This invention relates to internal comnustion engines and m particular to a cooling system for an internal combustion engine.

In a conventional water-cooled engine, the water-based coolant is pumped through the cylinder block and fed to an air-cooled heat exchanger in the form of a radiator before being returned to the engine at a lower temperature. Typically, during normal operation, the temperature of the coolant is reduced during its passage through the radiator by approximately 10°C, the coolant entering the radiator at approximately 110°C and leaving it at approximately 100°C. Such temperatures are required to ensure that the engine operates efficiently without damage to the cylinder block structure.

Sometimes a need arises for coolant at a distinctly lower temperature than that required to cool the engine such as for the cooling of the fuel supplied to the engine, the engine oil or transmission oil.

A particular need arises in the case of diesel fuel which is being supplied by an electronic unit injector to the engine as such injection systems result n considerable heat: transfer to the fuel. Without the provision for some form of cooling, the fuel would, in such circumstances, become unacceptably hot . The temperature of the coolant

that is returned to the engine is normally too high to be of use in cooling the fuel in such circumstances.

Although a separate fuel cooling system can be provided this would require its own pump and radiator system for cooling the cooling water and a fan to ensure sufficient air flow when the vehicle is stationary or travelling at low speeds . Such a system therefore adds to the complexity and cost of the engine package which is important if the engine is to be used in a motor vehicle.

It is an object of the present invention to provide a cooling system for a water-cooled engine having two cooling circuits one of which is capable of providing a supply of coolant at a lower temperature than that required for engine cooling purposes.

According to a first aspect of the invention there is provided an engine and a cooling system therefor in which the cooling system comprises a primary cooling circuit having an air-cooled radiator for cooling the primary coolant for the engine, a pump to circulate primary coolant through the engine to the radiator and back to the engine, a thermostat to regulate the flow of primary coolant within the primary circuit and a secondary cooling circuit having a fluid to fluid heat exchanger for cooling a second liquid, an air cooled heat exchanger for cooling a secondary coolant used to cool said second liquid and means to circulate said secondary coolant through said fluid to

fluid heat exchanger wherein the pump is used to circulate coolant through both primary and secondary circuits, said secondary coolant being extracted from the primary cooling circuit at a position prior to entry to the engine and being returned to the primary circuit upstream of the pump.

Advantageously, the secondary coolant may be extracted from the primary circuit at a position between the pump and a coolant inlet means to the engine.

Preferably, the air cooled radiator of the primary cooling circuit and the air cooled heat exchanger of the secondary cooling circuit may be formed as a single heat exchanger unit with separate flow paths therethrough for the primary and secondary coolants .

The air cooled heat exchanger may be a multipass heat exchanger causing the secondary coolant to flow two or more times through the cooling air before being returned to the fluid to fluid heat exchanger.

The secondary coolant may be returned to the primary circuit at a position between the thermostat and the entry position to the pump.

The secondary circuit may have a flow restrictor to slow the passage of the secondary coolant through the air cooled heat exchanger.

The flow restrictor may be included as part of the liquid to liquid heat exchanger.

The secondary circuit may have a thermostat to regulate the flow of secondary coolant rejoining the primary circuit.

Preferably, the thermostat may be arranged to directly sense and is regulated by the temperature of the secondary coolant.

Advantageously, the thermostat is included as part of the fluid to fluid heat exchanger.

The fluid to fluid heat exchanger may be a contra-flow multitube heat exchanger having a number of heat exchange tubes within a chamber.

The secondary coolant may pass through the tubes of the multi-tube heat exchanger and the second fluid may flow around the tubes within said chamber.

The fluid to fluid heat exchanger may be a fuel cooler for the engine.

The fluid to fluid heat exchanger may have a plastic end cap defining a return flow path from a heater circuit, a return flow path from a header tank associated with the primary cooling circuit and a flow path for the secondary coolant rejoining the primary coolant flow.

According to a second aspect of the invention there is provided a fluid to fluid heac exchanger for use in an engine cooling circuit in accordance with said first aspect of the invention in which the heat exchanger comprises a contra-flow multitube heat exchanger having a number of heat exchange tubes within a chamber formed by a cylindrical wall.

