Abstract:
A system and method is provided for use with a duel-fuel fumigation system. The system injects a gaseous-fuel flow into the air intake stream of a diesel engine. As presented, the system is operative to meter the gaseous-fuel flow into the diesel engine based on one or more engine parameters such as, for example, exhaust gas temperature, exhaust oxygen levels, engine speed, engine load and/or engine inlet pressure. Monitoring one or more engine parameters allows fine-tuning the flow of gaseous fuel into the engine and thereby prevents loss of engine power at high-end loads while maintaining favorable emission outputs over substantially the entire operating range of the engine.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application is a continuation in part of U.S. patent application Ser. No. 11/099,726 filed on Apr. 4, 2005, which is a continuation of U.S. patent application Ser. No. 10/797,803 filed on Mar. 10, 2004, now U.S. Pat. No. 6,901,889. 
     
    
     FIELD OF THE INVENTION  
       [0002]     The present invention relates generally to duel-fuel engine systems. More particularly, the present invention relates to a fumigation system for controlling the injection of a gaseous-fuel into an air intake of a diesel engine.  
       BACKGROUND  
       [0003]     Due to the high thermal efficiency achieved by compression-ignited engines (e.g., in comparison with spark-ignited engines), such engines are commonly utilized in industrial applications. Further, due to increasing fuel costs, such engines are also gaining popularity in the passenger vehicle and light truck markets. The high efficiency of compression-ignited engines, such as diesel engines, is due in part to the ability to use higher compression ratios than spark-ignited engines (i.e., gasoline engines) as well as the ability to control power output without a throttle. In the latter regard, the lack of a throttle eliminates throttling losses of premixed charges typical in spark-ignited engines thereby resulting in significantly higher efficiency at part load. However, compression-ignited engines and diesel engines in particular typically cannot achieve the low oxides of nitrogen (NO x ) and particulate emission levels that are possible with spark-ignited engines.  
         [0004]     Diesel engines typically inject diesel fuel into the engine&#39;s combustion chamber when that chamber&#39;s piston is near the end of the compression stroke. The high pressure present in the chamber ignites the diesel fuel. Due to the injection mixture of diesel fuel and compressed intake air within the combustion chamber, a large fraction of the fuel exists at a very fuel-rich equivalence ratio. That is, the fuel and air in the combustion chamber are not necessarily a homogenous mixture. This may result in incomplete combustion of the diesel fuel, which tends to result in high particulate emissions. Furthermore, the fuel-rich equivalence ratio can also lead to high flame temperatures in the combustion process, which results in increased NO x  emissions. As tougher environmental standards are being enacted for all internal combustion engines, users of diesel engines are looking for ways to lower emissions. One solution is to reduce the amount of diesel injected into the combustion chamber, which reduces the equivalence ratio and works to reduce particulate and NO x  emissions. Such a reduction in injected diesel, however, reduces engine power.  
         [0005]     Utilization of gaseous-fuels with diesel engines provides for more complete combustion of any diesel fuel consumed, can enhance fuel economy, and typically results in lower engine emissions. That is, in order to reduce particulate and NO x  emissions levels from diesel engines and/or to increase fuel economy, such engines may be partially or completely converted for use with gaseous-fuels such as, compressed natural gas (CNG), liquid natural fuels (LNG) such as ethanol, and liquid or liquefied petroleum gas (LPG), such as propane. However, such gaseous-fuels typically do not alone have the centane value required to allow for their ignition through compression. Accordingly, diesel engines must be modified to use such fuels.  
         [0006]     Methods for converting a diesel engine to consume gaseous-fuels typically fall into three categories. The first is to convert the engine to a spark-ignited engine; a second is to convert the engine to allow for the direct injection of gaseous-fuels into the combustion chamber with injected diesel; and a third is a dual-fuel technology, in which the gaseous-fuel is mixed with all or a portion of the intake air of the engine. As will be appreciated, the second and third methods utilize injected diesel (i.e., pilot diesel) to ignite the gaseous-fuel. In this regard, the combustion of the gaseous-fuel results in more complete combustion of the injected diesel. Furthermore, as the gaseous-fuel allows the engine to produce additional power less diesel is injected into the engine.  
         [0007]     Conversion to a spark-ignition system and/or a direct gaseous-fuel injection system for utilizing gaseous-fuels with a diesel engine each typically require substantial modification to the diesel engine. Such modifications may include replacement of cylinder heads, pistons, fuel injection system and/or duplication of many engine components (e.g., injection systems). Accordingly, these systems are typically expensive and oftentimes unreliable. On the other hand, dual-fuel systems require little modification to existing engines.  
         [0008]     Dual-fuel operation where gaseous-fuels are mixed with intake air prior to the introduction of that air-fuel mixture into the cylinders of the engine is known in the art as fumigation. That is, the mixture of gaseous-fuel and intake air is introduced into each cylinder of the engine during the intake stroke. During the compression stroke of the cylinder piston, the pressure and temperature of the mixture are increased. Near the end of the compression stroke, a small quantity of pilot diesel fuel from the engine&#39;s existing diesel fuel injection system is injected into the cylinder. The pilot diesel ignites due to compression and in turn ignites the mixture of gaseous-fuel and intake air. As will be appreciated, such fumigation systems may be retrofit onto existing diesel engines with little or no modification of the existing engine. Furthermore, engines using such fumigation systems may typically be operated in a dual-fuel mode or in a strictly diesel mode (e.g., when gaseous-fuel is not available).  
       SUMMARY OF THE INVENTION  
       [0009]     The inventors of the present invention have recognized that dual-fuel systems typically suffer two disadvantages that have prevented widespread use of such systems. The first disadvantage is typically encountered at high load operating conditions when elevated temperature and pressure in the engine during the compression strokes makes the intake air/gaseous-fuel mixture susceptible to premature detonation or knocking. Furthermore, at such high loads, some gaseous-fuels (e.g., natural gas) lack the thermal energy (i.e., BTUs) required to maintain a desired power output of the engine. The second disadvantage is encountered at low engine load where the gaseous-fuel and air mixture may be too lean for satisfactory combustion. In this instance, fuel consumption may actually increase, as may the emissions of hydrocarbons (i.e., unburned gaseous-fuels) and particulates. Each of these problems may be broadly termed a gaseous-fuel metering problem of a gaseous-fuel flow volume to the engine.  
         [0010]     Both of the above noted disadvantages are particularly acute in diesel engines, which run at varying load levels during operation (e.g., on-road and off-road vehicles). Such engines require the volume of gaseous-fuel injected into the intake airflow to vary with the varying requirements or demands of the engine in order to maintain desired power and emission outputs.  
