Abstract:
An early intake valve closing (EIVC) and variable valve timing (VVT) assembly and method are provided for an internal combustion engine. The method includes operating an intake valve to open and close an intake port and allow intake gas to enter a cylinder of the internal combustion engine containing a piston, where the timing of the closing of the intake valve occurs at or before bottom dead center (BDC) of the compression stroke of the piston. The method further includes selectively extending duration of the opening of the intake port by an amount that delays closing timing of the intake valve until after BDC of the piston compression stroke. An assembly for EIVC and VVT includes a cylinder head including an intake valve and intake port and a rotatably mounted camshaft including an intake lobe operatively connected to the intake valve to periodically move the valve into the first and second positions. The intake lobe is shaped such that the timing of the closing position occurs at or before BDC of a compression stroke of the piston. The assembly further includes a selectively activatable VVT assembly having an actuator that operatively engages the intake valve before the intake valve reaches the closed position to extend duration of the intake valve opening until after BDC of the piston compression stroke.

Description:
FIELD OF THE INVENTION 
       [0001]    An assembly and method for early intake valve closing and variable valve timing are disclosed. 
       BACKGROUND 
       [0002]    Internal combustion engines can utilize a Miller cycle to reduce parasitic pumping losses and fuel consumption rate. The Miller cycle has a greater expansion rate than compression rate because the mass of intake gas trapped in a cylinder of the engine during a compression stroke is reduced relative to an amount trapped during a standard intake closing event for a four stroke (e.g., Otto cycle) engine. A Miller cycle can be performed by controlling the intake valve closing using an early intake valve closing (EIVC) or late intake valve closing (LIVC) strategy. 
       SUMMARY 
       [0003]    An assembly and method to selectively achieve early intake valve closing (EIVC) in an internal combustion engine is provided by the invention. 
         [0004]    More particularly, embodiments consistent with the invention relate to a method of operating an early intake valve closing (EIVC) internal combustion engine, including processes of periodically operating an intake valve to open and close an intake port and allow intake gas to enter a cylinder of the internal combustion engine containing a piston, where the timing of the closing of the intake valve occurs at or before bottom dead center (BDC) of the compression stroke of the piston. The method further includes selectively extending duration of the opening of the intake port by an amount that delays a timing of the closing of the intake valve until after BDC of the piston compression stroke. 
         [0005]    In accordance with other embodiments consistent with the invention, an early intake valve closing (EIVC) and variable valve timing (VVT) assembly for an internal combustion engine is provided, and includes a cylinder head for covering a cylinder containing a piston movable in a reciprocating manner in the cylinder. The cylinder head includes an intake valve and intake port, and the intake valve is moveable between an open position allowing intake gas to enter the cylinder through the intake port and a closed position that prevents intake gas from entering the cylinder through the intake port. The assembly includes a rotatably mounted camshaft including an intake lobe operatively connected to the intake valve to periodically move the intake valve into the open and closed positions. The intake lobe is shaped such that a timing of the closing position occurs at or before bottom dead center (BDC) of a compression stroke of the piston. The assembly further includes a selectively activatable VVT assembly adapted to operatively engage the intake valve before the intake valve reaches the closed position to extend a duration of the intake valve in the open position until after BDC of the piston compression stroke. 
         [0006]    It is to be understood that both the foregoing general description and the following detailed description are exemplary and exemplary only and are not restrictive of the invention, as claimed. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]    The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in and constitute a part of this specification, illustrate embodiments of the invention that together with the description serve to explain the principles of the invention. In the drawings: 
           [0008]      FIG. 1  is a graph depicting valve lift versus engine crank angle with and without late intake valve closing (LIVC) Miller cycle operation. 
           [0009]      FIG. 2A  is a diagram of an internal combustion engine cylinder head and valve assembly when the camshaft is at a position of maximum lift according to an exemplary embodiment. 
           [0010]      FIG. 2B  is a diagram of the exemplary cylinder head and valve assembly of  FIG. 2A  after the camshaft has rotated past the position of maximum lift. 
           [0011]      FIG. 3  is a graph depicting valve lift versus engine crank angle with and without early intake valve closing (EIVC) Miller cycle operation in accordance with an exemplary embodiment. 
