Patent Publication Number: US-8991350-B2

Title: Reset type rocker braking method and device

Description:
TECHNICAL FIELD 
     The present application relates to the mechanical field, specifically to the valve actuation field for vehicle engines, particularly to method and device for a reset rocker arm braking. 
     BACKGROUND 
     In an engine braking method, an exhaust valve is opened at a later stage of a compression stroke of an engine piston and closed at an earlier stage of an expansion stroke (generally before the normal opening of the exhaust valve). An example of an engine brake was disclosed by Cummins in the disclosure of U.S. Pat. No. 3,220,392 in 1965. In the brake system, a mechanical input is transmitted to an exhaust valve to be opened through a hydraulic circuit. The hydraulic circuit generally includes a brake piston reciprocating in a brake piston bore. The reciprocating motion comes from the mechanical input of the engine, such as the motion of the engine&#39;s fuel injection cam or the neighboring exhaust cam. The motion of the brake piston is transmitted through hydraulic fluid to a slave piston located in the hydraulic circuit, causing the slave piston to reciprocate in a slave piston bore. The slave piston acts, directly or indirectly, on the exhaust valve, thereby generating the valve event for the engine braking operation. 
     The engine brake device disclosed by Cummins is a bolt-on accessory that fits above the engine. In order to mount the engine brake, a spacer needs to be provided between the cylinder and the valve cover, thus the height, weight and cost of the engine are additionally increased. Obviously, the solution to the above problems is to integrate the components of the braking device into the existing components of the engine, such as into the rocker arm of the engine, thereby forming an integrated brake. 
     An integrated rocker-arm brake was disclosed by the Mack Truck Company of the United States in U.S. Pat. No. 3,786,792 in 1974. The brake piston of the brake system is positioned in a rocker-arm cylinder arranged at one end close to a push rod and is hydraulically locked in a protruding position, so as to transmit the motion of the cam to an exhaust valve (there is only one valve per cylinder in an early engine), thereby producing the engine braking operation. A conventional cam lobe and a braking cam lobe are integrated in the above cam. The brake control valve mechanism (a combination of a funnel-shaped plunger valve and a one-way ball valve) in the above brake system was widely used after its disclosure. 
     Another integrated rocker-arm brake was disclosed by the Jacobs Company (JVS) of the United States in U.S. Pat. No. 3,809,033 in 1974. The brake piston of the brake system is positioned in a rocker-arm cylinder arranged at one end close to a valve bridge and is movable between a non-braking position and a braking position. In the braking position, the brake piston is hydraulically locked in a protruding position, so as to transmit the motion of the cam to the valve bridge to open two exhaust valves (the engine has two valves per cylinder), thereby producing the engine braking operation. The braking system uses two separate oil passages, one for supplying oil to the brake, and the other being a conventional engine lubrication oil passage. 
     An integrated rocker-arm brake system for an overhead cam four-valve engine was disclosed by Sweden&#39;s Volvo Company in U.S. Pat. No. 5,564,385 in 1996, which is very similar in both structure and principle to the integrated rocker-arm brake disclosed by Jacobs Company (JVS) in U.S. Pat. No. 3,809,033 in 1974. The hydraulic brake piston is positioned in a rocker-arm cylinder arranged at one end close to the valve bride, and is movable between a non-braking position and a braking position and forms a gap in the engine valve system. Oil with a certain pressure is supplied to the brake piston by a pressure control valve to fill the gap in the rocker arm so as to form a hydraulic linkage. The engine braking system adopted the combined structure having a funnel-shaped plunger valve and a one-way ball valve, and added an overload pressure relief mechanism and an oil supply device for providing dual oil pressures via a single oil passage, wherein a low oil pressure (below the engine lubricating oil pressure) is used for the engine lubrication, and a high oil pressure (equal to the engine lubricating oil pressure) is used for the engine brake. During engine braking, the brake piston pushes the valve bridge to open the two exhaust valves simultaneously for braking operation. 
     Another integrated rocker-arm brake was disclosed by the Mack Truck Company of the United States in U.S. Pat. No. 6,234,143 in 2001, which is quite different from the technology disclosed in U.S. Pat. No. 3,786,792 in 1974. First of all, an Exhaust Gas Recirculation (EGR) cam lobe was added to the integrated cam formed with the conventional cam lobe and the braking cam lobe, which facilitates improving the braking power. Secondly, the engine with a single valve per cylinder is changed into an engine with dual valves per cylinder, and a valve bridge (an air valve bridge or a cross arm) was added. Further, the brake piston in the rocker-arm piston bore is moved from the push rod side to the valve bridge side, and is located above the exhaust valve (an inner valve) next to the rocker-arm shaft. During braking, the brake piston opens one exhaust valve via a braking-push-rod or by a direct action on the valve bridge. However, since only one valve is opened for braking, the valve bridge is in an inclined state and an asymmetric load will be generated on the valve bridge and the rocker arm. Furthermore, the braking valve (the inner valve) lift profile is greater than the non-braking valve (an outer valve) or the conventional valve lift profile (larger opening and later closing). 
     An integrated rocker-arm brake system having a valve lift reset mechanism was disclosed by Cummins Engine Company in U.S. Pat. No. 6,253,730 in 2001 to resolve the problems of the one-valve (the inner valve) braking, such as the asymmetric load and the braking valve (the inner valve) lift profile being greater than the non-braking valve (the outer valve) or the conventional valve lift profile (larger opening and later closing). The valve lift reset mechanism resets or retracts the brake piston in the rocker arm before the braking valve reaches its peak valve lift, which allows the braking valve to return to the valve seat before the start of the main valve action, such that the valve bridge returns to the horizontal position, and the rocker arm can open the braking valve and the non-braking valve evenly, thereby eliminating any asymmetric load. 
