Volvo Penta D2-55 Workshop Manual page 20

Design and function
18
Cooling system, general
The engine is fresh water cooled with a closed cooling
system. The system is divided into two circuits.
In the inner circuit, the fresh watersystem, coolant is
pumped around by a circulation pump, driven via a
belt from the crankshaft pulley.
The fresh water system works under pressure, thus
reducing the risk of boiling if the temperature becomes
high. If the pressure becomes excessive, a pressure
valve opens in the filler cap.
The coolant temperature is regulated by a thermostat.
The flow in the sea water system is accomplished by
a gear-driven impeller pump.
The heat exchanger transfers heat from the coolant to
the sea water.
As extra equipment, the engine can be equipped with
a separate expansion tank.
Thermostat
The engine is equipped with a thermostat whose sen-
sor body contains wax.
When the engine is cold, the thermostat closes the
way to the heat exchanger. Coolant then passes
through a by-pass pipe, back directly to the suction
side of the pump. As the engine warms up, the vol-
ume of the wax increases and the thermostat progres-
sively opens the passage to the heat exchanger, at
the same time as the by-pass channel is closed.
Please refer to the "Technical Data" chapter for open-
ing temperatures.
Sea water pump
The sea water pump is driven by the gears in the tim-
ing gear. The impeller (pump wheel) is made from rub-
ber and is replaceable.
NOTE! The impeller will be damaged if the pump is run
dry.