Temperature control system for fuel cell electric vehicle cooling circuit

[0002] This application claims priority to a provisional patent application serial No. 60/325281 titled, “Temperature Control System for Fuel Cell Electric Vehicle Cooling Circuit,” filed Sep. 27, 2001. The entire disclosure of serial No. 60/325281 is incorporated herein by reference in its entirety.

[0003] The present invention relates generally to vehicle temperature control systems and specifically to a temperature control system for a fuel cell vehicle.

[0004] In an effort to find new energy sources, fuel cells using an electrochemical reaction to generate electricity are becoming an attractive energy alternative. Fuel cells offer low emissions, high fuel energy, conversion efficiencies, and low levels of noise and vibration. U.S. Pat. No. 5,248,566 to Kumar et al. These advantages make fuel cells useful in automotive applications. Of the various types of fuel cells, the proton electrolyte membrane (PEM) fuel cell appears to be the most suitable for use in automobiles, as it can produce potentially high levels of energy, but has low weight and volume.

[0005] Fuel cells, while simple in concept, are quite complex since many systems need to be considered to operate the fuel cell optimally. One of the design challenges of a vehicle with a PEM fuel cell stack is the high volume of heat it produces while in operation. Thermal management systems (cooling systems) are known for conventional internal combustion engine (ICE) vehicles. Unfortunately, fuel cell powered vehicles have unique thermal management requirements when compared to ICE vehicles. One of these requirements is that the aqueous-based coolant must be maintained at a specific temperature at the fuel cell stack inlet. At the same time, the inlet temperature of a separate, so-called low temperature cooling circuit must be limited to a specific temperature. Further, a temperature rise of the coolant across the stack needs to be maintained at a precise value. These requirements are needed to maintain the correct fuel cell stack efficiency and humidification that in turn effect the performance and durability of the stack.

[0006] Fuel cell thermal management systems are known in the art, but none appear to have dual controls for coolant temperature. A new fuel cell cooling system control strategy needs to be developed to address the deficiencies of the prior art. This new thermal management system for the fuel cell should include a cooling circuit with variable fan speed control and flow control actuators such as a radiator bypass valve, to deliver the precise temperature controls needed.

[0007] Accordingly, the present invention is a system and method to control coolant temperature within a fuel cell vehicle. Coolant inlet temperature is controlled by adjusting system fan speed and a radiator bypass valve position. For this invention, a general control solution with independent controllers for fan and bypass valve operating modes is proposed. The controllers can have different parameters and structures so they can provide better control performance for significantly different plant responses with respect to fan speed and bypass valve position plant inputs. For this invention, a plant involves what the controller is trying to control. The controllers are subsequently activated using simple and robust coordination logic. An additional control and coordination algorithm that provides limitation of the inlet temperature for the low temperature cooling circuit extends this control approach.

[0008] Further, inlet temperature control error is highly influenced by change of the net power and vehicle speed disturbance variables. The controller is extended by a feed-forward disturbance compensator, to mitigate the influence of the disturbances to improve the control system's performance. This improves performance in that rather than reacting to changes in coolant temperature, the controller is able to anticipate these changes before they occur. Gain scheduling is used to improve the performance of the feed-forward compensator that includes dependence on ambient temperature.

[0009] Further, fuel cell stack behavior in one mode (in which the fuel cell coolant inlet temperature is controlled by the operation of a radiator bypass valve) is characterized by a distinctive dead-zone nonlinearity, especially at low vehicle speeds. This dead-zone effect results in a large inlet coolant temperature dynamic control error during a significant portion of the radiator bypass valve's operation. Another disturbance effect is also caused by the abrupt change of fan speed requested from air-conditioning (A/C and cooling systems use the same fan for working fluid pressure and/or temperature control). The control strategy is extended by a controller-preset algorithm that efficiently compensates for the valve operation nonlinear effects and for A/C fan operation disturbance effects.

[0010] Other objects of the present invention will become more apparent to persons having ordinary skill in the art to which the present invention pertains from the following description taken in conjunction with the accompanying figures.

