Optimized cooling for vehicle with start-stop technology

An air conditioning system for a start-stop vehicle employs a compressor with a variable displacement. A controller determines a normal Climate Thermal Load (CTL) value for controlling a variable stroke of the variable displacement compressor. A stop event is detected in response to a speed of the vehicle and the occurrence of a predetermined deceleration. During the stop event, the variable stroke is increased by an amount determined in response to the normal CTL value, the speed of the vehicle, and a measure of the deceleration of the vehicle so that an evaporator temperature within the air conditioning system is allowed to decrease while the vehicle is stopping so that climate comfort can be maintained even while the engine is shut off during the subsequent time that the vehicle is stopped.

CROSS REFERENCE TO RELATED APPLICATIONS

Not applicable.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH

Not applicable.

BACKGROUND OF THE INVENTION

The present invention relates in general to vehicular air conditioning systems with variable displacement compressors, and, more specifically, to optimizing an increased cooling during a stop event so that passenger comfort is maintained during a stopped condition of the vehicle with the engine also stopped.

Fuel economy of automobiles is an important attribute of vehicle performance, and is determined by the technology employed in its design, by driver behavior and actions, and by conditions under which the vehicle is used (e.g., speed, road design, weather, and traffic). Manufacturers continuously strive to deliver better fuel economy. One technology being increasingly used is Stop-Start, wherein an internal combustion engine automatically shuts down when the vehicle comes to a stop and then restarts as needed to reduce the amount of time the engine spends idling (e.g., while waiting at a traffic light), thereby improving fuel economy and reducing emissions. Start-Stop technology can provide a 5% to 10% improvement in fuel economy.

In addition to vehicle propulsion, the engine drives other vehicle systems such as an air conditioning compressor. Occupant comfort must be maintained during the time that the engine is stopped. Since the air conditioning compressor runs on a front-end-accessory-drive (FEAD) belt driven by the engine, the compressor does not run when the engine is stopped. Thus, when the air conditioning system is actively being used and the engine stops during a vehicle stop, the cooling action is interrupted and the passenger cabin may become warmer. If the temperature increases by a certain amount, the engine is usually restarted so that cooling resumes, but some of the fuel economy improvement is lost.

For improved efficiency and performance, many vehicles now use variable displacement compressors. By varying the displacement of the compressor (e.g., by adjusting the piston stroke) to match the cooling needed at any particular time, the engine load is reduced when cooling demand is lower. The stroke can be electrically controlled via a swash plate, for example, within a range of 0% to 100% of the maximum stroke. By appropriately controlling the stroke, fuel economy improvements are realized during running of the engine.

In connection with the use of a variable displacement compressor in a vehicle with Start-Stop technology, it has been recognized that it can be beneficial to increase the stroke during coasting of the engine so that energy in the coasting engine that would otherwise be wasted can be regenerated into extra cooling. This allows the compressor to temporarily run at a lower stroke once the engine restarts. Examples of this regenerative cooling include U.S. Pat. No. 7,484,375 to Engel and U.S. Pat. No. 4,425,765 to Fukushima et al.

In known systems, however, a maximum cooling effect is obtained during coasting which leads to disadvantages. Unless a cold storage means is provided in the evaporator, the cooling of the passenger cabin may become uncomfortable. A sufficient cold storage capacity to avoid discomfort may be high. Moreover, many vehicles with Start-stop do not stop running during vehicle coasting, but only stop once the vehicle has come to a full stop. Therefore, the increased compressor stroke would actually increase engine load (and fuel consumption) during coasting.

It would be desirable to optimize the amount of extra cooling obtained during the slowing down and stopping of a vehicle equipped with a variable displacement compressor and start-stop technology. As a result, the time delay before the engine must be restarted during a vehicle stop due to increasing cabin temperature is optimized.

