Abstract:
Methods and apparatus for actuating an engine valve provided between an engine cylinder and an exhaust manifold to provide compression-release engine braking in combination with exhaust gas restriction and brake gas recirculation are disclosed. In a first embodiment of the present invention, the engine valve used to provide brake gas recirculation and compression-release braking may be maintained slightly open between the brake gas recirculation and compression-release events. In another embodiment of the present invention, the cam closing ramp for a main exhaust event may be extended to terminate near the beginning of a brake gas recirculation event to facilitate refilling a hydraulic valve actuation system used to in association with the exhaust valve.

Description:
FIELD OF THE INVENTION 
   The present invention relates generally to a method for operating an engine brake in internal combustion engines. 
   BACKGROUND OF THE INVENTION 
   In an internal combustion engine, engine valve actuation is required in order to produce positive power, and may also be used to produce engine braking and/or exhaust gas recirculation (EGR). During positive power, one or more intake valves may be opened to admit air into a cylinder for combustion during the intake stroke of the piston. One or more exhaust valves may be opened to allow combustion gases to escape from the cylinder during the exhaust stroke of the piston. 
   One or more exhaust valves may also be selectively opened to convert, at least temporarily, the engine into an air compressor for engine braking operation. This air compressor effect may be accomplished by either opening one or more exhaust valves near piston top dead center (TDC) position for compression-release type braking, or by maintaining one or more exhaust valves in a relatively constant cracked open position during much or all of the piston motion, for bleeder type braking. In either of these methods, the engine may develop a retarding force that may be used to help slow a vehicle down. This braking force may provide the operator with increased control over the vehicle, and may also substantially reduce the wear on the service brakes. Compression-release type engine braking has been long known and is disclosed in Cummins, U.S. Pat. No. 3,220,392 (November 1965), which is hereby incorporated by reference. 
   The braking power of a compression-release type engine brake may be increased by selectively actuating the exhaust valves to carry out brake gas recirculation in combination with compression release braking. Brake gas recirculation (BGR) can be accomplished by opening an exhaust or auxiliary valve near bottom dead center of the intake or expansion stroke of the piston and keeping the exhaust or auxiliary valve open during the first portion of the exhaust or compression stroke of the engine. Opening the exhaust or auxiliary valve during this portion of the engine cycle may allow exhaust gas to flow into the engine cylinder from the relatively higher-pressure exhaust manifold. The introduction of exhaust gases from the exhaust manifold into the cylinder may pressurize the cylinder with a charge faster than it would otherwise occur during the compression stroke. The increased gas pressure in the engine cylinder may increase the braking power produced by the compression-release event. 
   There are many different systems that may be used to selectively actuate an exhaust or auxiliary valve to produce BGR and compression-release events. One known type of actuation system is a lost motion system, described in the aforenoted Cummins patent. An example of a lost motion system and method used to obtain engine braking and brake gas recirculation is disclosed in Gobert, U.S. Pat. No. 5,146,890 (Sep. 15, 1992) which discloses a method of conducting brake gas recirculation by placing the cylinder in communication with the exhaust system during the first part of the compression stroke and optionally also during the latter part of the intake stroke, and which is hereby incorporated by reference. Gobert uses a lost motion system to enable and disable compression-release braking and brake gas recirculation. The system disclosed in Gobert opens the exhaust valve near bottom dead center of the intake stroke for a BGR event, closes the exhaust valve before the midway point of the compression stroke to terminate the BGR event, and opens the exhaust valve again near top dead center of the same compression stroke for a compression-release event. As a result, the exhaust valve actuated in accordance with the Gobert system must be rapidly seated and unseated between the BGR and compression-release events. 
   In many internal combustion engines, the intake and exhaust valves may be actuated by fixed profile cams, and more specifically, by one or more fixed lobes or bumps that are an integral part of each cam. The cams may include a lobe for each valve event that the cam is responsible for providing. The size and shape of the lobes on the cam may dictate the valve lift and duration which result from the lobe. For example, an exhaust cam profile for a system constructed in accordance with the aforenoted Gobert patent may include a lobe for a BGR event, a lobe for a compression-release event, and a lobe for a main exhaust event. 
