Abstract:
A method and apparatus for injecting pilot fuel in a combustion engine. The method and apparatus includes determining a load of the engine, determining a desired injection timing of the pilot fuel and a desired quantity of pilot fuel to be injected as a function of a desired homogeneous distribution of the pilot fuel based on the engine load, and adjusting the injection timing and quantity of the pilot fuel to the desired values.

Description:
[0001]    This application claims the benefit of prior provisional patent application Serial No. 60/384,311 filed May 30, 2002. 
     
    
     
       TECHNICAL FIELD  
         [0002]    This invention relates generally to a method and apparatus for providing distributed ignition of a combustion engine and, more particularly, to a method and apparatus for controlling the timing and amount of a pilot fuel injected into a combustion engine for distributed ignition.  
         BACKGROUND  
         [0003]    Low cetane, i.e., high octane, fuels, such as natural gas, have several advantages over other hydrocarbon fuels that are combusted in internal combustion engines. For example, natural gas is less expensive relative to other hydrocarbon fuels. Moreover, natural gas burns cleaner during operation of the internal combustion engine relative to other hydrocarbon fuels. By burning cleaner, a reduced amount of combustion byproducts such as carbon monoxide, oxides of nitrogen, and hydrocarbons are released into the environment during engine operation. In addition, because lubricants of the internal combustion engine become contaminated with combustion byproducts over time, the production of a reduced amount of combustion byproducts results in less contamination, thereby increasing the useful life of the lubricants.  
           [0004]    One type of internal combustion engine is a diesel engine. Diesel engines combust fuel by compressing a mixture of air and fuel to a point where the fuel is ignited by heat which results from such compression. When natural gas is used as a fuel in a diesel engine, the natural gas does not readily ignite as it is compressed. In order to overcome this problem, an ignition source is provided to ignite the natural gas. The ignition source may be provided by a spark plug similar to those used in spark ignition engines. However, in certain types of diesel engines, e.g., dual fuel engines, the ignition source is provided by injecting a small amount of pilot fuel, such as diesel fuel, into a mixture of air and natural gas (or other gaseous fuel). As the mixture of air, natural gas and pilot fuel is compressed, the pilot fuel ignites, which in turn provides a diesel type ignition of the natural gas.  
           [0005]    A disadvantage associated with using pilot fuel as an ignition source is the resulting generation of an increased amount of oxides of nitrogen (NO x ). In particular, the ratio of air to the combination of natural gas and pilot fuel in the combustion chamber varies with the proximity to the injected streams of pilot fuel. Rich mixtures are created near the location of injection of pilot fuel, while lean mixtures are created further away from the location of the injection. Combustion of the rich mixtures tend to produce more NO x  than does the combustion of the lean mixtures.  
           [0006]    One way to reduce the amount of NO x  produced during the combustion process is to create a lean homogeneous mixture of air, natural gas and pilot fuel throughout the combustion chamber prior to ignition of the pilot fuel. Because the homogeneous mixture is lean throughout the entire combustion chamber, only lean mixtures are combusted. Combustion of only lean mixtures produces a lesser quantity of NO x  than does combustion of a combination of rich mixtures and lean mixtures.  
           [0007]    In commonly-owned U.S. Pat. No. 6,095,102, Willi et al. (Willi) discloses a method for injecting a quantity of pilot fuel into a combustion chamber having a supply of gas/air mixture. The pilot fuel is injected during the compression stroke in the range from about 21 degrees to 28 degrees before top dead center (BTDC) and is used to provide distributed ignition of the gas/air mixture. Willi discloses that injection of the pilot fuel in advance of what has been typically done in the industry, e.g., from 5 to 20 degrees BTDC, provides for a homogeneous mixture of the pilot fuel with the main portion of the gas and air. Furthermore, Willi discloses that the exact desired timing of the injection is determined by sensing the amount of NO x  in the exhaust stream during each subsequent exhaust stroke and varying the timing until an optimal level of NO x  is attained.  
           [0008]    It has been found that, since Willi&#39;s initial disclosed method, variations in engines and engine operating conditions result in situations in which the optimal desired timing of the pilot fuel injection resides outside of the 21 to 28 degree BTDC range during the compression stroke. Furthermore, sensing the level of NO x  and responsively varying the pilot injection timing does not always yield the best results. For example, optimal results may be achieved by varying the timing of the pilot fuel injection as well as the amount of pilot fuel injected. This can only be accomplished by determining parameters other than merely sensing NO x , and responsively controlling both the timing and the amount of the pilot fuel injection.  
           [0009]    The present invention is directed to overcoming one or more of the problems as set forth above.  
