Abstract:
A valve control apparatus is provided for an internal combustion engine having a valve and a camshaft. The camshaft has an axis of rotation, a first lobe and a second lobe adjacent to the first lobe. The second lobe is angularly spaced-apart about the axis from the first lobe. The apparatus includes a follower operatively engagable with the camshaft and the valve. The follower has a first operational mode where the first lobe operatively engages the follower on each revolution of the camshaft to open the valve a first time on each revolution. There is a mechanism for selectively putting the follower in a second operational mode where the second lobe operatively engages the follower to open the valve a second time on each revolution of the camshaft. The mechanism puts the follower in the second operational mode on each revolution of the camshaft before the second lobe is fully aligned with the follower. The mechanism returns the follower to the first mode after the valve is opened by the second lobe and before the first lobe fully operatively engages the follower. Maximum opening and closing of the valve by the first lobe is thereby unaffected when the mechanism selectively puts the follower in the second operational mode.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to the valve control apparatuses and, in particular, to valve control apparatuses for diesel engine compression release brakes. 
     Compression release brakes are used to slow diesel powered vehicles such as large tractor trailer units. These brakes work by releasing compressed gases from each cylinder near top dead center of each compression stroke. This removes the rebound effect whereby the compressed gases would tend to drive the piston downwardly and thereby counter the braking effect otherwise created when the pistons compress gases during the compression stroke. Engine brakes are normally operated when a vehicle is coasting downhill and the fuel supply to the engine has been cut off. Wear on the wheel brakes is reduced since an engine brake significantly reduces the braking contribution required from the wheel brakes. 
     At least one exhaust valve on each cylinder is cracked open just before top dead center of each compression stroke when the brake is operational. Some mechanism must be provided, therefore, to open each exhaust valve twice during each engine cycle. The normal exhaust valve opening occurs during the exhaust stroke when the piston is moving upwardly towards the cylinder head. The second exhaust valve opening occurs during braking operation near the top dead center position at the end of the compression stroke. Various mechanisms have been devised to selectively crack open each exhaust valve the second time during each engine cycle. In many engines, for example, a fuel injector mechanism is used to crack open each exhaust valve at the required time. However such a mechanism is not available, nor suitable for all types of engines. Accordingly, alternative mechanisms have been devised. 
     U.S. Pat. Nos. 5,537,976 and 5,680,841, both to Hu, disclose the concept of providing a hydraulic linkage between the camshaft and the exhaust valves. The camshaft has two lobes for each exhaust valve, a first of the lobes opening each exhaust valve normally during the exhaust stroke. The system employs a cam follower hydraulically connected to each exhaust valve. Clearance between the cam follower and the camshaft is effectively changed whereby a second cam lobe, smaller than the first lobe, actuates the valve during brake operation. 
     One problem with such prior art engine brakes is that the normal operation of the exhaust valve is affected during brake operation. Clearance between the cam follower and camshaft is effectively reduced during brake operation. This means that the first lobe on the camshaft opens the exhaust valve further than normal for the exhaust stroke during exhaust brake operation. In some cases it is necessary to provide recesses in the pistons so that the exhaust valves do not strike the pistons when the brake is operational. These recesses, and the abnormally extended exhaust valves, interfere with optimal engine design from the point of view of other considerations such as emission controls. 
     Another problem with such prior art engine brakes is that the exhaust valve overlap at top dead center is increased during brake operation. This means that exhaust gas energy is lost from the exhaust manifold to the inlet stroke of the cylinder. Recovering the lost energy would be beneficial in order to drive the turbocharger to supercharge the compression stroke. 
     It is an object of the invention to provide an improved valve control apparatus which overcomes the disadvantages associated with the prior art. 
     It is also an object of the invention to provide an improved valve control apparatus which allows a camshaft to selectively open each exhaust valve near top dead center of each compression stroke, for engine braking purposes, without interfering with normal maximum lift and closing of each exhaust valve on each exhaust stroke. 
     Is a further object of the invention to provide an improved valve control apparatus which is rugged and economical in construction and reliable during operation. 
