EP2013461A1

EP2013461A1 - Gas and diesel powered compression ignition engine

Info

Publication number: EP2013461A1
Application number: EP06725739A
Authority: EP
Inventors: Gregory Wiedemeier; Carlos Gamez
Original assignee: Clean Air Power Ltd
Current assignee: Clean Air Power Ltd
Priority date: 2006-04-12
Filing date: 2006-04-12
Publication date: 2009-01-14
Also published as: AU2006341722A1; AU2006341722A2; WO2007115594A1; AU2006341722B2

Abstract

A liquid pilot ignited gaseous fueled engines compression ignition engine (GFE) capable of operating in at least a 'multi-fuel' mode in which it is fueled by a liquid pilot- ignited gaseous fuel. It may also selectively be fueled in a 'diesel-only” mode in which it is fueled solely by a liquid fuel or fuels. Engines operable in both modes are typically known as 'dual-fuel” engines. The GFE has a liquid fuel controller that is 'unmodified' in that it is not itself reprogrammed or otherwise modified from a stock liquid fuel controller for pilot fuel supply. Data signals supplied to the liquid fuel controller and used by it to control operation of the liquid fuel injector(s) instead are intercepted by a gaseous fuel controller and modified before being transmitted to the liquid fuel controller. The liquid fuel controller thereby is, in essence, 'tricked' into controlling the liquid fuel injector(s) to effect pilot fuel supply during multi-fuel operation.

Description

GAS AND DIESEL POWERED COMPRESSION IGNITION ENGINE

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to compression ignition fuel engines and, more particularly, relates to a system and method for controlling a compression ignition engine to operate in pilot ignition natural gas mode without having to modify a stock liquid fuel controller. The invention additionally relates to an engine incorporating such a control system.

2. Discussion of the Related Art Gaseous fuels are in strong demand as a primary fuel source in compression ignition engines. Gaseous fuels such as propane or natural gas are considered by many to be superior to diesel fuel and the like because gaseous fuels are generally less expensive, provide equal or greater power with equal or better mileage, and produce significantly lower emissions. This last benefit renders gaseous fuels particularly attractive because recently enacted and pending worldwide regulations may tend to prohibit the use of diesel fuel in many engines. The attractiveness of gaseous fuels is further enhanced by the fact that existing compression ignition engine designs can be readily adapted to burn gaseous fuels.

One drawback of gaseous fuels is that they exhibit significantly higher ignition temperatures than do diesel fuel, oil, and other liquid fuels traditionally used in compression ignition engines. In fact, the temperature of gaseous fuels does not increase sufficiently during operation of standard compression ignition engines for auto-ignition. This problem can be overcome by injecting limited mounts of pilot fuel, typically diesel fuel or lube oil, into the combustion chambers of the cylinders of the engine. The pilot fuel ignites upon injection and burns at a high enough temperature to ignite a gaseous fuel charge in the combustion chamber. These engines are referred to here in as 'liquid pilot ignited gaseous fueled compression ignition engines'^" or simply GFEs. They operate in at least a "multi-fuel mode" in which they are fueled by pilot liquid fuel ignited natural gas. Some GFE' s are also selectively additionally operable in a ^*'diesel-only mode^"' in which the engine's gaseous fuel supply system is selectively disabled and the liquid fuel supply system is controlled to effect a standard diesel cycle. These engines are typically known as "dual fuel^*' engines.

Due to the massive capital expenditure required to design and build an engine from scratch and the general suitability of existing diesel engines to operate as GFEs with relatively minimal modifications, most GFE' s are not designed from the ground-up but, instead, are constructed by modifying an existing diesel engine to retrofit it with a gaseous fuel supply system and related controls or at least modifying an existing diesel engine design to permit the engine to operate as a GFE. In the case of electronically controlled engines having electronic fuel injection (EFI), this retrofitting or modification includes the addition of a gaseous fuel controller that controls a gaseous fuel supply system and that coordinates with the original equipment manufacturer (OEM) supplied liquid fuel controller to control the supply of liquid fuel to the injectors. The liquid fuel controller is typically controlled in a master-slave relationship under control of the gaseous fuel controller. GFE^" s configured in this manner are disclosed, for example, in Published U.S. Pat. App. Ser. No. 2004/0111210 to Davis et al, U.S. Pat. No. 6,694,242 to Wong, and Published U.S. Pat. App. Ser. No. 2005/014005 to Edwards, the subject matter of each of which is incorporated by reference herein.

Reconfiguring an existing liquid fuel controller to cooperate with a gaseous fuel controller is a time consuming and potentially expensive process, particularly in the case of a retrofit operation in which the liquid fuel controller must be reprogrammed or replaced with one having the requisite gaseous fuel controller interface capabilities. Reconfiguring or redesigning an existing liquid fuel controller also may void the manufacturer's warranty. The reconfiguration procedures also tend to be highly OEM controller specific, requiring markedly different diesel/gaseous fuel controller interfaces for different engine brands and even different engine models within a brand. This additional complexity additionally hinders the production of GFE' s, particularly on a relatfvely low volume basis.

In light of the foregoing, the need has arisen to provide a technique for modifying a diesel engine designed to perform as a GFE, either on an initial assembly basis or a retrofit basis, without having to modify the design of the engine's OEM supplied liquid fuel controller, hence providing an at least somewhat generic GFE.

