HVAC control using modulated evaporator outlet temperature

A vehicle climate control system operates an evaporator at a minimum temperature adjusted by a temperature offset. The control system determines a desired temperature and a blend potentiometer position that is indicative of the desired temperature. The control system determines a target temperature for the evaporator according to the blend potentiometer position, and calculates the temperature offset according to the target temperature and the blend potentiometer position. A blend door position is controlled according to the blend potentiometer position, the target temperature, the temperature offset, and heater core air temperature in order to optimize the operation of the evaporator.

FIELD OF THE INVENTION

The present invention relates to vehicle climate control, and more particularly to a method for dynamically controlling vehicle climate according to a comfort zone.

BACKGROUND OF THE INVENTION

A climate control system10in a vehicle regulates temperature and humidity in the vehicle as shown inFIG. 1. The climate control system10includes a compressor12, a condenser14, an expansion element16, an evaporator18, and a heater core20. The compressor12moves a refrigerant through the refrigeration circuit of the climate control system10, which includes the compressor12, the condenser14, the expansion element16, and the evaporator18. The compressor12, which may be a cycling compressor or a variable displacement compressor, compresses and circulates the refrigerant vapor through the condenser14. The condenser14condenses the refrigerant vapor into a liquid and rejects heat, thereby cooling the liquid. The condenser14passes the liquid through the expansion element16, such as an expansion valve, into the evaporator18. The cooled refrigerant liquid passes through the evaporator18and returns to the compressor12as vapor.

A blower or fan22forces air24through the evaporator18and the heater core20into a passenger compartment26. As the refrigerant liquid inside the evaporator18cools the air24, the liquid absorbs heat from the air24and returns to vapor. The evaporator18cools the air24to a minimum temperature. Typically, the evaporator18is configured to cool the air24to a temperature just above freezing, effectively removing all humidity from the air24. A portion of the air24then passes through the heater core20. The heater core20is connected to the vehicle's coolant system28, which circulates water solution through the heater core20. For example, the water solution may include 50% water and 50% glycol. In this manner, the heater core20heats a portion of the air24according to a desired temperature, which is further determined from passenger feedback or thermostat controls. The heated air is recirculated with the cooled air to achieve the desired temperature and circulated into the passenger compartment26. Additionally, the climate control system10may include a controller (not shown) that communicates with components such as the compressor12, the evaporator18, and the blower22.

As described, the climate control system10operates to cool the air24to a low band temperature, such as a minimum temperature, regardless of the desired temperature, and then subsequently reheats the air. For example, the low band may be slightly above freezing, or 38° F. Alternatively, the climate control system10may adjust the low band according to other factors, such as the speed of the blower22, a mode setting, and the desired temperature. For example, the mode setting can be used to select panel, bi-level, floor, mixed, and defrost modes as are known in the art. Therefore, elements of the climate control system10, such as the compressor12, operate more than necessary and increase fuel consumption. In certain circumstances, maximum climate control usage may account for as much as a 4 mile per gallon increase of fuel consumption.

SUMMARY OF THE INVENTION

A vehicle climate control system comprises determining a desired temperature. A target temperature for air output from an evaporator that is indicative of the desired temperature is determined. A temperature offset is calculated according to the desired temperature and the target temperature. The air is cooled according to a minimum temperature adjusted by the temperature offset.

In another aspect of the invention, a vehicle climate control method comprises selecting a desired temperature. A blend door position that is indicative of the desired temperature is determined. An evaporator temperature that is indicative of at least one of the desired temperature and/or the blend potentiometer position is determined. A temperature offset is calculated according to at least one of the desired temperature, the blend potentiometer position, and/or the evaporator temperature. A heater core air temperature is determined. A blend door position is calculated according to the blend potentiometer position, the evaporator temperature, the temperature offset, and/or the heater core air temperature.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A passenger comfort zone30according to the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) as shown inFIG. 2. The comfort zone30is determined based on acceptable conditions for passengers according to factors such as temperature, humidity, season, and clothing. For example, clothing type worn by passengers, as well as humidity, may be different in the winter than in the summer, and the comfort zone30takes these factors into consideration. Hereinafter, “comfort zone” refers to the ASHRAE comfort zone30.

Rather than operating the evaporator to cool the air to the minimum temperature, the present invention operates the evaporator to cool the air to a temperature higher than the conventional minimum temperature in order to avoid excessive cycling of the compressor. In other words, instead of cooling the air to the minimum temperature and then reheating the air, the invention cools the air to a temperature that more accurately corresponds to the desired temperature of the passengers. In the preferred embodiment, the controller determines the low band according to the minimum temperature and an offset that is applied to the desired temperature, in addition to the blower speed and the mode setting.

The controller determines the offset40according to blend potentiometer position, which is indicative of the desired temperature, and a target evaporator air temperature42as shown inFIG. 3. Blend potentiometers partially control the position of a corresponding blend valve according to the desired temperature. The blend valves determine an amount of hot air that is blended with the cooled air from the evaporator. The blend potentiometer position is represented on the x-axis44in terms of a digital resolution of 256. Temperature is represented on the y-axis46in degrees Fahrenheit. As the blend potentiometer position increases, the target evaporator air temperature42increases linearly from 38° F. at blend potentiometer position 0 to 178° F. at blend potentiometer position 255. The offset40also increases, but reaches a maximum of 12° F. at blend potentiometer position 70. When the offset40is 12° F., the target evaporator air temperature is 76° F. The controller cools the air to the low band temperature48, which is equal to the conventional low band temperature of 38° F. adjusted by the offset40. For example, at blend potentiometer position 0, the offset40is 0. Therefore, the low band temperature48is 38° F. Similarly, at blend potentiometer position 70, the offset is 12° F., and the low band temperature48is 50° F.

