Control systems and methods to meet fuel cell fuel demand

Systems and methods for controlling fluid flow in a fuel cell circuit of a vehicle. A system may have a fuel cell stack configured to receive hydrogen gas. The system may have a current sensor configured to detect current flowing through the fuel cell stack. The system may have a plurality of actuators, which may include at least one injector, a pump, and a shut valve. The system may have an electronic control unit (ECU). The ECU may estimate pressures of the hydrogen gas and non-hydrogen gases in the circuit. The ECU may determine a current increase rate based on the detected current. The ECU may apply a compensatory hydrogen gas stoic to a base hydrogen gas stoic to meet a target hydrogen gas stoic by controlling one or more of the actuators based on the estimated pressures when the current increase rate is above a predetermined threshold value.

BACKGROUND

The present disclosure relates to systems and methods for controlling fluid flow through a fuel cell stack of a fuel cell circuit based on a real-time model of the circuit and, more particularly, to systems and methods for preventing fuel, or hydrogen, deficiency under transient and over-pressure conditions observed by the fuel cell circuit.

2. Description of the Related Art

As the push for conservation of natural resources and reduced pollution advances, various concepts have been discovered to achieve such goals. These concepts range from harvesting wind and sun-based energy to various improvements in vehicle design. The vehicle improvements include new engines designed to improve fuel economy, hybrid vehicles that operate using a combination of an engine and a motor-generator to further improve fuel economy, fully electric vehicles that operate based on power stored in a battery, and fuel cell vehicles that generate electricity by facilitating a chemical reaction.

Many fuel cell vehicles include a fuel cell stack that includes multiple fuel cells. The fuel cells may receive a fuel, which typically includes hydrogen, along with oxygen (via air) or another oxidizing agent. The fuel cell stack may facilitate a chemical reaction between the hydrogen and oxygen. This chemical reaction generates electricity and water as a byproduct. The electricity generated by the fuel cell stack may be stored in a battery or directly provided to a motor-generator to generate mechanical power to propel the vehicle. While fuel cell vehicles are an exciting advancement in the automobile industry, the technology is relatively new, providing space for improvements to the technology.

Electrical output of the fuel cell stack varies based on a pressure and flow of the gases (e.g., hydrogen and air) located therein. The desired electrical output may be determined based on a power request which may be based on starting or accelerating the vehicle. In that regard, it is desirable to accurately control the pressure and flow of hydrogen into the fuel cell stack from both a hydrogen supply and a hydrogen recirculation route to achieve the desired electrical output in a timely manner. However, it is important to know the pressure and flow of the various elements within the circuit (e.g., pipes, valves, pumps, etc.) in order to accurately and quickly control the fuel cell stack.

Thus, there is a need in the art for systems and methods for accurately controlling hydrogen circulation to a fuel cell stack of a vehicle.

SUMMARY

Systems and methods for controlling fluid flow in a fuel cell circuit of a vehicle to meet a fuel demand of a fuel cell stack of the fuel cell circuit in both steady and transient conditions. The system may have a current sensor configured to detect current flowing through the fuel cell stack. The system may have a plurality of actuators. The system may have an electronic control unit (ECU). The ECU may estimate pressures of the fuel and non-fuel gases in the circuit. The ECU may determine a current increase rate based on the detected current. The ECU may apply a compensatory fuel amount to a base fuel gas amount to meet a target fuel amount by controlling one or more of the actuators based on the estimated pressures when the current increase rate is above a predetermined threshold value.

In accordance with an embodiment of the present disclosure, there may be a system for controlling flow of fluids in a fuel cell circuit of a vehicle. The system may have a fuel cell stack having a plurality of fuel cells. The fuel cell stack may be configured to receive hydrogen gas. The system may further have a current sensor. The current sensor may be configured to detect a current flowing through the fuel cell stack. The system may have a plurality of actuators. The actuators may include at least one injector configured to supply the hydrogen gas to the fuel cell stack and adjust a pressure of the hydrogen gas. The actuators may further include a pump and/or a blower configured to facilitate the flow of the fluids in the fuel cell circuit. The fluids may comprise the hydrogen gas and non-hydrogen gases. The actuators may further include a shut valve configured to purge the fluids from the fuel cell circuit to adjust pressures of the non-hydrogen gases. The system may further have an ECU coupled to the fuel cell stack, the current sensor, and the actuators. The ECU may be configured to estimate the pressure of the hydrogen gas and the pressures of the non-hydrogen gases in real-time. The pressures of the hydrogen gas and the non-hydrogen gases may define a total system pressure. The ECU may be further configured to determine a current increase rate based on the detected current. The ECU may be further configured to apply a compensatory hydrogen gas stoic to a base hydrogen gas stoic to meet a target hydrogen gas stoic by controlling one or more of the actuators based on the estimated pressures of the hydrogen gas and the non-hydrogen gases when the fuel cell current rate is above a predetermined threshold value.

