Control apparatus for use in automotive air conditioning system

The present invention is directed to a control apparatus for use in an automotive air conditioning system which includes a variable capacity-type refrigerant compressor. The automotive air conditioning system comprises a refrigerant circuit including a refrigerant compressor with an externally controlled, variable capacity control mechanism; an evaporator connected to a suction chamber of the compressor; and a control apparatus which controls refrigerant circuit operation. The control apparatus includes an adjusting device for adjusting a control point of the compressor suction chamber pressure. During operation of the automotive air conditioning system, the control point of the compressor suction chamber pressure is adjusted to effectively maintain the temperature of air immediately downstream from the evaporator at the set temperature. In one situation in which the automotive air conditioning system is operated in a static thermodynamic condition of the evaporator, the control point of the compressor suction chamber pressure is adjusted to effectively converge the temperature of air immediately downstream from the evaporator to the set temperature. In another situation in which the automotive air conditioning system is operated in a dynamic thermodynamic condition of the evaporator, the control point of the compressor suction chamber is also adjusted to effectively converge the temperature of air immediately downstream from the set temperature. Accordingly, the passenger compartment of the automobile can be more effectively air conditioned during operation of the automotive air conditioning system.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to an automotive air conditioning system. 
More particularly, it relates to a control apparatus for controlling 
operation of an automotive air conditioning system which includes an 
externally controlled, variable capacity-type refrigerant compressor. 
2. Description of the Related Art 
Control apparatus for controlling operation of an automotive air 
conditioning system, which include an externally controlled, variable 
capacity-type refrigerant compressor, are well known in the art. In prior 
art embodiment. The capacity of a refrigerant compressor is adjusted to 
control air temperature T.sub.e immediately downstream from an evaporator 
during operation of an automotive air conditioning system. T.sub.e is 
maintained at the set temperature T.sub.set by sending an electric signal 
having an amperage, which is determined by the 
proportional-plus-integral-plus-derivative control action (hereinafter 
"PID control action") of a conventional control apparatus, to an 
externally controlled, variable capacity control mechanism of the 
compressor. 
In general, the operation of the automotive air conditioning system is 
divided into a first situation in which the automotive air conditioning 
system is operated in a static thermodynamic condition of the evaporator 
and a second situation in which the automotive air conditioning system is 
operated in a dynamic thermodynamic condition of the evaporator. In the 
first situation, heat load on the evaporator is slightly increased or 
decreased in response, for example, to the slight changes in the 
rotational speed of an evaporator fan caused by slight changes in electric 
load on the automobile's battery, or by slight changes in air temperature 
immediately upstream from the evaporator. On the other hand, in the second 
situation, heat load on evaporator is quickly increased or decreased by 
large amounts in response, for example, to changes in the rotational speed 
of the evaporator fan, e.g., changes in the amount of air flow which 
passes through an exterior surface of the evaporator, or a change in the 
automotive air conditioning mode, such as a switch from a passenger 
compartment air circulation mode to an outside air intake mode or vice 
versa. 
In the conventional automotive air conditioning system described above, a 
coefficient of the PID control action of the control apparatus is fixed at 
one constant value during operation of the automotive air conditioning 
system. If, however, the coefficient of the PID control action of the 
control apparatus is selected at one constant value to effectively control 
the first situation of the operation of the automotive air conditioning 
system, the air temperature T.sub.e overshoots the set temperature 
T.sub.set by a significant amount. This amount is significant enough to 
lengthen the time period required for the air temperature T.sub.e to 
approach the set temperature T.sub.set in the second situation of the 
operation of the automotive air conditioning system. Such a delay is 
indicated by a dashed line in FIG. 4. On the other hand, if the 
coefficient of the PID control action of the control apparatus is selected 
at another constant value to effectively control the second situation of 
the operation of the automotive air conditioning system, the air 
temperature T.sub.e can not be maintained at the set temperature 
T.sub.set, in the first situation of the operation of the automotive air 
conditioning system. This is due to the oversensitive control of the 
operation of the automotive air conditioning system. 
SUMMARY OF THE INVENTION 
Accordingly, it is an object of the present invention to provide an 
automotive air conditioning system which can adequately air condition a 
passenger compartment of an automobile. 