SUMMARY OF THE INVENTION

In one aspect of the invention, a vehicular air conditioning system is provided for a vehicle having a start-stop technology. An evaporator is monitored by an evaporator temperature sensor for generating an evaporator temperature signal according to a temperature of the evaporator. An exterior temperature sensor generates an exterior temperature signal according to a temperature outside of the vehicle. An occupant control element is used to set an occupant demand signal. A variable displacement compressor is driven by the engine and has a variable stroke for generating a variable amount of cooling of the evaporator in response to a stroke control signal. A vehicle speed sensor generates a speed signal according to an instantaneous speed of the vehicle. A controller receives the evaporator temperature signal and the speed signal. The controller determines a Climate Thermal Load (CTL) value in response to the exterior temperature signal and the occupant demand signal. The controller determines an evaporator target temperature in response to the CTL value. The controller varies the variable stroke to keep the evaporator temperature signal near the evaporator target temperature. The controller detects a stop event of the vehicle in response to the speed signal being less than or equal to a threshold speed and the occurrence of a deceleration. During the stop event, the controller determines the evaporator target temperature in response to a modified CTL value determined in response to the CTL value, the speed signal, and a magnitude of the deceleration.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The present invention is particularly applicable to vehicles having an internal combustion engine equipped with start-stop technology and an externally controlled variable displacement compressor (EVDC). When traveling at less than a threshold speed (e.g., 35 mph) and a predetermined deceleration is detected (e.g., the driver applies a predetermined amount of braking or the vehicle speed is dropping by a predetermined amount), then a stop event is detected by a vehicle controller. The predetermined amount of braking can be defined according to a certain angular position of the brake pedal or a certain rate of movement of the pedal. As used herein, the “stop event” refers to the time that the vehicle is reducing speed and comes to a stop. Preferably the engine continues to run during the stop event, but it is also possible to stop the engine prior to reaching a zero speed. A goal of a preferred embodiment is to optimize the evaporator temperature so that just the right amount of additional cooling is achieved during the stop event that maintains a comfortable cabin temperature during the time that the vehicle remains at zero speed with the engine off. Thus, the compressor is controlled by moving it into an optimum stroke rather than into full stroke. This will reduce the evaporator temperature to an optimum value rather than the lowest evaporator temperature possible before the engine shutoff. This strategy improves fuel economy by increasing the engine shutoff time before it becomes necessary to reactivate the compressor and by reducing compressor power consumption during the stop event since the compressor stroke is optimized instead of being maximized.

Referring now toFIG. 1, a vehicle10includes an engine11equipped with the start-stop feature wherein the engine can be automatically shut down during times when the vehicle is not moving and then automatically restarted as necessary when the vehicle begins to move again or when it becomes necessary to operate is accessories off of the engine. An engine controller12is connected to engine11for performing the start-stop functions. Engine11drives an externally controlled variable displacement compressor13which resides in an engine compartment14along with engine11and controller12.

Vehicle10includes a passenger cabin15which contains an evaporator16and a blower17of an automotive HVAC system. A controller20is connected to blower17for commanding a certain blower speed and to compressor13for commanding a desired stroke of the compressor. Controller20is connected to various sensors and other inputs for determining the appropriate value for the compressor stroke. A vehicle speed sensor21provides an instantaneous speed signal to controller20and to engine controller12. An exterior temperature sensor22, which may be located in engine compartment14, generates an exterior temperature signal according to a temperature outside of the vehicle which is provided to controller20. Similarly, an internal temperature sensor23generates an internal temperature signal and provides it to controller20. An evaporator temperature sensor24associated with evaporator16generates an evaporator temperature signal according to the actual temperature within the evaporator and provides it to controller20.

An occupant control element or control head25is used by the driver or other vehicle occupant to set a desired temperature and/or blower speed for the cooling of vehicle cabin15. Element25generates an occupant demand signal which is sent to controller20in a conventional manner.

A stop-event (i.e., the vehicle slowing to a stop) can be detected in response in part to the occurrence of a deceleration. In one embodiment, the deceleration can be detected by monitoring the position of a brake pedal26using an angle/position sensor27that provides an angle signal representing the instantaneous brake pedal angle to controller20.

In some embodiments of the invention, the desired compressor stroke may be determined in response to a sunload on the vehicle. Thus, a sunload sensor28such as a light intensity sensor provides a sunload signal to controller20. Controller20may use the foregoing input signals to determine a target evaporator temperature and/or compressor stroke as described below.

Operation of the air conditioning system has conventionally employed a measure known as the Climate Thermal Load (CTL). The CTL quantifies the amount of cooling needed from the air conditioning system based on the exterior temperature, occupant demand settings, actual interior temperature, and sunload, for example. Other variables may also be used in determining the CTL. As shown inFIG. 2, the CTL is used to determine a target temperature ttargetfor the evaporator. The higher the CTL, the more cooling work that must be done by the evaporator and the lower the evaporator target temperature is set. When the actual evaporator temperature is higher than ttarget, then the air conditioning controller increases the compressor stroke to obtain more cooling and reach the desired evaporator temperature. When the CTL has a lower value, the compressor stroke is controlled to obtain a higher evaporator ttarget. Thus, by lowering evaporator temperature ttarget, the compressor stroke will be correspondingly increased until the lower target is achieved. Furthermore, by forcing an increase in the value being used for CTL, ttargetis correspondingly reduced. Therefore, the present invention can proceed by either directly modifying the compressor stroke or by indirectly changing the compressor stroke by modifying the CTL.

