Control method for a vehicle having an engine and an accessory device

A method for controlling cycling of an air conditioning compressor coupled to an internal combustion engine interrupts normal cycling based on operation conditions. In addition, normal engaged and disengaged cycling durations are adaptively estimated in real-time. The method of the present invention achieves improved fuel economy and improved drive feel. As an example, improved fuel economy is achieved by engaging the compressor during braking or when the engine is being driven by the vehicle. As another example, improved drive feel is achieved by engaging the compressor during transient conditions when drive feel is unaffected.

FIELD OF THE INVENTION

The field of the invention relates generally to air conditioning system control coordinated with engine control.

BACKGROUND OF THE INVENTION

Vehicles are typically equipped with an air conditioning system to provide cabin cooling and to dry air for dehumidifying functions. Air conditioning systems typically include a compressor driven by a vehicle's internal combustion engine. The compressor can be either engaged, fully or partially, or disengaged to the engine via an electronically controlled clutch.

During air conditioning system operation under certain operating conditions, the compressor cycles between an engaged and disengaged state. Cycling is typically controlled based on refrigerant pressure in the air conditioning system. When the engine and clutch are coupled, pressure decreases and significantly cooled cabin air is circulated through the vehicle. Such operation continues until pressure reaches a minimum value where the clutch is controlled to disengage the engine and compressor. If air circulation is continued, pressure increases until it reaches a maximum value. At this maximum value, the compressor is then re-engaged via the clutch and cycling repeats.

It is also known to disengage the engine and compressor during vehicle launch conditions, thereby allowing more engine output. In this way, degraded vehicle launch performance is avoided, even when air conditioning is operational. Vehicle launch is determined based on vehicle speed, throttle position, and various other factors.

The inventors herein have recognized disadvantages with the above approaches. First, driver comfort is degraded during clutch engagements during some driving conditions. In other words, during some driving conditions, clutch engagements are felt strongly by vehicle operators and comfort is therefore degraded.

Second, optimum fuel economy is not obtained since compressor cycling engagement is not coordinated to vehicle and engine operating conditions. In other words, during some conditions, extra fuel is added to the engine to provide air conditioning while maintaining engine output at a desired level. During other conditions, no extra fuel is needed to provide air conditioning.

SUMMARY OF THE INVENTION

An object of the present invention is to provide methods for controlling engagements of an air conditioning compressor coupled to an internal combustion engine capable of improving fuel economy and/or improving drive feel.

The above object is achieved and disadvantages of prior approaches overcome by a control method for use with an internal combustion engine and an accessory device, the engine and device coupled to a vehicle, the method comprising: determining when the device is cycling between an engaged state where the engine is coupled to the device and a disengaged state where the engine is de-coupled from the device; and engaging the device based at least on an operating condition when the device is disengaged.

By engaging the device in response to an operating condition when the device is cycling between an engaged state and a disengaged, it is possible to coordinate cycling of the device with current driving conditions. In other words, rather than asynchronous operation between various control systems, the present invention provides a method to couple device cycling control to other conditions.

An advantage of the above aspect of the invention is that improved fuel economy is achieved.

Another advantage of the above aspect of the invention is that improved drive feel is achieved.

In another aspect of the invention, the above object is achieved and disadvantages of prior approaches overcome by a control method for use with an internal combustion engine and an air conditioning compressor, the engine and compressor coupled to a vehicle, the method comprising: indicating a transient vehicle driving condition while the vehicle moving; estimating a duration of a cycle in which the device is engaged and disengaged due to an air conditioning system parameter; and engaging the compressor in response to said indication when said duration is greater than a predetermined duration.

By coordinating engagement with a transient vehicle driving condition while the vehicle moving, it is possible to engage the compressor unbeknownst to the vehicle driver. Further, by performing engagement when a percentage disengaged duration is greater than a predetermined duration, it is possible to prevent excessive compressor cycling.

An advantage of the above aspect of the invention is that improved drive feel and improved customer satisfaction is achieved.