The automotive air conditioning system of the present invention includes a 
refrigerant circuit having a refrigerant compressor with an externally 
controlled variable capacity control mechanism and an evaporator connected 
to a suction chamber of the refrigerant compressor. A fan is associated 
with the evaporator to move air through an exterior surface of the 
evaporator. A control mechanism controls operation of the refrigerate 
circuit. 
The control mechanism includes the following devices. A sensing device 
senses a thermodynamic characteristic relating to the evaporator, such as 
the temperature of air immediately downstream from the evaporator. A first 
carrying out device carries out operation of a feedback control action, 
such as a PID control action. A second carrying out device, such as a 
gradient operation device, determines a thermal gradient of the air 
immediately downstream from the evaporator with respect to a time period. 
A storage device stores a relationship between the thermal gradient and a 
coefficient which subsequently is multiplied the result of the operation 
of the feedback control action of the first carrying out device. This 
multiplication is performed in a pressure adjusting device. A determining 
device determines a value of the coefficient by contrasting an operational 
result of the second carrying out device and the relationship stored in 
the storage device. The pressure adjusting device adjusts a control point 
of pressure in the suction chamber of the compressor according to the 
result of the operation of the feedback control action of the first 
carrying out device and the value of the coefficient determined by the 
determining device. 
Other objects, advantages, and features will be apparent when the detailed 
description of the invention and the drawings are considered.

DESCRIPTION OF A PREFERRED EMBODIMENT 
Referring to FIG. 1 the automotive air conditioning system includes 
refrigerant circuit 10 and control apparatus 20 which controls an 
operation of the automotive air conditioning system. Refrigerant circuit 
10 includes refrigerant compressor 11 with an externally controlled, 
variable capacity control mechanism (not shown), condenser 12, expansion 
device 13, and evaporator 14, which are connected in series. 
Electromagnetic clutch 111 is fixedly mounted on compressor 11, and 
intermittently transmits power derived from an external power source, such 
as engine 15 of an automobile, to a drive shaft of compressor 10 in order 
to intermittently operate compressor 10. Refrigerant circuit 10 further 
includes condenser fan 121 which is associated with condenser 12 and 
passes air through an exterior surface of condenser 12 and evaporator fan 
141 which is associated with evaporator 14 and passes air through an 
exterior surface of evaporator 14. Condenser fan 121 and evaporator tan 
141 receive electric power from DC battery 16 installed in the engine 
compartment of the automobile. 
Evaporator 14 is air-tightly disposed within a duct (not shown) the inlet 
of which is linked to the automobile passenger compartment and to the 
outside of the automobile through respective auxiliary ducts (not shown). 
The inlet of the duct is equipped with a damper (not shown). When air in 
the automobile passenger compartment and air outside the automobile are 
drawn into the inlet of the duct through the auxiliary ducts by the 
operation of evaporator tan 141, the air in the automobile passenger 
compartment and the air outside the automobile are mixed. Various mixture 
ratios are obtained by changing the pivotal position of the damper. The 
air mixed at the inlet of the duct passes through the exterior surface of 
evaporator 14 and flows into the automobile passenger compartment through 
an outlet of the duct. 
Control apparatus 20 includes thermosensor 17, gradient operation device 
21a, coefficient adjusting device 21b, coefficient-gradient characteristic 
storage device 21c, subtracter 22, set value generating device 23, 
proportion operation device 24, integration operation device 25, 
differential operation device 26, first adder 27a, second adder 27b, final 
operation device 28, final storage device 29a, final adder 29b, and 
amplifier 30. These elements are described in detail below. Further, 
subtracter 22, set value generating device 23, proportion operation device 
24, integration operation device 25, differential operation device 26, 
first adder 27a, second adder 27b, final operation device 28, final 
storage device 29a, final adder 29b, and amplifier 30 form a PID control 
action apparatus. 