Operation of the present invention will be described in further detail in connection withFIGS. 3-6. InFIG. 3, instantaneous vehicle speed is shown prior to, during, and after a stop event. A first example of vehicle speed is shown by a solid line30. Initially, the vehicle is moving at a speed higher than a threshold speed31. At a point32, the driver begins braking so that line30shows a deceleration that eventually reaches speed threshold31at a time33. The deceleration continues until a zero vehicle speed is obtained at a time34. The magnitude of the deceleration from time33until time34is sufficiently large to satisfy the predetermined condition of the present invention, i.e., either the drop in speed over time is greater than a threshold or the movement of the brake pedal is greater than a threshold. Therefore, both conditions are satisfied once the speed drops to threshold31. A stop event35between time33and time34is thus detected, and the invention accordingly adjusts the compressor stroke to obtain an optimized extra cooling of the evaporator during stop event35.

Another example of a vehicle velocity profile is shown inFIG. 3by a dashed line36which is below speed threshold31even before time33. However, the magnitude of deceleration is below the threshold for detecting a stop event prior to time33. At time33, however, the deceleration increases thereby resulting in the detection of a stop event37. Due to the fact that stop event37would be projected to end sooner than stop event35(as determined from the starting speed and deceleration), the increase in compressor stroke during stop event37would typically be larger than the increase adopted in stop event35.

Once the vehicle reaches a stop (i.e., instantaneous speed becomes zero), it remains stopped during a stop time38. The optimized extra cooling allows the engine to remain turned off for a longer time. When driving continues at a time40, vehicle speed increases at41and operation of the compressor can continue as soon as it become needed.

FIG. 4shows evaporator temperature as a function of time. Initially, the evaporator temperature is being controlled along a line segment45at a substantially constant temperature according to a conventional CTL value. Upon detection of a stop-event, prior art systems that maximize regenerative cooling would go to maximum cooling and would provide a dropping evaporator temperature as shown at segment46. The present invention that uses an optimized extra cooling which is typically less than maximum provides a dropping evaporator temperature along a segment47which reaches an optimized temperature48at the end of the stop event. When the engine is shut off during stop time38, the evaporator temperature gradually increases as shown at segments50and51for the invention and the prior art, respectively. Due to overcooling, prior art segment51shows the evaporator temperature may continue to increase even after the engine restarts, reflecting the is unnecessary load that was placed on the engine during the stop event. In contrast, when the engine restarts using the embodiment of the present invention, no overcooling was obtained and now additional cooling is needed upon restart of the engine so that a further recovery of the evaporator temperature is needed as shown at segment52.

An example compressor stroke is shown inFIG. 5as measured between 0% and 100% of full stroke. As an example, stroke may be running at about 50% while driving as shown at segment55. In the prior art, upon detection of a stop-event the stroke is raised to 100% as shown at segment56. In the present invention, stroke is raised to an intermediate value as shown at segment57. The actual amount of this increase can be determined by various methods as described below. Once the engine is shut off at time34, the compressor ceases operation and the last commanded stoke value begins to decrease because of loss of pressure as shown at line segments58and59. Once the engine restarts, pressure is restored and the compressor stroke can move back to its newly commanded value as shown at segments60and61.

The performance of the prior art maximization of the stroke may have many disadvantages. For example, if a target evaporator temperature is set to obtain maximum displacement in the compressor, then the target evaporator temperature may not have been reached by the time the vehicle actually comes to a stop. This may delay the shutting off of the engine because of the ongoing demand for cooling. By running the engine with the vehicle stopped, the fuel economy improvement sought by having a start-stop technology is compromised. Even if the engine is immediately shut off, additional unnecessary work is done by the compressor prior to engine shut-off, thereby increasing fuel consumption. Furthermore, the excessive cooling may cause discomfort to the occupants.

In order to achieve the optimized change in compressor stroke and evaporator temperature according to one embodiment, a quantity Y is determined as an adjustment factor.FIG. 6shows that Y has a value of zero shown prior to the stop-event at segment62, and a value of zero at segment64after the stop-event. During is the stop-event, Y has a non-zero value as shown at segment63. Since the vehicle deceleration may not stay constant during a stop-event, the value of Y during segment63may either increase or decrease accordingly. Y may have a value that increases a current compressor stroke of around 50% to a compressor stroke between 70% and 90%, depending upon the time until stop (i.e., the duration of the stop-event) and other conditions. A shorter projected duration of the stop event would result in a greater increase in compressor stroke. For any particular vehicle design, the quantitative values for Y under different conditions may be determined empirically during design and testing of the vehicle, for example.

