阅读说明：本技术 一种燃油系统的停机泄压控制方法及其控制装置 (Shutdown pressure relief control method and control device for fuel system ) 是由 李�东 陈铁 张鹏 王明卿 周海早 撒占才 于 2021-07-29 设计创作，主要内容包括：本发明涉及汽车发动机控制技术领域,公开一种燃油系统的停机泄压控制方法及其控制装置。燃油系统包括发动机、油轨和喷油器,喷油器能将油轨内的燃油喷至发动机内,该燃油系统的停机泄压控制方法包括：在发动机获取停机信号后,油轨处于怠速轨压至发动机停转之间为动态泄压阶段,在动态泄压阶段,喷油器进行多次喷射泄压,每次喷射泄压包括n次喷射,前(n-1)次为空喷,第n次为非空喷；在发动机停转至油轨处于停机轨压之间为静态泄压阶段,在静态泄压阶段,喷油器进行空喷,至轨压降至停机轨压。本发明设置动态泄压阶段,可以在发动机获取停机信号后的短时间内快速将轨压下降到停机轨压,泄压时间短,喷油器喷射次数少,使用寿命长。(The invention relates to the technical field of automobile engine control, and discloses a shutdown pressure relief control method and a control device of a fuel system. The fuel system comprises an engine, a fuel rail and a fuel injector, wherein the fuel injector can inject fuel in the fuel rail into the engine, and the shutdown pressure relief control method of the fuel system comprises the following steps: after the engine obtains a stop signal, a dynamic pressure relief stage is carried out when the oil rail is between idle rail pressure and engine stop, in the dynamic pressure relief stage, the oil injector carries out multiple injection pressure relief, wherein each injection pressure relief comprises n times of injection, the first (n-1) times are empty injection, and the nth time is non-empty injection; and in the static pressure relief stage, the oil injector performs air injection until the rail pressure is reduced to the shutdown rail pressure. The invention sets a dynamic pressure relief stage, can quickly reduce the rail pressure to the shutdown rail pressure in a short time after the engine acquires the shutdown signal, and has short pressure relief time, less injection times of the oil injector and long service life.)

1. A shutdown pressure relief control method of a fuel system is characterized in that the fuel system comprises an engine, a fuel rail and a fuel injector, the fuel injector can inject fuel in the fuel rail into the engine,

after the engine acquires a stop signal, a dynamic pressure relief stage is carried out when the oil rail is between idle rail pressure and engine stop, in the dynamic pressure relief stage, the oil injector carries out multiple injection pressure relief, wherein each injection pressure relief comprises n times of injection, the first (n-1) times are empty injection, and the nth time is non-empty injection;

and in the static pressure relief stage, the oil injector performs air injection until the rail pressure is reduced to the shutdown rail pressure.

2. The method for controlling shutdown and pressure relief of a fuel system according to claim 1, further comprising a pressure relief valve communicated with the fuel rail, wherein the pressure relief valve is used for judging the pressure in the fuel rail after the engine obtains the shutdown signal and before the dynamic pressure relief stage, when the pressure in the fuel rail is greater than the idle rail pressure, the pressure relief valve is controlled to relieve the rail pressure to the idle rail pressure, and when the pressure in the fuel rail is the idle rail pressure, the dynamic pressure relief stage is entered.

3. The fuel system shutdown pressure relief control method according to claim 1, wherein one injection pressure relief comprises a first pilot injection, a second pilot injection, a main injection, a first post injection and a second post injection, and the second post injection is a non-empty injection.

4. The fuel system shutdown pressure relief control method according to claim 3, wherein the starting angle of the second post injection is 20 ° after the piston of the engine moves to top dead center.

5. The fuel system shutdown pressure relief control method according to claim 1, wherein the injection pulse width of the nth non-empty injection of each injection pressure relief in the dynamic pressure relief stage is the maximum injection pulse width when the engine oil concentration is within a preset range and the engine does not reignite.

6. The fuel system shutdown pressure relief control method according to claim 1, wherein the pulse of the idle injection in the dynamic pressure relief stage and/or the static pressure relief stage is an injection pulse width of an idle injection oil return of a fuel injector under an idle condition.

7. The fuel system shutdown pressure relief control method according to claim 1, wherein the injection frequency of the idle injection in the static pressure relief stage is less than or equal to 100 Hz.

