Eine Studie in Südafrika hat ergeben, fehlende Schmiereigenschaft und Verunreinigung des Diesel-Kraftstoffes sind die Ursachen für defekte Injektoren und daraus folgenden Motorschäden.
NEW STANDARD FOR DIESEL UNDER NEGOTIATION
At von Wielligh, retired senior lecturer in
mechanical and aeronautical engineering
at the University of Pretoria, says newtechnology
engines are like racehorses that
require special feed and regular veterinary
care. Older engines meanwhile are like
carthorses that can eat any feed. Ultimately
though, it is the quality of the “feed” that
determines the damage to diesel engines,
which include score marks on pistons, heat
damage to engines and contaminated fuel,
which causes further damage.
In a recently presented paper, based on case
studies, Von Wielligh cites lubricity levels
and contamination as two significant causes
of engine failure. “A large number of engine
failures have recently occurred on these
modern diesel engines; these can be directly
blamed on the quality of the fuel used.
“Because of poor lubricity of the fuel, as well
as some particle contamination, injectors
failed prematurely, leading to poor combustion
and subsequent damage to the engine.
“Several failures were investigated and
eventually tests were conducted on the
lubricity and particle contamination of the
fuels used in these engines. The tests proved
that injector failures occur and engine failure
follows whenever the lubricity of the fuel is
lower than an accepted norm.
“Several cases were also studied in which
particle contamination of the fuel occurred
and this, in turn led to injector failure and
subsequent engine damage,” he says. Melted
pistons, as well as score-marks on the side of
the piston are caused by needle dribble and
poor spray pattern, as the damaged needle
gets sticky. This results in the fuel burning
directly on the piston. The reason the fuel
burns the piston is because the temperature
of the compression above the piston is
1500°C, while aluminium melts at 560°C.
Usually this is not a problem as stagnant air
above the piston protects the piston, but fuel
dripping from the jet erodes this air. Once the
protective layer of air has been eroded, the
piston starts to melt. The lubricity level of fuel
can be measured by a high-Frequency linear
oscillatory Friction and wear test machine in
the tribology laboratory of the department of
chemical engineering, says Von Wielligh.
Another method of testing is by using the
high frequency reciprocating rig (HFRR). “If the
lubricity of the fuel is above 700 newton on
the SRV, then there is no problem and the fuel
is a good enough lubricant.” Anything below
that figure will inevitably cause injector failure,
he says. Where the droplets from a spray
pattern are bigger than the fine mist they
should be, the fuel does not burn completely.
The droplets then reach the cylinder liner and
have the ability to thin out the lubrication film
on the liner. “This usually results in dilution
of the oil film and subsequent dilution of the
lubrication oil in the crankcase. Because the
lubricating oil dilutes in the crankcase, the
viscosity drops substantially and the oil loses
its ability to carry the heavy loads of the
engine,” says Von Wielligh.
The end result is bearing failure (usually the
big-end bearings) as well as rapid wear of
piston rings. “
Several cases were investigated
where the wear pattern on the ring almost
resembled the same situation as when dust
was inhaled by the engine,” he says.
Currently, there are no specifications for
lubricity in the South African Bureau of
Standards’ (SABS) SANS 342 specifications for
diesel fuel. In addition, the only specification
on contamination is that the fuel must be
visibly clean; and the human eye can typically
see down to 40 micrometers.
Yet Von Wielligh points out that dust particles
than can pass through a two-micrometer filter
can damage the injection parts of a diesel
engine. The present standard is therefore
not strict enough. All of this is currently
under negotiation at the SABS, and more
stringent specifications will hopefully soon
be agreed upon, says Von Wielligh. The SABS
specifications are likely to specify an HFRR to
determine the level of lubricity. This machine
works on the same principle as the SRV
machine except that it runs with a constant
load and it measures over a two-hour period.
Unfortunately, there are only two of these
almost R1-million machines available in the
country. Also stricter specifications do not
necessary mean that all fuel will immediately
meet these requirements, as they are ‘agreed
upon’ specifications. Essentially, consumers
will only have recourse in a court of law on a
contractual basis if the specifications are not
met. This is why several big companies now
go out on tender on the basis of fuel quality,
forcing suppliers to ensure that the supply
chain is faultless says Von Wielligh.
This is where most of the contamination
problems arise. Unscrupulous dealers also
sometimes add paraffin to diesel, earning
themselves an extra R1 profit per litre
and doing irreparable damage to engines.
The paraffin lowers the lubricity level, as
determined by the SRV machine, often
resulting in injector and engine failure. “It
is a question of policing, but policing the
supply chain is difficult.” Policing a chain
with as many links as the fuel supply chain
is a complex task. “
© Engineering News
by Nicola Mawson
> At von Wielligh
