Cooling System with Isolated Cooling Circuits

Abstract

A cooling system for an internal combustion engine, comprising a first, high temperature, cooling circuit coupled to an engine for cooling the engine, where a first coolant circulates in the first circuit; and a second, low temperature, cooling circuit coupled to a plurality of devices for cooling the plurality of devices, where a second coolant circulates in the second circuit, the second cooling circuit includes a plurality of radiator segments, the coolant circulating in the second cooling circuit may exit at different radiator segments of the plurality of radiator segments producing coolant streams of different cooling temperatures for cooling different devices of the plurality of devices.

Description

BACKGROUND AND SUMMARY

Internal combustion engines, in particular heavy duty and turbocharged diesel engines, may generate tremendous amounts of heat during combustion under some conditions. To address overheating of engine oil, cylinder walls, pistons, valves, and other components, various cooling systems have been applied. In one example system, WO 2005040574 provides a cooling system for an internal combustion engine that includes a first flow circuit that operates at a higher temperature and pressure range and serves to cool the engine and vehicle cab, and a second circuit that operates at a lower temperature and pressure range and primarily serves to cool various components including the gearbox, EGR, and charge air. The two flow circuits are interconnected via passages equipped with various one-way valves that open in the direction of the first flow circuit. The two cooling circuits allegedly reduce the likelihood of cavitation caused by large pressure drops in the coolant circuit.

However, such a system may not sufficiently address the individual cooling temperature demands for various components in the second, lower temperature, cooling circuit, since the radiator of the low temperature flow circuit only cools the coolant circulating in the low temperature flow circuit to a single temperature.

To address the above mentioned issue, a cooling system for an internal combustion engine may be used, the system comprising a first, high temperature, cooling circuit coupled to an engine for cooling the engine, where a first coolant circulates in the first circuit; and a second, low temperature, cooling circuit coupled to a plurality of devices for cooling the plurality of devices, where a second coolant circulates in the second circuit, and where the second cooling circuit includes a plurality of radiator segments, the coolant circulating in the second cooling circuit exiting at different radiator segments of the plurality of radiator segments producing coolant streams of different cooling temperatures for cooling different devices of the plurality of devices.

In this way, it may be possible to efficiently utilize two cooling loops while also tailoring the cooling of a plurality of devices in the low temperature circuit to the particular conditions of each individual device. In this, operation and efficiency of the individual devices may be improved. For example, heat exchangers may be optimized for minimum size and fan power may be optimized to accomplish the needed heat rejection and coolant temperatures.

In another example, a method for cooling an internal combustion engine of a vehicle may be used. The method may comprise circulating coolant through a first cooling circuit thermally coupled to combustion chambers of the engine, the first cooling circuit including at least a radiator; circulating coolant through a second cooling circuit thermally coupled to a plurality of devices, the second cooling circuit including a plurality of radiator segments, where the coolant is pumped via a pump coupled upstream of a radiator and downstream of the devices, where the coolant of the first cooling circuit does not mix with coolant of the second cooling circuit; distributing coolant in the second circuit to the plurality of devices at different temperatures via the plurality of radiators in the second circuit; and flowing a common stream of cooling air over at least the radiator in the first cooling circuit and at least one of the plurality of radiator segments in the second cooling circuit

By optionally maintaining two separate cooling circuits where the coolant streams do not mix, it may be possible to reduce heat transfer between the two cooling circuits. Further, by distributing coolant streams of different temperatures, as noted above, it may be possible to individualize the coolant temperature used for cooling a particular device, thereby improving cooling efficiency and performance of the device. Further still, by utilizing a common stream of cooling air over and to cool radiators in both the first and second coolant circuits, a more compact system design may be achieved. In addition, in the example of including successive ordering of the plurality of radiator segments in the second cooling circuit, it may be possible to further increase efficiency of the second cooling circuit. Finally, by pumping coolant in the second circuit via a pump coupled upstream of a radiator (e.g., on the hotter side and downstream of the various devices), it may be possible to utilize a single pump for the lower temperature circuit that is able to provide multiple coolant outlets with different coolant temperatures to serve the various devices with different temperature requirements. Alternatively, multiple pumps may also be used.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an embodiment of a cooling system for an internal combustion engine.

DETAILED DESCRIPTION

FIG. 1 illustrates a cooling system for an internal combustion engine, which includes two cooling circuits with their coolant flows isolated from each other: a high temperature cooling circuit 100 for primarily cooling the engine and a low temperature cooling circuit 102 for cooling a multitude of devices or components.

