PRESSURE REGULATION METHOD FOR A FUEL INJECTION SYSTEM

Abstract: Disclosed is a method of controlling the gas pressure in a dual fluid injection system wherein the gas is used to deliver a metered quantity of fuel to a combustion chamber (60) of an internal combustion engine (20). On detection of an abnormal fuel supply condition, gas pressure in the fuel injection system is regulated to a level less than the fuel pressure deliverable by a fuel supply system of the engine (20) under the abnormal fuel supply condition. In this way, fuel may be delivered as required to the dual fluid fuel injection system.

Description: PRESSURE REGULATION METHOD FOR A FUEL INJECTION SYSTEM FIELD OF THE INVENTION The present invention is generally directed to dual fluid fuel injection systems for internal combustion engines, and in particular to a method of regulating the air pressure required in such dual fluid fuel injection systems. The present invention is applicable for both two and four stroke engines and may be adapted for use on both single and multi-cylinder engines. BACKGROUND TO THE INVENTION Dual fluid fuel injection systems typically utilise compressed gas during each injection event to entrain and atomise a metered quantity of fuel for delivery into the combustion chambers of an internal combustion engine. The Applicant has developed such fuel injection systems and one version thereof is described in the Applicant's U.S. Patent No. RE36768, the details of which are incorporated herein by reference. Generally, a source of compressed gas, for example an air compressor, is required for these fuel injection systems to operate satisfactorily. The term "air" is used herein to refer not only to atmospheric air, but also to other gases including air and exhaust gas or fuel vapour mixtures. In operation, such dual fluid fuel injection systems typically rely on the existence of a differential pressure between the fuel which is metered for subsequent delivery and the compressed gas, typically air, which is used to deliver the fuel to the engine. In this regard, it is normal that the fuel pressure is slightly higher than the air pressure such that the fuel may be metered into a volume of compressed gas in a manner akin to that described in U.S. Patent No. RE36768 above. Without the existence of such a pressure differential, fuel would not be able to be metered for subsequent delivery. A typical dual fluid fuel direct injection system is capable of operating in both a homogenous charge mode and a stratified charge mode. A homogenous charge mode has a substantially homogenous distribution of fuel throughout the combustion chamber at ignition whereas a stratified charge mode has a substantially stratified distribution of fuel throughout the combustion chamber at ignition. To ensure effective operation under a wide range of operating conditions, both the fuel supply system and the air supply system of the dual fluid fuel injection system may be provided with excess capacity and appropriate regulation devices. This excess capacity and regulation devices ensure that the fuel supply system normally operates at a predetermined optimum pressure and similarly that the air supply system operates at a predetermined optimum pressure. The regulation devices control the pressure of the fuel and air in the fuel and air supply systems respectively so that the maximum pressure of these systems is capped at the predetermined optimum pressure. The fuel supply pressure and the air supply pressure are typically set so as to have a predetermined differential pressure so that known quantities of fuel are metered from the fuel supply system. However, it has been found that under certain operating conditions, such as when there exists an abnormal fuel supply condition, the pressure of the fuel in the fuel supply system may be significantly lower than the standard operational pressure referred to above, and hence may be lower than the pressure of the air in the air supply system. As a result, the necessary pressure differential between the fuel and the air will not be present and hence it will not be possible to meter fuel for delivery to the engine by way of the gas in the normal manner. Further, if the fuel pressure is significantly lower than the air pressure in the system, any attempt to meter fuel against the air pressure may result in a back-flow of air into the fuel supply which can result in air bubbles being trapped in the fuel supply and hence further fuel metering problems. One example of such an abnormal fuel supply condition may exist during cranking under cold ambient conditions, wherein the current drawn from the battery during cranking can cause the terminal voltage of the battery to drop from the standard 12V - 14V range, down to 8V or less. Such a drop in battery voltage may result in a voltage which is insufficient for a fuel pump of the fuel supply system to operate correctly so that fuel is supplied at a satisfactory pressure from a fuel reservoir to a fuel metering unit of the fuel injection system. Alternatively, it may result in fuel pressure in the fuel supply system rising at a lower rate than air pressure in the air supply system. This may result in the pressure of the air within an air supply system of a dual fluid fuel injection system being higher than the fuel pressure which is supplied by the fuel pump under such cold cranking and low battery voltage conditions, and hence may prevent any positive flow of fuel from the fuel injectors. This may be particularly problematic in many present day engine applications which require an engine to start under such cold ambient conditions when the battery voltage may drop to a less than optimum level. The problem of the delivered fuel pressure being lower than the prevailing air pressure in the fuel injection system, due to an abnormal fuel supply condition, may be further compounded in light of some of the control strategies that have been developed by the Applicant in which the air supply system may be rapidly pressurised at start-up through a "pump-up" sequence. Such "pump-up" strategies are disclosed in the Applicant's US Patent No. 4936279 and PCT Patent Application No. WO98/01667, the contents of which are incorporated herein by reference. Such strategies enable the quick establishment of a required air pressure in the air supply system of a dual fluid fuel injection system on start-up. In a typical pump-up sequence, the delivery injector(s) of the fuel injection system is (are) opened at an appropriate time to allow pressurised gas to flow from the combustion chamber(s) of the engine through the delivery injector(s) and into the air supply system so as to pressurise the air supply system. Unfortunately, the rate of rise in pressure of the fuel delivered by the fuel pump may lag behind the rate of rise in the air or gas pressure in the air supply system due to, for example, low battery voltage. As a result, the gas pressure during start-up may at times exceed the delivered fuel pressure and hence fuel may not be effectively metered and delivered to the fuel injection system and the engine may fail to start properly, if at all. Abnormal fuel supply conditions may also occur in other instances of operation, particularly in a failure situation, such as fuel pump failure or regulator failure. It would be advantageous to regulate the air pressure within a dual fluid fuel injection system under such conditions such that fuel may be delivered to the fuel injection system to enable starting of the engine and quick establishment of normal running. SUMMARY OF THE INVENTION It is therefore an object of the present invention to provide a method of regulating the gas pressure in a dual fluid fuel injection system that addresses the above noted disadvantages. With this object in view, the present invention provides a method of controlling gas pressure in a dual fluid fuel injection system wherein the gas is used to deliver a metered quantity of fuel to a combustion chamber of an internal combustion engine including, where an abnormal fuel supply condition is detected: regulating the gas pressure in the fuel injection system to a level less than the fuel pressure deliverable by a fuel supply system of the engine under the abnormal fuel supply condition such that fuel is delivered as required to the dual fluid fuel injection system. Preferably, the gas pressure is regulated to a level less than the expected gas pressure in the dual fluid fuel injection system when the engine is operating under normal running conditions. Conveniently, the gas pressure is provided by a compressor, typically an air compressor, arranged to supply air under pressure to the fuel injection system which, in certain applications, may be driven off the engine. Such control of the gas pressure in accordance with the above method may be especially advantageous upon start-up of the engine, particularly under cold ambient conditions, where the voltage of a battery of the engine may drop to a level which is insufficient to allow a fuel pump of the fuel supply system to develop a satisfactory fuel delivery pressure. Accordingly, in such instances, there may not exist a satisfactory pressure differential between the fuel and the air to enable fuel to be metered against the prevailing gas pressure in the fuel injection system. Such conditions might also occur with a wearing or faulty fuel pump which may not be able to deliver fuel at a required pressure. Other such situations which may cause an abnormally low fuel pressure include having a blocked or partially blocked fuel filter, damaged fuel pipe-work (e.g. if the vehicle is driven over rough ground and pipe-work is crushed), loss of fuel pressure due to the vehicle running out of fuel or starting to run out of fuel, a leak from the fuel system (e.g. due to a sealing O-ring failure), dirt in a fuel regulator of the fuel injection system; and in the case of a vehicle fitted with 2-off fuel pumps mounted in series, failure of 1-off of these pumps. Alternatively, the pressure