[0011] The foregoing objects, advantages, and features, as well as other objects and advantages, will become apparent with reference to the description and figures below, in which like numerals represent like elements and in which:

[0012] FIG. 1 illustrates a functional schematic of the fuel cell electric vehicle cooling circuit of the present invention.

[0013] FIG. 2 illustrates a block diagram of a temperature control system of the present invention.

[0014] FIG. 3 illustrates a block diagram of an inlet temperature controller of the present invention.

[0015] FIG. 4 illustrates a flow chart of the control strategy including controller-preset actions.

[0016] The present invention relates to fuel cell powered vehicles and their unique thermal management requirements (temperature control) when compared to conventional internal combustion engine (ICE) vehicles. The present invention coordinates management of a cooling system used to control the temperature of a fuel cell stack, the temperature of its accessories, and the cooling airflow for the air-conditioning (AC) system. Temperature control is needed to maintain correct fuel cell stack efficiency and humidification. These have dramatic effects on the performance and durability of the stack.

[0017] The temperature control strategy of the present invention delivers the precise temperature controls needed for a fuel cell powered vehicle. This control system has been designed based on transient and steady-state analyses and linearization of the cooling circuit mathematical model, and has been verified by computer simulation for different operating cycles and conditions.

[0018] FIG. 1 shows a functional schematic of a fuel cell vehicle cooling circuit of the present invention. Two independent cooling circuits (loops) are used to cool a fuel cell system 42 and all other liquid cooled components on the vehicle. They include a high temperature cooling loop 20 and a low temperature cooling loop 22. The fuel cell stack and several associated system components can be cooled with the high temperature cooling loop 20. Coolant is fed from the conduit into the fuel cell system 42 through an inlet and exits out of the fuel cell system 42 through an outlet. This illustrated and simplified high temperature cooling loop 20 is provided to assist in the understanding of the present invention. The low temperature cooling loop 22 has a low temperature cooling loop heat exchanger also known in the art as a radiator (“low temperature cooling loop radiator”) 28 with an inlet and an outlet to allow exit and entry of coolant and can be used to thermally manage some auxiliary vehicle components such as auxiliary fuel cell system 42 components, an electric drivetrain 24 and its power management hardware 26. The low temperature cooling loop 22 can also have a pump (not shown) to move coolant through a plurality of conduits from the low temperature cooling loop radiator 28 and through the various cooled components.

[0019] On the high temperature cooling loop 20, fuel cell system 42 waste heat (Q_in) is removed by coolant (not shown) and transported through the loop via several conduit means (as illustrated in FIG. 1) such as hoses, piping, etc. through the action of a variable speed pump 30 to a high temperature cooling loop heat exchanger also known in the art as a radiator (“high temperature cooling loop radiator”) 32 having an inlet and an outlet and/or a radiator bypass 40, where it is removed from the vehicle (Q_out) 44 by a cooling airflow 48. The cooling airflow 48 varies based on vehicle speed and ambient air temperature 34 (T_amb), and can be increased by the action of a fan 36. The fan 36 for the present invention has variable speeds. The flow of coolant is also controlled by a variable high temperature cooling loop radiator bypass valve 38. This bypass valve 38 controls the amount of coolant flow between the high temperature cooling loop radiator 32 and the high temperature cooling loop radiator bypass 40.

[0020] In one embodiment of the present invention, the temperature control requirements are: (i) a high temperature cooling loop fuel cell system inlet temperature (T_in) 50 should be approximately within ±2 degrees Celsius from a target temperature of 70 degrees Celsius at all times, (ii) coolant temperature rise across the fuel cell system 42 (i.e., T_ex52 minus T_in50=ΔT 54) should be as close to 10 degrees Celsius as possible, and (iii) a low temperature cooling loop inlet temperature (T_inLT) 56 should be lower than 50 degrees Celsius.

[0021] Inlet temperatures T_in50 and T_inLT56 are controlled by adjusting the fan 36 speed (N_f) and the bypass valve 38 position (X_v). The coolant temperature rise (ΔT) 54 is controlled by adjusting coolant mass flow through adjustment of the high temperature cooling loop pump 30 speed (N_p). All the commanded variables N_f, X_vand N_pare given in a normalized form. They change in the range from 0 to 1, where 0 corresponds to zero fan 36 or pump 30 speed and fully closed bypass valve 38 (i.e., full bypass operation), and 1 corresponds to the rated fan 36 or pump 30 speed and fully opened bypass valve 38 (i.e., no bypass operation).