The ECU may increase the pressure of the hydrogen gas by controlling the at least one injector to meet the target hydrogen gas stoic when the pump operates below an optimal speed under transient conditions. The ECU may decrease the pressure of the hydrogen gas once the pump operates at the optimal speed. The ECU may control the shut valve to regulate the total system pressure to ensure that the total system pressure does not exceed a maximum total system pressure. The ECU may increase a speed of the pump to meet the target hydrogen gas stoic. The compensatory hydrogen gas stoic may be 0.25, the base hydrogen gas stoic may be 1.00, and the target hydrogen gas stoic may be 1.25. The ECU may open one, some, or all of the at least one injector to control the pressure of the hydrogen gas.

The non-hydrogen gases may include water vapor. The system may further have a liquid vapor separator configured to direct the water vapor from the fuel cell stack to the pump and direct liquid water separated from the water vapor to be expelled through the shut valve to meet the target hydrogen gas stoic. The ECU may estimate a mole fraction of the hydrogen gas at an outlet of the liquid vapor separator to determine a speed of the pump.

In accordance with another embodiment of the present disclosure, there may be a method for meeting hydrogen gas demand in a fuel cell circuit of a vehicle under transient conditions. The method may include detecting, by a current sensor, a current flowing through a fuel cell stack. The method may further include estimating, by an ECU, a pressure of the hydrogen gas and pressures of non-hydrogen gases at an outlet of the fuel cell stack. The method may further include determining, by the ECU, a current increase rate based on the detected current. The method may further include determining, by the ECU, a target hydrogen gas stoic at an inlet of the fuel cell stack based on the detected current. The method may further include increasing, by at least one injector, the pressure of the hydrogen gas at the outlet based on the estimated pressures of the hydrogen gas and the non-hydrogen gases to meet the target hydrogen gas stoic when the current increase rate is above a predetermined threshold. The method may further include decreasing, by the ECU, the pressure of the hydrogen gas once a pump configured to recirculate the hydrogen gas into the fuel cell operates at a speed to meet the target hydrogen gas stoic when the current increase rate is above the predetermined threshold.

The method may further include purging, by a shut valve, a predetermined amount of the hydrogen gas and predetermined amounts of the non-hydrogen gases from the fuel cell circuit to keep a total system pressure at or below a maximum total system pressure. The total system pressure may be defined by the pressures of the hydrogen gas and the non-hydrogen gases. The non-hydrogen gases may comprise water. The method may further include separating, by a liquid vapor separator, liquid water from the water vapor. The method may further include exhausting, by the shut valve, the liquid water prior to the purging of the predetermined amount of the hydrogen gas and the predetermined amounts of the non-hydrogen gases.

The ECU may estimate the pressure of the hydrogen gas based on data from one or more pressure sensors configured to detect pressure at least at an outlet of the at least one injector. The ECU may open one, some, or all of the at least one injector to control the pressure of the hydrogen gas.

In accordance with another embodiment of the present disclosure, there may be a method for meeting hydrogen gas demand in a fuel cell circuit of a vehicle. The method may include determining, by an ECU, a target hydrogen gas recirculation stoic at an inlet of a fuel cell stack. The method may further include estimating, by the ECU, a mole fraction of the hydrogen gas at an outlet of a liquid vapor separator, a pressure of the hydrogen gas, and pressures of non-hydrogen gases at an outlet of the fuel cell stack. The pressures of the hydrogen gas and the non-hydrogen gases may define a total system pressure. The method may further include increasing, by the ECU, a speed of a hydrogen pump based on the mole fraction of the hydrogen gas to meet the target hydrogen gas recirculation stoic while minimizing excess hydrogen gas purging to improve overall efficiency of the fuel cell circuit. The method may further include purging, by a shut valve, a predetermined amount of the hydrogen gas, predetermined amounts of non-hydrogen gas, and a predetermined amount of liquid water directed from the liquid vapor separator to prevent the total system pressure from exceeding a maximum total system pressure. The predetermined amount of the hydrogen gas and the predetermined amount of non-hydrogen gas are purged to maintain the pressure of the hydrogen gas at the outlet of the fuel cell stack at a target pressure and the mole fraction of the hydrogen gas at the outlet of the liquid vapor separator at a minimum mole fraction. The ECU may determine the target hydrogen gas recirculation stoic based on a current flowing through the fuel cell stack. The method may further include determining, by the ECU, a target volumetric flow of the hydrogen gas to determine the speed of the hydrogen pump.

