Abstract:
A system and method are disclosed for operating an internal combustion engine in which the intake valves are electromechanical valves and the engine has direct fuel injection. By opening the intake valves during the intake stroke when the piston is moving at its maximum speed, the turbulence through the intake valve is enhanced, thereby increasing combustion speed, and hence combustion stability at low torque, low speed operating conditions. Furthermore, if the fuel injection interval occurs when flow of gases through the intake is highest, air-fuel mixing is improved.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates generally to a method for controlling electromechanical valves in an internal combustion engine with direct fuel injection.  
       BACKGROUND OF THE INVENTION  
       [0002]     An electromechanically operated poppet valve in the cylinder head of an internal combustion, as disclosed in U.S. Pat. No. 4,455,543, is actuated by energizing and de-energizing electromagnets acting upon an armature coupled to the poppet valve. Because the actuation of the electromagnets is controlled by an electronic control unit, valve opening and closing events occur independently of engine rotation. In conventional engines with camshaft actuated valves, which have timings based on engine rotation, air delivery to the cylinders is controlled by a throttle valve placed in the inlet duct of the engine. In contrast, electromechanical valves are capable of controlling air delivery based on valve timing, thereby providing a thermal efficiency improvement over throttled operation of a conventional engine.  
         [0003]     However, a drawback to electromechanical valves, particularly at low torque, is the undesirable noise generated when the valves impact upon opening and closing. Furthermore, because there is no throttling, or less throttling, the incoming air through the valves has very little turbulence. The ensuing combustion wave propagates very slowly through the relatively quiescent mixture, leading to combustion instability and rough operation. Furthermore, fuel-air mixing, particularly in engines with direct fuel injection, is insufficient at low turbulence levels.  
       SUMMARY OF THE INVENTION  
       [0004]     Disadvantages of prior methods are overcome by a method for operating an internal combustion engine, the engine having a plurality of engine cylinders with reciprocating pistons. Each cylinder has an electromechanically-actuated intake valve, an exhaust valve, and a fuel injector disposed in a cylinder head of the engine. The engine also has an electromechanical valve system with: an armature connected to the intake valve, a valve closing electromagnet capable of exhibiting an electromagnetic force for attracting the armature to open the intake valve, a valve opening spring for biasing the armature in a direction to open the intake valve, and a valve closing spring for biasing the armature in a direction to close the intake valve. The method includes de-energizing the valve closing electromagnet associated with a particular cylinder during an intake stroke such that the intake valve is fully open when a speed of the piston within the particular cylinder is near a maximum; opening the fuel injector so that fuel sprays into the particular cylinder during peak flow rate through the intake valve; and energizing the valve closing electromagnet after a predetermined time has elapsed.  
         [0005]     Also disclosed is an internal combustion engine with a plurality of cylinders. The engine has an electromagnetically-actuated intake valve disposed in each cylinder, a piston in each cylinder, an armature operatively connected to said intake valve, a valve closing electromagnet capable of exhibiting an electromagnetic force for attracting the armature to close the intake valve, a valve opening spring coupled to the armature for biasing the armature in a direction to open the intake valve, and a valve closing spring coupled to the armature for biasing the intake valve to a closed position. The engine is coupled to an electronic control until which is further coupled to the valve closing electromagnet. The electronic control unit de-energizes the valve closing electromagnet in a particular cylinder during an intake stroke in the particular cylinder. The electronic control unit energizes the valve closing electromagnet. The time of de-energizing is such that the intake valve is fully open near a time of a maximum speed of the piston in the particular cylinder.  
