Abstract:
A method and apparatus for operating an internal combustion engine is provided. The method comprises the steps of introducing a primary fuel into a main combustion chamber of the engine, introducing a pilot fuel into the main combustion chamber of the engine, determining an operating load of the engine, determining a desired spark plug ignition timing based on the engine operating load, and igniting the primary fuel and pilot fuel with a spark plug at the desired spark plug ignition timing. The method is characterized in that the octane number of the pilot fuel is lower than the octane number of the primary fuel.

Description:
GOVERNMENT RIGHTS 
   This invention was made with Government support under DE-FC26-01CH11079 awarded by the Department of Energy. The Government has certain rights in this invention. 

   TECHNICAL FIELD 
   This disclosure 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 ignition of a pilot fuel and primary fuel in a main combustion chamber of a combustion engine. 
   BACKGROUND 
   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. 
   One type of internal combustion engine is an auto-ignite engine, such as a typical 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 an auto-ignite 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, such as a spark plug. In other types of 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 non-auto-igniting fuel). As the mixture of air, natural gas, and pilot fuel is compressed, the pilot fuel ignites, which in turn provides an auto-type ignition of the natural gas. 
   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 tends to produce more NO x  than does the combustion of the lean mixtures. 
   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. 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. Once ignition is desired, a spark plug may be used to ignite the lean homogeneous mixture. 
   In commonly-owned U.S. Pat. No. 6,666,185 to Willi et al. (“Willi”), Willi discloses a method and apparatus for controlling the injection of pilot fuel to control ignition of a homogenous distribution of fuel in the engine. The method and apparatus of Willi comprises adjusting the injection timing and quantity of pilot fuel to control ignition as a function of engine load. In Willi, the fuel is auto-ignited. 
   In the present disclosure, a method and apparatus for controlling ignition of a homogenous distribution of pilot fuel and primary fuel using a spark plug is provided. 
   SUMMARY 
   In one aspect, a method for operating an internal combustion engine is disclosed. The method comprises the steps of introducing a primary fuel into a main combustion chamber of the engine, introducing a pilot fuel into the main combustion chamber of the engine, determining an operating load of the engine, determining a desired spark plug ignition timing based on the engine operating load, and igniting the primary fuel and pilot fuel with a spark plug at the desired spark plug ignition timing. The method is characterized in that the octane number of the pilot fuel is lower than the octane number of the primary fuel. 
   In another aspect, a method for providing distributed ignition of a combustion engine is provided. The method comprises 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 spark ignition timing based on the engine operating load, and igniting a spark plug to ignite the fuel/air mixture at the desired time. 
   In yet another aspect, a distributed ignition engine is provided. The engine comprises a cylinder assembly, which comprises an engine block having a piston cylinder defined therein, an engine head secured to the engine block, and a piston that translates within the piston cylinder, wherein the engine block, the engine head, and the piston cooperate to define a combustion chamber. The engine further comprises an intake port positioned in fluid communication with the combustion chamber during intake of a primary fuel and air mixture, a fuel injector operable to inject a pilot fuel for introduction into the combustion chamber of the engine, a spark plug configured to ignite the primary fuel and pilot fuel within the combustion chamber, an engine load determining device; and
         a controller configured to receive information from the engine load determining device and configured to responsively determine a desired spark plug ignition timing based on a desired homogeneous distribution of the pilot fuel within the combustion chamber.       

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a partial cross sectional, partial schematic view of a combustion engine which incorporates the features of the present disclosure; 
       FIG. 2  is a block diagram illustrating an embodiment of the present disclosure; and 
       FIG. 3  is a partial cross sectional, partial schematic view of a combustion engine, which incorporates features of an example of the present disclosure. 
   

   DETAILED DESCRIPTION 
   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 . 
   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 . 
   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). 
   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 . 
   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 . 
   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 disclosure. 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. 
   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. 
   The engine assembly  10  further comprises a spark plug  64 , configured to ignite the primary fuel, pilot fuel, and air mixture within the combustion chamber. Spark plug  64  enables engine assembly  10  to precisely control ignition of the fuel and air mixture, even during very lean fuel ratios. Ignition of spark plug  64  allows for precise control of combustion in a distributed ignition engine at low equivalence ratios, such as 0.5 and below. It should be appreciated that any type of spark plug may be used, such as a J-gap, multi-torch, pre-chamber, or laser, for example. It should also be appreciated that even a micro-pilot fuel injector, which auto-ignites the fuel in the absence of a spark, may be used. 
   In the embodiment shown in  FIGS. 1 and 3 , controller  90  sends a control signal  98  to spark plug  64  to precisely control ignition of the fuel and air mixture within combustion chamber  46 . 
   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), a rocker arm (not shown) driven by rotation of the crankshaft  50 , or any valve actuation system that may be operated hydraulically, electronically, or pneumatically, for example. 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 . 
   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), a rocker arm (not shown) driven by rotation of the crankshaft  50 , or any valve actuation system that may be operated hydraulically, electronically, or pneumatically, for example. 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 . 
   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 ”). 
   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 groups 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 that has a higher cetane number than the primary fuel, thus having the property of combusting more readily than the primary fuel. 
   In the embodiment shown in  FIGS. 1 and 3 , controller  90  sends a control signal  100  to injectors  62  and  302  to control the quantity of pilot fuel introduced into combustion chamber  46 . 
   It should be appreciated that the pilot fuel may be introduced into the combustion chamber  46  in any known manner, and is not limited to being directly injected, as shown in  FIG. 1 . For example, the pilot fuel may be port injected, as depicted in  FIG. 3 , or it may be introduced via a fuel supply valve, similar to fuel supply valve  41 . 
   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 quantity, spark plug ignition timing, 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. 
   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. 
   Referring to  FIG. 3 , an example of the present disclosure 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 above-discussed embodiment may 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. 
   INDUSTRIAL APPLICABILITY 
   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 disclosure may apply as well to other types of engines, such as a two-stroke engine. 
   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 . 
   At some point during either the intake or compression stroke, pilot fuel is injected into either the combustion chamber  46  or intake conduit  38  via injector  62 . The pilot fuel is injected far enough in advance to allow sufficient time for the pilot fuel to form a homogeneous mixture with the primary fuel and air mixture in the combustion chamber  46 . 
   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, pilot fuel, and air in the combustion chamber  46 . At a time during the compression stroke, spark plug  64  ignites so as to ignite the relatively homogenous mixture of primary fuel, pilot fuel, and air. 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 spark plug ignition timing based on a desired reduced amount of NO x  being exhausted. The controller  90  then delivers command signals via signal line  212 , which in turn controls the spark plug ignition timing. The controller may also deliver command signals via signal lines  208  and  210 , which control injection timing and injection quantity, respectively. 
   In addition to the reference maps, the controller  90  may determine the desired spark plug ignition timing, pilot fuel injection timing, and pilot fuel quantity by other methods. 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 spark plug ignition timing. 
   In the embodiment of  FIG. 3 , the pilot fuel is injected in the intake conduit  38 . 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. 
   Other aspects can be obtained from a study of the drawings, the disclosure, and the appended claims.