Thermosensor 17, which is associated with evaporator 14, detects air 
temperature T.sub.c immediately downstream from evaporator 14 within a 
predetermined short time interval .DELTA.t, e.g., one second, and 
generates first electric signal S.sub.1 representing the detected air 
temperature T.sub.c. Thermosensor 17 is connected to gradient operation 
device 21a. Gradient operation device 21a processes first electric signal 
S.sub.1 in accordance with the following equation: 
EQU .alpha..sub.n =.vertline.{(T.sub.e(n-3) +T.sub.e(n-2))/2-(T.sub.e(n-1) 
+T.sub.e(n))/2}/2.multidot..DELTA.t.vertline. (1) 
In equation (1), the first term of the numerator is the mean value of the 
(n-3)th detected air temperature T.sub.e(n-3) and the (n-2)th detected air 
temperature T.sub.e(n-2), and the second term of numerator is the mean 
value of the (n-1)th detected air temperature T.sub.e(n-1) and the (n)th 
detected air temperature T.sub.e(n). Therefore, .alpha..sub.n represents a 
leveled thermal gradient of the detected air temperature T.sub.e with 
respect to a time period from the (n-3)th detecting time to the (n)th 
detecting time, i.e., a three second time period. Accordingly, in one 
operational situation of the automotive air conditioning system in which 
heat load on the evaporator 14 is slightly increased or decreased, i.e., 
when the detected air temperature T.sub.e rises or falls slightly, the 
thermal gradient .alpha..sub.n becomes a very small value. On the other 
hand, in another operational situation of the automotive air conditioning 
system in which the heat load on the evaporator 14 increases or decreases 
by a large amount, i.e., when the detected air temperature T.sub.e rises 
or falls by a large amount, the thermal gradient .alpha..sub.n becomes a 
very large value. 
Gradient operation device 21a, which generates a second electric signal 
S.sub.2 representing the thermal gradient .alpha..sub.n, is connected to 
the coefficient adjusting device 21b. The coefficient adjusting device 21b 
is further connected to the coefficient-gradient characteristic storage 
device 21c which stores a coefficient-gradient characteristic as depicted 
in FIG. 2. The coefficient-gradient characteristic storage device 21c 
generates a third electric signal S.sub.3 representing the 
coefficient-gradient characteristic as depicted in FIG. 2. Third electric 
signal S.sub.3 is sent to coefficient adjusting device 21b from 
coefficient-gradient characteristic storage device 21c. 
Referring again to FIG. 2, when the thermal gradient .alpha..sub.n is equal 
to or greater than a predetermined first boundary value .alpha..sub.a, the 
coefficient A.sub.n is maintained at a predetermined maximum value 
A.sub.max. When the thermal gradient .alpha..sub.n is equal to or less 
than a predetermined second boundary value .alpha..sub.b, which is less 
than the predetermined first boundary value .alpha..sub.a, the coefficient 
A.sub.n is maintained at a predetermined minimum value A.sub.min. Further, 
when the thermal gradient .alpha..sub.n is less than the predetermined 
first boundary value .alpha..sub.a, but greater than the predetermined 
second boundary value .alpha..sub.b, coefficient A.sub.n varies within a 
range between the predetermined maximum and minimum values A.sub.max and 
A.sub.min. 
The coefficient adjusting device 21b processes second electric signal 
S.sub.2 sent from gradient operation device 21a and third electric signal 
S.sub.3 sent from the coefficient-gradient characteristic storage device 
21c by adjusting the coefficient A.sub.n in accordance with the 
coefficient-gradient characteristic as depicted in FIG. 2. Thus, the 
coefficient adjusting device 21b adjusts the coefficient A.sub.n in 
accordance with the following conditions. 
When .alpha..sub.n .gtoreq..alpha..sub.a, the coefficient A.sub.n is 
adjusted such that: 
EQU A.sub.n =A.sub.max. (2) 
When .alpha..sub.b &lt;.alpha..sub.n &lt;.alpha..sub.a, the coefficient A.sub.n 
is adjusted such that: 
##EQU1## 
In equation (3), {(A.sub.max -A.sub.min)/(.alpha..sub.a -.alpha..sub.b)} is 
a slope of straight line m which is depicted in FIG. 2, and (A.sub.min 
.multidot..alpha..sub.a -A.sub.max .multidot..alpha..sub.b)/(.alpha..sub.a 
-.alpha..sub.b) is an intercept at the ordinate with respect to straight 
line m. 
When .alpha..sub.b .gtoreq..alpha..sub.n, the coefficient A.sub.n is 
adjusted such that: 
EQU A.sub.n =A.sub.min. 
