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Fully synthetic oils
and mineral oils do not mix. In severe cases they can coagulate.
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If the engine's oil
pressure is low, the turbocharger will be the first engine
component to fail.
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Piston blow-by and
the resulting crankcase/sump pressure if not vented off
adequately is the most common cause of turbocharger oil
leakage.
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The most common contaminants
found in the oil are 'free floating' carbon deposits, fuel
and the by-products of combustion.
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A typical diesel turbocharger
with rotor speeds in excess of 200,000 R. P.M. will have
a blade speed on the compressor wheel of 850 miles per hour.
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Many operators assume,
quite wrongly, that if they run an engine with dirty or
contaminated oil, the oil filter will remove any foreign
matter before it reaches the engine and more importantly
the turbocharger.
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Oil delay of a mere
four seconds will start to cause journal and thrust bearing
wear. Delay of just eight seconds can cause irreparable
damage. This damage will not necessarily manifest itself
immediately, the final failure may occur after several days.
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The average temperature
of the exhaust gas, at the entry point to a diesel turbo,
is 800 degrees centigrade. A petrol engine can reach 1000
degrees, glowing bright yellow. Hot enough to melt window
glass.
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New generation turbocharger
impellers rotate at up to 220,000 revs per minute. The impellers
on a Boeing 747 engine rotate at about 7,000 revs in comparison.
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The air entering the
compressor impeller of the turbocharger can be traveling
at a speed close to mach 1.
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At average engine
revs, a medium size turbocharger will swallow 130 cubic
feet of air per minute, equivalent to the interior volume
of a transit van.
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Turbo shaft balance
is crucial – imbalance at maximum revs equivalent
to a 2 kilogram force is acceptable. We often find turbos
supplied for service with 6 kilos of imbalance. This is
equivalent to driving along with a house brick attached
to your wheel rim.
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The “hot end”
turbine blades in a turbocharger, are made from a high nickel
content alloy, as used in jet aircraft engines. A blade
will travel in the region of 820 mph at average engine speed,
and the exhaust gas entering it will be supersonic.
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The turbocharger rotor will accelerate
from 20,000 revs per minute to over 150,000 revs per minute
in less than one second.