The high temperature cooling circuit 100 may include an engine 104, a main pump 106 for circulating coolant in the high temperature cooling circuit 100, a main radiator 108 for dissipating the heat in the high temperature cooling circuit 100. The high temperature cooling circuit 100 may further include a high temperature exhaust gas recirculation (EGR) cooler 110 for cooling the EGR.

Coolant lines for circulating the coolant among the various components of the high temperature cooling circuit 100 may be provided. In this example, the coolant is shown to circulate from the main radiator 108 to the main pump 106 via a coolant line 112a, further to the engine 104 via a coolant line 112b, then back to a main radiator 108 via a coolant line 112c. In addition, the coolant may also circulate from the main pump 106 to the high temperature EGR cooler 110 via a coolant line 112d, and further to the main radiator 108 via a coolant line 112e.

The low temperature cooling circuit 102 may include an auxiliary pump 114 for circulating coolant in the low temperature cooling circuit 102, a low temperature heat exchanger 116 for dissipating the heat in the low temperature cooling circuit 100. It may further include various coolers or heat exchangers for cooling various devices, which in this example may include a transmission cooler 118 for cooling transmission, a charged air cooler (CAC) (e.g., an air-to-coolant CAC) 120 for cooling charged air, a fuel cooler 122 for cooling a fuel supply, and a low temperature EGR cooler 124 for cooling EGR. In other examples, the low temperature cooling circuit 102 may optionally include various other heat exchangers, such as one or more power steering coolers, condensers, interstage coolers, engine oil coolers. The low temperature cooling circuit 102 may also optionally include one or more additional condensers positioned outside the low temperature heat exchanger 116.

The low temperature heat exchanger 116 may include a condenser 116d, and a series of successively arranged two or more radiators or radiator segments, in this example, a warm temperature radiator 116a, an intermediate temperature radiator 116b, and a low temperature radiator 116c. These may be arranged as cross-counterflow segments providing maximum airflow face area to each radiator, or, if space allows, side-by-side arranged for u-flow or s-flow, for reduced cost and pressure drop.

The condenser 116d, the low temperature radiator 116c, the intermediate temperature radiator 116b, and the warm temperature radiator 116a, together with the main radiator 108 of the high temperature cooling circuit 100 are positioned successively in a common stream of air flow 126, with the main radiator 108 at the most downstream end and the condenser 116d at the most upstream end. The stream of air flow 126 may be generated by a moving vehicle and/or a fan.

In this way, the air flow 126 cools the condenser 116d, the low temperature radiator 116c, the intermediate temperature radiator 116b, the warm temperature radiator 116a, and the main radiator 108, successively in that order.

The temperature of the airflow 126 may increase as it passes the condenser and the various radiators. For example, the temperature of the air flow 126 may be 100° F. immediately upstream of the condenser 116d, 105° F. immediately downstream of the condenser 116d, 144° F. immediately downstream of the warm temperature radiator 116a, and 211° F. immediately downstream of the main radiator 108.

Various coolant lines may be provided to circulate the coolant in the low temperature heat exchanger 116. In this example, the coolant enters the low temperature heat exchanger 116 at the warm temperature radiator 116a, and then circulates to the intermediate temperature radiator (116b) via a coolant line 116e. The coolant further circulates to the low temperature radiator 116c via a coolant line 116f before exiting the low temperature heat exchanger 116.

Various coolant lines (e.g. 128a-j, 130, 132, 134a-c) may be provided for circulating the coolant in the low temperature cooling circuit 102. In this example, the coolant circulates from the auxiliary pump 114 to the low temperature heat exchanger 116 to be cooled. The cooled coolant may then circulate to various devices 118-122 in the low temperature cooling circuit. After picking up heat rejected from the various devices 118-122, the coolant may then circulate back to the auxiliary pump 114 via a common coolant line 132.

To be more specific, the coolant in this example may circulate from the auxiliary pump 114 to the low temperature heat exchanger 116 via a coolant line 128a. The coolant may then enter the low temperature heat exchanger 116 at the warm temperature radiator 116a. After being cooled by one or more radiators in the low temperature heat exchanger 116, the coolant may exit the low temperature heat exchanger 116 at various radiator segment locations, and may then further circulate to cool various devices in the low temperature cooling circuit.

