Patent Publication Number: US-11377980-B2

Title: Self-resetting single-valve double-piston hydraulic drive device and method for overhead cam engine

Description:
CROSS REFERENCE TO THE RELATED APPLICATIONS 
     This application is the continuation application of International Application No. PCT/CN2021/072988, filed on Jan. 21, 2021, which is based upon and claims priority to Chinese Patent Applications No. 202011384578.3, filed on Dec. 2, 2020, the entire contents of which are incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The present invention relates to the technical field of overhead cam engines, and more particularly, to a self-resetting single-valve double-piston hydraulic drive device and method for an overhead cam engine. 
     BACKGROUND 
     The concept and operation of the compression release engine brake are well known in the heavy commercial vehicle industry. The cost, power, reliability and engine modification requirements are often factors that determine whether to use the engine brake. There are several different types of compression release engine brakes in practical applications. Among them, the engine brake system integrated in the valve train has become more popular due to its low cost, high performance, reliability and compact structure. 
     One way to integrate the engine brake system is to integrate the drive valve lift into the positive cam, and add an “lost motion” device to the valve train to hide or prohibit the engine from implementing the brake lift of the valve in the positive mode. 
     In a first example, the US patent application U.S. Ser. No. 13/004,695 filed in January 2011 discloses a braking-type engine brake system and method. The braking-type engine brake system is provided in a valve bridge having one or more brake pistons and reset devices. In a second example, the US patent application U.S. 61/730,395 filed in November 2012 relates to a rocker arm brake reset device, which uses a cam and a reset pin to control the valve movement of a compression release brake. In a third example, the international patent application WO2016041600A1 filed in September 2014 discloses an exhaust valve rocker arm assembly. The exhaust valve rocker arm assembly can operate in an internal combustion engine mode and an engine braking mode, and a pressure relief valve for resetting is provided in the rocker arm assembly. Special reset devices are used in these examples, which reset the brake exhaust valve after the brake lift, thereby reducing or eliminating the increase in the positive-power exhaust lift caused by increasing the brake lift, the decrease in braking power caused by the increase in the overlap between the positive-power exhaust lift and the positive-power intake lift, and the increase in the possibility of contact between the exhaust valve and the cylinder piston. The reset function can also help the exhaust valve to be evenly closed according to a designed exhaust valve cam closing ramp so as to control the seating velocity of the valve towards the valve seat. These additional reset devices can help improve system performance, but can simultaneously increase the system complexity, and therefore takes up more space and increases the brake cost. In addition, if the reset function is completed in case of a high system load, it may damage the overall reliability and durability of the system. 
     SUMMARY 
     The present invention provides a self-resetting single-valve double-piston hydraulic drive device and method for an overhead cam engine. The present invention solves the following technical problems: the existing engine valve train has a compact structure, and it is hard to provide a hydraulic drive device; the engine drive function and the automatic valve clearance adjustment function cannot coexist; and an additional reset device will complicate the system and reduce the reliability. 
     In order to solve the above-mentioned technical problems, the present invention adopts the following technical solution: a self-resetting single-valve double-piston hydraulic drive device for an overhead cam engine. The device includes: 
     a rocker arm assembly, where the rocker arm assembly includes a rocker arm body and a driving oil passage; and one end of the rocker arm body is provided with a primary driving piston, and the other end of the rocker arm body is provided with a first elephant foot assembly; 
     an integrated cam assembly, where the integrated cam assembly includes an integrated cam, and the integrated cam assembly is provided below the primary driving piston and is configured to drive the rocker arm body to rotate; 
     an exhaust valve assembly, where the exhaust valve assembly includes an inner-side exhaust valve and an outer-side exhaust valve; 
     a valve bridge assembly, where the valve bridge assembly includes a valve bridge body; the valve bridge body is located below the first elephant foot assembly; the valve bridge body is provided with a secondary driving piston and an oil drain passage; the oil drain passage connects the secondary driving piston with the driving oil passage; and the secondary driving piston is connected to the inner-side exhaust valve or the outer-side exhaust valve; and 
     a limit assembly, where the limit assembly is located above the oil drain passage; 
     the driving oil passage connects the primary driving piston with the secondary driving piston; the driving oil passage is connected with an engine drive solenoid valve and a drive control valve, and the engine drive solenoid valve and the drive control valve are opened or closed simultaneously; 
     when the engine drive solenoid valve is opened and the drive control valve opens the driving oil passage: during a drive lift of the integrated cam, the limit assembly is in contact with the valve bridge body and seals the oil drain passage, the primary driving piston and the secondary driving piston are connected to form a hydraulic linkage, the secondary driving piston drives an exhaust valve connected to the secondary driving piston to open, and the rocker arm body and the valve bridge body do not move; during a positive-power exhaust lift of the integrated cam, the limit assembly is separated from the valve bridge body, the oil drain passage is opened, the secondary driving piston is automatically reset after oil is drained, the primary driving piston and the rocker arm body are rigidly connected, and the rocker arm body rotates to drive the valve bridge body to open the inner-side exhaust valve and the outer-side exhaust valve; and 
     when the engine drive solenoid valve is closed and the drive control valve closes the driving oil passage: during the drive lift of the integrated cam, the primary driving piston absorbs the drive lift of the integrated cam assembly for the rocker arm body, the rocker arm body does not move, and the drive lift of the integrated cam is not transmitted to an exhaust valve side; thus, a positive-power “lost motion” function of a drive mechanism is realized; during the positive-power exhaust lift of the integrated cam, the primary driving piston and the rocker arm body are rigidly connected, and the rocker arm body rotates to drive the valve bridge body to open the inner-side exhaust valve and the outer-side exhaust valve. 
     In the present invention, the primary driving piston is provided on the rocker arm body, and the secondary driving piston is provided on the valve bridge body. The secondary driving piston is connected to the inner-side exhaust valve or the outer-side exhaust valve. The driving oil passage connects the primary driving piston with the secondary driving piston, and the driving oil passage is connected with the drive control valve. The driving oil passage is separated from an opening oil passage of the drive control valve, and the flow rate of the driving oil passage is not limited by the source and flow rate of the opening oil passage of the drive control valve. There is no need for an additional hydraulic clearance adjustment device or rocker arm biasing device. The primary driving piston and the rocker arm body directly form a hydraulic clearance adjustment function, which eliminates the noise, impact and wear caused by the valve clearance, and reduces the frequency of maintenance. When the engine drive solenoid valve is closed and the drive control valve closes the driving oil passage, during the drive lift of the integrated cam, the primary drive piston absorbs the drive lift of the integrated cam assembly for the rocker arm body. The drive lift of the integrated cam will not be transmitted to the exhaust valve side. The rocker arm body will not swing, and the valve bridge will not tilt. Compared with other rocker arm drives, the valve stem is not subject to a lateral load. The rocker arm body is less worn, and a bushing-less rocker arm design can be adopted. The primary driving piston, the secondary driving piston and the driving oil passage are integrated on the rocker arm assembly and the valve bridge assembly, and no additional space is required. The secondary driving piston and the oil drain passage are connected with each other. The secondary driving piston can be automatically reset after the hydraulic oil is drained, and no special reset device is required. The secondary driving piston is connected to one of the exhaust valves in the exhaust valve assembly, and when driving, only one exhaust valve is opened per cylinder. Compared with other drives that open dual exhaust valves, the system of the present invention has a lower driving load. In addition, the drive valve is not restricted by the position, and it can be very adjacent to the rocker arm shaft or far away from the rocker arm shaft, which is hard for other rocker actuators. There is no need to establish a large back pressure in an exhaust passage through an exhaust gas, and the thermal load is less. The primary driving piston is used for both positive-power and driving operations. The valve bridge assembly is provided below the first elephant foot assembly. The first elephant foot assembly is low-cost and easily adjustable. The present invention is compact in structure, simple to be provided on the engine valve train, convenient to design, low driving load, and improves the reliability and durability of engine operation. 