         [0006]     An advantage of the present invention is that the valve is opened at such a time in the intake stroke to provide a high degree of turbulence to the intake gases in the cylinder. The higher turbulence increases the combustion rate 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0007]     The advantages described herein will be more fully understood by reading an example of an embodiment in which the invention is used to advantage, referred to herein as the Detailed Description, with reference to the drawings wherein:  
         [0008]      FIG. 1  is a schematic of an engine equipped with electromechanically-actuated poppet valves;  
         [0009]      FIG. 2  is a detail of an example of an electromechanically-actuated poppet valve in a closed position;  
         [0010]      FIG. 3  is a detail of an example of an electromechanically-actuated poppet valve in an open position;  
         [0011]      FIG. 4   a  is a graph of valve position for an electromechanically actuated poppet valve operated using both the valve closing electromagnet and the valve opening electromagnet;  
         [0012]      FIG. 4   b  is a graph of valve position for an electromechanically actuated poppet valve operated using only the valve closing electromagnet;  
         [0013]      FIG. 5   a  is a graph of piston position as a function of crank angle degree;  
         [0014]      FIG. 5   b  is a graph of piston speed as a function of crank angle degree; and  
         [0015]      FIG. 6  is a flowchart showing a method of operating the intake valve and fuel injector according to an aspect of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0016]     In  FIG. 1 , a single cylinder  13  of an internal combustion engine  10  with an electromechanical intake valve  20  and exhaust valve  19  is shown. Engine  10  contains a piston  14  which reciprocates within cylinder  13 . Intake valve  20 , disposed in cylinder head  22 , is opened to allow gases to communicate between the combustion chamber (the volume enclosed by cylinder  13 , piston  14 , and cylinder head  22 ) and intake port  70 . When exhaust valve  19  is opened, gases are released from the combustion chamber into exhaust port  72 . In the embodiment shown in  FIG. 1 , fuel is injected into the combustion chamber by injector  16 , a configuration commonly called direct fuel injection. Intake valve  20  and exhaust valve  19  are actuated electromechanically by valve actuators  18  and  17 , respectively. In a preferred embodiment, engine  10  is a spark-ignited engine, spark plug  12  initiates combustion in the combustion chamber. The present invention also applies to engines with other types of igniters and to compression ignition engines in which the fuel and air spontaneously ignite due to a compression-generated temperature rise in the combustion chamber. Both diesel and homogeneous charge compression ignition are examples of the latter type of engine.  
         [0017]     Continuing to refer to  FIG. 1 , electronic control unit (ECU)  60  is provided to control engine  10 . ECU  60  has a microprocessor  46 , called a central processing unit (CPU), in communication with memory management unit (MMU)  48 . MMU  48  controls the movement of data among the various computer readable storage media and communicates data to and from CPU  46 . The computer readable storage media preferably include volatile and nonvolatile storage in read-only memory (ROM)  50 , random-access memory (RAM)  54 , and keep-alive memory (KAM)  52 , for example. KAM  52  may be used to store various operating variables while CPU  46  is powered down. The computer-readable storage media may be implemented using any of a number of known memory devices such as PROMs (programmable read-only memory), EPROMs (electrically PROM), EEPROMs (electrically erasable PROM), flash memory, or any other electric, magnetic, optical, or combination memory devices capable of storing data, some of which represent executable instructions, used by CPU  46  in controlling the engine or vehicle into which the engine is mounted. The computer-readable storage media may also include floppy disks, CD-ROMs, hard disks, and the like. CPU  46  communicates with various sensors and actuators via an input/output (I/O) interface  44 . Examples of items that are actuated under control by CPU  46 , through I/O interface  44 , are fuel injection timing, fuel injection rate, fuel injection duration, throttle valve position, spark plug  12  timing, actuation of valve actuators  18  and  17  to control opening and closing of intake valve  20  and exhaust valve  19 , respectively, and others. Sensors  42  communicating input through I/O interface  44  may be indicating piston position, engine rotational speed, vehicle speed, coolant temperature, intake manifold pressure, pedal position, throttle valve position, air temperature, exhaust temperature, exhaust stoichiometry, exhaust component concentration, and air flow. Some ECU  60  architectures do not contain MMU  48 . If no MMU  48  is employed, CPU  46  manages data and connects directly to ROM  50 , RAM  54 , and KAM  52 . Of course, the present invention could utilize more than one CPU  46  to provide engine control and ECU  60  may contain multiple ROM  50 , RAM  54 , and KAM  52  coupled to MMU  48  or CPU  46  depending upon the particular application.  
         [0018]     In  FIG. 2 , an example of an electromechanical valve actuator  18  is shown in which intake valve  20  is in a closed position. Intake valve  20  closes off port  70  in cylinder head  22 . Valve actuator  18  is shown in detail in  FIG. 2 . A valve closing spring  24  biases valve  20  to the closed position. Armature  30  is disposed between two electromagnets: a valve closing electromagnet  32  and valve opening electromagnet  28 . Armature  30  is connected to shafts  26  and  34 . As shown in  FIG. 2 , armature  30  is next to valve closing electromagnet  32 . For this position to prevail, valve-closing electromagnet  32  is energized. Otherwise, armature  30  would act under the influence of valve closing spring  24  and valve opening spring  36 . In the embodiment shown in  FIG. 2 , valve opening spring is attached to shaft  34  at the lower end of valve opening spring  36 . Other alternative configurations may also provide the same functionality, e.g., an electrohydraulic system. If both electromagnets  28  and  32  are de-energized, armature  30  is influenced by springs  24  and  36  and attains a neutral position in between electromagnets  28  and  34 . Valve actuator  17  and exhaust valve  19  can also be represented by  FIG. 2 , by way of example.  