The need has additionally arisen to permit a liquid fuel controller to operate as the pilot fuel controller of a GFE engine without having to disable or replace sensors typically used to control diesel fuel supply, such as an accelerator pedal position detector and an engine position detector. The need has additionally arisen to provide a diesel-to-GFE conversion technique that is at least generally generic in its approach and, accordingly, is relatively inexpensive to implement on a low-volume basis.

SUMMARY OF THE INVENTION

An embodiment of the present invention provides methods and apparatus for producing a "gaseous fueled compression ignition engine" or simply a GFE having an unmodified liquid fuel controller. The GFE is capable of operating in at least a "'multi- fuel" mode in which it is fueled by a liquid pilot-ignited gaseous fuel. It may also selectively be fueled in a "diesel-only" mode in which it is fueled solely by a liquid fuel or fuels. Engines operable in both modes are typically known as "dual-fuel" engines. 4 "liquid fuel controller^"' is defined herein as either one that is supplied by the original equipment manufacturer (OEM) for controlling the diesel and'or other liquid fuel injectors or a controller designed to perform the functions of such a controller and supplied by a third party. The liquid fuel controller is "unmodified" in that it is not itself reprogrammed or otherwise modified for pilot fuel supply. Data signals supplied to the liquid fuel controller and used by it to control operation of the liquid fuel injector(s) instead are intercepted and modified to effect pilot fuel supply before being transmitted to the liquid fuel controller. The liquid fuel controller thereby is, in essence, "tricked^" into controlling the liquid fuel injector(s) to effect pilot fuel supply during multi-fuel operation.

For instance, in order to reduce pilot fuel quantity during multi-fuel operation from the much larger quantity that would be demanded at a prevailing engine speed and load conditions in diesel-only mode, the accelerator pedal position signal may be intercepted, reduced in magnitude to generate the appropriate pilot fuel supply quantity demand for the prevailing vehicular operating conditions, and transmitted to the liquid fuel controller. The liquid fuel controller thereafter controls the ϊnjector(s) to supply the reduced liquid fuel quantity without direct feedback to that effect to the gaseous fuel controller. If desired, the accelerator pedal position signal can be further modified to increase the liquid fuel quantity when insufficient combustion air is available for the engine to operate at the rich limit for the commanded total fuel quantity.

Similarly, in order to advance or retard the start of pilot fuel injection to obtain optimal pilot fuel ignition timing during multi-fuel operation for prevailing speed and load conditions, the existing engine position signal and pedal position signals may be intercepted by the gaseous fuel controller, modified to achieve the advancing or retarding required to obtain optimal pilot ignition timing for multi-fuel operation, and transmitted to the liquid fuel controller. The liquid fuel controller thereafter controls the injector(s) to initiate the pilot injection event at the desired timing.

The techniques described herein are applicable to both retrofit or conversion applications and to initial assembly applications. They are also applicable to all GFE engines, includes dual fuel engines and engines that operate exclusively multi-fuel mode. They are also applicable to systems in which the liquid fuel and/or gaseous fuels are supplied via single point or multipoint injection. They are also applicable to engines having either mechanical governors or electrical governors.

Other aspects and advantages of the invention will become apparent to those skilled in the art from the following detailed description and the accompanying drawings. It should be understood, however, that the detailed description and specific examples. while indicating preferred embodiments of the present invention, are given by way of illustration and not of limitation. Many changes and modifications could be made within the scope of the present invention without departing from the spirit thereof, and the invention includes all such modifications.

BRIEF DESCRIPTION OF THE DRAWINGS

A preferred exemplary embodiment of the invention is illustrated in the accompanying drawings in which like reference numerals represent like parts throughout, and in which:

FIG. 1 is a schematic view of a dual fuel engine constructed in accordance with a preferred embodiment of the invention and of gas and liquid fuel supply systems for the engine;

FIG. 2 is a partially schematic sectional side elevation view of a cylinder of the engine of FIG. 1 and of associated engine components;

FIG. 3 is a schematic control diagram of the engine of FIGS. 1 and 2 and of its attendant controllers and sensors;

FIG. 4 is a flowchart schematically illustrating a routine for modifying engine position and accelerator pedal position signals to cause the liquid fuel controller of the engine of FIGS. 1-3 to control pilot fuel injection timing;

FIGS, 5A-5F are a series of timing charts illustrating a technique for controlling the ignition timing by modifying the engine position signal going to the liquid fuel controller; FIG. 6 is a flowchart schematically illustrating a routine for modifying an accelerator position signal to cause the liquid fuel controller of the engine of FIGS. 1-3 to control pilot fuel injection quantity; and

FIG, 7 is a flowchart illustrating a routine for further modifying the accelerator pedal position signal to take limitations of the engine's combustion air supply system into account.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to FIGS. 1-3, an engine 10 (FIGS. 1 and 2) is illustrated that incorporates a control system 12 (FIG. 3) constructed in accordance with a preferred embodiment of the invention. Before discussing the engine 10 and the associated control system 12 in detail, it must be emphasized that they are exemplary only and that the invention as claimed herein is usable with a wide variety of dual fuel engines and other GFE^'s incorporating a wide variety of gaseous fuel supply systems, liquid fuel supply systems, and air supply systems. Some of these alternatives will be discussed when discussing the preferred embodiments, it being understood that the invention as defined in the clams encompasses these other alternatives as well.