Accordingly, the coefficient A.sub.n varies in a range from A.sub.max to 
A.sub.min response to changes in the thermal gradient .alpha..sub.n. 
Coefficient adjusting device 21b generates a fourth electric signal 
S.sub.4 representing the coefficient A.sub.n as it varies in the range 
from A.sub.max to A.sub.min in response to changes in the thermal gradient 
.alpha..sub.n. Coefficient adjusting device 21b is further connected to 
final operation device 28 to send fourth electric signal S.sub.4 to final 
operation device 28. Further, when the ordinal number n is less than four, 
the thermal gradient .alpha..sub.n is adjusted, such that .alpha..sub.n is 
equal to .alpha..sub.b in gradient operation device 21a. 
Subtracter 22, which is also connected to thermosensor 17, receives first 
electric signal S.sub.1. Set value generating device 23 generates a fifth 
electric signal S.sub.5 representing the set temperature T.sub.set. 
Subtracter 22 processes first electric signal S.sub.1 sent from 
thermosensor 17 and fifth electric signal S.sub.5 sent from set value 
generating device 23 by subtracting the nth detected air temperature 
T.sub.e(n) from the set temperature T.sub.set. This subtraction is shown 
by the following equation: 
EQU .DELTA.T.sub.v(n) =T.sub.set -T.sub.e(n) (5) 
In equation (5), the appended symbol n indicates the ordinal number of the 
detected air temperature T.sub.e. Subtracter 22 generates a sixth electric 
signal S.sub.6 representing the operational result of equation (5). 
Subtracter 22 is further connected to proportion operation device 24, 
integration operation device 25, and differential operation device 26. 
Proportion operation device 24 processes sixth electric signal S.sub.6 sent 
from subtracter 22 in accordance with the following equation: 
EQU p(.DELTA.T.sub.v(n) -.DELTA.T.sub.v(n-1) (6) 
In equation (6), p is an arbitrary coefficient and is selected to be p=1 in 
this embodiment. Accordingly, equation (6) is transformed to the following 
equation: 
EQU (.DELTA.T.sub.v(n) -.DELTA.T.sub.v(n-1) (6)' 
Proportion operation device 24 generates a seventh electric signal S.sub.7 
representing the operational result of equation (6)'. Proportion operation 
device 24 is connected to a first adder 27a. 
Integration operation device 25 processes sixth electric signal S.sub.6 
sent from subtracter 22 in accordance with the following equation: 
EQU .DELTA.T.sub.v(n) .multidot..DELTA.T/T.sub.I (7) 
In equation (7), 1/T.sub.1 is an arbitrary coefficient, and AT is a 
predetermined short time operation interval which is equal to .DELTA.t. 
Integration operation device 25 generates an eighth electric signal Ss 
representing the operational result of equation (7). Integration operation 
device 25 is also connected to first adder 27a. 
First adder 27a processes seventh electric signal S.sub.7 sent from 
proportion operation device 24 and eighth electric signal Ss sent from 
integration operation device 25 by adding the operational result of 
equation (6)' and the operational result of equation (7). First adder 27a 
generates a ninth electric signal S.sub.9 representing the result of the 
addition performed therein. First adder 27a is connected to the second 
adder 27b. 
Differential operation device 26 processes sixth electric signal S.sub.6 
sent from subtracter 22 in accordance with the following formula: 
EQU T.sub.D (.DELTA.T.sub.v(n) -2.multidot..DELTA.T.sub.v(n-1) 
+T.sub.v(n-2)/.DELTA.T (8) 
In equation (8), T.sub.D is an arbitrary coefficient. Differential 
operation device 26 generates a tenth electric signal S.sub.10 
representing the operational result of equation (8). Differential 
operation device 26 is also connected to second adder 27b. 
Second adder 27b processes ninth electric signal S.sub.9 sent from first 
adder 27a and tenth electric signal S.sub.10 sent from differential 
operation device 26 by adding the result of the addition in first adder 
27a and the operational result of equation (8). Accordingly, the result of 
the addition in second adder 27b is shown by the following equation: 
##EQU2## 
Second adder 27b generates an eleventh electric signal S.sub.1 1 
representing the operational result of equation (9), e.g., the feedback 
factor. Second adder 27b is further connected to final operation device 28 
to send eleventh electric signal S.sub.11 to final operation device 28. 