         [0011]     The inventors have realized that metering based on a load level of a diesel engine provides a good starting point for fine tuning a gaseous-fuel flow in order to achieve improved emissions and/or enhanced fuel economy over a wide range of operating conditions. For instance, in one method for metering the volume of a gaseous-fuel flow based on the varying requirements or demands of the engine, a fumigation system may utilize a valve that is actuated in response to boost pressure of a turbocharger interconnected to a diesel engine. In this regard, as a load on the diesel engine increases, the turbocharger generates increased boost pressure. Once this boost pressure reaches a predetermined level, the valve may be partially opened to permit gaseous-fuel to be supplied to the air intake. As the boost pressure further increases, the valve opens further to allow more gaseous-fuel flow to the intake. However, in this method no gaseous-fuel is supplied at low load levels (e.g., idle) as no boost pressure is generated at such load levels. Therefore, the system fails to provide any emissions benefits at low load levels. Furthermore, at high load levels, where the boost pressure may fully open such a valve, excess gaseous-fuel may flow into the intake air stream, which may result in knocking and/or lowered engine performance. In this regard, strict load-based metering is often, by itself, inadequate for diesel engines that experience varying load levels.  
         [0012]     Accordingly, the inventors have recognized that additional or alternate means for metering the volume of a gaseous-fuel flow in order to fine-tune that flow volume may, in many instances, be desirable. In this regard, it has been determined that by monitoring one or more engine parameters such as, for example, exhaust gas temperature, exhaust oxygen levels, and/or engine speed, the flow volume of the gaseous-fuel may be adjusted (i.e., in conjunction with or independent of load based metering) to prevent loss of power at high-end loads while maintaining favorable emission outputs over substantially the entire operating range of the engine.  
         [0013]     According to a first aspect, a gaseous-fuel fumigation system for interconnection to a diesel engine is provided. The system includes a first valve disposed in a flow path between a gaseous-fuel supply and an air intake stream of a diesel engine. The first valve is operative to regulate a flow volume of gaseous-fuel (i.e. from the gaseous-fuel supply) through the flow path based on a load level associated with the engine. A second valve, disposed in series with the first valve in the flow path is operative to further regulate the flow volume of the gaseous-fuel through the flow path prior to the gaseous-fuel reaching the air intake stream of the engine. The second valve is operative to further regulate the flow volume based on, at least in part, an oxygen content of exhaust gases in an exhaust stream of the engine.  
         [0014]     Various refinements exist of the noted features in relation to the subject first aspect of the present invention. These refinements and/or additional features may exist individually or in any combination. For instance, the gaseous-fuel system may be interconnected to any of a variety of differently configured diesel engines. That is, such a fumigation system may be incorporated into heavy industrial applications/engines (e.g., railroad engines), power generation systems and/or on and off-road diesel vehicles. Furthermore, such a system may be incorporated into original equipment manufacturer (OEM) equipment, or, such a system may be retro-fit onto an existing diesel engine. Additionally, it will be noted that the gaseous-fuel may be injected into the air stream of an engine in any appropriate manner. In this regard, the flow path may be directly coupled to an air intake stream in order to directly inject fuel into that stream, or, and outlet of the flow path may be disposed relative to an air intake such that the gaseous-fuel is drawn into the engine with ambient air.  
         [0015]     As used herein, the term valve is meant to include any mechanism for varying the flow of a fluid through a flow path. Such valves include, without limitation, mechanical valves operative to restrict a cross-sectional area of a flow path as well as pressure regulating valves that are operative reduce the pressure and/or volume of a gas passing across the valve. Accordingly, the pressure of the gaseous-fuel supply and/or the size (i.e., cross-sectional size) of the flow path may be selected for a particular application.  
         [0016]     It will be noted that while the first and second valves may be disposed in series, the order of those valves is unimportant. In this regard, the first and second valves may be operated independently and a total flow volume passing through the two valves will be the same irrespective of their order. Generally, however, the first valve will establish a first flow volume and the second valve will act as a leaning valve that is operative to establish a second flow volume between zero and 100% of the first flow volume. The second flow volume is then injected into the intake air stream. In this regard, the second valve can be utilized to prevent any gaseous-fuel flow from reaching the diesel engine if circumstances dictate eliminating the supply of gaseous-fuel.  
         [0017]     As noted, the first valve regulates the flow volume of the gaseous-fuel in relation to a load level associated with the engine. In one particular embodiment, this load level is determined from boost pressure produced by a turbo-charger attached to the diesel engine. In this regard, the boost pressure may be utilized to open a spring-actuated valve to variably open a flow path through the valve. As will be appreciated, as the engine load level increases, the boost pressure also increases. This increased boost pressure may further open the first valve such that increased flow volume of gaseous-fuel may pass through. As will be appreciated, use of such a spring-actuated valve may allow for preventing the passage of any gaseous-fuel through the valve until a predetermined boost pressure is achieved. Furthermore, the valve may be adjustable to allow a user to establish the engine load level necessary to initially and/or fully open the valve. Though discussed in relation to a spring-actuated valve, it will be appreciated that other valve types may be utilized. For example, an electronically actuated valve (e.g., using a stepper motor) operative in relation to the engine&#39;s governor and/or an RPM sensor may also be utilized to establish a gaseous-fuel flow volume based on such an engine load level.  
         [0018]     In order for the second valve to regulate (e.g., fine tune) the load based gaseous-fuel flow volume based on an oxygen content of exhaust gases, the system will typically include an oxygen sensor disposed within an exhaust stream of the engine. In this regard, a controller may receive an output signal from a sensor and/or convert such a signal into an oxygen content. This controller may then control the operation of the second valve in order to increase or decrease the volume of the gaseous-fuel flow through the second valve and thereby the flow volume reaching the air intake stream of the engine.  
         [0019]     To fine tune the gaseous-fuel flow, the controller may further include programming (e.g., software, hardware and/or firmware) that maintains one or more predetermined operating conditions for the engine. In this regard, the controller may be operative for engine performance and/or emissions purposes to maintain at least a predetermined minimum oxygen content in the oxygen stream. For example, if the exhaust stream is oxygen depleted (i.e., beneath a predetermined desired amount), unburned hydrocarbons are passing through the engine and hence, increasing emissions outputs. The controller may be operative to adjust the gaseous-fuel flow volume through the second valve (e.g., reduce the volume) to reestablish a desired oxygen content in the exhaust stream. Accordingly, ensuring that at least a minimum desired oxygen content is present within the exhaust gases allows for ensuring complete combustion of the gaseous-fuel and/or diesel fuel within the engine.  
         [0020]     According to another aspect of the invention, a gaseous-fuel fumigation system for interconnection to a diesel engine is provided wherein the volume of a gaseous-fuel flow through a flow path between a gaseous-fuel supply and an air intake stream is controlled based on the oxygen content of an exhaust stream of the engine. More particularly, the system includes an oxygen sensor disposed within the exhaust stream of the engine and a controller operative to adjust a first valve in the flow path to maintain a predetermined minimum oxygen content in the exhaust stream. In a further embodiment, the controller includes an oxygen content adjuster that is operative to allow selective setting of one or more predetermined minimum oxygen contents. As will be appreciated, this allows, for example, a user to select a minimum oxygen content based on one or more engine-specific parameters. Alternatively, the controller may select a minimum oxygen content based on one or more operating conditions. This allows customizing such a fumigation system for use with a wide variety of differently configured diesel engines.  