1. Engine Components The exemplary engine 10 illustrated in FIGS. 1-2 is a compression ignition-type internal combustion engine having a plurality of cylinders 12, each capped with a cylinder head 14 (FIG. 2). As is also shown in FIG. 2, a piston 16 is slidably disposed in the bore of each cylinder 12 to define a combustion chamber 18 between the cylinder head 14 and the piston 16. Piston 16 is also connected to a crankshaft 20 in a conventional manner. Conventional inlet and exhaust valves 22 and 24 are provided at the end of respective passages 26 and 28 in the cylinder head 14 and are actuated by a standard camshaft 30 that is rotated by a crankshaft 32 so as to control the supply of an air/fuel mixture to and the exhaust of combustion products from the combustion chamber 18. Gases are supplied to and exhausted from engine 10 via an air intake manifold 34 and an exhaust manifold 36, respectively. An intake air control system may also be provided. The engine 10 is also fitted with a gaseous fuel supply system, either in an OEM or a retrofit (conversion) process. The system includes a source of gaseous fuel 38 such as a compressed natural gas (CNG) fuel tank. Other sources, such as liquefied natural gas (LNG) could also be used. The gaseous fuel may be supplied to the cylinders 12 from the source 38 via any suitable mechanism. For instance, a separate electronically actuated external injector could be provided for each cylinder 12 or, in the case of a shared port intake system, for each pair of injectors. Or from a single point source for the entire engine. Injectors of this type are disclosed, for example, in U.S. Patent No.

5.673,673 and entitled Method and Apparatus for the High Mach Injection of a Gaseous Fuel into an Internal Combustion Engine, the subject matter of which is incorporated herein by reference. In the preferred embodiment, however, the gaseous fuel is supplied to the intake manifold 34 via a fuel metering device 40 and an air-gas mixer 42. The fuel metering device 40 may be any suitable electronlcallv controlled actuator capable of supplying gaseous fuel at times and quantities demanded by a gaseous fuel controller 70 (detailed below). One suitable fuel metering device is a gas injector available from the Clean Air Power gas injector. Part No, 619625. The air/gas mixer 42 may be any conventional mixer, such as the one disclosed in U.S. Patent No. 5, 408,978 and entitled Gaseous Fuel Entrainment Device and Method, the subject matter of which is incorporated by reference. Shut off valve(s) and other equipment for controlling the flow of gas to the metering device 40, all of which are known to those skilled in the art, are omitted for the sake of convenience.

Liquid fuel could be supplied to the cylinder 12 via either a pump/nozzle supply system or via a common rail supply system as described, for example, in U.S. Pat. No. 5,887,566, and entitled Gas Engine with Electronically Controlled Ignition Oil Injection, the subject matter of which is incorporated herein by reference. The illustrated engine 10 employs pump/nozzle supply system having multiple electronically controlled liquid fuel injectors 50. Each injector could comprise any electronically controlled injector, and preferably takes the form of an electro-hydraulic fuel injector such as the HUEI injector sold by Caterpillar Inc.. of Peoria, 111. or an accumulator-type injector of the type disclosed in Reissue U.S. Pat. No. 33,270. Referring to FIGS. 1 and 2. each injector 50 is fed with diesel fuel or the like from a conventional tank 52 via a supply line 54.

Disposed in line 54 are a filter 56, a pump 58. a high-pressure relief valve 60, and a pressure regulator 62. A return line 64 also leads from the injectors 50 to the tank 52.

2. Engine Control System Referring now to FIG. 3, all of the controlled components of the engine 10 are controlled via a control system 12 that includes a gaseous fuel controller 70 and an OEM supplied or at least OEM specified liquid fuel controller 72 connected to one another via a link 74. The link 74 may be a CAN or other broadband communications link or may be one or more conventional signal wires. In an especially preferred embodiment, it comprises a number of signal wires corresponding to the number of signal wires that would be present but for the presence of the gaseous fuel controller 70. For instance, if (as is the case in the illustrated embodiment) the gaseous fuel controller 70 intercepts signals from an accelerator pedal position sensor 76 and an engine position sensor 78. then the link 74 preferably consists if two signal wires, one of which supplies pedal position data and the other of which supplies engine position data. The gaseous fuel controller 70 is configured, based on information received directly from various sensors, to control operation of the gaseous fuel supply system. The liquid fuel controller 72 is configured, based on information received indirectly from sensors and indirectly from the gaseous fuel controller 70 via the link 74, to control operation of the liquid fuel supply system. The controllers 70 and 72 are also preferably programmed so that the engine 10 can be operated in both a multi-fuel mode and a diesel-only mode. In this case, gaseous fuel controller 70 is configured to control the liquid fuel controller 72 in a master-slave relationship when the engine 10 is operating in the multi-fuel mode, and the liquid fuel controller 72 is configured to control all aspects of engine operation when the engine 10 is operating in the diesel-only mode. Both controllers 70 and 72 may comprise any of a variety of commercially available programmable systems, preferably a programmable electronic control unit (ECU). A programmable ECU that is well-suited for use as the liquid fuel controller 72 is commercially available in the industry. Importantly, and as will be described in greater detail below, the gaseous fuel controller 70 is configured so that the other liquid fuel controller is configured so that the liquid fuel controller 72 need not be reprogrammed or otherwise modified to interact with the gaseous fuel controller. As a practical matter, the liquid fuel controller 72 Is unaware of the existence of the gaseous fuel controller 70 or even of the gaseous fuel supply system in general.