of the fuel delivered by the fuel pump may not rise at a rate required to achieve quick establishment of normal engine running or it may not rise at the same rate as the air pressure in the air supply system. Such situations are examples of abnormal fuel supply conditions which, as alluded to hereinbefore, may result, in the fuel pressure of the fuel supply system being less than the pressure of the gas delivered by the gas compressor. In a dual fluid fuel injection system of the kind developed by the Applicant, this circumstance may prevent satisfactory fuel metering and the proper operation of the fuel injection system. Preferably abnormal fuel system conditions are detected through use of at least one pressure detection means. Said pressure detection means is preferably a differential pressure detection means for detecting relative pressure between said fuel supply system and said air supply system. Said pressure detection means may also comprise a sensor for detecting fuel pressure in said fuel pressure supply system and/or an air pressure sensor for detecting air pressure in said air pressure supply system. Alternatively, abnormal fuel system conditions are determined from known characteristics of at least one of said internal combustion engine, said fuel supply system or said air supply system. Preferably said fuel supply system characteristics comprise at least one response time for fuel pressure in said fuel supply system to reach a predetermined pressure, preferably from a known starting pressure. Preferably said known characteristics further comprise engine operating conditions, such as battery voltage, ambient air temperature, engine temperature, coolant temperature and battery current draw at cranking and associated battery voltage response. Preferably said system comprises at least one electronic map for use by an electronic control unit, the map being of known characteristics and/or engine operating conditions. Preferably said at least one electronic map is at least one known characteristic over a range of at least one operating condition. For example the map may plot the rise in fuel pressure in the fuel supply system for various engine starting temperatures or various ambient air temperatures. From such a map of known engine response characteristics, the air pressure in the air supply system can be regulated to be less than the fuel pressure in the fuel supply system until the fuel pressure reaches a predetermined pressure. Upon reaching the predetermined pressure, standard fuel pressure and air pressure regulation devices may be relied upon. Alternatively said known engine characteristics in combination with said at least one pressure detection device may detect or estimate the occurrence of an abnormal fuel supply condition. Preferably, the rate of increase of the gas pressure within a gas supply system of the fuel injection system is controlled or regulated in accordance with the one or more of the methods detailed herein such that the pressure of the fuel delivered by the fuel pump exceeds the gas pressure by a sufficient amount to enable satisfactory engine starting and engine operation under such abnormal operating conditions, for example, cold ambient conditions. Such regulation of the gas pressure in the event of an abnormal fuel supply condition may be achieved in a number of different ways. In a first approach, a delivery injector of the dual fluid fuel injection system may be operated to control gas supply system pressure. For example, the delivery injector may be opened at an appropriate time to reduce the gas pressure within the gas supply system to a desired level below the deliverable fuel pressure. In this respect, the method shares some similarities with the method disclosed in the Applicant's co-pending PCT Patent Application No. WO 00/11337 filed August 20 1999 the contents of which were previously incorporated herein by reference. The opening of the delivery injector at an appropriate time results in a reduction in the gas pressure within the gas supply system of the fuel injection system, the degree of control or reduction being a function of the duration and/or timing of opening of the delivery injector, alternatively, it may be a function of the timing of closing of the injector relative to known cylinder gas pressure under known operating conditions. Preferably, such opening of the delivery injector occurs during engine operation where such an abnormal fuel supply condition is detected and the engine is operating, for example, during or shortly after start-up. Alternatively, the delivery injector may be controlled as suggested to provide a limp-home mode of operation for the engine in the event that the abnormal fuel supply condition occurs after engine start-up. The dual fluid fuel injection system may include one or more delivery injectors. The or each delivery injector may be arranged to inject directly into a cylinder of the engine, and one or more injectors may be selected to perform the gas pressure regulation function. The present invention is therefore applicable to engines having one or more cylinders. The duration of opening of the or each delivery injector may be sufficient to allow the gas pressure within the gas supply system to be maintained at a level below the fuel pressure. Conveniently, the control of the delivery injector may be such as to re-establish a desired pressure differential between the fuel and the air. The timing of the opening of the, some, or each of the delivery injectors for regulation of the gas pressure may conveniently be arranged to occur between the normal fuel delivery events of the injector. Hence, the opening of the injector(s) to effect gas pressure reduction in the fuel injection system preferably occur(s) at timings at which the delivery injectors (alternately referred to as air injectors) would not normally be open for fuel delivery into a combustion chamber. Nonetheless, in certain applications, the control of the injector(s) for gas pressure regulation may be immediately before, after, or in some cases overlapping slightly with a fuel delivery event. For example, one possible way of reducing air pressure may be to retain the same injection event as per normal, but to simply keep the delivery injector open for a longer period of time thus bleeding air pressure out of the gas supply system. In this case, closing of the delivery injector, often referred to as "End of Air" (EOA) is timed to coincide with cylinder pressures known to occur at specific points in an engine cycle, with the result that cylinder pressure is used to control air pressure in the air supply system. Preferably, the opening of the injector(s) for gas pressure regulation will be effected at a point in time at which the pressure in the corresponding cylinder is lower than the gas pressure within the gas supply system of the fuel injection system. Accordingly, the timing of opening of the injector(s) as well as the duration of opening may be used to control the degree of gas pressure regulation. That is, opening an injector for the same period of time, but, for example, at a point in a cylinder cycle when the cylinder pressure is lower will typically result in a greater decrease of the gas pressure within the fuel injection system. Alternatively, operating the injector for a longer than normal period of time could also be used to achieve the same result. Conveniently, in the case of engines operating on the four-stroke cycle, the injector(s) may be controlled to relieve gas pressure during the latter part of the intake stroke, the early part of the compression stroke and certain parts of the exhaust stroke. Conveniently, in the case of an engine operating on the two- stroke cycle, the injector(s) may be controlled to relieve gas pressure during the latter parts of the exhaust/intake stroke and/or an initial portion of the compression stroke. At appropriate times within these ranges of the engine cylinder cycle, the pressure within the engine cylinder is likely to be lower than the pressure within the gas supply system (ie: when pressure is required to be relieved therefrom) and as such the opportunity exists to reduce the gas pressure in the air supply system. Taking this concept a step further, at certain points in time within an engine cylinder cycle, the movement of a piston within the cylinder towards bottom dead centre causes a vacuum or low pressure region to be created within the cylinder. This vacuum or low pressure region induced in the cylinder may then assist in both relieving some gas pressure from the gas supply system and under certain conditions drawing fuel held within or being supplied to the delivery injector from a fuel metering injector into the cylinder while the delivery injector is held open. This is due to the pressure differential created across the open delivery injector which enables a net mass flow of fluid from an open fuel metering injector into the delivery injector and hence into the cylinder. This may assist delivery of sufficient fuel into the cylinder to sustain the subsequent combustion event in the cylinder. This latter concept is similar to the method as described in the Applicant's co- pending PCT Patent Application No. WO 00/11337 filed August 20, 1999, the contents of which are incorporated herein by reference. This technique has particular application, for example, under failure conditions where a fuel pump, fuel pressure regulator or other control means has caused an abnormal fuel supply condition. The opening of the injector(s), and operation of the delivery injector(s) and/or fuel injector or fuel metering means generally, may be controlled by an Electronic Control Unit (ECU) controlling the operation of the engine. Engine control systems utilising such ECUs are described in standard texts such as "The Motor Vehicle, Twelfth Edition (1996)" by K. Newton, W. Steeds and T.K. Garret and published by the Society of Automotive Engineers. Therefore, as the use of ECUs in engine control systems is well known to persons