     SUMMARY OF THE INVENTION 
     There is provided, according to one aspect of the invention, a valve control apparatus for an internal combustion engine having a valve and a camshaft. The camshaft has an axis of rotation, a first lobe and a second lobe. The second lobe is angularly spaced-apart about the axis from the first lobe. The first lobe extends further from the axis of rotation than the second lobe. The apparatus includes a follower which is operatively engagable with the camshaft and the valve. The follower is positioned to operatively engage the first lobe on each revolution of the camshaft and to open the valve a first time on each revolution of the camshaft. There is a mechanism for selectively changing clearance operatively between the follower and at least one of the camshaft and the valve. The mechanism selectively reduces the clearance on each revolution of the camshaft after the valve is opened by the first lobe. The follower operatively engages the second lobe and opens the valve a second time on each revolution of the camshaft when the clearance is so reduced. The mechanism increases the clearance on each revolution of the camshaft during the opening of the valve the first time and removes the clearance before the valve is opened by the second lobe. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the drawings: 
     FIG. 1 is a side view, partly in section, of a fragment of a diesel engine including two exhaust valves of one cylinder thereof, a camshaft and an exhaust valve opening mechanism including a valve control mechanism, according to an embodiment of the invention, shown at the position before the start of engine braking; 
     FIG. 2 is a top plan view thereof, also showing two intake valves of the one cylinder of FIG. 1, the intake valve opening mechanism and the fuel injector actuating mechanism; 
     FIG. 3 is a view similar to FIG. 1 near the top dead center of the compression stroke with the exhaust valves fully cracked open; 
     FIG. 4 is a graph which plots the lift of the exhaust valves against the crankshaft angle; 
     FIG. 5 is a view similar to FIG. 1, after the cracking open of the exhaust valves, as the exhaust valves begin to open on the normal exhaust stroke, and before resetting of the mechanism for the normal opening of the exhaust valves for the exhaust stroke; 
     FIG. 6 is a top plan view similar to FIG. 4, for the position of FIG. 5; 
     FIG. 7 is a view, similar to FIG. 5, showing the mechanism reset for the normal opening of the exhaust valves; 
     FIG. 8 is a view similar to FIG. 6, corresponding to the position of FIG. 3; 
     FIG. 9 is a view similar to FIG. 2 of a first alternative embodiment of the invention; 
     FIG. 10 is a view, similar to FIG. 1, of the third alternative embodiment; 
     FIG. 11 is a view, similar to FIG. 1, of a second alternative embodiment; 
     FIG. 12 is a view, similar to FIG. 2, of the second alternative embodiment; 
     FIG. 13 is a view, similar to FIG. 3, of the second alternative embodiment; and 
     FIG. 14 is a view, similar to FIG. 4, of the second alternative embodiment. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring first to FIGS. 1 and 2, these show a fragment of a diesel engine  20  including a camshaft  22 , a pair of exhaust valves  24  and  26 , a cross head  28  extending across tops  30  and  32  of the two exhaust valves, a rocker arm  34 , rocker arm supports  36  and  37  and a rocker arm shaft  38 . The rocker arm includes a cam follower in the form of a roller  40  rotatably mounted on a shaft  42 . There is a valve set screw  44  threadedly received at end  46  of the rocker arm above cross head  28 . A lock nut  48  is threadedly received on the set screw adjacent the rocker arm. The set screw has a concave recess  50  at its lower end which contacts hemispherical fitting  52  on cross head  28 . 
     Referring to FIG. 2, there is a pair of intake valves  56  and  58  on the same cylinder of engine  20  as the exhaust valves  24  and  26 . These are also provided with a cross head  60 , a rocker arm  62 , a valve set screw  64  and a lock nut  66 . There is also a fuel injector  68  actuated in this example by another rocker arm  70 . The supports are provided with bolts  72 ,  74 , 76  and  78 . As described thus far, the engine  20  is generally conventional. Camshaft  22  rotates in the direction of arrow  80  once for every two revolutions of the crankshaft (not shown) of the engine. A first lobe  23  is positioned in the conventional manner on camshaft  22  to open the exhaust valves  24  and  26  during the exhaust stroke of the particular cylinder of the engine where these valves are located. The lobe  23  contacts roller  40  and rotates rocker arm  34  in the direction indicated by arrow  86 , causing screw  44  to press downwardly on fitting  52  of the cross head and thus open the exhaust valves. 