       
    
    
     DETAILED DESCRIPTION 
       [0012]    The various aspects are described hereafter in greater detail in connection with a number of exemplary embodiments to facilitate an understanding of the invention. However, the invention should not be construed as being limited to these embodiments. Rather, these embodiments are provided so that the disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. Descriptions of well-known functions and constructions are omitted for clarity and conciseness. To aid in clarity of description, direction terms such as “ascending,” “clockwise,” “downward,” and “upward,” as used herein, provide reference with respect to orientation of the accompanying drawings and are not meant to be limiting. 
         [0013]      FIG. 1  is a graph showing an internal combustion engine intake valve lift profile over a typical duration of engine crank angle when performing a standard LIVC Miller cycle. The solid line  110  illustrates a cam profile of the internal combustion engine without the Miller cycle, and the dotted line  120  illustrates an extended duration of lift with a closing timing much after bottom dead center (ABDC) of the compression stroke. This closing timing can range from about 50° to 60° ABDC to an angle corresponding to more than one half of the compression stroke. During this extended intake valve opening duration, intake gas in the cylinder, which can include any one, or combinations of air, an air and fuel (e.g., diesel, gasoline or natural gas) mixture, and recycled exhaust gas (EGR), is forced to flow back into the intake manifold because the piston in the cylinder is ascending during the compression stroke, resulting in an intake gas mass being compressed by the compression stroke of the piston being reduced relative to an intake valve closing occurring closer to BDC of the compression stroke (i.e., BDC compression). 
         [0014]    However, the LIVC Miller cycle has been found to have the disadvantage of increased flow losses by intake gas flowing into and back out of the cylinder through the intake valve port while the valve is open during the compression stroke. Additionally, the gas flowing out of the cylinder during the compression stroke of the piston is reheated as it passes hot cylinder components, and this reheated gas flows back into the intake manifold. This reheated gas can undesirably and significantly increase the temperature of the intake manifold, for example, by an amount of about 15° F. or more. 
         [0015]      FIGS. 2A and 2B  show a portion of an internal combustion engine including an early intake valve closing (EIVC) and variable valve timing (VVT) assembly  200  according to an exemplary embodiment. The EIVC and VVT assembly  200  includes a selectively applied latching type VVT mechanism  202  in combination with camshaft  204  having an intake camshaft lobe  206  with a short lift duration profile. The EIVC and VVT assembly  200  avoids the significant flow losses and/or increases in intake manifold temperature present in LIVC Miller cycle internal combustion engines. 
         [0016]    With reference to  FIG. 2A , the EIVC and VVT assembly  200  includes a cylinder head  208  having at least one intake port  210  through which intake gas is provided to a cylinder  212  containing a piston  204  slidably movable within the cylinder  212 . The cylinder head  208  covers and seals a portion of an engine block  216  containing elements such as the cylinder  212 , the piston  214 , a piston connecting rod (not shown), and a rotating crankshaft (not shown). It is to be understood that  FIGS. 2A and 2B  only depict the portions of the cylinder head and engine block necessary for understanding the invention. The piston  214  is driven in a reciprocal motion in the cylinder  212  between top dead center (TDC) and BDC relative to the crankshaft position by the piston connecting rod connected at one end portion thereof to the piston  214  and at an opposite end portion thereof to the rotating crankshaft via lubricated bearings. 
         [0017]    The cylinder head  208 , cylinder  212 , and the piston  214  define a combustion chamber  217  into which intake gas is introduced for combustion. Intake gas is introduced into the combustion chamber by opening an intake valve  218  such that a facing of the valve  218  disengages a seating engagement with a valve seat  222 . Each cylinder in the cylinder head  208  also includes at least one exhaust port and respective exhaust valve (not shown) for allowing combusted gases to be removed from the cylinder after combustion in an exhaust stroke of the piston  214  just prior to an intake stroke when intake valve  218  opens to allow intake gas to enter the cylinder  212  through the intake port  210 . The number of cylinders and valves per cylinder associated with the cylinder head  208  depends on the kind of engine application. For example, an engine can include a total of four, six, eight or more cylinders, and can be arranged in an inline, V, radial or opposed configuration. Also, the supplied intake gas can be pressurized using a turbocharger (not shown) in fluid connection with the combustion chamber  217 . 
