Abstract:
The present invention provides a system for determining engine stop position and includes an engine tracking subsystem and a throttle control subsystem. The engine tracking subsystem is coupled to the engine and determines the engine position by sensing rotation of the crankshaft. Once the engine controller receives an engine shutdown signal, the throttle is controlled to lower the air pressure in the intake manifold of the engine. Lowered as such, the resulting reversal torque caused by compression of air in the cylinders is smaller than the friction load torque of the engine and engine reversal is eliminated or substantially reduced. When the engine has stopped, the engine tracking system stores the last engine position for use during the next engine startup.

Description:
BACKGROUND  
       [0001]     1. Field of the Invention  
         [0002]     The present invention generally relates to a system and method for tracking the angular position of an engine&#39;s crankshaft.  
         [0003]     2. Description of Related Art  
         [0004]     Various systems for tracking the angle of an engine are known. Known systems determine the engine position from a sensor that generally works only above a minimum speed. These systems are based on a profile of the rotation of two engine position wheels, one on the crankshaft and one on the camshaft. In addition, at start-up these systems require the engine to initially rotate through an angle before the engine position becomes known. The amount of requisite angular displacement is dependent on the initial engine position.  
         [0005]     It is desirable to know the engine position at engine startup, as this allows the system to fuel and ignite the very first possible cylinder. In the example of a port injected engine, the first possible cylinder would be the cylinder with an open or about to be opened intake valve. The benefits available from early ignition include minimization of tailpipe hydrocarbon emissions due to “crank-through” of fuel vapors from the intake manifold to the exhaust manifold, the minimization of crank time, and the reduction of crank time variability.  
         [0006]     Typically, determination of engine position or engine tracking begins at engine crank and is not complete until some amount of engine rotation. The requisite rotation can slightly exceed two revolutions, depending on configuration. People have proposed systems that leave the controller powered after the engine off command and track the engine position until it comes to rest. However, known sensors have difficulty identifying engine reversals as the engine slows to a stop. Further, methods to detect the reversals are complex and can become unreliable in the presence of missing teeth on the position encoder wheel.  
         [0007]     In view of the above, it is apparent that there exists a need for an improved engine position tracking system.  
       SUMMARY  
       [0008]     In satisfying the above need, as well as overcoming the enumerated drawbacks and other limitations of the related art, an embodiment of the present invention provides a system that includes an engine tracking subsystem for determining engine angle and a throttle configured to lower air pressure in the engine&#39;s intake manifold and thus lower the ingested air thereby reducing the cylinders compression torque based on an engine shutdown signal.  
         [0009]     The engine tracking subsystem is coupled to the engine and determines the angle of the engine by sensing rotation of the crankshaft. As the engine controller receives an engine shutdown signal, the throttle is controlled to lower the air pressure in the intake manifold of the engine. The air pressure is lowered such that the resulting reversal torque caused by compression of air in the cylinders is smaller than the friction torque of the engine thereby minimizing or eliminating engine reversal. To lower the air pressure, the throttle is closed and remains closed until the engine is stopped. Thereafter, the throttle is slightly opened increasing the air pressure in the engine to avoid the drawing of exhaust fumes back into the intake manifold. When the engine is stopped, the engine tracking system stores the engine angle for use during engine startup. Because engine reversal has been eliminated, the stored engine angle remains the correct engine position for the next startup. Alternatively, if valve actuation is available (Variable Cam Timing, or Electrically Actuated Valves) a cylinder&#39;s compression torque can be reduced by altering the valve timing, (for example: late closing of the intake valve).  
         [0010]     In a foot-operated throttle system, a throttle bypass valve provides air control when the driver&#39;s foot is off the accelerator pedal. Alternatively, instead of a throttle valve being commanded to close at the engine-off command, a throttle bypass valve could be commanded to close.  
         [0011]     In another aspect of the invention, the throttle is closed immediately upon key-off. The fuel injection system is configured to continue injecting for a predetermined time after key-off. Further, the ignition system is configured to continue sparking after the fuel injection has ceased. By allowing fuel injection and spark ignition for a short time after engine-off request, while still closing the throttle at the engine-off request, the intake manifold pressure is lower than it would otherwise be if all the actions were taken simultaneously.  
         [0012]     Further objects, features and advantages of this invention will become readily apparent to persons skilled in the art after a review of the following description, with reference to the drawings and claims that are appended to and form a part of this specification. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]      FIG. 1  is a diagrammatic view of an engine and controller including a system for determining the engine stop position according to the present invention;  
         [0014]      FIG. 2  is a flow chart of an engine shutdown sequence according to the present invention;  
         [0015]      FIG. 3  is a flow chart providing another embodiment of an engine shutdown sequence according to the present invention;  
         [0016]      FIG. 4  is a plot of engine angle travel with a normal intake manifold pressure according to the present invention; and  
         [0017]      FIG. 5  is a plot of engine angle travel with low intake manifold pressure according to the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0018]     Referring now to  FIG. 1 , a system  8  embodying the principles of the present invention is illustrated therein. The system generally includes an engine  10  and a controller  12 .  
