Patent Application
20120061069
1. Technical Field
The present disclosure relates to providing a desired cooling level in a liquid-to-air heat exchanger in an energy efficient manner.
2. Background Art
In most production vehicles, the water pump that causes engine coolant to circulate through the engine and radiator is driven by the engine and the speed of the pump is dictated by the rotational speed of the engine. To ensure that there is sufficient coolant flow at the most demanding operating condition, the amount of flow at most operating conditions is higher than necessary. To improve control over the pump speed, the pump is decoupled from the engine and is either driven by an electric motor, driven by a variable speed clutch, hydraulically driven, or driven by some other actively controllable means. The electrically driven variant is particularly suited to a vehicle with a significant capacity for electrical power generation such as a hybrid electric vehicle.
It is common for a fan to be provided to direct air flow across the fins and tubes of the radiator. The fan is commonly electrically driven, although it too may be driven by a variable speed clutch, hydraulically driven, or driven by some other actively controllable means. The flow across the radiator is due to movement of the vehicle and the fan.
When an increase in heat transfer rate is indicated, the fan speed or the coolant pump speed may be increased.
According to an embodiment of the disclosure, the choice of increasing the fan speed or increasing the pump speed is determined so that the power consumed is minimized. The broad concept is that dQ/dP, the gradient in heat transfer rate to power, is determined for both the fan and the pump at the present operating condition. The one with the higher gradient is the one that is commanded to increase speed.
A method to control cooling in a liquid-to-air heat exchanger with a fan and a pump forcing convection is disclosed including: determining a first gradient in heat transfer rate to fan power associated with adjusting fan speed, determining a second gradient in heat transfer rate to pump power associated with adjusting pump speed, and adjusting one of fan speed and pump speed based on the gradients. The method may further include determining whether a change in heat transfer is indicated and the adjusting one of fan speed and pump speed is further based on such change in heat transfer being indicated. The fan speed is increased when the first gradient is greater than the second gradient and an increase in heat transfer is indicated. The pump speed is increased when the second gradient is greater than the first gradient and an increase in heat transfer is indicated. The fan speed is decreased when the second gradient is greater than the first gradient and a decrease in heat transfer is indicated. The pump speed is decreased when the first gradient is greater than the second gradient and a decrease in heat transfer is indicated. The liquid is a coolant typically comprising water and ethylene glycol. The liquid is contained within a duct and the air may or may not be ducted. The liquid-to-air heat exchanger is called a radiator and the first and second gradients are determined by: evaluating a radiator performance relationship with radiator performance as a function of liquid coolant and air flows and/or velocities and transforming the radiator performance relationship into a heat transfer performance relationship with heat transfer rate as a function of liquid coolant and air flows and/or velocities. Radiator performance information may take one of several forms including: effectiveness, heat transfer per unit temperature difference between the bulk coolant and air flow streams entering the radiator, or any other suitable manner to capture performance. The performance relationships may be expressed as lookup tables, graphs, or empirical formulas. The first gradient is determined for increased fan speed and the second gradient is determined for increased pump speed when an increase in heat transfer is indicated. The first gradient is determined for decreased fan speed and the second gradient is determined for decreased pump speed when a decrease in heat transfer is indicated.
A method to control cooling in a liquid-to-air heat exchanger with a fan and a pump forcing convection is disclosed that includes determining a first gradient in heat transfer to power for increasing fan speed, determining a second gradient in heat transfer to power for increasing pump speed, increasing fan speed when the first gradient is greater than the second gradient, and increasing pump speed when the second gradient is greater than the first gradient. The method may further include determining whether an increase in heat transfer is desired. The choice of increasing fan speed and/or pump speed is further based on such a determination that an increase in heat transfer is desired. The first gradient is determined based on determining a gradient in heat transfer rate to air flow from a map of radiator performance and determining a gradient in air flow to fan power and the second gradient is determined based on determining a gradient in heat transfer rate to coolant flow from a map of radiator performance and determining a gradient in coolant flow to pump power.