The secondary coolant may pass through the tubes of the multi-tube heat exchanger and the second fluid may flow around the tubes within said chamber.

The fluid to fluid heat exchanger may be a fuel cooler for an engine.

The heat exchanger may have a thermostat separated from said chamber by a plate to sense the temperature of the fluid passing through the tubes.

The thermostat may be located within a plastic end cap at the exit end of the heat exchanger.

The thermostat may have a temperature sensitive body which reacts directly against a seat formed in said end cap.

The seat within the end cap may be in the form of an elastomeric seal attached to the inner surface of the end cap.

The thermostat may have a shaft extending therefrom for reaction against an abutment on the end cap.

The invention will now be described by way of example with reference to the accompanying drawing of which: -

Fig.l is a schematic diagram showing part of a cooling system according to a first aspect of the invention with an indication of the relative flow rates and temperatures ;

Fig.2 is a schematic diagram showing a cooling circuit according to said first aspect of the invention; and

Fig.3 is a scrap cross-section through a heat exchanger according to a second aspect of the invention.

With particular reference to Fig.l there is shown an air and liquid heat exchanger in the form of a radiator 10 forming part of a cooling system for an engine.

The cooling system comprises the radiator 10 for cooling hot liquid coolant coming from the engine through an inlet 12. The coolant entering through the inlet 12 passes into an inlet tank 14 forming one end portion of the radiator 10. Coolant passes from the inlet tank 14 through a multi-path main cooling section 16 to an outlet tank 18 at the opposite end of the radiator and then exits the radiator via an outlet 20 and is fed back to the engine via a thermostat (not shown) . The coolant is pumped around the

first or primary circuit so formed by an engine-driven pump (not shown) .

During passage through the radiator the temperature drop in the coolant in the primary circuit, between the inlet 12 and the outlet 20, is typically about 10°C, the coolant entering the inlet tank 14 at approximately 110° and leaving the outlet tank 18 at a temperature of around 100°C.

A secondary cooling circuit provides coolant to cool the fuel supplied to the engine.

The secondary circuit comprises a liquid to liquid heat exchanger in the form of a fuel cooler 22 which is provided with coolant taken from the engine cooling system but reduced to a lower temperature than the coolant leaving the outlet 20, as will next be described.

The radiator 10 is split to provides a first flow path comprising the main cooling section 16 through which approximately 98% of the hot coolant entering the inlet tank 14 passes through to the outlet tank 18 and leaves the outlet 20 as a first flow of cooled coolant and a second flow path for a second flow of coolant accounting for the remainder of the coolant entering the radiator 10.

The second flow path comprises of a double pass heat exchanger formed by two heat transfer passages 26,28 which

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are connected in series between the inlet tank 14 and a second outlet 24.

By making the coolant in the secondary path make two passes in opposite directions through the radiator the second flow path is a low flow path giving a relatively long dwell time for the coolant in the radiator. In addition a restriction of some form is included as part of the secondary cooling circuit to ensure that the flow of the fluid through the passages 26,28 is relatively slow relative to the speed of the flow through the cooling section 16. This may be achieved by restricting the flow as it enters the passage 26 or by other means as will be described hereafter.

The passages 26 and 28 are formed as part of the structure of the main cooling section 16 but are separated therefrom by separation plate between the outlet tank 18 and the first passage 26.

By providing this slow passage through the passages 26,28 the temperature of the coolant exiting via the outlet 24 is at a lower temperature than that at which the first coolant flow leaves the radiator through the first outlet 20. Although the temperature exiting the second outlet 24 will be determined by various parameters it is generally possible to obtain an outlet temperature of approximately

The second flow of coolant leaving the second outlet 24 is passed to and through the fuel cooler 22, typically leaving the fuel cooler at a temperature of around 100°C before being returned to a position adjacent to the first radiator outlet 20 to be combined with the first coolant flow being returned to the engine.

The fuel is passed through the fuel cooler 22 through a separate passage to that through which the secondary flow of coolant passes before being returned to a fuel supply for the engine.

To prevent thermosyphoning in cold running conditions, when the cooling system thermostat is still closed and the radiator flow is unpumped, the fuel cooler 22 is preferably positioned above the second flow path through the passages 26,28 in the radiator 10.