Based on a value of Y, the compressor stroke can be modified as shown inFIG. 8. Thus, a stroke command obtained from the air conditioning controller is provided to one input of an adder70. The value Y is provided to the other input of adder70and a modified stroke command is output to a compressor71. Steps may be taken (not shown) to ensure that the modified stroke command does not exceed 100% or does not reach a level that causes icing of the evaporator.

In a more preferred embodiment, the value of CTL being used to control air conditioning system operation may be modified as shown inFIG. 7. An unmodified CTL value obtained in a conventional manner is designated as having a value X. Control system operation is performed using a modified CTL value comprised of X+Y. As shown inFIG. 7, the sum X+Y has the conventional value prior to and after a stop-event since the value of Y is zero at those times. During the stop-event, however, the graph ofFIG. 7shows a modified CTL at a line segment72. The increased CTL value causes the compressor to do additional work, thereby providing extra cooling into the evaporator. During stop time38, the CTL value shown at segment73(after Y has returned to zero) may gradually increase in response to gradually increasing evaporator and internal temperatures. After engine restart, the CTL value may continue to rise temporarily until air conditioning operation recovers toward an equilibrium.

An embodiment using a modified CTL is shown in greater detail inFIG. 9. The conventional CTL value X is provided to one input of an adder75while adjustment factor Y is provided to another input. The output of adder70is coupled to a block76for transforming the modified CTL value to a target evaporator temperature. The actual evaporator temperature is compared to the target temperature in a controller77such as a proportional-integral-derivative (PID) controller. The resulting stroke command is provided to a compressor78. When the value of Y is zero, then the air conditioning control loop operates in a conventional manner. When the value of Y is non-zero during a stop event, the evaporator target temperature and stroke command become optimized to achieve additional cooling to extend the engine off time during a vehicle stop.

FIG. 10illustrates a preferred derivation for a value of Y. A calculation block80receives an instantaneous speed signal and a magnitude of deceleration, which together indicate the expected time until the vehicle stops (at which time it becomes desirable to shut off the engine). The value of Y can be determined using just these two variables. In alternative embodiments, a blower rate and/or an estimated duration of a stop may also be used in determining a value for Y. Calculation block80may be based on a look-up table having values empirically determined during a design effort, or may include calculations involving the following functions. In particular, Y may be a function of the conventional CTL value X, vehicle speed, and deceleration rate according to the following formula:

Y=C1⁢X+C2⁢dv
where d is the magnitude of deceleration and v is the instantaneous speed signal. C1and C2are constants determined for a particular system design.

The blower speed is an indication of the occupant's desire for cooling and also reflects the ability for storing cold in the evaporator over time. Thus, a higher blower speed would tend to increase the desired value for Y. Preferably, the blower speed may be expressed as a blower rate as a percentage of the maximum blower speed.

Values for Y may be determined either from a lookup table or by appropriate evaluation of functions according to an empirically chosen average engine off time during typical driving conditions. Alternatively, a dynamically estimated stop duration may be maintained by the vehicle in response to recent history of the vehicle usage, for example. The dynamically estimated duration can then be employed to determine the desired amount of extra cooling in order to maintain an engine off condition during the entire expected stop duration. An embodiment of a function for calculating a value of Y in response to all four of these variables is as follows.

Y=(C1⁢X+C2⁢dv+C3⁢Brate)⁢DestDnom
where Brateis the blower rate, Destis the estimated stop duration and Dnomis a nominal or default stop duration.

As previously mentioned, the magnitude of the deceleration can be determined in response to an absolute brake pedal angle or a rate of change in the pedal angle, which is especially to detect the initiation of a stop event. Alternatively, deceleration may be obtained by dividing the difference between two instantaneous speed measurements by the time difference between the measurements. By using an optimum compressor stroke based on an optimum evaporator temperature, less work is done by the compressor and less energy is used before the vehicle stops. Thus, the invention realizes fuel savings during the deceleration and during an extended off time after the vehicle stops.

The Climate Thermal Load as used herein can include any known factors for controlling the level of air conditioning cooling, including interior and exterior temperatures, sunload, and operator settings for a temperature and/or blower speed. The adjustment factor for modifying the CTL during a stop event preferably includes speed and deceleration, and may alternatively include blower rate and estimated duration of stop if applicable.