Final operation device 28 processes eleventh electric signal S.sub.11 sent 
from the second adder 27b and fourth electric signal S.sub.4 sent from the 
coefficient adjusting device 21b by carrying out the following equation: 
##EQU3## 
The operational result of equation (10) varies in response to changes in 
the value of the coefficient A. and the operational result of equation 
(9). Final operation device 28 generates a twelfth electric signal 
S.sub.12 representing the operational result of equation (10). Final 
operation device 28 is further connected to final storage device 29a and 
final adder 29b to send twelfth electric signal S.sub.12 to the final 
storage device 29a and final adder 29b. 
Final storage device 29a stores two twelfth electric signals S.sub.12 
representing the (n-1)th and (n)th operational results of equation (10). 
Final adder 29b processes the twelfth electric signal S.sub.12 
representing the (n-1)th operational result of equation (10) sent from 
final storage device 29a, and the other twelfth electric signal S.sub.12 
representing the (n)th operational result of equation (10) sent from final 
operation device 28 by adding the (n-1)th operational result of equation 
(10) and the (n)th operational result of equation (10). Accordingly, the 
result of the addition in final adder 29b is shown by the following 
equation: 
##EQU4## 
As the operational result of equation (10) varies, so does the operational 
result of the right side of equation (11). Nevertheless, if the 
operational result of the right side equation (11) is equal to or less 
than a predetermined minimum value I.sub.min, e.g., about 0 mA, the left 
side of equation (11) is adjusted, such that I.sub.n =I.sub.min. On the 
other hand, if the operational result of the right side of equation (11) 
is equal to or greater than a predetermined maximum value I.sub.max, e.g., 
about 100 mA, the left side of equation (11) is adjusted, such that 
I.sub.n =I.sub.max. Final adder 29b generates a thirteenth electric signal 
the amperage of which is identical to the operational result of equation 
(11). Final adder 29b is further connected to amplifier 30 to send 
thirteenth electric signal S.sub.13 amplifier 30. For example, amplifier 
30 amplifies the amperage of thirteenth electric signal S.sub.13 to 
10.multidot.I.sub.n. The electric current having amperage 
10.multidot.I.sub.n. is supplied from amplifier 30 to the solenoid of the 
externally controlled, variable capacity control mechanism of the 
compressor. 
In this embodiment of the present invention, when the electric current 
supplied to the solenoid of the externally controlled, variable capacity 
control mechanism of the compressor increases, the pressure control point 
of the compressor suction chamber pressure increases to a greater value. 
When the electric current supplied to the solenoid of the externally 
controlled, variable capacity control mechanism of the compressor 
decreases, the pressure control point in the compressor suction chamber 
pressure also decreases to a smaller value. 
Operation of the automotive air conditioning system in accordance with this 
embodiment of the present invention is described below. Referring to FIG. 
3, when it is desired to cool the passenger compartment of the automobile, 
the automotive air conditioning system is turned on as indicated in step 
201. When the automotive air conditioning system is turned on, a counter 
(not shown) which counts the number of times that air temperature T.sub.e 
has been detected is reset to zero as indicated in step 202. Operation of 
condenser fan 121 and evaporator fan 141 is initiated in step 203, and 
concurrently, operation of control apparatus 20 is initiated. 
As represented by step 204, when operation of control apparatus 20 is 
initiated, an electromagnetic coil (not shown) of electromagnetic clutch 
111 is energized to initiate operation of compressor 11. When compressor 
11 operates, compressed gaseous refrigerant flows to condenser 12 in which 
a first heat exchange operation occurs. Condensed refrigerant from 
condenser 12 then is expanded in expansion device 13 before evaporation 
occurs in evaporator 14. A second heat exchange operation also occurs in 
evaporator 14. Thereafter, vaporized refrigerant from evaporator 14 
returns to compressor 11. As long as compressor 11 operates, the 
above-mentioned operations are repeated. 