     However, there are a lot of problems with resetting the engine braking system before the braking valve reaching its peak valve lift. Firstly, during engine braking, both the opening time and the lift magnitude of the braking valve are very short, thus the time for resetting is very limited. Secondly, the resetting occurs when the engine braking load is close to the maximum (i.e. the top dead center of the compression stroke), thereby causing the reset valve of the valve lift reset mechanism to bear a high oil pressure or a large load. Thus, the engine brake resetting timing is essential. If the resetting occurs too early, the loss of braking valve lift is too much (causing a lower valve lift and the valve to be closed too early), which may reduces the braking performance. If the resetting occurs too late, the braking valve can not be closed before the start of the main valve action, which may results in an asymmetric load. 
     Tests show that the integrated rocker-arm brake cannot work properly at high engine speeds, because the resetting time is too short, the resetting height is too small, and the load or pressure on the reset valve is very high. 
     SUMMARY 
     An object of the present application is to provide a method for a reset rocker arm braking to solve the technical problems of the existing engine braking technologies, for example poor reliability and durability, an asymmetric braking load or unstable resetting, inconvenience in installation and adjustment, and increased height and weight of the engine. 
     The method for a reset rocker arm braking according to the present application includes a process of utilizing an exhaust valve actuator of an engine to open an exhaust valve, the engine including an engine brake control mechanism, the exhaust valve actuator including a cam and a rocker arm, the cam including an exhaust cam lobe and at least one brake cam lobe, the exhaust cam lobe being higher than the brake cam lobe, the rocker arm being provided with a brake oil supply passage, and the method for a reset rocker arm braking includes: arranging a brake piston bore opened downward at a lower side of one end of the rocker arm, slidably disposing a brake piston in the brake piston bore, the brake piston having an extended position and a retracted position in the brake piston bore, communicating the brake oil supply passage in the rocker arm with the brake piston bore, arranging a one-way oil supply valve between the brake piston bore and the brake oil supply passage or within the brake oil supply passage, the one-way oil supply valve having an oil supply direction from the brake oil supply passage to the brake piston bore, arranging an oil drain piston bore opened upwards at an upper side of the same end of the rocker arm, slidably disposing an oil drain piston in the oil drain piston bore, arranging an oil drain passage between the oil drain piston bore and the brake piston bore, wherein the process of utilizing the exhaust valve actuator of the engine to open the exhaust valve includes the following steps: firstly, turning on the brake control mechanism, supplying oil to the oil drain piston bore and the brake piston bore simultaneously through the brake oil supply passage, placing the brake piston at the extended position and placing the oil drain piston at a position for closing the oil drain passage; then, utilizing the brake cam lobe of the cam to drive the rocker arm and the brake piston at the extended position to open at least one exhaust valve under the brake piston; and then utilizing a rising section of the exhaust cam lobe of the cam which is higher than the brake cam lobe to keep driving the rocker arm, and at the same time, utilizing a motion of the rocker arm to change a position of the oil drain piston in the oil drain piston bore so as to open the drain oil passage to drain oil in the brake piston bore, moving the brake piston from the extended position to the retracted position, and skipping a part of the actuation onto the exhaust valve from a top portion of the exhaust cam lobe; finally, utilizing a descending section of the exhaust cam lobe of the cam to drive the rocker arm to rotate backwards, utilizing the backward motion of the rocker arm to change the position of the oil drain piston in the oil drain piston bore so as to close the oil drain passage, and at the same time, supplying oil to the brake piston bore through the brake oil supply passage and the one-way oil supply valve, re-placing the brake piston at the extended position, and starting a new engine braking cycle. 
     The method further includes: arranging a reset stopper mechanism at an upper side of the rocker arm at the end where the oil drain piston bore is located, and the reset stopper mechanism being fixed on the engine and configured to limit a motion of the oil drain piston in the oil drain piston bore. 
     The method further includes: utilizing a preload spring to maintain a gap in an exhaust valve drive chain formed by the retracted position and the extended position of the brake piston, and to eliminate any no-follow and impact within the exhaust valve drive chain. 
     Further, the process of utilizing the exhaust valve actuator of the engine to open the exhaust valve includes the following steps: 
     1) turning on the engine brake control mechanism, 
     2) supplying oil to the oil drain piston bore in the rocker arm through the brake oil supply passage, 
     3) placing the oil drain piston in the oil drain piston bore at a position for closing the oil drain passage, 
     4) supplying oil to the brake piston bore in the rocker arm through the brake oil supply passage and the one-way oil supply valve, 
     5) placing the brake piston at the extended position in the brake piston bore to form a locked hydraulic linkage, 
     6) driving the rocker arm and the extended brake piston by the brake cam lobe of the cam to open at least one exhaust valve, and producing a brake valve lift, 
     7) moving the cam into the rising section of the exhaust cam lobe which is higher than the brake cam lobe, and continuing to drive the rocker arm, the extended brake piston and the exhaust valve, 
     8) moving the oil drain piston in the oil drain piston bore from the position for closing the oil drain passage to a position for opening the oil drain passage so as to drain oil through the brake piston bore, 
     9) moving the brake piston in the brake piston bore from the extended position to the retracted position, and resetting and reducing an exhaust valve lift during an exhaust stroke of the engine, 
     10) descending the cam from the maximum lift of the exhaust cam lobe back to an inner base circle of the cam, and moving the rocker arm, the retracted brake piston and the exhaust valve backwards; 
     11) moving the oil drain piston in the oil drain piston bore from the reset position back to the braking position, and re-closing the oil drain passage; and 
     12) returning to step 4) and starting a new engine braking cycle. 