With particular reference to Figures 2 and 3 there is shown an internal combustion engine 30 and two interlinked but distinct cooling circuits. The first or primary cooling circuit comprises of an air cooled radiator 35, a bypass and thermostat assembly 33, a coolant pump 31, a coolant inlet means in the form of a manifold 38 and an expansion tank 36.

In use the coolant pump 31 pumps coolant through a supply passage 43 to the manifold 38 which is connected to coolant passages "R" (shown by a dotted line on Fig.2) to

cool the engine 30. The heated coolant is then transferred by means of a pipe 40 to the inlet end of tne air cooled radiator 35 n which it s cooled by the passage of air through a matrix forming part of the air cooled radiator 35. The cooled primary coolant is then transferred by means of a pipe 41 to the combined bypass and thermostat assembly 33 before being returned to the coolant pump 31 by means of a pipe 42.

The expansion tank 36 is connected via a pipe 46 to the air cooled radiator 35 to allow for the expansion of the primary coolant as it is heated from a normal ambient temperature to its normal operating temperature.

As is usual a bypass passage 49 is connected between the supply pipe 40 to the air cooled radiator 35 and the combined bypass and thermostat assembly 33 to allow for the circulation of some primary coolant through the engine 30 prior to opening of the main thermostat so as to assist with rapid warm up of the engine 30.

A secondary cooling circuit is also associated with the engine 30, the secondary cooling circuit comprising an air to liquid heat exchanger 37 and a fluid to fluid heat exchanger in the form of a fuel cooler 34.

Secondary coolant for the secondary cooling circuit is extracted from the primary cooling circuit at a position upstream from the coolant pump 31 but prior to entry to the

cooling passages "R" in the engine 30. In this case the secondary coolant is extracted from the manifold 38 and is then transferred by means of a pipe 44 to the air to liquid heat exchanger 37 in which it undergoes two passes through the cooling air stream before being supplied by means of a pipe 45 to the inlet end of the fuel cooler 34.

The fuel cooler 34 is of the contra-flow parallel tube heat excnange type and has a number of longitudinally extending heat exchange tubes 71 held in position by two end plates 70, 72 sealed within a chamber 61 defined by a cylindrical wall 60 forming the body of the fuel cooler 34. At one end the cylindrical wall 60 is swaged over to retain in position a first plastic end cap 73 n which is located an apertured plate 49 forming a flow restrictor for the secondary cooling circuit. At the other end of the fuel cooler 34 the cylindrical wall is swaged over to retain in position a second plastic end cap 62 which defines a secondary coolant thermostat housing and provides a junction for reuniting the flow from several parts of the cooling circuit.

, A thermostat "T" is located within the second end cap 62 and has a temperature sensitive body 66 urged by a spring 67 towards a seat in the form of an elastomeric seal 69 attached to part of the inner surface of the end cap 62. A split plate 68 is secured in the bore of the second end cap 62 to provide an abutment against which the thermostat spring 67 can react. A shaft 66a extends from the

temperature sensitive body 66 and reacts against a abutment formed in the end cap 62.

During normal operation secondary coolant enters the fuel cooler 34 via the first end cap 73 passes through the heat transfer tubes 71 and enters the space surrounding the temperature sensitive body 66 of the thermostat "T" .

In this way opening and closing of the thermostat "T" is controlled directly by the temperature of the secondary coolant and not by the temperature of the second fluid in the form of fuel passing through the fuel cooler 34.

The fuel to be cooled is supplied to the fuel cooler by means of a supply pipe 51 which is connected to part of the fuel system (not shown) of the engine 30. The fuel enters the fuel cooler 34 through an inlet port 75 whereupon it enters the chamber 61 surrounding the heat transfer tubes 71 and then flows in a contrary direction to the direction through which the secondary coolant flows through the tubes 71 to exit the chamber 61 via the outlet port 76. The cooled fuel is then returned by means of a return pipe 50 to the fuel system (not shown) associated with the engine 30.

The second end cap 62 defines three additional flow passages 63,64,65 and so acts as a juncture for several fluid flows.

