Air conditioning system for hybrid vehicles

Abstract

An air conditioning system for hybrid vehicles includes a control unit configured to re-operate an engine if an engine cooling water temperature is reduced to a lower limit value or less after entry into a motor drive mode, and configured to stop the engine if the engine cooling water temperature is increased to an upper limit value or more. The control unit is configured to change the upper limit value and the lower limit value depending on a target air injection temperature calculated according to an internal/external temperature condition and a user-set temperature so that, in the motor drive mode, a re-operation time point of the engine is actively changed according to the target air injection temperature.

Claims

1. An air conditioning system for hybrid vehicles, comprising: a control unit configured to re-operate an engine when an engine cooling water temperature is reduced to a lower limit value or less after entry into a motor drive mode, and configured to stop the engine if when the engine cooling water temperature is increased to an upper limit value or more; and an injected air temperature detection unit configured to detect a vehicle room injection air temperature, wherein the vehicle room injection air temperature is an actual temperature of air injected into a vehicle room; wherein the control unit controls the vehicle room injection air temperature so that the vehicle room injection air temperature is converged toward a target air injection temperature using a compensation value between the target air injection temperature and the vehicle room injection air temperature measured by the injected air temperature detection unit, wherein the control unit is configured to change the upper limit value and the lower limit value according to the target air injection temperature, wherein the target air injection temperature is calculated using an internal temperature condition or an external temperature condition and a user-set temperature so that, in the motor drive mode, a re-operation time point of the engine is actively changed according to the target air injection temperature, wherein the control unit is configured, in the motor drive mode, to re-operate the engine according to an opening position of a temperature door, and wherein the control unit is configured, in the motor drive mode, to re-operate the engine only when the target air injection temperature is equal to or higher than a value obtained by adding a predetermined temperature compensation value to the vehicle room injection air temperature, even when the engine cooling water temperature is reduced to the lower limit value or less.

2. The air conditioning system of claim 1, wherein the control unit is configured to calculate the lower limit value from the current target air injection temperature and a predetermined first compensation value using equation (1):
lower limit value=current target air injection temperature+predetermined first compensation value(1), and wherein the control unit is configured to calculate the upper limit value from the current target air injection temperature and a predetermined second compensation value using equation (2):
upper limit value=current target air injection temperature+predetermined second compensation value(2).

3. The air conditioning system of claim 2, wherein the predetermined first compensation value and the predetermined second compensation value are constant values for compensating a measurement error of the engine cooling water temperature, and the predetermined second compensation value is larger than the predetermined first compensation value.

4. The air conditioning system of claim 1, wherein the control unit is configured to, in the motor drive mode, re-operate the engine only when the temperature door is in a maximum heating position even if the engine cooling water temperature is reduced to the lower limit value or less.

5. The air conditioning system of claim 1, further comprising: a calculation unit configured to calculate a temperature difference E-now between the target air injection temperature and the vehicle room injection air temperature detected by the injected air temperature detection unit at a present time and a temperature difference E-prev between the target air injection temperature and the vehicle room injection air temperature detected by the injected air temperature detection unit at a previous time, in a predetermined time interval from a time point at which an engine drive mode is switched to the motor drive mode, wherein the control unit is configured to calculate an opening position correction value T-now of the temperature door with respect to a cold air path and a hot air path by processing the temperature differences E-now and E-prev calculated by the calculation unit in the predetermined time interval and a temperature door opening position T-prev available at the previous time point using equation (3): T-now=T-prev+Cp[Gp(E-nowE-prev)+GiE-now] . . . (1), where T-now is a temperature door opening position correction value, T-prev is a temperature door opening position available at the previous time, Cp is a proportional gain, Gp is a constant value with respect to a temperature difference, E-now is a temperature difference ( C.) between the target air injection temperature and the vehicle room injection air temperature available at the present time, E-prev is a temperature difference ( C.) between the target air injection temperature and the vehicle room injection air temperature available at the previous time, and Gi is an integral gain, and wherein the control unit is configured to correct the opening position of the temperature door based on the opening position correction value T-now of the temperature door.

6. The air conditioning system of claim 5, wherein the control unit is configured to, even in the motor drive mode, perform the change of the lower limit value and the upper limit value using equations (1), (2) and (3) and a correction of the opening position of the temperature door only when predetermined entry conditions are satisfied.

