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ntrol engine mperature (2)

12th February 1983
Page 65
Page 65, 12th February 1983 — ntrol engine mperature (2)
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Which of the following most accurately describes the problem?

THERMOSTAT controls the ine's temperature. It does so Ireventing coolant ulating through the radiator ;ri the engine is cool, and wing it to circulate when a letermined temperature is ;hed.

iere are two kinds of mostats in common use, the bellows type and the wax element type. It is the latter variety which will be examined in this article; the bellows type will be discussed later. I am indebted to Western Thomson Controls, whiuh specialises in the wax thermostat, for permission to reproduce the sketches which illuszrate this article and for much of the technical information used in its preparation.

Figure 1 shows the construction of the wax element. It consists of four main parts: the capsule body, the wax, a synthetic rubber boot and the piston or stem. The wax is contained between the rubber boot and the outer casing. Inside the rubber boot is the stem which is tapered at its lower end. When subjected to heat, the wax changes from a solid to a liquid and increases considerably in volume. This change results in the rubber sleeve being deformed in a manner which forces the stem out in much the same way as an orange pip is squeezed between the thumb and finger. This movement opens the valve allowing the coolant to flow through the radiator.

During normal driving in average weather conditions, the thermostat is open only a very small amount. It rarely ever reaches the fully open position.

A bypass is provided to allow partial coolant circulation around the engine cylinder block and head during the warm-up period when the thermostat is closed. Figure 2 shows a thermostat in the open and closed position and how the coolant passes through the bypass when the thermostat is closed. Even when it is fully open, there is always some flow through the bypass.

Figure 3 shows a refinement of the permanent bypass. Here, it will be seen, there is maximum circulation around the engine block and head during warm up and when the thermostat is partly open. When it is approaching its fully open position, the bypass is closed to allow full coolant flow through the radiator.

The wax employed is of the microcrystalline, paraffinic type; the risk of corrosion is obviated by the adoption of stainless steel for the stem and for the return spring.

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