CN112224093B - Low temperature start control method of fuel cell vehicle - Google Patents

Abstract

The invention relates to a low-temperature start-up control method for a fuel cell vehicle. The method includes the following steps: S1. Predicting the required power in the low-temperature start-up stage; S2. Setting the SOC target value of the battery pack at the last stop according to the required power; S3 , according to the SOC target value, perform a fuel cell shutdown purge operation; S4 , set the SOC target value to an initial state of startup, and the controller performs low temperature startup control of the fuel cell vehicle. The method optimizes the start-up process and the last shutdown process of the fuel cell vehicle by predicting the power demand of the fuel cell in the start-up stage and the initial stage of driving, so as to ensure the user's driving experience in the low-temperature scenario, and at the same time ensure the fuel cell and the fuel cell. Battery pack life requirements.

Description

Low temperature start control method of fuel cell vehicle

technical field

The invention relates to the field of fuel cell vehicles, in particular to a low-temperature startup control method for fuel cell vehicles.

Background technique

The fuel cell operates in a low temperature environment. If the preheating is not sufficient, the internal water will freeze, which will block the diffusion of the reaction gas and hinder the reaction. In addition, there will be volume expansion, which will cause structural damage to the porous medium. Therefore, during the low temperature start-up process of the fuel cell, a certain period of time is required for the warm-up operation to increase the internal temperature of the fuel cell, so as to reduce the attenuation of the performance and life of the fuel cell due to the temperature. At present, the international advanced mass production models generally use the self-heating method to warm up the inside of the fuel cell, so that the temperature of the fuel cell can quickly rise to a suitable temperature, and the warm-up operation is ended. The fuel cell enters the normal working mode and starts to output power externally. The self-heating method is to reduce the stoichiometric ratio of the air to generate a concentration voltage difference, so that the fuel cell works in a low-efficiency area, and more energy is dissipated in the form of heat to achieve a warm-up effect.

However, there are currently technical difficulties. If the fuel cell warm-up time is too short, the internal preheating of the fuel cell will be insufficient, resulting in the deterioration of the performance and life of the fuel cell. If the fuel cell warm-up time is too long, the fuel cell will wait for a long time to supply power to the drive motor, which will affect the customer's driving experience.

In view of this, the present invention is proposed.

SUMMARY OF THE INVENTION

The purpose of the present invention is to provide a low-temperature startup control method of a fuel cell vehicle, which optimizes the startup process and the last shutdown process of the fuel cell vehicle by predicting the power demand of the fuel cell in the startup stage and the initial driving stage, and realizes the In low temperature scenarios, it not only guarantees the user's driving experience, but also guarantees the life requirements of fuel cells and battery packs.

In order to achieve the above object, the present invention adopts the following technical solutions:

The invention provides a low-temperature startup control method for a fuel cell vehicle, comprising the following steps:

S1. Predict the required power in the low temperature start-up stage;

S2. According to the required power, set the SOC target value of the battery pack at the last stop;

S3, according to the SOC target value, perform a fuel cell shutdown and purge operation;

S4 , setting the SOC target value as an initial state of startup, and the controller performs low-temperature startup control of the fuel cell vehicle.

As a further preferred technical solution, in step S1, the demanded power includes driving demanded power, air conditioning demanded power, and accessory demanded power.

As a further preferred technical solution, the driving demand power is predicted using a Markov chain prediction method;

Preferably, the prediction formula of driving demand power is: Preg _-drive =δvma;

Where v is the speed of the vehicle, m is the mass of the vehicle, a is the acceleration, δ is the mass conversion factor, and δ is 1.03;

v and a use the v and a corresponding to the maximum value in the state transition matrix [T _l (k)] _ij ;

Preferably, within the historical duration k, the probability of the vehicle transitioning from state _xi to state x _j is

和[f_i ^l(1)，f_i ^l(2)，...，f_i ^l(k)]都被赋予了相应的指数加权，其中权重

取0.9。Among them, the matrix

and [f _i ^l (1), f _i ^l (2), ..., f _i ^l (k)] are given corresponding exponential weights, where the weights

Take 0.9.

As a further preferred technical solution, the required power of the air conditioner is 3kW;

Preferably, the power required by the accessories is 2kW.

As a further preferred technical solution, the SOC target value of the battery pack in step S2 is:

W _req =∫P _req dt;

Among them, SOC _min is the minimum allowable SOC of the battery pack, W _reg is the energy consumption of the fuel cell vehicle during the start-up phase, _Preq is the required power, _Preq-purge is the energy consumption of accessories during the fuel cell shutdown and purge process, and Q is the battery package rated power;

Preferably, the SOC _min is 30%.