skilled in this art, the ECU will not be further described herein in any detail. Conveniently, the ECU may determine the gas pressure within the gas supply system of the fuel injection system by means of a suitably located pressure sensor. The air supply system may include an air rail for supplying air to the delivery injector(s) and to which the compressed air is supplied from the air compressor. The pressure sensor may, for example, be located to measure the air pressure within the air rail. Hence, such an air pressure sensor is one means by which the gas pressure in the fuel injection system may be sensed to determine that it is above a desired level. Alternatively the ECU may estimate the air pressure from know air supply system characteristics and from known operating conditions. The ECU may further determine a desired air pressure level within the gas supply system, having reference to actual or predicted battery voltage and/or fuel pressure, and may compare, by way of an air pressure sensor, the actual measured gas pressure with the determined desired level. Such a desired level would be a pressure sufficiently lower than the deliverable fuel pressure to enable effective fuel metering and hence satisfactory operation of the fuel injection system. Using such a strategy, if the measured gas pressure is at least substantially the same as the desired level, then the ECU will take no further action to regulate the gas pressure. If the measured gas pressure is, however, above the desired level, then the ECU may determine a desired timing and duration of opening of a delivery injector(s) to seek to reduce the gas pressure within the gas supply system back towards or to the desired level. In this regard, the ECU may include a 'look-up' map or other suitable computational means to determine the required 'start of air' (SOA), being the start of the opening, of the injector, and the required duration of opening or to determine the required "end of air" (EOA) of the injector for a particular engine speed to thereby achieve the required drop in gas pressure. At the EOA, the injector is closed. The next viable 'window' period in the engine cycle when it is possible to open the injector is then determined, and the injector may then be actuated during that window period. The gas pressure may be measured after the actuation of the injector and the above-described procedure repeated until the air pressure reaches the desired level. Hence, use of such an air pressure sensor in the fuel injection system can essentially provide for closed loop control of the gas pressure within the fuel injection system. As an alternative to including a pressure sensor in the gas supply system, the method according to the present invention may simply rely on some means to detect that the compressor is delivering air at a higher pressure than the fuel pressure deliverable by the fuel pump. A differential pressure sensor, measuring the pressure differential between the fuel pressure in the fuel supply system and the air pressure in the air supply system may be used to detect an abnormal fuel supply condition. In an alternative strategy, the ECU may sense the actual voltage achievable by the battery at start-up of the engine. Alternatively, a likely voltage value, at start-up, may be predicted from information such as ambient temperature data. As the fuel pressure-battery voltage characteristic may be readily determined, the likely or actual fuel pressure deliverable by the fuel pump of the fuel injection system may be calculated or sensed by the ECU and the air pressure may accordingly be regulated to be less than the sensed or predicted fuel pressure. Preferably, the air pressure will be sufficiently lower than the fuel pressure to provide a sufficient pressure differential to properly operate the fuel injection system. In other words, the air pressure may be controlled as a function of battery voltage or ambient temperature. The method of the invention is also applicable in the case of an engine operated using one of the "pump-up" strategies as described in the Applicant's U.S. Patent No. 4936279 and/or co-pending PCT Patent Application No. WO98/01667. These patents describe a technique for using cylinder gas pressure to supply compressed air to an air supply system of a dual fluid fuel injection system. This supply of compressed air is achieved by opening delivery injectors to the combustion chamber on a compression stroke at a point where the cylinder gas pressure is greater than the air pressure in the air supply system. As a result, back flow of compressed air into the air supply system ensues and the gas pressure in the air supply system increases. As alluded to previously, when there exists an abnormal fuel supply condition as described hereinbefore, there is a certain likelihood that the gas supply system may be pressurised to a predetermined level at a faster rate than that at which the fuel pump of the engine can deliver fuel at a predetermined pressure to the fuel injection system. Accordingly, when a "pump up" strategy is employed, the air pressure within the gas supply system may be regulated such that, even whilst the air pressure is increasing, it does not exceed the fuel pressure available from the fuel pump at that particular point in time. Hence, fuel will be able to be metered for subsequent delivery to the engine. As described in the above mentioned PCT Patent Application No. WO 98/01667, in a "pump-up" sequence, the delivery injector(s) may be opened at progressively closer timings to the top dead centre position of a piston reciprocating in a cylinder of the engine. Accordingly, the timings of opening, and durations of opening, of the delivery injector(s) in such a pump-up sequence may be controlled, as may the rate of pressurisation of the gas supply, to the extent necessary to ensure that the air pressure does not exceed the fuel pressure. That is, where an abnormal fuel supply condition exists, sub-optimum pump-up events may be effected to reduce the rate at which the gas supply system is pressurised up to a desired level. Alternatively to this or together therewith, the frequency or sequence of pump-up events could be modified to provide for sub- optimum pressurisation of the gas supply system. For example, a pump-up event may be effected less frequently than every cylinder cycle, for example every second cylinder cycle rather than every cylinder cycle. In a variant of the above-described strategy, the rate of rise of air pressure could be made a function of the rate of rise of sensed battery voltage. As delivered fuel pressure is a function of sensed battery voltage, the rate of rise of the air pressure within the gas supply means may be controlled, for example, by techniques as described above, as a function of the rate of rise of battery voltage following start-up of the engine. As a further means of regulating the gas pressure within the fuel injection system when an abnormal fuel supply condition is prevalent, the ECU may simply not invoke any pump-up events such that no rapid pressurisation of the gas supply system results. Hence, the time for the gas pressure to increase to a desired level will be increased and is dependent upon the time taken for the compressor to complete an initial number of operating cycles (as referred to in the above-mentioned U.S. Patent and PCT Patent Application). A related variant of the above technique would be to simply delay invoking a pump-up sequence such that a similar result would ensue. In this respect, a pump-up sequence may be delayed until, for example, stable operation of the fuel pump is achieved. Alternatively, if a pump-up sequence has been invoked it may be ceased and depressurisation events may be included in the sequence as necessary to achieve the requisite control over gas or air pressure. In a system employing a differential pressure sensor between the fuel pressure and the air pressure, the air pressure in the system may be controlled so as to track changes in fuel pressure. In such a system optimum start time for "pump up" events to commence after initiation of cranking may be estimated by an ECU depending on operating conditions. Alternatively, an optimum differential pressure between the fuel pressure and the air pressure may be used. "Pump Up" events may then be controlled so as to regulate air pressure of the air supply system with reference to any change in the differential pressure between the air pressure and fuel pressure. Hence as the differential pressure starts to rise, due to a rise in fuel pressure, "pump up" events may commence and be controlled so that the air pressure tracks a predetermined differential pressure with the fuel pressure. As mentioned hereinbefore, the dual fluid fuel injection system includes a gas supply means for supplying gas as a propellant for fuel which assists with atomising fuel on injection of said fuel and gas to the engine wherein such air supply means typically takes the form of an air rail unit. It is the gas pressure in the rail unit that generally requires to be controlled to a value sufficiently less than the fuel pressure deliverable by the fuel supply system to enable effective operation of the fuel injection system. While the delivery injector(s) may be opened to reduce the air pressure in the air rail unit, it may also be feasible to alternatively or additionally provide a controllable gas pressure relief valve to enable gas pressure control in accordance with the method of the invention. Such a valve may be of a kind that opens to relieve pressure when gas pressure exceeds a certain preset value. Alternatively, the relief valve may be, for example, solenoid operated under control of the ECU. Thus, if the sensed gas pressure exceeds the deliverable fuel pressure, the relief valve may be opened under ECU control to reduce the pressure within the gas supply system to an acceptable value. The opening duration of such a relief valve may be controlled in an analogous manner to the delivery injector(s). In a further