External contamination of lube oil by silicon (dust) is a major cause of accelerated wear.
After oxygen, silicon is the most abundant element in the earth’s crust. Silicon does not occur naturally in elemental form but rather combined with oxygen in a compound called silica (silicon dioxide.) Silicon occurs in a free form (quartz, sand, etc.) or combined with a variety of metallic oxides, in which case it is called silicate (eg. Felspar).
Another class of silicon compounds that should not be confused with silica and silicates is silicones. Silicones are man-made organic compounds that find extensive application in the polish, paint and lubrication industries.
Silica and silicates make up a large proportion of the earth’s crust and as such are present at high concentrations in natural solids and dusts. It is for this reason that silicon is used as the main indicator of dust entry into a component. There have been several studies done on the causes of premature wear in components. The figures vary from study to study but one thing is clear: external contamination of lube oil by silicon (dust) is a major cause of accelerated wear.
The South African climate generally is one of low rainfall with resultant high dust levels. Particles of airborne sand and dust vary in size, shape and abrasive properties. In an engine the ingress of atmospheric dust takes place primarily through the air intake. Efficient air filters remove 99% of the dust that an engine ingests.
Image: Dust particles
The remaining 1% consists of very small dust particles that pass through the air filter. These vary between submicron size particles to particles up to 10 microns in size. The dust will pass between piston, rings and cylinder and eventually become suspended in the lubricating oil.
It is the particle that has its smallest dimension of a similar size to the clearance involved that does the maximum damage. A particle smaller than the clearance will pass straight through doing little harm. A particle larger than clearance will be unable to enter and do any damage. (See diagram above)
In an engine the clearance between the piston ring and liner bore is extremely small, therefore it is the small light airborne dust particles which are the biggest threat when a leak occurs in the induction system.
Under ideal conditions the working surfaces of a component are kept apart by a thin film of oil. This oil film prevents direct contact between the surfaces, reducing the amount of friction and the rate of wear. The oil film will also absorb shock loads and help distribute the load over the whole surface. The introduction of even a small amount of dust into this environment will seriously disrupt this. Once the dust particle has entered the oil film it forms a direct link between the two surfaces, nullifying the effects of the oil film.
The first and immediate effect is a “scratching” of the surface as the particle is dragged and rolled across the surfaces. The second and potentially more serious problem is that once the dust particles is introduced in between the two surfaces, it changes the loading of the surface from an even distribution to a point load concentration on the particle with tremendous increase in pressure at this point.
The increase in pressure causes a deflection of the surface, which will eventually result in metal fatigue and the surface breaking up.
The solution is to keep the dust out. To do this, design engineers use air cleaners, breathers and seals at any point that dust may enter.
Air cleaners filter air as it is drawn through a system, e.g. in engines, compressors, etc. They have to deal with large volumes of air, as an engine needs up to 2000 litres of air to burn 1 litre of fuel efficiently. The most common type of air cleaners have three main components: a centrifugal precleaner, a large pleated paper element, and a smaller paper or cloth “safety element”.
The precleaner consists of radial fins and a collection bowl. The fins cause incoming air to swirl rapidly around the inside of the housing. The heavier dust particles are thrown outward by centrifugal force and are channelled into the dust collection bowl. Some bowls have an automatic discharge valve; other bowls have to be removed from time to time to clean out the accumulated debris before its overflows into the filter housing.
The main filter element traps the remaining dust particles as the air is drawn through the paper pleats.
The smaller safety element is fitted after the main element in case the main element is damaged in any way. Its purpose is to trap any dust introduction when the main filter is changed. It does not filter as effectively, however, nor can it hold the same volume of dust.
Image: Air cleaner
Breathers are used to allow air to flow in and out of a component. As a components temperature increases in operation, the internal air expands with the increase in temperature with a resultant increase in pressure. If this air was not allowed to vent to the atmosphere, the build-up in pressure would force the air out at the weakest point, normally the seals. As the component cools down after operations, air is once again drawn in through the breather. If it is unable to draw in air, for example if the breather is blocked, then air will be pulled in past the seals with the danger that it may draw dust into the seal at the same time.
Seals prevent oil from entering or leaking out of a component wherever there is a danger of this happening. There are many different types of seals in use today, but as a rule of thumb “if oil can get out, dust can get in”. A leaking seal should always be replaced.
Images: Engine breather of turbocharged engine & front crackshaft oil seal.
End of part one...