         [0019]     The engine  10  is shown as an internal combustion engine having a throttle  30 , a piston  22 , and a cylinder  20 . As will be apparent from the discussion that follows, the engine  10  could be provided with any number of cylinders and the system  8  readily adapted thereto. Each cylinder  20  houses a piston  22  mounted for reciprocal movement therein. Combustion in the cylinder  20  will cause movement of the piston  22  resulting in a rotation of the crankshaft  48 , which is used to transfer power from the engine  10  to the drivetrain and other systems within the vehicle.  
         [0020]     Air entering the cylinder  20  from the intake manifold  28  is controlled by the throttle  30  and is combined with fuel, injected from a fuel injector  26 , to form a gas/air mixture in the cylinder  20 . The fuel injector may inject directly into the cylinder as shown or it may inject into the intake port. A spark is generated by a spark plug  24  to initiate combustion in the cylinder  20  thereby creating motion of the piston  22 . To create continuous rotation of the crank shaft  48 , the pistons  22  are positioned at varying engine angles relative to the crank shaft  48  and the controller  12  synchronizes combustion in each cylinder to cause a smooth rotation of the crank shaft  48 . After combustion, exhaust gasses are forced out of the cylinders  14 , as the piston  22  rises on the next part of its cycle and exit through the exhaust manifold  32 .  
         [0021]     As the engine  10  produces continuous rotation of the crankshaft  48 , a flywheel  52  is also rotated. Teeth  50  are provided at equally spaced positions around the circumference or perimeter of the flywheel  52  with one or two teeth missing. A sensor  54 , located proximate to the flywheel  52 , produces a signal as each tooth  50  is rotated therepast. This signal is provided to the controller  12  along line  56 . The controller  12  includes a microprocessor  40  which counts the number of signals provided from the sensor  54 . By counting the signals, the microprocessor  40  can keep track of the engine position or angle.  
         [0022]     Additionally, the microprocessor  40  optimizes the engine&#39;s performance by controlling the fuel injectors  26 , the timing of the spark plugs  24 , and the throughput of the throttle  30 . The position of the throttle  30  controls the amount of air allowed to flow through the intake manifold plenum  31  to the intake manifold  28  and into the cylinder  20 . The position of the throttle  30  is manipulated by the controller  12  through the throttle actuator  29 . The air flow into the cylinder  20  can also be controlled through cam timing. The timing of the cam shafts  66  can be manipulated by the controller  12  through the cam timing actuator  64 . The cam shafts  66  drive the opening and closing of the intake valve  67  and exhaust valve  68 .  
         [0023]     As a key switch  62  is switched to the off position, an engine shutdown signal is sent along line  60  to the controller  12  thereby initiating an engine shutdown sequence in the microprocessor  40 . During the shutdown sequence, engine position continues to be monitored by the sensor  54  and the controller  12 . After the engine has stopped, the last engine position is stored in a memory  46  of the controller  12  for use in the next engine startup.  
         [0024]     The engine shutdown sequence operates to reduce the engine&#39;s maximum compression torque to near or lower than the engine&#39;s friction torque in order to eliminate or reduce engine reversal on spin down. Lowering compression torque is readily accomplished by closing the throttle  30 .  
         [0025]     In addition, various forms of valve timing control are coming into use on automotive engines. Since valve timing influences the mass of gasses that are compressed in the cylinder  20 , valve timing is a way to either augment or substitute for closing the throttle  30 . While many compression torque reducing schemes are contemplated, the most readily accomplished scheme is to close the intake valve  67  later than normal. With ideal valving, the intake valve  67  is closed at the beginning of the compression stroke. If the intake valve  67  closing is delayed, then some gas consisting of air and residual combustion products can be pushed backwards out of the intake valve  67  instead of being compressed in the cylinder  20 . Effectively, this reduces the engine&#39;s compression ratio and compression torque is reduced, thus reducing the engine&#39;s propensity to reverse as it slows to a stop.  