alternative, a throttle may be located on the inlet side of a compressor used for supplying compressed air to the air supply system. Such an arrangement allows the compressor to regulate changes in air pressure relative to changes in fuel pressure when an abnormal fuel supply condition is present. Still further, rather than maintaining the gas supply system in a pressurised state on shutdown of the engine, the gas pressure within the gas supply system may be reduced or relieved prior to shutdown so that the gas supply system remains un-pressurised or at a low positive pressure thus preventing the air pressure from being higher than the fuel pressure under cold start conditions. That is, on start-up, the air pressure will already be at a low level and may not require to be reduced or regulated further. The method may also be made adaptive to particular fuel pumps. For example, for a given fuel pump, problems in terms of developing sufficient fuel pressure until a certain time after start-up has elapsed may be known. The ECU can accordingly be programmed to ensure that pump-up sequences automatically do not occur or that if they do occur, they do so in a manner to provide sub- optimum pressurisation of the gas supply system. Adaptive control of this kind may be possible in other situations. The aforementioned strategies and/or means for regulating the gas pressure in the fuel injection system may be effected individually or in accordance with any suitable combination of such strategies and/or means. For example, together with delaying a pump-up sequence for the fuel injection system, the method may simultaneously control the delivery injectors so as to relieve or reduce the gas pressure present in the gas supply system. Still further, the pressure of the gas in the gas supply system can be regulated on the basis of the measured air pressure, the measured fuel pressure or the measured differential pressure. Alternatively such pressures may be estimated from known system characteristics and from known operating conditions, such as battery voltage, ambient temperature, or engine temperature. Engines which implement the above described method as well as electronic control units to implement the method form further aspects of the invention. Though the above description has at times been made with reference to fuel pressure being not less than prevailing air pressure, in practice, effective running of the engine requires a differential pressure between the fuel and air sufficient to allow effective operation of the fuel injection system. The method of the invention allows such differential pressure to be maintained even under conditions wherein conventional engines may typically not be able to operate satisfactorily and this is an advantage of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS It will be convenient to further describe the invention with respect to the accompanying drawings that assist in describing various possible arrangements of the present invention. Other arrangements of the invention are however possible, and consequently, the particularity of the accompanying drawings is not to be understood as superseding the generality of the preceding description of the invention. In the drawings: Figure 1 is a schematic partial cross-sectional view of an internal combustion engine having a dual fluid fuel injection system operatively arranged with respect thereto; Figure 2 is a partial cross-sectional view of one form of a fuel metering and injector rail unit; Figure 3 is a flow chart showing a preferred arrangement of the method according to the present invention; Figure 4 is a graph of pressure within an air supply system of the dual fluid fuel injection system versus engine operating cycles from engine start-up for (a) an engine operated in accordance with a normal pump-up strategy and (b) an engine operated with a modified pump-up strategy in accordance with the method of the present invention; and Figure 5 is a series of schematic pressure traces for each cylinder of a four cylinder engine showing the timing of opening and closing of a respective delivery injector when the engine is operated with a (a) normal, and (b) modified pump-up strategy in accordance with the method of the present invention. DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION Figure 1 shows a direct injected four stroke internal combustion engine 20 comprising a fuel injection system, the engine 20 having an air intake system 22, an ignition means 24 (schematic representation only), a fuel pump 23, and fuel reservoir 28. An air compressor 29 is operatively arranged with respect to the engine 20 and typically driven off the engine crankshaft 33 or other drive-train by way of a suitable belt (not shown). Mounted in the cylinder head 40 of the engine 20 is a fuel and air rail unit 11. The fuel pump 23 draws fuel from the fuel reservoir 28 which is then supplied to the fuel and air rail unit 11 though a fuel supply line 55. Conventional inlet and exhaust valves 15 and 16 are also mounted in the cylinder head 40 in the known manner together with conventional cam means 17 for actuating the valves 15, 16. The valves 15, 16 are arranged to open and close corresponding inlet and exhaust ports 18 and 19 for admission of fresh air and the removal of exhaust gases from the engine cylinder in the known manner. Referring now to Figure 2, there is shown in detail a fuel and air rail unit 11 which, whilst being different in design from that shown in Figure 1 , shares all the same components thereof. The fuel and air rail unit 11 comprises a fuel metering unit 10 and an air or delivery injector 12 for the or each cylinder of the engine 20. The fuel metering unit 10 is commercially available in the form of a pressure time fuel injector well known in the field of multi-point fuel injection engines and requires no detailed description herein. Suitable ports are provided to allow fuel to flow through the fuel metering unit 10 and a metering nozzle 21 is provided to deliver fuel to a passage 120 and thence to the air injector 12. The body 8 of the fuel and air rail unit 11 may be an extruded component with a longitudinally extending air duct 13 and a fuel supply duct 14. As best seen in Figure 1 , at appropriate locations, there are provided connectors and suitable ducts communicating the rail unit 11 with air and fuel supplies: air line 49 communicating air duct 13 with the air compressor 29; air line 53 providing an air outlet which returns air to the air intake system 22; and fuel line 52 communicating the fuel supply duct 14 to the fuel reservoir 28 providing a fuel return passage. The air duct 13 communicates with a suitable air regulator 27 which regulates the air pressure of the compressed air provided by the air compressor 29 to the air duct 13. Referring again to Figure 2, the air injector 12 has a housing 30 with a cylindrical spigot 31 projecting from a lower end thereof, the spigot 31 defining an injection port 32 communicating with passage 90. The injection port 32 includes a solenoid operated selectively openable poppet valve 34 operating in a manner similar to that as described in the Applicant's U.S. Patent No. 4934329, the contents of which are hereby incorporated by reference. As best seen in Figure 1 , energisation of the solenoid in accordance with commands from an electronic control unit (ECU) 100 causes the valve 34 to open to deliver a fuel-gas mixture to a combustion chamber 60 of the engine 20, typically with the fuel entrained in the gas. However, it is not intended to limit the valve construction to that as described above and other valves, for example, pintle valve constructions, could be employed. The electronic control unit (ECU) 100 typically receives signals indicative of crankshaft speed and airflow from suitably located sensors within the engine (not shown). The ECU 100, which may also receive signals indicative of other engine operating conditions such as the engine temperature, ambient temperature and battery voltage (not shown), determines from all input signals received the quantity of fuel required to be delivered to each of the cylinders of the engine 20. As alluded to hereinbefore, this general type of ECU is well known in the art of electronically controlled fuel injection systems and will not be described herein in further detail. The opening of each injector valve 34 is controlled by the ECU 100 via a respective communicating means 101 in timed relation to the engine cycle to effect delivery of fuel from the injection port 32 to a combustion chamber 60 of the engine 20. By virtue of the two fluid nature of the system, fuel is delivered to the cylinder entrained in a gas. The passage 90 is in constant communication with the air duct 13 via the conduit 80 as shown in Figure 2 and thus, under normal operation, is maintained at a substantially steady air pressure. Upon energisation of the solenoid of the air injector 12, the valve 34 is displaced downwardly to open the injection port 32 so that a metered quantity of fuel delivered into the air injector 12 by the fuel metering unit 10 is carried by air through the injection port 32 into the combustion chamber 60 of a cylinder of the engine 20. In this regard, it is important that the pressure of the gas, typically air, within the passage 80 and hence the passage 90 within the air injector 12 is maintained at a predetermined pressure so as to enable satisfactory entrainment and delivery of a metered quantity of fuel to the engine 20. Further, it is also necessary that the pressure of the fuel that is metered into the passage 90 by the fuel injector 10 is greater than the prevailing air pressure therein otherwise no fuel will be able to be delivered by the fuel injector 10 for subsequent injection. In fact, it is preferable that a certain pressure differential exists between the fuel and air such that effective fuel delivery and entrainment can take place. Hence it is necessary that appropriate regulation of the air pressure within the unit 11 be effected. In