     It is known to provide clearance in the exhaust valve opening mechanism. Generally this is accomplished by adjusting screw  44  to provide a specified gap between the bottom of the screw and the cross head. Lock nut  48  maintains the proper gap. The gap however could be considered as being between the camshaft and roller depending upon the position of the rocker arm. Likewise it is known to provide clearance or play in other ways between the camshaft and the exhaust valves such that there is no actual clearance between the roller and the camshaft or the screw  44  and fitting  52 . For example, hydraulic devices can replace the rocker arm and the clearance or play can simply be lost motion in the hydraulic mechanism. Thus, the term “clearance” between the camshaft, the rocker arm and the exhaust valves is used herein in the operative sense to mean some type of operative clearance or play in the system. 
     Engine  20  is somewhat unconventional in that camshaft  22  has a second lobe  25  located on the same portion of the camshaft as lobe  23 . In other words, lobes  23  and  25  are axially aligned along axis of rotation  90  of the camshaft in this embodiment, but are angularly spaced-apart about the axis. It may be seen that lobe  23  extends further from axis  90  than lobe  25 . The second lobe  25  is positioned to crack open the exhaust valves  24  and  26  near top dead center of the compression stroke to provide a compression release brake for the engine. When lobe  25  reaches roller  40 , the rocker arm rotates in the direction of arrow  86 , cracking open the exhaust valves. 
     It is neither appropriate, nor desirable to have an engine brake operate at all times. Clearly the exhaust valves should not be cracked open at top dead center of the compression stroke when the engine is providing power. The exhaust brake should only be operational, as discussed above, when the fuel supply to the engine is cut off and the vehicle is coasting. Thus there must be some mechanism for selectively engaging the roller  40  with lobe  25  during engine brake operation only. It is known in the prior art discussed above to provide variable effective clearance between the roller and camshaft for this purpose. During normal engine operation, the clearance is increased such that the roller  40  operatively contacts only lobe  23  during rotation of the camshaft, so the exhaust valves are opened only during the exhaust stroke. When the engine brake is operational, there is means for decreasing this clearance such that the lobe  25  operatively contacts the roller  40 , rotates the rocker arm in the direction of arrow  86 , and cracks open the exhaust valves near top dead center of each compression stroke. 
     However, there is a problem associated with prior art devices of this nature. When the clearance is so reduced, the exhaust valves  24  and  26  are opened further than normal during the exhaust stroke as the lobe  23  contacts the roller  40 . This conceivably could cause the exhaust valves to contact the piston, causing serious damage to the engine. One way of countering this problem has been to provide pockets in the pistons to give additional clearance for the exhaust valves. However this can be detrimental to engine operation since the flows of gases to and from the cylinder can be adversely affected by the pockets. 
     It is not only the degree of opening of the exhaust valves which poses problems. Reducing the clearance also affects exhaust valve timing. In particular, the exhaust valves stay open longer than normal, increasing overlap with the intake valves (when both valves are open simultaneously). This may cause more exhaust energy to be dumped into the intake system instead of, for example, being available to help drive the engine turbocharger. 
     Another problem associated with these prior art apparatuses is that their typical rocker arm ratio is too high. The rocker arm ratio is the amount of opening of the exhaust valves divided by the amount of lift provided by lobe  23 . A typical range of ratios in prior art devices would be 1.6-1.9:1. Such ratios increase loading on the camshaft. The loading is typically reduced by timing the opening of the exhaust valves early, resulting in weak engine braking. 
     Engine  20  optimizes the rocker arm ratio by achieving a rocker arm ratio more nearly approaching 1:1 in this preferred embodiment as may be seen with reference to FIG.  1 . The distance between adjusting screw  44  and rocker arm shaft  38  is almost the same as the distance between the rocker arm shaft and point of contact  41  between the camshaft and roller  40 . The lever arms are therefore more equal in length and the amount of lift at the camshaft nearly equals the amount of opening of the exhaust valves. 