     Further, the driving oil passage may include a primary piston oil passage, a secondary piston oil passage and a control valve oil supply passage; the primary piston oil passage may be connected with the primary driving piston and the drive control valve; the secondary piston oil passage may be connected with the secondary driving piston and the drive control valve; the control valve oil supply passage may be connected with the drive control valve; and the engine drive solenoid valve may be connected on the control valve oil supply passage. 
     Further, the inner-side exhaust valve may be connected to the secondary driving piston. 
     When the inner-side exhaust valve is connected to the secondary driving piston, the limit assembly is the rocker arm body, and the limit surface is provided on the rocker arm body. When the limit surface is in contact with the valve bridge body, the limit surface limits the valve bridge body and seals the oil drain passage. The present invention uses the rocker arm body for limiting, which simplifies the overall structure and realizes a compact structure. 
     Further, the secondary piston oil passage may connect the drive control valve with the limit surface; and when the limit surface is in contact with the valve bridge body, the secondary piston oil passage may be connected with the oil drain passage. 
     Further, the secondary piston oil passage may be connected with the first elephant foot assembly; the valve bridge body may be provided with an inner-side connection passage; the first elephant foot assembly may be connected with the secondary driving piston through the inner-side connection passage; and when the limit surface is in contact with the valve bridge body, the limit surface may seal the oil drain passage. 
     Further, in order to lower the height of the rocker arm assembly on the integrated cam assembly side, the secondary piston oil passage may be connected with a second elephant foot assembly; the second elephant foot assembly may be provided on the rocker arm body and may be located directly above the oil drain passage; and when a lower end of the second elephant foot assembly is in contact with the valve bridge body, the second elephant foot assembly may be connected with the oil drain passage. 
     Further, the outer-side exhaust valve may be connected to the secondary driving piston; the secondary piston oil passage may be connected with the first elephant foot assembly; the valve bridge body may be provided with an outer-side connection passage; and the first elephant foot assembly may be connected with the secondary driving piston through the outer-side connection passage. 
     When the outer-side exhaust valve is connected to the secondary driving piston, the limit assembly may be a limit rod; the limit rod may be located directly above the oil drain passage; and when the limit rod is in contact with the valve bridge body, the limit rod may seal the oil drain passage. 
     In order to adjust the clearance between the limit assembly and the valve bridge assembly, the limit assembly further includes a bracket. The limit rod is adjustably provided on the bracket, and the position of the limit rod on the bracket is adjustable, such that the clearance between the limit assembly and the valve bridge assembly is adjustable. 
     Further, the primary driving piston may include a primary piston body; one end of the rocker arm body may be provided with a primary piston hole; the primary piston hole may be connected with the primary piston oil passage; the primary piston body may be coaxially and slidably provided in the primary piston hole; a primary elastic element and an anti-dropping assembly may be provided between the primary piston body and the primary piston hole; the anti-dropping assembly may restrict the primary piston body from moving out of the primary piston hole. 
     When the primary piston body is in contact with the integrated cam, the friction between the primary piston body and the integrated cam is sliding friction. In order to adapt to some heavy-load engine models and protect the primary piston body and the integrated cam, rolling friction is used. A roller pin is fixedly provided at the lower end of the primary piston body, and a roller is rotatably provided on the roller pin. An anti-rotation assembly is further provided between the primary piston body and the primary piston hole. The anti-rotation assembly restricts the primary piston body from rotating along the central axis of the primary piston body. The central axis of the roller and the central axis of the integrated cam are located on an identical plane. By providing the roller, the sliding friction between the primary piston body and the integrated cam becomes rolling friction. This avoids the friction and wear of the primary piston body and reduces the friction and wear of the integrated cam, thereby protecting the primary piston body and the integrated cam. 
     In order to adjust the movement stroke of the primary piston body in an axial direction of the primary piston hole, the primary driving piston further includes an adjusting bolt. The adjusting bolt is threadedly provided on the rocker arm body. The adjusting bolt is axially inserted in the primary piston hole. 
     Further, the secondary driving piston may include a secondary piston body; the secondary piston body may be connected to the inner-side exhaust valve or the outer-side exhaust valve; a secondary piston hole may be provided on the valve bridge body; the secondary piston hole may be connected with the oil drain passage; the secondary piston body may be coaxially and slidably provided in the secondary piston hole; and a secondary elastic element may be provided between the secondary piston body and the secondary piston hole. 
     Further, the rocker arm assembly may further include a rocker arm shaft; the drive control valve may be provided on the rocker arm body; the control valve oil supply passage includes a rocker arm shaft oil supply passage and a connecting oil supply passage; the rocker arm shaft oil supply passage may be provided at the rocker arm shaft; and the connecting oil supply passage may be provided at the rocker arm body; the connecting oil supply passage may be connected to the rocker arm shaft oil supply passage and the drive control valve. The rocker arm body is able to rotate on the rocker arm shaft. Therefore, in order to ensure an uninterrupted supply of lubricating oil during the rotation of the rocker arm body on the rocker arm shaft, the connecting oil supply passage may include a connecting oil supply section and an annular oil supply section. The annular oil supply section may be connected with the rocker arm shaft oil supply passage. The connecting oil supply section may be connected with the drive control valve. 
     Further, the drive control valve may include a control valve body and a return assembly; the return assembly may be in contact with the control valve body; the rocker arm body may be provided with a control valve hole; the control valve body may be coaxially and slidably provided in the control valve hole; and the control valve body may be provided with a primary control valve oil passage; 
     when the engine drive solenoid valve is opened, the rocker arm shaft oil supply passage, the control valve oil supply passage and the bottom of the control valve hole may be filled with oil, an oil pressure force of the bottom of the control valve hole may be greater than a force of the return assembly acting on the control valve body, the control valve body moves up to an opened position under the action of the oil pressure, and the primary control valve oil passage connects the primary piston oil passage with the secondary piston oil passage; and 
     when the engine drive solenoid valve is closed, no oil pressure exists in the rocker arm shaft oil supply passage and the control valve oil supply passage, the control valve body may be in a closed position at the bottom of the control valve hole under the action of the return assembly, and the primary control valve oil passage may be always connected with the primary piston oil passage and is not connected with the secondary piston oil passage. 
     In order to supply oil to the driving oil passage and lubricate the first elephant foot assembly, the rocker arm assembly may further include a lubricating oil passage; the lubricating oil passage may be connected with the first elephant foot assembly and the control valve hole; in order to realize the communication between the lubricating oil passage and the driving oil passage, the control valve body may be further provided with a secondary control valve oil passage; the secondary control valve oil passage may be connected with the primary control valve oil passage through a communication passage provided at the control valve body; and a one-way valve may be coaxially and fixedly provided in the communication passage; 
     when the engine drive solenoid valve is opened and the drive control valve is in the opened position: when the integrated cam is in a section of a cam base circle, an oil pressure of the secondary control valve oil passage may be greater than an oil pressure of the primary control valve oil passage, the one-way valve opens the communication passage, and the lubricating oil passage may be connected with the secondary control valve oil passage, the primary control valve oil passage and the driving oil passage; during the drive lift of the integrated cam, the oil pressure of the primary control valve oil passage may be greater than the oil pressure of the secondary control valve oil passage, the one-way valve seals the communication passage, the lubricating oil passage may be connected with the secondary control valve oil passage, and may be not connected with the primary control valve oil passage and the driving oil passage; during the positive-power lift of the integrated cam, the oil pressure of the secondary control valve oil passage may be greater than the oil pressure of the primary control valve oil passage, the one-way valve opens the communication passage, and the lubricating oil passage may be connected with the secondary control valve oil passage, the primary control valve oil passage and the driving oil passage; and 
     when the engine drive solenoid valve is closed and the drive control valve is in the closed position: the lubricating oil passage may be connected with the primary control valve oil passage, the oil pressure of the primary control valve oil passage may be greater than the oil pressure of the secondary control valve oil passage, and the one-way valve seals the communication passage; after lubricating oil fills the primary piston hole, the rocker arm assembly may be biased to a side of the valve bridge assembly; the primary driving piston absorbs the drive lift of the integrated cam assembly for the rocker arm body, and the primary driving piston and the rocker arm body form a hydraulic clearance adjuster to adjust a valve clearance. 
