Abstract:
An engine particularly suited to powering work machines operated under varying loads. The engine includes a plurality of combustion cylinders operable in a homogenous charge compression ignition mode, each adapted to operate in both two-stroke and four-stroke cycles. Variable compression ratios and boost are available to improve performance.

Description:
TECHNICAL FIELD  
         [0001]    The present invention relates generally to the field of internal combustion engines, and more specifically to engines operated under homogenous charge compression ignition principles.  
         BACKGROUND  
         [0002]    In a direct injection compression ignition engine, such as a diesel engine, it is common to use either a two-stroke or a four-stroke operating sequence. Each has certain advantages and disadvantages.  
           [0003]    On the downward stroke of a piston in a combustion cylinder of a two-stroke engine, ports for air intake are opened and a charge of air is received in the combustion cylinder. Turbochargers are often used to supply the charge air at higher pressure and density than existing ambient conditions. On the upward stroke of the cylinder, the air intake ports are closed and the air is highly compressed. At the desired point of compression, fuel is sprayed into the cylinder by a fuel injector. The fuel ignites immediately, as a result of the heat and pressure inside the cylinder. The pressure created by the combustion of the fuel drives the piston downward in the power stroke of the engine. As the piston nears the bottom of its stroke, all of the exhaust valves open. Exhaust gases rush from the cylinder, relieving pressure in the cylinder. The intake ports are opened, and pressurized air fills the cylinder, forcing out the remaining exhaust gases. The exhaust valves close and the piston starts traveling back upward, the intake ports are closed and the fresh charge of air is compressed in the cylinder, in preparation for fuel injection.  
           [0004]    In a four-stroke engine, the various functions are separated into individual cycles of the piston. Combustion forces the piston downward in a power stroke, the following upward stroke of the piston is associated with expelling combustion gases in an exhaust stroke. The next downward stroke is an intake stroke for combustion air, followed by an upward compression stroke.  
           [0005]    Thus, in a two-stroke engine, each downward cycle of the piston is a power stroke, immediately following initiation of combustion. In a four-stroke engine, only every second downward stroke is a power stroke following combustion.  
           [0006]    Engine emission standards have led to the investigation of engine operating and ignition alternatives. In one such alternative, referred to as homogenous charge compression ignition (HCCI), significant emission reductions have been experienced during initial testing. In an engine operating under HCCI concepts, the air and fuel are intimately mixed, typically at a high air/fuel ratio, before maximum compression in the combustion cylinder. This can be achieved, for example, by the use of a fuel system having an injector able to vary the angle of injection as the piston moves from a bottom dead center position to a top dead center position. By way of further example, this also can be achieved by the use of an injector injecting a homogenous mixture of fuel and air into the combustion cylinder.  
           [0007]    As used herein, including the claims to follow, operation under HCCI concepts should be understood to encompass features and concepts whereby air and fuel are intimately mixed before maximum compression, and each droplet of fuel is surrounded by combustion air in excess of the amount required for combustion. In a compression ignition engine, as compression occurs, the air temperature increases, and ultimately combustion is initiated at numerous locations throughout the cylinder. Typically, combustion commences at lower temperatures than for direct charge ignition, leading to reduced NO x  emissions.  
           [0008]    The use of homogenous charge compression ignition results in reduced NO x  and particulate matter emissions. However, while homogenous charge compression ignition combustion presents advantages from emissions and fuel consumption view points, it presents challenges in control strategies, especially for engines operated under a wide range of load conditions. Four-stroke engines using homogenous charge compression ignition concepts typically exhibit good operating patterns under light to part load conditions. However, for high power density at high load, two-stroke engine cycles are preferred. Two-stroke engine cycles provide additional benefits in higher levels of internal EGR control.  
           [0009]    Multi-fuel, hybrid engines are known. U.S. Pat. No. 5,010,852 discloses an engine operable in both two-stroke and multi-stroke working cycles.  
           [0010]    The present invention is directed to overcoming one or more of the problems as set forth above.  