     Further, the process of utilizing the exhaust valve actuator of the engine to open the exhaust valve further includes the following steps: 
     1) turning off the engine brake control mechanism, 
     2) stopping supplying oil to the oil drain piston bore and the brake piston bore in the rocker arm, 
     3) opening the oil drain passage by the oil drain piston to drain oil, 
     4) removing a hydraulic linkage between the brake piston and the rocker arm, and forming a gap, 
     5) rotating the cam upwards from an inner base circle, 
     6) keeping the exhaust valve stationary, and 
     7) rotating the cam into the rising section higher than the brake cam lobe, and driving the rocker arm to open the exhaust valve, and producing a conventional exhaust valve lift. 
     Further, the brake cam lobe includes a compression release cam lobe. 
     Further, the brake cam lobe includes an exhaust gas recirculation cam lobe. 
     The present application also provides a reset rocker arm brake device including a brake control mechanism, a brake actuation mechanism, an exhaust valve actuator and at least one exhaust valve, wherein the exhaust valve actuator includes a cam and a rocker arm, the cam includes an exhaust cam lobe and at least one brake cam lobe, and the exhaust cam lobe is higher than the brake cam lobe, and the brake control mechanism includes a control valve connected to a hydraulic pressure generating device, the brake actuation mechanism includes an oil supply mechanism, an oil drain mechanism and a brake piston, the brake piston has an extended position and a retracted position in a brake piston bore in the rocker arm, a lower end of the brake piston is connected to at least one exhaust valve, the oil supply mechanism includes a brake oil supply passage and a one-way oil supply valve, the control valve of the brake control mechanism is connected to an inlet of the brake oil supply passage, and an outlet of the brake oil supply passage is communicated with the brake piston bore, the one-way oil supply valve is arranged between the brake oil supply passage and the brake piston bore, or within the brake oil supply passage, the one-way oil supply valve has an oil supply direction from the brake oil supply passage to the brake piston bore, the oil drain mechanism includes an oil drain valve and an oil drain passage, the oil drain valve is communicated to the brake piston bore through the oil drain passage, and the opening and closing of the oil drain valve is controlled by a distance between the rocker arm and the engine. 
     Further, the brake actuation mechanism further includes a preload spring configured to maintain a gap in an engine exhaust valve drive chain formed by the retracted position and the extended position of the brake piston, so as to eliminate any no-follow and impact among members of the exhaust valve drive chain. 
     Further, the brake actuation mechanism further includes a position-limiting mechanism configured to limit a stroke of the brake piston in the brake piston bore. 
     Further, the oil drain valve includes an oil drain piston disposed in an oil drain piston bore in the rocker arm, the oil drain piston has a braking position and a reset position in the oil drain piston bore, the oil drain piston bore has a bottom portion communicated with the brake oil supply passage and a middle portion communicated with one end of a drain oil passage, and the other end of the oil drain passage is communicated with the brake piston bore; at the braking position, the oil drain piston closes the oil drain passage; and at the reset position, the oil drain piston opens the oil drain passage. 
     Alternatively, the oil drain valve includes an oil drain piston disposed in an oil drain piston bore in a valve lash adjusting screw, the oil drain piston has a braking position and an oil drain position in the oil drain piston bore, the valve lash adjusting screw further includes an oil drain passage having one end communicated with a bottom of the oil drain piston bore and the other end communicated with the brake piston bore; at the braking position, the oil drain piston is located at the bottom of the oil drain piston bore and the oil drain passage is closed; and at the reset position, the oil drain piston is located at a top of the oil drain piston bore and the oil drain passage is opened. 
     Further, the brake actuation mechanism further includes a reset stopper mechanism which is fixed on the engine above one end of the rocker arm having an oil drain piston bore and configured to limit a motion of an oil drain piston in the oil drain piston bore. 
     The working principle of the present application is described as follows. When engine braking is required, the brake control mechanism is turned on and the control valve supplies oil to the brake actuation mechanism. Engine oil with low pressure (i.e. the engine lubrication oil) flows into the brake piston bore through the oil supply passage and the one-way oil supply valve. The brake piston is at the extended position in the brake piston bore in the rocker arm, the oil drain piston is at the braking position in the oil drain piston bore of the rocker arm, and the oil drain passage between the brake piston bore and the oil drain piston bore is closed. The brake cam lobe of the cam moves upwards from the inner base circle to drive the rocker arm and the brake piston which is at the extended position and hydraulically locked in the brake piston bore so as to open the exhaust valve for braking. 
     When the exhaust cam lobe of the cam moves upward to a position higher than the brake cam lobe, the oil drain piston is moved from the braking position to the reset position in the oil drain piston bore in the rocker arm, thereby opening the oil drain passage between the brake piston bore and the oil drain piston bore to drain oil out of the brake piston bore. The brake piston is moved from the extended position to the retracted position, and the exhaust valve lift is reset and reduced to the conventional exhaust valve lift profile without the engine brake. 