     Further, the return assembly may include an elastic return element and a control valve limit ring; the control valve limit ring may be coaxially and fixedly provided at an open end of the control valve hole; and the elastic return element may be located between the control valve body and the control valve limit ring; and 
     when the drive control valve is in the closed position, the control valve body and the control valve limit ring may be spaced apart to form an open chamber; the open chamber may be connected with the secondary piston oil passage; and the open chamber may be connected with an outside environment. 
     Further, the rocker arm assembly may further include a rocker arm shaft; the drive control valve may be provided on the rocker arm shaft; and each of an end of the primary piston oil passage and an end of the secondary piston oil passage connected with the drive control valve may be an annular passage. 
     In order to meet the requirements of different manufacturing processes, the rocker arm body may be provided with an insert in an interference fit manner, and a lower surface of the insert may be a limit surface. The insert may be separately hardened or treated by other process to enhance the flexibility of process design. 
     Further, when the secondary piston oil passage does not pass through the first elephant foot assembly, the lubricating oil passage may include a rocker arm shaft lubricating oil passage and an elephant foot lubricating oil passage; the rocker arm shaft lubricating oil passage may be provided on the rocker arm shaft; and the elephant foot lubricating oil passage may be provided on the rocker arm body. The rocker arm body is able to rotate on the rocker arm shaft. Therefore, when the rocker arm body rotates on the rocker arm shaft, in order to ensure an uninterrupted supply of lubricating oil, the elephant foot lubricating oil passage may include a connecting lubrication section and an annular lubrication section. The annular lubrication section may connect the rocker arm shaft lubricating oil passage to the connecting lubrication section. The connecting lubrication section may be connected with the first elephant foot assembly. 
     Further, when the secondary piston oil passage passes through the elephant foot assembly, the lubricating oil passage may include a rocker arm shaft lubricating oil passage and an injection lubricating oil passage; the rocker arm shaft lubricating oil passage may be provided on the rocker arm shaft; the injection lubricating oil passage may be provided on the rocker arm body and include an injection lubrication section and an annular lubrication section; the annular lubrication section may be connected to the rocker arm shaft lubricating oil passage and the injection lubrication section; after the injection lubrication section penetrates through the rocker arm body, an oil outlet of the injection lubrication section may be opposite to the first elephant foot assembly, so the lubricating oil is directly injected on the first elephant foot assembly to lubricate the first elephant foot assembly. 
     Further, the integrated cam includes a cam base circle; above the cam base circle is provided with a positive-power exhaust lift lobe, a drive exhaust gas recirculation lift lobe and a compression release drive lift lobe in sequence. 
     A self-resetting single-valve double-piston hydraulic drive method for an overhead cam engine, includes: allowing an engine drive solenoid valve to operate in two working states, namely an opened state and a closed state; 
     when the engine drive solenoid valve is in the opened state, a rotation process along an integrated cam is implemented by the following steps: 
     step 1: when the engine drive solenoid valve is opened, a rocker arm shaft oil supply passage, a connecting oil supply passage and the bottom of a control valve hole are filled with oil, such that an oil pressure of the bottom of the control valve hole is greater than a force of an elastic return element acting on a control valve body, the control valve body moves up to an opened position under the action of the oil pressure, and a primary control valve oil passage connects a primary piston oil passage with a secondary piston oil passage; 
     step 2: when the integrated cam rotates to a cam base circle, a one-way valve ball is opened under an oil pressure of a secondary control valve oil passage to connect the primary control valve oil passage; lubricating oil flows into a drive control valve and an entire driving oil passage, a primary piston hole is filled with oil; a rocker arm body is biased to a side of a valve bridge body under the action of an oil pressure of the primary piston hole, a limit assembly is tightly attached to an upper surface of the valve bridge body, such that the secondary piston oil passage is connected with an oil drain passage on the upper surface of the valve bridge body, and the primary piston hole and a secondary piston hole are connected through the primary piston oil passage, the primary control valve oil passage, the secondary piston oil passage and the oil drain passage, and are filled with the lubricating oil simultaneously; 
     step 3: during a drive lift of the integrated cam, an integrated cam assembly pushes a primary piston body upward, such that the primary piston body moves upward along the primary piston hole, the one-way valve ball seals a communication passage, and the drive control valve is in a locked state; the lubricating oil in the primary piston hole is pressed into the secondary piston hole, such that a primary driving piston and a secondary driving piston form a hydraulic linkage, the secondary driving piston pushes away an inner-side exhaust valve connected to the secondary driving piston; the rocker arm body and the valve bridge body do not move; and thus, a drive function of a drive mechanism is realized; and 
     step 4: during a positive-power exhaust lift of the integrated cam, the integrated cam assembly pushes the primary piston body upward, such that the primary piston body moves upward along the primary piston hole; when the primary piston body is pressed against the bottom of an adjusting bolt or the bottom of the primary piston hole, the primary piston body and the rocker arm body are rigidly connected; the rocker arm body starts to rotate, and a valve bridge assembly is pushed down through a first elephant foot assembly; a limit surface is separated from the upper surface of the valve bridge body, and the oil drain passage on the upper surface of the valve bridge body is automatically opened; the secondary piston body is in contact with the bottom of the secondary piston hole; and the valve bridge assembly is automatically reset to restore normal valve movement; 
     when the engine drive solenoid valve is in the closed state, the rotation process along the integrated cam is implemented by the following steps: 
     step 1: when the engine drive solenoid valve is closed, no oil pressure exists in the rocker arm shaft oil supply passage; the control valve body is in the closed position at the bottom of the control valve hole under the action of the elastic return element; the primary piston oil passage and the secondary piston oil passage are not connected; no oil pressure exists in a control valve oil supply passage; the primary piston oil passage and the primary piston hole are filled with the lubricating oil; and the secondary piston oil passage is connected with an open chamber, and no oil pressure exists in the secondary piston oil passage; 
     step 2: when the integrated cam rotates to the cam base circle, the lubricating oil fills the primary piston hole; the rocker arm assembly is biased to a side of the valve bridge assembly; the primary driving piston absorbs a drive lift of the integrated cam assembly for the rocker arm body; and the primary driving piston and the rocker arm body form a hydraulic clearance adjuster to adjust a valve clearance; 
     step 3: during the drive lift of the integrated cam, the integrated cam assembly pushes the primary piston body upward, such that the primary piston body moves upward along the primary piston hole; the rocker arm body is still biased to the side of the valve bridge body under the action of the oil pressure of the primary piston hole, but does not rotate; the drive lift of the integrated cam is absorbed by the primary driving piston and is not transmitted to the side of the valve bridge body; and thus, a positive-power “lost motion” function of the drive mechanism is realized; and 
     step 4: during the positive-power exhaust lift of the integrated cam, the integrated cam assembly pushes the primary piston body upward, such that the primary piston body moves upward along the primary piston hole; when the primary piston body is pressed against the bottom of the adjusting bolt or the bottom of the primary piston hole, the primary piston body and the rocker arm body are rigidly connected; and the rocker arm body rotates, and the valve bridge assembly is pushed down through the first elephant foot assembly to achieve positive valve movement. 
