Abstract:
A method of operating an internal combustion engine includes the steps of: igniting a diesel fuel and air mixture in a combustion cylinder using spark ignition; sensing knock in the combustion cylinder; adjusting a spark timing and a fuel injection amount in the combustion cylinder dependent upon the sensed knock; and igniting the diesel fuel and air mixture in the combustion cylinder using compression ignition.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates to internal combustion engines, and, more particularly, to internal combustion engines operated using homogenous charge compression ignition.  
       BACKGROUND OF THE INVENTION  
       [0002]     Internal combustion (IC) engines are typically operated using spark ignition or compression ignition. With spark ignition, a mixture of fuel and air is introduced into a combustion cylinder and compressed. A spark plug initiates combustion through the creation of an open spark sufficient to ignite the air and fuel mixture in the cylinder. With compression ignition, fuel is injected into the combustion chamber and the heat generated during compression causes the fuel and air mixture to ignite.  
         [0003]     Increasingly restrictive engine emission standards have caused more efficient engine operation and reduced emissions from such engines. Homogenous charge compression ignition (HCCI) may result in significant emission reductions. This same process is also known by the names PCI, PCCI and CAI, which stand for Premixed Compression Ignition, Premixed Charge Compression Ignition, and Controlled Auto-Ignition, respectively. In an engine operating under HCCI, the fuel is introduced into the cylinder earlier in the compression cycle than is typical. The air and fuel are intimately mixed, typically at a high air/fuel ratio or with considerable exhaust gas recirculation (EGR), before compression in the combustion cylinder. As compression occurs, the air temperature increases, and ultimately combustion is initiated at numerous locations throughout the cylinder, as the fuel droplets auto-ignite from the heat of the surrounding air. Typically, combustion occurs at lower temperatures leading to reduced noxious oxides (NOx) emissions.  
         [0004]     The use of HCCI has apparent benefits in substantial reduction of NOx emissions. However, difficulties have been encountered in implementing HCCI. Fuel preparation is important for peak operating performance of an HCCI engine. The air/fuel mixture must be intimately and thoroughly mixed. Preferably, fuel breakup occurs early in the compression cycle, allowing for intimate mixture of the air and fuel. It is desirable to create droplets of fuel as small as possible in a combustion cylinder operating under HCCI concepts. High pressure injection of the fuel can be used to create surface instabilities on the fuel droplets, causing the fuel spray to breakup and disperse.  
         [0005]     In addition to the problem of control of HCCI combustion, there is the problem of starting the engine and warming it up to arrive at a state with stable HCCI combustion. With a cold engine, HCCI combustion is very difficult to achieve and would require heating the air, obtaining very high compression ratio, or using a very easily ignited fuel or additive. The starting and warm up of an engine (whether diesel, spark-ignition, or HCCI) is affected by the ease with which the fuel can be ignited and its volatility.  
       SUMMARY OF THE INVENTION  
       [0006]     The invention comprises, in one form thereof, a method of operating an internal combustion engine, including the steps of: igniting a diesel fuel and air mixture in a combustion cylinder using spark ignition; sensing knock in the combustion cylinder; adjusting a spark timing and a fuel injection amount in the combustion cylinder dependent upon the sensed knock; and igniting the diesel fuel and air mixture in the combustion cylinder using compression ignition.  