         [0021]     Variations and refinements exist of the present aspect. Such variations and refinements may exist in any combination. For instance, the controller may be a stand-alone unit or incorporated into one or more existing processors associated with the engine. What is important is that the controller is operable to determine an oxygen content from the sensor output and provide an output signal operative to adjust (e.g., open and/or restrict) a valve disposed within the flow path between a gaseous-fuel supply and an air intake stream of the engine.  
         [0022]     In order to set a predetermined minimum oxygen content for the exhaust stream of an engine, the oxygen sensor disposed within that exhaust must be operable to accurately read a range of oxygen contents for a wide range. In this regard, a wide band oxygen sensor is utilized that is capable of accurately identifying oxygen content between about 1% and about 22%. Furthermore, use of such an oxygen sensor allows for establishing and maintaining a desired oxygen content within the exhaust stream. As will be appreciated, oxygen sensors utilized for automotive catalytic converter applications utilize an oxygen sensor having a preset value. Typically, the operation of the engine is adjusted to be within a percentage value of the preset value of the oxygen sensor. This results in cyclic engine operation. That is, the exhaust gases are either above or below the predetermined set point, but never necessarily converge on that set point. The wide band oxygen sensor allows for more accurate oxygen sensing and therefore the ability to establish and maintain an oxygen level within an exhaust stream (i.e., non-cyclic operation).  
         [0023]     According to another aspect of the present invention, a method for operating a gaseous fumigation system interconnected to a diesel engine is provided. The method includes establishing a first fluid flow of gaseous-fuel based on a load level of a diesel engine. An oxygen content in the exhaust stream of the engine is identified and, based on that oxygen content, the flow volume of gaseous-fuel is regulated to establish a second flow volume that is between zero and 100% of the first flow volume. The second flow volume is then injected into the air intake stream of the diesel engine. This method allows for the fine-tuning (i.e., metering) of a load based gaseous-fuel flow such that performance and/or emissions outputs may be maintained at high and/or low load levels in a diesel engine.  
         [0024]     The present method may be utilized with diesel engines that run at constant operating speeds (i.e., point source engines) and/or with diesel engines having varying load requirements (e.g., on and/or off-road vehicles). In the latter regard, the load level of the diesel engine may fluctuate in an almost continuous manner. Accordingly, the first flow volume will likewise fluctuate. Furthermore, the regulation of the first gaseous-fuel flow will likewise fluctuate to maintain one or more desired parameters including, for example, the oxygen content of the exhaust, engine speed and/or exhaust gas temperatures. In this regard, it will be noted that one or more variables may be monitored to provide a system that can shut down gas injection to the engine when any of a number of predetermined variables are exceeded.  
         [0025]     In this regard, logic within a controller may monitor one or more such variables such that gaseous-fuel flow may be adjusted accordingly. For example, if an exhaust gas temperature exceeds a predetermined maximum amount, the gaseous-fuel supplied to the diesel engine may be reduced. If the exhaust gas temperature continues to exceed the predetermined maximum value (i.e., the system will not stabilize at a desired value), the gaseous-fuel flow may be shut off entirely. Similar systems may be set up for engine rpm such that the engine doesn&#39;t over-speed, which may damage the engine. As presented, the method provides for a multiple redundancy system for preventing damage to an engine utilizing a fumigation system.  
         [0026]     According to another aspect of the invention, a gaseous-fuel fumigation system is provided wherein a volume of gaseous fuel flow through a flow path between a gaseous fuel supply and an air intake stream is controlled utilizing a single electronically controlled valve. The flow path through the valve is controlled by a controller. This controller is electrically interconnected to an oxygen content sensor that is operative to generate an output that is indicative of an oxygen content of an exhaust stream of an engine. The controller is also interconnected to a pressure sensor that is fluidly interconnected to the input of the diesel engine. This pressure sensor is operative to generate an output signal that is indicative of the input pressure into the engine. In one arrangement, the pressure sensor is fluidly connected to a turbocharger of the engine. In any arrangement, the controller receives oxygen content information and pressure information associated with the input pressure into the engine, which may correspond to boost pressure of the turbocharger. The controller may then utilize such information to generate control signals for controlling the flow of gaseous fuel through the valve. Such a system allows for controlling the gaseous fuel flow based on engine load level and oxygen content without requiring mechanical connection of a turbocharger to a fluidly actuated valve assembly.  
         [0027]     In one arrangement, the controller may utilize oxygen content information and pressure information to control different operating ranges of the engine. For instance, at low operating speeds or powers, the exhaust stream may have more than a predetermined minimum oxygen content. Accordingly, the controller may, while the oxygen content is above the minimum level, utilize only pressure information to generate control signals for controlling the valve and, hence, gaseous fuel flow to the engine. In contrast, when the oxygen content of the exhaust falls below the predetermined minimum, the controller may utilize the oxygen content information alone and/or in combination with the pressure information to generate control signals for controlling the valve. Stated otherwise, the controller is operative to generate control signals based on a first engine operating parameter for a first set of operating conditions and generate control signals based on a second engine operating parameter for a second set of operating conditions. It will be appreciated that, in addition to exhaust oxygen content and engine input pressure, other engine operating parameters may be utilized including, without limitation, rpm, exhaust temperature, gaseous fuel parameters (e.g., BTU, centane, etc).  
         [0028]     In another arrangement, the controller can also use pressure from sensor to adjust the oxygen set point up or down to achieve a non-linear result of gaseous fuel flow. Alternatively, the controller may be programmed to provide linear increases of gaseous fuel based on input pressure from the transducer. In either arrangement, limitations may be imposed by exhaust oxygen presets. This allows for proportional or non-proportional fuel flows as may be required by a specific application.  
         [0029]     The inventors have also recognized that dual-fuel systems are often susceptible to engine knocking at high load operating conditions and/or in response to rapid changes in operating conditions of the engine. Such engine knocking is an uncontrolled combustion process that can damage engines and is typically caused by elevated temperature and pressure in the engine during the compression strokes that makes the intake air/gaseous-fuel mixture susceptible to premature detonation (i.e., engine knocking).  
         [0030]     The occurrence of engine knocking in dual-fuel systems is particularly acute for engines that run at varying load levels during operation (e.g., on-road and off-road vehicles). That is, due to the changes in operating conditions, the load applied to the engine may increase or decrease rapidly. Accordingly, the mixture of gaseous fuel and intake air that was appropriate prior to a change in operating conditions may be excessively rich or excessively lean after the change in operating conditions. This may lead to the mixture being out of balance for the current operating conditions and thus the initiation of engine knocking. Further, such engine knocking may increase in intensity (e.g., oscillation due to harmonic amplification) and can result in the engine shaking and/or jumping (i.e., rough engine operation) while the gaseous fuel/intake air mixture is readjusted for the current operating conditions.  