         [0019]     Continuing to refer to  FIG. 2 , valve actuator  18  preferably includes a valve position-sensing device, such as a linear variable differential transformer (LVDT)  38 . The tip of shaft  34  forms the core of the position sensor. The inductance of the LVDT varies when the position of the shaft  34  is altered with respect to the LVDT  38  windings. LVDT  38  is connected to ECU  60  (connection not shown). LVDT  38  is shown by way of example; other types of position sensing devices may also be used.  
         [0020]      FIG. 3  shows the same hardware as shown in  FIG. 2  with the difference being that  FIG. 2  shows valve  20  in the fully closed position and  FIG. 3  shows valve  20  in the fully open position. Thus, in  FIG. 2 , valve closing electromagnet  32  is energized and, in  FIG. 3 , valve opening electromagnet  28  is energized. In  FIG. 2 , valve opening spring  36  is compressed. Holding current is applied to valve closing electromagnet  32  to act against the spring tension of valve opening spring  36 . Analogously, in  FIG. 3 , valve closing spring  24  is compressed. Holding current is applied to valve opening electromagnet  28  to act against the spring tension of valve closing spring  24 .  
         [0021]     Before discussing aspects of the present invention, an example of prior art control of an electromechanical valve is described. Typically, a valve, whether an intake or exhaust valve, of an internal combustion engine is normally closed, i.e., the valve is in the closed position for more of the time than the open position. Thus, the description of valve opening begins with a closed valve, i.e., with a holding current be applied to valve closing electromagnet  32 . Actuating the valve proceeds by: de-energizing valve closing electromagnet  32  which causes the valve to open under the influence of valve opening spring  36 ; applying a peak current to valve opening electromagnet  28  to grab armature  30  when it is near its fully open position; applying a holding current to valve opening electromagnet  28  after armature  30  is attracted to valve opening electromagnet  28 ); applying holding current for as long as the desired open duration of the valve; de-energizing valve opening electromagnet  28  which causes the valve to close under the influence of valve closing spring  24 ; and, applying a peak current to valve opening electromagnet  32  to grab armature  30  when it is near its fully closed position. The terms peak current and holding current are concepts known to those skilled in the art and refer to a higher current level (peak current) used to catch a moving armature  30  and a lesser current (holding current) used to prevent a stationary armature  30  from moving.  
         [0022]     The neutral position, i.e., the position that valve  20  attains when both electromagnets  28  and  34  are de-energized, is about halfway between the fully closed position,  FIG. 2 , and fully open position,  FIG. 3 . The exact neutral position would depend, though, on the relative spring tensions of valve opening spring  36  and valve closing spring  24 .  
         [0023]     The valve lift profiles for normal valve operation are shown in  FIG. 4   a . The valve opens and is held open for a variable duration and then the valve is closed. Three example durations are shown in  FIG. 4   a . The minimum duration is the sum of the opening time and the closing time and the maximum duration is infinite.  
         [0024]     In  FIG. 4   b , a plot of valve position as a function of time is shown for valve  20  under the situation that the valve at time T 0  is at the fully closed position by virtue of holding current being applied to valve closing electromagnet  32 . At time T 0 +, valve closing electromagnet  32  is de-energized. The valve lifts from the fully closed position and proceeds to a nearly open position by action of the valve opening spring  36 . As valve  20  progresses to a nearly open position, valve closing spring  24  becomes compressed. Valve  20  then returns to a nearly closed position under the influence of the valve closing spring  24 . The period of time that it takes for the valve to leave the fully closed position, travel to a nearly open position, and return to a nearly closed position is called a valve period and is indicated as T 1  in  FIG. 4   b . The oscillation of valve  20  continues, with each successive peak and trough being closer to the neutral position than the prior peak or trough, due to irreversibilities in the system. Eventually, valve  20  stops oscillating and attains the neutral position (not shown in  FIG. 4 ). Period T 2  is twice period T 1  and period T 3  is three times period T 1 , etc. The first three troughs of the curve in  FIG. 4   b  are lower than the maximum grabbing distance dotted line with the 4 th  trough being above the maximum grabbing distance. The maximum grabbing distance is the maximum distance away from the fully closed position that armature  30  may be and still allow valve closing electromagnet  32  to attract armature  30 . If armature  30  is farther away from the fully closed position than the maximum grabbing distance, valve closing electromagnet  32  cannot attract armature  30 , that is, at the peak current of the driving system (not shown). For the example shown in  FIG. 4   b , after de-energizing valve closing electromagnet  32 , armature  30  may be allowed to oscillate three periods and still allow valve closing electromagnet  32  to catch armature  30  at around the end of period T 3 . If valve closing electromagnet  32  were not caught before valve  20  begins the fourth oscillation, valve  20  would not come to a position where valve closing electromagnet  32  could exert enough attractive force to catch valve  20 . The discrete times at which the valve can be grabbed are designated with an X on the abscissa of  FIG. 4   b.    