         SUMMARY OF THE INVENTION  
         [0010]    In one aspect of the present invention a method for injecting pilot fuel in a combustion engine is disclosed. The method includes the steps of determining a load of the engine, determining a desired injection timing of the pilot fuel and a desired quantity of pilot fuel to be injected as a function of a desired homogeneous distribution of the pilot fuel based on the engine load, and adjusting the injection timing and quantity of the pilot fuel to the desired values.  
           [0011]    In another aspect of the present invention a method for providing distributed ignition of a combustion engine is disclosed. The method includes the steps of introducing a quantity of fuel/air mixture into a combustion chamber of the engine, determining an operating load of the engine, determining a desired injection timing of a pilot fuel and a desired quantity of the pilot fuel to be injected as a function of a desired homogeneous distribution of the pilot fuel with the fuel/air mixture based on the engine load, and injecting the pilot fuel at the desired time.  
           [0012]    In yet another aspect of the present invention an apparatus for providing distributed ignition of a combustion engine is disclosed. The apparatus includes a cylinder assembly which includes (1) an engine block having a piston cylinder defined therein, (2) an engine head secured to the engine block, and (3) a piston which translates within the piston cylinder, wherein the engine block, the engine head, and the piston cooperate to define a combustion chamber. The apparatus further includes an intake port positioned in fluid communication with the combustion chamber during intake of a primary fuel and air mixture, a fuel injector positioned in the engine head and operable to inject pilot fuel into the combustion chamber during a compression stroke of the engine, an engine load determining device, and a controller which receives information from the engine load determining device and responsively determines a desired injection timing of the pilot fuel and a desired quantity of pilot fuel to be injected based on a desired homogeneous distribution of the pilot fuel. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]    [0013]FIG. 1 is a partial cross sectional, partial schematic view of a combustion engine which incorporates the features of the present invention;  
         [0014]    [0014]FIG. 2 is a block diagram illustrating a preferred embodiment of the present invention; and  
         [0015]    [0015]FIG. 3 is a partial cross sectional, partial schematic view of a combustion engine which incorporates features of a preferred embodiment of the present invention.  
     
    
     DETAILED DESCRIPTION  
       [0016]    Referring to FIG. 1, there is shown an engine assembly  10 . The engine assembly  10  includes a plenum member  12 , and an air source  14 . The plenum member  12  has an inlet opening  16 , and an exit opening  15  defined therein. The air source  14  supplies air to the inlet opening  16 . Air from the air source  14  advances into a plenum chamber  24  defined in the plenum member  12  via the inlet opening  16 .  
         [0017]    The engine assembly  10  further includes a cylinder assembly  26 . The cylinder assembly  26  includes a block  28  having a piston cylinder  30  defined therein. An engine head  32  is secured to the block  28 . The engine head  32  has an intake port  34 , an exhaust port  36 , and a fuel injector opening  60  defined therein. An intake conduit  38  places the intake port  34  in fluid communication with the exit opening  15  of the plenum member  12 . An exhaust passage  52  places the exhaust port  36  in fluid communication with an exhaust manifold  54 .  
         [0018]    The engine assembly  10  further includes a piston  40  which translates in the piston cylinder  30  in the general direction of arrows  42  and  44 . As the piston  40  moves downwardly in the general direction of arrow  44  to the position shown in FIG. 1, a connecting rod  43  urges a crankshaft  50  to rotate in the general direction of arrow  51 . Subsequently, as the crankshaft  50  continues to rotate in the general direction of arrow  51 , the crankshaft  50  urges the connecting rod  43  and the piston  40  in the general direction of arrow  42  to return the piston  40  to the uppermost position (not shown).  
         [0019]    The piston  40 , the piston cylinder  30 , and the engine head  32  cooperate so as to define a combustion chamber  46 . In particular, when the piston  40  is advanced in the general direction of arrow  42 , the volume of the combustion chamber  46  is decreased. On the other hand, when the piston  40  is advanced in the general direction of arrow  44 , the volume of the combustion chamber  46  is increased as shown in FIG. 1.  
         [0020]    The engine assembly  10  further includes a primary fuel source  18  in fluid communication with the intake conduit  38 . A primary fuel supply valve  41  controls the amount of primary fuel, such as natural gas, advanced to the intake conduit  38 . In particular, the primary fuel supply valve  41  moves between an open position, which advances primary fuel to the intake conduit  38 , and a closed position, which prevents advancement of primary fuel to the intake conduit  38 . It should be appreciated that the amount of primary fuel advanced by the primary fuel valve  41  controls the ratio of air to primary fuel, or air/fuel ratio, advanced to the combustion chamber  46 . Specifically, if it is desired to advance a leaner mixture to the combustion chamber  46 , a primary fuel control signal received via a signal line  96  causes the primary fuel supply valve  41  to operate so as to advance less primary fuel to the intake conduit  38 . On the other hand, if it is desired to advance a richer mixture of air and primary fuel to the combustion chamber  46 , a primary fuel control signal received via the signal line  96  causes the primary fuel supply valve  41  to operate so as to advance more primary fuel to the intake conduit  38 .  