the embodiment as shown, one form of regulation is performed by the air regulator 27, which ensures that the air pressure does not exceed a predetermined maximum value. Typically, the air injector 12 is located within the cylinder head 40 of the engine 20, and is directly in communication with the combustion chamber 60 defined by the reciprocation of a piston 61 within the engine cylinder. As above described, when the injection port 32 is opened and the air supply available via the conduit 80 is above the pressure in the engine cylinder, air will flow from the air duct 13 through the passage 80, passage 90 and, injection port 32, into the engine combustion chamber 60. Typically fuel is entrained by gas exiting into the combustion chamber 60. As stated above, in the arrangement shown in Figures 1 and 2, the air pressure is regulated to a maximum value by the air regulator 27. However, alternative air regulation means may also be used to provide for the necessary regulation of air pressure within the rail unit 11. For example, the Applicant's co- pending Provisional Patent Application No. PQ7526, the contents of which are hereby incorporated by reference, describes an electronic inlet valve arrangement for a compressor which may alternatively be implemented to regulate the pressure in the air duct 13 and adjoining passages. In this arrangement, a solenoid controlled air intake valve is operated to enable the air supplied to a dual fluid fuel injection system to be closely matched to specific engine operating conditions. In the above-described dual fluid fuel injection system, a typical air pressure for the gas or air within the air duct 13 is 650kPa (although this depends on the particular engine application). A corresponding fuel pressure in this case may be, for example, 700kPa, again depending upon the particular engine application. This would then provide for a 50kPa differential pressure that is sufficient to enable fuel to be satisfactorily metered into the air injector 12 by the fuel injector 10. However, under certain abnormal fuel supply conditions, the fuel pressure deliverable by the fuel pump 23 and hence the fuel injector 10 may be significantly less than the normal value hence reducing the differential pressure to an unsatisfactory low or negative value (ie: when the fuel pressure drops below the level of the normal air pressure). In such circumstances, the fuel pump 23 and hence fuel injector 10 are not capable of flowing any fuel and hence the engine 20 will be unable to operate properly or perhaps even start as the fuel pump 23 will not be able to produce the level of pressure necessary to enable satisfactory fuel metering by the fuel injector 10. In fact, such a scenario may result in a back-flow of fuel through the fuel supply duct 14 and fuel supply line 55. Such an occurrence may result from a low battery voltage due to a high current being drawn from the battery when cranking the engine under cold ambient conditions wherein the fuel pump 23 may not be able to operate in a satisfactory manner. For example, under such operating conditions the fuel pump 23 may be required to operate from a battery voltage of 8.0V. However, the fuel pump 23 may be incapable of flowing any fuel when driven by an 8.0V operating voltage if there exists a 650kPa air back pressure on the fuel injector 10 and hence the fuel pump 23. Accordingly, to enable the fuel pump 23 to flow fuel, the back pressure thereon (as constituted by the pressure of the air in the air duct 13 and adjoining passages 80 and 90) will need to be reduced or eliminated. Hence, the delivery injector (alternately referred to as an air injector) 12 may be controlled by the ECU 100 to actuate between open and closed positions so as to regulate the air pressure in the air duct 13 and adjoining passages 80 and 90 whenever the fuel pressure delivered by the fuel pump 23 is insufficient, for example, due to low voltage operation, to allow effective operation of the fuel injection system. Such low pressure operation being an example of an abnormal fuel supply condition. As alluded to hereinbefore, one or a number of air injector(s) 12 may be actuated until the gas pressure in the fuel injection system has been suitably relieved or reduced. Such abnormal fuel supply conditions may be detected in any number of ways, for example through use of a differential pressure sensor that monitors the differential pressure between the fuel supply system and the air supply system. Alternatively, a fuel pressure sensor may be used in combination with an air pressure sensor or in combination with an estimate of air supply pressure. An estimate of fuel supply pressure may also be used. Such estimates of fuel supply pressure and air supply pressure may be derived from known engine characteristics or known response characteristics of the fuel supply system or the air supply system to certain operating conditions. For example, pressure / time profile or other pressure related characteristics of the fuel supply system at start-up of the engine under a range of operating conditions may be known. Such operating conditions may be one or more of battery voltage, ambient temperature, engine or engine coolant temperature, or battery current drawn at cranking. Such an estimate of fuel pressure may be used in combination with measured air pressure or with estimated air pressure of the air supply system in order to regulate this air pressure. Use of such maps to control engine operation is known as "open loop control". In a system utilising a "pump up" strategy at start-up, air pressure in the air supply system may be controlled with reference to cylinder gas pressures known to exist during cranking. Again these pressures may be mapped and an open loop control system implemented for regulation of air pressure in the air supply system with reference to estimated or measured pressure of the fuel supply system. Referring to Figure 3 which shows one possible control procedure provided by the ECU 100, on the basis of a known abnormal fuel supply condition (ie: such as may be known to exist at engine start-up), the pressure within the fuel and air rail unit 11 , and typically within the air duct 13 is periodically measured (step 120). The air pressure may, for example, be measured by a pressure sensor (not shown) supported at an appropriate location on the fuel and air rail unit 11. However, and as alluded to hereinbefore, other means for determining that the gas pressure within the fuel injection system is too high relative to the deliverable fuel pressure may employed. Alternatively, other control procedures may not require to know the level of the air pressure in the rail unit 11 and may simply seek to reduce the air pressure on the basis of other known factors such as the initial operating characteristics of the fuel pump 23. At step 121 , if the measured air pressure is substantially equal to or slightly less than the desired air pressure for correct operation of the fuel injection system, taking into account the actual fuel pressure or the fuel pressure predicted from sensed battery voltage or the known operating characteristics of the fuel supply system, then no further action is taken by the ECU 100 to regulate the air pressure, and the ECU 100 waits for the next cycle or next occurrence of an abnormal fuel supply condition (step 127) before next measuring the air pressure (step 120) in the fuel injection system. If, however, the measured air pressure is greater than the desired pressure (step 121 ) (ie: typically more than the pressure of the fuel deliverable by the fuel pump), then steps are required to be taken to reduce or relieve the level of air pressure in the air duct 13 (and hence the passages 80 and 90). In this regard, the magnitude of the pressure differential between the fuel and air may be determined (step 122) so as to provide information to the ECU 100 as to the level of subsequent pressure regulation that is required. Furthermore, the current speed of the engine 20 may be measured (step 123) such that the ECU 100 may be able to accurately determine how the air injector(s) 12 need to be subsequently controlled. On the basis of this information, the ECU 100 can then open the air injector(s) 12 to regulate the air pressure supplied to the fuel and air rail unit 11 to a suitable value less than the fuel pressure available from the fuel pump 23. As previously described, the pressure of the fuel delivered by the fuel pump 23 may be sensed and the air pressure adjusted accordingly in accordance with the above strategy. However, such fuel pressure sensing may be avoided due to the fact that the delivered fuel pressure may be predicted from the prevailing battery voltage because the fuel pressure-battery voltage relationship may be readily determined for a given fuel pump with known operating characteristics. The relationship may in certain circumstances also be a function of temperature and the air pressure may thus be regulated as a function of battery voltage for a given temperature. Hence, rather than or as well as following the flow-chart above described in which air pressure is used as a measured and controlled variable, battery voltage and/or fuel pressure could be used as the measured variable with the air pressure being regulated accordingly. Further, rather than operating on the basis of measured air pressure, the pressure differential between the fuel and air pressures may be measured and the air pressure regulated accordingly if the differential is too low to allow effective operation of the fuel metering and injection system. Irrespective of how the ECU 100 determines that an abnormal fuel supply condition has occurred and hence that the air pressure needs to be reduced or relieved, once this determination has been made, the ECU 100 firstly determines the start of air (SOA) and duration of the period of opening for the air injector 12 (step 124). This information may be obtained from an appropriate air pressure regulation look-up map provided in the ECU 