As a further preferred technical solution, the fuel cell shutdown purge operation in step S3 includes the following steps: first, determine whether the SOC target value is higher than the SOC threshold value, and if it is determined to be yes, then carry out the first purge mode; if it is determined to be no, then carry out the purge Method 2: End the purging until the purging stop conditions are met;

Preferably, the first purging method includes: the fuel cell stops outputting power to the outside, enters an idle mode, and the battery pack provides all power consumption during the purging process;

Preferably, the second purging method includes: the fuel cell charges the battery pack until the SOC of the battery pack is higher than the SOC threshold; then the fuel cell stops outputting power to the outside, enters an idle mode, and the battery pack provides all power consumption during the purging process;

Preferably, the SOC threshold is 50%.

As a further preferred technical solution, in step S4, the controller performs the low temperature start-up control of the fuel cell vehicle including the following steps: first, the driver makes a start request, then the fuel cell performs a pre-start purging operation, and after the pre-start purging operation is completed, the fuel cell The battery enters the warm-up mode, and after the warm-up is over, the fuel cell enters the normal power generation mode.

As a further preferred technical solution, the condition for completing the purge operation before starting is that the voltage of the fuel cell is higher than 0.6V.

As a further preferred technical solution, the warm-up mode includes: reducing the air stoichiometric ratio, generating a concentration difference and a voltage difference, so that the fuel cell works in a low-efficiency region to achieve warm-up.

As a further preferred technical solution, the warm-up ending condition is that the temperature of the fuel cell cooling fluid is higher than 55°C.

Compared with the prior art, the beneficial effects of the present invention are:

(1) A low-temperature startup control method for fuel cell vehicles is proposed, which simultaneously considers fuel cell life, battery pack life, and user experience, and divides the entire startup process into three stages. The first-stage battery pack is used as a separate energy source; the second-stage battery pack is used as a separate energy source; The battery is used as the main energy source, and the fuel cell is used as the auxiliary energy source; the three-stage fuel cell is used as the main energy source, and the battery pack is used as the auxiliary energy source. Among them, the main optimization measures for the life of the fuel cell are to delay the external high power output time of the fuel cell as much as possible, and to make the internal temperature of the fuel cell reach an appropriate temperature as soon as possible through the warm-up mode. The main optimization measure for the life of the battery pack is to set the SOC target value before the last shutdown, so that the battery pack can meet the subsequent power output requirements while always keeping it above the minimum limit. The main optimization measure for the user experience is that the battery pack can provide enough energy for vehicle driving, air conditioning, and accessories in the initial stage, without affecting the user's use.

(2) A prediction method based on Markov chain is proposed to predict the driving power demand of the fuel cell vehicle during the start-up phase in a low-temperature environment, set the acceleration-vehicle speed as a state variable, and predict the low-temperature start-up process through historical data. energy consumption. In addition, this method has the advantage of updating and learning the state probability transition matrix according to the user's usage habits, geographical location, traffic conditions and other factors as the user uses the vehicle, so as to improve the prediction accuracy of the Markov chain model in a personalized manner.

(3) A battery pack SOC management method is proposed, which can design the SOC target value before the fuel cell vehicle stops by using the predicted energy consumption and the minimum SOC requirement of the battery pack, and use different shutdown purging methods to make the battery pack SOC meets the requirements.

Description of drawings

In order to illustrate the specific embodiments of the present invention or the technical solutions in the prior art more clearly, the following briefly introduces the accompanying drawings that need to be used in the description of the specific embodiments or the prior art. Obviously, the accompanying drawings in the following description The drawings are some embodiments of the present invention. For those of ordinary skill in the art, other drawings can also be obtained based on these drawings without creative efforts.

Fig. 1 is the flow chart of the low temperature start control method of fuel cell vehicle provided by the present invention;

Figure 2 is a flow chart of battery pack target SOC determination and realization;

Fig. 3 is the low temperature start control flow chart of fuel cell vehicle;

Figure 4 is a schematic structural diagram of a fuel cell vehicle;

Figure 5 is a graph of key signal changes before and after the low temperature start of the fuel cell vehicle.

Icons: 1-fuel cell stack; 2-first DCDC; 3-motor controller; 4-drive motor; 5-power battery pack; 6-second DCDC; 7-accessory; 8-main reducer; 9- Differential and axle shafts.