     The present application has positive and significant effects over the prior art. The present application integrates the engine braking mechanism and the reset oil drain mechanism into the existing rocker arm of the engine, thereby simplifying the design, forming a compact structure, reducing the weight and height of the engine, increasing the engine braking power, and improving reliability and durability of the engine operation. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram showing a reset rocker arm brake device according to a first embodiment of the present application when an engine braking device is at an “OFF” position; 
         FIG. 2  is a schematic diagram showing the reset rocker arm brake device according to the first embodiment of the present application when the engine braking device is at an “ON” position; 
         FIG. 3  is a schematic diagram showing an oil drain piston of a reset rocker arm brake device of the present application; 
         FIG. 4  is a schematic diagram showing a brake control mechanism at an “ON” position for the reset rocker arm brake device of the present application; 
         FIG. 5  is a schematic diagram showing the brake control mechanism at an “OFF” position for the reset rocker arm brake device of the present application; 
         FIG. 6  is a schematic diagram showing a cam profile for the reset rocker arm brake device of the present application; 
         FIG. 7  is a schematic diagram showing an exhaust valve lift profile and an intake valve lift profile for the reset rocker arm brake device of the present application; 
         FIG. 8  is a schematic diagram showing a reset rocker arm brake device according to a second embodiment of the present application when a cam is on an inner base circle; 
         FIG. 9  is a schematic diagram showing a reset rocker arm brake device according to a third embodiment of the present application when an engine braking device is at an “OFF” position; and 
         FIG. 10  is a schematic diagram showing a reset rocker arm brake device according to a fourth embodiment of the present application when an engine braking device is at an “OFF” position. 
     
    
    
     DETAILED DESCRIPTION 
     First Embodiment 
       FIGS. 1 and 2  show a reset rocker arm brake device according to a first embodiment of the present application with an engine braking device at an “OFF” position and an “ON” position respectively. There are four major parts: an exhaust valve actuator  200 , an exhaust valve mechanism  300 , an engine brake actuation mechanism  100  and a reset stopper mechanism  150 . 
     The exhaust valve actuator  200  includes a cam  230 , a cam follower  235 , a push rod  201  and a rocker arm  210 . The exhaust valve actuator  200  and the exhaust valve mechanism  300  are collectively referred to an exhaust valve drive chain. The rocker arm  210  has a valve lash adjusting system on an end near the cam  230 . A valve lash adjusting screw  110  is fixed on the rocker arm  210  via a lock nut  105 . The rocker arm  210  is swingably mounted on a rocker arm shaft  205 . 
     The exhaust valve  301  is held onto a valve seat  320  in an engine block  500  via a valve spring  310  to prevent gas (air during engine braking) from flowing between an engine cylinder and an exhaust duct  600 . The exhaust valve actuator  200  transmits the mechanical motion of the cam  230  to the exhaust valve  301  through the rocker arm  210 , so as to periodically open and close the exhaust valve  300 . 
     The cam  230  integrates dual functions of conventional exhaust and braking of the engine. On its inner base circle  225 , the cam  230  has an enlarged cam lobe  220  mainly used for the conventional exhaust operation of the engine. The enlarged cam lobe  220 , also referred to as an integrated exhaust cam lobe, is larger than a conventional exhaust cam lobe (without engine braking) because the cam  230  also has small cam lobes  232  and  233  for engine braking. A bottom of the enlarged cam lobe  220  must have a transitional portion having about the same height as the small cam lobes  232  and  233  so as to skip the braking cam lobes  232  and  233  during the engine conventional operation (i.e. an ignition operation). A top portion of the enlarged cam lobe  220  is equivalent to the conventional exhaust cam lobe. The small cam lobe  232  is used for an Exhaust Gas Recirculation (EGR) during the braking, and the small cam lobe  233  is used for compression release during the braking. Cam lift profiles generated by the enlarged cam lobe  220  and the small cam lobes  232  and  233  of the cam  230  are described in detail in  FIG. 6 . 
     The brake actuation mechanism  100  includes an oil supply mechanism, an oil drain mechanism and a brake piston  160 . 
     The brake piston  160  is placed in a brake piston bore  190  in the rocker arm  210 . The brake piston  160  has an extended position and a retracted position in the brake piston bore  190 . A lower end of the brake piston  160  is connected to the exhaust valve  301  via an elephant foot pad  114 . Of course, the brake piston  160  can also directly act on the exhaust valve  301 . A preload spring  198  is placed between the brake piston  160  and the rocker arm  210 . The preload spring  198  can be a coil spring or other forms of springs, and can be installed in different ways or at different locations, to achieve an object of maintaining a gap  234  in the exhaust valve drive chain generated by the brake piston  160  moving between the extended position and the retracted position, and eliminating any no-follow and impact between the components in the exhaust valve drive chain. The brake piston  160  is also provided with a stopping groove  137  which is combined with a stopping pin  142  in the rocker arm  210  to form a position-limiting mechanism so as to limit a stroke of the brake piston  160  in the brake piston bore  190 . 
     The oil supply mechanism includes brake oil supply passages and a one-way oil supply valve  172 . For simplicity, the engine lubrication oil passages in the rocker arm shaft  205  and in the rocker arm  210  are not shown. The brake oil-supply passages include an axial hole  211  and a radial hole  212  both arranged in the rocker arm shaft  205 , a notch  213  and an oil passage  214  both arranged in the rocker arm  210 . An outlet of the oil passage  214  is communicated with the brake piston bore  190 . The one-way oil supply valve  172  is placed between the oil passage  214  and the brake piston bore  190 , and has an oil supply direction from the oil passage  214  into the brake piston bore  190 . A valve ball of the one-way oil supply valve  172  is biased on a valve seat via a spring  156 . In practical application, the one-way oil supply valve  172  can be further provided with a spring seat or be mounted in different manners. 