In yet another aspect of the invention, the above object is achieved and disadvantages of prior approaches overcome by an article of manufacture comprising a computer storage medium having a computer program encoded therein for use with an internal combustion engine and an air conditioning compressor, the engine and device coupled to a vehicle having brakes. The computer storage medium comprises code for determining when the compressor is cycling between an engaged state where the engine is coupled to the compressor and a disengaged state where the engine is de-coupled from the compressor, code for indicating when the brakes are actuated, code for estimating a percentage disengaged duration of a cycle in which the compressor is engaged and disengaged due to an air conditioning system parameter, and code engaging the compressor based at least on said indication when said percentage disengaged duration is greater than a predetermined value.

By engaging the compressor in response to brake actuation when a percentage disengaged duration is greater than a predetermined value, it is sometimes possible to operate the compressor without added fuel to the engine since kinetic energy from the vehicle can be used to power the compressor. In other words, this added coordination between compressor cycling control and vehicle braking conditions provides more opportunities to operate the compressor without excess fuel to the engine.

An advantage of the above aspect of the invention is that improved fuel economy is achieved.

Other objects, features and advantages of the present invention will be readily appreciated by the reader of this specification.

DESCRIPTION OF THE INVENTION

Referring toFIG. 1A, internal combustion engine10, further described herein with particular reference toFIG. 1B, is shown coupled to torque converter11via crankshaft13. Torque converter11is also coupled to transmission15via turbine shaft17. Torque converter11has a bypass clutch (not shown) which can be engaged, disengaged, or partially engaged. When the clutch is either disengaged or partially engaged, torque converter11is said to be in an unlocked state. Turbine shaft17is also known as transmission input shaft. Transmission15comprises an electronically controlled transmission with a plurality of selectable discrete gear ratios. Transmission15also comprises various other gears such as, for example, a final drive ratio (not shown). Transmission15is also coupled to tire19via axle21. Tire19interfaces the vehicle (not shown) to the road23.

Internal combustion engine10comprising a plurality of cylinders, one cylinder of which is shown inFIG. 1B, is controlled by electronic engine controller12. Engine10includes combustion chamber30and cylinder walls32with piston36positioned therein and connected to crankshaft13. Combustion chamber30communicates with intake manifold44and exhaust manifold48via respective intake valve52and exhaust valve54. Exhaust gas oxygen sensor16is coupled to exhaust manifold48of engine10upstream of catalytic converter20. In a preferred embodiment, sensor16is a HEGO sensor as is known to those skilled in the art.

Intake manifold44communicates with throttle body64via throttle plate66. Throttle plate66is controlled by electric motor67, which receives a signal from ETC driver69. ETC driver69receives control signal (DC) from controller12. Intake manifold44is also shown having fuel injector68coupled thereto for delivering fuel in proportion to the pulse width of signal (fpw) from controller12. Fuel is delivered to fuel injector68by a conventional fuel system (not shown) including a fuel tank, fuel pump, and fuel rail (not shown).

Engine10further includes conventional distributorless ignition system88to provide ignition spark to combustion chamber30via spark plug92in response to controller12. In the embodiment described herein, controller12is a conventional microcomputer including: microprocessor unit102, input/output ports104, electronic memory chip106, which is an electronically programmable memory in this particular example, random access memory108, and a conventional data bus.

Controller12receives various signals from sensors coupled to engine10, in addition to those signals previously discussed, including: measurements of inducted mass air flow (MAF) from mass air flow sensor110coupled to throttle body64; engine coolant temperature (ECT) from temperature sensor112coupled to cooling jacket114; a measurement of throttle position (TP) from throttle position sensor117coupled to throttle plate66; a measurement of transmission shaft torque, or engine shaft torque from torque sensor121, a measurement of turbine speed (Wt) from turbine speed sensor119, where turbine speed measures the speed of shaft17, and a profile ignition pickup signal (PIP) from Hall effect sensor118coupled to crankshaft13indicating an engine speed (We). Alternatively, turbine speed may be determined from vehicle speed and gear ratio.

Continuing withFIG. 1, accelerator pedal130is shown communicating with the driver's foot132. Accelerator pedal position (PP) is measured by pedal position sensor134and sent to controller12.

In an alternative embodiment, where an electronically controlled throttle is not used, an air bypass valve (not shown) can be installed to allow a controlled amount of air to bypass throttle plate62. In this alternative embodiment, the air bypass valve (not shown) receives a control signal (not shown) from controller12.