Detailed ways

In order to make the objectives, technical solutions and advantages of the present invention clearer, the technical solutions of the present invention will be described clearly and completely below. Obviously, the described embodiments are only some, but not all, embodiments of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative work fall within the protection scope of the present invention.

It should be noted that: the "battery pack" mentioned in the present invention refers to a power battery pack.

Example 1

In order to ensure the life of the fuel cell, during the low temperature start-up process, the time point when the fuel cell enters the normal power generation mode should be moved as far back as possible to make the interior of the fuel cell fully preheated. In order to ensure the user's driving experience, the time point when the vehicle enters the drivable state should be moved forward as far as possible. Therefore, in the start-up stage of the fuel cell, the battery pack needs to be the main energy source to meet the power requirements for driving the vehicle and heating the air conditioner.

During the start-up phase of the fuel cell, the battery pack continues to supply external power with a large power, and the working range of the SOC is too low, which will lead to the deterioration of the battery pack life. Therefore, the SOC of the battery pack needs to be predicted and set a target value in advance to meet the external output demand during the startup phase and the life expectancy of the battery pack itself.

Considering the above design ideas, as shown in FIG. 1 , this embodiment provides a low-temperature startup control method for a fuel cell vehicle, including the following steps:

S1. Predict the required power in the low temperature start-up stage;

S2. According to the required power, set the SOC target value of the battery pack at the last stop;

S3, according to the SOC target value, perform a fuel cell shutdown and purge operation;

S4 , setting the SOC target value as an initial state of startup, and the controller performs low-temperature startup control of the fuel cell vehicle.

The present invention has the following advantages:

(1) A low-temperature startup control method for fuel cell vehicles is proposed, which simultaneously considers fuel cell life, battery pack life, and user experience, and divides the entire startup process into three stages. The first-stage battery pack is used as a separate energy source; the second-stage battery pack is used as a separate energy source; The battery is used as the main energy source, and the fuel cell is used as the auxiliary energy source; the three-stage fuel cell is used as the main energy source, and the battery pack is used as the auxiliary energy source. Among them, the main optimization measures for the life of the fuel cell are to delay the external high power output time of the fuel cell as much as possible, and to make the internal temperature of the fuel cell reach an appropriate temperature as soon as possible through the warm-up mode. The main optimization measure for the life of the battery pack is to set the SOC target value before the last shutdown, so that the battery pack can meet the subsequent power output requirements while always keeping it above the minimum limit. The main optimization measure for the user experience is that the battery pack can provide enough energy for vehicle driving, air conditioning, and accessories in the initial stage, without affecting the user's use.

(2) A prediction method based on Markov chain is proposed to predict the driving power demand of the fuel cell vehicle during the start-up phase in a low-temperature environment, set the acceleration-vehicle speed as a state variable, and predict the low-temperature start-up process through historical data. energy consumption. In addition, this method has the advantage of updating and learning the state probability transition matrix according to the user's usage habits, geographical location, traffic conditions and other factors as the user uses the vehicle, so as to improve the prediction accuracy of the Markov chain model in a personalized manner.

(3) A battery pack SOC management method is proposed, which can design the SOC target value before the fuel cell vehicle stops by using the predicted energy consumption and the minimum SOC requirement of the battery pack, and use different shutdown purging methods to make the battery pack SOC meets the requirements.

In a preferred embodiment, in step S1, the demanded power includes driving demanded power, air conditioning demanded power and accessory demanded power. In the low temperature start-up stage, the total demand power is composed of driving demand power, air conditioning demand power and accessory demand power. Among them, the required power of accessories mainly includes the work of air compressors, hydrogen pumps, cooling water pumps and other related accessories during the start-up process of the fuel cell. The required power value is small and the fluctuation is not large. The required power of the air conditioner is mainly related to the ambient temperature. The required power value of this part can be obtained by the calibration test, and stored in the controller according to the test result. The above two parts are composed of required power. The power value is small and the fluctuation range is not large, so it can be estimated through experience. As for the driving power of the vehicle, energy consumption accounts for the largest proportion, and is affected by many factors such as user habits, geographical environment, and traffic congestion.

In a preferred embodiment, the driving demand power is predicted using a Markov chain prediction method.

Preferably, the prediction formula of driving demand power is: Preg _-drive =δvma;

Where v is the speed of the vehicle, m is the mass of the vehicle, a is the acceleration, δ is the mass conversion factor, and δ is 1.03;

v and a adopt v and a corresponding to the maximum value in the state transition matrix [T _l (k)] _ij .