     The present invention has the following beneficial effects: In the present invention, the primary driving piston is provided on the rocker arm body, and the secondary driving piston is provided on the valve bridge body. The secondary driving piston is connected to the inner-side exhaust valve or the outer-side exhaust valve. The driving oil passage connects the primary driving piston with the secondary driving piston, and the driving oil passage is connected with the drive control valve. When the drive control valve closes the driving oil passage, during the drive lift of the integrated cam, the primary drive piston absorbs the drive lift of the integrated cam assembly for the rocker arm body. The drive lift of the integrated cam will not be transmitted to the exhaust valve side. The rocker arm body will not swing, and the valve bridge will not tilt. Compared with other rocker arm drives, the valve stem is not subject to a lateral load. The rocker arm body is less worn, and a bushing-less rocker arm design can be adopted. The primary driving piston, the secondary driving piston and the driving oil passage are integrated on the rocker arm assembly and the valve bridge assembly, and no additional space is required. The secondary driving piston and the oil drain passage are connected with each other. The secondary driving piston can be automatically reset after the hydraulic oil is drained, and no special reset device is required. The secondary driving piston is connected to the inner-side exhaust valve or the outer-side exhaust valve, and when driving, only one exhaust valve is opened per cylinder. Compared with other drives that open dual exhaust valves, the system of the present invention has a lower driving load. In addition, the drive valve is not restricted by the position, and it can be very adjacent to the rocker arm shaft or far away from the rocker arm shaft, which is hard for other rocker actuators. There is no need for exhaust brake, and the thermal load is less. The primary driving piston is used for both positive-power and driving operations. The driving oil passage is separated from an opening oil passage of the drive control valve, and the flow rate of the driving oil passage is not limited by the source and flow rate of the opening oil passage of the drive control valve. There is no need for an additional hydraulic clearance adjustment device or rocker arm biasing device. The primary driving piston and the rocker arm body directly form a hydraulic clearance adjustment function, which eliminates the noise, impact and wear caused by the valve clearance, and reduces the frequency of maintenance. The present invention features a simple drive principle, a compact structure, convenient optimization, low driving load, and improves the operational reliability and durability of the engine drive. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention is described in further detail below with reference to the drawings and embodiments. 
         FIG. 1  is a two-dimensional (2D) view illustrating an integrated cam rotating to a cam base circle when an engine drive solenoid valve is closed and a drive control valve closes a driving oil passage in a positive work state of an engine according to Embodiment 1 of the present invention. 
         FIG. 2  is a 2D view illustrating a rocker arm assembly according to Embodiment 1 of the present invention. 
         FIG. 3  is a 2D view illustrating a valve bridge assembly according to Embodiment 1 of the present invention. 
         FIG. 4  is a 2D view illustrating the integrated cam rotating to a drive lift lobe when the engine drive solenoid valve is closed and the drive control valve closes the driving oil passage in the positive work state of the engine according to Embodiment 1 of the present invention. 
         FIG. 5  is an enlarged view of A in  FIG. 4 . 
         FIG. 6  is a 2D view illustrating the integrated cam during a positive-power exhaust lift when the engine drive solenoid valve is closed and the drive control valve closes the driving oil passage in the positive work state of the engine according to Embodiment 1 of the present invention. 
         FIG. 7  is a 2D view illustrating the integrated cam rotating to the cam base circle when the drive control valve is in an opened position in a driving state of the engine according to Embodiment 1 of the present invention. 
         FIG. 8  is an enlarged view of B in  FIG. 7 . 
         FIG. 9  is a 2D view illustrating a drive exhaust valve opened during the drive lift of the integrated cam when the drive control valve is in the opened position in the driving state of the engine according to Embodiment 1 of the present invention. 
         FIG. 10  is a 2D view illustrating a secondary driving piston automatically decompressing and reset in the driving state of the engine according to Embodiment 1 of the present invention. 
         FIG. 11  is a 2D view of Embodiment 2 of the present invention. 
         FIG. 12  is a 2D view of Embodiment 3 of the present invention. 
         FIG. 13  is a 2D view of Embodiment 4 of the present invention. 
         FIG. 14  is a 2D view of Embodiment 5 of the present invention. 
         FIG. 15  is a 2D view of Embodiment 6 of the present invention. 
         FIG. 16  is a 2D view of Embodiment 7 of the present invention. 
         FIG. 17  is a 2D view of Embodiment 8 of the present invention. 
     
    
    
     Reference Numerals:  100 . rocker arm assembly: 
       110 . rocker arm body;  111 . limit surface;  112 . primary piston hole;  113 . control valve hole;  114 . rocker arm shaft hole; 
       120 . driving oil passage:  121 . primary piston oil passage;  122 . secondary piston oil passage;  123 . control valve oil supply passage;  123   a . rocker arm shaft oil supply passage;  123   b . connecting oil supply passage; b 1 . connecting oil supply section; b 2 . annular oil supply section; 
       130 . primary driving piston:  131 . primary piston body;  131   a . limit opening;  132 . primary elastic element;  133 . adjusting bolt;  134 . roller pin;  135 . roller;  136 . limit pin; 
       140 . first elephant foot assembly; 
       150 . drive control valve:  151 . control valve body;  151   a . primary control valve oil passage;  151   b . secondary control valve oil passage;  151   c . communication passage;  152 . one-way valve;  152   a . one-way valve ball;  152   b . one-way valve spring;  153 . elastic return element;  154 . control valve limit ring;  155 . control valve limit circlip; 
       160 . second elephant foot assembly; 
       170 . rocker arm shaft; 
       180 . lubricating oil passage:  181 . rocker arm shaft lubricating oil passage;  182 . elephant foot lubricating oil passage;  182   a . connecting lubrication section;  182   b . annular lubrication section;  183 . injection lubricating oil passage;  183   a . injection lubrication section;  183   b . annular lubrication section; 
       200 . integrated cam assembly: 
       210 . integrated cam:  211 . cam base circle;  212 . positive-power exhaust lift lobe;  213 . drive exhaust gas recirculation lift lobe;  214 . compression release drive lift lobe; 
       300 . exhaust valve assembly:  310 . inner-side exhaust valve;  314 . upper lubricating oil passage;  320 . outer-side exhaust valve; 
       400 . valve bridge assembly: 
       410 . valve bridge body;  411 . inner-side connection passage;  412 . outer-side connection passage;  413 . secondary piston hole;  414 . valve groove; 
       420 . secondary driving piston:  421 . secondary piston body;  422 . secondary elastic element;  423 . secondary piston valve base; 
       430 . oil drain passage; 
       500 . limit assembly:  510 . limit rod;  520 . bracket. 
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     The present invention is described in more detail below with reference to the drawings. These drawings are all simplified schematic diagrams, which merely illustrate the basic structure of the present invention in a schematic manner, and thus only show the parts associated with the present invention. 
     Embodiment 1 
     As shown in  FIGS. 1 to 10 , a self-resetting single-valve double-piston hydraulic drive device for an overhead cam engine includes a rocker arm assembly  100 , an integrated cam assembly  200 , an exhaust valve assembly  300 , a valve bridge assembly  400  and a limit assembly  500 . The rocker arm assembly  100  includes a rocker arm shaft  170 , a rocker arm body  110  and a driving oil passage  120 . The rocker arm shaft  170  is provided in a rocker arm shaft hole  114 . The rocker arm body  110  is rotatably provided on the rocker arm shaft  170 . One end of the rocker arm body  110  is provided with a primary driving piston  130 , and the other end of the rocker arm body  110  is provided with a first elephant foot assembly  140 . The primary driving piston  130  includes a primary piston body  131 . The primary piston body  131  may be cylindrical or may be a stepped post with steps. One end of the rocker arm body  110  is provided with a primary piston hole  112 . The primary piston hole  112  is connected with a primary piston oil passage  121 . The primary piston body  131  is coaxially and slidably provided in the primary piston hole  112 . A primary elastic element  132  and an anti-dropping assembly are provided between the primary piston body  131  and the primary piston hole  112 . The anti-dropping assembly restricts the primary piston body  131  from moving out of the primary piston hole  112 . A roller pin  134  is fixedly provided at a lower end of the primary piston body  131 . A roller  135  is rotatably provided on the roller pin  134 . An anti-rotation assembly is further provided between the primary piston body  131  and the primary piston hole  112 . The anti-rotation assembly restricts the primary piston body  131  from rotating along the central axis of the primary piston body. Central axes of the roller  135  and an integrated cam  210  are located on an identical plane. The primary elastic element  132  may be a compression spring, and the compression spring serves as a primary piston spring. In order to simplify the overall structure of the primary driving piston  130 , all the functions of the anti-dropping assembly and the anti-rotation assembly may be realized by a simple structure. In the present invention, the simple structure includes a limit pin  136 . The limit pin  136  is fixedly provided on an inner circumferential surface of the primary piston hole  112 . The primary piston body  131  is provided with a limit opening  131   a . The limit opening  131   a  is provided along an axial direction of the primary piston body  131 . The limit pin  136  is inserted in the limit opening  131   a . The length of the limit opening  131   a  is greater than or equal to a movement stroke of the primary piston body  131 . The limit pin  136  restricts the primary piston body  131  from moving out of the primary piston hole  112 . The width of the limit opening  131   a  is equal to the diameter of the limit pin  136 . The limit pin  136  is clamped in the limit opening  131   a . The limit pin  136  is able to limit the rotation of the primary piston body  131 . 