     The oil drain mechanism includes an oil drain valve and an oil drain passage  219 . The oil drain valve includes an oil drain piston  170  arranged in an oil drain piston bore  183  in the exhaust rocker arm  210 . The oil drain piston  170  has three different positions in the oil drain piston bore  183 : a non-braking position, a braking position and an oil drain position. The oil drain piston bore  183  has a bottom portion communicated with the oil passage  214 , and a middle portion communicated with one end of the oil drain passage  219 , and the other end of the oil drain passage  219  is communicated with the brake piston bore  190 . In the non-braking position as shown in  FIG. 1 , the oil drain piston  170  is located at the bottom portion of the oil drain piston bore  183 , and the oil drain passage  219  is open. In the braking position as shown in  FIG. 2 , the oil drain piston  170  is located at the middle portion of the oil drain piston bore  183 , and the oil drain passage is closed. In a reset position, the oil drain piston  170  is located at a top portion of the oil drain piston bore  183 , and the oil drain passage is open again. Therefore, the opening and closing of the oil drain valve is determined by the position of the oil drain piston  170  in the oil drain piston bore  183 . 
     The reset stopper mechanism  150  is located above the rocker arm  210  at a side having the oil drain piston bore  183 , and includes a stopper support  125  fixed on the engine, an adjusting screw  1102  and a lock nut  1052 . The reset stopper mechanism limits the motion of the oil drain piston  170  in the oil drain piston bore  183 , thereby controlling the opening and closing of the oil drain valve. That is, the opening and closing of the oil drain valve is controlled by a distance between the rocker arm  210  and the engine or between the rocker arm  210  and the reset stopper mechanism fixed on the engine. The maximum stroke of the oil drain piston  170  in the oil drain piston bore  183  is controlled by a screw  179  mounted on the rocker arm  210 . The screw  179  can also be replaced by a snap ring or other positioning parts. In addition, if desired, a spring can also be arranged on the oil drain piston  170 . 
     When engine braking is required, the brake control mechanism  50  is turned on as shown in  FIG. 4 . A control valve  51  of the brake control mechanism is communicated with an entrance of an axial oil passage  211  in the rocker arm shaft  205  and supplies oil to the brake actuation mechanism  100  through other brake oil supply passages. Engine oil with low pressure pushes the oil drain piston  170  from the non-braking position (as shown in  FIG. 1 ) to the braking position (as shown in  FIG. 2 ) in the oil drain piston bore  183 . The oil drain piston  170  stops at a bottom of the adjusting screw  1102  of the reset stopper mechanism  150 , thereby closing the oil drain passage  219 . At the same time, engine oil is supplied to the brake piston bore  190  through the one-way oil supply valve  172 . The brake piston  160  is at the extended position in the brake piston bore  190  in the rocker arm  210 , thus a gap  234  is formed between the brake piston  160  and a bottom surface of the brake piston bore  190  (i.e. the rocker arm  210 ). 
     When the small brake cam lobe  233  (i.e. the compression release cam lobe) on the cam  230  rises from the inner base circle  225 , the rocker arm  210  rotates clockwise, driving downward the brake piston  160  which is hydraulically locked at the extended position in the brake piston bore  190  of the rocker arm  210 , and opening the exhaust valve  301  below the brake piston  160 . Although there is the gap  234  between the brake piston  160  and the rocker arm  210 , a hydraulically locked linkage is generated between the brake piston  160  and the rocker arm  210  by the engine oil due to the one-way oil supply valve  172  and the closed oil drain passage  219 , such that the motion of the small cam lobes  233  and  232  can be transmitted to the exhaust valve  301 . 
     In the process of rotating clockwise driven by the cam  230 , the rocker arm  210  is moved away from a contact position with the reset stopper mechanism  150  as shown in  FIG. 2 , and the oil drain piston  170  in the oil drain piston bore  183  is also moved upwards from the braking position. However, due to the structure of the oil drain valve, the motion from the small cam lobe  233  is insufficient to open the oil drain passage  219 . Thus, a top surface  147  of the oil drain piston  170  (as shown in  FIG. 3 ) is still in contact with the adjusting screw  1102 , and the oil drain passage  219  remains blocked by an outer wall of the oil drain piston  170 . 
     In a case that the brake cam lobe of the cam  230  also includes a small cam lobe  232  for exhaust gas recirculation (EGR), the process for driving the exhaust valve  301  by the small cam lobe  232  is the same as the process for driving the exhaust valve  301  by the small cam lobe  233 , which will not be described herein. 
     When the cam  230  rotates into a rising segment of the exhaust cam lobe  220  which is higher than the small cam lobe  233 , the rocker arm  210  is separated from the reset stopper mechanism  150  with an enough distance, the oil drain piston  170  is moved upwards to the reset position in the oil drain piston bore  183 , and an annular groove  180  on the oil drain piston  170  (as shown in  FIG. 3 ) opens the oil drain passage  219  in  FIG. 2  to drain oil out of the brake piston bore  190 . The brake piston  160  is moved upwards to eliminate the gap  234 , and the exhaust valve motion generated by the small cam lobe  233  is lost, and the valve lift of the exhaust valve  301  is reset to the smaller conventional exhaust valve lift profile without an engine brake. 