   It may also be desirable to increase the exhaust back pressure in the exhaust manifold during engine braking. Higher exhaust back pressure may increase gas mass and pressure in the engine cylinder available for engine braking, and thereby increase braking power. Increased exhaust back pressure, however, may undesirably increase the force required to open the exhaust valve for a compression-release event because the opening force applied to the exhaust valve must exceed the increased pressure in the engine cylinder resulting from the increased exhaust back pressure. To some extent the increased exhaust back pressure may also increase the pressure applied to the back of the exhaust valve, which may counter-balance the increased pressure in the cylinder and thus reduce the loading on the exhaust valve opening mechanism used for the compression-release event. 
   Increasing the pressure of gases in the exhaust manifold may be accomplished by restricting the flow of gases through the exhaust manifold. Exhaust manifold restriction may be accomplished through the use of any structure that may, upon actuation, restrict all or partially all of the flow of exhaust gases through the exhaust manifold. The exhaust restrictor may be in the form of an exhaust engine brake, a turbocharger, a variable geometry turbocharger, a variable geometry turbocharger with a variable nozzle turbine, and/or any other device which may limit the flow of exhaust gases. 
   Exhaust brakes generally provide restriction by closing off all or part of the exhaust manifold, thereby preventing the exhaust gases from escaping. This restriction of the exhaust gases may provide a braking effect on the engine by providing a back pressure when each cylinder is on the exhaust stroke. For example, Meneely, U.S. Pat. No. 4,848,289 (Jul. 18, 1989); Schaefer, U.S. Pat. No. 6,109,027 (Aug. 29, 2000); Israel, U.S. Pat. No. 6,170,474 (Jan. 9, 2001); Kinerson et al., U.S. Pat. No. 6,179,096 (Jan. 30, 2001); and Anderson et al., U.S. Pat. Appl. Pub. No. US 2003/0019470 (Jan. 30, 2003) disclose exhaust brakes for use in retarding engines. 
   Turbochargers may similarly restrict exhaust gas flow from the exhaust manifold. Turbochargers often use the flow of high pressure exhaust gases from the exhaust manifold to power a turbine. A variable geometry turbocharger (VGT) may alter the amount of the high pressure exhaust gases that it captures in order to drive a turbine. For example, Arnold et al., U.S. Pat. No. 6,269,642 (Aug. 7, 2001) discloses a variable geometry turbocharger where the amount of exhaust gas restricted is varied by modifying the angle and the length of the vanes in a turbine. An example of the use of a variable geometry turbocharger in connection with engine braking is disclosed in Faletti et al., U.S. Pat. No. 5,813,231 (Sep. 29, 1998), Faletti et al., U.S. Pat. No. 6,148,793 (Nov. 21, 2000), and Ruggiero et al., U.S. Pat. No. 6,866,017 (Mar. 15, 2005), which are hereby incorporated by reference. 
   Compression-release engine braking is not the only type of engine braking known. The operation of a bleeder type engine brake has also long been known. During bleeder type engine braking, in addition to the normal exhaust valve lift, the exhaust valve(s) may be held slightly open continuously throughout the remaining engine cycle (full-cycle bleeder brake) or during a portion of the cycle (partial-cycle bleeder brake). The primary difference between a partial-cycle bleeder brake and a full-cycle bleeder brake is that the exhaust valve is closed for the former during most of the intake stroke. 
   Usually, the initial opening of the braking valve(s) in a bleeder braking operation is far in advance of the compression TDC (i.e., early valve actuation) and then lift is held constant for a period of time. As such, a bleeder type engine brake may require much lower force to actuate the valve(s) due to early valve actuation, and generates less noise due to continuous bleeding instead of the rapid blow-down of a compression-release type brake. Moreover, bleeder brakes often require fewer components and can be manufactured at lower cost. Thus, an engine bleeder brake can have significant advantages. 
   BRIEF SUMMARY OF THE INVENTION 
   One embodiment of the present invention is directed to an innovative method of actuating an engine valve provided between an engine cylinder and an exhaust manifold to provide compression-release engine braking comprising the steps of: opening the engine valve for a brake gas recirculation event; increasing the lift of the engine valve during an initial portion of the brake gas recirculation event; reducing the lift of the engine valve during a later portion of the brake gas recirculation event; maintaining the engine valve open between the brake gas recirculation event and a compression-release event; and increasing the lift of the engine valve during an initial portion of the compression-release event. 