8. The fuel system shutdown pressure relief control method according to any one of claims 1-7, further comprising accumulating an idle injection time when the idle injection is performed in the static pressure relief stage, and stopping the idle injection when the idle injection time is accumulated to a preset time and the rail pressure is not reduced to the shutdown rail pressure.

9. An apparatus for controlling shutdown pressure relief of a fuel system, characterized in that the method for controlling shutdown pressure relief of a fuel system according to any of claims 1-8 can be performed.

10. The fuel system shutdown pressure relief control device according to claim 9, further comprising a fuel tank, a fuel pump and a filter disposed between the fuel tank and the fuel pump, wherein a fuel return end of the fuel injector is communicated with the fuel tank, and the fuel pump is communicated with the fuel rail.

Technical Field

The invention relates to the technical field of automobile engine control, in particular to a shutdown pressure relief control method and a shutdown pressure relief control device for a fuel system.

Background

The oil injector of the common rail fuel system in the field of traditional commercial vehicles is a common oil return oil injector, namely, oil return exists no matter whether the oil injector works or not. With research, diesel engines with higher horsepower and higher rail pressure (high horsepower usually means power exceeding 441.3kW, and ultrahigh rail pressure usually means a rail pressure control range exceeding 2000bar) can save fuel consumption and reduce pollutant emission more effectively. In order to ensure the fuel supply efficiency of the ultra-high rail pressure fuel system (mainly by reducing the rail pressure loss caused by the frequent oil return of the fuel injector), the ultra-high rail pressure fuel system usually uses a non-static oil return fuel injector (when the fuel injector does not work, the oil return amount is very small). The common rail fuel system adopting the fuel injector needs an additional control strategy to ensure that the rail pressure is in a reasonable range under the shutdown working condition (the normal oil return fuel injector can realize shutdown pressure relief through normal oil return without the additional control strategy).

Taking the ultrahigh-pressure fuel system as an example, the ultrahigh-pressure fuel system is provided with a high-pressure oil rail with an electric control pressure relief valve while adopting a non-static oil return oil injector. The electronic control pressure relief valve is configured to meet the control requirement of rail pressure sudden drop when a large-load working condition is switched to a small-load working condition, but in order to ensure that the vehicle can realize a limping home function (namely, when an electrical fault exists in a fuel supply system, the vehicle can also run at a low speed), the normally-open electronic control pressure relief valve can also ensure that the fuel pressure of 500-600 bar exists in the fuel rail under the working condition of non-power supply. However, the fuel pressure for limping home exists in the fuel rail, so that the electric control pressure release valve is not suitable for occasions needing quick pressure release after shutdown working conditions, such as shutdown maintenance and the like.

In the prior art, in order to realize rapid pressure relief after shutdown, a common rail fuel system based on a non-static oil return injector generally only adopts a static idle injection technology after shutdown under a shutdown working condition, wherein the static state refers to a section from the time when an engine executes a shutdown request to the time when a controller loses the driving capability of the injector from the time when the rotating speed is just 0, the oil return ball valve is controlled to move by adopting the technology, an injection needle valve is not moved, the injector only returns oil but does not inject oil after the engine is shutdown, and the pressure relief is realized in an oil rail by returning oil. The disadvantages of this approach are: because the pulse width of power-on is very short (the idling working condition is about 450 mus generally), the pressure relief effect is limited, and the oil injector can be driven only under the condition that the engine has no rotating speed, the pressure relief time is long; meanwhile, the oil sprayer is additionally driven thousands of times when the engine is stopped every time, the control technology has high requirements on the service life of the oil sprayer, and the requirement on the normal opening times of the oil sprayer is several times that of the normal oil return oil sprayer.

Therefore, a shutdown pressure relief control method and a control device thereof for a fuel system are needed to solve the above problems.

Disclosure of Invention

Based on the above, the invention aims to provide the shutdown pressure relief control method and the shutdown pressure relief control device for the fuel system, which have the advantages of short pressure relief time and less oil injection times and are beneficial to prolonging the service life of an oil injector.