The gaseous fuel controller 70 manages fuel delivery for reliable power and emissions control over a wide range of operating conditions. For example, the engine 10 may run entirely on diesel or another liquid fuel until activation of a manual switch and/or automatically upon attainment of threshold operating conditions indicates that multi-fuel mode operation is desired. At this time, the controller 70 operates to deliver sufficient natural gas to run the engine 10 and to control the liquid fuel controller 72 to cause the injectors 50 to inject the minimum amount of diesel fuel required to initiate combustion (i.e., the minimum amount required for pilot ignition of the natural gas) while meeting other engine design criteria, such as maintaining a desired air fuel ratio (lambda). In adjusting the amounts of diesel and natural gas delivered to the cylinder 12. the controller 70 strives to minimize emissions while maximizing performance. The engine control system 12 may be governed either mechanically or electronically. The illustrated engine control system 12 Is electronically governed. As shown in FIG. 3, engine operation is monitored by an accelerator pedal position sensor 76, an engine position sensor 78, an intake manifold pressure sensor 80, and an intake manifold temperature sensor 82. Other sensors, such as a coolant, temperature sensor, an ambient pressure sensor, an ambient temperature sensor, and a vehicle speed sensor may be provided as well. These sensors are collectively denoted 84 and are connected to the gaseous fuel controller 70 by appropriate signal line(s). Still other sensors that are needed only when the engine 10 is operating in diesel-only mode are denoted as 88 and connected to the liquid fuel controller 72. They could alternatively be connected to ihe gaseous fuel controller 70, in which case the information contained therein would simply be relayed in an unmodified fashion to the liquid fuel controller 72 via the data link 74. The gaseous fuel controller 70 also is connected to the gas metering device 40, and to other controlled equipment, such as high-pressure and/or low pressure gas shut off valves, denoted by reference numeral 86, The liquid fuel controller 72 is connected to each of the injectors 50. It could also control other components of the engine, as denoted by reference numeral 90.

Based on the measured operating characteristics, the gaseous fuel controller 70 may calibrate itself by determining the "governing characteristics" of the engine 10 in a process known as "mapping. " Governing characteristics are engine parameters that define multiple fuel system performance and correspond to each operating characteristic recorded. Thus, for a given operating characteristic such as engine speed, the gaseous fuel controller 70 may map a corresponding governing characteristic for engine speed that defines dual fuel operation. The gaseous fuel controller 70 uses the governing characteristics to determine when to adjust the delivery of each fuel and by what amount. A target ratio at maximum load and torque is. for example, 90 percent natural gas and 10 percent diesel. Alternatively, the gaseous fuel controller 70 can be pre-programmed for a particular engine brand or even each individual model.

As discussed briefly above, the gaseous fuel controller 70 is operable to control the liquid fuel controller 72 in a master-slave relationship so as to cause the liquid fuel controller 72 to control the fuel injectors 50 to inject pilot fuel into the cylinders 12 at a timing and quantity that achieves the desired effect at prevailing speed and load conditions. This control need not be with feedback from the liquid fuel controller 72 to the gaseous fuel controller 70. It instead is performed by intercepting signals that, in an OEM engine, would have been bound for the liquid fuel controller 72 and modifying those signals to effect pilot fuel injection for multi-fuel operation rather than diesel-only injection for diesel-only operation. Routines will now be described for pilot fuel timing control and pilot fuel quantity control.

a. Pilot Injection Timing Control

Pilot fuel injection timing control is desired because optimal injection timing differs in multi-fuel mode when compared to diesel-only mode because combustion characteristics differ between the two modes. A great deal of work has been performed in optimizing pilot fuel injection timing for various engine operating conditions. A sophisticated control algorithm is disclosed in U.S. Pat. No. 6,598,584, entitled "Gas Fueled, Compression Ignition Engine with Maximized Pilot Ignition Intensity," the subject of which is incorporated herein by reference. Less sophisticated but more common and more easily adaptable systems simply empirically map desired ignition timing with prevailing engine speed and load conditions. This latter, mapping technique is well known in the industry and need not be described in greater detail.

Turning to FIG. 4, one Routine 100 for selecting and effecting pilot fuel injection timing is illustrated. Routine 100 should be thought of a process as opposed to a computer program. Indeed, since portions of Routine 100 are performed by each of the controllers 70 and 72, no single controller could perform all of the functions discussed below in conjunction with Routine 100. Routine 100 proceeds from START in Block 102 to Block 104, where the gaseous fuel controller 70 reads data including accelerator pedal position and engine position from sensors 76 and 78. Then, in Block 106, the Routine 100 determines whether the engine 10 is operating in multi-fuel mode or in dϊesel-only mode. As discussed above. the changeover between modes may occur differently in different systems or even in a particular system. For instance, changeover may occur upon closure of a manually operated switch and/or may occur automatically upon obtaining threshold engine speed, load, and/or temperature operating conditions.