         SUMMARY OF THE INVENTION  
         [0011]    In one aspect of the invention, an internal combustion engine is provided with a plurality of combustion cylinders operable under homogenous charge combustion concepts. Each cylinder has at least one intake valve, at least one exhaust valve and separate independent operators for each valve. Each combustion cylinder is adapted for operation alternatively in two-stroke and four-stroke cycles. A control unit connected to each operator selectively operates each valve alternatively in two-stroke and for stroke cycles.  
           [0012]    In another aspect of the invention, a work machine is provided with a frame carrying an engine. The engine includes a block and a head defining a plurality of combustion cylinders operable under homogenous charge combustion concepts. Each cylinder has at least one intake valve, at least one exhaust valve and separate independent operators for each valve. Each combustion cylinder is adapted for operation alternatively in two-stroke and four-stroke cycles. A control unit is connected to each operator for selectively operating each valve alternatively in two stroke and four stroke cycles based on engine operating data.  
           [0013]    In a further aspect of the invention, a method for operating an engine is provided, with steps of providing a plurality of combustion cylinders operable in a homogenous charge compression ignition mode; operating the plurality of combustion cylinders in a two-stroke cycle homogenous charge compression ignition mode under high load conditions; and operating the plurality of combustion cylinders in a four-stroke cycle homogenous charge compression ignition mode under low load conditions. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]    [0014]FIG. 1 is a sectional, partially fragmentary view of an embodiment of an internal combustion engine of the present invention within a work machine; and  
         [0015]    [0015]FIG. 2 is a cross-sectional view taken along line  2 - 2  of FIG. 1. 
     
    
     DETAILED DESCRIPTION  
       [0016]    Referring now to the drawings, and more particularly to FIGS. 1 and 2, there is shown an embodiment of an internal combustion engine  10  of the present invention which is incorporated within a work machine such as an on-road vehicle, off-road vehicle, tractor, excavator or the like. Engine  10  also may be immobile, such as for a generating facility or the like, wherein the work machine is stationary. The work machine includes a frame  12  that carries internal combustion engine  10 , as designated schematically by phantom line  14 .  
         [0017]    Engine  10  includes an engine block  16  that defines one or more combustion cylinders  18 , and typically defines a plurality of combustion cylinders  18 , which in preferred embodiments may be between one and twenty combustion cylinders. A head  22  is provided on block  16  above all cylinders  18 . In accordance with the present invention, combustion cylinders  18  are designed and controlled to be operable under homogenous charge compression ignition (HCCI) concepts, in both two-stroke and four-stroke cycles. As stated previously herein, operation under HCCI concepts is achieved through the use of a fuel system that provides an intimate mixture of air and fuel, typically at a high air/fuel ratio, before maximum compression is reached.  
         [0018]    While in most applications of the present invention a plurality of cylinders  18  are operable under HCCI concepts, for purposes of simplicity, only one such cylinder  18  operable under HCCI concepts is shown in the drawings. A primary piston  24  is reciprocally disposed within combustion cylinder  18 , and movable between a top dead center position adjacent head  22  (as shown in FIG. 1) and a bottom dead center position at an opposing end of combustion cylinder  18 . Primary piston  24  includes a rod  26  coupled therewith on a side opposite from head  22 .  
         [0019]    Primary piston  24  also includes a crown  28  having a predefined contour that assists in mixing the fuel and air mixture that is injected into combustion cylinder  18 . The particular contour of crown  28  may vary, depending on the particular application. Fuel also can be provided premixed, as in a stationary natural gas engine. Primary piston  24  also includes one or more annular piston ring grooves  30  in the exterior periphery thereof, which each carry a respective piston ring  32 . Piston rings  32  prevent blow-by of combustion products during a combustion cycle, as is known. Primary piston  24  may also be configured without piston ring grooves  30  and piston rings  32 , depending upon the particular application.  