DETAILED DESCRIPTION

The present disclosure describes systems and methods for controlling fluid flow, such as hydrogen gas and non-hydrogen gas (e.g., nitrogen, water vapor), within a fuel cell circuit to meet a fuel (e.g., hydrogen gas) demand of the fuel cell under various conditions, including steady and transient conditions. The systems provide various advantages and benefits such as controlling various actuators of the circuit based on a real-time model of the circuit. This advantageously provides for accurate state determination of each element of the circuit using relatively few sensors, which beneficially reduces a cost of the system. The systems further advantageously utilize high level state target values to generate coarser actuator values to then control each actuator with specificity to meet the state target values.

An exemplary system may have a fuel cell stack configured to receive hydrogen gas. The system may have a current sensor configured to detect current flowing through the fuel cell stack. The system may have a plurality of actuators, which may include at least one injector, a pump, and/or a shut valve. The at least one injector may be configured to supply the hydrogen gas to the fuel cell stack and adjust a pressure of the hydrogen gas. The at least one pump may be configured to facilitate the flow of the fluids in the fuel cell circuit. The fluids may include the hydrogen gas and non-hydrogen gases. The shut valve may be configured to purge the fluids from the fuel cell circuit to adjust pressures of the non-hydrogen gases. The system may have an electronic control unit (ECU) coupled to the fuel cell stack, the current sensor, and the actuators. The ECU may estimate the pressures of the hydrogen gas and non-hydrogen gases in the circuit in real-time. The ECU may determine a current increase rate based on the detected current. The ECU may apply a compensatory hydrogen gas stoic to a base hydrogen gas stoic to meet a target hydrogen gas stoic by controlling one or more of the actuators based on the estimated pressures when the current increase rate is above a predetermined threshold value.

FIG.1is a block diagram illustrating various components of a vehicle100having a fuel cell circuit102capable of generating electricity based on a chemical reaction according to an embodiment of the present disclosure. The vehicle100includes components of a system104for controlling a flow of one or more fluids (i.e., gas and/or liquid) into and out of fuel cells. The gas may be hydrogen gas or a non-hydrogen gas, such as nitrogen, water and/or water vapor and/or a combination thereof that permeates into the fuel cells. The liquid may be water. The system104may control the flow of the fluids to maintain a target hydrogen stoic, or a ratio of hydrogen gas to mole of hydrogen consumed by the fuel cells to operate the fuel cell more efficiently.

The vehicle100and the system104may include an ECU106and a memory108. The vehicle100further includes a power source110which may include at least one of an engine112, a motor-generator114, a battery116, and/or the fuel cell circuit102. The fuel cell circuit102may be a part of the system104.

The ECU106may be coupled to each of the components of the vehicle100and may include one or more processors or controllers, which may be specifically designed for automotive systems. The functions of the ECU106may be implemented in a single ECU or in multiple ECUs. The ECU106may receive data from components of the vehicle100, may make determinations based on the received data, and may control the operation of components based on the determinations.

In some embodiments, the vehicle100may be fully autonomous or semi-autonomous. In that regard, the ECU106may control various aspects of the vehicle100, such as steering, braking, accelerating, or the like, to maneuver the vehicle100from a starting location to a destination location.

The memory108may include any non-transitory memory known in the art. In that regard, the memory108may store machine-readable instructions usable by the ECU106and may store other data as requested by the ECU106or programmed by a vehicle manufacturer or operator. The memory104may store a model of the fuel cell circuit102. The model may include equations or other information usable to estimate various parameters of the fuel cell circuit102. That is, the model of the fuel cell circuit may determine a current or present state of each component (e.g., actuators, pipes, or the like) of the fuel cell circuit102. The state of each component may include a stoic (e.g., hydrogen stoic), a pressure value (e.g., both at an inlet and at an outlet of the component), and a flow value (e.g., a volumetric flow rate) through the component. The model may be a real-time model or a near-real-time model which continuously or periodically (e.g., at least every second, at least every half second, at least every tenth of a second, every one hundredth of a second, or the like) determines new states for each component. In some embodiments, the system100may determine the various parameters via one or more sensors instead of or in addition to estimating the parameters via the ECU106.

The engine112may convert a fuel into mechanical power. In that regard, the engine112may be a gasoline engine, a diesel engine, or the like. The battery116may store electrical energy. In some embodiments, the battery116may include any one or more energy storage device including a battery, a flywheel, a super-capacitor, a thermal storage device, or the like.