In step 205, thermosensor 17, which is associated with evaporator 14, 
detects air temperature T.sub.e immediately downstream from evaporator 14 
within a predetermined short time interval .DELTA.t. Thermosensor 17 
generates first electric signal S.sub.1 representing the detected air 
temperature T.sub.e. The first electric signal S.sub.1 is sent from 
thermosensor 17 to subtracter 22 and gradient operation device 21a. 
In step 206, it is determined whether the number of times that air 
temperature T.sub.e has been detected is equal to or greater than four, 
i.e., n.gtoreq.4. If the number of times that air temperature T.sub.e has 
been detected is equal to or greater than four, i.e., n.gtoreq.4, 
operation of the air conditioning system proceeds from step 206 to step 
207. On the other hand, if the number of times that air temperature 
T.sub.e has been detected is less than four, i.e., n&lt;4, operation proceeds 
from step 206 to step 209. In step 207, operation of equation (1) is 
carried out in gradient operation device 21a. 
Operational results in step 207 is classified by steps 208 and 210 as 
described below. In step 208, it is determined whether the thermal 
gradient .alpha..sub.n is equal to or less than .alpha..sub.b. If 
.alpha..sub.n is equal to or less than .alpha..sub..sub.b, operation 
proceeds from step 208 to step 209. In step 209, the coefficient A.sub.n 
is adjusted, such that A.sub.n =A.sub.min, by coefficient adjusting device 
21b. On the other hand, if .alpha..sub.n is greater than .alpha..sub.b, 
operation proceeds from step 208 to step 210. In step 210, it is 
determined whether 60 .sub.n is less than .alpha..sub.a. If .alpha..sub.n 
less than .alpha..sub.a, operation proceeds from step 210 to step 211. In 
step 211, operation of equation (3) is carried out in coefficient 
adjusting device 21b. On the other hand, if .alpha..sub.n is equal to or 
greater than .alpha..sub.a, operation proceeds from step 210 to step 212. 
In step 212, the coefficient A.sub.n is adjusted, such that A.sub.n 
=A.sub.max, by coefficient adjusting device 21b. 
Step 213 follows steps 209, 211, and 212. In step 213, the amperage I.sub.n 
of thirteenth electric signal S.sub.13 adjusted by the operational result 
of equation (11) is amplified at amplifier 30 to 10.multidot.I.sub.n. The 
electric current having amperage 10.multidot.I.sub.n is supplied from 
amplifier 30 to the solenoid of the externally controlled, variable 
capacity control mechanism of the compressor to control the pressure 
control point in the compressor suction chamber. 
Step 214 follows step 213. In step 214, it is determined whether the 
predetermined short time interval .DELTA.t has elapsed from the time when 
the air temperature T.sub.e(n) was detected. When the predetermined short 
time interval .DELTA.t has elapsed from the time when the air temperature 
T.sub.e(n) was detected, operation proceeds from step 214 to step 215. In 
step 215, the number of times that air temperature T.sub.e is detected 
increases to n+1. When the number of times that air temperature T.sub.e 
has been detected becomes n+1, operation returns from step 215 to step 
205. The sequence of steps from step 205 to step 215 continues until 
operation of the automotive air conditioning system is terminated. 
FIG. 4 depicts a change in the rotational speed of evaporator fan 141; a 
change in the air temperature T.sub.e immediately downstream from 
evaporator 14; a change in the electric current supplied to the solenoid 
of the externally controlled, variable capacity control mechanism of the 
compressor; and a change in the thermal gradient .alpha..sub.n of the 
detected air temperature T.sub.e(n) during operation of the automotive air 
conditioning system. More specifically, in FIG. 4, except for the change 
in the rotational speed of evaporator fan 141, solid lines indicate the 
above changes in accordance with an embodiment of the present invention, 
and dashed lines indicate the above changes in accordance with a prior art 
embodiment. 
Referring to FIG. 4, when the rotational speed of evaporator fan 141 is 
decreased quickly by a large amount, e,g., when the rotational speed of 
evaporator fan 141 is decreased from about 3500 rpm to about 1000 rpm, at 
a time t.sub.1, the amount or air flow which passes through the exterior 
surface of the evaporator 14 also quickly decreases by a large value. 