     In order to adjust the movement stroke of the primary piston body  131  in the axial direction of the primary piston hole  112 , the primary driving piston  130  further includes an adjusting bolt  133 . The adjusting bolt  133  is threadedly provided on the rocker arm body  110 . The adjusting bolt  133  is axially inserted into the primary piston hole  112 . The length of the adjusting bolt  133  inserted into the primary piston hole  112  is adjustable. In this embodiment, the first elephant foot assembly  140  used is an existing rocker arm elephant foot. 
     The integrated cam assembly  200  is provided below the primary driving piston  130  and is configured to drive the rocker arm body  110  to rotate. The integrated cam assembly  200  includes the integrated cam  210 . The integrated cam  210  includes a cam base circle  211 . The cam base circle  211  is provided with a positive-power exhaust lift lobe  212 , a drive exhaust gas recirculation lift lobe  213  and a compression release drive lift lobe  214  in sequence. 
     The exhaust valve assembly  300  includes an inner-side exhaust valve  310  and an outer-side exhaust valve  320 . The inner-side exhaust valve  310  is an exhaust valve on a side adjacent to the rocker arm shaft  170 , and the outer-side exhaust valve  320  is an exhaust valve on a side away from the rocker arm shaft  170 . The inner-side exhaust valve  310  is connected to the secondary driving piston  420 . 
     The valve bridge assembly  400  includes a valve bridge body  410 . The valve bridge body  410  is located below the first elephant foot assembly  140 . The valve bridge body  410  is provided with a secondary driving piston  420  and an oil drain passage  430 . The oil drain passage  430  connects the secondary driving piston  420  with the driving oil passage  120 . The secondary driving piston  420  is connected to the inner-side exhaust valve  310 . The secondary driving piston  420  includes a secondary piston body  421 . The secondary piston body  421  is connected to the inner-side exhaust valve  310 . A secondary piston hole  413  is provided on a side of the valve bridge body  410  adjacent to the rocker arm shaft  170 . The secondary piston hole  413  is connected with the oil drain passage  430 . The secondary piston body  421  is coaxially and slidably provided in the secondary piston hole  413 . A secondary elastic element  422  is provided between the secondary piston body  421  and the secondary piston hole  413 . The secondary elastic element  422  is a compression spring, and the compression spring serves as a secondary piston spring. A valve groove  414  is provided on a side of the valve bridge body  410  away from the rocker arm shaft  170 . An upper lubricating oil passage  314  is provided above the valve groove  414 . A mounting hole is provided at a lower end of the secondary piston body  421 , and a secondary piston valve base  423  is formed. The inner-side exhaust valve  310  or the outer-side exhaust valve  320  is fixedly provided in the secondary piston valve base  423 . 
     The limit assembly  500  is the rocker arm body  110 . The rocker arm body  110  is provided with a limit surface  111 . When the limit surface  111  is in contact with the valve bridge body  410 , the limit surface limits the valve bridge body  410  and seals the oil drain passage  430 . 
     The driving oil passage  120  connects the primary driving piston  130  with the secondary driving piston  420 . The driving oil passage  120  is connected with an engine drive solenoid valve and a drive control valve  150 , and the engine drive solenoid valve and the drive control valve  150  are opened or closed simultaneously. The driving oil passage  120  includes a primary piston oil passage  121 , a secondary piston oil passage  122  and a control valve oil supply passage  123 . The primary piston oil passage  121  connects the primary driving piston  130  with the drive control valve  150 . The secondary piston oil passage  122  connects the secondary driving piston  420  with the drive control valve  150 . The control valve oil supply passage  123  is connected with the drive control valve  150 . The engine drive solenoid valve is connected on the control valve oil supply passage  123 . The control valve oil supply passage  123  includes a rocker arm shaft oil supply passage  123   a  and a connecting oil supply passage  123   b . The rocker arm shaft oil supply passage  123   a  is provided at the rocker arm shaft  170 . The connecting oil supply passage  123   b  is provided at the rocker arm body  110 . The connecting oil supply passage  123   b  connects the rocker arm shaft oil supply passage  123   a  with the drive control valve  150 . The rocker arm body  110  is able to rotate on the rocker arm shaft  170 . Therefore, in order to ensure an uninterrupted supply of lubricating oil during the rotation of the rocker arm body  110  on the rocker arm shaft  170 , the connecting oil supply passage  123   b  includes a connecting oil supply section b 1  and an annular oil supply section b 2 . The annular oil supply section b 2  is connected with the rocker arm shaft oil supply passage  123   a . The connecting oil supply section b 1  is connected with the drive control valve  150 . 
     The drive control valve  150  is provided on the rocker arm body  110 . A control valve hole  113  is provided at a middle position of the rocker arm body  110 . The drive control valve  150  includes a control valve body  151  and a return assembly. The return assembly is in contact with the control valve body  151 . The control valve body  151  is coaxially and slidably provided in the control valve hole  113 . The control valve body  151  is provided with a primary control valve oil passage  151   a  and a secondary control valve oil passage  151   b . The secondary control valve oil passage  151   b  is connected with the primary control valve oil passage  151   a  through a communication passage  151   c  provided at the control valve body  151 . A one-way valve  152  is coaxially and fixedly provided in the communication passage  151   c . The one-way valve  152  includes a one-way valve ball  152   a  and a one-way valve spring  152   b . The one-way valve spring  152   b  is located between the one-way valve ball  152   a  and the control valve body  151 . The control valve body  151  is provided with a blind mounting hole for mounting the one-way valve spring  152   b . The one-way valve spring  152   b  is confined in the blind mounting hole. When the one-way valve  152  is closed, the one-way valve ball  152   a  seals the communication passage  151   c . The return assembly includes an elastic return element  153 , a control valve limit ring  154  and a control valve limit circlip  155 . The elastic return element  153  may be a return spring. The control valve limit ring  154  is coaxially and fixedly provided on an open end of the control valve hole  113  through the control valve limit circlip  155 . The elastic return element  153  is located between the control valve body  151  and the control valve limit ring  154 . When the drive control valve  150  is in a closed position, the control valve body  151  and the control valve limit ring  154  are spaced apart to form an open chamber. The open chamber is connected with an outside environment. The open chamber is also connected with the secondary piston oil passage  122 . 