         [0021]    It is noted that other methods of introducing the primary fuel and air mixture to the combustion chamber  46  may be used without deviating from the spirit and scope of the present invention. For example, the primary fuel may be mixed with air at any point from the air source  14  through the intake conduit  38 , including upstream of a turbocharger (not shown). Alternatively, the primary fuel may be injected directly into the combustion chamber  46 , and subsequently mixed with the intake of air.  
         [0022]    The primary fuel is typically a fuel having a high octane number, i.e., low cetane number. Preferably, the primary fuel is natural gas. However, the primary fuel may be of some other type, such as gasoline, methanol, ethanol, and the like, and may be either gaseous or liquid.  
         [0023]    An intake valve  48  selectively places the plenum chamber  24  in fluid communication with the combustion chamber  46 . The intake valve  48  is actuated in a known manner by a camshaft (not shown), a pushrod (not shown), and a rocker arm (not shown) driven by rotation of the crankshaft  50 . When the intake valve  48  is placed in the open position (shown in FIG. 1), air and primary fuel are advanced from the intake conduit  38  to the combustion chamber  46  via the intake port  34 . When the intake valve  48  is placed in the closed position (not shown), primary fuel and air are prevented from advancing from the intake conduit  38  to the combustion chamber  46  since the intake valve  48  blocks fluid flow through the intake port  34 .  
         [0024]    An exhaust valve  56  selectively places the exhaust manifold  54  in fluid communication with the combustion chamber  46 . The exhaust valve  56  is actuated in a known manner by a camshaft (not shown), a pushrod (not shown), and a rocker arm (not shown) each of which are driven by the rotation of the crankshaft  50 . When the exhaust valve  56  is placed in the open position (not shown), exhaust gases are advanced from the combustion chamber  46  to the exhaust manifold  54  via a fluid path that includes the exhaust port  36  and the exhaust passage  52 . From the exhaust manifold  54 , exhaust gases are advanced to an exhaust conduit  55 . When the exhaust valve  56  is placed in the closed position (shown in FIG. 1), exhaust gases are prevented from advancing from the combustion chamber  46  to the exhaust manifold  54  since the exhaust valve  56  blocks fluid flow through the exhaust port  36 .  
         [0025]    Combustion of the mixture of primary fuel and air in the combustion chamber  46  produces a number of exhaust gases. After the mixture of primary fuel and air is combusted in the combustion chamber  46 , exhaust gases are advanced through the exhaust conduit  55 . Included among the exhaust gases are quantities of oxides of nitrogen (NO x ).  
         [0026]    The engine assembly  10  further includes a fuel reservoir  70 . A fuel pump  72  draws low pressure fuel from the fuel reservoir  70  and advances high pressure fuel to a fuel injector  62  via a fuel line  74 . The fuel injector  62  is positioned in the injector opening  60  and is operable to inject a quantity of fuel into the combustion chamber  46  through the injector opening  60 . In particular, the fuel injector  62  injects fuel into the combustion chamber  46  upon receipt of an injector control signal on a signal line  100 . Furthermore, the fuel can be any one of the following group of fuels: diesel fuel, crude oil, lubricating oil, or an emulsion of water and diesel fuel. More generally, the fuel may be any type of fuel which has a higher cetane number than the primary fuel, thus having the property of combusting more readily than the primary fuel.  
         [0027]    The engine assembly  10  further includes a controller  90 . The controller  90  is preferably a microprocessor-based engine control unit. As FIG. 2 illustrates, the controller  90  preferably includes a set of maps  202 . Each map  202  is a three-dimensional map of fuel injection timing, fuel injection quantity, and NO x  for a determined engine operating load. A change in engine load would result in a new map  202  being referenced. Furthermore, the changes in loads, and hence maps, are based on a determined constant engine speed. A change in engine speed would require reference to additional maps.  
         [0028]    The engine speed is determined by an engine speed determining device  206 , such as a speed sensor or some such device well known in the art. The engine load is determined by an engine load determining device  204 . Examples of engine load determining devices include, but are not limited to, cylinder pressure transducers to measure work per cycle, estimation based on measurement of intake pressure and oxygen in the exhaust, and estimation based on measured fuel mass flow rate.  