[0022] The fuel cell system 42 main plant disturbances are: fuel cell system output power (approximately proportional to net power P_n), vehicle speed (V_v), and the ambient temperature T_amb34. The first disturbance (P_n) influences fuel cell system waste heat Q_in, while the last two (V_vand T_amb) influence the heat Q_out44 removed by the cooling airflow 48. The plant of the present invention refers to what the controllers are trying to control.

[0023] In the illustrated embodiment of the present invention, fan 36 is also used by an air-conditioning (A/C) system 62 to cool an A/C condenser 60. The A/C system 62 can command stepwise changes of the fan 36 speed (N_fAC) to a few distinct levels. However, an A/C system control unit (not shown) can work independently of the fuel cell system 42 temperature control of the present invention. A final fan 36 speed command is selected as a maximum value of the A/C system 62 and fuel cell system 42 commanded variables N_fACand N_f, respectively. Hence, the change of the A/C system 62 commanded speed to a level N_fAC>N_fcan be regarded as an external disturbance action, which increases the inlet coolant temperature dynamic control error.

[0024] In summary, the present invention as illustrated in FIG. 1 can have the following:

[0025] [t1]

[0026] Controlled Variables

[0027] [t2]

[0028] Command Variables

[0029] [t4]

[0030] Disturbance Variables

[0031] All functions can be placed in separate control units with each vehicle system, under a vehicle system controller (VSC) 64, or in various combinations. For the illustrated embodiment, the vehicle including the fuel cell plant is under the control of the VSC 64.

[0032] A block diagram of the temperature control system of one possible embodiment of the present invention is illustrated in FIG. 2. Note: in FIG. 2 a vector form for the T_in−T_inLTfeedback paths is used and indicated by a thick line 68. Alternatively two lines (not shown) could have been used (scalar form) having two summation elements and two target/output variables. This system consists of three control loops: (1) an inlet temperature control loop for the high temperature cooling loop T_in68, (2) an inlet temperature control loop for the low temperature cooling loop T_inLT(included in 68), and (3) a temperature rise across the fuel cell system (ΔT) 54 control loop 72. An inlet temperature controller 82 also receives vehicle input in the form of measurable disturbance variables 74 as described above (P_n, V_v, T_amb, and N_fAC) using various sensors contained within the vehicle to obtain the measurements.

[0033] The part of the plant that represents the temperature rise across the fuel cell system ΔT 54 has a very fast time-response that is predominantly determined by the disturbance action of the net power P_n.Therefore, a simple integral ΔT controller 76 can be utilized.

[0034] The high temperature cooling loop inlet temperature T_in50 is controlled by changing fan 36 speed N_fand the bypass valve 38 position X_v. To minimize energy consumption, it is desirable to only change one of these commanded variables at any one time. Namely, the inlet temperature controller 82 can operate either in a fan operating mode 78 (N_fchanges, X_v=X_{v max}=1) or in a bypass valve operating mode 80(X_vchanges, N_{f min}=0). The present invention can easily accommodate a controller that can operate in both fan operating mode (fan speed) 78 and the bypass valve operating mode,(bypass valve position) 80.

[0035] The present invention is the use of an operating mode coordination strategy that can incorporate flip-flop control logic and has a fast response. A controller for the bypass valve 38 is initially utilized to provide temperature control. The valve control is active until the bypass valve 38 reaches a wide open position to allow all coolant flow to pass through the high temperature cooling loop radiator 32. Once the bypass valve 38 is in its full open position, the strategy flip-flops from bypass valve 38 control to fan 36 speed control. The advantage of not allowing fan 36 operation until the bypass valve 38 is fully open is that energy is saved by only operating the fan 36 when it is most effective (i.e., when all the coolant is flowing through the radiator).