     In order to supply oil to the driving oil passage  120  and lubricate the first elephant foot assembly  140 , the rocker arm assembly  100  further includes a lubricating oil passage  180 . The lubricating oil passage  180  is connected with the first elephant foot assembly  140 , and is also connected with the driving oil passage  120  through the drive control valve  150 . The lubricating oil passage  180  includes a rocker arm shaft lubricating oil passage  181  and an elephant foot lubricating oil passage  182 . The rocker arm shaft lubricating oil passage  181  is provided on the rocker arm shaft  170 . The elephant foot lubricating oil passage  182  is provided on the rocker arm body  110 . The rocker arm body  110  is able to rotate on the rocker arm shaft  170 . Therefore, when the rocker arm body  110  rotates on the rocker arm shaft  170 , in order to ensure an uninterrupted supply of lubricating oil, the elephant foot lubricating oil passage  182  includes a connecting lubrication section  182   a  and an annular lubrication section  182   b . The annular lubrication section  182   b  connects the rocker arm shaft lubricating oil passage  181  to the connecting lubrication section  182   a . The connecting lubrication section  182   a  is connected with the first elephant foot assembly  140 . The annular lubrication section  182   b  may also be provided on the rocker arm shaft  170 . 
     When the inner-side exhaust valve  310  is connected to the secondary driving piston  420 , the limit assembly  500  is the rocker arm body  110 . A limit surface  111  is provided on the rocker arm body  110 . The limit surface  111  is located on an inner upper side of the valve bridge body  410 , and the limit surface  111  is located directly above the oil drain passage  430 . The present invention uses the rocker arm body  110  for limiting, which simplifies the overall structure and realizes a compact structure. The secondary piston oil passage  122  connects the drive control valve  150  with the limit surface  111 . When the limit surface  111  is in contact with the valve bridge body  410 , the secondary piston oil passage  122  is connected with the oil drain passage  430 . 
     The specific working principle of the present invention is as follows: 
     When the engine drive solenoid valve is opened, the rocker arm shaft oil supply passage  123   a , the control valve oil supply passage  123  and the bottom of the control valve hole  113  are filled with oil. An oil pressure of the bottom of the control valve hole  113  is greater than a force of the return assembly acting on the control valve body  151 . The control valve body  151  moves up to an opened position under the action of the oil pressure; and the primary control valve oil passage  151   a  connects the primary piston oil passage  121  with the secondary piston oil passage  122 . 
     When the integrated cam  210  rotates to the cam base circle  211 , an oil pressure of the secondary control valve oil passage  151   b  is greater than an oil pressure of the primary control valve oil passage  151   a . The one-way valve  152  opens the communication passage  151   c . The lubricating oil passage  180  is connected with the secondary control valve oil passage  151   b , the primary control valve oil passage  151   a  and the driving oil passage  120 . The lubricating oil flows into the drive control valve  150  and the entire driving oil passage  120 , and the primary piston hole  112  is filled with oil. The primary piston body  131  supplements the clearance of the valve bridge assembly  400 , and moves downward to push the roller  135  to tightly attach to the cam base circle  211 . The rocker arm body  110  is biased to a side of the valve bridge body  410  under the action of the oil pressure of the primary piston hole  112 . The limit surface  111  is tightly attached to the upper surface of the valve bridge body  410 , such that the secondary piston oil passage  122  is connected with the oil drain passage  430  on the upper surface of the valve bridge body  410 . The primary piston hole  112  and the secondary piston hole  413  are connected through the primary piston oil passage  121 , the primary control valve oil passage  151   a , the secondary piston oil passage  122  and the oil drain passage  430 , and are filled with the lubricating oil simultaneously. 
     When the integrated cam  210  continues to rotate to the drive exhaust gas recirculation lift lobe  213  and the compression release drive lift lobe  214 , that is, during the drive lift of the integrated cam  210 , the integrated cam  210  pushes the roller  134  and the primary piston body  131  upward, such that the primary piston body  131  moves upward along the primary piston hole  112 . The oil pressure of the primary control valve oil passage  151   a  is greater than the oil pressure of the secondary control valve oil passage  151   b . The one-way valve  152  seals the communication passage  151   c . The lubricating oil passage  180  is connected with the secondary control valve oil passage  151   b  and is not connected with the primary control valve oil passage  151   a  and the driving oil passage  120 . The drive control valve  150  is in a locked state. Thus, the lubricating oil in the primary piston oil passage  121  and the primary control valve oil passage  151   a  connected to the drive control valve is caused to flow back. The lubricating oil in the primary piston hole  112  is pressed into the secondary piston hole  413 , such that the drive control valve  150  seals the driving oil passage  120  between the primary driving piston  130  and the secondary driving piston  420 . The primary driving piston  130  and the secondary driving piston  420  form a hydraulic linkage. The secondary driving piston  420  pushes away the inner-side exhaust valve  310  connected to the secondary driving piston  420 . The rocker arm body  110  and the valve bridge body  410  do not move. Thus, a drive function of the drive mechanism is realized. It should be noted that in this case, the rocker arm body  110  is still biased to the side of the valve bridge body  410  under the action of the oil pressure of the primary piston hole  112 , but does not rotate, and the valve bridge body  410  basically maintains a horizontally balanced position without deflection. 
     When the integrated cam  210  continues to rotate to an initial section of the positive-power exhaust lift lobe  212 , the integrated cam  210  pushes the roller  135  and the primary piston body  131  upward, and the primary piston body  131  moves upward along the primary piston hole  112 . When the integrated cam  210  continues to rotate on the positive-power exhaust lift lobe  212 , the oil pressure of the secondary control valve oil passage  151   b  is greater than the oil pressure of the primary control valve oil passage  151   a . The one-way valve  152  opens the communication passage  151   c , and the lubricating oil passage  180  is connected with the secondary control valve oil passage  151   b , the primary control valve oil passage  151   a  and the driving oil passage  120 . When the primary piston body  131  is pushed to the bottom of the adjusting bolt  133 , the primary piston body  131  and the rocker arm body  110  are rigidly connected. The rocker arm body  110  starts to rotate, and the valve bridge assembly  400  is pushed downward through the first elephant foot assembly  140 . At this time, the limit surface  111  is separated from the upper surface of the valve bridge body  410 , and the oil drain passage  430  on the upper surface of the valve bridge body  410  is automatically opened. The secondary driving piston  420  retracts along the secondary piston hole  413  under the action of the secondary elastic element  422 , and the excess lubricating oil in the secondary piston hole  413  is discharged through the oil drain passage  430 . The secondary driving piston  420  is reset after the oil is drained. When the secondary piston body  421  contacts the bottom of the secondary piston hole  413 , the primary driving piston  130  and the rocker arm body  110  are rigidly connected. The rocker arm body  110  rotates, and the valve bridge assembly  400  is automatically reset. The valve bridge body  410  is driven to open the inner-side exhaust valve  310  and the outer-side exhaust valve  320  to restore normal valve movement. 
     When the engine drive solenoid valve is closed, no oil pressure exists in the rocker arm shaft oil supply passage  123   a . The control valve body  151  is in the closed position at the bottom of the control valve hole  113  under the action of the return assembly. The lubricating oil passage  180  is connected with the primary control valve oil passage  151   a . The oil pressure of the primary control valve oil passage  151   a  is greater than the oil pressure of the secondary control valve oil passage  151   b . The one-way valve  152  seals the communication passage  151   c.    
     The primary piston oil passage  121  and the secondary piston oil passage  122  are not connected. No oil pressure exists in the control valve oil supply passage  123 . The primary piston oil passage  121  and the primary piston hole  112  are filled with the lubricating oil. The secondary piston oil passage  122  is connected with an open chamber, and no oil pressure exists in the secondary piston oil passage  122 . Since there is no oil supplied to the oil passage of the secondary driving piston  420  in the valve bridge body  410 , the driving oil passage  120  is closed. 