If the answer to the inquiry of Block 106 is NO such that the engine 10 is operating in diesel-oniy mode, the signals from the sensors 76 and 78 are transmitted in an unmodified form to the liquid fuel controller 72 as seen in Block 108. The Routine 100 then proceeds to Block 110, where liquid fuel controller 72 determines the desired injection timing for prevailing accelerator pedal and engine positions. As is typical in conventional electronically controlled fuel injection systems, this determination preferably utilizes a previously prepared look-up table or map to determine the appropriate injection timing based on a prevailing engine speed and load conditions as determined using signals from the sensors 76 and 78. The look-up table or map typically is preprogrammed into the controller 72 by the OEM.

If the answer to the inquiry to Block 106 is YES. indicating that the engine 10 is operating in multi-fuel mode, the Routine 100 causes the gaseous fuel controller 70 to intercept the signals from sensor 76 and 78 and to modify those signals to '"trick" the liquid fuel controller 72 to cause the injectors 50 to effect the desired pilot fuel injection timing. More specifically, the Routine 100 proceeds to Block 116 where the gaseous fuel controller 70 determines a desired pilot injection timing for prevailing speed and load conditions as determined by signals from the sensors 76 and 78. Once again, this determination preferably comprises accessing a previously prepared look-up table or map containing data tabulated via routine experimentation. This timing may be significantly retarded or advanced when compared to that which would be employed for diesel-only operation at the existing speed and load conditions. The Routine 100 then proceeds to Block 1 18 to determine the modified values of the signals from sensors 76 and 78 that are required to obtain the desired pilot injection timing as determined in Block 116. This determination can be performed using a duplicate of the look-up table or map discussed above in connection with Block 110 since that table or map will already have stored therein a full range of information correlating injection timing, pedal position slants, and engine position signals. The signals from sensors 76 and 78, or separate signals representative thereof, are then modified on Block 120.

The thus modified signals are then forwarded to the liquid fuel controller 72 In Block 108, whereupon the Routine 100 proceeds to Block 110, where the liquid fuel controller 72 determines diesel injection timing using the look-up table or map as referenced above in connection with Block 110. This timing is the same as that determined by the gaseous fuel controller 70 in Block 116. The Routine 100 then proceeds to Block 112 to output the appropriate control signal to the injector 50 and then proceeds to END in Block 114. The Routine 100 cycles through Blocks 102-114 on a full range, full load, cycle-by -cycle basis for so long as the engine is operating.

Determining a modified engine position signal requires that the signal be advanced or retarded when compared to that obtained directly from the sensor 78. However, the manner in which a typical inductive type engine position sensor operates and the corresponding control logic in a standard liquid fuel controller make it difficult or even impossible to significantly advance the engine position signal. This problem, and a technique for overcoming it using the gaseous fuel controller 70. can be appreciated with reference to the timing diagrams of FIGS. 5A-5E.

Referring first to FIG. 5 A, distinct pulses Pl are generated by magnetic teeth on a rotating wheel of an inductive type position sensor as the teeth pass a Hall Effect detector or the like. Specifically, as each rotating tooth passes the Hall Effect detector, a pulse Pl is generated that first rises from a baseline BL as seen at curve portion LEl when the leading edge of the tooth passes the detector, and then falls past the baseline BL to a negative limit as seen by curve portion TEl when the trailing edge of the tooth passes the detector. As seen in FIG. 5B, conventional diesel injection control systems detect engine position by detecting a trigger point TPD at which each pulse Pl traverses the baseline BL with a negative slope. This occurs at the point where LEl passes the baseline BL in FIG. 5 A when the trailing edge of the tooth passes the detector

The manner in which engine position signals are generated and manipulated makes it difficult to artificially advance the engine position signals, particularly under transient conditions in which the position of the engine cannot be easily predicted. The gaseous fuel controller 70 cannot output a modified engine position signal until it receives the signal from the sensor 76, manipulates that signal to determine the actual engine position, and performs the signal modification calculations discussed above in connection with Block 116 of Routine 100. The time required to perform these tasks and to physically transmit the modified signal to the liquid fuel controller 72 would prevent artificial advancement of the position signal if the gaseous fuel controller detected the same portion of the pulse as the liquid fuel controller. Gaseous fuel controller 70 detects the leading edge LEl of the pulse Pl as indicated by the gas controller trigger pulses TPG in FIG. 5C. Then, as seen by FIG. 5D. the gaseous fuel controller 70 generates square wave pulse signals P2 each having leading edges LE2 that correspond to the gaseous fuel trigger pulses TPG, offset by an amount O determined to achieve the desired change in pilot fuel injection timing as discussed above in connection with Block 118 of FIG. 4. The offset O will always comprises a delay when compared to the trigger pulse point TPG because the trigger point TPG represents an advancement from the diesel trigger point PTD equal to LEI- TEI. Hence, any advancement or retractment from the point TPD will still constitute a retractment from point TPG. Then, as seen by FIG. 5E, urversion of the signals P2 In FIG. 5C produces conditioned signal pulses P3 whose leading edges LE3 extend downwardly from the baseline BL. just as the trailing edge TEl of the actual sensor- generated pulses Pl extend downward from the baseline BL. When the resulting pulses P3 are transmitted to the liquid fuel controller in Block 108 of Routine 100. the liquid fuel controller 72 will detect the leading edges LE3 of the conditioned pulses P3 as pilot trigger points(not really pilot trigger points, but engine position. If the OEM controller thinks the position of the engine is 3 degrees earlier than it is, the actual timing will be retarded 3 degrees) TPP as seen in FIG. 5E. The offset "O^" between TPD and TPP reflects the modification in the engine position signal determined in Block 118 of Routine 100.