         [0020]    Head  22  includes a secondary cylinder  34  that is in communication with HCCI combustion cylinder  18 . A secondary piston  36  is reciprocally disposed in secondary cylinder  34 . Head  22  also includes at least one intake port  40  and one exhaust port  42 , and, as shown, includes a pair of intake ports  40  and exhaust ports  42  in which a corresponding pair of intake valves  44  and exhaust valves  46  are reciprocally disposed. In some uses of the present invention, head  22  can include more intake ports  40  than exhaust ports  42 , or more exhaust ports  42  than intake ports  40 , and is not limited to two but may include more than two intake ports  40  or exhaust ports  42 . Intake ports  40  with intake valves  44  operatively disposed therein, and exhaust ports  42  with exhaust valves  46  operatively disposed therein, are optimized for good two-stroke scavenging as well as for four-stroke operation, having appropriate intake and exhaust porting and variable valve timing, as will be described more fully hereinafter.  
         [0021]    Secondary cylinder  34  has a generally cylindrical shape in the embodiment shown, and preferably is positioned generally concentrically with combustion cylinder  18  and primary piston  24 . However, it is also possible to position secondary cylinder  34  offset relative to a longitudinal axis of primary piston  24 , depending upon the particular application. Secondary cylinder  34  is positioned adjacent to, and in communication with combustion cylinder  18 , so as to affect the fluid dynamics and chemical kinetics of the fuel and air mixture during the combustion process in combustion cylinder  18 .  
         [0022]    Secondary piston  36  is reciprocally disposed within secondary cylinder  34 , and movable between a top dead center position adjacent combustion cylinder  18  (as shown in FIG. 1) and a bottom dead center position at an opposite end of secondary cylinder  34 . Secondary piston  36  includes a crown  48  with a predefined contour, depending upon the particular application. In the embodiment shown, crown  48  is generally flat, but may also have a curved surface or compound curvature, depending upon the particular application.  
         [0023]    Secondary piston  36  includes a pair of piston ring grooves  50  that respectively carry a pair of piston rings  52 . Piston rings  52  are configured to inhibit blow-by of combustion products during combustion of the fuel and air mixture within combustion cylinder  18 . A rod  54  is coupled with secondary piston  36 , and is directly or indirectly coupled with an actuator  56  as indicated by line  58 . Secondary piston  36  is reciprocated within secondary cylinder  34  to affect the combustion timing of the fuel and air mixture within combustion cylinder  18 , as primary piston  24  reciprocates within combustion cylinder  18 .  
         [0024]    Actuator  56  controls the reciprocating position of secondary piston  36 , depending upon a position of primary piston  24  and operating conditions of engine  10 . Actuator  56  is configured as a hydraulic actuator, and acts as a plunger shaft for reciprocating secondary piston  36  between the top dead center position and the bottom dead center position. When configured as a hydraulic actuator, it will be appreciated that secondary piston  36  may be moved to or through any desired location within secondary cylinder  34 . Thus, the top dead center position and bottom dead center position of secondary piston  36  may vary. By varying the top dead center position of secondary piston  36 , the effective compression ratio of primary piston  24  and combustion chamber  18  may likewise be varied.  
         [0025]    In the embodiment shown, secondary piston  36  and secondary cylinder  34  each have a generally cylindrical shape (i.e., generally circular cross-sectional shape). However, depending upon the particular application, it is possible to configure secondary piston  36  and secondary cylinder  34  with a different cross-sectional shape, while still allowing effective reciprocation of the secondary piston within the secondary cylinder.  
         [0026]    In accordance with the present invention, engine  10  is operable in both two-stroke and four-stroke cycles of each combustion cylinder  18 . A variable valve timing system is used, without cam operation of intake valves  44  and exhaust valves  46 . Thus, each intake valve  44  is connected to an intake valve operator  60 , and each exhaust valve  46  is connected to an exhaust valve operator  62 . Operators  60  and  62  are hydraulic actuators or the like, such that each intake valve  44  and each exhaust valve  46  is operable independently of the other intake valves  44  and exhaust valves  46 , and independently of the movement and positions of primary piston  24  and secondary piston  36 .  