[0036] A further feature of the present invention can be when the A/C system 62 independently commands increased fan 36 speed for improved A/C condenser 60 performance, the strategy can flip-flop again to the bypass valve 38 control to reduce the flow of coolant through the high temperature cooling loop radiator 32. This allows the fuel cell system 42 to maintain its temperature, even when fan 36 speed is higher than required to accomplish this.

[0037] Another general feature of the present invention can be the use of feed forward disturbance compensation. This compensation allows the temperature control system to work faster and more efficiently (a faster response) using input from the measurable disturbance variables 74. For example, the system can detect a requested large increase in net power (P_n) from the VSC 64. The strategy will anticipate that this request for large increase in P_nwill result in a large heat load. Therefore, the strategy can be configured to, for example, increase fan 36 speed as P_nincreases. This feature allows the overall temperature control strategy to reduce temperature spikes (i.e., variance of temperature changes) therefore allowing the plant to operate more frequently within its optimum operating temperature range.

[0038] FIG. 3 illustrates one possible configuration of the present invention by way of a block diagram of the inlet temperature controller 82 utilizing features of the present invention. As stated above this controller can be housed within the VSC 64 or a separate controller. This controller generally has feedback controllers 86 and 88, feed-forward disturbance compensation 84, and a fan/valve coordination logic within the inlet temperature controller 82, which includes controller preset actions (algorithm) 90. This description will first describe an inlet temperature controller for the high temperature cooling loop control 92 utilizing a first flip-flop 96 in FIG. 3 (a second flip-flop 98 is reset: Q=0, N_f=N _fHT) followed by a description in FIG. 3 of an extension of this controller including an additional low temperature cooling loop control 94.

[0039] The high temperature cooling loop inlet temperature T_in50 can be controlled by a first proportional integral (PI) controller 86 that controls fan operating mode (fan speed) N_f78 and a second PI controller 88 that controls the bypass valve operating mode (bypass valve position) X_v80. In other words, the first PI controller 86 is active in the fan operating mode 78(X_v32 X_{v max}=1), while the second PI controller 88 operates in the valve operating mode 80 (N_f=N_{f min}=0). Both first 86 and second 88 PI controllers are coordinated by a first flip-flop logic 96 generally described above.

[0040] An advantage of the proposed structure is based on the use of two controllers, first PI controller 86 and second PI controller 88. This differs significantly from prior art single-controller-based split range control structures. Advantages of the two controllers include: (i) the controllers can have different parameters or even structures, so that they can provide better control performance in the general case of significantly different plant models with respect to inputs N_fand X_v, and (ii) the overall control algorithm becomes more transparent in the presence of additional controller preset actions and additional T_inLT-limitation logic.

[0041] The high temperature cooling loop 20 plant dynamics contained in the inlet temperature control loop 92 is predominantly determined by a relatively large system time constant (τ) due to a large thermal capacity of the system, and by the coolant transport delay (τ_td) which is dependent on the high temperature cooling loop pump 30 speed N_pand is at least 10 times smaller than τ. The first 86 and second 88 PI feedback controllers are utilized, to achieve a fast, well-damped and steady-state-accurate inlet temperature response.

[0042] The feed-forward disturbance compensator 84 reacts to a change in measured disturbance variables 74 (such as the net power P_nand the vehicle speed V_v) by generating feed-forward commanded signals that are added to the feedback controllers output signals. This compensates for the disturbance action, i.e., leads to decrease of the variance of inlet temperature variations in view of preventing temperature spikes as described above. Based on the insight to the linearized plant model structure, proportional feed-forward actions are adequate to accomplish the present invention. Actually, the feed-forward gains represent an inverse plant dynamics with respect to the disturbance signals P_nand V_v. The measured disturbance signals P_nand V_v(especially P_n) are filtered, in order to attenuate the transfer of noise from disturbance to commanded signals. Good performance of the feed-forward disturbance compensation 84 for the wide range of ambient air 34 temperatures T_ambcan be provided by applying feed-forward gain scheduling with respect to measured ambient temperature signal. PI controllers 86 and 88 gain scheduling known in the art may also be convenient.