b. Pilot Fuel Quantity Control Returning now to FIG. 6, a flowchart is illustrated illustrating a Routine 150 for effecting pilot fuel injection quantity control. Routine 150 is similar in its approach to Routine 100 described above and, like Routine 100, should be thought of as a process rather than a computer program because aspects of it are performed by each of the controllers 70 and 72. Routine 150 proceeds from START in Block 152 to Block 154» where the gaseous fuel controller 70 reads data including the accelerator pedal position signal from sensor 76. Routine 150 then proceeds to Block 156. where gas fuel controller 70 inquires whether the engine is operating in multi-fuel mode. If not, the signal from sensor 76 is simply forwarded to the liquid fuel controller 72 in Block 158, which then determines diesel injection quantity for the prevailing pedal position in Block 160. As with injection timing control, the determination preferably takes the form of simply referring to a look-up table or map in which Is empirically recorded the desired injection quantity for the detected accelerator pedal position and the prevailing engine RPM as determined in a conventional manner using signals from the sensors 76 and 78. The generation and use of such a look-up table or map Is well-known to those skilled in the art, and is typically pre-programmed Into the liquid fuel controller 72 by the OEM. The Routine 150 then proceeds to Block 162, where the liquid fuel controller 72 controls the appropriate injector 50 to inject the desired quantity of fuel. The Routine 150 then proceeds to END in Block 164, If, on the other hand, the answer to the inquiry of Block 156 is YES, indicating that the engine 10 is operating in multi-fuel mode, the Routine 150 proceeds to Block 166 where the gaseous fuel controller 70 determines the desired pilot fuel injection quantity for prevailing speed and load conditions. As with pilot injection timing determination, this determination preferably involves the use of a map or look-up table that maps, for each speed/load condition, a desired pilot fuel quantity in a range that achieves a percentage or percentage range of the total fuel supplied for the prevailing gaseous fuel quantity. Typically, pilot fuel quantity will be between 5 and 20 percent, and more typically 10-15 percent, of the total fuel quantity. Various techniques for mapping pilot fuel quantity for prevailing speed and load conditions, ranging significantly in sophistication, are known to those skilled in the art. Any such techniques that are compatible with the system described herein are suitable.

After determining the desired pilot injection quantity, the Routine 150 proceeds to Block 168 where the gaseous fuel controller 70 determines the accelerator position signal resulting in the desired pilot injection quantity using another look-up table or map. As with injection timing, the data contained in that map or look-up table is preferably identical to the one discussed above in connection with Block 160. The Routine 150 then proceeds to Block 170, where a modified signal is generated that corresponds to the signal from pedal position sensor 76, modified to reflect the determined desired pedal position signal as discussed above in Block 168. The gaseous fuel controller 70 then forwards the modified pedal position signal to liquid fuel controller 72 in Block 158. Liquid fuel controller 72 then determines the desired injection quantity for the prevailing (now modified) pedal position signal. The resulting control signal is then output to the appropriate injector 50 in Block 162, and Routine 150 proceeds to END in Block 164. As with Routine 100 discussed above, the Routine 150 is repeated on a full-time, full range, cycle-by-cycle basis.

An optional modification of the Routine 150 as described above resides in the operation of the gaseous fuel controller 70 to additionally modify the pedal position signal to take practical limitations on available combustion air volume into account. Specifically, due to differences in stochiometric air fuel ratios (lambdas) of diesel fuel and natural gas, multi-fuel combustion of a given total quantity of fuel requires more air for combustion then does diesel-only combustion. This effect can limit the maximum output of the engine in multi-fuel mode. Previously, when insufficient air was available to achieve the desired torque while operating in multi-fuel mode, the engine would simple switch to diesel-only operation. While permitting the engine to continue operating without any power reduction, switch over to diesel-only operation significantly increases emissions. Pursuant to this embodiment of the invention, practical limits on combustion air volume are taken into account, while still at least substantially minimizing emissions, by increasing the amount of diesel fuel at high loads only as much as is necessary to do so without sacrificing power.

Specifically, in the preferred embodiment, a Routine 200 of FIG. 7 is acti\ated after the modified pedal position signal is generated in Block 170 of Routine 150. Routine 200 therefore can be considered a subroutine of Routine 150. Subroutine 200 proceeds to Block 204. where gaseous fuel controller 70 determines the actual quantity of available combustion air AMPC and a rich limit of combustion air AMPCrichlimit. AMPC can be determined in a conventional manner using prevailing speed and load and data from the intake manifold pressure sensor 80. The rich limit of any engine is one in which the engine operates at an unacceptably low lambda, resulting in undesirable combustion characteristics, such as detonation, or high NOx emissions. AMPCrichlimit is the amount of air. which would result in the rich limit of combustion of a gaseous fuel quantity and liquid fuel quantity selected for the prevailing load conditions as detected by the pedal sensor 76. AMPCrichlimit for a given quantity of natural gas and diesel can be easily calculated by those skilled in the art using known stochiometric data and/or empirical data.

The Routine 200 then proceeds to Block 206 and compares to the calculated AMPCrichlimit to the actual AMPC. IfAMPC is greater than AMPCrichlimit, then there is no need to increase the pilot fuel injection quantity, and Routine 200 simply proceeds to Block 158 of Routine 150 as seen in Block 208.