     When the cam  230  rotates into a top portion of the exhaust cam lobe  220 , the top surface  147  of the oil drain piston  170  (as shown in  FIG. 3 ) will be separated from the adjusting screw  1102 , a stepped surface  148  of the oil drain piston  170  (as shown in  FIG. 3 ) is in contact with a stopping screw  179  (as shown in  FIG. 2 ) on the rocker arm, and the oil drain passage  219  is fully opened by the annular groove  180  on the oil drain piston  170 . 
     When the cam  230  rotates over the highest position of the exhaust cam lobe  220  and descends back to the inner base circle  225 , the rocker arm  210 , the brake piston  160  retracted in the brake piston bore  190  and the exhaust valve  301  below the brake piston  160  are all moved backwards under the action of the valve spring  310 . The oil drain piston  170  in the oil drain piston bore  183  is moved downwards from the reset position back to the braking position, thereby re-closing the oil drain passage  219 . Engine oil is again supplied to the brake piston bore  190  through the one-way oil supply valve  172 , the brake piston  160  in the brake piston bore  190  is moved from the retracted position back to the extended position, thereby starting a new cycle of engine braking. 
     When the engine brake is not required, the brake control mechanism  50  is turned off as shown in  FIG. 5 . The control valve  51  stops supplying oil to the brake actuation mechanism  100 . Without the oil pressure, the oil drain piston  170  is at the non-braking position in the oil drain piston bore  183  as shown in  FIG. 1 , and the oil drain passage  219  is opened, thereby eliminating the hydraulic linkage formed between the brake piston  160  and the rocker arm  210 . 
     When the small cam lobe  233  (i.e. the compression release cam lobe) of the cam  230  rises from the inner base circle  225 , the rocker arm  210  rotates clockwise. However, due to the gap  234  between the brake piston  160  and the rocker arm  210 , there is only relative motion between the rocker arm  210  and the brake piston  160 , and the exhaust valve  301  remains stationary. That is, in the non-braking state as shown in  FIG. 1 , the motion of the small cam lobes  233  and  232  is lost due to the gap  234 , and will not be transmitted to the exhaust valve  301 , thereby removing the engine braking operation. 
     When the cam  230  rotates into the rising segment of the exhaust cam lobe  220  which is higher than the small cam lobe  233 , the gap  234  between the brake piston  160  and the rocker arm  210  begins to disappear, and the rocker arm  210  will act directly on the brake piston  160  to open the exhaust valve  301 . That is, in the non-braking state as shown in  FIG. 1 , only the motion of the top portion of the exhaust cam lobe  220  is transmitted to the exhaust valve  301  to create the conventional exhaust valve motion. 
     As shown in  FIGS. 4 and 5 , the brake control mechanism  50  for the reset rocker arm brake device of the present application is at the “ON” position and the “OFF” position respectively. The control valve  51  as shown is a two-position three-way solenoid valve. When the brake control mechanism  50  is turned on (as shown in  FIG. 4 ), a valve body of the control valve  51  is moved downwards to open an oil supply port  111  and to close an oil drain port  222  at the same time, and engine oil with low pressure (i.e. the lubrication oil) flows to the brake actuation mechanism  100  through the brake fluid passages (as shown in  FIGS. 1 and 2 ). When the brake control mechanism  50  is turned off (as shown in  FIG. 5 ), the valve body of the control valve  51  is moved upwardly to close the oil supply port  111  and to open the oil drain port  222  at the same time, and engine oil with low pressure (i.e. the lubrication oil) stops flowing to the brake actuation mechanism  100  (as shown in  FIGS. 1 and 2 ), and the brake actuation mechanism  100  drains oil through the brake fluid passages and the oil drain port  222 . Since the oil drain passage  219  is arranged between the brake piston bore  190  and the oil drain piston bore  183  in the rocker arm  210  (as shown in  FIGS. 1 and 2 ), oil may be drained once per cycle, thus a two-position two-way solenoid valve may be used to replace the three-way solenoid valve, that is, the drain port  222  is not required. 
       FIG. 6  shows a cam profile of the reset rocker arm brake device of the present application, which includes brake cam lobes and an integrated exhaust cam lobe  220 , wherein a reference numeral  225  denotes the inner base circle of the cam. The brake cam lobes include small cam lobes  233  and  232 . The integrated exhaust cam lobe  220  is divided into a top portion and a bottom portion (separated by a double dotted line as shown in  FIG. 6 ). The bottom portion of the integrated exhaust cam lobe  220  is the transitional portion and has about the same height as the brake cam lobes. The top portion of the integrated exhaust cam lobe  220  is nearly identical to the conventional cam lobe of an engine. Thus, in a non-braking operation (for example a conventional ignition), the motion from the bottom portion of the integrated exhaust cam lobe as well as the motion from the brake cam lobes (i.e. the small cam lobes  233  and  232 ) are skipped or lost due to the gap  234  in the exhaust valve drive chain (as shown in  FIG. 1 ), and will not be transmitted to the exhaust valve  301 ; and only the motion from the top portion of the integrated exhaust cam lobe  220  is transmitted to the exhaust valve  301 , thereby generating the conventional valve lift motion. 