   Another embodiment of the present invention is directed to an innovative internal combustion engine cam for compression-release engine braking comprising: a main exhaust lobe including an extended closing ramp portion; a brake gas recirculation lobe; a compression-release lobe; and a base circle portion extending approximately 15 cam angle degrees or less between the main exhaust lobe extended closing ramp portion and the brake gas recirculation lobe. 
   Yet another embodiment of the present invention is directed to an innovative internal combustion engine cam for compression-release engine braking comprising: a base circle portion; a brake gas recirculation lobe; a compression-release lobe; and a depressed region between the brake gas recirculation lobe and the compression-release lobe, wherein said depressed region has a height greater than the base circle portion of the cam. 
   It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only, and are not restrictive of the invention as claimed. The accompanying drawings, which are incorporated herein by reference, and which constitute a part of this specification, illustrate certain embodiments of the invention and, together with the detailed description, serve to explain the principles of the present invention. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will now be described in connection with the following figures in which like reference numbers refer to like elements and wherein: 
       FIG. 1  is schematic diagram of a valve actuation system that may be used to actuate an exhaust or auxiliary engine valve in accordance with embodiments of the present invention; 
       FIG. 2  is a plot of cam follower lift versus cam angle degrees in accordance with an embodiment of the present invention; 
       FIG. 3  is a plot of valve lift versus crank angle degrees produced in accordance with an embodiment of the present invention; and 
       FIG. 4  is a plot of cam follower lift versus cam angle degrees in accordance with an alternative embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Reference will now be made in detail to an example of a system that may be used to actuate an exhaust or auxiliary valve in accordance with an embodiment of the present invention. An engine cylinder  40  in a portion of an engine  20  is shown in  FIG. 1 . The engine  20  may have any number of similar cylinders  40  in which a piston  45  may reciprocate upward and downward repeatedly, during the time the engine is used for positive power and engine braking. At the top of the cylinder  40  there may be at least one intake valve  32  and one exhaust valve  34 . It is common for there to be two or more intake valves  32  and exhaust valves  34  each in an engine cylinder, and only one each is shown for ease of illustration. The intake valve  32  and exhaust valve  34  may be opened and closed to provide communication with an intake gas passage  22  and an exhaust gas passage  24 , respectively. The exhaust gas passage  24  may communicate with an exhaust manifold  26 , which may also have inputs from other exhaust gas passages (not shown) from other engine cylinders. Downstream of the exhaust manifold  26  there may be an exhaust restriction means  70  which may be selectively activated to restrict the flow of exhaust gas from the manifold  26 . Exhaust restriction means  70  may be provided by various means, such as a turbocharger turbine, a variable geometry turbocharger, a butterfly valve  72  in the exhaust pipe, shown, or other restriction means. The exhaust restriction means, when closed partially or fully, may selectively develop exhaust back pressure in the exhaust manifold  26  and the exhaust gas passage  24  which may be used for BGR. 
   The engine  20  may include an exhaust valve actuating subsystem  38  and an intake valve actuating subsystem  36 , for actuating the engine valves during positive power and engine braking modes of operation. The engine could optionally include an auxiliary valve and auxiliary valve actuating subsystem (not shown) to provide auxiliary communication between the engine cylinder  40  and the exhaust gas passage  24 . There are several known subsystems  36  and  38  that may be used for opening intake and exhaust valves for intake and exhaust events, including, but not limited to mechanical valve trains, electrical actuators, and hydraulic (such as lost motion) actuators. It is contemplated that any such subsystem or combination of subsystems, and/or new subsystems developed by the Applicant or others may be used to provide engine valve actuation for the intake and exhaust valves. 