In a preferred embodiment, controller12controls engine according to a torque based control system. In such a system, a desired wheel torque, or engine torque, is determined based on pedal position (PP). Then, position of throttle66is controlled so that actual wheel torque, or engine torque, approaches the desired engine torque. The system can be configured based on engine brake torque, which is the available torque at the engine output, taking into account torque losses.

Referring now toFIG. 2, an air conditioning (A/C) system is shown. Arrows201indicate direction of refrigerant, or working fluid, flow. Arrows200indicate direction of air flow that is circulated through the engine compartment (not shown). Arrows206indicate direction of air flow that is circulated through the cabin (not shown). Solid shading202indicates working fluid is a high pressure gas, left handed cross-hatching203indicates working fluid is a high pressure liquid, right handed cross-hatching204indicates working fluid is a low pressure liquid, and no shading205indicates working fluid is a low pressure gas. Working fluid is circulated through the A/C system via line207.

Compressor220, which can be coupled to engine10via a clutch219, is located between high pressure gas202and low pressure gas205. Upstream of compressor220is low pressure service port222and A/C cycling pressure switch223. Upstream of cycling switch223is suction accumulator/drier224. Further upstream of suction accumulator/drier224is A/C evaporator core226, which is coupled to blower motor225. Continuing upstream of A/C evaporator core226is A/C evaporator orifice227and A/C condenser core228, which is coupled to radiator fan233. Upstream of A/C condenser core228is high pressure service port229, compressor relief valve230, and A/C pressure cut-off switch231.

A description of an A/C thermodynamic process is now presented. Starting at compressor220, low pressure gas205is compressed to high pressure gas202, rising in temperature due to compression. Compressor relief valve230prevents high pressure gas202from reaching a maximum allowable high pressure gas pressure. A/C pressure cut-off switch231disengages compressor200from engine10via clutch219.

High pressure gas202sheds heat to the atmosphere at A/C condenser core228, changing phase to high pressure liquid203as it cools. At A/C evaporator orifice227, high pressure liquid204expands to low pressure liquid204. At A/C evaporator core226low pressure liquid204passes through a jet (not shown) and evaporates into low pressure gas205. This action cools the working fluid, A/C evaporator core226, and cabin airflow206.

Low pressure liquid204continues to suction accumulator/drier224and A/C cycling pressure switch223A/C cycling pressure switch223signals to engage compressor220to engine10via clutch219when measured pressure is above a predetermined maximum pressure. A/C cycling pressure switch223also signals to disengage compressor220from engine10via clutch219when measured pressure is below a predetermined minimum pressure. These setpoint pressures are typically 45 psi and 24.5 psi, respectively. They are designed to keep A/C evaporator core226just above freezing. When compressor220cycles between engaged and disengaged due solely to A/C cycling pressure switch223, it is referred to herein as normal, or uninterrupted, cycling. Stated another way, this normal/uninterrupted cycling is when the compressor cycles to control cabin temperature, or cooling air temperature, based on air conditioning parameters such as pressure or temperature. However, according to the present invention, engagement of compressor220is controlled due to various factors as described later herein.

Referring toFIG. 3, a routine is shown for learning the on and off duration of A/C compressor201. First, in step300, a determination is made as to whether the A/C system is presently cycling. In other words, engagement due to engine operating conditions is not enabled unless compressor201has cycled a predetermined number of times. When the answer to step300is YES, the routine continues to step302. In step302, a determination is made as to whether A/C compressor201has just disengaged. In other words, a determination is made as to whether A/C compressor201has just been disconnected from engine10. When the answer to step302is YES, a determination is made in step304as to whether the A/C compressor was engaged due to normal cycling. In other words, a determination is made as to why the compressor was previously engaged. If it was engaged due to normal cycling, which means pressure measured by sensor223was greater than a predetermined value, then the routine continues to step306. Stated another way, if an uninterrupted cycle was completed, it is possible to learn normal on and off durations. In step306, the routine calculates temporary values A′ and B′ from the previous cycle. Value A′ represents the duration that A/C compressor201was engaged and B′ represents the duration A/C compressor201was disengaged. In other words, A′ and B′ respectively represent on and off durations for normal cycling under present operating conditions. Next, in step308, the learned values A and B are updated based on the calculated temporary values A′ and B′ using filter coefficients γ1and γ2. In other words, the learned on and off durations are filtered to remove measurement noise. When the answer to step304is NO, the routine continues to step310, where values A and B are not updated. In addition, if compressor201was disengaged due to vehicle launch conditions, the routine continues to step310. In this way, it is possible to learn the on and off durations of uninterrupted (or normal) A/C compressor cycling with the present conditions. In other words, the on and off durations are adaptively learned for normal (uninterrupted) A/C operation.