The fuel cell circuit102may include a plurality of fuel cells that facilitate a chemical reaction to generate electrical energy. For example, the fuel cells may receive hydrogen and oxygen, may facilitate a reaction between the hydrogen and oxygen, and may output electricity in response to the reaction. In that regard, the electrical energy generated by the fuel cell circuit102may be stored in the battery116or directly utilized by the motor-generator114or another component of the vehicle100(e.g., a heating-ventilation-air conditioning (HVAC) unit). In some embodiments, the vehicle100may include multiple fuel cell circuits including the fuel cell circuit102.

The motor-generator114may convert the electrical energy stored in the battery116, or electrical energy received directly from the fuel cell circuit102, into mechanical power usable to propel the vehicle100. The motor-generator114may further convert mechanical power received from the engine112or wheels of the vehicle100into electricity, which may be stored in the battery116as energy and/or used by other components of the vehicle100(e.g., an HVAC system). In some embodiments, the motor-generator114may also or instead include a turbine or other device capable of generating thrust.

FIG.2is a block diagram illustrating various features of the fuel cell circuit102according to an embodiment of the present disclosure. The fuel cell circuit102may include a fuel cell stack200having a plurality of fuel cells, one or more injectors202, a hydrogen pump204, a liquid vapor separator206, a shut valve208, and one or more sensors.

The one or more sensors may include a pressure sensor210that measures the pressure of the gas at an outlet of the injectors202. A pressure sensor212may measure the ambient pressure at an outlet of the shut valve208. A speed sensor214may measure the speed of the hydrogen pump204, and a power sensor216may measure the power of the hydrogen pump204. A current sensor218may measure the current flowing through the fuel cell stack200and a temperature sensor220may measure a coolant temperature of the fuel cell stack200. A shut valve gate sensor221may detect whether the shut valve208is open (i.e., purging fluids) or closed (i.e., not purging fluids). The aforementioned components of the fuel cell circuit102may be interconnected with one or more pipes222.

The fuel cell stack200converts chemical energy from a fuel (i.e., hydrogen gas) into electricity through an electrochemical reaction of hydrogen gas with oxygen or another oxidizing agent. The fuel cells of the fuel cell stack200require a continuous source of fuel and oxygen to sustain the chemical reaction to produce electricity continuously for as long as fuel and oxygen are supplied. Additionally, some fuel may permeate out of the fuel cell stack200without reacting. When the vehicle100(seeFIG.1) is turned on or when the vehicle100accelerates, the vehicle100demands more electricity from the fuel cell stack200. Hence, more hydrogen gas is needed to fuel the fuel cell stack200to meet this demand while preventing hydrogen starvation.

The one or more injectors202, the hydrogen pump204, and the shut valve208may be actuators. As actuators, these components may be controlled by the ECU106to meet system target states. The actuators may then control stoic, pressure, and fluid values at various locations of the fuel cell circuit102(seeFIG.1) by actuation. The injectors202may inject hydrogen gas into the fuel cell circuit102. The hydrogen pump204may be a pump, a compressor, or other blower that moves the hydrogen gas through the fuel cell circuit102. The shut valve208may work in conjunction with the liquid vapor separator206. The liquid vapor separator206separates a vapor-liquid mixture, such as water from gas, or liquid water from water vapor, which flows through the fuel cell circuit102. Then, the shut valve208purges the excess water and gas to regulate a total system pressure out of the fuel cell circuit102.

The injectors202may be natural-gas injectors with solenoid valve control or other open and close device. The ECU106may position the injectors202to control the flow of gas into one or more components of the fuel cell circuit102(see alsoFIG.1). The ECU106may open, partially open, close, and/or otherwise position the injectors202to control the quantity or amount of hydrogen gas injected by the injectors202. The injectors202inject the hydrogen gas to meet a hydrogen gas pressure target at the outlet of the fuel cell stack200. The injectors202have to meet a flow rate target the achieve the hydrogen gas pressure target.

The hydrogen pump204recirculates the hydrogen gas back to the fuel cell stack200to meet the target hydrogen gas stoic of approximately (1+α):1, where 0<α<1.5. α is a compensatory hydrogen gas stoic. Preferably, a may be 0.25 to yield a target hydrogen gas stoic of 1.25:1. To account for other variables (e.g., transient conditions), the ECU106(seeFIG.1) may use the hydrogen pump204to pump enough hydrogen gas to provide the compensatory hydrogen gas stoic (i.e., recirculation hydrogen gas stoic) and meet the target hydrogen gas stoic. By increasing the speed of the hydrogen pump204, the total volumetric flow rate at the inlet of the fuel cell stack200is increased, and more hydrogen gas is pumped or recirculated into the fuel cell stack200to increase the hydrogen gas stoic. The hydrogen pump204may meet the recirculation hydrogen gas stoic based on the hydrogen gas mole fraction at the liquid vapor separator206outlet. The hydrogen gas mole fraction at the liquid vapor separator206may be estimated by the ECU106. In some embodiments, the hydrogen gas mole fraction may be obtained through sensor data. The ECU106may set a speed for the pump204that can maintain a minimum hydrogen gas mole fraction that can meet the recirculation hydrogen gas stoic.

