant supply of oil at reasonable-pressure, to allow this phenomenon to continue whilst the crankshaft rotates.
HYDRO-STATIC lubrication takes place when a fluid-film is formed by extremely-high-pressure. A high-pressure pumping arrangement is required to maintain the high-pressures required for hydro-static lubrication. To clear the fog, engine designers use this method where relative-speeds between surfaces is too-slow for hydro-dynamic lubrication to take place, or where there's a change in the speed or direction of rotation. ( As an example, some of the ships I've worked on have HUGE slow-speed-diesels that have "cross-head-bearing assemblies", that require this type of high-pressure pumping arrangement, since there's no rotating shaft here, only "slipper-guide-bearings" & a "cross-head pin" that has a limited oscillating movement.
BOUNDARY LUBRICATION
Boundary lubrication takes place when conditions are such that a layer of lubricant of finite thickness cannot be maintained. The oil-film that's formed will only be a few molecules thick & can be pierced by surface-asperities, which can lead to micro-welds & eventual seizure of components.
Boundary lubrication takes place whenever there is slow-relative-speed between surfaces, change-in-speed or direction, or at extremely high-loads...Boundary lubrication always takes place at the beginning of rotation of components before a "hydro-dynamic-wedge" has formed - ( eg, when your first cranking your engine at start-up & no oil-pressure has established itself ).
Certain properties of lube-oils such as viscosity & oiliness become extremely important during boundary lubrication ( engine start-up ). Oiliness is the ability of the oil to adhere to a surface, which it is in contact with. The bond that is formed between the oil-film & the surface prevents the oil from being squeezed-out, maintaining "some" lubrication.
In finishing, it's hoped that it's understood that at the beginning of "motion", boundary lubrication takes place, & passes through a transitional period ( partial-film-lubrication), before attaining fluid-film-lubrication ( hydro-dynamic ). So there's quite a lot of things that need to get-going quickly inside your engine in the first couple of seconds when you start-up. It's these repeated first-few-seconds of start-up that leads to most of the wear & tear inside ! Keep your oil clean / fresh to keep your engine going longer !
Cheers,
Rastus
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"Only an alert & knowledgeable citizenry can compel the proper meshing of the huge industrial & military machinery of defense with our peaceful methods & goals, so that security & liberty may prosper together". Dwight D.Eisenhower.
"Only an alert & knowledgeable citizenry can compel the proper meshing of the huge industrial & military machinery of defense with our peaceful methods & goals, so that security & liberty may prosper together". Dwight D.Eisenhower.