         [0031]     To prevent such rough engine operation, which can temporarily reduce engine performance and/or damage the engine, the inventors have determined it is desirable to identify engine knocking at early stages and rapidly readjust the ratio of gaseous fuel to intake air. Accordingly, systems and methods (i.e., utilities) are presented herein for operating a dual-fuel system while reducing the occurrence, duration and/or intensity of engine knocking conditions. More specifically, the utilities herein utilize an engine knock sensor to identify engine knocking conditions such that remedial measures may be taken.  
         [0032]     According to another aspect, a gaseous fuel fumigation system for use with a diesel engine is provided. The system includes a valve disposable in a flow path between a gaseous fuel supply and an air intake of a diesel engine. The valve is operative to regulate a flow volume of gaseous fuel through the flow path. The system further includes a knock sensor that is operative to monitor a motion level (e.g., vibrations and/or acceleration) of the diesel engine and generate an output signal that is indicative of the motion level. Finally, the system includes a controller that is operative to receive the output signal from the knock sensor and compare the output signal to at least a first threshold value. If the output signal of the knock sensor exceeds the first threshold value, the controller generates control signals that are operative to: move the valve from its current or initial open position, which defines a first gaseous fuel flow volume, to a substantially closed position, which defines a second gaseous fuel flow volume, and back to a reopened position, which defines a third gaseous fuel flow volume. Stated otherwise, the controller is operative to cycle the valve from its current open position to a substantially or fully closed position and back to a reopened position. Such movement may be substantially continuous and may permit reestablishing gaseous-fuel flow based on present engine operating conditions.  
         [0033]     Cycling of the valve allows the engine to momentarily run in near or full diesel mode. That is, while the valve is substantially closed, the engine operates free or substantially free of gaseous fuel flow. Accordingly, while running in full/near full diesel mode, the knocking conditions caused by an out of balance mixture of gaseous-fuel and intake air are eliminated. Accordingly, the valve may be reopened based on the present operating conditions of the diesel engine. In this regard, it will be appreciated that the flow volumes associated with the initial valve position and the reopened valve position may be different. By cycling the valve upon initially identifying the knocking conditions, visibly rough engine operation may be totally avoided. Furthermore, as the valve is reopened almost immediately after being closed, the benefits of dual fuel operation are only briefly interrupted.  
         [0034]     In one arrangement, the position of the valve is adjusted based on an oxygen content of the exhaust stream of the diesel engine. In such an arrangement, the system may further include an oxygen sensor that is disposable within the exhaust path of the diesel engine. An oxygen content value generated by this oxygen sensor may be received by the controller. Accordingly, the controller may utilize the oxygen content value to control the position of the valve. Likewise, the controller may utilize this oxygen content value to establish a reopened position for the valve. In addition to an oxygen content of the exhaust, the controller may also utilize an engine load level to control/reestablish a position of the valve. Such engine load level may correspond to, for example and without limitation, engine rpm and/or boost pressure of a turbo-charger. In one arrangement, a boost pressure sensor in fluid communication with the turbocharger may be utilized to provide a signal to the controller that is indicative of boost pressure. In any arrangement, an appropriate gaseous fuel flow may be established for the present/current operating conditions of the engine.  
         [0035]     The knock sensor may be any sensor that is operative to provide an output indicative of movement associated with engine knocking. For instance, various different accelerometers and/or velocity sensors may be utilized. Furthermore, it will be appreciated that a plurality of such sensors may be utilized. In this regard, a knock sensor may be positioned adjacent to each cylinder of the engine to provide information regarding early states of engine knocking. In one arrangement, the knock sensor is a piezoelectric device. However, it will be appreciated that other types of devices may be utilized, without limitation. In any case, it may be desirable that the knock sensor is attached to a solid surface of the engine (e.g., head and/or block) such that, for example, high frequency oscillations (e.g., vibrations) associated with engine knocking may be detected.  
         [0036]     The threshold value to which the output signal the knock sensor is compared may be generated by the controller during the operation of the engine. For instance, the threshold value may be determined as an average motion value over a preceding and predetermined period of time. Alternatively, the threshold value may be a predetermined threshold value that may be stored in a storage structure. In this regard, the threshold values may include, inter alia, tabulated threshold values and/or threshold value equations for calculating an appropriate threshold based on one or more current operating conditions of the engine.  
         [0037]     According to another aspect of the present invention, a method for use of a gaseous fuel fumigation system with a diesel engine is provided. The method includes establishing a gaseous fuel flow through a valve disposed between a gaseous fuel supply and an air intake stream of a diesel engine. The valve is operative for regulating a volume of the gaseous fuel flow. A motion level of the engine is monitored and, upon the motion exceeding at least one threshold value, the valve is substantially closed and reopened to at least partially interrupt the flow of gaseous fuel to the diesel engine.  
         [0038]     Various refinements exist of the noted features in relation to the present aspect. These refinements and/or additional features may exist individually or in any combination. For instance, the valve may be entirely closed and then reopened, or the valve may be substantially closed in relation to its initial flow volume. What is important is that the flow volume of gaseous fuel to the diesel engine is substantially reduced to establish full or rear full diesel operation to reduce the motion level of the engine (e.g., to eliminate knocking conditions).  
         [0039]     Generally, monitoring a motion level of the engine includes generating an output signal from a motion sensor that is attached to the engine. This output signal may then be compared to one or more threshold values to determine if the current motion of the engine is acceptable and/or exceeds the threshold value. If the output signal exceeds the threshold value (e.g., by a predetermined amount), remedial measures may be taken. That is, the valve may be cycled to temporarily interrupt the supply of gaseous fuel to the engine. In this regard, closing and reopening the valve may include generating a first valve control signal that is operative to move the valve from an initial valve position to a substantially closed position and generating a second control signal that is operative to move the valve from the substantially closed position to a reopened position. The reopened position of the valve may be based on one ore more current operating conditions of the engine. In one arrangement, the reopened position may be defined based at least in part on the oxygen content of the exhaust gas of the engine and/or based on a engine load value.  