The shut valve208exhausts or releases water accumulated within the liquid vapor separator206and gases in the fuel cell circuit102(seeFIG.1) to regulate the total system pressure. Specifically, the shut valve208controls the non-hydrogen gas pressure at the outlet of the fuel cell stack200. The ECU106(seeFIG.1) may position the shut valve208into an open, a close, and/or a partially open position to release an amount or quantity of water that has accumulated within the liquid vapor separator206. Further, the ECU106may control the shut valve208to exhaust or release the water and/or other non-hydrogen gas to control the amount of non-hydrogen gas at the outlet of the fuel cell stack200. The shut valve208may have to exhaust the water prior to purging the gases. Purging the gases regulates the total system pressure so that it does not exceed a maximum total system pressure. Otherwise, exceeding the total maximum pressure may cause components of the fuel cell circuit102to burst and/or malfunction. Purging assists the fuel cell circuit102in maintaining a stoic of approximately (1+α) moles of hydrogen gas where 0<α<1.5 for every 1 mole of hydrogen gas consumed by the fuel cell stack.

The shut valve208may operate while maintaining a hydrogen gas mole fraction at the liquid vapor separator206outlet that is sufficient to meet a recirculation hydrogen gas stoic target under both steady state and transient conditions. The shut valve208may minimize unnecessary hydrogen gas purging in order to preserve the hydrogen gas supply of the fuel cell circuit102. Hence, the shut valve208may only purge gases when it is necessary to not exceed the maximum total system pressure.

The one or more pipes222interconnect to form one or more pipe junctions224and connect the components of the fuel cell circuit102. A single pipe222may branch off into multiple pipes222or multiple pipes222may unite to form a single pipe222. The pipes222that split or branch off from the original pipe222may run in parallel and may have the same pressure drop across parallel running pipes222. The one or more pipes222allow the gas to flow through the fuel cell circuit102. For example, the injectors202may be connected to and inject the gas to the fuel cell stack200through the pipe222aand the pipe junction224. In the fuel cell stack200, the hydrogen gas reacts with the oxygen to generate electricity. The fuel cell stack200exhausts and/or emits a gas stream including water and/or water vapor as a by-product of the chemical reaction to generate electricity. The gas stream then passes through the pipe222bto the liquid vapor separator206. The liquid vapor separator206separates the water and/or water vapor from the gas stream, and the shut valve208purges the water and/or water vapor. The hydrogen pump204recirculates the gas stream including any remaining hydrogen gas through the pipe222cand the pipe junction224to the fuel cell stack200.

The ECU106may estimate, predict, or model parameters, such as pressure, flow rate, mole fraction, or temperature at various locations of the fuel cell circuit102, including the injectors202, the hydrogen pump204, the shut valve208, the one or more pipes222, the liquid vapor separator206, and the one or more pipe junctions224(see alsoFIG.1). The ECU106may control the speed of the hydrogen pump204, the number of hydrogen gas injections and the pressure of hydrogen gas supplied by the injectors202, and open and close the shut valve208to maintain the hydrogen stoic.

FIG.3is a block diagram illustrating various logic components of the ECU106of the vehicle100that control actuators of the fuel cell circuit102to meet fuel cell hydrogen demand according to an embodiment of the present disclosure (see alsoFIG.1). The actuator controller300may include a state estimator302, a state governor306, a feedforward/feedback control308, and an upper controller310. The actuator controller300may receive an input, such as a power request312, and may generate an output, such as actuator commands314a-c.

The upper controller310may receive the power request312. The upper controller310may then identify a target hydrogen gas stoic at the inlet of the fuel cell stack200(seeFIG.2), a target hydrogen gas pressure at the outlet of the fuel cell stack200, and a target minimum volumetric flow rate at the inlet of the fuel cell stack200. The upper controller310may identify these target values based on calibration achieved by conducting bench tests with the fuel cell circuit102(seeFIG.2). The target values may depend on the current generated by the fuel cell stack200. The upper controller310may adjust the target values based on information316from the state estimator302. A time signal outputted from the state estimator302may be sent back to the upper controller for control purposes by means of a “unit delay” or other time delay logic, visible in block318. The upper controller310may transmit the target values to the state governor306as shown by an arrow320.