         [0007]     The invention comprises, in another form thereof, a method of operating an internal combustion engine, including the steps of: igniting a diesel fuel and air mixture in a plurality of combustion cylinders using spark ignition; sensing knock independently in each combustion cylinder; adjusting a spark timing and a fuel injection amount independently in each combustion cylinder, dependent upon the sensed knock in that combustion cylinder; as the engine warms up, igniting the diesel fuel and air mixture in at least one combustion cylinder using compression ignition prior to the spark ignition; and transitioning ignition of the diesel fuel and air mixture in the plurality of combustion cylinders to only compression ignition. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]      FIG. 1  is a schematic illustration of an IC engine which may be used to carry out an embodiment of the method of operation of an IC engine of the present invention;  
         [0009]      FIG. 2  is fragmentary, sectional view of a combustion cylinder shown in  FIG. 1 ;  
         [0010]      FIG. 3  is a flowchart of an embodiment of the method of operation of an IC engine of the present invention; and  
         [0011]      FIG. 4  is a schematic illustration of the input signals to the electronic controller shown in  FIG. 1 . 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0012]     Referring now to the drawings, and more particularly to  FIG. 1 , there is shown an embodiment of an IC engine  10 , which may be used to carry out the method of operating an IC engine of the present invention. IC engine  10  generally includes a block  12  defining a plurality of combustion cylinders  14 . In the embodiment shown, IC engine  10  includes six combustion cylinders, but may include a different number such as three, four, eight, ten or twelve combustion cylinders, depending upon the application. IC engine  10  also includes an intake manifold  16  and an exhaust manifold  18  coupled with block  12 . Intake manifold  16  and exhaust manifold  18  are each in fluid communication with the plurality of combustion cylinders  14 , in known manner. A turbocharger  20  includes a variable geometry turbine (VGT)  22  which is rotatably driven by exhaust gas from exhaust manifold  18  via fluid line  24 . VGT  22  includes an actuatable element such as a plurality of actuatable turbine vanes or an actuatable orifice for controlling the air flow therethrough, as indicated by diagonal arrow  26 .  
         [0013]     VGT  22  rotatably drives an output shaft  28 , which in turn rotatably drives compressor  30 . Compressor  30  receives ambient combustion air, and provides compressed combustion air to intake manifold  16  via fluid line  32 . An aftercooler (not shown) may be optionally provided in communication with fluid line  32  between compressor  30  and intake manifold  16  to cool the compressed combustion air which is heated during the compression process.  
         [0014]     An electronic controller  34  is coupled with and controls operation of fuel injectors in each combustion cylinder  14 . In the embodiment shown, electronic controller  34  is coupled with fuel injectors via electric lines  38 . These fuel injectors receive fuel from a fuel tank (not shown) for supplying to combustion cylinders  14 .  
         [0015]     Electronic controller  34  is also electrically coupled with an actuator  40  for controlling exhaust gas flow through VGT  22  using, e.g., actuatable vanes or an orifice as described above.  
         [0016]     Electronic controller  34  receives input signals from an engine speed/crankshaft position sensor  42  associated with crankshaft  44  which is rotatably driven by reciprocating motion of pistons within combustion cylinders  14 .  
         [0017]     Electronic control  34  is also electrically coupled with an air flow sensor  46  via electric line  48 . Air flow sensor  46  senses the air flow rate of compressed combustion air which is introduced at intake manifold  16 .  
         [0018]      FIG. 2  is a fragmentary, sectional view of a combustion cylinder and piston arrangement shown in  FIG. 1 . Piston  50  is coupled with crankshaft  44  by a connecting rod (not shown) and reciprocates in known manner within combustion cylinder  14  between top dead center (TDC) and bottom dead center (BDC) positions. Piston  50  has a contoured head  52  which assists in HCCI compression ignition during operation. Piston  50  includes a number of piston ring grooves near head  52  which carry respective piston rings (not shown). An intake port  54  is in communication with intake manifold  16 , and an exhaust port  56  is in communication with exhaust manifold  18 . It will be appreciated that the valves for selectively opening and closing intake port  54  and exhaust port  56  are not shown for simplicity sake. A fuel injector nozzle  58  sprays a selected amount of diesel fuel into combustion cylinder  14  and is combined with combustion air from intake port  54  for ignition and combustion within combustion cylinder  14 . The combustion of the fuel and air mixture is ignited through spark ignition using spark plug  60  during warm up periods, and is ignited using compression ignition upon movement of piston  50  to the TDC position after an initial warm up period.  