[0043] The fuel cell plant dynamic behavior is different for each of the two inlet temperature controller output variables N_fand X_v. Namely, it is necessary to have different feedback and feed-forward controller parameters (or even structures) for the fan and bypass valve operating modes 78 and 80. Ultimately, this is achieved by realization of separate controllers for each operating mode, which are coordinated by a logic algorithm described in FIG. 3.

[0044] When the first flip-flop 96 in FIG. 3 is set (Q=1), the bypass valve operating mode 78 is active. The first PI controller 86 in the fan 36 path is disabled by setting its upper limit N_{f max}to the lower limit N_{f min}(usually 0). If the second PI controller 88 in the bypass valve 38 path requires full opening of the valve (X_v=X_{v max}=1), the first flip-flop 96 is reset, i.e. it is switched to the fan operating mode 78 (Q=0). During the fan operating mode 78 period, the bypass valve 38 is fully opened (X_vmin=X_{v max}=1), i.e., the second PI controller 88 in the bypass valve 38 path is disabled. The first flip-flop 96 is switched back to the bypass valve operating mode 80, when the first PI controller 86 in the fan 36 path requires minimum (usually zero) fan 36 speed (N_f=N_fmin=0). The preset coordination algorithm may be extended by appropriate supervision of the switching logic to avoid frequent switching in the presence of noise. For example, it may be requested that the conditions N_f=N_{f min}and X_v=X_{v max}need to be satisfied for a certain, relatively short time, before the operating mode is changed.

[0045] The coordination logic presented in FIG. 3 illustrates an embodiment of the present invention in the form of a digital circuit. A corresponding algorithm for purposes of software implementation is included in the flowchart diagram shown in FIG. 4. Note that only one of the first PI 86 or second PI controllers 88 is activated at any one time.

[0046] Additional preset actions can be included in the coordination algorithm illustrated in FIG. 4 to compensate for distinct nonlinear plant model effects. They include overshoot compensation, undershoot compensation, and A/C action compensation.

[0047] First is overshoot compensation. During a transition from bypass valve operating mode 80 to fan operating mode 78, the fan 36 is turned off. If the vehicle speed V_vis low at the same time, the output heat is also low and T_inis practically independent of the bypass valve 38 position X_v(“dead-zone effect”). This implies loss of the plant controllability, and finally results in a large inlet temperature overshoot after transition to the fan operating mode 78. To prevent this, the control algorithm of the present invention can detect an overshoot tendency potential, and command full opening of the bypass valve 38 (X_v=X_vmax=1) and presetting the fan 36 speed N_fto a value dependent on ambient air 34 temperature (T_amb). Note that controller output presetting always includes presetting the controller integral term to avoid windup problem.

[0048] Second is undershoot compensation. Steady-state analysis of the fuel cell plant model has discovered the presence of a bypass valve 38 insensitivity zone (“dead-zone effect,” see above) that occurs approximately between X_vvalues between 0.5 and 1.0. Here a change of bypass valve 38 position X_vhas a relatively small influence on the change of inlet temperature (T_in). Inside the “dead-zone” the disturbance rejection is inefficient, i.e. the dynamic control error is relatively large. Compensation of the inlet temperature rise (overshoot) is already provided (above). Additionally, inlet temperature drop (undershoot) can be compensated by skipping the “dead-zone” (presetting of X_v) after transition to the bypass valve operating mode 80, as well as during the bypass valve operating mode 80 if a control error (ε) exceeds the given maximum value (ε_lim) and X_vis decreasing.

[0049] And, third is A/C action compensation. The fan 36 speed stepwise change commanded from the A/C system 62 represents one of the measurable disturbance variables 74 (N_fAC). This can cause a temporary rise or drop of the inlet temperature of the high temperature cooling loop T_in50. The algorithm can be configured to detect the change of A/C fan 36 speed command N_fAC,and reacts immediately by changing the operating mode if required (and resetting the controller output value), thus reducing the disturbance influence. In addition, even if N_fAC=constant, the controller needs to skip the fan 36 control “dead-zone”(N_f<N_fAC) caused by the fan 36 input selector function max (N_f, N_fAC).