In an alternative embodiment, values A and B are learned as a function of air conditioning operating conditions such as, for example, blower speed, desired cabin temperature, desired cooling level, ambient temperature, cabin humidity, and/or ambient humidity. By including variation in these air conditioning operating conditions, values A and B for current operating conditions can be used to include an open loop estimate to account for quickly changing driver requests or quickly changing ambient conditions.

Referring now toFIG. 4A, a routine is described for determining whether normal A/C cycling can be interrupted to engage A/C compressor201. First, in step410, the time A/C compressor201has been off, or the time since A/C compressor201was last disengaged, is measured (cur_b). Next, in step412, the percent of an uninterrupted cycle in which the A/C compressor has been off, is calculated. In other words, the routine calculates the percent of an uninterrupted cycle that A/C compressor201has been off (pb) at the present calculation point. This value is calculated based on the time measured in step410(cur_b) and the learned off-time (B). Next, in step414, a determination is made as to whether the value pb is greater than a limit value (pb_limit). Stated another way, engagement due to operating conditions is prevented until compressor201has been disengaged for a predetermined duration. In this particular example, the duration is a relative percentage of the presently estimated off duration (B). This prevents excessive cycling. For example, if compressor201is engaged right after it was disengaged, it will again be disengaged since measured pressure will quickly reach the maximum limit value. When the answer to step414is NO, an engagement flag (engage_flg) is set equal to zero in step416. Otherwise, in step418, an engagement flag is set equal to 1. In other words, in step418, the routine enables A/C engagement due to various conditions described later herein.

In an alternative embodiment of the present invention, step414can be modified to determine whether time measured in step410(cur_b) is greater than a predetermined limit time (cur_b_limit). Those skilled in the art will recognize various other methods to prevent excessive cycling such as determining if compressor201has been off for a predetermined number of engine rotations.

Referring now toFIGS. 4B–4D, several graphs show an example of operation according to the present invention.FIG. 4Bshows whether A/C compressor201is engaged or disengaged as well as on and off durations A′ and B′, respectively.FIG. 4Cshows the corresponding percent of an uninterrupted cycle that A/C compressor201has been off (pb). Also, limit value (pb_limit) is shown by a dash dot line.FIG. 4Dshows corresponding engagement flag (engage_flg). According to the present invention as described with particular reference toFIG. 4A, when pb is greater than pb_limit, engage_flg is set equal to one. Otherwise, engage_flg is set equal to zero.

The A/C compressor cycling of the present invention is controlled by various parameters. Uninterrupted A/C compressor cycling, as defined herein, represents when the A/C compressor is cycled on and off based on pressure measured by A/C cycling pressure switch203. This uninterrupted cycling is also referred to herein as normal cycling. In this normal cycling, the A/C compressor engages and disengages so that the driver is provided with requested cooling. Further, in this normal cycling, the A/C compressor is engaged when the A/C cycling pressure switch203measures a pressure greater than a first predetermined value. The A/C compressor stays on until the A/C cycling pressure switch203measures a pressure less than a second value. At this point, the A/C compressor is disengaged. The A/C compressor remains disengaged until, once again, A/C cycling pressure switch203measures a pressure greater than the first value. In this way, the A/C cycles normally on and off based on environmental conditions and driver requests.

According to the present invention, engagement of the A/C compressor is also performed under various other conditions. These conditions can be transient vehicle operating conditions; conditions where the A/C compressor can be driven with minimal fuel economy impact; and conditions where the potential for minimum drive impact during the engagement is possible. The following figures describe such operation.