Preferably, within the historical duration k, the probability of the vehicle transitioning from state _xi to state x _j is

和[f_i ^l(1)，f_i ^l(2)，...，f_i ^l(k)]都被赋予了相应的指数加权，其中权重

取0.9。Among them, the matrix

and [f _i ^l (1), f _i ^l (2), ..., f _i ^l (k)] are given corresponding exponential weights, where the weights

Take 0.9.

The invention adopts the machine learning algorithm to predict the power change of the vehicle driving demand in the fuel cell start-up stage. Considering that in the process of vehicle driving, longitudinal dynamic variables, such as vehicle speed, acceleration, torque, power, etc., can be regarded as time series, so Markov chain prediction method can be used to establish a prediction model for time series samples, and find out the system. features and forecasts current values and future trends based on historical values of the time series. A Markov chain is a discrete-time random process with Markov properties in mathematics. In each step, the system can transition from one state to another according to the probability distribution. Using its current state and the probability distribution of state transitions, the most likely state of the system at the next moment can be predicted.

In order to predict the power, the present invention uses the vehicle speed-acceleration two-dimensional vector to describe the state at each moment, that is, as the element feature description of the state probability transition matrix. Define X as the Markov chain state space, X={x ₁ , x ₂ ,...x _s };

Since the vehicle speed-acceleration is used to describe the state, when t=k, the Markov chain state can be expressed as: x _k =(v(k), a(k));

Then the probability distribution of t=k+l is determined by the l-step state transition probability matrix. The present invention recommends l to be 3, then the conditional probability of the state x _i corresponding to time k transitioning to the state x _j corresponding to time k+l [T _l ] _ij can be represented by the following formula:

i, j ∈ {1, 2, ..., s};

为观测到的经过l时间步长，从x_i转移到x_j的转移频数；N_i ^l为观测到的经过l时间步长，从x_i转移到其他状态的频数。Among them, T _l ∈ R ^s×s , l=1, 2, . . . , N _T is the time scale of the Markov chain.

is the observed transition frequency from x _i to x _j after l time step; N _i ^l is the observed frequency of transition from x _i to other states after l time step.

To make personalized predictions based on user habits, geographical environment, traffic congestion and other factors, it is necessary to update the state transition probability matrix based on the current vehicle historical data. The method is as follows: within the historical duration k, transfer from x _i to x _j The probability can be expressed by the following formula:

为历史时长k下，发生x_i经过l步长转移到x_j的频率；

为历史时长k下，状态由x_i转移到其他状态的频率。

为事件x_i经过l步长转移到x_j发生的指示器，若该事件发生，则

为1，否则为0。f_i ^l为事件x_i经过l步长转移到其他状态发生的指示器，若该事件发生，则f_i ^l为1，否则为0。in,

is the frequency at which x _i is transferred to x _j after l steps under the historical duration k;

is the frequency of state transition from _xi to other states under the historical duration k.

is the indicator that event x _i is transferred to x _j after l step size, if the event occurs, then

1, otherwise 0. f _i ^l is an indicator that event _xi transitions to other states after l steps, if the event occurs, f _i ^l is 1, otherwise it is 0.

The above formula can be further written as:

来替代

最终表达式为：Considering better personalized prediction, the impact of historical observations on future system output is weakened over time, so according to the magnitude of the impact, historical observations are given different weights chronologically, so forgetting is used. factor

to replace

The final expression is:

和[f_i ^l(1)，f_i ^l(2)，...，f_i ^l(k)]都被赋予了相应的指数加权，其中权重

建议

取0.9。where the matrix

and [f _i ^l (1), f _i ^l (2), ..., f _i ^l (k)] are given corresponding exponential weights, where the weights

Suggest

Take 0.9.

According to the above derivation process, the state corresponding to the maximum value is selected from [T _l (k)] _ij as the prediction result. After obtaining the predicted values of acceleration and vehicle speed, the required power can be predicted by the following formula: Preg _-drive = δvma;

Where v is the speed of the vehicle, m is the mass of the vehicle, a is the acceleration, and δ is the mass conversion factor, which is recommended to be 1.03.

In the early stage of vehicle use, due to the lack of training data, sufficient historical data cannot be provided to accurately predict vehicle speed and acceleration. Offline training of standard cycle conditions, such as NEDC conditions, CLTC conditions, WLTC conditions, UDDS conditions, etc., to obtain the state probability transition matrix of the standard cycle conditions, which is used as the initial transition matrix in the control unit to solve the problem of the vehicle as much as possible. Lack of historical data at the beginning of use. As the user uses the vehicle, according to the user's usage habits, geographical location, traffic conditions and other factors, the state probability transition matrix is updated and learned to improve the prediction accuracy of the Markov chain model individually.