In particular, the coolant may exit the low temperature heat exchanger 116 at the low temperature radiator 116c after being cooled by the low temperature radiator 116c. It may then circulate to cool the charged air cooler (CAC) 120 via a coolant line 128d before circulating back to the auxiliary pump 114 via a coolant line 128e and then the common coolant line 132; or it may circulate to cool the fuel cooler via a coolant line 128b before circulating back to the auxiliary pump 114 via a coolant line 128c and then the common coolant line 132.

The coolant may additionally exit at the intermediate temperature radiator 116b, after being cooled by the intermediate temperature radiator 116b. It may then circulate to cool the low temperature EGR 124 via a coolant line 128h before circulating back to the auxiliary pump 114 via a coolant line 128i.

The coolant may further exit the low temperature heat exchanger 116 at the warm temperature radiator 116a after being last cooled by the warm temperature radiator 116a. It may then circulate to cool the transmission 118 via a coolant line 128f before circulating back to the auxiliary pump 114 via a coolant line 128g and then the common coolant line 132.

A bypass coolant line 128j with various branches (e.g., 134a, 134b, 134c) may also be provided for bypassing the low temperature heat exchanger 116 under certain operating conditions, for example when the coolant temperature exiting the auxiliary pump is lower than 59° F. For example, as provided in this example, the coolant may circulate directly from the auxiliary pump 114 to cool the various devices 118, 120, 122, 124 of the low temperature cooling circuit without passing through the low temperature heat exchanger 116. In particular, the coolant may circulate from the auxiliary pump 114 via coolant line 128j, and then via a coolant line 134a, to a coolant line 128h which leads to the low temperature EGR cooler 124. The coolant may circulate from the auxiliary pump 114 via the coolant line 128j, and then via a coolant line 134c, to the coolant line 128f which leads to the transmission 118. The coolant may circulate from the auxiliary pump 114 via coolant line 128j, and then coolant line 134b, to the coolant line 130 which leads to the fuel cooler 122 and the charged air cooler 120.

Various modifications or adjustments may be made to the above example systems. For example, the cooling system may include no fan, one fan or multitude of fans (not shown) for generating air flow for cooling the various radiators of the cooling system. In case there is no fan, the cooling system may rely solely on ram air generated when the vehicle is moving for cooling the various radiators of the cooling system.

The cooling system may include various sensors (not shown) for sensing various operating parameters of the engine, vehicle, and/or cooling system, such as one or more temperature sensors, pressure sensors, and coolant flow rate sensors. Theses sensors may be located in various locations in the cooling system.

The cooling system may also include various additional pumps, filters, bypasses, valves, meters, controls and actuators, etc. For example, additional pumps may be provided for the high temperature cooling circuit and the low temperature cooling circuit. The cooling system may also include valves for adjusting and/or directing the flow rates of coolant down various coolant lines or pipes.

The cooling system may further include a control unit (not shown). The control unit (not shown) may be an engine control unit or may be a unit separate from the engine control unit. It may be configured to send and receive information from various sensors, such as temperature sensors and pressure sensors. It may also be coupled to and control operation of various pumps, such as coolant pumps (e.g., 106 & 114), and various fans, such as an engine cooling fan. It may be used to receive information from various other sensors, pumps, actuators and valves etc.

Although the cooling system includes a main radiator 108 for the high temperature cooling circuit 100, the radiators (e.g., the various radiators 116a-c of the low temperature heat exchanger116), and coolers (e.g. fuel cooler, charge air cooler, EGR cooler) may be any suitable types of heat exchangers for heat transfer, such as air-to-coolant heat exchangers, which serve to exchange heat between air and coolant, and coolant-to-coolant exchangers, which serve to exchange heat between one coolant to another.

The coolant flow rates in the various coolant lines of the cooling system may be adjusted according to engine and/or vehicle specifications. For example, the rate of coolant flow and the dimension of the coolant line may be increased to accommodate an increased cooling need, or decreased to accommodate a decreased cooling need. For example, the rate of coolant flow for cooling the engine, which has the largest cooling need among all engine and vehicle components, (e.g. 100) may be relatively large (e.g., 150 gpm). The EGR cooler has a comparatively smaller cooling need compared to the engine. Therefore, the coolant flow rate for cooling the high temperature EGR (e.g. 106) may be comparatively smaller (e.g., 20 gpm).