7. The air conditioning system of claim 6, wherein the entry conditions include a condition that the engine cooling water temperature is equal to or lower than the target air injection temperature, a condition that the temperature door is moved to a maximum heating position, and a condition that the vehicle room injection air temperature is equal to or lower than the target air injection temperature, and wherein the control unit is configured to, after entry into the motor drive mode, perform the change of the lower limit value and the upper limit value using equations (1), (2) and (3) and the correction of the opening position of the temperature door only when the condition that the engine cooling water temperature is equal to or lower than the target air injection temperature, the condition that the temperature door is moved to the maximum heating position, and the condition that the vehicle room injection air temperature is equal to or lower than the target air injection temperature, are satisfied.

8. The air conditioning system of claim 6, wherein the entry conditions include a condition that the engine cooling water temperature is equal to or lower than a first entry reference temperature obtained by adding a predetermined first temperature compensation value to the target air injection temperature, a condition that the temperature door is moved to a maximum heating position, and a condition that the vehicle room injection air temperature is equal to or lower than a second entry reference temperature obtained by subtracting a predetermined second temperature compensation value from the target air injection temperature, and wherein the control unit is configured to, after entry into the motor drive mode, perform the change of the lower limit value and the upper limit value using equations (1), (2) and (3) and the correction of the opening position of the temperature door (9) only when the condition that the engine cooling water temperature is equal to or lower than the first entry reference temperature, the condition that the temperature door is moved to the maximum heating position, and the condition that the vehicle room injection air temperature is equal to or lower than the second entry reference temperature, are satisfied.

9. The air conditioning system of claim 6, wherein if the motor drive mode is cancelled or a predetermined cancellation condition is satisfied while performing variable control of the lower limit value and the upper limit value and correction control of the opening position of the temperature door after entry into the motor drive mode, the control unit is configured to cancel the variable control of the lower limit value and the upper limit value and the correction control of the opening position of the temperature door.

10. The air conditioning system of claim 9, wherein the cancellation condition includes a condition that the engine cooling water temperature is equal to or higher than a cancellation reference temperature obtained by adding a predetermined third temperature compensation value to the target air injection temperature, and wherein if the condition that the engine cooling water temperature is equal to or higher than the cancellation reference temperature, the control unit is configured to cancel the variable control of the lower limit value and the upper limit value and the correction control of the opening position of the temperature door.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a graph showing an engine cooling water temperature and a vehicle room injection air temperature when the drive mode of a hybrid vehicle of the related art is switched from an engine drive mode to a motor drive mode.

(2) FIG. 2 is a graph showing an engine cooling water temperature and a vehicle room injection air temperature when the drive mode of another hybrid vehicle of the related art is switched from an engine drive mode to a motor drive mode.

(3) FIG. 3 is a graph showing a state in which an engine is on-off controlled depending on an engine cooling water temperature when a hybrid vehicle of the related art is controlled in a motor drive mode.

(4) FIG. 4 is a view illustrating the configuration of an air conditioning system for hybrid vehicles according to the present invention.

(5) FIG. 5 is a graph showing a state in which an engine is on-off controlled depending on an engine cooling water temperature when the hybrid vehicle of the present invention is controlled in a motor drive mode.

(6) FIG. 6 is a graph showing a state in which an engine is on-off controlled depending on an engine cooling water temperature and an opening angle of a temperature door when the hybrid vehicle of the present invention is controlled in a motor drive mode.

(7) FIG. 7 is a flowchart illustrating an operation example of an air conditioning system for hybrid vehicles according to the present invention, in which example an opening position of a temperature door is compensated in a motor drive mode.

(8) FIG. 8 is a flowchart illustrating an operation example of an air conditioning system for hybrid vehicles according to the present invention, in which example a re-operation of an engine is controlled in a motor drive mode.

MODE FOR CARRYING OUT THE INVENTION

(9) A preferred embodiment of an air conditioning system for hybrid vehicles according to the present invention will now be described in detail with reference to the accompanying drawings. Components similar to those of the related art described earlier will be designated by like reference symbols.

(10) Prior to describing features of an air conditioning system for hybrid vehicles according to the present invention, an air conditioning system for motor vehicles will be briefly described with reference to FIG. 4.

(11) The air conditioning system for motor vehicles includes an air conditioner case 1 within which a blower 3, an evaporator 5 and a heater core 7 are installed.