         [0027]    An engine control unit  64  is used to control and monitor various operations and functions of engine  10 . Control unit  64  is capable of monitoring various functions of engine  10 , by use of one or more sensors in a sensor system  66  associated with engine  10 . Each sensor of sensor system  66  is connected to control unit  64  via a signal connection  68 , which may be an electrically conductive wire. Sensors of sensor system  66  may be employed at various locations in engine  10 , to sense various engine operating conditions, such as engine speed, intake manifold air temperature, intake manifold pressure, fueling rate, cylinder pressure, exhaust back-pressure, and various other load, boost and speed conditions, all of which are known to those skilled in the art. Sensor system  66  provides data signals with regard to the various conditions to control unit  64 . Control unit  64  provides control signals to actuator  56 , intake valve operators  60  and exhaust valve operators  62  via control operating connections  70 ,  72  and  74 , respectively.  
         [0028]    Engine  10  further includes a turbocharger  80 , having a turbine  82  and a compressor  84  connected by a turbocharger shaft  86 . Turbine  82  includes an inlet  88  and an outlet  90  connected in known manner to the exhaust system of engine  10 . Turbine  82  is thereby powered by an exhaust gas stream from engine  10 , and provides power to compressor  84  via shaft  86 . Compressor  84  includes an inlet  92  for receiving combustion air, and an outlet  94  for supplying compressed combustion air to each combustion cylinder  18  of engine  10 . Turbocharger  80  has a variable geometry turbine/compressor arrangement, schematically illustrated in FIG. 1 by variable inlet nozzle  96  on turbine  82 . Those skilled in the art will recognize that variable inlet nozzle  96  is one example of a suitable arrangement to achieve variability in performance of turbocharger  80 , in accordance with the present invention. Variability also can be achieved by the use of controllable nozzles on turbine  82  and/or compressor  84 , other than variable inlet nozzle  96 . Further, turbocharger  80  can be a multistage turbocharger, including a plurality of compressors  84 . Control unit  64  is connected to turbocharger  80 , via a control operating connection  98 , to control the performance thereof, as necessary. A supercharger may also be used in addition to or in place of turbocharger  80 .  
       Industrial Applicability  
       [0029]    During operation of engine  10 , under light to part load conditions, cylinders  18  may be operated in a four-stroke cycle. Engine control unit  64  provides control signals to intake valve operators  60  and exhaust valve operators  62  to control opening and closing of intake ports  40  and exhaust ports  42  in the desired four-stroke operating mode. If control unit  64  determines from signal data received from sensor system  66  that operating in a two-stroke cycle would be advantageous, conversion from four-stroke cycles to two-stroke cycles occurs from one combustion cycle to the next. Engine control unit  64  provides control signals to intake valve operators  60  and exhaust valve operators  62  to control opening and closing of intake ports  40  and exhaust ports  42  in the desired two-stroke operating mode.  
         [0030]    Secondary piston  36  can be positioned to establish the desired compression ratio, and intake and exhaust valve timing is controlled as desired, for both two-stroke and four-stroke cycles in that secondary piston  36 , intake valves  44  and exhaust valves  46  all are independently controllable. Actuator  56 , intake valve operator  60  and exhaust valve operator  62  are separately and individually operable, as desired. Advantages may be obtained if operation follows so-called Miller cycle principles, with late intake valve closing, as those skilled in the art will readily understand. Further, with the variable compression ratio available, Melchior cycles and other advantageous operating principles are readily available in the present invention.  
         [0031]    During operation under HCCI concepts in either two-stroke or four-stroke cycles, primary piston  24  is reciprocated within combustion cylinder  18  between the bottom dead center position and the top dead center position as shown in FIG. 1, and vice versa. As primary piston  24  moves from the bottom dead center position to the top dead center position, intake valves  44  are actuated to draw in combustion air and/or an air and fuel mixture. A separate fuel injector (not shown) may also be provided. When primary piston  24  is at or near the top dead center position, and preferably shortly before the top dead center position, secondary piston  36  is likewise actuated and moved to the top dead center position adjacent combustion cylinder  18 . This effectively causes a rapid decrease in the combined volumes of combustion cylinder  18  and its associated secondary cylinder  34 , causing rapid compression of the air/fuel mixture. Sufficient energy is imparted to the fuel and air mixture within combustion cylinder  18  to cause the fuel and air mixture to combust. Secondary piston  36  is preferably held at the top dead center position for a predetermined period of time to maintain the total volume at a minimum.  