If. on the other hand, the Routine 200 determines in Block 206 that AMPC is less than AMPCrichlimit, the Routine 206 proceeds Io Block 210 where gaseous fuel controller 70 determines a pilot quantity Qnewpilot required to make AMPCrichlimit approximately equal to AMPC. This determination is based on the fact that it is known that the required combustion air for a given combined fuel quantity Qcom will decrease a known amount for each incremental volume of gaseous fuel replaced by liquid fuel. That is, because diesel fuel requires less air for combustion on an energy constant basis than natural gas, the required air for combustion drops incrementally as one replaces incremental portions of the gaseous fuel with diesel fuel on an energy content basis. Hence, it is possible to determine and map the quantity AIRSAVED for each cubic millimeter replacement of liquid fuel for natural gas. The quantity Qnewpilot of diesel fuel required to make AMPCrichlimit drop below AMPC thus Is easily calculated when Qcom, AMPC, and AMPCrichlimit are all known.

The Subroutine 200 then proceeds to Block 212, where the look-up table or map discussed above in connection with Block 160 is accessed to determine an accelerator pedal position signal resulting in the Qnewpilot. The pedal position signal is then modified accordingly in Block 214. The Subroutine 200 then proceeds to Block 208, where the doubly modified signal produced in Block 150 is forwarded to the liquid fuel controller 72 in Block 158.

To the extent that they might not be apparent from the above, the scope of variations falling within the scope of the present invention will become apparent from the appended claims.

Claims

We claim:

1. A method of operating a compression ignition internal combustion engine having first and second controllers, the first controller being configured to control a liquid fuel injector based on a signal from a sensor so as to cause the engine to operate in a diesel- only mode in which the engine is powered at least primarily by compression ignition of liquid fuel, the method comprising: using the second controller, intercepting a signal from the sensor, modifying the intercepted signal to produce a modified signal which causes the first controller to control the liquid fuel injector to cause the engine to operate in a multi-fuel mode in which the engine is powered at least in substantial part by a liquid pilot-ignited gaseous fuel, and transmitting the modified signal to the first controller; using the first controller, relying on the modified signal to control the liquid fuel injector to operate in the pilot-ignited gaseous fuel mode.

2. The method of claim 1, wherein the transmitting step is performed without feedback to the second controller from the first controller.

3. The method of claim 1 or 2, wherein the engine is a dual fuel engine operable in both the diesel-only operating mode the multi-fuel mode.

4. The method of claim 1, 2 or 3, wherein the signal is an accelerator pedal position signal, and wherein the modifying step comprises determining 1) a magnitude of the

ZJ pedal position signal that results in the control of the liquid injector to inject a desired quantity of liquid pilot fuel and 2) modifying the pedal position signal accordingly.

5. The method of claim 4, wherein the second controller determines the desired quantity of liquid pilot fuel based on a detected engine speed and a detected accelerator pedal position.

6. The method of claim 5, further comprising determining the desired quantity of liquid pilot fuel based on the maximum quantity of available combustion air, AMPC.

7. The method of claim 6, further comprising determining a quantity of liquid pilot fuel that results in an AMPCrichlimit that is less than AMPC, where AMPCrichiimit is the amount of air which would result in the rich limit of combustion of a fuel quantity selected for the prevailing load conditions, and wherein the determining step comprises determining a desired quantity of liquid pilot fuel that maintains AMPCrichlimit below AMPC.

8. The method of any one of the preceding claims, wherein the signal is an engine position signal and an accelerator pedal position sensor, and wherein the modifying step comprises 1 ) determining a timing of the engine position signal and the magnitude of the pedal position signal that result in the control of the liquid fuel injector to effect a liquid fuel injection timing desired for pilot fuel ignition and 2) modifying the engine position signal and the pedal position signal accordingly.

9. The method of claim 8, wherein the second controller determines a desired liquid fuel pilot injection timing based on a detected engine speed and a detected accelerator pedal position.

10. The method of claim 8 or 9, wherein the engine position signal is an engine position signal having a leading edge portion, and further comprising generating, using the second controller, a conditioned signal from a gaseous fuel trigger signal and outputting the conditioned signal to the first controller, and generating, using the first controller, a pilot fuel trigger signal from the conditioned signal.

1 1. The method of claim 10, wherein the step of generating the conditioned signal comprises generating a gaseous fuel trigger signal by detecting the leading edge portion of the engine position signal, generating a pulsed signal from the trigger signal, the pulsed signal having a leading edge corresponding to the trigger signal, offset an amount required to obtain a desired pilot injection timing, and inverting the pulsed signal to produce the conditioned signal.

12. A method of constructing a compression ignition engine, comprising:

(A) electronically coupling a first controller to a liquid fuel injector, the first controller being a liquid fuel controller configured receive a signal from a sensor and, based on the received signal, controlling the liquid fuel injector to operate in a diesel-oniy mode in which the engine is powered at least primarily by compression ignition of a liquid fuel;

(B) without modifying software in the liquid fuel controller, electronically coupling a second controller to the sensor and to an input of the first controller, wherein, as a result of the coupling, the first controller controls the liquid fuel injector to cause the engine to operate in a multi-fuel mode in which the engine is powered at least in substantial part by a liquid pilot-ignited gaseous fuel.

13. The method of claim 12, wherein the coupling between the first and second controllers lacks feedback from the first controller to the second controller.