In order to achieve the purpose, the invention adopts the following technical scheme:

a shutdown pressure relief control method of a fuel system comprises an engine, an oil rail and a fuel injector, wherein the fuel injector can inject fuel in the oil rail into the engine,

after the engine acquires a stop signal, a dynamic pressure relief stage is carried out when the oil rail is between idle rail pressure and engine stop, in the dynamic pressure relief stage, the oil injector carries out multiple injection pressure relief, wherein each injection pressure relief comprises n times of injection, the first (n-1) times are empty injection, and the nth time is non-empty injection;

and in the static pressure relief stage, the oil injector performs air injection until the rail pressure is reduced to the shutdown rail pressure.

As an optimal scheme of a shutdown pressure release control method of a fuel system, the fuel system further comprises a pressure release valve communicated with the oil rail, the engine further comprises judgment of pressure in the oil rail after obtaining a shutdown signal and before a dynamic pressure release stage, when the pressure in the oil rail is larger than the idle rail pressure, the pressure release valve is controlled to release the rail pressure to the idle rail pressure, and when the pressure in the oil rail is the idle rail pressure, the fuel system enters the dynamic pressure release stage.

As a preferable scheme of the shutdown pressure relief control method of the fuel system, the first injection pressure relief comprises a first pilot injection, a second pilot injection, a main injection, a first post injection and a second post injection, and the second post injection is non-empty injection.

As a preferable scheme of the shutdown pressure relief control method of the fuel system, the starting angle of the second post-injection is 20 degrees after the piston of the engine moves to the top dead center.

As a preferable scheme of the shutdown pressure relief control method of the fuel system, the injection pulse width of the nth non-empty injection of each injection pressure relief in the dynamic pressure relief stage is the maximum injection pulse width when the engine oil concentration is within a preset range and the engine does not reignite.

As a preferable scheme of the shutdown pressure relief control method of the fuel system, the pulse of the idle injection in the dynamic pressure relief stage and/or the static pressure relief stage is the injection pulse width of the idle injection return oil of the fuel injector under the idle working condition.

As a preferable scheme of the shutdown pressure relief control method of the fuel system, the injection frequency of the idle injection in the static pressure relief stage is less than or equal to 100 Hz.

As a preferable scheme of the shutdown pressure release control method of the fuel system, when the idle injection is performed in the static pressure release stage, the method further comprises accumulating the idle injection time, and when the idle injection time is accumulated to the preset time and the rail pressure is not reduced to the shutdown rail pressure, the idle injection is stopped.

A shutdown pressure relief control device of a fuel system can execute the shutdown pressure relief control method of the fuel system in any scheme.

As a preferred scheme of the shutdown pressure relief control device of the fuel system, the shutdown pressure relief control device further comprises an oil tank, an oil pump and a filter arranged between the oil tank and the oil pump, wherein an oil return end of the oil injector is communicated with the oil tank, and the oil pump is communicated with the oil rail.

The invention has the beneficial effects that:

according to the invention, by setting the dynamic pressure relief stage, after the engine acquires the stop signal and when the pressure of the oil rail is between the idling rail pressure and the engine stalling, the oil injector is controlled to perform multiple injection pressure relief so as to reduce the pressure in the oil rail, then the air injection is continuously performed between the engine stalling and the oil rail being at the stop rail pressure until the rail pressure is reduced to the stop rail pressure, the dynamic pressure relief stage and the static pressure relief stage are matched for pressure relief, the rail pressure can be quickly reduced to the stop rail pressure in a short time after the engine acquires the stop signal, and the control method can be used in occasions where the quick pressure relief is urgently needed after the stop working condition, such as stop maintenance and the like. Specifically, each injection pressure relief in the dynamic pressure relief stage comprises n times of injection, the former (n-1) times are idle injection, the nth time is non-idle injection, and the nth time of injection for pressure relief is set to be non-idle injection, so that the pressure relief effect is further increased, the pressure relief time is further shortened, the injection times of the oil injector are reduced, and the service life of the oil injector is prolonged.

Drawings

In order to more clearly illustrate the technical solutions in the embodiments of the present invention, the drawings used in the description of the embodiments of the present invention will be briefly described below, and it is obvious that the drawings in the following description are only some embodiments of the present invention, and for those skilled in the art, other drawings can be obtained according to the contents of the embodiments of the present invention and the drawings without creative efforts.

