Cooling System; Thermostat - Kubota WSM 07-E3B Series Workshop Manual

07-E3B SERIES, WSM

4. COOLING SYSTEM

[1] THERMOSTAT

Conventional thermostatically-controlled
(outlet water temperature control type) open against the
flow of coolant. In this design, the pressure (steam
pressure + water pump's discharge pressure) affects the
open/close performance of such valve. In other words,
the valve may be delayed in opening at a preset opening
temperature opening suddenly, above the preset
temperature. This is called the overshoot phenomenon.
The overshoot problem invites the undershoot
phenomenon too. Too much water cooled by the radiator
flows through the water passage, which suddenly closes
the valve below the thermostat's preset valve closing
temperature.
A repeated cycle of such overshoot (3) and
undershoot phenomena is called the water temperature
hunting. This hunting problem may adversely affect the
cooling system parts, and also the engine and its related
components.
To cope with this trouble, the 07 series engine is
equipped with the flow control thermostat. The valve has
a notch (4) to control the coolant flow-rate smoothly in
small steps.
(1) Coolant Temperature
(2) Time
(3) Overshoot
(4) Notch
M-9
ENGINE
valves
(A) Valve Lift Versus Flow-rate
(B) Flow-rate
(C) At Short Valve Lift
(D) At Medium Valve Lift
(E) At High Valve Lift
(F) Valve Lift
W1013684
KiSC issued 06, 2020 A