     The engine also includes a valve control apparatus  100  which selectively reduces the operative clearance between the camshaft  22  and the exhaust valves  24  and  26  in order to operate the engine brake by cracking open the valves, near top dead center of the compression stroke, with lobe  25  of the camshaft. There is a solenoid valve  102  operatively connected to controls  104 . The controls are conventional and include a switch operatively associated with the throttle of the engine such that the brake is only operational when the throttle is closed. There is also a manual switch in the cab of the vehicle, allowing the operator to operate the engine brake when the vehicle is coasting downhill. The solenoid valve allows engine oil to enter a passageway  110  when the operator closes the switch and the valve opens. 
     Rocker arm  34  is unconventional in that it comprises a first portion  112  and a second portion  114 . Both portions are rotatably mounted on rocker arm shaft  38  as best shown in FIG.  1 . Portion  112  operatively contacts the camshaft  22  by means of roller  40  and portion  114  operatively contacts the exhaust valves via screw  44 , fitting  52  and cross head  28 . As discussed above, both portions have nearly the same effective length measured by the distance from the center of the rocker arm shaft to the point of contact with camshaft  22  and fitting  52  respectively, providing a rocker arm ratio of nearly 1:1 for this example of the invention. 
     There is a mechanism  130  for selectively changing the operative clearance between the camshaft and the valves. Normally the rocker arm  34  is in a first operational mode, illustrated in FIG. 7, where on each revolution of the camshaft the first lobe  23  only operatively contacts roller  40 , causing the valves  24  and  26  to open in the normal manner during the exhaust stroke only. The mechanism  130  can selectively put the rocker arm  34  in a second operational mode, illustrated in FIGS. 1,  3  and  5 , where, on each revolution of the camshaft, the roller  40  is lifted by the second lobe  25  to crack open the exhaust valves near top dead center of the compression stroke. This second mode is selected by opening solenoid valve  102  with controls  104  to provide engine oil to the passageway  110  extending through rocker arm support  36  from oil line  111 . 
     The adjusting mechanism  130  includes a hydraulic cylinder  132  with a piston  134  reciprocatingly received therein. There is a pin  136  extending through the cylinder and a bore  138  in the piston. The bore  138  is substantially wider than the pin, allowing for reciprocation of the piston in the cylinder, but limiting its movement. 
     As seen in FIG. 7, there is a first coil spring  140  biased between end  142  of the cylinder and recess  144  in the piston. The spring biases the piston to the right from the point of view of FIGS.  1 , 3 ,  5  and  7 . There is a smaller coil spring  148  coaxially within spring  140  and biased between the recess  144  in the piston and a ball  150 . The spring biases the ball towards a position to close passageway  152 . 
     There is a second cylinder  160 , integral with cylinder  132  in this embodiment and located coaxially to the left thereof from the point of view of FIG.  7 . There is a second piston  162  in the cylinder having a stem  164  extending to the right, from the point of view of FIGS. 1,  3 ,  5  and  7 , into the passageway  152 . 
     There is a further hydraulic passageway  170  which, from the point of view of FIGS. 1,  3 ,  5  and  7 , extends downwardly through portion  112  of the rocker arm and then angles to the right to intersect with cylindrical bore  174  which receives the rocker arm shaft  38 . Passageway  110  in rocker arm support  36  and passageway  170  in portion  112  are both aligned with a passageway  113  in the rocker arm shaft  38  for the positions of the rocker arm portions illustrated in FIG.  1  and FIG.  7 . This allows oil to pass through the passageways  110 ,  113 ,  170  and  152  when the solenoid is open. 
     There is a chamber  180  formed in the cylinder  132  between the piston  134  and end  142  of the cylinder. Oil can pass from passageway  152  and into the chamber  180 , unseating ball  150 , when the rocker arm portions are in this position. The ball  150  acts as a check valve, trapping the oil within the chamber  180 . At the same time, the spring  140  biases the piston  134  to the right and against upward extension  190  on portion  114  of the rocker arm, to rotate the two portions  112  and  114  to the positions shown in FIGS. 1,  3  and  5 , with the piston  134  projecting outwardly from the cylinder  132 . The two portions of the rocker arm are thus moved away from each other and reduce operative clearance between the camshaft and the exhaust valves during brake operation. 