The dimensions of the various coolant lines of the cooling system may be set according to coolant flow rates. For example, the CAC 120 which may have a smaller flow rate compared to that of CAC, has a comparatively smaller coolant line dimension than that of the low temperature EGR cooler 124. Various additional dimensions and/or flow rates are indicated in the Figures.

Although one condenser is provided in this example, no condenser or a multitude of condensers may be provided in other examples. For example, an additional condenser may be added to the low temperature cooling circuit to further cool the coolant temperature in the low temperature cooling circuit.

Although the high temperature cooling circuit 100 serves to cool only the engine 104 and the high temperature EGR 110 in this example, the high temperature cooling circuit 100 may serve to cool other components or devices in other examples.

Although the engine and other components and/or devices in the high temperature cooling circuit 100, such as the engine 104 and the high temperature EGR cooler 110 are arranged parallel to each other in this example. In other examples, they may be arranged in series, or in a combination of in parallel and in series.

Similarly, although the low temperature cooling circuit 102 serves to cool only the transmission 118, the charged air cooler 120, fuel cooler 122, and the low temperature EGR cooler 124 in this example, in other examples, other components or devices, such as Interstage Cooler, Engine Oil Cooler, Turbocharger, Electronic Controls, or a power supply cooler, may be cooled by the low temperature cooling circuit.

As shown in FIG. 1, the various components and/or devices to be cooled in the low temperature cooling circuit 102 are arranged in parallel with each other. To be more specific, in this example, the transmission cooler 118, the charged air cooler (CAC) 120, the fuel cooler 122, and the EGR cooler 124 are arranged in parallel with each other. In other examples, the various components and/or devices to be cooled by the low temperature cooling circuit may be arranged in series or in a combination of series or parallel.

The coolant lines serving the various components and/or devices to be cooled by the low temperature cooling circuit 102 may exit from one or more of the radiators in the low temperature heat exchanger, depending on the cooling need of the particular component and/or devices to be cooled. For example, and as described in part previously, the charged air cooler 120 and the fuel cooler 122 need to be cooled to relatively lower temperatures, the coolant line serving the fuel cooler and may exit the low temperature radiator 116c, which provides the coolest coolant among all the radiators of the low temperature heat exchanger.

In other examples, it may also be possible for a coolant serving to cool a particular device to be a mixture of coolants exiting one or more radiators. The coolant mixing ratio and flow rates may be adjusted depending on the cooling need of the particular device. The flow rates of the coolant may be determined depending on the temperature of the coolant exiting the various radiators and the cooling temperature requirement of the particular device. For example, although not provided in this example, the coolant serving to cool the CAC 120 may be a mixture of coolants exiting the low temperature radiator 116c and the intermediate temperature radiator 116b.

By maintaining two separate cooling circuits where the coolant streams do not mix, it may be possible to reduce heat transfer between the two cooling circuits. To be more specific, it may better insulate the engine heat of the high temperature cooling circuit from the low temperature cooling circuit, achieving lower coolant temperatures in the low temperature cooling circuit to achieve more optimal cooling of the various devices in the second cooling circuit.

By providing at least a condenser in the second cooling circuit, it may be possible, in some situations, to further cool the coolant streams of the second cooling circuit to a lower temperature when it is needed for an increased cooling capacity.

By successively ordering the plurality of radiator segments in the second cooling circuit so that increased cooling of the circulating coolant may be achieved at a radiator positioned downstream in the path of the circulating coolant.

By allowing coolant to exit at the various successively arranged radiator segments, it may be possible to provide coolant streams of different temperatures for cooling the various devices in the second cooling circuit. In one example, the coolant stream exiting the low temperature radiator, which is the most downstream radiator segment, has the lowest temperature (e.g., 127° F.); the coolant stream exiting the intermediate temperature radiator, which is positioned upstream of the low temperature radiator, has the next lowest temperature (e.g., 150° F.); and the coolant stream exiting the warm temperature radiator, which is positioned upstream of the intermediate temperature radiator, has the highest temperature (e.g. 180° F.).