(12) The blower 3 is configured to draw an internal or external air and to blow the air toward an internal path 1a of the air conditioner case 1. The evaporator 5 is configured to cool the air blown toward the internal path 1a. The heater core 7 is configured to heat the air blown toward the internal path 1a. Particularly, the heater core 7 is configured to receive high-temperature cooling water from an engine 8. The heater core 7 allows the cooling water to exchange heat with the ambient air, thereby heating the air blown into a vehicle room.

(13) The air conditioning system includes a temperature door 9 installed in the internal path 1a of the air conditioner case 1. The temperature door 9 is installed at a branch point of a cold air path 1b and a hot air path 1c. The temperature door 9 is configured to adjust the opening amounts of the cold air path 1b and the hot air path 1c while swinging between the cold air path 1b and the hot air path 1c. Thus, the temperature door 9 adjusts a cold air amount or a hot air amount supplied into the vehicle room.

(14) In the air conditioning system for hybrid vehicles, as shown in FIG. 2, if the drive mode of a hybrid vehicle is switched from an engine drive mode to a motor drive mode, the engine 8 is stopped (see curve A) and the engine cooling water temperature is lowered (see curve B).

(15) The temperature of the heater core 7 is also lowered due to the lowered cooling water temperature (see curve C). Thus, the temperature of the air injected into the vehicle room is lowered (see curve D). As a result, the vehicle room heating efficiency is reduced.

(16) Next, some features of the air conditioning system for hybrid vehicles according to the present invention will be described in detail with reference to FIGS. 4 to 8.

(17) Referring first to FIG. 4, the air conditioning system of the present invention includes an injected air temperature detection unit 10 configured to detect an actual temperature of an air injected into a vehicle room. The injected air temperature detection unit 10 is formed of temperature sensors installed in some of vehicle room air injection vents. For example, the injected air temperature detection unit 10 is formed of a center vent temperature sensor installed in a vehicle room center vent and a floor vent temperature sensor installed in a vehicle room floor vent.

(18) The injected air temperature detection unit 10 is configured to directly detect an actual temperature of an air injected into the vehicle room and to input data on the detected air temperature to a control unit 20 which will be described below.

(19) The air conditioning system of the present invention includes a control unit 20. The control unit 20 is formed of a microprocessor and is provided with a calculation unit 22.

(20) If the drive mode of the hybrid vehicle is switched from an engine drive mode to a motor drive mode when the data on a vehicle room injection air temperature is inputted from the injected air temperature detection unit 10, the calculation unit 22 calculates a temperature difference E-now between a target air injection temperature and a vehicle room injection air temperature at the present time and a temperature difference E-prev between a target air injection temperature and a vehicle room injection air temperature at the previous time, for example, 5 seconds earlier than the present time, in a predetermined time interval, for example, in a several millisecond interval from the time point at which the engine drive mode is switched to the motor drive mode.

(21) For reference, the target air injection temperature is a value for optimally controlling an evaporator 5, a PTC heater 7a and various kinds of doors D and is set based on a user-set temperature, an evaporator temperature, an external air temperature, an internal air temperature, a solar radiation amount and so forth. This means that the target air injection temperature indirectly contains those factors that cause a change in the vehicle room injection air temperature, namely the user-set temperature, the evaporator temperature, the external air temperature, the internal air temperature, the solar radiation amount and so forth.

(22) After entry into the motor drive mode, if the temperature difference E-now between the target air injection temperature and the vehicle room injection air temperature at the present time and the temperature difference E-prev between the target air injection temperature and the vehicle room injection air temperature at the previous time are calculated in a predetermined time interval (an interval of 5 seconds) by the calculation unit 22, the control unit 20 finally calculates an opening position correction value T-now of the temperature door 9 with respect to the cold air path 1b and the hot air path 1c by processing the temperature differences E-now and E-prev and a temperature door opening position T-prev using the following equation (1) pre-stored in the control unit 20.
T-now=T-prev+Cp[Gp(E-nowE-prev)+GiE-now](1)

(23) In equation (1), T-now is an opening position correction value (V) of the temperature door, T-prev is an opening position (V) of the temperature door at the previous time, Cp is a proportional gain, Gp is a constant value with respect to the temperature difference, E-now is a temperature difference ( C.) between the target air injection temperature and the vehicle room injection air temperature at the present time, E-prev is a temperature difference ( C.) between the target air injection temperature and the vehicle room injection air temperature at the previous time, and Gi is an integral gain.