A cooling system for an automotive engine includes a radiator coupled to an engine cooling circuit in which the engine is disposed, a fan forcing air past the radiator, a pump disposed in the cooling circuit, and an electronic control unit electronically coupled to the fan and the pump. The electronic control unit commands the fan and/or the pump to change operating speed when an adjustment in heat transfer rate is indicated. In some situations, the adjustment in heat transfer may be realized by increasing either the fan speed or the pump speed. The electronic control unit determines which of the fan and the pump to command based on a first gradient of heat transfer rate to power for adjusting fan speed and a second gradient of heat transfer rate to power for adjusting pump speed. The fan and the pump may be electrically driven, driven by a variable speed clutch, hydraulically driven, or driven by some other actively controllable means. The system may have various sensors and actuators coupled to the electronic control unit including: an ambient temperature sensor electronically coupled to the electronic control unit, an engine coolant sensor electronically coupled to the engine coolant circuit, and a vehicle speed sensor electronically coupled to the electronic control unit. The first and second gradients may further be based on inputs from the sensors which include the ambient temperature, the engine coolant temperature, and the vehicle speed.
The fan speed is commanded to increase when the first gradient is greater than the second gradient and an increase in heat transfer is indicated. The pump speed is commanded to increase when the second gradient is greater than the first gradient and an increase in heat transfer is indicated. The fan speed is commanded to decrease when the second gradient is greater than the first gradient and a decrease in heat transfer is indicated. The pump speed is commanded to decrease when the first gradient is greater than the second gradient and a decrease in heat transfer is indicated. The amount of the fan speed increase or decrease and the amount of the pump speed increase or decrease is based on an amount of a change in heat transfer rate that is indicated. In some situations, both fan and pump speeds may be increased simultaneously. These situations may include situations when increasing one or the other in isolation may not provide the desired increase in heat transfer performance. Further, in these situations, the aforementioned logic may be utilized to determine the speed increase for each actuator so as to realize the least combined usage of energy between them for increasing heat transfer by changing both fan and pump speed simultaneously.
As those of ordinary skill in the art will understand, various features of the embodiments illustrated and described with reference to any one of the Figures may be combined with features illustrated in one or more other Figures to produce alternative embodiments that are not explicitly illustrated and described. The combinations of features illustrated provide representative embodiments for typical applications. However, various combinations and modifications of the features consistent with the teachings of the present disclosure may be desired for particular applications or implementations. Those of ordinary skill in the art may recognize similar applications or implementations consistent with the present disclosure, e.g., ones in which components are arranged in a slightly different order than shown in the embodiments in the Figures. Those of ordinary skill in the art will recognize that the teachings of the present disclosure may be applied to other applications or implementations.
According to an embodiment of the disclosure, the decision to increase the speed of a fan or a pump associated with a liquid-to-air heat exchanger is based on evaluating the gradient in heat transfer to power input, dQ/dP.
One example of a liquid-to-air heat exchanger to which the present disclosure applies is commonly called a radiator. Although the predominant heat transfer mode associated with the radiator is actually convention, it is commonly referred to as a radiator. For convenience and simplicity, the liquid-to-air heat exchanger is referred to as a radiator in the following description.
In
An electronic control unit (ECU) 30 is coupled to a variety of sensors and actuators, which may include, but is not limited to: ambient air temperature sensor 32, engine coolant temperature sensor 34, engine 14, water pump 18, fan 20, vehicle speed sensor 36, and other sensors and actuators 38.
For a radiator having a particular architecture and deploying specific heat transfer media, a map of its heat transfer performance characteristics can be determined experimentally, analytically, or by a combination of the two. The resultant heat transfer performance map may take on the form of a dimensionless, heat-exchanger effectiveness. An example two-dimensional lookup table is shown in Table 1 in which the heat transfer media are engine coolant and air and the effectiveness is based on the flows and/or resultant velocities of the two heat transfer media:
The heat transfer rate is related to effectiveness:
Q=ε*C*v*(Tcoolant,in−Tair,in)
where Q is the heat transfer rate in W, ε is the effectiveness, C is the heat capacity of the lower heat capacity fluid in J/kg-K, v is the mass flow rate of the lower heat capacity fluid in kg/s, Tcoolant,in is the temperature of engine coolant as it enters the radiator in K, and Tair,in is the temperature of the air as it approaches the radiator in K. From the above equation, the heat transfer as a function of fluid flows can be computed and an example of which is shown in Table 2:
In an automotive application, the air provided to the radiator may or may not be ducted and the temperature may be ambient temperature. In some applications, however, the temperature of the air is heated upstream of the radiator, i.e., it is exposed to other heat loads prior to being supplied to the radiator. In the automotive application, the velocity of the air blowing across the radiator is based on several factors including both the speed of the fan and the velocity of the vehicle. Temperatures may be inferred from provided engine sensors, such as engine coolant temperature and ambient temperature where applicable. Coolant velocity or mass flowrate is based on the pump speed and system architecture. Additional modeling may be required to account for the factors specific to the particular application and the particular present operating condition. The results of these models may be utilized in the ECU, or the models may themselves reside in the ECU and may be exercised in real time to provide the necessary information.