[0050] The algorithm of the present invention as illustrated in FIG. 3 generally functions as described above if the low temperature loop inlet temperature (T_inLT) 56 does not exceed a limit value of 50 degrees Celsius. If the temperature T_inLTexceeds the value T*_inLT(set to 50 degrees Celsius, or somewhat lower to avoid overshoot), the second flip-flop 98 as shown in FIG. 3 is set, and activates a low temperature (LT) control mode using a third PI controller 100 (N_f=N_fLT). This third PI controller 100 adjusts the fan 36 speed with the aim to keep the temperature T_inLTat a target (limit) value T*_inLT≈50 degrees Celsius, thus spending a minimum fan 36 power to reach the requirements T_inLT<50 degrees Celsius. Concurrently, the first flip-flop control 96 switches to the bypass valve operating mode 80, so that the inlet temperature T_infor the high temperature cooling loop 20 is controlled by changing the bypass valve 38 position X_v. When the commanded bypass valve 38 position reaches its upper limit value (X_{v max}), the overall strategy determines that temperature T_incannot be controlled by the bypass valve 38. Therefore, additional fan 36 speed needs to be commanded. In this case (X_v=X_vmax) the first flip-flop 96 is reset, and the fan operating mode 78 is reactivated. Concurrently, the second flip-flop 98 is also reset, thus disabling the low temperature cooling loop control mode (N_f=N_fHT). Note, the low temperature cooling loop control mode is now unnecessary, because the temperature T_inLTwill decrease due to fan 36 speed increase. This low temperature cooling loop control mode strategy is included in the overall controller algorithm illustrated in FIG. 4 in sections “LT loop control mode trigger” and “LT loop control” described below.

[0051] The parameters of the first 86, second 88, and third 100 PI controllers illustrated in FIG. 3 are determined according to the following simple equations: $K_R=\frac{4{\zeta }^2}{{\tau }_e}\frac{\tau }{K},$

[0052] and where the ratio τ/K between system (fuel cell plant) time constant τ and gain K is determined by plant model linearization, while the closed-loop system equivalent time constant τ_eand damping ratio ζ are arbitrarily chosen. The damping ratio ζ is convenient to set to a value in the range 0.7-1.0 to provide a fast and well-damped control system response. The minimum value of the equivalent time constant τ_e(i.e., the minimum response time) for a well-damped response is given by: ${\tau }_{e\min }=4\left(\frac{{\tau }_0}{2}+{\tau }_{\mathrm{td}\max }+{\tau }_{\mathrm{actuator}}+{\tau }_{\mathrm{sensor}}\right),$

[0053] where τ₀is the sampling time, τ_{td max}is maximum coolant transport delay, and τ_actuatorand τ_sensorare the equivalent time constants of the fan/valve actuator and the temperature sensor, respectively. The case ζ=0.7, τ_e=τ_{e min}corresponds to the symmetric optimum. Increase of τ_{e>τe min}results in decrease of noise sensitivity and control effort.

[0054] Similarly, the ΔT-controller integral gain is given by: $K_{l\Delta T}=\frac{1}{{\tau }_{e\Delta z}{K}_{\Delta T}},$ ${\tau }_{e\Delta T}\ge 2\left(\frac{{\tau }_0}{2}+{\tau }_{\mathrm{td}\Delta T\max }+{\tau }_{\Delta T\mathrm{actuator}}+{\tau }_{\Delta T\mathrm{sensor}}\right).$

[0055] The feed-forward disturbance compensator 84 gains K_nand K_vare determined as the ratios of two characteristic gains of linearized fuel cell plant model: the gains with respect to disturbance variables P_nand V_v, and the system gain K (with respect to N_fand X_v). These ratios vary with P_nand V_v. Approximately minimum values of the ratios are chosen for gains K_nand K_v, to avoid an overcompensation effect.

[0056] According to the above optimization procedure, the controller parameters are calculated from the parameters of the linearized fuel cell plant model. The final controller tuning is carried out on the real plant.

