Waste gate valve. If
the turbocharged air pressure becomes too high, knocking occurs and
engine output actually decreases. To prevent this, the turbocharger uses
a waste gate valve. This valve allows a certain amount of exhaust gas to
bypass the turbine once the ideal boost is exceeded. The waste gate is
usually operated by an actuator that senses the air pressure in the
induction system. When the pressure becomes too high, the actuator opens
the waste gate valve. The flow of exhaust gas reaching the turbine
decreases. This reduces turbine and compressor wheel speed. The result
is a drop in turbocharged air pressure.
intercooler cools the turbocharged air before it reaches the combustion
chamber. Cooling the air makes it denser. It also lower the temperature
produced in the combustion chamber. These factors help reduce engine
knock and increase engine output. Intercoolers are like radiators in
that heat from the air passing through them is removed and dissipated to
the atmosphere. Intercoolers can be air or water cooled.
Most turbochargers are lubricated by pressurized and filtered engine oil
that is line-fed to the unit¡¯s oil inlet. The oil drains back to the
engine (by gravity) through a separate line. The lubrication system is
critical to the center bearing of a turbocharger. This bearing has the
high heat of the exhaust on one side of it and the oil tends to keep it
cool as well as lubricate it.
A turbocharger should never be operated under load if
the engine has less than 30 psi oil pressure. A turbocharger is much
more sensitive to a limited oil supply than an engine, due to the high
rotational speed of the shaft and the relatively small area of the
bearing surfaces. Low oil pressure and slow oil delivery during engine
starting can destroy the bearings in a turbocharger. During normal
engine starting, this should not be a problem. There are, of course,
abnormal starting conditions. Oil lag conditions will most often occur
during the first engine start after an engine oil and filter change.
Before the engine is put under load and the turbo activated, the engine
should be run for 3 to 5 minutes at idle to prevent oil starvation to
the turbo. Similar conditions can also exist if an engine has not been
operated for a long period of time. Engine lube systems have a tendency
to bleed down. Before allowing the engine to start, the engine should be
cranked over until a steady oil pressure reading is observed. This is
called priming the lubricating system. The same starting procedure
should be followed in cold weather. The thick engine oil will take a
longer period of time to flow.