         [0040]     In any of the above aspects, the emissions of a diesel engine may be significantly reduced without affecting the power output of that engine. This is due in part to more complete combustion of diesel fuel within the engine due to the inclusion of the gaseous-fuel as well as the reduction of diesel fuel utilized by the engine. Of note, any gaseous fuel may be utilized including, without limitation, natural gas, methane, propane, hydrogen, vaporized ethanol, vaporized methanol, and/or any other gas or vapor having adequate combustion properties. Further, it will be noted that the above-noted fumigation systems may be set to minimize the diesel fuel utilized by an engine. This has significant benefits in natural gas and/or methane gas well production applications. In such applications, natural gas engines or diesel are often utilized for pumping and/or compressing gas from wells into pipelines. Due to the low BTU value of the natural gas, such natural gas engines are often considerably larger and more expensive than a diesel engine having a similar power rating. In this regard, it is desirable to utilize diesel engines though these diesel engines may be remotely located and require frequent servicing (e.g., replenishment of diesel fuel). By incorporating a fumigation system onto these remotely located engines such that they utilize gaseous-fuel, of which there is a ready supply, the rate of diesel usage may be significantly reduced thereby lowering the service requirement of such engines. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0041]      FIG. 1  is a schematic view of a first embodiment of a fumigation system;  
         [0042]      FIG. 2   a  illustrates a first cross-sectional view of a boost-pressure valve utilized with the system of  FIG. 1 ;  
         [0043]      FIG. 2   b  illustrates a second cross-sectional view of the boost-pressure valve of  FIG. 2   a;    
         [0044]      FIG. 3  illustrates one embodiment of a leaning valve utilized with the system of  FIG. 1 ; and  
         [0045]      FIG. 4  illustrates a process that may be utilized with a fumigation system.  
         [0046]      FIG. 5  illustrates a second embodiment of a fumigation system.  
         [0047]      FIG. 6  illustrates a process that may be utilized with the fumigation system of  FIG. 5 . 
     
    
     DETAILED DESCRIPTION  
       [0048]     The present invention will now be discussed in relation to the accompanying drawings, which at least partially assist in illustrating its various pertinent features.  FIG. 1  shows a schematic view of a first embodiment of a dual-fuel fumigation system  10  interconnected to a turbocharged diesel engine  20 . The fumigation system  10  is applicable to a variety of diesel engine applications and may be utilized in any of a variety of environments where diesel engines are utilized. For example, the fumigation system  10  may be utilized with stationary power generation systems as well as on-road and off-road diesel powered vehicles. That is, the system may be utilized with diesel engines designed to operate at a set load level as well as with diesel engines having varying load requirements due to, for example, idling needs, acceleration needs, cruising needs, etc. Furthermore, it will be noted that the fumigation system  10  is discussed in relation to its applicability to turbocharged diesel engines, however, it will be appreciated that certain aspects of the invention are not limited to such applications.  
         [0049]     The particular internal characteristics of the turbocharged internal combustion engine  20  are not of particular importance for the present invention. In this regard, it will be noted that the fumigation system  10  may be interconnected to a multitude of differently configured diesel engines  20 . As shown, the engine  20  includes a turbocharger  30 , which provides pressurized intake air to the engine  20  during operation at load. The turbocharger  30  includes an impeller  32  that is interconnected to a turbine  34 . The impeller  32  is disposed within an air inlet line  26  of the engine  20  while the turbine  34  is disposed within an exhaust output  38  of the engine  20 . As is conventional, the turbine  34  receives exhaust gas from an exhaust manifold  28  of the diesel engine  20 . The exhaust gas rotates the blades of the turbine  34 , which causes the impeller  32  to likewise rotate, thereby compressing air supplied to the impeller  32  through the air inlet line  26 . The compressed air is discharged from the impeller  32  through the inlet manifold  24  where it is supplied to air inlet ports within the diesel engine  20 . By means of the turbocharger  30 , air supplied to the engine  20  is compressed, which enhances the performance of the engine  20 .  
         [0050]     The exact internal configuration of the engine diesel  20  is unimportant for the present invention as the gaseous fumigation system  10  disclosed herein may be interconnected to a variety of differently configured engines. Typically, the engine  20  will include a number of cylinder assemblies, each having an intake port, an exhaust port, and a fuel injector therein. Each cylinder assembly will further include a piston for compressing gasses within the cylinder. The fuel injector is utilized to inject diesel fuel into the cylinder (e.g., when the piston is at or near top dead center), causing combustion of the fuel/air mixture.  
         [0051]     Irrespective of the exact configuration of the diesel engine  20 , the engine  20  may be equipped and/or retrofit with the dual-fuel fumigation system  10 . As shown in  FIG. 1 , the fumigation system includes a gaseous-fuel supply  40  for providing a fuel flow to the air intake  36  of the engine  20  via a gas supply conduit  42 . Of note, the outlet of the gas supply conduit  42  may be disposed relative to the air intake  36  such that gas emitted from the end of this gas supply conduit  42  may be drawn into the air intake  36 . In this regard, the gaseous-fuel may pass through a filter associated with the air intake  36  prior to entering the engine  20 . As will be discussed herein, a series of three valves are disposed between the gaseous-fuel supply  40  and the air intake  36 . It will be noted that other embodiments, the fumigation system may utilize the valves in different order and/or combine the functioning of two or more valves into a single valve assembly.  
         [0052]     In the present embodiment, the three valves include a lock-off valve  46 , a boost-pressure valve  50 , and a leaning valve  100 . The boost-pressure valve  50  is a mechanical valve that opens in relation to boost-pressure received from the turbocharger  30  via a boost-pressure conduit  52 . The lock-off valve  46  and the leaning valve  100  are interconnected to a programmable electronic control unit  80 . As shown, the programmable electronic control unit (ECU)  80  is further interconnected to an oxygen sensor  84  disposed within the exhaust output  38  of the engine  20 , an exhaust gas temperature (EGT) sensor  88 , which is also disposed within the exhaust output  38 , and an RPM sensor  92 , which is operative to determine the engine speed at the flywheel. Further, a knock sensor for detecting motion levels of the engine  20  is also interconnected to the ECU  80 , as will be discussed herein.  
         [0053]     The lock-off valve  46  is operative to prevent any gas flow between the gaseous-fuel supply  40  and the air intake  36  when the engine  20  is not operating. That is, when the electrical system of the engine  20  is inactive, the lock off valve  46  is closed to prevent the typically pressurized gaseous-fuel  40  from traveling through the gas supply conduit  42 . Of note, the gaseous-fuel supply  40  may further include regulators in order to provide a predetermined gas pressure to the conduit  42 . Furthermore, it will be noted that the size of the gas supply conduit  42  and/or pressure of the gaseous-fuel may be selected to provide a desired flow rate/volume based on one or more requirements of the engine  20 .  
         [0054]     Upon starting the engine  20 , the lock off valve  46  is opened to allow gaseous-fuel flow between the gas supply  40  and the boost-pressure valve  50 .  FIG. 2   a  shows a cross-sectional view of the boost-pressure valve  50 . As shown, the boost-pressure valve  50  includes a gas inlet  54  and a gas outlet  56 . Disposed between the inlet  54  and the outlet  56  is a diaphragm operated valve assembly. The valve assembly includes a valve body  58 , a valve seat  60 , a spring  62 , and a spring tension adjuster  64 . As shown, the valve body  58  and valve seat  60  are generally cylindrical and are disposed in a mating relationship when boost pressure is not present. That is, the spring  62  maintains the valve body  58  against valve seat  60  in the absence of boost pressure from the turbocharger  30 , thereby preventing flow of gas through the main portion of the boost-pressure valve  50 .  