(24) In this regard, Cp, Gp and Gi are the constant values pre-stored in the control unit 20 and are found through several experiments.

(25) The opening position correction value T-now of the temperature door 9 represented by equation (1) is a value calculated in view of the target air injection temperature, the vehicle room injection air temperature, the temperature difference at the present time and the temperature difference at the previous time. Use of equation (1) makes it possible to find an actual change in the vehicle room injection air temperature, which is generated by injection air temperature changing factors after entry into the motor drive mode. Thus, it is possible to calculate the opening position correction value T-now of the temperature door 9 that can compensate the actual change in the vehicle room injection air temperature.

(26) The opening position correction value T-now of the temperature door 9 represented by equation (1) is calculated as a voltage value V applied to the temperature door 9 which adjusts the opening amount of the cold air path 1b or the hot air path 1c.

(27) Upon calculating the opening position correction value T-now of the temperature door 9, the control unit 20 corrects the opening position of the temperature door 9 based on the opening position correction value T-now of the temperature door 9. This makes it possible to correct the opening position of the temperature door 9 in view of all the injection air temperature changing factors. Specifically, it is possible to correct the opening position of the temperature door 9 in view of the blower air volume, the evaporator temperature, the user-set temperature, the internal or external temperature, the solar radiation amount and the like, which may directly affect the change in the injection air temperature.

(28) Accordingly, in the motor drive mode, it is possible to optimally correct the opening position of the temperature door 9 in conformity with the actual change in the vehicle room injection air temperature. As a result, the vehicle room injection air temperature can be accurately compensated in the motor drive mode. This makes it possible to optimally control the vehicle room temperature in the motor drive mode.

(29) Referring again to FIG. 4, even when the opening position correction value T-now of the temperature door 9 is calculated in real time after entry into the motor drive mode, the control unit 20 does not unconditionally correct the opening position of the temperature door 9 based on the opening position correction value T-now. Only when all the predetermined entry conditions are met, the control unit 20 corrects the opening position of the temperature door 9.

(30) The entry conditions include a condition that the engine cooling water temperature is equal to or lower than the target air injection temperature, a condition that the temperature door 9 is moved to a maximum heating position to open the hot air path 1c at the maximum, and a condition that the vehicle room injection air temperature is equal to or lower than the target air injection temperature.

(31) The control unit 20 is configured to correct the opening position of the temperature door 9 according to the opening position correction value T-now of the temperature door 9 represented by equation (1), only when the condition that the engine cooling water temperature is equal to or lower than the target air injection temperature, the condition that the temperature door 9 is moved to a maximum heating position, and the condition that the vehicle room injection air temperature is equal to or lower than the target air injection temperature, are met.

(32) The reason for employing this configuration is that if the engine cooling water temperature exceeds the target air injection temperature, the engine cooling water temperature is sufficiently high and the opening position of the temperature door 9 need not be compensated.

(33) Furthermore, if the vehicle room injection air temperature exceeds the target air injection temperature, the vehicle room injection air temperature is sufficiently high and the opening position of the temperature door 9 need not be compensated.

(34) In the case where the condition that the temperature door 9 is moved to a maximum heating position is not met, the vehicle room injection air temperature can be sufficiently compensated by merely controlling the opening angle of the temperature door 9 with respect to the hot air path 1c without having to use equation (1).

(35) Other examples of the entry condition include a condition that the engine cooling water temperature is equal to or lower than a first entry reference temperature obtained by adding a predetermined first temperature compensation value to the target air injection temperature, a condition that the temperature door 9 is moved to a maximum heating position, and a condition that the vehicle room injection air temperature is equal to or lower than a second entry reference temperature obtained by subtracting a predetermined second temperature compensation value from the target air injection temperature.

(36) The control unit 20 may be configured to correct the opening position of the temperature door 9 according to the opening position correction value T-now of the temperature door 9 calculated by equation (1), only when the condition that the engine cooling water temperature is equal to or lower than the first entry reference temperature obtained by adding the predetermined first temperature compensation value to the target air injection temperature, the condition that the temperature door 9 is moved to a maximum heating position, and the condition that the vehicle room injection air temperature is equal to or lower than the second entry reference temperature obtained by subtracting the predetermined second temperature compensation value from the target air injection temperature, are met.

(37) In this regard, the first temperature compensation value is a value set in view of a measurement error of the engine cooling water temperature and may preferably be set at 5 C. The second temperature compensation value is a value set in view of a measurement error of the vehicle room injection air temperature and may preferably be set at 2 C.