[0057] FIG. 4 illustrates the strategy of the present invention. Specifically, the algorithm outputs fan speed N_f78 and bypass valve position X_v80 and are calculated based on actual and target temperatures for T_in-inlet temperature of high temperature cooling loop 20 and T_inLT-inlet temperature of low temperature cooling loop 22, and inputs for measurable disturbance variables 74. Initial values can include: fan 36 speed (N_f)=N_fmin(i.e., the fan 36 speed is set to its lower limit); bypass valve 38 position (X_v)=X_vmin(i.e., the bypass valve 38 position is closed); valve_mode=TRUE (i.e., the bypass valve control mode 80 is on using second PI controller 88 ); LT_mode=FALSE (i.e.,the low temperature cooling loop controller, third PI controller 100 is off); and the fan 36 speed required from A/C N_fAC=N_fAC-old=0(i.e., the A/C does not require fan 36 ). The strategy can be divided into following sections: Overshoot and A/C ON→OFF compensation; undershoot compensation during bypass valve operating mode 80 (VOM), and low temperature cooling loop (LT) control; regular control in VOM and a VOM→FOM (valve to fan operating mode 78 ) trigger; A/C OFF→ON compensation; undershoot compensation during FOM→VOM transition; regular control in FOM; and LT_loop control mode trigger.

[0058] The strategy begins at step 110 where determinations are made for: ε, ε_LT,ΔN_fAC,N_fAC-old, ΔX_v. In this step ε and ε_LTare the control errors of the high and low temperature cooling loops 20 and 22 using: T* _in−T_inand T*_inLT−T_inLTrespectively using target and actual values.

[0059] The strategy next determines at step 112 whether LT_mode=FALSE and ε_LT<−Δ_LT. In other words, it determines if the low temperature control loop third PI controller 100 is deactivated and the low temperature loop inlet temperature 56 exceeds its limit value. If no, at step 112 the strategy next makes a determination of whether valve_mode=TRUE at step 118.

[0060] If yes at step 112, the strategy commands LT_mode=TRUE at step 114 (i.e., the low temperature cooling loop controller 100 is activated). Next at step 116 the strategy determines if valve_=mode FALSE. If yes, the strategy commands at step 120 for valve_mode=TRUE, and X_v=X_v0for Preset Controller (X_v). This activates the bypass valve operating mode 80 and presets the bypass valve 38 to its predetermined position to avoid the valve “dead-zone” and proceeds to step 122. If no at step 116, the strategy just proceeds to step 122 (the bypass valve operating mode 80 remains active).

[0061] At step 122 the strategy commands N_f=max(N_f, N_fAC)for preset controller LT(N_f) to the actual fan 36 speed N_for to the speed required from A/C N_fACif it is greater than N_f. From here the strategy proceeds to step 118.

[0062] At step 118 if the determination for valve_mode=TRUE is yes, the strategy proceeds to step 124 to determine if N_f<N_fmaxAND (−ε>ε_limAND V_v<V_v0OR ΔN_fAC<0 AND N_fAC=0 AND LT_mode=FALSE). Here the strategy determines if the fan 36 speed is not at its upper limit, and the inlet temperature T_inexceeds its upper limit at low vehicle speeds V_v(overshoot compensation condition), or A/C commands turning off the fan 36 in the high temperature loop operating mode (A/C ON→OFF compensation condition). If no at step 124, the strategy proceeds to step 126. If yes at step 124, the strategy proceeds to step 128.

[0063] At step 128, the strategy commands the valve_mode=FALSE, LT_mode=FALSE, N_f0=f₁(T_amb), N_f=max(N_f, N_f0,N_fAC), and X_v=X_vmax. These commands essentially deactivate the bypass valve operating mode 80 and the low temperature cooling loop mode, thus skipping to the fan operating mode 78. Further, they set the bypass valve 38 position X_vto its upper limit X_vmax, and determine the fan 36 speed to a value N_f0which is a function of the ambient air 34 temperature, or to the A/C commanded value N_fACif it is greater than N_f0. From step 128 the strategy proceeds to step 130 to Preset Controller (N_f). From here, the output is sent to the fan 36 and bypass valve 38.