         [0055]     The boost-pressure conduit line  52  has a first end interconnected to the turbocharger  30  and a second end interconnected to the boost-pressure inlet  66  of the boost-pressure valve  50 . The boost-pressure inlet  66  is connected to a pressure chamber  68  that is partitioned by a flexible diaphragm  70 . The pressure within the pressure chamber  68  from the turbocharger  30  applies upward pressure to the diaphragm  70 . The diaphragm  70  is interconnected to the valve body  58  by a pushrod  71 . When upward pressure is sufficient, the diaphragm  70  moves upward and the pushrod  71  lifts the valve body  58  from the valve seat  60 , thereby opening a fluid path between the gas inlet  54  and outlet  56 . That is, when the boost pressure in the pressure chamber  68  overcomes the force applied to the valve body  58  by the spring  62 , gaseous-fuel from the gaseous-fuel supply  40  may be delivered through the boost-pressure valve  50  and on to the air intake  36  via the leaning valve  100 . As will be appreciated, as the boost pressure increases, the spring  62  and valve seat  60  may be further displaced, thereby progressively increasing the flow of gaseous-fuel through the boost-pressure valve in response to changes in engine operating conditions. Further, the adjustor  64  allows for setting a minimum boost pressure that will open the boost-pressure valve  50 .  
         [0056]     At low load levels, the diesel engine  20  does not produce boost pressure and likewise cannot open the boost-pressure valve  50 . In this regard, at low engine loads such as idle, no gaseous-fuel is able to pass through the main portion of the boost-pressure valve  50 . However, the present configuration of the boost-pressure valve  50  includes an idle bypass port  72 . As shown in  FIG. 2   b,  a portion of the gas flow passing through the boost-pressure valve  50  may pass through the idle bypass port  72 , through the main valve body and through the outlet  56  while the valve body  58  is seated on the valve seat  60 . Furthermore, in the embodiment shown, the idle bypass port  72  includes an idle bypass adjustment screw  74 . This idle bypass adjustment screw  74  allows for varying the flow of the gaseous-fuel through the boost-pressure valve  50  when the boost pressure is insufficient to open the main valve assembly. In this regard, gaseous-fuels may be provided to the engine at low load levels such that the benefits of the gaseous-fuel injection (e.g., reducing NO x  and/or particulates) may be realized at low load settings. Stated otherwise, the idle by-pass port  72  may be set to maintain a minimum gas flow through the boost pressure valve  50 .  
         [0057]      FIGS. 2   a  and  2   b  illustrate the flow of the gaseous-fuels through the boost-pressure valve  50 . As shown in  FIG. 2   b,  the gaseous-fuel enters the gas inlet  54  from the left side of the valve  50  and passes through the main valve assembly. Returning to  FIG. 2   a,  it will be noted that when the valve assembly is open, the gaseous-fuel passes up through the center portion of the valve  50  and out through the outlet  56 . Furthermore, it will be noted in  FIG. 2   b  that the boost-pressure valve  50  includes a fuel mix adjuster screw  76  that is operative to limit the maximum flow of the gaseous-fuel through the boost-pressure valve  50 . In this regard, the boost-pressure valve  50  may be utilized with engines having varying flow requirements. That is, when the boost pressure fully opens the main valve assembly, the fuel mix adjuster screw  76  may be set for a particular engine such that a desired maximum flow through the boost-pressure valve  50  is established.  
         [0058]      FIG. 3  shows one embodiment of the leaning valve  100 . In the embodiment shown, the inlet  102  of the leaning valve  100  receives gas flow from the outlet  56  of the boost pressure valve  50 . As shown, the leaning valve  100  has an inlet  102 , an outlet  104 , a valve seat  106 , and a needle assembly  108 . Interconnected to the needle valve  108  is a stepper motor  110 . The stepper motor  110  is operative to, in response to signals received from the ECU  80 , adjust the position of the needle assembly  108  in order to vary the volume of gaseous-fuel flow (i.e., as received from the boost-pressure valve  50 ) through the leaning valve  100  prior to being received by the air intake  36 . In this regard, it should be noted that at high loads, dual-fuel fumigation systems often suffer from a lack of power. This is due in part to the fact that most gaseous-fuels contain less thermal energy than diesel fuel. Accordingly, at high loads the gaseous-fuel may not provide enough BTUs to the diesel engine  20  for efficient operation. In this regard, the flow of the gaseous-fuel may be restricted by the leaning valve  100 .  
         [0059]     That is, at high loads, the boost-pressure valve  50  is fully opened and the gaseous-fuel flow may be leaned to provide improved engine performance (i.e., resulting in higher diesel usage to maintain power output). Furthermore, the leaning valve  100  may be utilized to fine-tune the gaseous-fuel flow in order to achieve desired emission outputs. Of note, at such high load conditions, elevated temperature and pressure in the engine during the compression strokes of the pistons may make the intake air/gaseous-fuel mixture susceptible to premature detonation (i.e., engine knocking). Such engine knocking is an uncontrolled combustion process that can damage the engine. While such engine knocking may initially be of low intensity, oscillation (e.g., harmonic amplification) may occur resulting in the engine shaking an/or bouncing (i.e., rough engine operation). Discussed herein are systems and methods to reduce and/or eliminate such engine knocking.  
         [0060]     As noted, the leaning valve  100  is operated by the ECU  80 . In one arrangement, the ECU  80  utilizes the oxygen sensor  84 , which is disposed in the exhaust output  38  of the engine  20 , to generate control signals for adjusting the leaning valve  100 . In this regard, the oxygen content of the exhaust gases is monitored by the oxygen sensor  84 . The oxygen sensor  84  generates a signal based on this oxygen content, which signal is transmitted to the ECU  80 . The ECU  80  may comprise any computer, logic, firmware, computer chip, microprocessor, and/or circuit board. In any case, the ECU  80  controls the operation of the leaning valve  100  in relation to the received oxygen signal. Accordingly, the ECU  80  monitors the signal from the oxygen sensor  84  at a predetermined rate.  
         [0061]     Upon receipt of an oxygen signal showing an amount of oxygen below a desired amount in the exhaust conduit  38 , the ECU  80  sends a control signal to the leaning valve  100 , thereby causing the leaning valve  100  to advance the needle assembly  108  towards the valve seat  106 , which provides a reduced gaseous-fuel flow to the air intake  36 . Accordingly, this reduced gaseous-fuel flow to the air intake results in a leaner gaseous-fuel/intake air mixture. Conversely, upon receipt of an oxygen content signal showing an excess of oxygen in the exhaust conduit  38 , the ECU  80  sends a control signal to the leaning valve  100  causing the needle assembly  108  to be opened relative to the valve seat  106 . This causes an increased gaseous-fuel flow and thereby a richer gaseous-fuel/intake air mixture to be provided to the engine  20 .  