Using the Markov chain model prediction method, the fuel cell vehicle controller performs online self-learning, and calculates the value of _Preq in the initial stage of driving through historical data, so as to calculate the energy consumption W _reg in the initial stage of driving, and finally get the shutdown The target value of the front battery pack SOC.

Preferably, the required power of the air conditioner is 3kW.

Preferably, the power required by the accessories is 2kW.

In a preferred embodiment, the SOC target value of the battery pack in step S2 is:

W _req =∫P _req dt;

Among them, SOC _min is the minimum allowable SOC of the battery pack, W _reg is the energy consumption of the fuel cell vehicle during the start-up phase, _Preq is the required power, _Preq-purge is the energy consumption of accessories during the fuel cell shutdown and purge process, and Q is the battery Package rated power.

Preferably, the SOC _min is 30%.

Step S1 predicts the required power P _req of the fuel cell vehicle in the start-up stage through the method of Markov chain model prediction, and the energy consumption value W _req in this stage can be expressed as: W _req =∫P _req dt;

Since the battery pack becomes the main energy source during the start-up phase of the fuel cell, it needs to meet the power requirements for driving the vehicle and heating the air conditioner. The battery pack continues to supply external power with high power, and the working range of the SOC is too low, which will cause the life of the battery pack to be attenuated. Therefore, it is necessary to preset the SOC target value before the vehicle is turned off. The SOC target value is represented by SOC _int , which should meet the following conditions : SOC _int ≥ΔSOC+SOC _min ;

The SOC _min represents the minimum SOC allowed for the battery pack to work, which is used as a threshold value for the life protection strategy of the battery pack, which is obtained by pre-calibration through experiments. In the present invention, the reference value is 30%. ΔSOC represents the change in the SOC of the battery pack during the startup process, and this value can be approximately represented by the energy consumption during the startup phase: W _reg ≈ΔSOC*Q; where Q represents the rated power of the battery pack.

The SOC target value can be obtained from the above two formulas:

Among them, _Preq-purge is the energy consumption of accessories during the shutdown and purge process of the fuel cell, which can be obtained in advance through calibration tests of different ambient temperatures and different internal water contents of the fuel cell, and stored in the controller.

In a preferred embodiment, the fuel cell shutdown purge operation in step S3 includes the following steps: first determine whether the SOC target value is higher than the SOC threshold value; Purging mode 2; end purging until the purging stop conditions are met.

Preferably, the first purging method includes: the fuel cell stops outputting power to the outside, enters an idle mode, and the battery pack provides all power consumption during the purging process.

Preferably, the second purging method includes: the fuel cell charges the battery pack until the SOC of the battery pack is higher than the SOC threshold; then the fuel cell stops outputting power to the outside, enters an idle mode, and the battery pack provides all power consumption during the purging process.

Preferably, the SOC threshold is 50%.

The determination and realization process of the target SOC of the battery pack is shown in Figure 2 below. After the driver requests the vehicle to stop, the controller inquires about the ambient temperature and the state of the fuel cell, and calculates the power required by the accessories during the shutdown and purge process. Based on the Markov chain prediction model, the driving demand power of the fuel cell in the startup phase can be obtained, and the air conditioning demand power can be obtained according to the month information, and the SOC target value can be calculated by the method in the previous step. In order to reach the SOC set target value, the battery pack needs to be charged by the fuel cell power generation before the last fuel cell shutdown. Considering that in a low temperature environment, a shutdown purge operation is performed before the fuel cell is shut down, and the internal liquid water is discharged to prevent the internal freezing. According to the SOC state of different battery packs before shutdown and purging, there are two purging working modes.

When the SOC is higher than the given threshold, the reference value is 50%, and it is considered that the battery pack can provide sufficient power to meet the energy consumption of purging accessories during shutdown, the driving energy consumption of the next fuel cell vehicle starting stage, the energy consumption of air conditioners, and the energy consumption of accessories. And the SOC will not be lower than the minimum threshold SOC _min , which will seriously affect the life of the battery pack. In this case, in consideration of reducing the hydrogen consumption of the fuel cell, the first purge mode is entered. In this mode, the fuel cell stops outputting power to the outside, and enters the idle mode, and the battery pack provides all the power consumption during the purging process. When the purge shutdown condition is met, for example, the water content in the fuel cell is lower than a certain threshold, the purge is ended.