     Referring to FIG. 5, this shows a point after the lobe  25  has rotated past roller  40 , and before lobe  23  has completed the lifting of the rocker arm  34  to open the exhaust valves  24  and  26  for the exhaust stroke. There is another hydraulic passageway  200 . 1  in portion  112  of the rocker arm which becomes aligned with passageway  115  in the shaft which is connected to drain. This allows pressurized oil to flow through passageway  200 . 1  from chamber  204  of cylinder  160 , allowing spring  206  to move piston  162  to the right, from the point of view of FIG. 5, so stem  164  unseats ball  150  to the right, compressing spring  148 . The force of projection  190  on piston  134 , as the roller  40  rides up on lobe  23 , forces the piston  134  to the left, from the point of view of FIG. 5, dumping oil through passageways  152 , 170 ,  113  and  110  back through the solenoid valve. Thus the two portions  112  and  114  of the rocker arm rotate closer together, increasing operative clearance between the exhaust valves and camshaft to the same amount as occurs when the engine brake is not operational. 
     Operation 
     To summarize the operation of each cylinder of engine  20 , FIG. 1 is first referenced. This shows the position of camshaft  22  as the roller  40  on the rocker arm  34  is on the dwell surface  21  of the camshaft, with its second lobe  25  approaching. Solenoid valve  102  has been opened using the controls  104 . In this position passageways  110  and  170  in the rocker arm support and portion  112  of the rocker arm respectively are aligned with passageway  113  in shaft  38  such that engine oil is forced through passageway  152 , past ball  150  and into the chamber  180  when piston  134  is moved to the right under the action of spring  140 . The piston is prevented from moving to the left by the ball  150  which blocks the oil in the chamber  180 . Thus the two portions  114  and  112  of the rocker arm are rotated away from each other, increasing the gap  200  between them and decreasing the operative clearance between the roller  40  and camshaft such that the lobe  25  on the camshaft rotates the rocker arm clockwise cracking open the exhaust valves  24  and  26 , as shown in FIG. 3, as the roller rides up on lobe  25 . 
     FIG. 5 shows the position of the apparatus after lobe  25  has passed the roller  40  and the roller is riding up on lobe  23 . At this point passageway  200 . 1  in portion  112  of the rocker arm becomes aligned with passageway  115  in the shaft, which is connected to drain, allowing pressurized oil from chamber  204  of cylinder  160  to escape so spring  206  forces piston  162  to the right. This causes stem  164  to unseat ball  150 . As roller  40  begins to ride up on lobe  23 , portion  112  of the rocker arm is pushed upwardly by the camshaft, forcing projection  190  of portion  114  against piston  134  and forcing oil out from chamber  180  toward solenoid  102  through passageways  170 ,  111  and  110 . 
     When the camshaft  22  has rotated such that the roller  40  is past the lobe  23  and is approaching lobe  25 , as shown in FIG. 1, passageway  200 . 1  is aligned with passageway  113 . 1  in shaft  38 . As seen, this receives oil from passageway  113  connected thereto. The hydraulic pressure pushes piston  162  to the left, along with stem  164 , from the point of view of FIG.  1 . Spring  148 , shown in FIG. 7, biases ball  150  to the left so it reseats itself Passageways  110  and  170  are both aligned with passageway  113  in shaft  38  in this position such that oil again fills chamber  180  in cylinder  132  as piston  134  is biased to the right by spring  140 . The oil is locked in chamber  180  by ball  150  so the portions  112  and  114  of the rocker arm are held in the relative position shown in FIG. 5 with the gap  200  increased, and the operative clearance between the roller  40  and the camshaft  22  decreased, so lobe  25  again cracks open the exhaust valves as it reaches roller  40 . 
     Alternative Embodiments 
     FIG. 9 show an alternative embodiment which is generally similar to the previous embodiment and like parts have like numbers with the additional designation “0.1”. Like engine  20 , engine  20 . 1  has a camshaft  22 . 1  with two lobes  23 . 1  and  25 . 1 . Rocker arm  34 . 1  has two portions  112 . 1  and  114 . 1 . There is a piston  134 . 1  which contacts projection  190 . 1  of portion  114 . 1 . There is a ball  150 . 1  which normally seals passageway  170 . 1  against a back flow of oil from chamber  180 . 1 . 
