Abstract:
A method of controlling airflow through a grille on a front end of a vehicle comprising the steps of: detecting vehicle operating conditions; determining a desired percentage of airflow through the grille based on the desired operating conditions; actuating grille airflow shutters to an open position for the determined desired percentage of a predetermined time interval; and actuating the grille airflow shutter to a closed position for a remaining percentage of the predetermined time interval.

Description:
BACKGROUND OF INVENTION 
     The present invention relates generally to vehicles having grille airflow shutters and more particularly to grille airflow shutter systems and methods of controlling such systems. 
     Grille airflow shutters are devices that control the amount of airflow through a front grille of a vehicle. The airflow control is typically based on cooling or warming needs for heat exchangers in a condenser, radiator, fan module (CRFM) located behind the grille. A greater airflow provides for better heat transfer out of the heat exchangers in the CRFM. When reduced or no airflow is desired, the grille airflow shutters are adjusted to block airflow through the grille, and, conversely, when additional air flow is desired the airflow shutters can be adjusted to allow full airflow through the grille. 
     Grille airflow shutters have been controlled using one of two methods. There are discrete open/shut shutters that only move between the open and shut positions, and the decision to open is based on reaching a particular temperature threshold, with the shutters remaining open continuously above this threshold, and the decision to close is based on dropping below this threshold, where the shutters remain continuously closed. However, this type of system provides less precise airflow control than is desired and may end up with repeated open/closed cycling if the temperature fluctuates around an open/close temperature threshold. 
     The other type of grille airflow shutter system overcomes the precision concern by using shutters that are variably controlled. That is, the shutters can be controlled to be in various positions of partial closure. With this control, the precise amount of airflow can be achieved by opening the shutters only the desired amount. These variable shutters are controlled with pulse width modulation. However, pulse width modulation, while allowing for the variable control of the shutters, adds significantly to both the cost of the physical hardware and to the complexity of the control algorithm and driver. 
     SUMMARY OF INVENTION 
     An embodiment contemplates a method of controlling airflow through a grille on a front end of a vehicle, the method comprising the steps of: detecting vehicle operating conditions; determining a desired percentage of airflow through the grille based on the desired operating conditions; actuating grille airflow shutters to an open position for the determined desired percentage of a predetermined time interval; and actuating the grille airflow shutter to a closed position for a remaining percentage of the predetermined time interval. 
     An embodiment contemplates a vehicle comprising a front end having a grille with grille openings therethrough; a condenser, radiator, fan module located behind the grille openings; and a grille airflow shutter system located proximate the grille openings, including shutters movable to selectively block airflow through the grille openings and including a discrete shutter actuator controllable to move the shutters between an open position and a closed position. The vehicle also includes a controller in communication with the discrete shutter actuator to move the shutters between the open position and the closed position, wherein the controller is configured to detect vehicle operating conditions; determine a desired percentage of airflow through the grille based on the desired operating conditions; actuate grille airflow shutters to an open position for the determined desired percentage of a predetermined time interval; and actuate the grille airflow shutter to a closed position for a remaining percentage of the predetermined time interval. 
     An advantage of an embodiment is that an effective variable airflow control is achieved in order to precisely control airflow through the grille while employing a discrete open/shut shutter actuator and controls, which minimizes the cost and complexity of the grille airflow shutter system. This grille airflow shutter system allows for overall improved fuel economy for the vehicle by reducing overall aerodynamic drag while still providing for airflow to the CRFM as needed. The aerodynamic improvements averaged over time can be essentially the same as with a variable pulse width modulated controller but without the cost and complexity. The modulated airflow through the grille can be precisely controlled to meet powertrain and air conditioning cooling requirements without requiring variable position shutters. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a schematic view of a vehicle including a grille airflow shutter system. 
         FIG. 2  is a flow chart illustrating a method of operating the grille airflow shutter system. 
         FIGS. 3A and 3B  are graphs showing an example of operating the grille airflow shutter system and the resulting engine coolant temperature. 
     
    
    