         [0062]     Another feature of the illustrated fumigation system  10  is the ability for a user to selectively control an oxygen set point for the ECU  80 . That is, a user may control the oxygen content of the exhaust gas. In this regard, the ECU  80  includes an exhaust oxygen content adjuster  82  that may be set by a user to establish a desired oxygen content in the exhaust gases. As will be appreciated, engines from different manufacturers may run at different consumption and/or power levels. This is true even for engines having the same power rating (e.g., horsepower). For example, one 300 hp rated engine may run at 25 cubic feet of intake air per second, whereas another 300 hp rated engine may utilize 50 cubic feet of air per second. Likewise, these engines may have varying boost-pressure levels. Furthermore, there may also be variations in the thermal energy of the gaseous-fuel utilized with these engines. For example, there may be thermal energy and/or combustion differences between propane and natural gas. In this regard, the ability to select an oxygen level for the exhaust gases of a particular engine allows for adjusting the fumigation system  10  in accordance with a particular set of operating conditions that may be engine specific.  
         [0063]     The oxygen sensor  84  allows the ECU  80  to ensure that the fuel provided to the engine  20  (i.e., diesel fuel and the gaseous-fuel) is more fully combusted. That is, if the oxygen level in the exhaust conduit  38  drops below a predetermined minimum, it is known that there is not enough oxygen within the intake air/gaseous-fuel mixture to completely combust the fuel. In this instance, fuel is being utilized inefficiently and hydrocarbons are passing unburned through the engine  20 , which results in increased emissions. Accordingly, by adjusting the flow of the gaseous-fuel such that a predetermined minimum oxygen level is maintained within the exhaust output  38 , more complete combustion of the diesel fuel and gaseous-fuel mixture is realized and emissions may be reduced.  
         [0064]     Though the ECU  80  is discussed as being operable to control the fumigation system  10  with only a sensor input from the oxygen sensor  84 , additional and/or different sensors may be utilized control the fumigation system and/or to enhance performance of the engine. For example, an exhaust gas temperature gauge  88  and RPM sensor  92  may be incorporated into the system  10  to control and/or fine-tune the operation of the fumigation system  10 .  
         [0065]     As noted above, in some instances an engine utilizing the gaseous-fuel fumigation system may experience engine knocking conditions. In order to identify such engine knocking conditions, the gaseous-fuel fumigation system also includes at least a first knock sensor  86  that is operatively connected to the engine  20 . See  FIG. 1 . Further, it will be appreciated that a plurality of such knock sensors  86  may be utilized. The knock sensor  86  may take the form of an accelerometer (e.g., a piezoelectric sensor) that is attached to a solid surface of the engine  20  to provide feedback signals to the ECU  80 . What is important is that the knock sensor be operative to generate an output that is indicative of engine motion (e.g., vibrations).  
         [0066]     Generally, the knock sensor(s) may be attached to any location that provides a solid base though which engine vibrations may be detected. Such locations include, without limitation, the engine block and/or cylinder head. In one arrangement, knock sensors  86  may be disposed adjacent to each cylinder of the engine  20 . It is intended that knock sensors  86  detect high motion frequency oscillations of pressure in the cylinders of engine  20  that can lead to mechanical breakdown of engine  20 . As will be appreciated, all diesel engines produce a “rattling” sound when running. This is caused when the diesel fuel ignites upon injection into the combustion chamber. That is, the rapid combustion of the diesel fuel and intake air produces very high pressures generating a rumble or dull clattering sound. Accordingly, for purposes of identifying knocking conditions, it is engine motion/vibration (e.g., engine knocking) above the motion/vibration caused by the rattling of the diesel engine that needs to be identified by the knock sensor  86  and the ECU  80 .  
         [0067]     To identify such knocking conditions, knock sensors  86  may be connected to the engine  20  adjacent to corresponding cylinders. For each cylinder firing, the closest knock sensor  86  may be monitored over a specific time interval during which the output signals of the knock sensors are gathered/sampled by the ECU  80 . That is, the ECU  80  may sample the outputs of the knock sensor(s)  86  and generate a baseline/threshold value(s) that is indicative of normal motion/vibration levels. As will be appreciated, such a threshold value may be generated utilizing a predetermined number of outputs samples of the knock sensor(s)  86 . Further, such a threshold value may be continually updated. Stated otherwise, the threshold value may be a moving average. Alternatively and/or additionally, the ECU  80  may be operative to store one or more threshold values determined during operation of the engine  20  and/or utilize predetermined threshold values. For instance, such threshold values may be stored in a lookup table or other memory structure (e.g., equations) that are indexed to one or more engine operating parameters. A non-inclusive list of such parameters may include, exhaust oxygen level, engine rpm, exhaust temperature, gaseous fuel supply pressure, engine load and/or boost pressure.  
         [0068]     Irrespective of whether the ECU  80  generates the threshold values and/or utilizes predetermined/stored threshold values, the ECU  80  is operative to compare output signals from the knock sensor(s)  86  to one or more threshold values. For instance, the ECU  80  may count the number of knock sensor outputs above a threshold value during a specific time period. If the number of outputs above the threshold value for the time period is unacceptable, remedial measures may be taken. In such circumstances, the ECU  80  adjusts the flow of gaseous fuel to the air intake  36  in response to identifying a predetermined number of outputs from the knock sensor(s)  86  that are above the threshold value (i.e., outputs indicating engine knocking conditions).  
         [0069]     In one particular arrangement, upon identifying engine knocking conditions, the ECU  80  is operative to close the leaning valve  100  to shut off the flow of gaseous fuel to the air intake  36 . Upon shutting the leaning valve  100 , the ECU  80  immediately begins reopening the leaning valve  100  based on one or more current operating parameter (e.g., exhaust oxygen levels). This closing and reopening of the leaning valve  100  (i.e., cycling of the valve) allows for reestablishing proper operating conditions for the engine  20 . That is, engine knocking conditions are eliminated and those conditions are generally eliminated prior to the engine oscillation (i.e., prior to visibly rough operation).  