         [0025]     Referring now to  FIG. 5   a , piston position as a function of crank angle degree is shown.  FIG. 5   b  shows piston speed as a result of the change in piston position. As the piston travels from top dead center (TDC) to bottom dead center (BDC) accomplished during the 0-180 crank degrees of crank rotation, the piston speed is a 0 speed at 0 degrees, at a maximum at approximately 90 crank degrees, and returns to 0 speed at about 180 degrees. Peak piston speed occurs during the middle of the intake stroke. Because flow through the valve is influenced by the vacuum generated in the combustion chamber which is induced by the piston movement, peak flow through the valve is related to the maximum piston speed.  
         [0026]     In both  FIGS. 5   a  and  5   b , purely sinusoidal piston movement and speed are shown. The actual piston move and piston movement deviate slightly from a sinusoid, actual movement being a function of crank throw and stroke length.  FIGS. 5   a  and  5   b  are approximations to true piston movement.  
         [0027]     It is well known to those skilled in the art, that combustion stability is poor at low torque engine conditions, partially to low turbulence levels in the combustion chamber. Turbulence is enhanced when the speed of gases flowing through the intake valve is increased. By timing the opening of intake valve  20  such that piston  14  is near its maximum speed increases the flow velocity through intake valve  20 . With a direct fuel injected engine, such as shown in  FIG. 1 , fuel air mixing is enhanced when fuel injection occurs concurrent with maximum flow through intake valve  20 . Intake flow blows by the injector shearing the fuel jet and causing the air to entrain fuel droplets.  
         [0028]     Referring to  FIG. 6 , a method by which the present invention can be used to advantage is illustrated with a flow chart. The algorithm starts in block  80 . Control passes to block  82  in which it is determined if piston speed is in an appropriate range, meaning whether the piston is moving sufficiently fast to cause a high intake flow when intake valve is opened. If not, wait until a positive result in block  82 , from which control passes to block  84 . The valve closing magnet is de-energized allowing intake valve  20  to open. Control passes to block  86  in which it is determined whether the flow of gases through intake valve  20  is appropriate for beginning fuel injection. If a negative result in block  86 , wait until a positive result, from which control passes to block  88 . In block  88  the fuel injector is actuated. Control then passes to block  90  in which it is determined whether intake valve  20  is in an appropriate range to catch intake valve  20 . If so, control passes to block  92  in which the valve closing electromagnet is actuated to close intake valve  20 .  
         [0029]     In an alternative embodiment, decision blocks  82 ,  86 , and  90  of  FIG. 6  are supplanted by an model of valve dynamics, flow through the intake valve, piston speed, etc. That is, a model is used to determine at what crank angle the piston speed is appropriate, based on current operating conditions, to send out a signal to de-energize the valve closing electromagnet, and similarly for blocks  86  and  90 . In an alternative embodiment, lookup tables are used in place of a model of the system to determine when to perform the de-energization, fuel injection, and energization. The lookup table is a function of one or more of engine speed, manifold vacuum pressure, desired torque  
         [0030]     Another factor in determining the time at which the intake valve is caused to open is the amount of air desired in the cylinder. There are situations in which the intake valve is opened earlier or later than the exact optimal time for inducing intake turbulence so that the appropriate amount of air is inducted into the cylinder; the desire amount of air is determined so as to supply the desired amount of engine torque.  
         [0031]     In one embodiment, it is desirable to open the intake valve more than once during the intake stroke. Some air is inducted during the first opening and when the intake valve is closed, further downward motion of the piston causes a vacuum to develop in the cylinder. When the intake valve is opened a second time, the pressure difference across the valve induces a greater degree of turbulence than if the valve were left open. Because of the rush of air that is induced when the valve is opened, mixing is enhanced.  
         [0032]     In yet another embodiment, fuel is injected between the first opening and the second opening. If the rush of air is too forceful, the injected fuel may be pushed against cylinder walls. Thus, by injecting the fuel in between the two intake periods, the fuel is injected into highly turbulent air, but not pushed against the wall.  
         [0033]     While several modes for carrying out the invention have been described in detail, those familiar with the art to which this invention relates will recognize alternative designs and embodiments for practicing the invention. The above-described embodiments are intended to be illustrative of the invention, which may be modified within the scope of the following claims.