(38) When the opening position of the temperature door 9 is compensated after entry into the motor drive mode, the control unit 20 is configured to stop opening position compensation control of the temperature door 9 using equation (1), if the motor drive mode is cancelled or a predetermined cancellation condition is met, for example, if a condition that the engine cooling water temperature is equal to or higher than a cancellation reference temperature obtained by adding a predetermined third temperature compensation value to the target air injection temperature, is met. In this regard, the third temperature compensation value may preferably be set at 10 C.

(39) Referring again to FIGS. 4, 5 and 6, the control unit 20 is configured to re-operate the engine 8 when the engine cooling water temperature is lowered to a lower limit value L or less after entry into the motor drive mode. Furthermore, the control unit 20 is configured to stop the engine 8 when the engine cooling water temperature is increased to an upper limit value U or more after the re-operation of the engine 8.

(40) In this case, the lower limit value L for re-operating the engine 8 is calculated by adding up a current target air injection temperature M and a first compensation value as in equation (2) indicated below. The upper limit value U for stopping the re-operation of the engine 8 is calculated by adding up a current target air injection temperature M and a second compensation value as in equation (3) indicated below. Equation (2) and equation (3) are stored in the control unit 20.

(41) After entry into the motor drive mode, the control unit 20 monitors the current target air injection temperature M, the first compensation value and the second compensation value in real time using equation (2) and equation (3) and calculates the lower limit value L and the upper limit value U in real time. Then, the control unit 20 compares the lower limit value L and the upper limit value U with the engine cooling water temperature in real time and controls the re-operation and stop of the engine 8 in real time.
Lower limit value L=current target air injection temperature M+first compensation value (2)
Upper limit value U=current target air injection temperature M+second compensation value (3)

(42) In this regard, the first compensation value and the second compensation value are used to compensate a measurement value of the engine cooling water temperature and are found by several experiments. The second compensation value is larger than the first compensation value .

(43) In the air conditioning system of the present invention configured as above, after entry into the motor drive mode, the engine 8 is re-operated depending on the engine cooling water temperature. As illustrated in FIGS. 5 and 6, the lower limit value L and the upper limit value U as reference values for the determination of the re-operation of the engine 8 are changed based on the current target air injection temperature M. This makes it possible to, in the motor drive mode, actively change the re-operation time point of the engine 8 depending on the target air injection temperature.

(44) Unlike the technique of the related art in which the re-operation time point of the engine 8 is controlled depending on one lower limit value and one upper limit value, it is possible in the present invention to actively control the re-operation time point of the engine 8.

(45) Thus, in the case where the target air injection temperature is set low so that the vehicle room injection air temperature can be sufficiently compensated even under a relatively-low engine cooling water temperature, the re-operation time point of the engine 8 can be delayed in conformity with the low target air injection temperature.

(46) As a result, it is possible to optimally control the vehicle room injection air temperature in conformity with the target air injection temperature, thereby suppressing the re-operation of the engine 8. This makes it possible to prevent unnecessary frequent re-operation of the engine 8 and to significantly improve the fuel efficiency.

(47) Referring again to FIGS. 4 and 6, the control unit 20 does not unconditionally re-operate the engine 8 even when the engine cooling water temperature is lowered to the lower limit value L or less (see curve X in FIG. 6) after entry into the motor drive mode

(48) Determination is first made as to whether the temperature door 9 is moved to a maximum heating position to open the hot air path 1c at the maximum. Then, the control unit 20 re-operates the engine 8 only when the temperature door 9 opens the hot air path 1c at the maximum.

(49) The reason for employing this configuration is that even if the engine cooling water temperature is lowered to the lower limit value L or less (see curve X in FIG. 6), the vehicle room injection air temperature can be sufficiently compensated by controlling the opening angle of the temperature door 9 with respect to the hot air path 1c.

(50) After the vehicle room injection air temperature has been compensated by controlling the opening angle of the temperature door 9, the engine 8 is re-operated to control the vehicle room injection air temperature. This makes it possible to sufficiently compensate the vehicle room injection air temperature while delaying the re-operation of the engine 8 as far as possible. As a result, it is possible to prevent the unnecessary frequent re-operation of the engine 8 and to significantly improve the fuel efficiency of the hybrid vehicle.