     When the integrated cam  210  rotates to the cam base circle  211 , the lubricating oil fills the primary piston hole  112 , and the primary driving piston  130  is filled with oil. After the lubricating oil fills the primary piston hole  112 , the rocker arm assembly  100  is biased to a side of the valve bridge assembly  400 . The primary driving piston  130  absorbs the drive lift of the integrated cam assembly  200  for the rocker arm body  110 . The primary driving piston  130  and the rocker arm body  110  form a hydraulic clearance adjuster to adjust a valve clearance. The valve clearance refers to a clearance between a lower end surface of the first elephant foot assembly  140  and the upper end surface of the valve bridge body  410 . 
     When the integrated cam  210  continues to rotate to the drive exhaust gas recirculation lift lobe  213  and the compression release drive lift lobe  214 , that is, during the drive lift of the integrated cam  210 , the integrated cam assembly  210  pushes the roller  135  and the primary piston body  131  upward, such that the primary piston body  131  moves upward along the primary piston hole  112 . The rocker arm body  110  is still biased to the side of the valve bridge body  410  under the action of the oil pressure of the primary piston hole  112 , but does not rotate. The drive lift of the integrated cam  210  is absorbed by the primary driving piston  130  and is not transmitted to the side of the valve bridge body  410 . Thus, a positive “lost motion” function of the drive mechanism is realized. 
     When the integrated cam  210  continues to rotate to an initial section of the positive-power exhaust lift lobe  212 , the integrated cam  210  pushes the roller  135  and the primary piston body  131  upward, such that the primary piston body  131  moves upward along the primary piston hole  112 . When the primary piston body  131  is pressed against the bottom of the adjusting bolt  133 , the primary piston body  131  and the rocker arm body  110  are rigidly connected. When the integrated cam  210  continues to rotate on the positive-power exhaust lift lobe  212 , the rocker arm body  110  is rotated. The valve bridge assembly  400  is pushed down through the first elephant foot assembly  140  and drives the valve bridge body  410  to open the inner-side exhaust valve  310  and the outer-side exhaust valve  320 , so as to achieve positive valve movement. 
     In the present invention, the primary driving piston  130  is provided on the rocker arm body  110 , and the secondary driving piston  420  is provided on the valve bridge body  410 . The secondary driving piston  420  is connected to the inner-side exhaust valve  310  or the outer-side exhaust valve  320 . The driving oil passage  120  connects the primary driving piston  130  with the secondary driving piston  420 , and the driving oil passage  120  is connected with the drive control valve  150 . The driving oil passage  120  is separated from an opening oil passage of the drive control valve  150 , and the flow rate of the driving oil passage  120  is not limited by the source and flow rate of the opening oil passage of the drive control valve  150 . There is no need for an additional hydraulic clearance adjustment device or rocker arm biasing device. The primary driving piston  130  and the rocker arm body  110  directly form a hydraulic clearance adjustment function, which eliminates the noise, impact and wear caused by the valve clearance, and reduces the frequency of maintenance. When the engine drive solenoid valve is closed and the drive control valve  150  closes the driving oil passage  120 , during the drive lift of the integrated cam  210 , the primary drive piston  130  absorbs the drive lift of the integrated cam assembly  200  for the rocker arm body  110 . The drive lift of the integrated cam  210  will not be transmitted to the exhaust valve side. The rocker arm body  110  will not swing, and the valve bridge  410  will not tilt. Compared with other rocker arm drives, the valve stem is not subject to a lateral load. The rocker arm body  110  is less worn, and a bushing-less rocker arm design can be adopted. The primary driving piston  130 , the secondary driving piston  420  and the driving oil passage  120  are integrated on the rocker arm assembly  100  and the valve bridge assembly  400 , and no additional space is required. The secondary driving piston  420  and the oil drain passage  430  are connected with each other. The secondary driving piston  420  can be automatically reset after the hydraulic oil is drained, and no special reset device is required. The secondary driving piston  420  is connected to the inner-side exhaust valve  310  or the outer-side exhaust valve  320 , and when driving, only one exhaust valve is opened per cylinder. Compared with other drives that open dual exhaust valves, the system of the present invention has a lower driving load. In addition, the drive valve is not restricted by the position, and it can be very adjacent to the rocker arm shaft  170  or far away from the rocker arm shaft  170 , which is hard for other rocker actuators. There is no need for exhaust brake, and the thermal load is less. The primary driving piston  130  is used for both positive-power and driving operations. The driving oil passage  120  is separated from an opening oil passage of the drive control valve  150 , and the flow rate of the driving oil passage  120  is not limited by the source and flow rate of the opening oil passage of the drive control valve  150 . There is no need for an additional hydraulic clearance adjustment device or rocker arm biasing device. The primary driving piston  130  and the rocker arm body  110  directly form a hydraulic clearance adjustment function, which eliminates the noise, impact and wear caused by the valve clearance, and reduces the frequency of maintenance. The present invention features a simple drive principle, a compact structure, convenient optimization, low driving load, and improves the operational reliability and durability of the engine drive. 
     Embodiment 2 
     Embodiment 2 shown in  FIG. 11  only differs from Embodiment 1 in the following. The secondary piston oil passage  122  is connected with the first elephant foot assembly  140 . The valve bridge body  410  is provided with an inner-side connection passage  411 . The first elephant foot assembly  140  is connected with the secondary driving piston  420  through the inner-side connection passage  411 . When the limit surface  111  is in contact with the valve bridge body  410 , the limit surface  111  seals the oil drain passage  430 . 
     When the secondary piston oil passage  122  passes through the elephant foot assembly  140 , the lubricating oil passage  180  includes a rocker arm shaft lubricating oil passage  181  and an injection lubricating oil passage  183 . The rocker arm shaft lubricating oil passage  181  is provided on the rocker arm shaft  170 . The injection lubricating oil passage  183  is provided on the rocker arm body  110  and includes an injection lubrication section  183   a  and an annular lubrication section  183   b . The annular lubrication section  183   b  connects the rocker arm shaft lubricating oil passage  181  to the injection lubrication section  183   a . After the injection lubrication section  183   a  penetrates through the rocker arm body  110 , an oil outlet of the injection lubrication section is opposite to the first elephant foot assembly  140 , such the lubricating oil is directly injected on the first elephant foot assembly  140  to lubricate the first elephant foot assembly  140 . 
     Embodiment 3 
     Embodiment 3 shown in  FIG. 12  only differs from Embodiment 2 in the following. In order to lower the height of the rocker arm assembly  100  on the side of the integrated cam assembly  200 , the secondary piston oil passage  122  is connected with a second elephant foot assembly  160 . The second elephant foot assembly  160  is provided on the rocker arm body  110  and is located directly above the oil drain passage  430 . The second elephant foot assembly  160  may be screwed or fixedly provided on the rocker arm body  110 . When a lower end of the second elephant foot assembly  160  is in contact with the valve bridge body  410 , the second elephant foot assembly  160  is connected with the oil drain passage  430 . In this embodiment, the second elephant foot assembly  160  used is an existing rocker arm elephant foot. 
     Embodiment 4 
     Embodiment 4 shown in  FIG. 13  only differs from Embodiment 1 in the following. The outer-side exhaust valve  320  is connected to the secondary driving piston  420 . The secondary piston oil passage  122  is connected with the first elephant foot assembly  140 . The valve bridge body  410  is provided with an outer-side connection passage  412 . The first elephant foot assembly  140  is connected with the secondary driving piston  420  through the outer-side connection passage  412 . 
     When the outer-side exhaust valve  320  is connected the secondary driving piston  420 , the limit assembly  500  is a limit rod  510 . The limit rod  510  may be provided with a flat or spherical end surface, and the limit rod  510  may also be provided with an elephant foot end surface structure. The limit rod  510  is located directly above the oil drain passage  430 . When the limit rod  510  is in contact with the valve bridge body  410 , the limit rod  510  seals the oil drain passage  430 . In order to adjust the clearance between the limit assembly  500  and the valve bridge assembly  400 , the limit assembly  500  further includes a bracket  520 . The limit rod  510  is adjustably provided on the bracket  520  by screwing, that is, the limit rod  510  is threadedly provided on the bracket  520 . The position of the limit rod  510  on the bracket  520  is adjustable, such that the clearance between the limit assembly  500  and the valve bridge assembly  400  is adjustable. 