100. The next viable window during which the air injector 12 may be opened without effecting the operation of the engine is then determined (step 125). This window may be between respective injection events of the air injector 12 and as such would be different to the timing at which the air injector 12 is normally opened to affect fuel delivery to the engine 20. Further, the opening of the air injector 12 will typically occur at a point in the engine cylinder cycle when the cylinder pressure is less than the gas pressure in the fuel injection system. Alternatively, a timing for closing of the air injector may be used, as well as either a timing for opening of the injector or a duration of opening, any of which may be obtained from a map in the ECU as detailed above. Finally, the air injector 12 is actuated (step 126) on the basis of the determined operational criteria. In this way, gas pressure may be relieved from the fuel injection system into the engine cylinder, typically by way of a pressure differential that exists across the port 32 of the air injector 12. Further, and as alluded to hereinbefore, the air injector 12 may be opened at a point in time at which the downward motion of the piston 61 causes a vacuum to be drawn at the port 32. As well as facilitating the reduction of air pressure in the air duct 13, this vacuum may also be used to draw fuel into the combustion chamber 60 for subsequent combustion in accordance with the method disclosed in the Applicant's co-pending PCT Patent Application No. WO 00/11337. The pressure within the fuel and air rail unit 11 is then again measured (step 120) and the operational procedure repeated as required until the gas pressure is lowered to a workable level. Adoption of such a control strategy, involving a cycle of delivery injector open events, hence ensures that the air pressure within the fuel injection system is prevented from rising above the expected or achievable fuel pressure. As alluded to hereinbefore, the method of the invention may also be used in conjunction with a pump-up strategy for pressurising the air supply system of the fuel injection system. In this regard, control may also or alternatively be exercised over the rate of pressurisation of the air supply system of the fuel injection system. In this way, the problem of the pressure rise rate of the fuel pump 23, particularly during cold starts, not being known and possibly not being able to 'keep up' with any pump-up events that are carried out during engine cranking to pressurise the air duct 13 may be suitably dealt with. In other words the fuel supply system is assumed to always come up to pressure at a slow rate, for example corresponding to very cold ambient conditions, and the air supply system is regulated to match this rate. Referring now to Figure 4, there is shown a plot 71 of the pressure in the air duct 13 versus the number of engine cycles for an engine operating under normal starting conditions in accordance with the Applicant's co-pending PCT Application No. WO98/01667, the contents of which are hereby incorporated by reference. Under cold starting conditions, as identified by sensed or predicted battery voltage, the pump-up sequence may be modified such that developing air pressure in the air duct 13 does not overtake the fuel pressure which is achievable during such low voltage operation at start-up as is shown by the plot 73. Thus, under cold starting conditions, the characteristic may have a lesser, or equal slope to that shown in plot 73 and/or may commence later as shown by plot 72. Such a modified characteristic may be obtained by simply delaying the start time of the pump-up strategy. In this way, the fuel pump 23 is given some time to raise the delivered fuel pressure up to a suitable level. Further, in certain circumstances, the pump-up sequence might not be invoked at all such that the fuel pump 23 is only required to meter fuel against the existing pressure in the fuel injection system upon start-up. Accordingly, the situation where the air pressure is greater than the fuel pressure as is represented by the shaded area 74 in Figure 4 is avoided. More preferably, the modified pressure rise rate characteristic may be achieved by opening the delivery injector(s) 12 for lesser durations or at different, more appropriate timings in during a respective engine cycle. For example, where a low battery voltage is sensed on start-up of the engine 20 under cold ambient conditions, the ECU 100 may set opening times for the delivery injector(s) 12 which correspond to a lower pressure rise rate in the air duct 13. That is, whilst the air injector(s) 12 are still controlled to provide a certain degree of 'pump-up' or pressurisation of the gas supply system, and in particular the air duct 13, sub-optimum air injector 12 event timings are used so as to limit the rate of rise of the air pressure in light of the deliverable fuel pressure. Hence, as sensed battery voltage varies, the ECU 100 may determine different timings for end of air (EOA) and start of air (SOA) events for the air injector(s) 12, as appropriate, such that gas pressure may be increased at the maximum possible rate consistent with effective fuel pump 23 operation. These timings are identified by primes in Figure 5 whilst otherwise normal pump-up sequence timings for the air injector(s) 12 are denoted by the non-prime references. Alternatively or further to the above, the frequency or sequence of pump-ups could be modified. Battery voltage variation may advantageously demonstrate an increase following start-up, however, battery voltage and gas pressure fluctuations may occur and the ECU may be suitably programmed to manage such fluctuations. The desired consequence of controlling the start of air (SOA) and end of air (EOA) timings in a sub-optimum manner is the provision of a controlled rate of increase of the air pressure in the gas supply system, but one which also allows for the slower rate of rise in the delivered fuel pressure under cold ambient conditions as shown by plot 72 in Figure 4. As stated hereinabove, the shaded area 74 beneath plot 71 shows the situation where the differential pressure between the fuel and air would be negative, Hence without any suitable air pressure regulation, fuel starvation of the fuel injector 10 or back-flow problems would exist in such situations. Operating the engine in a homogenous mode of operation during an abnormal fuel supply event may prove advantageous under certain circumstances, particularly where a pump-up strategy is employed. A homogenous mode of operation is generally achieved by metering fuel into the delivery injector on an exhaust or power stoke and then actuating the delivery injector during an intake stroke as this provides sufficient time for the fuel to mix efficiently within the combustion chamber. During a pump-up strategy, gas from the combustion chamber may be transferred into the gas supply system on a compression stroke of the engine, even after fuel has been delivered into the combustion chamber. Some small amounts of fuel may transfer into the gas supply system, however this is not believed to be a problem. In addition to the above-mentioned control strategies, on shutdown of the engine 20, the air pressure within the gas supply system may be reduced to atmospheric pressure or to a slight positive pressure such that the pressure barrier to effective fuel pump operation may be substantially lowered or in some cases removed. One possible methodology for achieving such an objective is described in the Applicant's U.S. Patent No. 5730108, the contents of which are hereby incorporated by reference. By way of the present invention, the fuel injection system may be made to operate in the event of an abnormal fuel supply condition such as when a fuel pump is unable to deliver fuel at a sufficient pressure to enable satisfactory operation of the fuel injection system. That is, the air pressure is controlled so as to be maintained below the fuel pressure deliverable at the time. This is most advantageous in the case of cold starting where low battery voltages typically associated with such start-ups may prevent engine starting. However, the method may also be implemented following start-up, for example, under cold ambient conditions. In this respect, while the fuel pump 23 may deliver fuel even whilst being operated with a low voltage during early cycles of engine operation, a problem may occur where the pressurisation of the gas supply system rises at a quicker rate than the deliverable fuel pressure. In such a case, pressure regulation may be required even following initial engine start-up. Further, the present invention is not limited in its applicability to cold starting circumstances. For example, as fuel pumps wear they may also have less capability to deliver fuel at a desired pressure. In such circumstances, where the ECU 100 detects such an unsatisfactory lower fuel pressure, or lower fuel-air pressure differential, the strategy described herein may be employed to ensure a kind of 'limp-home' mode in which the air pressure is controlled to allow functioning of the fuel injection system. In its simplest form, the method of the present invention can be implemented such that the air pressure within the fuel injection system is limited to a value less than the deliverable fuel pressure hence avoiding the undesirable consequences mentioned hereinbefore. Further, any of or a combination of the control strategies discussed may be implemented in order to regulate the air pressure within the fuel injector system. The invention is equally applicable to single cylinder configurations and multi-cylinder engines of any number of cylinders. The method according to the present invention is applicable to both two stroke and four stroke engines, whether single or multi-cylinder, incorporating dual fluid fuel injection systems. Modifications and variations as would be deemed obvious to the person skilled in the art are included within the ambit of the present invention.