   The actuation of the exhaust valve  34  may be controlled by the subsystem  38  to open the exhaust valve for brake gas recirculation and engine braking, such as compression-release braking, bleeder braking, or partial bleeder braking. The exhaust valve actuating subsystem  38  may comprise various hydraulic, hydro-mechanical, and electromagnetic actuation means, including but not limited to means which derive the force to open the valve from a common rail, lost motion, rocker arm, cam, push tube, or other mechanisms. The exhaust valve actuating subsystem  38  and the intake valve actuating subsystem  36  may be electronically controlled by an ECM  50  to vary the valve actuation events that are provided by the exhaust valve  34  and intake valve  32  during positive power and/or engine braking. 
   During engine braking, the exhaust restriction means  70  may be closed or partially closed to increase exhaust back pressure. Increased back pressure may be used to increase the charge and pressure of gas in the cylinder  40  for braking when increased back pressure is provided in combination with a brake gas recirculation event. 
   During brake gas recirculation, gas flow may temporarily reverse from the exhaust manifold  26  into the engine cylinder  40  and potentially even back past the intake valve  32  and into the intake passage  22 . Control of this backward gas flow through the exhaust and intake valves determines the system exhaust pressure profile and the resulting mass charge that is delivered to the cylinder on intake. The mass charge may affect the power of engine braking because, generally, the greater the pressure of the gas in the cylinder  40 , the greater the amount of braking that may be realized from the reciprocating piston  45  as it is opposed by the high pressure gas. 
     FIG. 2  is an example of the cam follower lift that may result from the system shown in  FIG. 1  to actuate an exhaust valve to produce engine braking in accordance with an embodiment of the present invention shown in  FIG. 3 .  FIG. 2  is a plot of the cam follower lift produced from a cam having a number of lobes extending from the cam base circle which may be used to provide main exhaust, BGR and compression-release valve events. Cam base circle is indicated by zero (0) lift in  FIG. 2 . The exhaust cam profile may include a main exhaust lobe  100 , a BGR lobe  110  and a compression-release lobe  120 . 
   The cam may be connected to a lost motion system that is inoperative during positive power operation of the engine the cam lobes with a height less than the threshold  130  (which may be the height of the valve or cam lash) are absorbed or “lost”. Thus, during positive power operation, cam motion from the BGR lobe  110  and the compression-release lobe  120  is not transferred to the exhaust valve. Only motion from the main exhaust event  100  may be transferred to the exhaust valve during positive power, just as it would be in an engine that did not include an engine brake. 
   During engine braking, the lost motion system may be turned on and provided with hydraulic fluid so that the motion imparted by the BGR lobe  110  and the compression-release lobe  120  may cease to be “lost,” and motion from all cam lobes may be transferred to the exhaust valve. As a result, during engine braking, the cam may impart the following additional motions to the exhaust valve. Region  102  of the cam corresponds to the closing ramp portion of the main exhaust lobe  100  used during engine braking. The closing ramp portion  102  of the main exhaust lobe is shown to return to base circle in region  104  between about 210 and 240 cam angle degrees, or more preferably between about 225 and 235 cam angle degrees. 
   The BGR lobe  110  may begin after region  104  between about 230 and 270 cam angle degrees, and more preferably between about 240 and 260 cam angle degrees. The BGR lobe  110  may reach a maximum height between about 270 and 300 cam angle degrees and then return toward the cam base circle. Region  112  of the cam corresponds to the intersection of the BGR lobe  110  with the compression-release lobe  120 . The lowest point of region  112  may be elevated above the cam base circle a minimum height  114  which is sufficient to keep the exhaust valve from seating (i.e., completely closing) between the BGR event and the compression-release event. The lowest point of region  112  may be between about 300 and 340 cam angle degrees, and more preferably between about 310 and 330 cam angle degrees. The minimum height  114  may be selected such that the exhaust valve is very nearly, but not quite closed between the BGR event and the compression-release event shown in  FIG. 3 . 
   The compression-release engine braking lobe  120  may follow the BGR lobe  110 . The compression-release lobe  120  may be provided on the cam so as to open the exhaust valve near the point that the engine cylinder piston reaches its top dead center position. The compression-release lobe  120  may reach a maximum height as early as 350 cam angle degrees or after zero cam angle degrees (i.e., by top dead center) and return towards base circle thereafter. Region  122  of the cam corresponds to the intersection of the compression-release lobe  120  with the main exhaust lobe  100 . The lowest portion of region  122  may be elevated above the cam base circle by a minimum distance  124  such that the exhaust valve does not close between the compression-release event and the main exhaust event. Alternatively, the lowest portion of the region  122  may return all the way to cam base circle by following alternative cam profile  124 . 