         [0070]     As will be appreciated, closing of the leaning valve  100  allows the engine  20  to briefly run in full diesel mode. As noted above, the cause of knocking in dual-fuel systems often stems from a rich/excessive gaseous fuel mixture at high load levels. That is, a lack of BTUs in the gaseous-fuel/intake air mixture at high load levels. Knock can also occur due to changes between gaseous fuel types as different gaseous fuels (e.g., natural gas and propane) may have different BTU values. Running in full diesel mode may eliminate the rich gaseous fuel condition. Further, the brief operation in full diesel mode allows the boost pressure valve position to stabilize for current operating conditions (e.g., changes in type of gaseous fuels, engine speed, etc.) as well as for oxygen levels in the exhaust stream to be established for the current operating conditions. Accordingly, when the ECU  80  reopens the leaning valve, the valve may be opened to reestablish a desired oxygen level in the exhaust stream. Due to the changes in the operating conditions, the leaning valve will typically reopen to a different position (e.g., gaseous fuel flow volume) than existed prior to the change in operating conditions. Though discussed in relation to fully closing the leaning valve  100  to correct knocking conditions, it will be appreciated that in some instances, substantially reducing the fuel flow though the leaning valve  100  may produce similar results. That is, the leaning valve  100  may, after the detection of knocking conditions, be closed to reduce gaseous fuel flow to a predetermined percentage of the pre-knocking fuel flow (e.g., 20%) and then be reopened. Such partial closure may, for some applications, be sufficient to eliminate knocking conditions and may also reducing cycling time. Accordingly such partial closure is considered within the scope of the present invention.  
         [0071]     Referring to  FIG. 4 , a process for implementing the above-noted valve cycling to counteract engine knocking conditions is provided. Initially, the ECU  80  monitors ( 210 ) the output of the knock sensor(s)  86  over a predetermined period of time. The output of the knock sensor(s)  86  is compared ( 210 ) to at least a first threshold. If the output is below or within a predetermined amount of the threshold, monitoring continues. If the output is greater than the threshold or a predetermined percentage greater than the threshold (e.g., 1.2 times the threshold) it may be determined that engine knocking conditions exist. Once the determination is made that engine knocking conditions exist, the supply of gaseous fuel flow may be shut off ( 230 ) or substantially restricted to allow the engine to operate in full or near full diesel mode. Once the gaseous fuel is shut off/restricted ( 230 ) the gaseous fuel flow may be reopened ( 240 ) in accordance with current operating conditions. Likewise, monitoring ( 210 ) may continue until operation of the engine is terminated.  
         [0072]      FIG. 5  shows another embodiment of the gaseous-fuel fumigation system  10 . As shown, the second embodiment does not utilize the lock-off valve  46  or the boost-pressure valve  50 . Rather, the system of  FIG. 5  relies upon a single ECU controlled valve  200  to regulate the flow between the gas supply  40  and the air intake  36 . Of note, in  FIG. 5  the gas supply  40  is received from a gas well  44 . In this regard, it will be appreciated the in the production of natural and methane gas, diesel engines are commonly utilized for pumping and compressing gas from wells into pipelines. In this regard, these diesel engines have a ready supply of natural gas that may be utilized with the above-noted systems. However, it will be appreciated that gas received from the well  44  may require dehydration and/or compression prior to use with the diesel engine  20 . It will be further appreciated that the system of  FIG. 5  may be utilized with other gas sources.  
         [0073]     The dual-fuel fumigation system  10 , as shown in  FIG. 5 , again utilizes an oxygen sensor  84  that is disposed within the exhaust output  38  of the engine  20 . However, in this embodiment, the dual-fuel fumigation system  10  utilizes a pressure sensor  90  that is fluidly interconnected to the output stream of the turbocharger  30 . Accordingly, the pressure sensor  90  may provide an output that is indicative of the input pressure (e.g., boost pressure) into the engine  20 .  
         [0074]     The sensor  90  may be any sensor that is operative to generate an output signal that is indicative of pressure. Non-limiting examples of such a sensor include pressure transducers such as diaphragm transducers, strain gage transducers and/or piezoelectric transducers. In any case, the pressure sensor  90  is electrically interconnected to the ECU  80 . In this regard, the output of the sensor  90  is provided to the ECU  80 , which may then utilize the signal from the pressure sensor  90  to generate a pressure value. The controller may then generate a control signal for controlling the valve  200  and thereby regulate the fluid flow between the gas supply  40  and the air intake  36  based on the input pressure of the engine. In addition, the ECU  80  may utilize the output signal from the pressure sensor  90  in conjunction with the output signal from the oxygen sensor  84  (i.e., that is indicative of the oxygen content of the engine exhaust) in order to control the valve  200  based on both (e.g., a combination) engine input pressure and exhaust oxygen content. The ECU  80  can also use pressure from the sensor  90  to adjust the oxygen set point up or down to achieve a non-linear result of gaseous fuel flow. Alternatively, the ECU  80  may be programmed to provide linear increases of gaseous fuel based on input pressure from the sensor  90 . In either arrangement, limitations may be imposed by exhaust oxygen presets. This allows for proportional or non-proportional fuel flows as may be required by a specific application.  
         [0075]      FIG. 6  illustrates a process in accordance with the embodiment of  FIG. 5 . Initially, the ECU  80  monitors ( 310 ) the output signals of first and second engine parameter sensors, which in the present embodiment are the pressure sensor  90  and oxygen sensor  84 . The ECU  80  then compares ( 320 ) at least one of the signals with a reference value/threshold. In this regard, comparing may include use of the signals with predetermined equations to determine, for example, oxygen content values and/or pressure values. In the present arrangement, when the oxygen value of the exhaust stream is above a predetermined minimum (i.e, threshold value), the controller may utilize primarily or entirely the signals from the pressure sensor  90  (e.g., pressure values) to generate ( 330 ) control signals for setting the valve  200 . That is, the controller computes a desired gaseous fuel flow rate based primarily on the intake pressure. Accordingly, the ECU  80  the valve control signals that are relayed to the valve  200  and an actuator associated with the valve  200  then adjusts ( 350 ) the valve  200  in accordance with the signals from the ECU  80  to establish a desired gaseous fuel flow rate. Alternatively, when the oxygen content of the exhaust stream is below a predetermined minimum, the controller may utilize primarily or entirely the signals form the oxygen sensor  84  to generate ( 340 ) control signals for setting the valve  200 . For instance, the oxygen level dropping below the predetermined minimum may indicate that excess gaseous fuel is present and the controller may generate control signals to restrict the valve  200  and thereby reduce the gaseous fuel flow to the engine. This may reestablish a desired oxygen content to the exhaust stream and may thereby result in subsequent operation based on the pressure sensor.  
         [0076]     Of note, the embodiment of  FIG. 5  may also incorporate the knock sensor  86 , as discussed above. In this regard, it will be appreciated that the utilization of the knock sensor may be incorporated into the single valve embodiment of  FIG. 5  as well. That is, the valve  200  may be cycled (e.g., between open and substantially closed positions) to account for knocking conditions.  
         [0077]     The foregoing description of the present invention has been presented for purposes of illustration and description. Furthermore, the description is not intended to limit the invention to the form disclosed herein. Consequently, variations and modifications commensurate with the above teachings, and skill and knowledge of the relevant art, are within the scope of the present invention. The embodiments described hereinabove are further intended to explain best modes known of practicing the invention and to enable others skilled in the art to utilize the invention in such, or other embodiments and with various modifications required by the particular application(s) or use(s) of the present invention. It is intended that the appended claims be construed to include alternative embodiments to the extent permitted by the prior art.