FIG. 1 is a graph of pressure relief time versus engine speed for a method of controlling fuel system shut down pressure relief according to an embodiment of the present invention;

FIG. 2 is a schematic diagram of a shutdown pressure relief control device of a fuel system according to an embodiment of the present invention.

Detailed Description

In order to make the technical problems solved, technical solutions adopted and technical effects achieved by the present invention clearer, the technical solutions of the embodiments of the present invention will be described in further detail below with reference to the accompanying drawings, and it is obvious that the described embodiments are only a part of the embodiments of the present invention, and not all embodiments. All other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present invention.

As shown in fig. 1, the present embodiment provides a shutdown pressure relief control method for a fuel system, where the fuel system includes an engine, an oil rail and a fuel injector, and the fuel injector can inject fuel in the oil rail into the engine, and the shutdown pressure relief control method for the fuel system includes that, after the engine obtains a shutdown signal, a dynamic pressure relief stage is performed when the oil rail is between an idle rail pressure and an engine stall, and in the dynamic pressure relief stage, the fuel injector performs multiple injection pressure relief, where each injection pressure relief includes n injections, where the first (n-1) injections are idle injections and the nth (n-1) is non-idle injections; and in the static pressure relief stage, the oil injector performs air injection until the rail pressure is reduced to the shutdown rail pressure.

Through setting up the developments pressure release stage, after the engine acquires the shut down signal, and when oil rail pressure is in idle rail pressure to the engine stall, carry out the injection pressure release many times through controlling the sprayer, in order to reduce the pressure in the oil rail, then stop to continue to carry out the air injection between the oil rail is in the rail pressure of shutting down at the engine stall, until rail pressure drops to the rail pressure of shutting down, the cooperation pressure release in developments pressure release stage and static pressure release stage, can be in the engine short time after acquiring the shut down signal with rail pressure drop to the rail pressure of shutting down fast, this control method can be used to the occasion that needs quick pressure release after the operating mode of shutting down, like shutting down maintenance etc.. Specifically, each injection pressure relief in the dynamic pressure relief stage comprises n times of injection, the former (n-1) times are idle injection, the nth time is non-idle injection, and the nth time of injection for pressure relief is set to be non-idle injection, so that the pressure relief effect is further increased, the pressure relief time is further shortened, the injection times of the oil injector are reduced, and the service life of the oil injector is prolonged.

It can be understood that the advantage of non-empty injection for the nth time of injection pressure relief is that the pressure relief efficiency can be improved, but the problems of engine oil dilution and engine afterburning are also existed, the adhesion of the engine oil can be reduced after the engine oil dilution, the effective components of the engine oil can be damaged, the nonmetal sealing parts of the engine body can be corroded, the possibility of combustion can be caused, residues can be volatilized, oil dirt can be generated, and the lubricating system can be damaged; the reignition of the engine caused by the stop signal is contrary to the intention of the driver, and serious potential safety hazard is easily caused. Therefore, the pulse width setting of the nth non-air injection needs to ensure that the concentration of engine oil is within a preset range and the engine does not re-ignite while the pressure relief efficiency can be improved. In summary, in the embodiment, the injection pulse width of the nth non-empty injection during each injection pressure relief in the dynamic pressure relief stage is the maximum injection pulse width when the engine oil concentration is within the preset range and the engine does not reignite. For example, the nth injection pulsewidth may range from 1500 μ s to 3000 μ s, which has little effect on oil dilution while also allowing for a rapid rail pressure drop. The present example uses a more conservative injection pulsewidth of 1500 mus.

It should be noted that, in the air injection technology, the oil return ball valve is moved by controlling the power-on pulse width of the oil injector, while the injection needle valve is not moved, and the oil injector only returns oil without injecting oil after the engine is stopped. Meanwhile, the oil injector in the embodiment is a non-static oil return oil injector.

Preferably, the fuel system further comprises a pressure release valve communicated with the oil rail, the pressure inside the oil rail is judged after the engine obtains the stop signal and before the dynamic pressure release stage, when the pressure inside the oil rail is greater than the idling rail pressure, the pressure release valve is controlled to release the rail pressure to the idling rail pressure, and when the pressure inside the oil rail is the idling rail pressure, the dynamic pressure release stage is started. Illustratively, the idle rail pressure is 500bar-600bar, when the vehicle is stopped from an idle state, the rail pressure is the idle rail pressure, and the dynamic pressure relief stage can be directly entered for pressure relief; when the vehicle was shut down from direct execution under other operating modes, the rail pressure can be greater than idle rail pressure this moment, and this just needs earlier to carry out the pressure release through the relief valve, and it continues the pressure release to get into the dynamic pressure release stage again after pressing the rail to idle rail pressure to this. The pressure relief valve is an electrically controlled pressure relief valve, and the pressure relief process and principle of the pressure relief valve are the prior art and are not specifically limited herein.