By distributing coolant streams of different temperatures, it may be possible to individualize the temperature of the coolant for cooling a particular device, thereby improving cooling efficiency and performance of the device. In a particular example, the temperature of the coolant used for cooling a transmission is 180° F., achieved by allowing the coolant used to cool the transmission to exit a temperature heat exchanger after it is cooled by only one radiator; the temperature of the coolant used to cool a low temperature EGR cooler is 150° F., achieved by allowing the coolant used for cooling the low temperature EGR cooler to exit a low temperature heat exchanger after it is cooled by two radiators; and the temperature of the coolant used to cool a fuel cooler and a charged air cooler is 127° F., achieved by allowing the coolant used to cool the fuel cooler and the charger air cooler to exit a low temperature heat exchanger after it is cooled by three radiators.

An improved cooling efficiency may allow smaller coolant flows and smaller coolant line dimensions, which may lead to a more compact cooling system design.

By utilizing a common stream of cooling air over to cool radiators in both the first and the second coolant circuits, it may further help to achieve a more compact system design.

By pumping coolant in the second cooling circuit via a pump coupled upstream of a radiator (e.g., on the hotter side and downstream of the various devices), it may be possible to utilize a single pump for the lower temperature circuit that is able to provide multiple coolant outlets with different coolant temperatures to serve the various devices with different temperature requirements.

Further, the dimension of the cooling lines or pipes of the two cooling circuits and the coolant flow rates may be individualized to suit the cooling needs of each component, and thereby minimize the cooling system package size. For example, utilizing a larger coolant line or pipe dimension in the high temperature cooling circuit may help to meet the increased cooling need of the engine, which generates a tremendous amount of heat during combustion. On the other hand, utilizing a smaller coolant line dimension in the low temperature cooling circuit, which serves engine and vehicle components with lower cooling needs, may help to minimize the overall package size of the cooling system.

By providing a high temperature EGR cooler and a low temperature EGR cooler with different operating temperatures, it may be possible for a finer tuning of the EGR temperature.

Further, by utilizing two thermally separate cooling loops, it may be possible to reduce cavitation, even when using a pump on the hotter side of a radiator of the low temperature loop.

The following claims particularly point out certain combinations and subcombinations regarded as novel and nonobvious. These claims may refer to “an” element or “a first” element or the equivalent thereof. Such claims should be understood to include incorporation of one or more such elements, neither requiring nor excluding two or more such elements. Other combinations and subcombinations of the disclosed features, functions, elements, and/or properties may be claimed through amendment of the present claims or through presentation of new claims in this or a related application. Such claims, whether broader, narrower, equal, or different in scope to the original claims, also are regarded as included within the subject matter of the present disclosure.

Claims

1. A cooling system for an internal combustion engine, comprising

a first, high temperature, cooling circuit coupled to an engine for cooling the engine, where a first coolant circulates in the first circuit; and

a second, low temperature, cooling circuit coupled to a plurality of devices for cooling the plurality of devices, where a second coolant circulates in the second circuit, and where the second cooling circuit includes a plurality of radiator segments, the coolant circulating in the second cooling circuit exiting at different radiator segments of the plurality of radiator segments producing coolant streams of different cooling temperatures for cooling different devices of the plurality of devices.

2. The cooling system of claim 1, wherein the plurality of radiator segments are positioned in a path of a common stream of air flow.

3. The cooling system for an internal combustion engine of claim 1, wherein the coolant of the first cooling circuit does not mix with the coolant of the second cooling circuit.

4. The cooling system according to claim 1, wherein the first cooling circuit includes at least a radiator for cooling the coolant circulating in the first cooling circuit.

5. The cooling system of claim 3, wherein the radiator of the first cooling circuit is positioned in the path of an air flow downstream from the plurality of radiator segments of the second cooling circuit.

6. The cooling system of claim 1, wherein the plurality of radiator segments of the second cooling circuit include at least two radiator segments, where the coolant circulating in the second cooling circuit exits at each of the two radiator segments to produce coolant streams of two different temperatures.

7. The cooling system of claim 1, wherein the plurality of radiator segments of the second cooling circuit includes at least three radiator segments, where the coolant circulating in the second cooling circuit exits at each of the three radiator segments to produce coolant streams of three different temperatures.

8. The cooling system of claim 1, wherein the first cooling circuit includes at least a coolant pump for circulating the coolant in the first cooling circuit, and where the second cooling circuit includes at least a coolant pump for circulating the coolant in the second cooling circuit.

9. The cooling system of claim 8, wherein the coolant pump of the second cooling circuit is positioned downstream of the devices and upstream of the radiator segments, the coolant exiting the coolant pump first traveling to the radiator segments and then to the plurality of devices of the second cooling circuit.