Next, gradients of heat transfer vs. fluid flow, dQ/dv can be determined for each of the fluids, as shown in Tables 3 and 4:
The pump power and coolant flow are shown as a function of pump speed in
Based on the data in the tables above, gradients in coolant flow to pump power and air flow to fan power can be determined, as in Tables 6 and 7:
At this point, dQ/dv and dv/dP are known for each fluid. From these, two values of dQ/dP, i.e., for coolant and air, can be determined. Examples of these tables are shown in Tables 8 and 9:
Based on the data in Tables 8 and 9, the more efficient device, fan or pump, can be commanded to increase output to respond to a demand for additional cooling. For example, if the present coolant flow is 1.25 kg/s and the present air velocity is 2.8 m/s, dQ/dP for the pump is 2.36 and for the fan, 10.28. In this example, the fan provides the greater heat transfer rate for the same input power.
The selection of which device to actuate to provide improved heat transfer is described above in terms of two-dimensional lookup tables. However, this is a non-limiting example. The determination can be based on data in graphical form, a set of empirical relationships of the data, a comprehensive model including all of the relevant factors, or any other suitable alternative. In regards to the above discussion, heat transfer leading to energy being removed from the coolant is considered to be positive and power supplied to the device (either fan or pump) is considered to be positive.
A flowchart showing an embodiment of the disclosure is shown. After the vehicle is started in 100, control passes to 102 in which it is determined whether there is an increased demand for cooling. In so, control falls through to block 104 in which dQ/dP for the fan and dQ/dP for the pump are determined. In block 106, the two are compared. If dQ/dP for the pump is greater, control passes to 108 for increasing pump speed. If dQ/dP for the fan is greater, the fan speed is increased in block 110. Control from block 108 and 110 returns to block 102.
The discussion above focuses on selecting the appropriate actuator to employ to meet a demand for additional cooling. It is also within the scope of the present disclosure to select the appropriate device to reduce heat transfer. In this case, dQ is negative and dP are negative because the rate of heat transfer is decreasing as well as the power input decreasing. In this situation, the device which has the lesser dQ/dP associated with it is the one that is commanded to reduce speed. The determination of the gradients dQ/dP for this situation can be determined analogously as for the situation where an increased heat transfer rate is indicated.
A flow chart showing both increases and decreases in heat transfer rate is shown in
In the embodiment in
The data in Tables 8 and 9 can be utilized to determine a region in which the gradient in dQ/dP is equal for the fan and the pump, shown as 150 in
The tables above are shown for a specific arrangement and a specific set of operating conditions. The tables are updated continuously to reflect present conditions by a real time running model, results from such a model, test data, or a suitable combination. Also, in the above tables, coolant is provided as a mass flowrate and airflow as a velocity. However, any measure of flow can be used for either: mass flowrate, volumetric flowrate, velocity, as examples. As described herein, sensors may be used to provide input to models. However, there is a desire to minimize the sensor set to reduce cost. Thus, some of the quantities used in the models may be inferred based on sensor signals, actuator settings, or inferred from other sensor signals.
While the best mode has been described in detail, those familiar with the art will recognize various alternative designs and embodiments within the scope of the following claims. Where one or more embodiments have been described as providing advantages or being preferred over other embodiments and/or over background art in regard to one or more desired characteristics, one of ordinary skill in the art will recognize that compromises may be made among various features to achieve desired system attributes, which may depend on the specific application or implementation. These attributes include, but are not limited to: cost, strength, durability, life cycle cost, marketability, appearance, packaging, size, serviceability, weight, manufacturability, ease of assembly, etc. The embodiments described as being less desirable relative to other embodiments with respect to one or more characteristics are not outside the scope of the disclosure as claimed.
