NL2011321C2 - Modification of a thermostat to prevent thermal cycling. - Google Patents

Description

Title: Modification of a thermostat to prevent thermal cycling

The invention relates to a modification of a thermostat of a fluid cooled internal combustion engine to prevent or reduce thermal cycling.

Introduction: 5 A combustion engine generates heat. To control the temperature a cooling system is used. A cooling system normally consists of components as water pump, radiator, fan, thermostat, several, hoses, reservoir tank.

10 Thermal cychng:

Depending on a combination of variables like ambient temperature, coolant fluid temperature, thermostat characteristic, delay time in the system, coolant flow rate, radiator dimensions, etc. a situation of thermal cychng 15 may occur. Thermal cycling (or temperature cycle) is defined as a situation where the temperature at a location in the system e.g. the radiator shows a cycling between two extreme temperatures, typically at high frequencies of change when compared with the heat emissions of the engine related to the varying engine load (see figure 1 for an example of this phenomenon). Fig. 1: 20 time plot of thermo cycling at constant vehicle speed.

Mechanical failures resulting from cyclical thermo mechanical loading are detected as fatigue. Temperature cycling accelerates fatigue failures.

25 Accordingly it is an object of the present invention to propose an improved thermostat. In a more general sense it is an object of the invention to overcome or ameliorate at least one of the disadvantages of the prior art. It is also an object of the present invention to provide alternative structures which are less cumbersome in assembly and operation and which moreover 2 can be made relatively inexpensively. Alternatively it is an object of the invention to at least provide the public with a useful alternative.

To this end the invention provides as a solution a thermostat for opening and closing a fluid passage of a heat exchange system, the thermostat 5 including: a housing; a closing member movable with respect to the housing for opening a passage in the housing; a thermal expansion element for moving the closing member; and a fluid flow guiding element associated with the closing member for fluid flow reduction upon initial opening of the closing member. Such a thermostat will gives a steep opening characteristic.

10

Advantageously a flow restriction may be created by adding a ring to a thermostat plate of a thermostat. Alternatively the flow restriction (or threshold) may advantageously be integrated in the thermostat plate by an adaptation to the mould for forming the thermostat plate.

15

Further advantageous aspects of the invention will become clear from the appended description and in reference to the accompanying figures, in which:

Figure 1 shows a time plot of a phenomenon referred to as thermal cycling 20 at constant vehicle speed;

Figure 2 shows an initial concept of thermostat according to the invention with its thermostat plate lifted from its housing sleeve that will gives a steep opening characteristic;

Figure 3 shows a ring element attached to the thermostat plate of the 25 thermostat of Figure 2;

Figure 4 shows the thermostat of Figure 2 in its closed position;

Figure 5 shows a detail of the ring for adding to the thermostat plate;

Figure 6 shows a detail of an alternative flow restriction (threshold) integrated in the thermostat plate by an adaptation to the mould; and 3

Figure 7 shows a variation of the threshold ring, which similar to the initial test sample of Figures 3 and 5 is creates the flow restriction by adding a ring to the thermostat plate.

5 The concept of thermostat as shown in figure 2 gives a steep opening characteristic. For the example thermostat on the picture, the initial increase of coolant flow for each mm of opening due to the circumference length of the thermostat valve, is massive. This is equivalent to a high gain controller in the cooling system. The valve makes changes to the coolant 10 flow that are too large and overshoot the target. The result is an oscillating system around a set point. This typically happens when the need for cooling is small and the capacity of the cooler (radiator) is far more than needed at that moment. Figure 2 a thermostat with its thermostat plate lifted from its housing sleeve.

15 A solution is to dampen out the oscillation (thermal cycling) by smoothening the characteristic of the thermostat by reducing the initial flow through the gap at opening of the thermostat.

20 For this purpose a small barrier (threshold) was created over the first few mm’s when the thermostat valve (sleeve) opens. This barrier is designed inside the valve and has a tight fit (few tenth of a millimetre) with the valve sleeve. This creates a flow restriction at opening of the valve. While further opening of the thermostat, this restriction is decreasing as the overlap of the 25 threshold barrier and the sleeve is reducing until the valve is fully open when the valve sleeve opens more than the height of the threshold. Important variables to adjust the valve characteristic at opening are the height and the curved shape of the threshold, the chosen fit between valve sleeve and threshold and the curved shape of the sleeve in the area of 4 overlap with the threshold. Figure 3 shows a ring element attached to a thermostat plate of a thermostat.

The advantage of this type of thermostat that closes fully without leaking, 5 remains unchanged. When fully open the effect of the threshold is minor relative to the total opening and so the reduction of the maximum flow through the valve is not limiting its function.

The threshold should be shaped in such a way that the sleeve is guided to 10 its close position. For that purpose some guiding cams can be positioned on the threshold barrier. Figure 4 shows the thermostat in its closed position.

In the initial test sample the threshold was created by adding a ring to the thermostat plate. See figures 3, 5 and 7. For a production solution this 15 threshold should preferably be integrated in the thermostat plate. This means an adaptation to the mould, as shown in figure 6.

This solution against thermal cycling can be used for situations with only one single thermostat or with two or more thermostats. When more than 20 one thermostats are applied the characteristic of the total set of thermostats can be optimized by applying this threshold on only one or on all thermostats as required for the characteristic of the total assembly (system).

Fig. 5: Detail of the ring for adding to the thermostat plate 25

Fig. 6: Detail flow restriction (threshold) integrated in the thermostat plate.

Hence is described a thermostat for opening and closing a fluid passage in a heat exchange system. The thermostat includes a housing that 30 defines the fluid passage for the heat exchange system. The thermostat further includes a closing member that is movable with respect to the 5 housing for opening the fluid passage, and a thermal expansion element for moving the closing member. According to the invention a fluid flow guiding element is associated with the closing member for reducing the fluid flow upon initial opening of the closing member.

5

It is thus believed that the operation and construction of the present invention will be apparent from the foregoing description and drawings appended thereto. It will be clear to the skilled person that the invention is not limited to any embodiment herein described and that modifications are 10 possible which should be considered within the scope of the appended claims.

Claims (3)

A thermostat for opening and closing a fluid passage for a heat exchanging system, the thermostat comprising: a housing; a closing member movable relative to the housing for opening the passage in the housing; a thermal expansion element for moving the closing member; and a fluid flow guiding element associated with the shut-off member to reduce the fluid flow after the first shut-off of the shut-off member. The thermostat according to claim 1, wherein the closing member is a thermostat plate, and wherein the fluid flow guide element is a ring element attached to the thermostat plate. 15 The thermostat according to claim 1, wherein the closing member is a thermostat plate, and wherein the fluid flow guide element is integrated in the thermostat plate.