Referring now toFIG. 5, a routine is described for determining whether to engage the A/C compressor. First, in step500, a determination is made as to whether the A/C system is presently cycling. In other words, engagement due to engine operating conditions is not enabled unless compressor201has cycled a predetermined number of times. When the answer to step500is YES, in step510, a determination is made as to whether the engagement flag (engage_flg) is set equal to 1. When the answer to step510is YES, a determination is made in step512as to whether enabling conditions have been detected based on engine or vehicle conditions (seeFIG. 6). When the answer to step512is YES, the A/C compressor is engaged in step514and interrupt flag (int_flag) is set equal to 1.

When the answer to step510is NO, a determination is made in step516as to whether A/C cycling pressure switch203indicates that A/C engagement is necessary. When the answer to step516is YES, in step518the A/C compressor is engaged, interrupt flag (int_flag) is set equal to zero, and normal cycling will follow.

Referring now toFIG. 6, a routine is described for determining whether enabling conditions have been detected. First, in step610, it is determined whether vehicle speed is greater than vehicle speed threshold (pvs). When the answer to step610is YES, a determination is made as to whether transient conditions have been detected in step612. The detection of transient conditions is described later herein. When the answer to step612is NO, a determination is made in step614as to whether there is a potential for more efficient A/C operation. Determining whether more efficient A/C operation is possible is described later herein. When the answer to step614is NO, a determination is made in step616as to whether there is a potential for minimum drive impact during engagement. When the answer to either step612,614or616is YES, the routine indicates in step618that enabling conditions have been detected.

Other conditions can also be used in determining whether to enable engagement according to the present invention. For example, during high ambient temperatures, cycling is minimal. Stated another way, if compressor220if cycled off only for less than a minimal off time, enabling conditions would not be detected.

Referring now toFIG. 7, a routine is described for determining whether potential for more efficient A/C operation has been detected. First, in step710, a determination is made as to whether torque converter11is unlocked. When the answer to step710is YES, a speed ratio (sr) is calculated across torque converter11based on engine speed (We) and turbine speed (Wt). Next, in step714, a determination is made as to whether the calculated speed ratio is less than 1. When the answer to step714is YES, the routine indicates in step716that there is a potential for more efficient A/C operation. In other words, when torque converter speed ratio is less than 1, the engine is absorbing torque, or engine brake torque is less than zero, and therefore it is possible to engage the A/C compressor and use the force transmitted from the road through the engine powertrain to power the A/C compressor. In this way, less fuel is used since the A/C compressor is not being powered by engine combustion. Stated another way, excess fuel does not need to be added to the engine to power the A/C compressor when the engine is producing negative engine brake torque, i.e., when the engine is being driven.

Referring now toFIG. 8, a routine is described for detecting transient conditions. First, in step810, a determination is made as to whether an antilock braking system is activated. When the answer to step810is YES, the routine indicates in step812that transient conditions have been detected In other words, when antilock braking systems are activated the hydraulic pulsing that applies the hydraulic brake actuator interrupts normal drive feel experienced by the vehicle operator. Therefore, if the A/C compressor is engaged while the antilock braking system is activated, the driver will not notice the A/C engagement In this way, the A/C compressor can be engaged less often during normal drive situations where the driver may feel the A/C compressor engagement. Thus, drive feel is improved.

Referring now toFIG. 9, a routine is described for detecting transient conditions. First, in step910, a determination is made as to whether traction control is engaged. When the answer to step910is YES, the routine indicates in step912that transient conditions have been detected. In other words, when traction control systems are activated, application of brakes and/or reduction in engine torque interrupts the normal drive feeling experienced by the vehicle operator. Therefore, if the A/C compressor is engaged while the traction control system is activated, the driver will not notice the A/C engagement. In this way, the A/C compressor can be engaged less often during normal drive situations where the driver may feel the A/C compressor engagement. Thus, drive feel is improved.

Referring now toFIG. 10, a routine is described for detecting transient conditions. First, in step1010, a determination is made as to whether cruise control was commanded to be engaged or disengaged. When the answer to step1010is YES, the routine indicates in step1012that transient conditions have been detected. In other words, when cruise control is activated or deactivated, the change in control from the driver to the automatic control system or from the automatic control system to the driver can interrupt normal drive feel experienced by the vehicle operator. Therefore, if the A/C compressor is engaged concurrently with engagement or disengagement of the cruise control system, the driver will not notice the A/C engagement. In this way, the A/C compressor can be engaged less often during normal drive situations where the driver may feel the A/C compressor engagement. Thus, drive feel is improved.

