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Additional recommended knowledge
Since that time several universities and research institutions have involved themselves in the determination of the mechanisms of wear and lubrication and the result is the knowledge of boundary lubrication whith numerous references for instance 
It is boundary lubrication, where the meaning of Lubricity should be found. It is not a mistical property. It is due to chemical action between the lubricant or some components of it and the rubbing surfaces in the physical conditions there, due to which a tenatious layer develops, which is able to support the load that the bulk lubricant alone by virtue of its viscous resistance could not.
The case of the high speed diesel engine f.i.e.
Originally the operating speed of the diesel engines (those in which ignition occurs due to the heat of compression of the air into which the fuel is injected) was a lot lower than that of spark ignition engines.
The modern high speed diesel engines, widely used in road transport, though following the same principles, require highly sophisticated fuel injection equipment or f.i.e. (pumps and injectors or pump-injectors, or unit injectors) to inject the fuel, with accurate metering into the cylinder air space. These have to be manufactured with extremely close tolerances with respect to the distances between moving parts (so that the proportion of the pumped liquid which is lost by backleak is minimised), and the injection timing as well as the rate of injection has to be closely controlled with a specialised equipment called a governor. All this at varying and sometimes very high speeds.
The tolerances have to be preserved if operability is to be maintained, that is, they have to be lubricated. The lubricating oil must not have a high viscosity, because that would hinder operation and it was found that up to recent times petroleum based diesel fuels contained enough chemically active components (e.g. sulphur compounds) to carry out the job of boundary lubrication and no external lubrication was employed.
In modern road service it is an important requirement from the diesel engine that the SO2 content of its exhaust emission is minimised, because this is a harmful air pollutant and it causes what is called acid rain. For this aim a refining process which is called hydrotreatment is employed, which removes the sulphur, but with that it also removes those compounds which are beneficial for boundary lubrication, so the lubricity of the fuel becomes worse.   
One solution is the addition of a lubricity improver additive, which is usually done at the final distribution point of bulk sale (bulk fuels terminals) just prior to entering the chain of commerce (the gas station).
|This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Lubricity". A list of authors is available in Wikipedia.|