     There is a passageway  350  which connects chamber  180 . 1  to chamber  352  in a cylinder  354 . There is a piston  356 , 0.225″ in diameter in this example, which slidingly extends through aperture  357  at end  359  of cylinder  354 . A larger diameter, tubular piston  358 , 0.250″ in diameter in this example, extends slidingly and sealingly through aperture  361  at opposite end  360  of the cylinder. There is a screw  380  with a nylon insert  381  on the end which provides resistance against the movement of piston  358 . 
     There is a larger diameter spring  371  pressing against the disk-shaped member  370  and which biases the piston assembly to the left, from the point of view of FIG.  10 . When chamber  180 . 1  is supplied with pressurized oil, as the lobe  25 . 1  approaches roller  40 . 1 , pistons  356  and  358  are moved to the right due to the larger diameter of piston  358 . This compresses spring  371 . The pressure builds up as the roller  40 . 1  rides up on the lobe, causing piston  358  to project outwardly beyond the right end of cylinder  354  from the point of view of FIG.  10 . 
     However, once the lobe  25 . 1  has caused the exhaust valves to crack open, the pressure in the engine cylinder rapidly drops due to the escape of the compressed gases through the exhaust valves. This reduces the pressure in cylinder  354 , causing larger spring  371  to force member  370  to the left against the pressure of smaller spring  373 , moving piston  356  to the left. However tubular piston  358  lags behind due to the resistance of nylon insert  381  pressing against the piston under the action of screw  380 . Member  370  therefore separates from the tubular piston  358 , allowing oil to escape from chamber  180 . 1  through the center of the tubular piston  358  and outwardly to the right from the point of view of FIG.  10 . Thus piston  134 . 1  is forced towards chamber  180 . 1  by projection  190 . 1  as the roller  40 . 1  starts to ride on lobe  23 . 1 , so the apparatus resumes its normal operational mode, equivalent to its position when the brake is not operational, prior to each exhaust stroke. 
     FIGS. 11-14 show another alternative embodiment wherein like parts have like numbers as in the previous embodiments with the additional designation “0.2”. In this example rocker arm  34 . 2  has only a single portion instead of the two portions of the previous embodiments. However, rocker arm  34 . 2  is unconventional in that includes a mobile hydraulic finger  201 , reciprocatingly received in a hydraulic cylinder  202 . The finger has a convex outer end  205  which contacts crosshead  28 . 2 . Rocker arm shaft  38 . 2  is provided with two passageways  210  and  212 , the former aligning with passageway  110 . 2  to provide pressurized oil via solenoid  102 . 2 . The latter is connected to drain. 
     There is a passageway  220  in the rocker arm equipped with a check valve  222  including a ball  224  biased against a seat  226  via spring  228 . There is another passageway  230  which intersects passageway  221  between the check valve and cylinder  202 . 
     As in the previous embodiments, lobe  25 . 2  serves to crack open the valves  24 . 2  and  26 . 2  near top dead center of the compression stroke. FIG. 11 shows lobe  25 . 2  approaching roller  40 . 2  of the rocker arm. It may be seen that passageway  220  is connected to passageway  110 . 2  via passageway  210  in the rocker arm shaft and thereby receives pressurized boil oil which passes through check valve  222  to enter cylinder  202  and thereby extend finger  201 . The same time, passageway  230  is not aligned with the passageway  212  and thereby not connected to drain. Thus any oil entering cylinder  202  is trapped by the check valve and the nonalignment of passageway  230  with drain. 
     Referring to FIG. 13, this shows the valves  24 . 2  and  26 . 2  fully cracked open near top dead center of the compression stroke. This is achieved with finger  201  fully extended. 
     Referring to FIG. 14, this shows the position of the camshaft  22 . 2  after lobe  25 . 2  has rotated past roller  40 . 2  and as the roller begins to ride up on lobe  23 . 2  for normal opening of the valves for the exhaust stroke. In this position, passageway  230  becomes aligned with passageway  212  and, thereby, to drain. This allows oil from cylinder  202  drain outwardly from the cylinder through passageway  230 , thereby allowing finger  201  to retract until it contacts set screw  44 . 2 . This is the position for normal valve opening where the lash and amount of valve opening are dictated by the position of screw  44 . 2 . 
     It will be understood by someone skilled in the art that many of the details provided above are by way of example only and can be deleted or altered without departing from the scope of the invention as set out in the following claims.