   The cam profile shown in  FIG. 2  may provide the exhaust valve actuation shown in  FIG. 3  during engine braking operation. A valve lift of zero (0) in  FIG. 3  indicates that the exhaust valve is closed and seated. With reference to  FIG. 3 , the exhaust valve may be actuated for a main exhaust event  200  and seated in accordance with valve seating event  202 . The exhaust valve may remain seated during period  204  until it is actuated for a BGR event  210 . During the period that the exhaust valve is seated, no exhaust gas exchange may occur between the engine cylinder and the exhaust manifold. 
   Next, the exhaust valve may be actuated for the BGR event  210 . The BGR event may overlap partially or entirely with an intake event. During the BGR event, exhaust gas in the exhaust manifold may flow back into the engine cylinder and potentially back through the open intake valve into the intake manifold. This may result in increased exhaust mass in the cylinder for the subsequent compression-release event. After reaching a maximum lift for the BGR event, the exhaust valve may return towards its seat, but not close at a point  212  between the BGR event  210  and the compression-release event  220 . The amount of lift that the exhaust valve maintains at point  212  may vary in different embodiments of the present invention. It may even be zero and thus the exhaust valve may seat between the BGR event and the compression-release event in some embodiments of the present invention with greater compliances, and/or larger valve lash settings. 
   The compression-release event  220  may follow the BGR event  110 . During the compression-release event, the lift of the exhaust valve is increased as the engine cylinder piston approaches and reaches a top dead center position. Gas pressure in the cylinder may be released to the exhaust manifold by increasing the lift of the exhaust valve near the end of the compression stroke of the piston. This compression energy of the exhaust gas in the cylinder may be released to the exhaust manifold instead of doing positive work by pushing the engine piston downward during the expansion stroke. After reaching a maximum lift for the compression-release event  220 , the exhaust valve may return towards its seat during period  222  between the compression-release event  220  and the main exhaust event  200 . The exhaust valve may maintain some lift and not close during period  222 , or alternatively, the exhaust valve may seat in accordance with the valve actuation  224 . 
   An alternative cam follower lift shown in  FIG. 4  may include a closing ramp that is better able to draw hydraulic fluid into the lost motion system with a valve lift reset function. The cam follower lift shown in  FIG. 4  differs from that shown in  FIG. 2  as follows. Region  102  of the cam, corresponding to the closing ramp portion of the main exhaust lobe  100 , may be extended from that shown in  FIG. 2 , all the way or almost all the way to the BGR event  110 . The valve closing velocity produced by the region  102  of the main exhaust lobe may be designed to match the hydraulic fluid refill speed to optimize hydraulic refill for a lost motion system with a reset function. The closing ramp portion  102  of the main exhaust lobe is shown to return to base circle in region  104  between about 230 and 265 cam angle degrees. 
   The BGR lobe may return to base circle such that the exhaust valve closes between the BGR event and the compression-release event. Alternatively, the BGR lobe may approach base circle, but not reach it in region  112  such that the exhaust valve remains open between the BGR event and the compression-release event. 
   A cam with the extended closing ramp  102  shown in  FIG. 4  may be used in a hydraulic valve actuation system that also includes a resetting device, such as disclosed in U.S. Pat. Nos. 5,460,131 to Usko and 4,399,787 to Cavanaugh, for example. The resetting device may cause the exhaust valve to close before the cam follower reaches the cam base circle in region  104 . The extended closing ramp  102  may improve the ability of the hydraulic valve actuation system to refill with hydraulic fluid for the next hydraulic valve actuation, namely the BGR event. 
   While various embodiments of the present invention have been described herein, it is evident that many alternatives, modifications and variations will be apparent to those skilled in the art. Accordingly, the preferred embodiments of the invention as set forth herein we intended to be illustrative, not limiting. Various changes may be made without departing from the spirit and scope of the invention as defined in the following claims.