10. The cooling system of claim 1, wherein the second cooling circuit includes a bypass for allowing the coolant circulating in the second cooling circuit to bypass the plurality of radiator segments.

11. The cooling system of claim 1, wherein the first cooling circuit cools a high temperature EGR cooler, and the second cooling circuit cools a low temperature EGR cooler.

12. The cooling system of claim 1, wherein the second cooling circuit includes a condenser positioned in a path of air flow.

13. The cooling system of claim 1, wherein the cooling system includes a condenser positioned away from a path of air flow.

14. The cooling system of claim 1, wherein rates of coolant flow passing through each of the plurality of devices of the second cooling circuit are less than or equal to 15 grams per minute.

15. The cooling system of claim 1, wherein coolant flow rates of the second cooling circuit are less than or equal to 6 grams per minute.

16. A cooling system for an internal combustion engine, comprising

a first, high temperature, cooling circuit coupled to an engine and an high temperature EGR cooler for cooling the engine and the high temperature EGR cooler, where a first coolant circulates in the first cooling circuit, the first cooling circuit includes a main coolant pump for circulating the coolant in the first cooling circuit and a main radiator for cooling the coolant circulating in the first cooling circuit, the main coolant pump is positioned downstream of the main radiator and upstream of the engine and the high temperature EGR cooler in the path of the circulating coolant, the engine and the high temperature EGR are arranged in parallel in the path of the coolant circulating in the first cooling circuit with respect to each other;

a second, low temperature, cooling circuit coupled to a plurality of devices for cooling the plurality of devices, where a second coolant circulates in the second cooling circuit, where the second cooling circuit includes an auxiliary coolant pump and a condenser, where the second cooling circuit includes three radiator segments: a warm temperature radiator, a intermediate temperature radiator, and a low temperature radiator, arranged successively in the path of the coolant circulating in the second cooling circuit, where the low temperature radiator is downstream of the intermediate temperature radiator, and the intermediate temperature radiator is downstream of the warm temperature radiator, where the circulating coolant of the second cooling circuit exits the warm temperature radiator to form a first a coolant stream with a first coolant temperature, exits the intermediate temperature radiator to form a second coolant stream with a second coolant temperature, exits the low temperature radiator to form a third coolant stream with a third coolant temperature, where the temperature of the first coolant stream is higher than the temperature of the second coolant stream, and the temperature of second coolant stream is higher than the temperature of the third coolant stream, where the first coolant stream further circulates to a transmission cooler to cool the transmission cooler, the second coolant stream further circulates to a low temperature EGR cooler to cool the low temperature EGR cooler, the third coolant stream further circulates to a fuel cooler and a charged air cooler to cool the fuel cooler and the charged air cooler, where coolant exiting each of the plurality of devices of the second cooling circuit circulates to the auxiliary coolant pump before circulates further to the plurality of radiator segments, and where the second cooling circuit includes a bypass for the coolant circulating in the second coolant circuit to bypass the plurality of radiator segments; where a common stream of air flow flows pass the first condenser, the plurality of radiators of the second cooling circuit, and the radiator of the first cooling circuit, where the first condenser is upstream of the low temperature radiator, the low temperature radiator is upstream of the intermediate temperature radiator, the intermediate temperature radiator is upstream of the warm temperature radiator, the warm temperature radiator is upstream of the main radiator of the first cooling circuit.

17. A method for cooling an internal combustion engine of a vehicle, comprising:

circulating coolant through a first cooling circuit thermally coupled to combustion chambers of the engine, the first cooling circuit including at least a radiator;

circulating coolant through a second cooling circuit thermally coupled to a plurality of devices, the second cooling circuit including a plurality of radiator segments, where the coolant is pumped via a pump coupled upstream of a radiator and downstream of the devices, where the coolant of the first cooling circuit does not mix with coolant of the second cooling circuit;

distributing coolant in the second circuit to the plurality of devices at different temperatures via the plurality of radiators in the second circuit; and

flowing a common stream of cooling air over at least the radiator in the first cooling circuit and at least one of the plurality of radiator segments in the second cooling circuit.

18. The method of claim 17 wherein the radiator of the first cooling circuit and the plurality of radiator segments of the second cooling circuit are positioned in a common stream of cooling air, with the said radiator of the radiator of the first cooling circuit downstream of the plurality of radiators; and the said plurality of radiator segments positioned successively in the path of the common stream of cooling air.