         [0018]    The opening and closing of the intake port  210  is controlled by a valve assembly that includes an intake valve  218 , a valve spring  220 , and a rocker arm  224  that moves in a reciprocal manner about a pivot  226  as a camshaft  204  rotates at half the rate of the crankshaft (not shown). The valve spring  220  biases the outer end of the stem of valve  218  away from the cylinder head  208 , thereby biasing a facing of the valve  218  toward seating engagement with the valve seat  222 . As long as the intake valve  218  is in an open position, i.e. any position that intake valve  218  takes when it is not engaged with the valve seat  222 , intake gas can pass through the intake port. 
         [0019]    The profile of the intake camshaft lobe  206  is depicted in an exaggerated manner in  FIG. 2A  for purpose of explanation, and the lift of the intake profile as it relates to crankshaft angle is represented by the solid line plot  310  of  FIG. 3 . It will be appreciated that the actual profile of the lift of intake valve  218  can take one of numerous shapes, but that the duration of each would be relatively short to provide Miller effects when desired. As the camshaft  204  turns, the intake camshaft lobe  206  periodically engages the rocker arm  224  to cause the rocker arm  224  to pivot about pivot  226 , which in turn forces the intake valve  218  in a downward direction such that the intake valve  218  disengages the seat  222  and allows intake gas to pass through the intake port  210 .  FIG. 2A  shows the camshaft lobe  206  in a position providing maximum lift to the intake valve  218 . As camshaft rotates from the position depicted in  FIG. 2A , the rocker arm  224  pivots in a clockwise direction about pivot  226  because the spring  220  forces the stem of the intake valve  218 , and thus an end portion of the rocker arm engaging the stem, in an upward direction, as depicted in  FIG. 2A , until the intake valve  218  closes in a seating engagement with the valve seat  222 . 
         [0020]    As shown in  FIG. 3 , an exemplary profile of the intake camshaft lobe  206  permits the engine to be operated according to a Miller cycle because the intake valve  218  closes “early” (i.e., EIVC) due to the shape of the camshaft lobe  206 . For example, an early closing can be provided when the intake valve is at 1 mm lift in the range of −275° to about −180° before top dead center (BTDC) of the crankshaft at the power stroke. In an exemplary embodiment, the shape of the intake camshaft lobe  206  can cause the intake valve  218  to close at about 235° BTDC of the power stroke in a Miller cycle operation mode. 
         [0000]    Rick: Is the “at 1 mm lift” a standard way to describe a closing timing event? 
         [0021]    With reference to  FIG. 2A , the EIVC and VVT assembly  200  includes a VVT apparatus  202  to permit an internal combustion engine mode of operation without a Miller cycle. The VVT apparatus  202  includes a rod  230  that extends in the direction of the rocker arm to prevent the intake valve  218  from closing for a predetermined duration of crank rotation angle later than the closing defined by the profile of the intake camshaft lobe  206 . This extended duration is shown by dashed plot  320  in  FIG. 3 . As can be seen in  FIG. 2B , when the VVT apparatus  202  is activated and engages the rocker arm, the valve  218  is held open until it closes ABDC, as indicated where graph  320  of  FIG. 3  crosses the abscissa. 
