Yanmar 4LHA-DTP, Operation Manual

Page: 531 / 670

Chapter 6 Cooling Water System
6. Thermostat
6. Thermostat
4LHA Serie.
6-1 Functioning of thermostat
The thermostat opens and closes a valve according to
changes in the temperature of the fresh water inside the
engine, controlling the volume of water flowing to the heat
exchanger from the cylinder head, and in turn maintaining
the temperature of the fresh water in the engine at a con-
stant level.
The thermostat is bottom bypass type. It is located in a
position connected with the cylinder head outlet line at
the top of the top of fresh water pump unit.
When the fresh water temperature is low (75.0 ~ 78.0
o
C or
less), the thermostat is closed, and fresh water goes from
the bypass line to the fresh water pump intake and cir-
culates in the engine.
When the fresh water temperature exceeds the above
temperature, the thermostat opens, and a portion of the
water is sent to the heat exchanger and cooled by sea
water, the other portion going from the bypass line to the
fresh water pump intake.
The bypass line is closed off as the thermostat valve
opens, and is completely closed when the fresh water
temperature reaches 81.5
0
C (valve lifts 4mm (0.1575in.)),
sending all of the water to the heat exchanger.
Thermostat cover
Thermostat valve
From cylinder head
Impeller
To heat exchanger
Thermostat valve
From cylinder head
When valve is opened (by-pass passage is closed)
6-2 Thermostat construction
The thermostat used in this engine is of the wax pellet
type, with a solid wax pellet located in a small chamber.
When the temperature of the cooling water rises, the wax
melts and increases in volume. This expansion and con-
struction is used to open and close the valve.
6-3 Characteristics of thermostat
Opening temperature 71 "C
Full open temperature 85
0
C
Valve lift at full open 8 mm (0.31 in)
When valve is closed (by-pass passage is opened)
6-14