(51) After entry into the motor drive mode, the control unit 20 does not unconditionally re-operate the engine 8 even if the engine cooling water temperature is reduced to the aforementioned lower limit value L or less (see curve X in FIG. 6). The control unit 20 is configured to re-operate the engine 8 only when the target air injection temperature is equal to or higher than a value obtained by adding a predetermined temperature compensation value to the vehicle room injection air temperature. In this case, it is preferred that the temperature compensation value is set at 2 C.

(52) After entry into the motor drive mode, the control unit 20 is configured to variably control the lower limit value L and the upper limit value U, which serve as criteria of the re-operation of the engine 8. If the motor drive mode is cancelled or if the aforementioned cancellation condition is satisfied, for example, if a condition that the engine cooling water temperature is equal to or higher than a cancellation reference temperature obtained by adding a predetermined third temperature compensation value to the target air injection temperature is satisfied, the control unit 20 stops the variable control of the lower limit value L and the upper limit value U and returns the lower limit value L and the upper limit value U to the original state.

(53) Next, an operation example of the present air conditioning system configured as above will be described with reference to FIGS. 4, 7 and 8.

(54) First, descriptions will be made on an operation example of compensating the opening position of the temperature door 9 in the motor drive mode.

(55) Referring to FIGS. 4 and 7, the hybrid vehicle is operated in a heating mode (S101). In this state, determination is made as to whether the drive mode of the hybrid vehicle is switched from the engine drive mode to the motor drive mode (S103).

(56) If the result of determination reveals that the drive mode of the hybrid vehicle is switched to the motor drive mode, the control unit 20 further determines whether the engine cooling water temperature and the state of the air conditioning system satisfy the predetermined entry conditions (S104). Specifically, the control unit 20 determines whether a condition that the engine cooling water temperature is equal to or lower than the first entry reference temperature obtained by adding the predetermined first temperature compensation value to the target air injection temperature, a condition that the temperature door 9 is moved to a maximum heating position, and a condition that the vehicle room injection air temperature is equal to or lower than the second entry reference temperature obtained by subtracting the predetermined second temperature compensation value from the target air injection temperature, are satisfied.

(57) If the result of determination reveals that all the entry conditions are satisfied, the control unit 20 calculates a temperature difference E-now between a target air injection temperature and a vehicle room injection air temperature at the present time and a temperature difference E-prev between a target air injection temperature and a vehicle room injection air temperature at the previous time, for example, 5 seconds earlier than the present time, in a predetermined time interval, for example, in a several seconds interval from the time point at which the engine drive mode is switched to the motor drive mode (S105).

(58) If the calculation of the temperature difference E-now and the temperature difference E-prev is completed, the control unit 20 processes the temperature differences E-now and E-prev and the temperature door opening position T-prev using equation (1) (S107) and calculates an opening position correction value T-now of the temperature door 9 with respect to the cold air path 1b and the hot air path 1c (S109).

(59) If the opening position correction value T-now of the temperature door 9 is calculated, the control unit 20 corrects the opening position of the temperature door 9 in real time based on the opening position correction value T-now of the temperature door 9 (S111).

(60) Then, the opening position of the temperature door 9 is corrected in view of the target air injection temperature and the injection air temperature changing factors. Thus, the opening position of the temperature door 9 is optimally corrected in conformity with the actual change in the vehicle room injection air temperature caused by the target air injection temperature and the injection air temperature changing factors.

(61) As a result, it is possible to accurately compensate the vehicle room injection air temperature in the motor drive mode and to optimally control the vehicle room temperature in the motor drive mode.

(62) While compensating and controlling the opening position of the temperature door 9 after entry into the motor drive mode, the control unit 20 determines whether the motor drive mode is cancelled or whether the predetermined cancellation condition is satisfied (S113). Specifically, determination is made as to whether a condition that the engine cooling water temperature is equal to or higher than the cancellation reference temperature obtained by adding the predetermined third temperature compensation value to the target air injection temperature, is satisfied.

(63) If the result of determination reveals that the motor drive mode is cancelled or the cancellation condition is satisfied, the control unit 20 cancels the compensation control of the opening position of the temperature door 9 using equation (1) (S115).

(64) Next, an operation example of controlling the re-operation of the engine 8 in the motor drive mode will be described with reference to FIGS. 4 and 8.

(65) The hybrid vehicle is operated in a heating mode (S201). In this state, determination is made as to whether the drive mode of the hybrid vehicle is switched from the engine drive mode to the motor drive mode (S203).