In particular, the multiple injection technique has been a prior art widely used in high pressure common rail diesel engine systems, the multiple injection including a pre-injection, a main injection, and a post-injection. The calibration of the injection times is determined according to the type of an oil injector configured for the engine and the processing technology of a fuel system of a mass production engine. The number of multiple injections ranged from 1 to 7. The higher the processing technology of the engine fuel system is, the better the sealing and pressure stabilizing effect is, and the larger the calibrated multiple injection frequency value is; conversely, the smaller the calibrated multiple injection count value. In this embodiment, the number of injections in the dynamic pressure release process is set to five. Specifically, the primary injection pressure relief comprises a first pre-injection, a second pre-injection, a main injection, a first post-injection and a second post-injection, and the pre-injection mainly aims at reducing noise; the purpose of post-injection is to reduce soot and increase exhaust temperature. The first pilot injection, the second pilot injection, the main injection and the first after injection are idle injections, and the second after injection is non-idle injection.

Further, the injection pressure relief process also needs to set the injection pulse width of the first (n-1) empty injection and the initial angle of each injection. The pulse of the idle injection is the injection pulse width of the idle injection oil return of the oil injector under the idle working condition, and specifically, the injection pulse width of the idle injection is 450 mus. The initial angle of the second post-injection is 20 degrees after the piston of the engine moves to the top dead center, specifically, the initial angle of the first pre-injection is 20 degrees before the piston of the engine moves to the top dead center, the initial angle of the second pre-injection is 5.5 degrees before the piston of the engine moves to the top dead center, the initial angle of the main injection is 1 degree before the piston of the engine moves to the top dead center, and the initial angle of the first post-injection is 10 degrees after the piston of the engine moves to the top dead center.

It should be noted that the pulse width and the injection angle of each injection in the dynamic pressure relief stage are obtained through experiments, and the experimental mode is the prior art and is not limited herein. In other embodiments, the idle-injection return oil injection pulse width of the oil injector can be obtained by looking up a table, the idle-injection return oil injection pulse width of the oil injector can be calculated in a mode that a one-dimensional graph can be inquired about the pulse width of the previous (n-1) injection, the abscissa of the one-dimensional graph is the pressure of the common rail of the fuel oil, the ordinate is the idle-injection return oil injection pulse width of the oil injector, and the injection pulse width is the idle-injection return oil injection pulse width measured by a single injection instrument through traversing different rotating speeds; the calibration of the nth injection pulse width needs to continuously look up the table and calculate the nth injection pulse width look-up table value on the basis of the existing look-up table value. The table is a one-dimensional chart, the abscissa is the pressure of the fuel common rail, and the ordinate is the pulse width value of the nth injection; the starting angle of each injection is the sum of the basic look-up table value of the starting angle of injection and the corrected value of the starting angle of injection. And the basic table lookup value adopts a three-dimensional table lookup, wherein the x axis of the three-dimensional table is the engine rotating speed, the y axis is the final fuel injection quantity of multiple injection, and the z axis is the injection initial angle. The injection start angle correction value is a product of a correction value base look-up table value and a correction coefficient look-up table value. The correction value basic table checking value adopts a three-dimensional table, the x axis of the three-dimensional table is the engine rotating speed, the y axis is the final fuel injection quantity of multiple injection, and the z axis is the correction value basic value; the correction coefficient table lookup value adopts a two-dimensional table lookup, the abscissa of the two-dimensional table is the engine temperature, and the ordinate of the two-dimensional table is the correction coefficient. The corresponding table and table lookup method are prior art and are not described in detail herein.

Further, the air injection pulse width of the static pressure relief stage is the same as the air injection pulse width of the dynamic pressure relief stage, and both the air injection pulse width of the oil injector under the idle working condition and the air injection pulse width of the oil injector under the idle working condition are the same, and specifically, the air injection pulse width is 450 μ s.