         [0026]     An engine shutdown sequence in accordance with the present invention is shown in  FIG. 2 . Referring thereto, the process begins in block  80 . In block  81 , the controller  12  determines if an engine shutdown signal has been received, for example, by key switch  62  being moved to its “off” position. If an engine shutdown signal has not been received, the engine continues to run normally as indicated by the loop of line  82 . If an engine shutdown signal has been received,  10541 - 1917  the sequence flows along line  84  and the air pressure in the intake manifold  28  is decreased by fully closing the throttle  30  to prevent engine reversals, as denoted by block  86 . In the case of a foot operated throttle, the throttle  30  is referred to as an idle bypass valve. As shown in box  88 , the engine tracking system continues to track the engine position during the shutdown sequence. Next, in block  90 , the system determines if the engine  10  is fully stopped. If the engine  10  is not fully stopped, the sequence follows the loop of line  92  allowing the system to maintain a low intake manifold pressure with the throttle  30  closed (block  86 ) and continue to track the engine position (block  88 ). However, if the engine  10  has stopped, the logic flow follows line  94  and the engine position is recorded for use in a subsequent engine startup, as denoted by box  96 . After the engine position has been recorded or simultaneous therewith, the throttle  30  is opened, generally equalizing pressure in the system  8  to prevent the intake manifold  28  from filling with exhaust gas. Preferably, the default throttle position at engine stop is open between 3° and 8°. The process then ends at block  99 .  
         [0027]     Now referring to  FIG. 3 , another embodiment of an engine shutdown sequence according to the present invention is provided therein. At block  100  the engine shutdown sequence begins. In block  101 , the controller  12  determines whether an engine shutdown signal has been received. If an engine shutdown signal has not been received, the engine  10  continues to run as normal, as denoted by the loop of line  102 . However, if an engine shutdown signal has been received, the engine shutdown sequence flows along line  104  where air pressure in the intake manifold  28  is reduced, by fully closing the throttle  30 , to prevent engine reversals. This is denoted by block  106 . Block  108  indicates that a predetermined delay, either time based (for example 0.1 seconds), or fuelling event based (for example, 2 fuel injection events) is provided after which the controller  12  stops scheduling new fuel injection events, as denoted by block  110 . As indicated by block  112 , the controller  12  continues to track the engine position as is normally done. In block  114 , the controller  12  determines whether the engine  10  has fully stopped. If the engine  10  has not stopped, the shutdown sequence flows along the loop of path  116  where the controller  12  continues to maintain low intake air pressure and to track the engine position, as denoted by block  117 . However, if the engine  10  has fully stopped, the shutdown sequence follows along line  118  and the spark ignition is fully shutdown, as denoted by block  120 . The engine position is then recorded for use in the next engine startup, as denoted by block  122 . In block  124 , the throttle  30  is open to prevent the intake manifold  28  from filling with exhaust gas. The process then ends at block  125 .  
         [0028]     As noted above, the lowering of the air pressure in the intake manifold  28  is instrumental in preventing engine reversals. Now referring to  FIG. 4 , line  60  shows the travel of the engine as measured with a laboratory instrument, a quadrature encoder, with each vertical transition indicating a 0.25° movement of the engine; line  62  denotes the direction of travel of the engine (either forward or reverse); line  64  denotes conventional manifold pressure; all the above represented as typically provided by known systems. With conventional manifold pressure during engine shutdown, the engine moves forward slowing down (as seen with line  60  generally at 2.2-2.3s) and reversing as line  62  goes high. The change in the direction of engine travel is due to the reversal torque of the air compressed in the cylinders overcoming the engine inertial torque and friction torque. Thereafter, the engine reverses again, as denoted by line  62  going low (between 2.4 and 2.5s) as the air in the opposite cylinders is compressed and overcomes the engine inertial torque and the friction torque to move in the reverse or forward direction. Inspection and analysis of the signal represented by line  60 , indicates that the full reverse travel of the engine is approximately 90.75° under conventional manifold pressure.  
         [0029]     When closing the throttle  30  to lower the manifold pressure in accordance with the present invention, referring to  FIG. 5 , the manifold pressure is represented by line  74 ; line  72  represents the direction of engine travel, by line  72  transitioning high, and indicates the direction of the engine  10  did reverse once; and line  70  represents the rotation of the engine  10  where each vertical transition represents a 0.25° increment of movement. As can be seen from line  70 , the engine  10  progressively slowed and, although it reversed slightly as line  72  indicates by its high transition, the amount of reverse rotation was smaller than 0.25° in that there is no corresponding vertical component to line  70 . Further analyzing the signal represented by line  70 , it was determined the engine had produced a reverse rotation of approximately 0.25°. The reduced engine reversal provides an accurate engine position that can be used to optimize engine startup thereby reducing hydrocarbon emission, minimizing crank time, and reducing crank time variability.  
         [0030]     As a person skilled in the art will readily appreciate, the above description is meant as an illustration of implementation of the principles this invention. This description is not intended to limit the scope or application of this invention in that the invention is susceptible to modification, variation and change, without departing from spirit of this invention, as defined in the following claims.