The state estimator302may receive inputs including sensor values322and current actuator positions324(or commanded actuator positions) and may estimate conditions at various locations of the fuel cell circuit102(seeFIG.2). The sensor values322may be obtained from the sensors210,212,214,216,218,220, and221(see alsoFIG.2). The sensor values322may include hydrogen gas pressure at the outlet of the injectors202, the ambient pressure at the outlet of the shut valve208, the speed of the hydrogen pump204, the power of the hydrogen pump204, the current flowing through the fuel cell stack200, the coolant temperature of the fuel cell stack200, and the position of the shut valve208(see alsoFIG.2). The fuel cell circuit102may include relatively few sensors. Additional data is desirable to provide optimal control of the actuators326. In that regard, the state estimator302may calculate, predict, or estimate the additional data (i.e., current conditions) based on the sensor values322and the actuator positions324. For example, the state estimator302may estimate hydrogen gas and non-hydrogen gas pressures at various locations of the fuel cell circuit102. Specifically, the state estimator302may estimate pressures of hydrogen gas, nitrogen gas, and water vapor. As another example, the state estimator302may estimate a mole fraction of a gas at various locations of the fuel cell circuit102. Specifically, the state estimator302may predict the mole fraction of hydrogen gas at the outlet of the liquid vapor separator206(seeFIG.2). The state estimator302may output state estimates328to the state governor306. In some embodiments, addition of more sensors that can detect the additional data may be utilized in lieu of the state estimator302.

The estimates328of the state estimator302may have an accuracy tolerance. The accuracy tolerance may have to be taken into account within the state governor306to prevent hydrogen gas starvation in the fuel cell circuit102(seeFIG.2). Estimates based on a sensor measurement may have a low error potential while estimates that do not have an anchoring sensor measurement may have a high error potential. Error protection margins may be applied to controls relying on high error potential estimates in feed forward control308. For example, a non-hydrogen gas pressure at the outlet of the fuel cell stack200(seeFIG.2) may have an error protection margin that may be applied to pump control332and shut valve control334. As another example, an estimated amount of liquid water within the liquid vapor separator206(seeFIG.2) may have an error protection margin that may be applied to the shut valve control334.

The state governor306may receive the estimates328from the state estimator302. The state governor306may also receive the target values from the upper controller310. Based on the estimates328, the state governor306may convert the target values into actuator values336a-c, or commands. The actuator values336a-cmay then be transmitted to the feedforward/feedback control308. The actuator values336a-cmay be based on meeting the target recirculation hydrogen gas stoic under both steady state and transient conditions by controlling a mole fraction of hydrogen gas at the outlet of the liquid vapor separator206(i.e., inlet of the pump204) (seeFIG.2). The actuator values336atransmitted to the injector control330may include a final hydrogen gas pressure target at the outlet of the fuel cell stack200and a flow rate target at the injectors202to achieve the final hydrogen gas pressure target at the fuel cell stack200(see alsoFIG.2). The actuator values336btransmitted to the pump control332may include a target recirculation hydrogen gas flow rate stoic. The actuator values336ctransmitted to the shut valve control334may include a non-hydrogen gas pressure maximum threshold at the outlet of the fuel cell stack200and a liquid water maximum threshold of the liquid vapor separator206.

The feedforward portion of the feedforward/feedback control308may transmit actuator commands314a-cto the actuators326to achieve the target values based on the actuator values336c. The feedforward control may advantageously achieve a fast response time from the actuators326. The feedback portion of the feedforward/feedback control308may work to close any existing gap between one or more of the actual state values and one or more of the corresponding target values by increasing the actuator commands314a-cuntil the gap eventually closes. The feedback control may advantageously achieve a stable convergence of the gap. However, the feedback control may function relatively slower than the feedforward control. In some embodiments, only one of the feedforward and the feedback control may be utilized.

The feedforward/feedback control308may include the injector control330, the pump control332, and the shut valve control334. The injector control330may transmit the actuator command314ato an injector driver338of the actuators326. The pump control332may transmit the actuator command314bto a pump driver340of the actuators326. The shut valve control334may transmit the actuator command314cto a shut valve driver342of the actuators326.

The pump control332may calculate a pump speed target from the volumetric flow target obtained from the state governor306and the mole fraction of hydrogen gas at the outlet of the liquid vapor separator206obtained from the state estimator302. The error margin of the state estimator302may decrease the mole fraction of hydrogen gas and increase the pump speed target. The pump control332may command the pump driver340to set the pump speed to the calculated pump speed target by transmitting the actuator command314b.