Claims: THE CLAIMS DEFINING THE INVENTION ARE AS FOLLOWS: 1. A method of controlling gas pressure in a dual fluid fuel injection system wherein the gas is used to deliver a metered quantity of fuel to a combustion chamber of an internal combustion engine including, where an abnormal fuel supply condition is detected: regulating the gas pressure in the fuel injection system to a level less than the fuel pressure deliverable by a fuel supply system of the engine under the abnormal fuel supply condition such that fuel is delivered as required to the dual fluid fuel injection system. 2. The method of claim 1 wherein gas pressure in the dual fluid fuel injection system is regulated to a level less than expected gas pressure in the dual fluid fuel injection system when the engine is operating under normal running conditions. 3. The method of claim 1 or claim 2 wherein said method is implemented at start-up of the engine. 4. The method of claim 3 wherein said method is implemented at start-up of the engine under cold ambient conditions. 5. The method of any one of the preceding claims wherein a delivery injector of the dual fluid fuel injection system is operated at timing to maintain the gas pressure within the gas supply system at a desired level below the deliverable fuel pressure of the fuel supply system. 6. The method of claim 5 wherein the gas pressure in said gas supply system is controlled as a function of at least one of the duration of opening of said delivery injector, timing of opening of said delivery injector timing of opening of said delivery injector. 7. The method of claim 5 or 6 wherein actuation of said delivery injector is controlled to establish a desired pressure differential between fuel and gas. 8. The method of any one of claims 5 to 7 wherein actuation of said delivery injector occurs between normal fuel delivery events thereof. 9. The method of any one of claims 5 to 8 wherein actuation of said delivery injector overlaps with a normal fuel delivery event. 10. The method of any one of the preceding claims wherein said engine is a four stroke engine and said delivery injector is actuated to reduce gas pressure during a portion of at least one of a latter portion of an intake stroke, an early portion of a compression stroke and an exhaust stroke. 11. The method of any one of claims 1 to 9 wherein said engine is a two stroke engine and said delivery injector is actuated to control gas pressure in said gas supply system during a portion of at least one of a latter portion of an exhaust/intake stroke and an initial portion of a compression stroke. 12. The method of any one of the preceding claims wherein gas pressure level is determined by an engine control means as a function of actual or predicted engine battery voltage. 13. The method of any one of claims 5 to 12 wherein start of opening of said delivery injector and open duration is determined by an engine control means as a function of at least one engine operating condition. 14. The method of claim 13 wherein said control means programs a cycle of open events for said delivery injector, determining the start of opening and open duration for each one. 15. The method of claim 14 wherein said cycle is terminated when desired gas pressure in the gas supply system is reached. 16. The method of any one of claims 1 to 4 including at start-up of the engine: (a) determining battery operating voltage by a sensor or as a function of ambient temperature; (b) determining deliverable fuel pressure of said fuel supply system as a function of battery operating voltage; and (c) controlling the gas pressure in the gas supply system to a level less than said deliverable fuel pressure. 17. The method of any one of the preceding claims wherein the gas supply system is pressurised by a pump-up sequence including actuation of at least one delivery injector in at least one pump-up event when cylinder pressure is greater than gas supply pressure, the gas supply pressure being regulated so as not to exceed the fuel pressure during gas supply system pressurisation. 18. The method of claim 17 wherein a predetermined differential between fuel and gas supply pressure is maintained during gas supply system pressurisation by a pump-up sequence. 19. The method of claim 17 or claim 18 wherein the timing and duration of opening of said delivery injector is controlled such that the gas supply system pressure does not exceed the deliverable fuel pressure during gas supply system pressurisation by a pump-up sequence. 20. The method of claim 19 wherein delivery injector opening for pressurising the gas supply system is enabled less frequently than once per cylinder cycle. 21. The method of any one of the preceding claims including controlling the rate of increase of gas pressure as a function of sensed battery voltage. 22. The method of any one of claims 17 to 21 wherein a pump-up sequence is not invoked or is interrupted when said abnormal fuel supply condition prevails. 23. The method of any one of claims 17 to 22 wherein said pump-up sequence is delayed when said abnormal fuel supply condition prevails. 24. The method of any one of claims 17 to 23 wherein a delivery injector is opened to reduce gas supply pressure during a pump up sequence. 25. The method of any one of the preceding claims wherein said gas supply system is provided with a relief valve which is operated to control gas supply system pressure. 26. The method of claim 25 wherein timing and duration of relief valve opening is controlled to control the pressure in said gas supply system. 27. The method of any one of the preceding claims wherein the pressure of said gas supply system is reduced prior to shutdown. 28. The method of any one of the preceding claims including adaptive control to implement said method for a specific fuel pump of said engine. 29. An engine operated in accordance with the method of any one of the preceding claims. 30. The engine of claim 29 being a four-stroke engine having at least one cylinder. 31. The engine of claim 30 being a two-stroke engine having at least one cylinder. 32. An electronic control unit for the engine of any one of claims 29 to 31 being programmed to control said method of controlling gas pressure. 33. A method of operating an internal combustion engine upon starting of the engine, the engine having dual fluid fuel injection system which includes a gas supply system adapted to supply pressurised gas to said fuel injection system and a fuel supply system adapted to supply pressurised fuel to said fuel injection system, said method comprising the steps of determining the fuel supply pressure during at least start-up of the engine and regulating said gas supply pressure to be less than said fuel supply pressure. 34. A method according to claim 33 wherein the pressure of said pressurised fuel is determined by use of a pressure sensor in said fuel supply system. 35. A method according to any one of claim 33 or 34 wherein said fuel pressure is measured relative to the pressure of said pressurised gas. 36. The method according to claim 33 wherein the pressure of said pressurised fuel is estimated from at least one engine operating condition. 37. The method according to claim 36 wherein said at least one engine operating condition comprises ambient temperature, engine temperature, coolant temperature, battery voltage, or battery current at cranking. 38. The method according to any one of claim 36 or 37 wherein the pressure of said pressurised fuel is estimated from an electronic look up map stored in an electronic control unit of said engine, said look up map having been derived from characteristics of at least one of said engine, said fuel injection system, said fuel supply system or said gas supply system. 39. The method according to any one of claims 33 to 38 wherein a delivery injector of said dual fluid fuel injection system is operated with a timing to maintain the gas pressure within the gas supply system at a desired level below the fuel pressure of the fuel supply system. 40. The method according to claim 39 wherein the gas pressure in said gas supply system is controlled as a function of at least one of the duration of opening of the delivery injector, the timing of opening of the delivery injector or the timing of closing of the delivery injector. 41. The method according to claim 39 or 40 wherein actuation of said delivery injector is controlled to establish a desired pressure differential between the pressure of fuel in the fuel supply system and the pressure of gas in the gas supply system. 42. The method according to any one of claims 39 to 41 wherein said engine is a two stroke engine and said delivery injector is actuated to control the pressure of gas in said gas supply system during a portion of at least one of a latter portion of an exhaust stroke, a latter portion of an intake stroke, or an initial portion of an intake stroke. 43. The method according to any one of claims 33 to 41 wherein said engine is a four stroke engine and said delivery injector is actuated to reduce gas pressure during a portion of at least one of a latter portion of an intake stroke, an early portion of a compression stroke or an exhaust stroke. 44. The method according to any one of claims 33 to 43 wherein the gas supply system is pressurised by a pump-up sequence including actuation of a delivery injector in a number of pump-up events when cylinder pressure is greater than gas supply pressure, the gas pressure in the gas supply system being regulated so as not to exceed the pressure of fuel in the fuel supply system. 