     Embodiment 5 
     Embodiment 5 shown in  FIG. 14  only differs from Embodiment 1 in the following. The primary driving piston  130  does not include the adjusting bolt  133 , and the movement stroke of the primary piston body  131  along the axial direction of the primary piston hole  112  is adjustable through the first elephant foot assembly  140 . 
     Embodiment 6 
     Embodiment 6 shown in  FIG. 15  only differs from Embodiment 1 in the following. The drive control valve  150  is provided on the rocker arm shaft  170 ; and each of an end of the primary piston oil passage  121  and an end of the secondary piston oil passage  122  connected with the drive control valve  150  is an annular passage coaxial with the rocker arm shaft  170 . 
     Embodiment 7 
     Embodiment 7 shown in  FIG. 16  only differs from Embodiment 1 in the following. The lower end of the primary piston body  131  is not provided with the roller pin  134  and the roller  135 . The lower end surface of the primary piston body  131  directly contacts the integrated cam  210 . There is no need to provide an anti-rotation assembly between the primary piston body  131  and the primary piston hole  112 , and only an anti-dropping assembly is required. In this embodiment, the anti-dropping assembly is a limit pin  136 . The limit pin  136  is fixedly provided on an inner circumferential surface of the primary piston hole  112 . The primary piston body  131  is provided with a limit opening  131   a . The limit opening  131   a  is provided along an axial direction of the primary piston body  131 . The limit pin  136  is inserted in the limit opening  131   a . The length of the limit opening  131   a  is greater than or equal to a movement stroke of the primary piston body  131 . The limit pin  136  restricts the primary piston body  131  from moving out of the primary piston hole  112 . 
     Embodiment 8 
     Embodiment 8 shown in  FIG. 17  only differs from Embodiment 1 in the following. In order to meet the requirements of different manufacturing processes, the rocker arm body  110  is provided with an insert  115  in an interference fit manner, and a lower surface of the insert  115  is a limit surface  111 . The insert  115  may be separately hardened or treated by other process to enhance the flexibility of process design. 
     Embodiment 9 
     A self-resetting single-valve double-piston hydraulic drive method for an overhead cam engine, where an engine drive solenoid valve has two working states, namely an opened state and a closed state. 
     When the engine drive solenoid valve is in the opened state, a rotation process along an integrated cam  210  is implemented by the following steps: 
     Step 1: When the engine drive solenoid valve is opened, a rocker arm shaft oil supply passage  123   a , a connecting oil supply passage  123   b  and the bottom of a control valve hole  113  are filled with oil. An oil pressure of the bottom of the control valve hole  113  is greater than a force of an elastic return element  153  acting on a control valve body  151 . The control valve body  151  moves up to an opened position under the action of the oil pressure. A primary control valve oil passage  151   a  connects a primary piston oil passage  121  with a secondary piston oil passage  122 . 
     Step 2: When the integrated cam  210  rotates to a cam base circle  211 , a one-way valve ball  152   a  is opened under an oil pressure of a secondary control valve oil passage  151   b  to connect the primary control valve oil passage  151   a . Lubricating oil flows into a drive control valve  150  and an entire driving oil passage  120 . A primary piston hole  112  is filled with oil. A rocker arm body  110  is biased to a side of a valve bridge body  410  under the action of an oil pressure of the primary piston hole  112 . A limit assembly  500  is tightly attached to an upper surface of the valve bridge body  410 , such that the secondary piston oil passage  122  is connected with an oil drain passage  430  on the upper surface of the valve bridge body  410 . The primary piston hole  112  and a secondary piston hole  413  are connected through the primary piston oil passage  121 , the primary control valve oil passage  151   a , the secondary piston oil passage  122  and the oil drain passage  430 , and are filled with the lubricating oil simultaneously. 
     Step 3: During a drive lift of the integrated cam  210 , an integrated cam assembly  200  pushes the primary piston body  131  upward, such that the primary piston body  131  moves upward along the primary piston hole  112 . The one-way valve ball  152   a  seals a communication passage  151   c , and the drive control valve  150  is in a locked state. The lubricating oil in the primary piston hole  112  is pressed into the secondary piston hole  413 , such that a primary driving piston  130  and a secondary driving piston  420  form a hydraulic linkage. The secondary driving piston  420  pushes away an inner-side exhaust valve  310  connected to the secondary driving piston  420 . The rocker arm body  110  and the valve bridge body  410  do not move. Thus, a drive function of a drive mechanism is realized. 
     Step 4: During a positive-power exhaust lift of the integrated cam  210 , the integrated cam assembly  200  pushes the primary piston body  131  upward, such that the primary piston body  131  moves upward along the primary piston hole  112 . When the primary piston body  131  is pressed against the bottom of an adjusting bolt  133  or the bottom of the primary piston hole  112 , the primary piston body  131  and the rocker arm body  110  are rigidly connected. The rocker arm body  110  starts to rotate, and a valve bridge assembly  400  is pushed down through a first elephant foot assembly  140 . A limit surface  111  is separated from the upper surface of the valve bridge body  410 , and the oil drain passage  430  on the upper surface of the valve bridge body  410  is automatically opened. The secondary piston body  421  is in contact with the bottom of the secondary piston hole  413 . The valve bridge assembly  400  is automatically reset to restore normal valve movement. 
     When the engine drive solenoid valve is in the closed state, the rotation process along the integrated cam  210  is implemented by the following steps: 
     Step 1: When the engine drive solenoid valve is closed, no oil pressure exists in the rocker arm shaft oil supply passage  123   a . The control valve body  151  is in the closed position at the bottom of the control valve hole  113  under the action of the elastic return element  153 . The primary piston oil passage  121  and the secondary piston oil passage  122  are not connected. No oil pressure exists in the control valve oil supply passage  123 . The primary piston oil passage  121  and the primary piston hole  112  are filled with the lubricating oil. The secondary piston oil passage  122  is connected with an open chamber, and no oil pressure exists in the secondary piston oil passage  122 . 
     Step 2: When the integrated cam  210  rotates to the cam base circle  211 , the lubricating oil fills the primary piston hole  112 . The rocker arm assembly  100  is biased to a side of the valve bridge assembly  400 . The primary driving piston  130  absorbs the drive lift of the integrated cam assembly  200  for the rocker arm body  110 . The primary driving piston  130  and the rocker arm body  110  form a hydraulic clearance adjuster to adjust a valve clearance. 
     Step 3: During the drive lift of the integrated cam  210 , the integrated cam assembly  200  pushes the primary piston body  131  upward, such that the primary piston body  131  moves upward along the primary piston hole  112 . The rocker arm body  110  is still biased to the side of the valve bridge body  410  under the action of the oil pressure of the primary piston hole  112 , but does not rotate. The drive lift of the integrated cam  210  is absorbed by the primary driving piston  130  and is not transmitted to the side of the valve bridge body  410 . Thus, a positive-power “lost motion” function of the drive mechanism is realized. 
     Step 4: During the positive-power exhaust lift of the integrated cam  210 , the integrated cam assembly  200  pushes the primary piston body  131  upward, such that the primary piston body  131  moves upward along the primary piston hole  112 . When the primary piston body  131  is pressed against the bottom of the adjusting bolt  133  or the bottom of the primary piston hole  112 , the primary piston body  131  and the rocker arm body  110  are rigidly connected. The rocker arm body  110  rotates, and the valve bridge assembly  400  is pushed down through the first elephant foot assembly  140  to achieve positive valve movement. 
     Under the inspiration of the above ideal embodiment of the present invention, a skilled person can absolutely make various changes and modifications through the above description content without departing from the scope of the technical idea of the present invention. The technical scope of the present invention is not limited to the content of the description, which must be determined according to the scope of the claims.