       FIG. 7  shows an exhaust valve lift profile and an intake valve lift profile for the reset rocker arm brake device of the present application. An engine conventional exhaust valve lift profile  220   m  has a starting point  225   a , an end point  225   b , and the highest lift  220   b . In a case that no oil drain passage  219  (the oil drain passage  219  is shown in  FIGS. 1 and 2 ) is arranged between the brake piston bore  190  and the oil drain piston bore  183  in the rocker arm  210 , an enlarged main valve lift profile  220   v  generated by the integrated exhaust cam lobe  220  during engine braking has a starting point  225   h , an end point  225   c , and the highest lift  220   e  which is a summation of  220   a  and  220   b . Due to the reset effect of the oil drain valve generated through the oil drain passage  219 , the valve lift profile of the exhaust valve  301  begins to transit to the main valve lift profile  220   m  at a transitional point  220   t  between the bottom portion  220   a  and the top portion  220   b  of the enlarged main valve lift profile  220 V, merges into the main valve lift profile  220   m  at a point  220   s , and closes at the end point  225   b  earlier than the case without the oil drain passage. The enlarged main valve lift profile  220   v  is reset and reduced to the conventional valve lift profile  220   m , and the reset point  220   s  is between  220   t  and  220   m.    
     During the engine braking operation, the motions of the braking cam lobes (i.e. the small cam lobes  232  and  233 ) are transmitted to the exhaust valve  301  under the brake piston  160  (as shown in  FIG. 2 ) by the rocker arm  210  through a hydraulic linkage  234  and the brake piston  160 , thereby producing a brake valve lift profile  232   v  for exhaust gas recirculation and a brake valve lift profile  233   v  for compression release as shown in  FIG. 7 . The brake valve lift profile  232   v  for exhaust gas recirculation has a starting point  225   d  located in a later stage of the intake stroke of the engine, that is, near a place when an intake valve lift profile  280   v  ends. The brake valve lift profile  232   v  for exhaust gas recirculation has an end point  225   e  located in an earlier stage of the compression stroke of the engine. The brake valve lift profile  233   v  for compression release has a starting point  225   f  located in a later stage of the compression stroke of the engine, and an end point  225   g  located in an earlier stage of the expansion stroke of the engine. The valve lift profile recycles between 0° to 720°, wherein 0° and 720° are the same point. 
     When the integrated exhaust cam lobe  220  of the cam  230  rises from the inner base circle  225  (as shown in  FIG. 7 ), the rocker arm  210  pushes the exhaust valve  301  downwards through the hydraulic linkage  234  and the brake piston  160  (as shown in  FIG. 2 ). When the cam  230  rotates into the top portion of the integrated exhaust cam lobe  220  (as shown in  FIG. 7 , which is greater than the maximum lift of the small cam lobe  233 ), the rocker arm  210  is moved further away from the reset stopper mechanism  150  as shown in  FIG. 2 . The oil drain piston  170  is further moved upwards in the oil drain piston bore  183  to open the oil drain passage  219 , the brake piston bore  190  starts to drain oil, and the brake piston  160  is moved upwards to the retracted position in the brake piston bore  190 . The valve lift profile of the exhaust valve  301  transits to the main valve lift profile  220   m  after the transitional point  220   t  (as shown in  FIG. 7 ), and ends at the end point  225   b  which is significantly ahead of the end point  225   c  in the case without the oil drain passage. In this way, the exhaust valve lift at the top dead center in the engine exhaust stroke is reduced, which avoids the collision between the exhaust valve  301  and the engine cylinder piston, and also increases the braking power and reduces the temperature inside the cylinder. 
     Second Embodiment 
       FIG. 8  shows the reset rocker arm brake device according to a second embodiment of the present application when the cam  230  is on the inner base circle. The present embodiment can be applied on an overhead cam engine, and there is no push rod between the cam  230  and the rocker arm  210 , thus the valve lash adjusting mechanism is placed on the rocker arm  210  at an end close to the exhaust valve  301 . There is no elephant foot pad under the brake piston  160 , and the brake piston  160  acts directly on the exhaust valve  301 . Another difference between this embodiment and the first embodiment is that the one-way oil supply valve  172  of the present embodiment is placed in the oil supply passage  214  and biased to a closed position by a spring  156 . The spring  156  has one end located on the valve ball and the other end located on a spring seat  157  fixed on the rocker arm  210 . 
     Yet another difference between this embodiment and the first embodiment is that the oil drain valve of the present embodiment is placed in the valve lash adjusting mechanism. The oil drain piston  170  is slidably disposed in the oil drain piston bore  183  in the adjusting screw  110 , and an oil drain passage  197  is further arranged in the adjusting screw. 
     When engine braking is required, the brake control mechanism  50  is turned on as shown in  FIG. 4 , and the control valve  51  supplies oil to the brake actuation mechanism  100 . Engine oil with low pressure flows into the brake piston bore  190  through the oil supply passage and the one-way oil supply valve  172  shown in  FIG. 8 . The brake piston  160  is located at the extended position in the brake piston bore  190  to form a hydraulic linkage with the rocker arm  210 . When the brake cam lobe  233  (i.e. the compression release cam lobe) of the cam  230  rises from the inner base circle  225 , the rocker arm  210  rotates clockwise to push down the brake piston  160  which is hydraulically locked at the extended position in the brake piston bore  190  in the rocker arm  210 , thereby opening the exhaust valve  301  under the brake piston  160 . 