The shut valve control334may command the shut valve driver342to purge liquid water from the liquid vapor separator206and purge gas from the fuel cell circuit102to reduce non-hydrogen gas pressure, and thereby reduce the total system pressure (see alsoFIG.2). The shut valve driver342may open and close the shut valve208(seeFIG.2) based on the actuator command314c. If either liquid water purge or gas purge is required for efficient operation of the fuel cell circuit102and to avoid system failure, the shut valve control334may command the shut valve driver342to open the shut valve208. Otherwise, the shut valve208may be kept closed. If an estimated amount of liquid water in the liquid vapor separator206by the state estimator302exceeds a maximum threshold value calculated by the state governor306, the shut valve control334may command the shut valve driver342to open the shut valve208. If a time determined by a time counter exceeds a shut valve open time target calculated for the liquid water in the liquid vapor separator206and the non-hydrogen gas at the outlet of the fuel cell stack200(seeFIG.2), the shut valve control334may command the shut valve driver342to close the shut valve208. The open time target may be based on a purge target both for the liquid water and the non-hydrogen gas. The purge target for the liquid water may be calculated from the actuator values336cobtained from the state governor306by including the error margin of the state estimator302. The open time target for the liquid water may then be calculated from the purge target and a liquid water purge capability and liquid water accumulation rate of the shut valve208obtained from the state estimator302. If the open time target to purge the liquid water is projected to take longer than a maximum allowable wait time to begin purging the gas, the amount of liquid water accumulated within the liquid vapor separator206must be reduced.

If an estimated non-hydrogen gas pressure at the outlet of the fuel cell stack200obtained from the state estimator302is equal to or greater than a maximum non-hydrogen gas pressure at the outlet of the fuel cell stack200obtained from the state governor306with the state estimator302error protection applied, the shut valve control334may command the shut valve driver342to open the shut valve208(see alsoFIG.2). The shut valve control334may command the shut valve driver342to close to shut valve208to maintain a hydrogen gas concentration below a threshold percentage for a given time period. For example, the hydrogen gas concentration may not exceed four percent (4%) average by volume during any moving three second time interval. In another example, the hydrogen gas concentration may not exceed eight percent (8%) at any time. The timer that determines the open time for non-hydrogen gas purging may not start timing until the liquid water is removed from the liquid vapor separator206.

The injector control330may control the hydrogen gas pressure rate of change and the duty cycle of the injectors202(seeFIG.2) by transmitting actuator commands314ato the injector driver338. The injectors202may be binary state devices that open and close. The injectors202may be controlled through latching hysteresis switch between a minimum and a maximum pressure target. The injectors202may be controlled more precisely by setting a number and a timing of the injectors202to open and close. The number of injections conducted by opening the injectors202may regulate the total system pressure rate of change. One, some, or all injectors202may be opened or closed simultaneously. The injector control330may target a number of pressure states to control the injectors202. For example, a targeted state may be a pressure rise state where the total system pressure is below a target and must be increased. Another targeted state may be a pressure fall state where the total system pressure is above a target and must be decreased. A minimum and a maximum injector flow rate threshold may ensure that the total system pressure is controlled within the targeted states. The minimum and the maximum injector flow rate thresholds may be a threshold for a number of required injectors to raise pressure and a threshold for a number of required injectors to drop pressure, respectively.

FIG.4is a graph400illustrating an exemplary operation of the fuel cell circuit102(seeFIG.2) in a transient condition according to an embodiment of the present disclosure. The graph400may be a time plot of pressure values, speed values, flow rate values, and current values. The X-axis illustrates time, and the Y-axis illustrates the following: values of pressure at the outlet of the fuel cell stack200(seeFIG.2); specifically, a hydrogen gas pressure illustrated by a line402, a water vapor pressure illustrated by a line404, and a nitrogen gas pressure illustrated by a line406, a pump speed illustrated by a line408, a flow rate of hydrogen gas at the inlet of the fuel cell stack200illustrated by a line410, and a current request from the fuel cell stack200illustrated by a line412.

As shown, current request, the flow rate, the pump speed, and the pressure values all have target and estimated values that are correct from the beginning of the graph400until a first time414. At the time414, the target pump speed begins to increase due to the current request beginning to increase, while an estimated pump speed also begins to increase, but at a slower rate. The current request rate may have an increase rate above a predetermined threshold value (e.g., 300 ms). The difference between the target pump speed and the estimated pump speed corresponds to a gap416. In response, the ECU106(seeFIG.3) over increases the hydrogen gas pressure to account for the gap caused by the slow pump speed between times414,418. Dashed lines417show the hydrogen gas pressure target prior to compensation, and a line419shows the compensated hydrogen gas pressure target. By accounting for the gap, the ECU106achieves a hydrogen gas recirculation stoic illustrated by a line420. A line422illustrates a total of the hydrogen gas recirculation stoic and a base hydrogen gas stoic from the injectors202(seeFIG.2). The hydrogen gas recirculation stoic prevents hydrogen gas starvation of the fuel cell stack200. Once the desired hydrogen gas recirculation stoic is achieved, the ECU106begins to decrease the hydrogen gas pressure at the time418. The estimated pump speed catches up to the target pump speed at a time424and the gap416is closed. At the same time, the hydrogen gas pressure stops compensating. At a time426, the current request begins to drop, which leads to a drop in the stoic and the hydrogen gas pressure. At a time428, the current request becomes stagnant, which leads to the pump speed to stagnate and the pressure values as well as the stoic to drop further and eventually stagnate.