In particular, the static pressure relief phase also requires the determination of the injection frequency of the empty jets. The injection frequency is determined according to the type of an oil injector configured for the engine and the processing technology of a fuel system of a mass production engine. The range of the injection frequency is less than or equal to 100 Hz. The higher the processing technology of an engine fuel system is, the better the sealing and pressure stabilizing effect is, and the larger the injection frequency is; conversely, the smaller the injection frequency. Illustratively, in the present embodiment, the injection frequency of the air injection in the static pressure relief stage is 1 Hz.

Preferably, when the air injection is carried out in the static pressure relief stage, the method further comprises the step of accumulating the air injection time, and when the air injection time is accumulated to the preset time and the rail pressure is not reduced to the shutdown rail pressure, the air injection is stopped. By setting the accumulated air injection time, the situation that the oil injector fails or is abnormal and the like can be prevented from being always in a static pressure relief stage, and the reliability of pressure relief is favorably ensured

As shown in fig. 2, the present embodiment further discloses a shutdown pressure relief control device for a fuel system, which is capable of implementing the shutdown pressure relief control method for a fuel system according to any one of the above aspects. Shut down the pressure release controlling means through fuel oil system and shut down the pressure release, can effectual improvement shut down pressure release efficiency, shorten the latency after the parking, generally need about 8 minutes from idle speed rail pressure to shut down rail pressure than prior art, adopt dynamic pressure release stage and static pressure release stage combination pressure release technique can shorten the pressure release time to about 1 minute. Compared with the prior art which needs the oil sprayer to perform idle injection for more than 500 times, the injection frequency of the oil sprayer can be controlled to be about 100 times by adopting the combined pressure relief technology of the dynamic pressure relief stage and the static pressure relief stage, and the service life of the oil sprayer is effectively prolonged.

The shutdown pressure relief control device of the fuel system comprises a controller, wherein a pressure relief valve and a fuel injector are electrically connected with the controller, and the controller can receive shutdown information of a vehicle and send the shutdown information to an engine; the shutdown pressure relief control device of the fuel system further comprises a pressure detection piece arranged in the oil rail, and the pressure detection piece is also electrically communicated with the controller.

Furthermore, the shutdown pressure relief control device of the fuel system also comprises an oil tank, an oil pump and a filter arranged between the oil tank and the oil pump, wherein the oil return end of the oil injector is communicated with the oil tank, and the pressure relief valve is also communicated with the oil tank and used for returning oil and relieving pressure; the oil pump is communicated with the oil rail and is used for pumping fuel oil in the oil tank into the oil rail; the filter can be used for filtering the fuel in the oil filter box, so that the fuel entering the oil rail is cleaner.

It is to be noted that the foregoing is only illustrative of the preferred embodiments of the present invention and the technical principles employed. It will be understood by those skilled in the art that the present invention is not limited to the particular embodiments described herein, but is capable of various obvious changes, rearrangements and substitutions as will now become apparent to those skilled in the art without departing from the scope of the invention. Therefore, although the present invention has been described in greater detail by the above embodiments, the present invention is not limited to the above embodiments, and may include other equivalent embodiments without departing from the spirit of the present invention, and the scope of the present invention is determined by the scope of the appended claims.

In the description of the present invention, it should be noted that the terms "center", "upper", "lower", "left", "right", "vertical", "horizontal", "inner", "outer", etc., indicate orientations or positional relationships based on the orientations or positional relationships shown in the drawings, and are only for convenience of description and simplicity of description, but do not indicate or imply that the device or element being referred to must have a particular orientation, be constructed and operated in a particular orientation, and thus, should not be construed as limiting the present invention. Furthermore, the terms "first," "second," and the like are used for descriptive purposes only and are not to be construed as indicating or implying relative importance. Wherein the terms "first position" and "second position" are two different positions.

In the description of the present invention, it should be noted that, unless otherwise explicitly specified or limited, the terms "mounted," "connected," and "connected" are to be construed broadly, e.g., as meaning either a fixed connection, a removable connection, or an integral connection; can be mechanically or electrically connected; they may be connected directly or indirectly through intervening media, or they may be interconnected between two elements. The specific meanings of the above terms in the present invention can be understood in specific cases to those skilled in the art.