(66) If the result of determination reveals that the drive mode of the hybrid vehicle is switched to the motor drive mode, the control unit 20 further determines whether the engine cooling water temperature and the state of the air conditioning system satisfy the predetermined entry conditions (S204). Specifically, the control unit 20 determines whether a condition that the engine cooling water temperature is equal to or lower than the first entry reference temperature obtained by adding the predetermined first temperature compensation value to the target air injection temperature, a condition that the temperature door 9 is moved to a maximum heating position, and a condition that the vehicle room injection air temperature is equal to or lower than the second entry reference temperature obtained by subtracting the predetermined second temperature compensation value from the target air injection temperature, are satisfied.

(67) If the result of determination reveals that all the entry conditions are satisfied, the control unit 20 processes the current target air injection temperature M and the first compensation value at the entry time of the motor drive mode using equation (2) (S205) and calculates the lower limit value L in real time (S207).

(68) After the lower limit value L is calculated in real time, the control unit 20 determines whether the engine cooling water temperature is reduced to the lower limit value L or less (S209).

(69) If the result of determination reveals that the engine cooling water temperature is reduced to the lower limit value L or less, the control unit 20 re-operates the engine 8 (S211).

(70) As the engine 8 is re-operated, the engine cooling water temperature is increased. As a result, the vehicle room injection air temperature is increased to heat the interior of the vehicle room.

(71) When the engine 8 is re-operated, the control unit 20 processes the current target air injection temperature M and the second compensation value using equation (3) (S213) and calculates the upper limit value U in real time (S215).

(72) After the upper limit value U is calculated in real time, the control unit 20 determines whether the engine cooling water temperature is increased to the upper limit value U or more (S217).

(73) If the result of determination reveals that the engine cooling water temperature is increased to the upper limit value U or more, the control unit 20 stops the engine 8 (S219). The stoppage of the engine 8 limits consumption of fuel, thereby improving the fuel efficiency of the hybrid vehicle.

(74) After entry into the motor drive mode, the control unit 20 variably controls the lower limit value L and the upper limit value U. In this state, the control unit 20 determines whether the motor drive mode is cancelled or whether the cancellation condition is satisfied (S221). Specifically, the control unit 20 determines whether a condition that the engine cooling water temperature is equal to or higher than the cancellation reference temperature obtained by adding the predetermined third temperature compensation value to the target air injection temperature, is satisfied.

(75) If the result of determination reveals that the motor drive mode is cancelled or the cancellation condition is satisfied, the control unit 20 stops the variable control of the lower limit value L and the upper limit value U (S223).

(76) According to the present air conditioning system configured as above, in the motor drive mode, the opening position of the temperature door 9 is corrected in response to the change in the vehicle room injection air temperature and in view of different factors that affect the actual change in the vehicle room injection air temperature. It is therefore possible to optimally correct the opening position of the temperature door 9 in conformity with the actual change in the injection air temperature.

(77) Furthermore, in the motor drive mode, the opening angle of the temperature door 9 is optimally controlled in conformity with the actual change in the injection air temperature. It is therefore possible to, in the motor drive mode, accurately compensate the vehicle room injection air temperature and to optimally control the vehicle room temperature in the motor drive mode.

(78) Furthermore, in the motor drive mode, the engine 8 is re-operated depending on the engine cooling water temperature. The engine re-operation time point is variably controlled according to the target air injection temperature. It is therefore possible to suppress the engine re-operation while optimally controlling the vehicle room injection air temperature in conformity with the target air injection temperature and to prevent the unnecessary re-operation of the engine 8.

(79) Furthermore, in the motor drive mode, the engine 8 is re-operated depending on the engine cooling water temperature. The engine re-operation time point is controlled in association with the opening angle of the temperature door 9. It is therefore possible to sufficiently compensate the vehicle room injection air temperature by merely controlling the opening angle of the temperature door 9 without having to re-operate the engine 8 and to prevent the unnecessary re-operation of the engine 8.

(80) In addition, since the unnecessary re-operation of the engine 8 is prevented, it is possible to prevent the frequent re-operation of the engine 8 and to improve the fuel efficiency.

(81) While some preferred embodiments of the present invention have been described above, the present invention is not limited to these embodiments. It is to be understood that various changes and modifications may be made without departing from the scope of the invention defined in the claims.