         [0022]    Referring again to  FIGS. 2A and 2B , the VVT apparatus  202  includes an actuator  228 , for example, a hydraulically or electromagnetically based actuator such as those described in U.S. Pat. Nos. 5,775,203 and 5,983,863, each of which is hereby incorporated by reference in its entirety. The VVT apparatus  202  is activated when no Miller cycle is desired by activating actuator  228  to extend the rod  230  while the lift of the valve  218  is greater than the prescribed lift at which the valve  218  is to be held in the open state. As shown in  FIG. 2A , the rod  230  extends while the valve is open, but the rod  230  does not yet contact the land  232  on the rocker arm  224 . In this way, the VVT apparatus  202  essentially acts in parallel to the actuation flow path. That is, the camshaft lobe  206 , rocker arm  224  and valve  218  operate in parallel, or simultaneously with the VVT apparatus  202  so that the rod  230  of the VVT apparatus  202  contacts the rocker arm  224  to catch the rocker arm, and thus the valve  218 , and prevent the valve from closing, i.e., “latches” the valve  218  during the retraction portion of the intake camshaft lobe  206 , i.e., along a valve retraction portion of the camshaft lobe profile. At a prescribed later time, the actuator  228  is deactivated and rocker arm  224  retracted to allow the valve  218  to seat with the valve seat  222  under the force of the spring  220 . In an embodiment, the valve  218  is caught on a land  232  of the rocker arm  224  to halt movement of the rocker arm  224  as it rotates to close the valve  218 , thus locking the valve  218  into an open state until the crankshaft rotates through the predetermined duration of crank angle.  FIG. 2B  shows the rod  230  extended and having engaged the land  232  at a point along the valve retraction portion of the camshaft lobe  206  after the camshaft  204  has rotated about 45° past the position where it would provide maximum lift, although different applications can include different intake cam profiles and valve train configurations. 
         [0023]    When Miller cycle effect is not desired, for example, during engine startup or when the engine is idling, the VVT latching mechanism can be engaged to hold the intake valve open, for example, until nearly full volumetric efficiency is achieved. Some embodiments can include a single non-Miller cycle intake valve closing timing that occurs after a prescribed timing duration (i.e., VVT duration value). In other embodiments, the intake valve closing timing can be selected from plural possible predetermined VVT duration values. For example, an embodiment can include an engine controller or processor (not shown), such as an engine control unit (ECU, not shown), that can select a valve closing timing from one or more possible crank duration values. Such an embodiment can include plural stored VVT duration values linked to specific operating characteristics, such as a look up table storing plural crank duration values for the VVT latching mechanism to allow selection of a particular stored value based on the current engine operation and/or other environmental parameters, power demands. Alternatively, a VVT duration value can be determined by way of the engine controller or some other processor performing a calculation based on operating parameters such as load, speed, power demand, temperature etc. 
         [0024]    Greater levels of internal exhaust gas recirculation (EGR) can be attained by optionally providing a slight opening of an exhaust valve just after BDC compression of the cylinder  212 . This can be carried out because the exhaust manifold pressure is usually pulsing high just after BDC compression and with EVIC, and the cylinder pressure is depressed at this time (i.e., just after BDC compression).  FIG. 3  shows a plot  330  representing lift after BDC compression of at least one exhaust valve (not shown) provided for each cylinder in the cylinder head  208 . Opening the exhaust valve can be achieved by providing a bump on the exhaust camshaft lobe, although other known valve actuation mechanisms such as electromagnetic or hydraulic actuators can be used to open the exhaust valve during the compression stroke. 
         [0000]    Rick: could you explain the EGR embodiment in more detail? For example, how and why is the pressure in the cylinder lower than that of the exhaust manifold in the compression stroke? How long is the exhaust valve open (preferably a range of values), and is there a range within the compression stroke where this would be optimized (or would it vary based on operating conditions)? 
         [0025]    It will be appreciated that the embodiments described and shown herein may be modified in a number of ways. For instance, while the exemplary embodiments described above include an overhead camshaft, other embodiments may have a camshaft positioned in the engine block and include intermediate rocker arm driving mechanisms, such as pushrods, tappets etc. Furthermore, each cylinder of an internal combustion engine more than one intake valve driven separately or in a unified way with one another. Also, instead of a camshaft and rocker arm assembly driving each valve, embodiments can have each intake valve driven by electromagnetic actuation, or VVT latching provided by dedicated valves in addition to intake valves driven by a camshaft providing EIVC. 
         [0026]    The EIVC and VVT assembly and method described herein can be used in any of a variety of applications requiring an internal combustion engine, such as mobile and stationary heavy duty machines, vehicles including heavy and light duty trucks and automobiles, construction equipment and the like. 
         [0027]    Although a limited number of embodiments is described herein, one of ordinary skill in the art will readily recognize that there could be variations to any of these embodiments and those variations would be within the scope of the appended claims. Thus, it will be apparent to those skilled in the art that various changes and modifications can be made to the EIVC and VVT assembly and method described herein without departing from the scope of the appended claims and their equivalents.