The first flow passage 63 has, in use, a pipe 53 connected thereto forming a flow path for the secondary coolant rejoining the primary coolant flow. The pipe 53 is connected to the return pipe 42 from the combined bypass and thermostat assembly 33 to the coolant pump 31. The second passage 65 forms part of a return flow path from the header tank 36 and is connected thereto by a pipe 55.

The third passage 64 is connected in use to a return pipe 48 from a heater assembly 32.

The heat exchange media for the heater assembly is obtained through a pipe 47 connected to the pipe 40 extending between the engine and the input side to the air cooled radiator 35. In this way heat exchange media of a high temperature is obtained as it is taken from a position where it has just exited the normally hot engine 30.

In use primary coolant is circulated around the primary circuit by the coolant pump 31 in the normal manner to effect cooling of the engine 30. The operation of such a cooling circuit and an associated heater circuit is well known in the art and will not be described herein in detail.

In respect of the secondary cooling circuit with the engine 30 being started from cold the thermostat "T" will be closed and no secondary coolant will flow through the secondary circuit. As the temperature of the fuel passing

through the chamber 61 increases it will increase the temperature of the secondary coolant in the tubes 71 and also in the space surrounding the temperature sensitive body 66 of the thermostat "T" .

When the temperature of the secondary coolant reaches a predetermined temperature the thermostat "T" will open, the temperature sensitive body 66 moving away from the resilient seal 69 against the action of the spring 67. In this condition secondary coolant will begin to flow through the secondary cooling circuit from the manifold 38 through the air to liquid heat exchanger 37 and then into the fuel cooler 34. However due to the presence of the flow restrictor 39 and the fact that the secondary coolant undergoes two passes through the air to liquid heat exchanger 37 the coolant supplied to the fuel cooler 34 via the pipe 45 is considerably cooler than that returning to the engine from the air cooled radiator 35. This is important because it is necessary to keep the temperature of the fuel below approximately 80°C so that it operates efficiently.

The presence of the thermostat "T" in the secondary circuit and its ability to directly sense the temperature of the secondary coolant is important particularly when the engine 30 is first started from cold. This is because when the engine 30 is cold the bypass and thermostat assembly prevents flow through the radiator 35 and through the bypass passage 49 so that the temperature of the engine 30

can be brought rapidly to its normal operating temperature. In practice there is normally a very small leakage through the- bypass side of the bypass and thermostat 33 to prevent the engine from being starved of coolant.

It will therefore be appreciated that the admittance of any coolant from the secondary circuit during start up is to be avoided as the temperature of the secondary coolant is generally. If the temperature of the fuel is directly used to effect opening of the thermostat then because the temperature of the fuel increases rapidly there is the potential for cold coolant from the secondary circuit being admitted prematurely to the primary circuit thereby slowing engine warm up. However by using the temperature of the secondary coolant a slight time delay is introduced due to the time it takes for the fuel to heat the coolant in the pipes 71 and the time it takes for that that increase in temperature to effect opening of the thermostat "T" . In addition because it is the temperature of the coolant that is sensed the system is self regulating even under normal running conditions for if the secondary circuit admits too much cold coolant to the primary circuit this will result in the temperature of the primary coolant falling which is undesirable beyond certain limits. However because the secondary circuit then begins to admit colder coolant from the primary circuit and further cool it this will very rapidly result in the thermostat "T" sensing the consequential reduction in temperature and closing the

return path to the primary circuit. In this way the secondary circuit cannot cause serious overcoolmg of the engine 30.

By extracting the coolant for the secondary cooling circuit at a position prior to entry to the engine 30 the coolant entering the secondary coolant circuit is not only at virtually the lowest possible temperature to be found m the primary circuit it is also at virtually the highest possible pressure. Less cooling s therefore required to obtain the lower temperature required by the fuel cooler 34 and the flow rate through the air to liquid heat exchanger 37 can be optimised by means of a restrictor 39 which would not be possible if a lower pressure is used.

It will be appreciated that although the invention has been described w th reference to its use to cool fuel it could be used for other uses such as for cooling oil or any other system where a temperature lower than that used for engine cooling purposes is required.