     During the process of the rocker arm  210  rotating clockwise driven by the cam  230 , the valve lash adjusting screw  110  on the rocker arm  210  is moved away from a contact position with the reset stopper mechanism  150  shown in  FIG. 8 , and the oil drain piston  170  is also moved upwards in the oil drain piston bore  183 . However, due to the structural design of the oil drain valve, the motion of the small cam lobe  233  is insufficient to open the oil drain passage  219 . Thus, the top surface  147  of the oil drain piston  170  is still in contact with the adjusting screw  1102 , and the oil drain passage  219  is still blocked by the oil drain piston  170 . 
     When the cam  230  rotates into the rising segment of the exhaust cam lobe  220  higher than the small cam lobe  233 , the valve lash adjusting screw  110  on the rocker arm  210  is moved away from the reset stopper mechanism  150  far enough, and the oil drain piston  170  is moved upwards to the reset position in the oil drain piston bore  183  to open the oil drain passage  219 , and the brake piston bore  190  drains oil through the oil drain passages  197  and  219 . The brake piston  160  is moved upwards to eliminate the gap  234  between the adjusting screw  110  and the brake piston  160 , such that the exhaust valve motion produced by the small cam lobe  233  is lost, and the valve lift of the exhaust valve  301  is reset and reduced to the conventional exhaust valve lift profile without the engine brake. 
     When the cam  230  rotates over the highest position of the exhaust cam lobe  220  and descends back to the inner base circle  225  of the cam, the rocker arm  210 , the brake piston  160  retracted in the brake piston bore  190 , and the exhaust valve  301  under the brake piston  160  are all moved backwards under the action of the valve spring  310 . The oil drain piston  170  in the oil drain piston bore  183  is pushed back to the original position from the reset position, thereby re-closing the oil drain passage  219 . Engine oil is again supplied to the brake piston bore  190  through the one-way oil supply valve  172 , and the brake piston  160  is moved back to the extended position from the retracted position in the brake piston bore  190 , thereby starting a new cycle of engine braking. 
     When the engine brake is not required, the brake control mechanism  50  is turned off as shown in  FIG. 5 , and the control valve  51  stops supplying oil to the brake actuation mechanism  100 . Engine oil is drained out of the brake piston bore  190  through the oil drain valve and is not refilled by the oil supply mechanism, thus the hydraulic linkage is no longer formed with the rocker arm  210 . The motions from the small cam lobe  233  and the brake cam lobe  232  are lost due to the gap  234 , and will not be transmitted to the exhaust valve  301 , thereby removing the engine braking operation. Only the motion of the top portion of the exhaust cam lobe  220  is transmitted to the exhaust valve  301  to generate the conventional exhaust valve motion. 
     Third Embodiment 
       FIG. 9  shows the reset rocker arm brake device according to a third embodiment of the present application when the engine brake device is at an “OFF” position. A major difference between the present embodiment and the first embodiment is the engine exhaust valve mechanism  300 . The exhaust valve mechanism  300  of the present embodiment includes two exhaust valves, and therefore is further arranged with a valve bridge  400  (also referred to as a valve cross arm). The elephant foot pad  114  acts on the top of the valve bridge  400  at a central position, such that the rocker arm  210  can open the two exhaust valves simultaneously through the valve bridge  400 . In the present embodiment, the spring  156  for the one-way oil supply valve  172  has a spring seat  157 . 
     Except for opening the two exhaust valves simultaneously during braking, the working principle of the present embodiment is similar to the first embodiment, thus will not be described herein. 
     Fourth Embodiment 
       FIG. 10  shows the reset rocker arm brake device according to a fourth embodiment of the present application when the engine brake device is at an “OFF” position. A main difference between the present embodiment and the third embodiment is that, in the present embodiment only one of the two exhaust valves is opened during braking. An exhaust valve lash adjusting mechanism is further arranged on the rocker arm  210 , and includes a valve lash adjusting screw  110  locked on the rocker arm  210  by a nut  105 . An elephant foot pad  1142  is placed under the adjusting screw  110 . A gap  130  is arranged between the elephant foot pad  1142  and the valve bridge  400 , and has the same function as the gap  234 , which is to skip the motions of the small cam lobes  232  and  233  during the normal operation of the engine. 
     The above description discloses a new reset rocker arm brake device and a method thereof. The above-described embodiments should not be regarded as limiting the scope of the present application, but rather as specific exemplifications representing the present application. Many other variations may be derived from the above embodiments. For example, the reset rocker arm brake device and the method thereof can be applied to both of an overhead cam engine and a push-rod engine, as well as a single-valve engine and a dual-valve engine. For the double-valve engine, the braking operation may be realized by opening only one valve or double valves. 
     Also, the one-way oil supply valve  172  may be in other forms, such as a butterfly valve. The one-way oil supply valve  172  can be placed at different locations, for example, in the brake piston  160  or in the oil supply passage. In addition, the reset stopper mechanism  150  may also in other forms. The oil drain valve may also have different structure and arrangement. Also, the brake piston  160  may be in other forms, such as an “H” form or a “T” form. The bottom of the brake piston  160  may be further arranged with a spring or be connected to an elephant foot pad  114 . 
     In addition, the preload spring  198  may have various forms and arrangements, for example may be a coil spring, or a leaf spring, and can be placed between the rocker arm  210  and the engine, or between the rocker arm  210  and the exhaust valve  301 , or between the rocker arm  210  and the valve bridge  400 , or between the rocker arm  210  the push rod  201 . Therefore, the scope of the present application should not be limited by the above-described specific examples, but is defined by the claims.