FIG.5is a graph500illustrating an exemplary operation of the fuel cell circuit102(seeFIG.2) in an over-pressure condition according to an embodiment of the present disclosure. The graph500may be a time plot of position values, pressure values, speed values, mole fraction values, and stoic target values. The X-axis illustrates time, and the Y-axis illustrates the following: position of the shut valve208(seeFIG.2) illustrated by a line501, values of pressure at the outlet of the fuel cell stack200(seeFIG.2); specifically, a hydrogen gas pressure illustrated by a line502, a water vapor pressure illustrated by a line504, and a nitrogen gas pressure illustrated by a line506, a pump speed illustrated by a line508, a hydrogen gas mole fraction at the outlet of the liquid vapor separator206(seeFIG.2) illustrated by a line510, and a hydrogen gas recirculation stoic target at the outlet of the fuel cell stack200illustrated by a line512.

At a time514, the hydrogen gas recirculation stoic target begins to increase to meet hydrogen gas demand and prevent hydrogen starvation of the fuel cell stack200(seeFIG.2). At the same time, the pump speed begins to increase to help achieve this higher stoic target. The hydrogen gas partial pressure increases rapidly, and the non-hydrogen gas partial pressures also begin to increase slowly at the time514. This drives a rapid increase in the H2 mol fraction at the outlet of the liquid vapor separator206(seeFIG.2). The shut valve208(seeFIG.2) opens to purge water only from the liquid vapor separator206at the time514. The shut valve208opens again to purge water periodically and increase the frequency of the purging of water to prepare for purging gas within a period516.

At a time518, the hydrogen gas recirculation stoic target reaches a peak value and stabilizes. The hydrogen gas partial pressure target has been achieved and remains flat. The estimated hydrogen gas mole fraction at the outlet of the liquid vapor separator206(seeFIG.2) begins to decrease with some time delay as the partial pressure of other non-hydrogen gasses continue to rise. At the same time, the pump speed continues to increase at a slower rate to maintain the stoic target and compensate for the decreasing mole fraction of hydrogen. The shut valve208(seeFIG.2) opens to purge water only from the liquid vapor separator206at the time518.

At a time520, the maximum system combined pressure threshold illustrated by dashed lines522is reached with the continued increase of non-hydrogen gas pressures. At the same time, the non-hydrogen gas pressure threshold illustrated by dashed lines524is reached with the increase of the non-hydrogen gas pressure. This triggers activation of the shut valve208(seeFIG.2) to purge water as well as gas and prevent the combined pressure threshold from being exceeded. By not exceeding the non-hydrogen gas pressure threshold, the minimum mole fraction of hydrogen gas and the hydrogen gas pressure is maintained. At the same time, the mole fraction of the hydrogen gas reaches a minimum mole fraction of the hydrogen gas at the outlet of the liquid vapor separator206illustrated by dashed lines526for the pump204to maintain the hydrogen gas recirculation stoic target (see alsoFIG.2). This corresponds to the pump speed hitting its maximum pump speed threshold illustrated by dashed lines528. The mole fraction of the hydrogen gas, the pump speed, the hydrogen gas pressure, and the non-hydrogen gas pressures are stable between the time520and time530, or during a period532. The stoic target remains constant during this time. Effectively, this control allows for maintaining a constant H2 stoic value while minimizing gas purging by carefully coordinating the relationship between the total gas pressure, H2 and non-H2 partial pressures, and hydrogen pump speed.

At the time530, the hydrogen gas recirculation stoic target begins to decrease. At the same time, the pump speed and the hydrogen and non-hydrogen gas pressure begin to decrease, and the mole fraction of the hydrogen gas at the liquid vapor separator206(seeFIG.2) begins to increase. At a time534, the power generation target of the FC has dropped, and the hydrogen gas recirculation stoic target and the mole fraction of the hydrogen gas become stable, and the shut valve208purges water only. The non-hydrogen gas pressure continues to drop due to crossover back from the anode to the cathode, as the cathode pressure drops to reflect the reduced FC power generation target.