Thus, heat load on the evaporator 14 is decreased by a large amount. As a 
result, air temperature T.sub.e immediately downstream from evaporator 14 
is quickly reduced by a large amount, so that the thermal gradient 
.alpha..sub.n of the detected air temperature T.sub.e(n) increases to a 
large value. Thus, the thermal gradient .alpha..sub.n exceeds the 
predetermined first boundary value .alpha..sub.n, as well as the 
predetermined second boundary value .alpha..sub.b, immediately after time 
t.sub.1. 
In this situation, as long as thermal gradient .alpha..sub.n is greater 
than the predetermined second boundary value .alpha..sub.b, but is less 
than the predetermined first boundary value .alpha..sub.a, the coefficient 
A.sub.n is adjusted in the operational manner represented by step 211 of 
FIG. 3. As long as the thermal gradient .alpha..sub.n is equal to or 
greater than the predetermined first boundary value .alpha..sub.a, the 
coefficient A.sub.n is adjusted in the operational manner represented by 
step 212 of FIG. 3, such that A.sub.n =A.sub.max. Accordingly, the value 
of the second term of the right side of equation (11) increases to a large 
value. In particular, the value of the second term of the right side of 
equation (11) continues to increase as long as the thermal gradient 
.alpha..sub.n is equal to or greater than the predetermined first boundary 
value .alpha..sub.a. Therefore, amperage of the electric current supplied 
from amplifier 30 to the solenoid of the externally controlled, variable 
capacity control mechanism of the compressor is quickly increased by a 
large amount, so that the pressure control point in the compressor suction 
chamber pressure greatly increases immediately after time t.sub.1. As a 
result, a fall in air temperature T.sub.e from the set temperature 
T.sub.set levels off at a low amount within a short time period. Once the 
fall in air temperature T.sub.e has leveled off, air temperature T.sub.e 
rises to approach the set temperature T.sub.set in accordance with the 
operational result of equation (11), so that T.sub.e, converges on the set 
temperature T.sub.set. When air temperature T.sub.e has converged at the 
set temperature T.sub.set, air temperature T.sub.e is slightly increased 
or decreased in accordance with the operational manner of step 209 of FIG. 
3. 
As described above, even when the heat load on the evaporator 14 is 
decreased quickly by the large amount due to the a large and rapid 
decrease in the rotational speed of evaporator fan 141, the fall in air 
temperature T.sub.e from the set temperature T.sub.set levels off at a low 
amount within the short time period, so that air temperature T.sub.e 
quickly ceases to fall and begins to rise. Thus, T.sub.e effectively 
converges at the set temperature T.sub.set. The air in the passenger 
compartment of the automobile, therefore, is adequately air conditioned 
even when the heat load on evaporator 14 is decreased quickly and by the 
large amount due to the large and rapid decrease in the rotational speed 
of evaporator fan 141. 
The above embodiment is applied to an operational situation of the 
automotive air conditioning system in which the heat load on evaporator 14 
is decreased quickly by the large amount due to the large and rapid 
decrease in the rotational speed of evaporator fan 141. Nevertheless, the 
present invention can be applied to any other operational situations of 
the automotive air conditioning system in which the heat load on the 
evaporator is decreased or increased quickly by a large amount due to a 
drastic change in the thermodynamic condition of the evaporator, such as 
in an operational situation of the automotive air conditioning system in 
which the heat load on the evaporator is increased quickly by the large 
amount due to a switch from a passenger compartment air circulation mode 
to an outside air intake mode. 
Further, in the embodiment described above, air temperature T.sub.e 
immediately downstream from the evaporator is detected as a thermodynamic 
characteristic relating to the evaporator. Nevertheless, in the present 
invention, pressure in an outlet of the evaporator also may be detected as 
the thermodynamic characteristic relating to the evaporator. 
Moreover, although the PID control action apparatus is used in the 
embodiment described above, the present invention is not restricted to 
this embodiment. In another embodiment of the present invention, for 
example, a proportional-plusintegral (PI) control action apparatus or a 
proportional (P) control action apparatus can be used in place of the PID 
control action apparatus. 
This invention has been described in detail in connection with a preferred 
embodiment. It will be understood, however, by those skilled in the art 
that other variations and modifications can be easily made within the 
scope of this invention. Although a detailed description of the present 
invention is provided above, it is to be understood that the scope of the 
invention is not to be limited thereby, but is to be determined by the 
claims which follow.