14. A compression ignition internal combustion engine comprising:

(A) a cylinder;

(B) a sensor that monitors an operational characteristic of the engine; (C) a liquid fuel injector that supplies a compression ignitable liquid fuel to the cylinder; (D) a gaseous fuel source that supplies gaseous fuel to the cylinder; (E) a liquid fuel controller that is configured to receive signals from the sensor and to control the liquid fuel injector to cause the engine to operate in a diesei-only mode in which the engine is powered at least primarily by compression ignition of a liquid fuel: and (F) a gaseous fuel controller that is coupled to the sensor, the gaseous fuel source, and the liquid fuel controller, the gaseous fuel controller intercepting signals from the sensor, modifying the intercepted signals, and transmitting the modified signals to the liquid fuel controller so as to cause the liquid fuel controller to inject pilot liquid fuel into the engine while the engine operates in a pilot-ignited multi-fuel mode in which the engine is powered at least in substantial part by a liquid pilot-ignited gaseous fuel.

15. The engine of claim 14. wherein the engine lacks feedback to the gaseous fuel controller from the liquid fuel controller.

16. The engine of claim 14 or 15, wherein the sensor is an acceleration pedal position sensor and wherein the gaseous fuel controller 1) determines a magnitude of modification of the pedal position signal required to result in the control of the liquid fuel injector to inject a desired quantity of liquid pilot fuel and 2) modifies the pedal position signal accordingly.

17. The engine of claim 16, wherein the gaseous fuel controller determines the desired quantity of liquid pilot fuel based on a detected engine speed and a detected accelerator pedal position.

18. The engine of claim 16, wherein, the gaseous fuel controller determines the desired quantity of liquid fuel based on a determined quantity of available air AMPC.

19. The engine of claim 18, wherein the gaseous fuel controller determines the desired quantity of liquid fuel that results in AMPCrichlimit being less than a prevailing AMPC, where AMPCrichlimit is the amount of air which would result in the rich limit of combustion of a fuel quantity selected for the prevailing load conditions.

20. The engine of any one of claims 14 to 19. wherein the sensor is an engine position sensor and an accelerator pedal position sensor, and wherein the gaseous fuel controller 1 ) determines a magnitude of modification of an engine position signal and a pedal position signal required to result in the control of the liquid fuel injector to effect a liquid fuel injection timing resulting in a desired pilot fuel ignition timing and 2) modifies the engine position signal and the pedal position signal accordingly.

21. The engine of claim 20, wherein the gaseous fuel controller determines the desired liquid fuel injection timing based on a detected engine speed and a detected accelerator pedal position.

22. The engine of claim 21 , wherein the engine position signal has a leading edge portion, wherein the gaseous fuel controller generates a conditioned signal from a gaseous fuel trigger signal and outputs the conditioned signal to the liquid fuel controller, and wherein the liquid fuel controller generates a pilot fuel trigger signal from the conditioned signal.

23. The engine of claim 22, wherein the gaseous controller generates a gaseous fuel trigger signal by detecting the leading edge portion of the engine position signal, generates a pulsed signal from the trigger signal, the pulsed signal having a leading edge corresponding to the gaseous fuel trigger signal, offset by an amount required to obtain a desired pilot injection timing, and inverts the pulsed signal to produce the conditioned signal, and wherein the liquid fuel controller generates the pilot fuel trigger by detecting the leading edge of the conditioned signal.

24. A method of generating a modified engine position signal for the purposes of determining pilot fuel injection timing in a gaseous fueled compression ignition engine. comprising: (A) using an engine position sensor, generating an engine position signal having a leading edge,

(B) transmitting the engine position signal to a first controller;

(C) generating, using the first controller, a conditioned signal from a gaseous fuel trigger signal;

(D) transmitting the conditioned signal to a second controller; and

(E) generating, using the second controller, a pilot fuel trigger signal from the conditioned signal.

25. The method of claim 24, wherein the step of generating the conditioned signal comprises generating a gaseous fuel trigger signal by detecting a leading edge portion of the engine position signal, generating a pulsed signal from the trigger signal, the pulsed signal having a leading edge corresponding to the trigger signal, offset bv an amount required to obtain a desired pilot fuel injection timing, and inverting the pulsed signal to produce the conditioned signal.

26. The method of claim 24 or 25, wherein the step of generating the pilot fuel trigger signal includes detecting a leading edge portion of the conditioned signal.

27. A method of operating a compression ignition internal combustion engine having first and second controllers, the first controller being configured to control a liquid fuel injector based on a signal from an accelerator pedal position sensor so as cause the engine to operate in a diesei-only mode in which the engine is powered at least primarily bv compression ignition of liquid fuel, the method comprising: using the second controller

(A) determining a desired quantity of liquid pilot fuel based on a detected engine speed and a detected accelerator pedal position, wherein the determining step includes ϊ. determining maximum quantity of available combustion air, AMPC, and ii. determining a quantity of liquid pilot fuel that results in an

AMPCrichlimit that is less than AMPC, where AMPCrichlimit is the amount of air which would result in the rich limit of combustion of a fuel quantity selected for the prevailing speed engine operation conditions;

(B) intercepting a signal from the pedal position sensor;

(C) determining a magnitude of the pedal position signal that results in the control of the liquid fuel injector to inject the desired quantity of liquid pilot fuel, and

(D) modifying the pedal position signal accordingly.