     DETAILED DESCRIPTION 
     Referring to  FIG. 1 , a vehicle, indicated generally at  20 , is shown. The vehicle  20  may include a power plant, such as an internal combustion engine  22 , and transmission  24 . Coolant pumped through a radiator  26  by a water pump  27  may be employed to cool the engine  22 . Also, transmission oil may flow through a transmission oil cooler  28  to cool the transmission. The transmission oil cooler  28  and radiator  26  may be part of a condenser radiator, fan module (CRFM)  30 , which may also include a condenser  32  of an air conditioning system  34 . Other heat exchangers, for example, a powertrain electronics cooling heat exchanger (not shown), may also be part of the CRFM. The CRFM  30  may also include an engine fan  36  that may be driven by a motor  38 . 
     One or more controllers  40  may be in communication with and control the engine  22  and transmission  24  as well as the air conditioning system  34  and motor  38 . The controller  40  may be in communication with various sensors, such as, for example, an engine coolant temperature sensor  42 , an air conditioning system pressure sensor  44 , a transmission oil temperature sensor  46 , as well as other sensors known to those skilled in the art. The controller  40  may be a single controller or multiple discrete controllers in communication with one another and may be various combinations of hardware and software as is known to those skilled in the art. The controller  40  may also be in communication with a discrete shutter actuator  48 , which is part of a grille airflow shutter system  50 . 
     The grille airflow shutter system  50  cooperates with a grille  52  mounted in the front of the vehicle  20  that has grille openings  54 , which allow airflow through to the CRFM  30 . The system  50  also includes grille airflow shutters  56  that mount adjacent to the grille  52  (shown above the grille  52  in  FIG. 1  for clarity in showing the various elements). The grille airflow shutters  56  may include shutters  58  that can be selectively aligned with the grille openings  54  to block flow through the grille openings  54  and shutter openings  60  that can be selectively aligned with the grille openings  54  to allow airflow through the openings  54 . The grille airflow shutters  56  may, for example, slide up and down to selectively allow and block airflow, with a discrete shutter actuator  48  that raises and lowers the grille airflow shutters  56 , or they may rotate or pivot to selectively block and allow airflow, with a discrete shutter actuator that causes pivoting of the shutters  56 . 
     The controller  40  and discrete shutter actuator  48  as defined in the present application cooperate to move the shutters  56  between one or the other of only two positions. That is, the grille airflow shutters  56  are either positioned to block all (or essentially all) airflow through the grille openings  54 , a shutter closed position, or are positioned to allow full (or essentially full) airflow through the grille openings  54 , a shutter open position. This binary shutter positioning allows for reduced cost and complexity because no variable or pulse width modulated actuator and actuator driver are needed—a simple on/off driver and a two position actuator are all that is required for grille airflow shutter control. 
       FIG. 2  illustrates a method of operating the grille airflow shutter system  50  for the vehicle shown in  FIG. 1 . Sensors detect the various operating conditions of vehicle components, block  100 . For example, the controller  40  may read, among other things, the engine coolant temperature, the transmission oil temperature, and whether the air conditioning system is operating. Other readings, such as, for example, engine load, vehicle speed and ambient temperature may also be read. The vehicle speed may be applicable because the variable shutter operation may be used only at vehicle speeds above a predetermined speed threshold. For example, the predetermined speed threshold may be a speed in the range of forty-five to sixty kilometers per hour. 
     Based on the vehicle operating conditions, the controller  40  determines the desired percentage of flow through the grille openings  54 , block  102 . The controller  40  may employ a lookup table or other means to determine the desired percentage, with the lookup table based on the particular vehicle and powertrain combination. For example, the controller  40  may determine that a thirty-three percent airflow rate through the grille  52  is desired to achieve the appropriate thermal cooling. 
     After determining the desired percentage, the controller  40  actuates the discrete shutter actuator  48  to achieve the desired percentage, block  104 . In the example given above, to achieve the thirty-three percent airflow rate, the discrete shutter actuator  48  may be actuated to move the grille airflow shutters  56  to the open position for ten seconds and then move the shutters  56  to the closed position for twenty seconds. The thirty second interval is just an example, and other predetermined intervals may be employed instead. If twenty percent airflow is required, the shutters may be opened for twelve seconds out of each minute. Thus, the discrete shutter actuation, while producing periods of full airflow and full blockage of airflow, in effect produces a variable airflow of the desired percentage needed for the particular vehicle conditions. 
     The process repeats itself since the controller  40  continually monitors the vehicle operating conditions. As a result, the controller  40  may detect a significant change in operating conditions while the grille airflow shutters  56  are going through a particular cycle of percentage open/shut time. In this case, the controller  40  may change the percentage open shut times in mid-cycle to account for this change. The end result though is that the shutters  56  are still controlled based on a percentage of time open and a corresponding percentage of time closed even if this percentage requires changing before the end of the previous cycle. In this way precise airflow control is achieved but with only a discrete shutter actuator  48  and corresponding control electronics. 
       FIGS. 3A and 3B  are graphs both having a corresponding time line for elapsed time (T) in minutes along the horizontal axis.  FIG. 3A  shows engine coolant temperature in degrees Celsius (° C.) along the vertical axis and  FIG. 3B  shows the binary open/closed position of the grille airflow shutter system  50 , with level  0  being a shutter open position and level  1  being a shutter closed position. Initially, under a light engine load condition, the shutter position is switched to the shutter closed position  200 . Under this condition, with the shutters closed, the engine coolant temperature slowly rises. As the cooling load increases the shutter pulses with increased frequency  202 . The engine coolant temperature tends to rise and fall based on the open or closed position of the shutters. While the shutters may pulse with increasing frequency, the shutter position at any given time is based on the percentage of discrete open/closed shutter position as determined by the controller at that time. Under heavy load, the shutters remain open  204  to provide the needed cooling capacity for the engine coolant or other vehicle components needing this cooling capacity. 
     The overall thermal control may also have a threshold temperature of engine coolant where the controller activates the engine cooling fan to increase the airflow through the CRFM. For example, the engine fan may be activated if the engine coolant exceeds a predetermined temperature threshold. This further adds to the ability to maintain precise thermal control. 
     While certain embodiments of the present invention have been described in detail, those familiar with the art to which this invention relates will recognize various alternative designs and embodiments for practicing the invention as defined by the following claims.