When the SOC is lower than the given threshold and the reference value is 50%, it is considered that the battery pack cannot provide sufficient power to meet the energy consumption for purging accessories during shutdown, the driving energy consumption in the next start-up phase of the fuel cell vehicle, the energy consumption for air conditioning, and the energy consumption for accessories. , and the SOC will not be lower than the minimum threshold SOC _min , which will seriously affect the battery pack life. In this case, considering the working range of the battery pack SOC, the second purge mode is entered. In this mode, the stack charges the battery pack before purging until the SOC is higher than the given threshold, the reference value is 50%, and then the stack stops outputting power to the outside world and enters idle mode, where the battery pack provides all the power consumption during the purging process. . When the purge shutdown condition is met, for example, the water content in the fuel cell is lower than a certain threshold, the purge is ended.

In a preferred embodiment, in step S4, the controller performs the low temperature start-up control of the fuel cell vehicle including the following steps: first, the driver makes a start request, then the fuel cell performs a pre-start purge operation, and after the pre-start purge operation is completed , the fuel cell enters the warm-up mode, and after the warm-up is over, the fuel cell enters the normal power generation mode.

Preferably, the condition for completing the purge operation before starting is that the voltage of the fuel cell is higher than 0.6V.

The low temperature start control process is shown in Figure 3 below. When the driver makes a start request, the accessories of the fuel cell system, such as the air compressor and the hydrogen pump, perform the preheating operation, and perform the purging operation to discharge the hydrogen-oxygen mixture inside the fuel cell, so as to prevent the oxidation of the carbon carrier and reduce the life of the fuel cell. At this stage, the fuel cell does not output power externally, and the battery pack acts as a separate energy source. When the fuel cell meets the purge end condition, the fuel cell enters the warm-up mode. The setting of the purging end condition needs to satisfy all the discharge of the hydrogen-oxygen mixed interface. In the present invention, the determination is made by monitoring the voltage of the fuel cell cell. When the cell voltage is higher than a certain threshold, the reference value is 0.6V, and it is considered that the purging is achieved. end condition.

Preferably, the warm-up mode includes: reducing the air stoichiometric ratio, generating a concentration difference and a voltage difference, so that the fuel cell works in a low-efficiency region to achieve warm-up. The fuel cell enters the warm-up mode. By reducing the stoichiometric ratio of the air, the concentration voltage difference is generated, so that the fuel cell works in the low-efficiency area, and more energy is dissipated in the form of heat to achieve the warm-up effect. At this stage, the fuel cell is in a low-efficiency working area and can only output small power, and the battery pack is the main energy source. Since the SOC of the battery pack is reserved in the previous step, it has sufficient output capability so as not to affect the user's driving experience.

The above-mentioned "air stoichiometric ratio" refers to the ratio of the oxygen consumption in the air to the reference oxygen consumption, wherein the reference oxygen consumption is calculated by the current, which is the theoretical oxygen consumption of the complete reaction, which can be converted into the actual oxygen consumption by the oxygen content in the air. The ratio of air consumption to the base air consumption.

The above-mentioned "concentration difference" refers to the oxygen concentration difference generated at the cathode of the fuel cell.

The above "voltage difference" refers to the difference between the voltage corresponding to the polarization characteristic curve of the stack and the current working voltage of the stack under the same current density.

The above-mentioned "low-efficiency region" refers to a region deviating from the polarization characteristic curve.

Preferably, the warm-up ending condition is that the temperature of the fuel cell coolant is higher than 55°C. The setting of the warm-up end condition requires that the internal temperature of the fuel cell reaches an appropriate temperature, such as 55°C. When the warm-up end condition is reached, the fuel cell enters the normal power generation mode.

The fuel cell enters the normal power generation mode, works in the high-efficiency area, and becomes the main energy source. The battery pack acts as an auxiliary energy source to "cut peaks and fill valleys" for the output power, reduce the rate of change of fuel cell power, and prolong the life of the fuel cell. .