An offset lookup table50is shown inFIG. 4. The offset lookup table50provides an offset and low band temperature according to the data inFIG. 3. The controller receives the blend potentiometer position and consults the lookup table50to determine the offset and the corresponding low band. The controller then controls the evaporator to cool the air to the low band temperature. It should be noted that the values used inFIGS. 3 and 4are exemplary, and other suitable values may be used to achieve similar results.

It is to be understood that although the preferred embodiment uses offset and low band values as shown inFIG. 3andFIG. 4, other values are possible. For example, the maximum value for the low band temperature is 50° F. as shown inFIG. 4. The maximum low band temperature is chosen to be a predetermined amount, such as 4° F., below a maximum average evaporator temperature. In the preferred embodiment, the maximum average evaporator temperature is 54° F. Therefore, the maximum offset is chosen to be 12° F. to ensure that the maximum low band temperature is 50° F., or 4° F. below the maximum average evaporator temperature. Additionally, the use of the offset may be phased in rather than used immediately. For example, the controller may operate conventionally for a specific duration after an initial AC request by the user. The offset can then be phased in over a period of time. In this manner, moisture is removed from the vehicle interior more quickly. In another embodiment, the controller may be adjusted according to different temperature requests from one or more passengers. For example, if only a driver is present, the offset is determined based solely on the driver's temperature request. In contrast, if a passenger is present, a modified offset may be used. Additionally, other suitable values for the maximum low band temperature and the maximum average evaporator temperature may be used.

A modulated evaporator outlet temperature method60is shown inFIG. 5. At step62, a desired temperature is sent to the controller. For example, a user requests the desired temperature with conventional air conditioner controls, such as a knob or dial. Alternatively, the desired temperature may be generated upon ignition or initial powering on of the vehicle. The controller monitors various factors affecting the operation of the climate control system at step64. These factors include, but are not limited to, the evaporator, ambient temperature, and the desired temperature. The controller calculates the position of the blend valves according to the blend potentiometers and the desired temperature, the evaporator temperature, and the temperature of the hot air from the heater core at step66.

The controller determines the offset based on the data inFIGS. 3 and 4at step68. If any blend potentiometer is set at zero, or full cold, the offset is zero, and the climate control system may operate in a conventional manner. The controller may limit evaporator temperatures based on a combination of a configurable offset below the current ambient temperature, a configurable upper bound, and a configurable minimum temperature at step70. For example, the controller limits compressor operation to prevent window fog. Typically, if the interior cabin dew point temperature is below the glass surface temperature, window fogging does not occur. Therefore, the controller ensures that the compressor and/or evaporator operation are limited to maintain a maximum temperature that is a fixed amount below the current ambient temperature. In one embodiment, the controller operates the compressor to release the compressor clutch, thereby turning off the compressor, at a temperature that is a configurable value below the ambient temperature, accounting for hysteresis. Additional factors may include a maximum allowable compressor clutch off temperature and a minimum compressor off temperature relative to ambient temperature.

At step72, the controller operates the climate control system with an offset of zero during initial operation, and phases in the offset according to compressor cycles. For example, if the desired temperature is full cold, the offset is initially zero. If the desired temperature is increased, the evaporator temperature is increased gradually rather than immediately to mitigate the effects from the change in humidity. At step74, the controller continues to monitor system variables, determines proper control of the compressor and evaporator, and adapts as necessary in order to maintain the desired temperature.

The controller calculates the blend valve position to provide the desired air temperature according to the blend potentiometer position as shown inFIG. 6. An energy balance diagram80includes an input node82, a blower node84, an evaporator node86, a blend valve node88, a heater core node90, and a mix node92. The total of all air in the system leaves the blower node84and is cooled at the evaporator node86. The blend valve node receives the cold air and diverts a portion of the cold air to the heater core node90and a portion of the cold air to the mix node92. Hot air from the heater core node90and the cold air are mixed together at the mix node92. The controller calculates energy balance at the mix node92. The controller calculates the temperature output from the mix node92according to Tout=Tc—expected+[(Th—expected−Tc—expected)*(% above full cold)], where Toutis temperature output from the mix node92, Tc—expectedis expected temperature output of the blend valve node88, Th—expectedis expected temperature output of the heater core node90, and % above full cold corresponds to an amount the desired temperature is above the minimum possible temperature. The output of the heater core node90is Thot=Th—water*10, where Th—wateris the temperature of the water solution in the heater core. Therefore, the desired blend valve position is Blend Pos %=(Tmix−Tcold)/(Thot−Tcold), where Tmixis temperature output of the mix node92.

The modulation method of the present invention may include a rain sensing and adjustment algorithm100as shown inFIG. 7. The algorithm100monitors the state of the windshield wipers to determine if the wipers are on or off at step102. If the wipers are off, the algorithm100continues to step104. If the wipers are on, the algorithm100determines the duration of time that the wipers have been on at step106. If the duration is above a threshold, the algorithm100continues to step108. At step108, the offset is adjusted according to a configurable rain factor. For example, the offset can be set to 0 if it is determined that the offset is unnecessary during rain conditions. If the duration is below the threshold, the algorithm100continues to step110, and the current offset is used. At step104, the algorithm100determines if the wipers were previously on. If the wipers have been off for a significant length of time, the algorithm100continues to step110. If the wipers were recently on, the algorithm phases out the rain factor at step112.