Referring now toFIG. 11, a routine is described for detecting potential for minimum drive impact during engagement. First, in step1110, a determination is made as to whether torque converter11is unlocked. When the answer to step1110is YES, the routine continues to step1112, where a determination is made as to whether the percent off-time (pb) is greater than limit value (pb_limit_uc). When the answer to step1112is YES, the routine indicates in step1114that the potential for minimum drive impact during engagement has been detected. In other words, it is less likely that a vehicle operator will feel A/C compressor engagements when torque converter11is unlocked since additional damping is provided by an unlocked torque converter. Thus, if the A/C compressor has been disengaged for greater than limit value (pb_limit_uc), improved drive feel can be achieved by taking advantage of the current situation and engaging the A/C compressor, rather than waiting until the A/C cycling pressure switch203indicates that the A/C compressor should be engaged due to measured pressure.

Referring now toFIG. 12, a routine is described for detecting transient conditions. First, in step1210, a determination is made as to whether a transmission shift has been commanded or detected. When the answer to step1210is YES, the routine indicates in step1212that transient conditions have been detected. In other words, during a transmission shift vehicle acceleration or deceleration can occur, interrupting normal drive feel experienced by the vehicle operator. Therefore, if the A/C compressor is engaged during a transmission shift, the driver will not notice the A/C engagement since the driver expects vehicle feel to change. In this way, the A/C compressor can be engaged less often during normal drive situations where the driver may feel the A/C compressor engagement. Thus, drive feel is improved.

Referring now toFIG. 13, a routine is described for detecting the potential for minimum drive impact during engagement. First, in step1310, a determination is made as to whether desired engine brake torque is less than zero. For example, to control vehicle speed to a desired vehicle speed during cruise control on a steep downgrade, it may be necessary to provide engine braking. Alternatively, if vehicle acceleration is controlled to a desired acceleration, negative engine brake torque may be requested When the answer to step1310is YES, the routine indicates in step1312that potential for more efficient A/C operation is detected. In other words, when engine10is absorbing torque, it is possible to engage the A/C compressor and use force transmitted from the road through the engine powertrain to power the A/C compressor. In this way, less fuel is used since the A/C compressor is not being powered by engine combustion. Stated another way, excess fuel does not need to be added to the engine to power the A/C compressor when the engine is producing negative engine brake torque.

In an alternative embodiment, potential for more efficient A/C operation can be determined directly from a desired vehicle acceleration. For example, if desired vehicle acceleration (which can be determined based on pedal position (PP)) is negative, or is less than a predetermined acceleration, potential for more efficient A/C operation can be indicated.

Referring now toFIG. 14, a routine is described for detecting the potential for minimum drive impact during engagement. First, in step1410, a determination is made as to whether deceleration fuel shut-off (DFSO), or partial cylinder deactivation, is active (or requested). For example, to control vehicle speed to a desired vehicle speed during cruise control on a steep downgrade, it may be necessary to provide significant engine braking to a point where combustion in some engine cylinders is terminated. When the answer to step1410is YES, the routine indicates in step1412that potential for more efficient A/C operation is detected. In other words, when engine10is absorbing torque, it is possible to engage the A/C compressor and use force transmitted from the road through the engine powertrain to power the A/C compressor. In this way, less fuel is used since the A/C compressor is not being powered by engine combustion. Stated another way, excess fuel does not need to be added to the engine to power the A/C compressor when the engine is producing negative engine brake torque.

Referring now toFIG. 15, a routine is described for detecting more efficient A/C operation. First, in step1510, a determination is made as to whether pedal position (PP) is less than minimum pedal position (MPP). In other words, if the driver has tipped-out, this can be viewed as a request for some deceleration and reduced engine torque. One method for reducing engine torque in an efficient manner is to engage the A/C compressor. Thus, when the answer to step1510is YES, the routine indicates in step1512that potential for more efficient A/C operation is detected. In other words, when engine10is absorbing torque to decelerate, it is possible to engage the A/C compressor and use the deceleration force to power the A/C compressor. In this way, less fuel is used since the A/C compressor is not being powered by engine combustion.