[0064] Returning to a no determination at step 124 (no overshoot nor A/C ON→OFF compensation), the strategy proceeds to step 126 where a determination is made as to whether ε>ε_limAND X_v>X_v1AND ΔX_v<−ΔX_vlim. Here the strategy determines if the inlet temperature T_inexceeds its lower limit, the bypass valve 38 position X_vis greater than some level X_v1inside the dead-zone, and the bypass valve 38 position (negative) change is sufficiently large. If yes, the strategy proceeds to steps 132 and 134 and presets the bypass valve 38 controller to a predetermined bypass valve 38 position X_v2<=X_v1which is around the “dead-zone” lower boundary.

[0065] Next the strategy determines whether LT_mode=TRUE at step 136. If no, the strategy output is sent to the fan 36 and bypass valve 38. If yes at step 136, a determination of whether N_fAC>N_fis made at step 138. If no, the strategy executes (commands) the LT PI controller 100 (N_f=ExecController LT ( )) at step 140 and its output is sent to the fan 36 and bypass valve 38. If yes, the strategy commands N_f=0 and LT_mode=FALSE at step 142 and its output is sent to the fan 36 and bypass valve 38 (Note that the external fan 36 speed selector max(N_f,N_fAC) (see FIG. 2) will set N_fto N_fmaxin this case).

[0066] Returning to step 126, if the determination is no (no undershoot compensation), the strategy proceeds to step 144 and executes the bypass valve 38 PI controller 88 (X_v=ExecController( . . . )). From step 144 the strategy proceeds to step 146 and determines whether X_v=X_vmax, which determines whether the bypass valve 38 is in its maximum open position. If no, the strategy proceeds to step 136 and considers LT loop control as explained above. If yes, the strategy commands valve_mode=FALSE, LT_mode=FALSE, and N_f=max(N_fAC, N_f) at step 148. This deactivates the bypass valve 38 controller and LT PI controller 100, and determines the fan 36 speed preset value N_fas a maximum between actual fan 36 speed value N_fand the value N_fACcommanded from A/C. These commands are followed by step 150 that presets the fan 36 speed N_f(Preset Controller, (N_f)), and the outputs are sent to the fan 36 and bypass valve 38.

[0067] Returning to step 118, if the determination is no (the fan operating mode 78 is active) the strategy proceeds to step 152 where N_f=ExecController( . . . ) is commanded, which executes the HT fan 36 PI controller 86.

[0068] From step 152, the strategy proceeds to step 154 to determine whether N_f<N_fAC, which means whether the calculated fan 36 speed N_fis lower than the speed N_fACcommanded from A/C. If no, the strategy proceeds to step 156 and determines whether N_f=N_fmin, which means whether the calculated fan 36 speed N_fis at its lower limit value. If yes, the strategy proceeds to step 160 and commands valve_mode=TRUE. At step 156 if the determination is no the output is sent to the fan 36 and bypass valve 38.

[0069] After step 160, the strategy proceeds to step 162 and determines whether ΔN_fAC>0 AND N_fAC>N_f, which means whether A/C commanded a higher A/C speed N_fACwhich was greater than the calculated fan 36 speed N_f. If yes, the strategy determines the bypass valve 38 preset value X_v=f₂(N_fAC−N_f) at step 164, which is a function of the difference between A/C-required and calculated fan 36 speed. After step 164, the strategy presets the bypass valve 38 controller (PresetController(X_v)) in order to compensate for A/C OFF→ON disturbance, and at step 166 the output is sent to the fan 36 and bypass valve 38.

[0070] Returning to step 162, if the determination is no (no A/C OFF→ON disturbance), the strategy proceeds to step 168 to determine whether ε>ε_lim,(i.e., whether the HT inlet temperature T_inis below its lower limit). If yes, the strategy determines at step 170 the valve preset value X_v=f₃(T_amb), which depends on the ambient air 34 temperature and is around the dead-zone boundary. Then the strategy proceeds to step 166. If no at step 168, the strategy sets a lower preset value X_v=X_v0at step 172. The strategy then proceeds to step 166, where the bypass valve 38 controller is preset to the previously calculated preset value X_v, in order to compensate for undershoot effect during FOM→VOM transition. The output is then sent to the fan 36 and bypass valve 38.

[0071] The above-described embodiment of the invention is provided purely for purposes of example. Many other variations, modifications, catalysts, and applications of the invention may be made.