45. The method according to claim 44 wherein the gas pressure of said gas supply system is controlled so as not to exceed a predetermined differential pressure relative to the fuel pressure of said fuel supply system. 46. The method according to any one of claim 44 or 45 wherein said pump-up event comprises actuating at least one delivery injector so as to transfer gas from a cylinder into said gas supply system. 47. An electronic control unit for controlling the operation of an internal combustion engine having a dural fluid fuel injection adapted to supply fuel entrained in gas into a combustion chamber of said engine, said electronic control unit adapted to operate according to the following: determining that an abnormal fuel supply condition exists; regulating the gas pressure in the fuel injection system to a level less than the fuel pressure in the fuel injection system under said abnormal fuel supply condition such that fuel is delivered as required to said fuel injection system. 48. An electronic control unit as claimed in claim 47 wherein said dual fluid fuel injection system comprises a gas supply system, a fuel supply system and at least one delivery injector, said gas supply system communicating gas under a pressure to said delivery injector and said fuel supply system communicating fuel under pressure to said delivery injector and wherein actuation of said delivery injector delivers said fuel communicated to said delivery injector entrained in said gas communicated to said delivery injector. 49. An electronic control unit as claimed in any one of claim 47 or 48 wherein said gas pressure is regulated to a level less than expected gas pressure in the fuel injection system when the engine is operating under normal running conditions. 50. An electronic control unit as claimed in any one of claim 47, 48 or 49 further adapted to monitor said engine at start-up to determine that said abnormal fuel supply condition exists. 51. An electronic control unit as claimed in claim 50 wherein said monitoring occurs under cold ambient conditions. 52. An electronic control unit as claimed in any one of claims 47 to 51 further adapted to operate a delivery injector of said fuel injection system with a timing to maintain the gas pressure within the gas supply system at a desired level below the deliverable fuel pressure of the fuel supply system. 53. An electronic control unit as claimed in claim 52 wherein said delivery injector is actuated to regulate the pressure of the gas in said gas supply system as a function of at least one of the duration of opening of said injector, the timing of opening of said injector, or the timing of closing of said injector. 54. An electronic control unit as claimed in any one of claims 47 to 53 wherein actuation of said of said delivery injector is controlled to establish a desired pressure differential between the fuel pressure and the gas pressure. 55. An electronic control unit as claimed in any one of claims 47 to 54 further adapted to receive input from a pressure sensor measuring said fuel pressure whereby said electronic control unit determines that an abnormal fuel supply condition exists. 56. An electronic control unit according to any one of claims 47 to 55 wherein said fuel pressure is measured relative to the pressure of said pressurised gas. 57. An electronic control unit according to any one of claims 47 to 54 wherein the pressure of said pressurised fuel is estimated from at least one engine operating condition whereby said electronic control unit determines that an abnormal fuel supply condition exists. 58. An electronic control unit according to claim 57 wherein said at least one engine operating condition comprises ambient temperature, engine temperature, coolant temperature, battery voltage, or battery current at cranking. 59. An electronic control unit according to any one of claims 57 or 58 wherein the pressure of said pressurised fuel is estimated from an electronic look up map stored in said electronic control unit, said look up map having been derived from characteristics of at least one of said engine, said fuel injection system, said fuel supply system or said gas supply system. 60. An electronic control unit according to any one of claims 47 to 59 further adapted to pressurise said gas supply by a pump-up sequence including actuation of at least one delivery injector in at least one pump-up event when cylinder pressure is greater than gas supply pressure. 61. An electronic control unit according to claim 61 further adapted to control pressurisation of said gas supply by a pump-up sequence whereby gas supply pressure is regulated so as not to exceed the fuel pressure of said fuel supply system. 62. An electronic control unit as claimed in claim 60 or 61 further adapted to regulate said gas supply by a sequence of pump-up events during pressurisation of said fuel supply system after start-up of said engine. 63. An electronic control unit as claimed in any one of claims 60 to 62 further adapted to regulate said gas supply by a sequence of pump-up events whereby a predetermined differential between fuel and gas pressure is maintained. 64. An electronic control unit for controlling operating an internal combustion engine upon starting of the engine, the engine having dual fluid fuel injection system which includes a gas supply system adapted to supply pressurised gas to said fuel injection system and a fuel supply system adapted to supply pressurised fuel to said fuel injection system, said electronic control unit adapted to operate so as to determine the fuel supply pressure during at least start-up of the engine and regulating said gas supply pressure to be less than said fuel supply pressure. 65. An electronic control unit according to claim 64 wherein said electronic control unit is adapted to receive pressure signal input whereby said electronic control unit is adapted to determine said fuel supply pressure. 66. An electronic control unit according to any one of claims 64 or 65 wherein said fuel pressure is measured relative to the pressure of said pressurised gas. 67. An electronic control unit according to claim 64 further adapted to determine the pressure of said pressurised fuel from at least one engine operating condition. 68. An electronic control unit according to claim 67 wherein said at least one engine operating condition comprises ambient temperature, engine temperature, coolant temperature, battery voltage, or battery current at cranking. 69. An electronic control unit according to any one of claims 67 or 68 wherein the pressure of said pressurised fuel is estimated from an electronic look up map stored in said electronic control unit, said look up map having been derived from characteristics of at least one of said engine, said fuel injection system, said fuel supply system or said gas supply system. 70. An electronic control unit according to any one of claims 64 to 69 further adapted to actuate a delivery injector of said dual fluid fuel injection system with a timing to maintain the gas pressure within the gas supply system at a desired level below the fuel pressure of the fuel supply system. 71. An electronic control unit according to claim 70 further adapted to actuate the delivery injector as a function of at least one of the duration of opening of the delivery injector, the timing of opening of the delivery injector or the timing of closing of the delivery injector so as to regulate the pressure of gas in the gas supply system. 72. An electronic control unit according to claim 70 or 71 wherein actuation of said delivery injector is controlled to establish a desired pressure differential between the pressure of fuel in the fuel supply system and the pressure of gas in the gas supply system. 73. An electronic control unit according to any one of claims 64 to 72 wherein said engine is a two stroke engine and said electronic control unit is further adapted to actuate said delivery injector so as to control the pressure of gas in said gas supply system during at least one of a latter portion of an exhaust stroke, a latter portion of an intake stroke, or an initial portion of an intake stroke. 74. An electronic control unit according to any one of claims 64 to 72 wherein said engine is a four stroke engine and said electronic control unit is further adapted to actuate said delivery injector so as to reduce gas pressure during at least one of a latter portion of an intake stroke, an early portion of a compression stroke or an exhaust stroke. 75. An electronic control unit according to any one of claims 64 to 74 further adapted to actuate at least one delivery injector whereby the pressure of the gas in the gas supply system is regulated by at least one pump-up event when cylinder pressure is greater than gas supply pressure such that the gas pressure in the gas supply system does not to exceed the pressure of fuel in the fuel supply system. 76. An electronic control unit according to claim 75 further adapted to control the gas pressure of said gas supply system so as not to exceed a predetermined differential pressure relative to the fuel pressure of said fuel supply system. 77. An electronic control unit according to claim 75 or 76 wherein said pump- up event comprises actuating at least one delivery injector so as to transfer gas from a cylinder into said gas supply system.

Inventors: ARCHER MARK DOUGLAS, FRAHM JEFFRY ROGER, ARCHER, MARK, DOUGLAS, FRAHM, JEFFRY, ROGER
Applicants: ARCHER MARK DOUGLAS, FRAHM JEFFRY ROGER, ORBITAL ENG PTY, ARCHER, MARK, DOUGLAS, FRAHM, JEFFRY, ROGER, ORBITAL ENGINE COMPANY (AUSTRALIA) PTY LIMITED
Application year: 2000
Main classification: F02M67
IPC codes: F02B13, F02B23, F02B43, F02D41, F02F1, F02M67