2.1.5
Friction Modifiers
Desulphurisation of modern fuels has instigated the need for use of friction
modifiers (FM). Low sulphur levels are mandated in gasoline and diesel fuels both,
with Ultra Low Sulphur Diesel consisting as little as 15 ppm or less of sulphur [90].
However, it is argued that sulphur itself does not directly contribute towards the
lubricant properties of the fuel. Wei et al. [91] describe a study which concluded that
the drop in fuel lubricity results from removal of oxygen-containing and other polar
materials that are inevitably removed as bi-products during the desulphurisation
process. In vehicle fuel delivery systems, pumps and injectors rely nearly solely on
fuels for lubrication. However, hydrocarbon fuels have a low viscosity and consist of
2.1 Fuel Additive Review
38
nearly no polar components resulting in poor hydrodynamic and boundary layer
lubricating properties.
Whilst in some heavy duty diesel machinery significant efficiency gains could
be experienced as a result of FM use, in gasoline engines, where fuel pressures only
reach a fraction of the pressure, key benefits lie within reliability and extended life
expectancy of fuel pumps and injectors. Gustavsson et al. [92] showed with different
fuels that factory coatings used in high pressure fuel system components exhibit good
to excellent lubricating characteristics, but under sufficient contact pressure can wear
out in as little as 10,000 piston strokes.
As described by Williams [93], hydrodynamic lubrication is where a viscous
film is created between two surfaces, keeping them apart completely. Friction can be
expressed as:
𝜇 = 𝑘√
𝐿𝑈𝜂
𝑊
2.1
where W/L stands for the specific load, U the relative sliding speed of the given two
surfaces, η is the Newtonian viscosity of the liquid and k a constant that depends on
the surface geometry of the given surfaces. Reducing speed or increasing specific load
would enable lower friction coefficient but to a limit, after which the viscous film
becomes unsustainable, reaching thicknesses of only a few molecules. Although
remarkable progress has been experienced in engine tribology and possible lubricant
film thicknesses can be as little as 1 x 10
-7
m, sliding surfaces touching each other can
eventually occur [94].
This leads to boundary lubrication mode. Williams [93] states that fuel
properties such as viscosity and density play little role in this mode while significance
of chemical composition rises. Fuels themselves are unable to support lubrication in
this mode, with an increase in friction coefficient as much as 100 times and hence FM
additives are used [94]. Their mode of action is very similar to that of lubricant oil
additives and aims to provide a very fine film on all surfaces with film thicknesses
about 0.0025 µm. According to Totten et al. [35], FM can interact with available
surfaces through physical absorption or chemical reaction.
In case of physical absorption, the polar head groups associate themselves with
metal surfaces and keep their structural integrity. In case of chemical reaction, new
molecules are created on the metal surfaces, changing the structure of the additive as
2.1 Fuel Additive Review
39
well as the surface on which the reaction takes place. The hydrocarbon chains of the
additives extend into the fuel and also associate with each other which results in a fine
but strong film on the metal surfaces.
Do'stlaringiz bilan baham: |