Referring now toFIG. 16, a routine is described for detecting transient conditions. First, in step1610, a change in pedal position (Δpp) is calculated. In step1612, a change in desired engine torque (ΔT_des) is calculated. Next, in step1614, a change in throttle position (ΔTp) is calculated. Next, in step1616, a change in fuel injection amount (Δfpw) is calculated. In step1618, a determination is made as to whether the absolute value of any of these changes is greater than corresponding threshold values. When the answer to step1618is YES, the routine continues to step1620where the routine indicates that transient conditions have been detected. In this way, when the vehicle operator makes a change in power or torque delivered by engine10, the A/C compressor can be engaged. Also, in this way, it is possible to mask engagement of the A/C compressor, since the driver will be expecting a significant change in vehicle operation.

Referring now toFIG. 17, a routine is described for detecting the potential for more efficient A/C operation. First, in step.1710, a determination is made as to whether vehicle brakes are actuated, for example by detecting whether the driver pressed a brake pedal. In other words, if the driver has applied the brakes this can be viewed as a request for some deceleration and reduced engine torque. One method for reducing engine torque in an efficient manner is to engage the A/C compressor. Thus, when the answer to step1710is YES, the routine indicates in step1712that potential for more efficient A/C operation is detected. In other words, when engine10is absorbing torque to decelerate, it is possible to engage the A/C compressor and use the deceleration force to power the A/C compressor. In this way, less fuel is used since the A/C compressor is not being powered by engine combustion.

Referring now toFIG. 18, a graph depicting operation according to the present invention is shown. The graph shows whether the A/C compressor is engaged or disengaged. The graph starts at time t0where the A/C compressor is disengaged. The compressor is then engaged after duration B′ and remains engaged for duration A′. At time t1, the A/C compressor is disengaged. At time t1, the routine is able to measure values A′ and B′ and update values A and B, since the normal A/C cycling occurred and was not interrupted. At time t2, another uninterrupted A/C compressor cycle is completed, and again values A′ and B′ are measured and values A and B updated. At time t3, the vehicle brakes are applied. At time t3, since the percent of an uninterrupted A/C compressor cycle (pb) is greater than the limit value (pb_limit), the A/C compressor is engaged. At time t4, the A/C compressor is disengaged due to pressure measured by A/C cycling pressure switch203. At time t4, the parameters A and B are not updated since the value of A′ and B′ cannot be measured since a normal cycle was not completed. At time t5, a vehicle gear shift is performed. At time t5, the A/C compressor is not engaged since the off time (pb) is too small. At time t6, another uninterrupted A/C compressor cycle has been completed and the values A′ and B′ can be measured so the values A and B can be updated. At time t7, engine braking is detected and the A/C compressor is engaged since the off time (pb) is greater than the limit value (pb_limit). At time t8, the A/C compressor is disengaged based on the pressure measured by A/C cycling pressure switch203. At time t9, the brakes are actuated. However, the A/C compressor is not engaged since the off time is too small. At time t10, torque converter11is unlocked due to vehicle driving conditions. At time t10, the A/C compressor is engaged since the off time (pb) is greater than the limit value (pb_limit) and greater than torque converter unlocked limit value (pb_limit_uc). At time t11, the A/C compressor is disengaged due to pressure measured by A/C cycling pressure switch203. Parameter A′ and B′ are not measured at time t11nor are parameters A and B updated. Next, at time t12, another uninterrupted A/C compressor cycle has been completed and values A′ and B′ can be measured so that values A and B can be updated.

The graph inFIG. 18shows an example of operation in which a portion of the conditions which can cause the A/C compressor to be engaged are described.

Although several examples of embodiments which practice the invention have been described herein, there are numerous other examples which could also be described. For example, the invention can also be used with direct injection engines wherein fuel is injected directly into the engine cylinder. Also, the invention is applicable with various types of accessory devices that can cycle between an engaged state and a disengaged state. In another example, potential for minimum drive impact can also be indicated when a clutch is depressed (or disengaged) in a manual transmission vehicle. During such a condition, it is may be possible to engage compressor220without affecting drive feel since engine10is not coupled to the wheels or transmission of the vehicle. The invention is therefore to be defined only in accordance with the following claims.