The present invention is particularly suitable for the fuel cell vehicle configuration as shown in FIG. 4 . The fuel cell vehicle includes a fuel cell stack 1, a first DCDC2, a motor controller 3 (DCAC), a drive motor 4, a power battery pack 5, a first Two DCDC6, accessory 7, main reducer 8, differential and half shaft 9, etc., among which, the first DCDC refers to the back-end supercharger of the fuel cell, and the second DCDC refers to the two-way DCDC at the rear end of the battery pack. The cathode of the fuel cell stack is fed with air, and the anode is fed with hydrogen, which reacts on the membrane electrodes to generate electricity. The fuel cell supercharger can boost the output voltage of the fuel cell to meet the needs of the electric drive system, thereby reducing the number of fuel cells. The back end of the battery pack is connected to a two-way DCDC. During the driving process, the battery pack voltage can be boosted to the load voltage, and the external output power can be coordinated with the fuel cell to effectively reduce the dynamic load of the fuel cell. During the energy recovery process, the motor can be recovered. The energy is stepped down to charge the battery pack. Accessories include high-pressure accessories such as air compressors, hydrogen pumps, cooling water pumps, and air conditioners, as well as low-pressure accessories such as various valve bodies, sensors, and actuators.

The changes of key signals before and after optimization using the above control method are shown in Figure 5. From top to bottom in the figure, the changes of battery pack SOC, total required power _Preq and fuel cell output power _PFC over time are shown. The SOC curve is schematically represented by a straight line, and the actual change is nonlinear. The solid line represents before optimization, and the dotted line represents after optimization.

Among them, t1 is the time when the fuel cell is shut down and purged, and t1-t2 is when the vehicle is in a shutdown state. At this stage, the temperature of the fuel cell is gradually reduced by the influence of the ambient temperature. When the vehicle starts next time, when the internal temperature of the fuel cell is lower than 0 °C, The low temperature start control function of the fuel cell vehicle is activated.

The low-temperature start-up process of the fuel cell before optimization is the stage t2-t4 in the figure. Among them, t2-t3 is the fuel cell preparation and purging stage. The air compressor and hydrogen pump are preheated and the hydrogen-oxygen mixture inside the fuel cell is discharged by purging. At this stage, the fuel cell cannot output power externally. , all the energy is provided by the battery pack, and the time is set according to the different ambient temperature. In the present invention, the t2-t3 corresponding to the low temperature start-up process at -20°C is 30s. Since the battery pack can provide energy, it will not affect the driving of the vehicle and the opening of the air conditioner.

t3-t4 is the fuel cell warm-up stage. In this stage, the fuel cell works in the low-efficiency region and only outputs a small power. In the present invention, the reference value is 5kW, and most of the energy is provided by the battery pack. The time is set according to the internal temperature of the fuel cell. In the present invention, the t3-t4 corresponding to the low temperature start-up process at -20°C is 200s. At this stage, the required power is high, and the SOC of the battery pack decreases rapidly. When reaching t4, the SOC should be greater than SOC _min . Wherein, SOC _min represents the minimum allowable value of the suggested SOC considering the life of the battery pack. In the present invention, the reference value is 30%.

At time t4, the fuel cell enters the normal working mode from the warm-up mode, and the fuel cell becomes the main power source.

The optimized low-temperature start-up process of the fuel cell is the stage t2-t5 in the figure. Among them, t2-t3 are the fuel cell preparation and purging stages, which are unchanged from those before optimization.

t3-t5 is the fuel cell warm-up stage. In this stage, the fuel cell works in the low-efficiency region and only outputs a small power. In the present invention, the reference value is 5kW, and most of the energy is provided by the battery pack. The time is set according to the internal temperature of the fuel cell. In the present invention, the t3-t5 corresponding to the low temperature start-up process at -20°C is 300s. At this stage, the required power is high, and the SOC of the battery pack decreases rapidly. When reaching t5, the SOC should be greater than SOC _min . Wherein, SOC _min represents the minimum allowable value of the suggested SOC considering the life of the battery pack. In the present invention, the reference value is 30%. It can be seen that the optimized control method, on the premise of meeting the requirements of the SOC variation range, prolongs the fuel cell warm-up time and makes the internal temperature of the fuel cell closer to the appropriate temperature, which is beneficial to the optimization of the performance and life of the fuel cell.

At time t5, the fuel cell enters the normal working mode from the warm-up mode, and the fuel cell becomes the main power source.

Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, but not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those of ordinary skill in the art should understand that: The technical solutions described in the foregoing embodiments can still be modified, or some or all of the technical features thereof can be equivalently replaced; and these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the technical solutions of the embodiments of the present invention.