         [0029]    Referring to FIG. 3, a preferred embodiment of the present invention is shown. The embodiment of FIG. 3 differs from the embodiment of FIG. 1 in that the pilot fuel is introduced into the combustion chamber  46  by way of the intake port  34 , rather than by means of direct injection. For example, a port injector  302  may inject pilot fuel into the intake conduit  38 , as shown. Alternatively, other devices may be used to deliver the pilot fuel into the intake port  34 , such as an acoustic atomizer, an air assisted injector, and the like. Alternatives to the preferred embodiment include introducing the pilot fuel at some other location upstream of the intake conduit  38 , for example upstream of the supply of primary fuel and air.  
         [0030]    When the pilot fuel is introduced through the intake port  34 , the desired timing of pilot fuel injection is no longer an issue. However, the desired amount of pilot fuel to use is still of concern, and is still determined based on engine load, such as determined by use of the maps  202 . The maps  202 , however, would not include fuel injection timing as a parameter.  
       INDUSTRIAL APPLICABILITY  
       [0031]    In operation, the typical engine assembly  10  operates in a four stroke cycle which includes an intake stroke, a compression stroke, a power stroke, and an exhaust stroke. Although the below discussion pertains specifically to a four stroke engine, the principles of the present invention may apply as well to other types of engines, such as a two stroke engine.  
         [0032]    The first stroke is the intake stroke, during which the exhaust valve  56  is positioned in the closed position and the intake valve  48  is positioned in the open position as shown in FIG. 1. During the intake stroke, the piston  40  is advanced downwardly in the general direction of arrow  44  thereby creating a low pressure in the combustion chamber  46 . This low pressure draws primary fuel and air from the intake conduit  38  downwardly into the combustion chamber  46  so as to form a homogeneous mixture of air and primary fuel in the combustion chamber  46 .  
         [0033]    Advancing to the compression stroke, the intake valve  48  and the exhaust valve  56  are both positioned in their respective closed positions. As the piston  40  moves upwardly in the general direction of arrow  42 , it compresses primary fuel and air in the combustion chamber  46 . At a time during the compression stroke, the fuel injector  62  injects pilot fuel into the combustion chamber  46  so as to ignite the mixture of primary fuel and air. The pilot fuel is injected in advance of 20 degrees before top dead center (BTDC) to allow sufficient time for the pilot fuel to form a homogeneous mixture with the fuel/air mixture already present in the combustion chamber  46 .  
         [0034]    The controller  90  receives information from the engine load determining device  204  and the engine speed determining device  206  and responsively accesses a relevant map  202 . The map  202  provides an indication of a desired injection timing of the pilot fuel and a desired quantity of pilot fuel to be injected based on a desired homogeneous distribution of the pilot fuel and a desired reduced amount of NO x  being exhausted. The controller  90  then delivers command signals via signal lines  208  and  210 , which in turn control, respectively, the pilot fuel injection timing and the pilot fuel injection quantity.  
         [0035]    Alternatively, the controller  90  may determine the desired pilot fuel injection timing and quantity by methods other than reference to maps. For example, the controller  90  may receive information from a cylinder pressure transducer (not shown) or information relevant to engine speed fluctuations and responsively determine a desired injection quantity based on combustion variability. Furthermore, the controller  90  may receive information relevant to cylinder pressure rise rate, e.g., from measurement of cylinder pressure or the use of a “knock” sensor (not shown), and responsively determine a desired injection timing. The above two alternatives may be used in cooperation with each other to determine both the desired injection timing and the desired injection quantity.  
         [0036]    It is noted that the pilot fuel is injected in advance of 20 degrees BTDC. The exact timing, as determined above, is indicative of a reduced amount of NO x  emissions. For example, it is found that NO x  increases as timing is advanced to a point. However, as timing is further advanced, NO x  begins to decrease until the level of NO x  reaches a transition point, i.e., the amount of decrease of NO x  does not change significantly for additional advances in timing. It is desired to control the timing, and also the quantity, of the pilot fuel to attain NO x  emissions at about the transition point. It is found that, with various engines and under various operating conditions, the optimal timing varies anywhere from 20 degrees BTDC to the initiation of the compression stroke, i.e., about 180 degrees BTDC.  
         [0037]    In the preferred embodiment of FIG. 3, the pilot injection quantity is desired and the timing of the pilot fuel is not an issue. For example, it may be determined by the above maps or alternative means that the desired injection quantity may be somewhere in the range of 0.5% to 1% of the total fuel introduced into the combustion chamber  46 . It is noted, however, that these quantities are exemplary only and may differ in value.  
         [0038]    Other aspects can be obtained from a study of the drawings, the disclosure, and the appended claims.