         [0019]     In the embodiment shown, spark plug  60  includes a first electrode  62  and a second electrode  64  spaced apart by a spark gap therebetween. During warm up periods, a spark is generated between first electrode  62  and second electrode  64  as piston  50  is moving toward the TDC position shown in  FIG. 2 . The spark ignites the fuel and air mixture in combustion cylinder  14 . When spark plug  60  is not being used to generate a spark for ignition of the fuel and air mixture, first electrode  62  and second electrode  64  are used at selected points in time as an ionization sensor for sensing ionization of the fuel and air mixture which occurs upon ignition.  
         [0020]     Referring now to  FIG. 3 , there is shown a flow chart of an embodiment of the method of operation of an IC engine of the present invention. In general, the IC engine is assumed to be started at a cold state during which spark ignition of the fuel and air mixture occurs. The spark timing and the fuel injection amount are adjusted as the engine warms up from the cold state to an operating, steady state condition. After initial warm up, compression ignition occurs prior to the spark ignition being initiated, and therefore the spark ignition is no longer used when in a steady state condition after warm up.  
         [0021]     At initial start up of the IC engine, the fuel quantity per cycle as well as the spark timing are determined from a lookup table (block  66 ). The specified quantity of fuel is injected into combustion cylinder  14  when piston  50  is near a BDC position (block  68 ). During warm up, the fuel and air mixture within combustion cylinder  14  will not ignite upon movement of piston  50  toward the TDC position. First electrode  62  and second electrode  64  of spark plug  60  are used to monitor ionization of the fuel and air mixture within combustion cylinder  14  indicating compression ignition (decision block  70 ). If compression ignition has not occurred and the spark ignition flag is turned on (decision block  72 ), then spark plug  60  is fired at a specified crank angle of crank shaft  40  using crank shaft position sensor  42  (block  74 ). After firing of spark plug  60 , electrodes  62  and  64  are again used to monitor for ionization within combustion cylinder  14  to determine if knock has occurred (block  76 ). If combustion started upon generation of the spark (decision block  78 ) and the cylinder did not knock (decision block  80 ) then control loops back to block  68  for the next fuel injection cycle via control line  82 . On the other hand, if combustion occurred upon generation of the spark but the cylinder did knock, then the spark ignition timing signal is retarded (block  84 ) and control loops back to block  68  for the next fuel injection cycle via line  86 .  
         [0022]     In the event that combustion occurred before generation of the spark using spark plug  60 , a determination is made as to whether the combustion initiation was too early in the cycle (decision block  88 ). If combustion did occur too early, then the injected fuel quantity is reduced an appropriate amount (block  90 ) and control loops back to block  68  via line  92 . On the other hand, if combustion was not too early, then another query is made as to whether combustion was too late in the cycle (decision block  94 ). If combustion did not occur too late, then the IC engine has warmed up a suitable amount to turn off spark generation using spark plug  60  (block  96 ) and control loops back to block  68  for the next fuel injection cycle. On the other hand, if combustion did occur to late in the cycle, then the amount of fuel which is injected into the next cycle is increased (block  98 ). If the spark ignition flag is off, the spark ignition flag is turned on and a spark timing is obtained from a look up table (block  100 ). Control then loops back to block  68  via line  86 .  
         [0023]      FIG. 4  illustrates inputs and outputs for the six cylinder embodiment of IC engine  10  shown in  FIG. 1 . It should be noted that the spark signals and the ion sensor signals are each generated from the six spark plugs  60  associated with the respective combustion cylinders  14 . In addition to the crankshaft position/speed sensor  42  and airflow sensor  46  shown in  FIG. 1 , it should be noted that other sensors corresponding to the camshaft position, engine air flow rate, intake manifold pressure and/or intake manifold temperature may also optionally be used.  
         [0024]     Having described the preferred embodiment, it will become apparent that various modifications can be made without departing from the scope of the invention as defined in the accompanying claims.