Mercury MerCruiser D2.8L D-Tronic Service Manual page 807

SERVICE MANUAL NUMBER 22
Description
Turbocharger
All diesel engines covered in this manual are equipped with a turbocharger to boost intake
pressure resulting in increased horsepower. In one casing on the turbocharger housing
exhaust gases are used to spin the turbine up to 100,000 rpm. The compressor, which is
installed on the same shaft but in a separate casing, draws in filtered air, compresses it, and
delivers it to the engine through an intake duct and intercooler.
The turbo bearings are lubricated by engine oil.
Coolant from the engine's closed cooling system cools the turbocharger housing, while the
exhaust elbow (or exhaust riser) is cooled by seawater flowing through it from the seawater
system.
Turbocharger pressure is limited by a boost pressure control valve (commonly called a was-
tegate) is mounted on the combined exhaust / intake manifold.
Wastegate
When used in combination with the turbocharger a higher torque output even at low engine
speeds is obtained and allows the engine a wider operating speed range.
The function of the wastegate is to limit the boost pressure generated by the turbocharger
within a controlled tolerance band.
When factory set boost pressure is exceeded, the valve opens and bypasses a part of the
exhaust gas flow around the turbine. The resulting reduced mass-flow produces a lower
power output. The compressor output is reduced in proportion and the boost pressure falls
to the predetermined level. This control process is repeated for each change in engine load.
This allows the use of a smaller turbocharger turbine which provides better acceleration and
torque, as boost comes on more rapidly and turbo lag is reduced.
90-860074--1 FEBRUARY 2002
TURBOCHARGER
Page 7C-5