Claims (14)

1. a low-temperature starting control method for a fuel cell vehicle, characterized in that, comprising the following steps: S1. Predict the required power in the low temperature start-up stage; S2. According to the required power, set the SOC target value of the battery pack at the last stop; S3, according to the SOC target value, perform a fuel cell shutdown and purge operation; S4, setting the SOC target value as an initial state of startup, and the controller performs low-temperature startup control of the fuel cell vehicle; In step S1, the demanded power includes driving demanded power, air-conditioning demanded power, and accessory demanded power; The driving demand power is predicted using a Markov chain prediction method; The prediction formula of driving demand power is: _Preq-drive = δvma; Where v is the speed of the vehicle, m is the mass of the vehicle, a is the acceleration, δ is the mass conversion factor, and δ is 1.03; v and a adopt v and a corresponding to the maximum value in the state transition matrix [T _l (k)] _ij . 2 . The low-temperature start-up control method for a fuel cell vehicle according to claim 1 , wherein, within the historical duration k, the probability of the vehicle transitioning from the _xi state to the x _j state is: 2 .

in,

is the frequency at which x _i is transferred to x _j after l steps under the historical duration k;

is the frequency at which x _i is transferred to x _j after l steps under the historical duration k-1;

is the frequency of state transition from _xi to other states under the historical duration k;

is the frequency of state transition from _xi to other states under the historical duration k-1;

is the indicator that event x _i is transferred to x _j after l step size, if the event occurs, then

is 1, otherwise it is 0; f _i ^l is the indicator that event _xi transitions to other states after l steps, if the event occurs, f _i ^l is 1, otherwise it is 0; matrix

and [f _i ^l (1), f _i ^l (2), ..., f _i ^l (k)] are given corresponding exponential weights, where the weights

Take 0.9. 3 . The low-temperature startup control method for a fuel cell vehicle according to claim 1 , wherein the required power of the air conditioner is 3kW. 4 . 4 . The low-temperature start-up control method for a fuel cell vehicle according to claim 1 , wherein the required power of the accessories is 2kW. 5 . 5 . The low-temperature start-up control method for a fuel cell vehicle according to claim 1 , wherein the target SOC value of the battery pack in step S2 is: 6 .

W _req =∫P _req dt; Among them, SOC _min is the minimum allowable SOC of the battery pack, W _req is the energy consumption of the fuel cell vehicle during the start-up phase, _Preq is the required power, _Preq-purge is the energy consumption of accessories during the fuel cell shutdown and purge process, and Q is the battery Package rated power. 6 . The low-temperature starting control method for a fuel cell vehicle according to claim 5 , wherein the SOC _min is 30%. 7 . 7 . The low-temperature start-up control method for a fuel cell vehicle according to claim 1 , wherein the fuel cell shutdown purge operation in step S3 comprises the following steps: first determine whether the SOC target value is higher than the SOC threshold value, and if it is determined to be yes, then Carry out the purging mode 1; if the judgment is no, then carry out the purging mode 2; until the purging stop conditions are met, the purging is ended. 8 . The low-temperature startup control method for a fuel cell vehicle according to claim 7 , wherein the first purging method includes: the fuel cell stops outputting power to the outside, enters an idle mode, and the battery pack provides all power consumption during the purging process. 9 . . 9 . The low-temperature start-up control method for a fuel cell vehicle according to claim 7 , wherein the second purging method comprises: the fuel cell charges the battery pack until the SOC of the battery pack is higher than the SOC threshold; then the fuel cell stops outputting power to the outside. 10 . , enter the idle mode, and the battery pack provides the full power consumption during the purging process. 10 . The low-temperature starting control method for a fuel cell vehicle according to claim 7 , wherein the SOC threshold is 50%. 11 . 11. The low-temperature start-up control method for a fuel cell vehicle according to any one of claims 1 to 10, wherein in step S4, the controller performing the low-temperature start-up control of the fuel cell vehicle comprises the following steps: first, the driver puts forward a start request, Then, the fuel cell performs a pre-start purging operation. After the pre-start purging operation is completed, the fuel cell enters a warm-up mode, and after the warm-up is completed, the fuel cell enters a normal power generation mode. 12 . The low-temperature startup control method of a fuel cell vehicle according to claim 11 , wherein the condition for completing the purge operation before startup is that the voltage of the fuel cell is higher than 0.6V. 13 . 13 . The low-temperature start-up control method for a fuel cell vehicle according to claim 11 , wherein the warm-up mode comprises: reducing the air stoichiometric ratio, generating a concentration difference and a voltage difference, making the fuel cell work in a low-efficiency region, and realizing a warm-up operation. 14 . machine. 14 . The low-temperature start-up control method for a fuel cell vehicle according to claim 11 , wherein the warm-up ending condition is that the temperature of the fuel cell coolant is higher than 55° C. 15 .

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