Abstract:
In an internal combustion engine having both a port injector and a direct injector supplying fuel to a cylinder of the engine, a method is disclosed for avoiding deposit formation on and/or inside the tip of the direct injector. The tip temperature is estimated. When the tip temperature exceeds a threshold temperature at which deposits are formed, the amount of fuel delivered by the direct injector is increase.

Description:
FIELD OF THE INVENTION 
       [0001]    Deposits can form on and in injectors which are disposed within a combustion chamber of a gasoline-fuelled engine. The present invention concerns mitigating such deposit formation. 
       BACKGROUND OF THE INVENTION 
       [0002]    Direct injection (DI) for gasoline-fuelled engines present a fuel economy benefit by providing charge cooling, thereby allowing a modest increase in compression ratio. A drawback of direct injection, however, is that there is less time available for the fuel injection to take place compared to port injection. That is, with a port injected engine, the fuel injection pulse width can comprise almost 720 crank degrees. The fuel sprayed in the port during a period when the intake valve is closed is inducted during the next induction stroke. However, DI is not so flexible. For example, fuel which is to participate in the combustion event cannot be injected during a period in which exhaust gases are flowing out of the cylinder. Furthermore, there are mixing limitations placed upon fuel injection during the intake and compression strokes in that the injection timing affects the homogeneity achieved at the time of spark firing. Due to the limitations on DI timing, obtaining the appropriate amount of fuel for the lowest fuel delivery and highest fuel delivery requirements is a challenge with DI. That is, due to DI&#39;s limitations in injection pulse width to meet the highest injection demands causes the pulse widths at the lowest injection demands to be in a nonlinear range of the injector, meaning a high degree of variability in the pulse-to-pulse fuel delivery quantity. 
         [0003]    To overcome such problems, it is known to provide both port and direct injectors. This can be accomplished by providing a central injector (or multiple injectors) upstream of the intake manifold branches leading to the cylinders or by providing a port injector in the intake port for each cylinder. At the lowest fuel demands, the port injector can be used alone. At higher fuel demands, the direct injector can be used alone. This provides for less compromise in the design of the direct injector in that it no longer is called upon to provide a repeatable quantity of fuel from injection to injection at the lowest fuel demands. 
         [0004]    During periods in which the direct injector has no fuel flow through it, the injector is no longer provided cooling by the fuel flow. The fuel trapped at the injector tip can get very hot and undergo chemical reactions which cause deposit buildup. These deposits can occur within the injector tip thereby effectively reducing the cross-sectional area of the injector orifice or orifices, depending on whether the injector has a single hole or multiple holes. Additionally, deposits can from on the tip&#39;s external surface also having the effect of reducing the effective cross-sectional area of the injector orifices and/or interfering with the injector spray pattern. 
         [0005]    The inventors of U.S. Pat. No. 6,988,490 have recognized such a problem and propose increasing the tip temperature of the direct injector to enable periodic burning of the accumulated deposits. The inventor of the present invention has recognized several problems with this solution. First, such a proposal can only remove accumulated deposits that form on the outside surfaces of the injector, i.e., deposits that are in communication with oxygen so that they can be burned. Deposit formation within the orifices of the injector, forming in areas within the injector having limited access to oxygen would be exacerbated by the even higher temperatures experienced during the cleaning operation. Secondly, depending on the operating condition of the engine when such a requirement for increasing injector tip temperature is demanded can lead to a reduction in fuel economy. 
       SUMMARY OF THE INVENTION 
       [0006]    The inventor of the present invention recognizes that it is advantageous to prevent the formation of deposits as opposed to burning off the deposits once formed. To mitigate the formation of deposits, the temperature of the injector tip is maintained below a threshold temperature so that the fuel at the tip is sufficiently cool so that the reactions which lead to deposit formation do not occur. 
         [0007]    A method to operate an internal combustion engine having both a port injector and a direct injector supplying fuel to a cylinder of the engine is disclosed in which an estimate of the tip temperature of the direct injector is determined. If the tip temperature exceeds a threshold temperature, the fuel delivered by the direct injector is increased and the fuel delivered by the port injector is decreased. By providing more fuel through the direct injector, the direct injector is cooled by the fuel flowing through it. 
         [0008]    In one embodiment, the tip temperature is estimated based on temperature measured in the vicinity of the injector tip. Alternatively, the tip temperature is modeled based on one or more of: engine coolant temperature, engine speed, engine torque, vehicle speed, time since the direct injector was last commanded a pulse width, and ambient temperature. 
         [0009]    Also disclosed is a method to operate an internal combustion engine having both a port injector and a direct injector supplying fuel to a cylinder of the engine in which the engine is operated according to a normal operating mode when a tip temperature of the direct injector is below a threshold temperature and the normal engine operating mode is interrupted when a tip temperature of the direct injector is above the threshold temperature. The interruption of the normal engine operation involves increasing fuel delivered by the direct injector and decreasing fuel delivered by the port injector. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]    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: 
           [0011]      FIG. 1  is a schematic of an engine having two PI and DI injectors; 
           [0012]      FIG. 2  is an engine operating map of torque and engine rpm showing an example of a normal engine operating mode; and 
           [0013]      FIG. 3  is a flowchart indicating an embodiment of the present invention in which the direct injector tip temperature is maintained below a threshold temperature. 
       