         [0032]    After combustion, primary piston  24  is moved from the top dead center position toward the bottom dead center position. Secondary piston  36  is concurrently moved toward its bottom dead center position to effectively increase the total communicating volume area. In using hydraulic actuator  24 , the bottom dead center position of secondary piston  36  may also be varied to in turn vary the compression ratio of internal combustion engine  10 . The process repeats for each combustion cycle of primary piston  24  between the bottom dead center position and top dead center position, and vice versa.  
         [0033]    As primary piston  24  moves toward the bottom dead center position, exhaust valves  46  are actuated to allow exhaust gas to exit from the combustion chamber within combustion cylinder  20 .  
         [0034]    By varying the timing of secondary piston  36 , it is possible to likewise vary the timing of the combustion sequence occurring within combustion cylinder  18 . Thus, it is possible to indirectly control the combustion sequence of the fuel and air mixture within combustion cylinder  18  using secondary piston  36 . Alternatively, once a desired compression ratio has been determined and achieved, secondary piston  36  can be held at a fixed position to provide the desired compression ratio. Such steady state operation can continue for numerous combustion cycles.  
         [0035]    Engine control unit  64  also provides control signals to turbocharger  80 , such that variable geometry components thereof, such as variable inlet nozzle  96 , are adjusted to obtain the desired performance of turbocharger  80 . EGR volumes, back-pressure to boost ratio, inlet manifold temperature and other parameters are all controllable to establish both advantageous steady-state operating conditions and suitable transition operating conditions as operation is changed from two-stroke to four-stroke, or from four-stroke to two-stroke operation  
         [0036]    Operation of engine  10  in accordance with the present invention is fuel independent, and any conventional fuel for internal combustion engines can be used.  
         [0037]    In accordance with the present invention, it is possible to maximize the performance of engine  10  by selectively operating under two-stroke and four-stroke cycles. While relatively complex control strategies can be implemented through control unit  64 , it can be understood generally that control unit  64  will control operators  60  and  62  to four-stroke operation mode during low load conditions, and to two-stroke operation mode during high load conditions. Control unit  64  can alter compression ratios and valve functions between two cycles, while also controlling secondary piston  36  to switch between low and high compression, as desired. With the related, and independent control of turbocharger  80 , appropriate boost can be provided.  
         [0038]    During the transition from four-stroke, to two-stroke operation, or vice versa, the valve events are retarded or advanced to phase combustion and ultimately change engine operating condition between two-stroke and four-stroke as needed. For example, in the transition from four-stroke to two-stroke, intake valves  44  are opened following the opening of exhaust valves  46  during cylinder  18  expansion. Exhaust valves  46  are closed during late expansion or early compression. The residual gas fraction may be higher than typical in four-stroke operation, since the exhaust valve event is truncated. To counteract this effect, the exhaust valve is closed later in the cycle than in the normal two-stroke operation. At the same time, depending on the boost to back-pressure ratio, a blower can be activated to increase cylinder scavenging. As two-stroke operation is phased in, intake valves  44  and exhaust valves  46  are closed earlier and earlier in compression, until the desired valve timing is reached. The rate of shifting the intake valve closing can be made to depend on the response of various sensed parameters (i.e. pressure, 50% burned fraction, speed, load, etc.) relative to the desired values.  
         [0039]    The present invention combines two-stroke and four-stroke, with variable compression ratios and appropriate boost to provide improved controllability of the HCCI combustion mode for the specific conditions under which engine  10  is operating.  
         [0040]    Other aspects, objects and advantages of this invention can be obtained from a study of the drawings, the disclosure and the appended claims.