    
    
     DETAILED DESCRIPTION 
       [0014]    A 4-cylinder internal combustion engine  10  is shown, by way of example, in  FIG. 1 . Engine  10  is supplied air through intake manifold  12  and discharges spent gases through exhaust manifold  14 . An intake duct upstream of the intake manifold  12  contains a throttle valve  32  which, when actuated, controls the amount of airflow to engine  10 . Sensors  34  and  36  installed in intake manifold  12  measure air temperature and mass air flow (MAF), respectively. Sensor  31 , located in intake manifold  14  downstream of throttle valve  32 , is a manifold absolute pressure (MAP) sensor. A partially closed throttle valve  32  causes a pressure depression in intake manifold  12  compared to the pressure on the upstream side of throttle valve  32 . When a pressure depression exists in intake manifold  12 , exhaust gases are caused to flow through exhaust gas recirculation (EGR) duct  19 , which connects exhaust manifold  14  to intake manifold  12 . Within EGR duct  19  is EGR valve  18 , which is actuated to control EGR flow. Fuel is supplied to engine  10  by fuel injectors  30 , injecting directly into cylinders  16 , and port injectors  26  supply fuel into intake manifold  12 . Each cylinder  16  of engine  10  contains a spark plug  28 . The crankshaft (not shown) of engine  10  is coupled to a toothed wheel  20 . Sensor  22 , placed proximately to toothed wheel  20 , detects engine  10  rotation. Other methods for detecting crankshaft position may alternatively be employed. 
         [0015]    In one embodiment, the engine is pressure charged by a compressor  58  in the engine intake. By increasing the density of air supplied to engine  10 , more fuel can be supplied at the same equivalence ratio. By doing so, engine  10  develops more power. Compressor  58  can be a supercharger which is typically driven off the engine. Alternatively, compressor  58  is connected via a shaft with a turbine  56  disposed in the engine exhaust. Turbine  56 , as shown in  FIG. 1 , is a variable geometry turbine; but, may be, in an alternative embodiment, a non-variable device. In another embodiment, the engine is naturally aspirated, in which embodiment elements  56  and  58  are omitted. Downstream of turbine  56  is three-way catalyst  66 . Three-way catalyst  66  can alternatively be placed upstream of turbine  56  for faster light-off. Alternatively, catalyst  66  is a lean NOx trap or lean NOx catalyst having the capability to reduce NOx at a lean equivalence ratio. 
         [0016]    Continuing to refer to  FIG. 1 , electronic control unit (ECU)  40  is provided to control engine  10 . ECU  40  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  32  position, spark plug  28  timing, EGR valve  18 . Various other sensors  42  (such as a humidity sensor, an engine block accelerometer, an in-line torque sensor, cylinder pressure transducer sensor, an ionization sensor, as examples) and specific sensors (engine speed sensor  22 , engine coolant sensor  38 , manifold absolute pressure sensor  31 , exhaust gas component sensor  24 , air temperature sensor  34 , and mass airflow sensor  36 ) communicate input through I/O interface  44  and may indicate engine rotational speed, vehicle speed, coolant temperature, manifold pressure, pedal position, cylinder pressure, throttle valve position, air temperature, exhaust temperature, exhaust stoichiometry, exhaust component concentration, and air flow. Some ECU  40  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. 
         [0017]    In  FIG. 2 , one embodiment of an operating map is shown in which the upper curve, labeled WOT for wide open throttle, shows the maximum torque that the engine can develop over the speed range. At the lowest speed and torque conditions, PI only is used. At moderate speeds and torques, DI and PI are used. At the highest speeds and/or torques, DI is used.  FIG. 2  is shown by way of example and is in no way intended to be limiting. It is simply one example of a normal engine operating mode. A wide variety of strategies could be employed as the normal engine operating mode, which strategies are not the subject matter of the present invention. 
         [0018]    In  FIG. 3 , engine operation starts in step  80  according to a normal engine operating mode in step  82 . Control passes to step  84  in which the injector tip temperature is estimated based on measured temperatures and/or modeled based on operating conditions. In step  86 , it is determined whether the tip temperature exceeds the threshold temperature, i.e., that temperature at which deposit formation occurs. If the temperature exceeds the threshold, DI fuel supply is increased. If not, control passes back to step  82  to operate at the normal engine operating mode. Both operating modes: normal mode and the injector cooling mode in which fuel is preferentially supplied by the DI injector return to step  84  to continue to monitor the injector tip operating temperature. 
         [0019]    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-describe embodiments are intended to be illustrative of the invention, which may be modified within the scope of the following claims.