Abstract:
A flow control system for use in a compressible fluid-fueled internal combustion engine includes a fluid metering device comprising a plurality of fluid lines for communication between an inlet fluid manifold and a metered fluid manifold. Bistable valves of various orifice sizes are provided, one in each of the fluid lines except for at least two lines, which are provided with pulsing valves. The device also includes sensors for determining parameters of the inlet fluid sufficient to determine potential mass flow of fluid through each of the valves, and valve controls for activating valve actuators to open the valves in combinations and for time intervals to give a desired total mass flow of fluid through the valves. The total fluid flow passing from the metering device comprises a base flow component established by opening selected bi-stable control valves to provide a continuous predetermined flow, discrete step changes in the base flow component being provided by opening and closing combinations of bi-stable control valves. Increments of fluid flow between the discrete steps are provided by opening at least one pulsing valve for a fraction of a predetermined time period to provide a desired average flow over the time period.

Description:
FIELD OF THE INVENTION 
     This invention relates to the control of the flow rate of a fluid, and in particular to the control of the supply of compressible fluid fuels for internal combustion engines. 
     BACKGROUND OF THE INVENTION 
     Systems for converting conventional gasoline and diesel fuelled internal combustion engines to run on compressible fluid fuels, such as natural gas and propane, have been available for some time. Under current fiscal policy, there is generally a considerable retail price advantage to using propane or natural gas as a fuel, such that, for example, taxi operators and police forces can rapidly recoup the extra cost of providing a vehicle with the capability to run on propane or natural gas. Even without advantageous tax structuring the cost of a natural gas is generally lower than that of gasoline and diesel fuels, natural gas requiring relatively minor processing before it is in a saleable form, whereas conventional liquid gasoline fuels are often produced by &#34;cracking&#34; and processing of other longer chain hydrocarbons. Also, spark ignition engines running on natural gas do not suffer from compression ignition or &#34;knocking&#34; (except at very high compression ratios and intake air temperatures on large bore engines), and thus, the need for the provision of natural gas with different characteristics, similar to octane rated gasoline, is obviated. This also removes the requirement to provide knock resisting additives, such as the lead based additives used in some gasoline. Further, the major component of natural gas is methane, a &#34;clean&#34; fuel, which produces substantially less carbon dioxide on burning than does conventional gasoline or diesel fuel. 
     Despite these advantages the use of natural gas has only met with limited acceptance. This may be linked, in part at least, to a number of areas where natural gas powered vehicles compare unfavourably with conventional gasoline fuelled vehicles. The technology for utilizing natural gas fuel in this area is not as well developed as that used in conventional gasoline fueling systems and existing products are generally relatively expensive to produce, install and service. Also, the conversion of a gasoline engine to run on natural gas normally results in a decrease in power output and a corresponding drop in vehicle performance. 
     A typical gaseous fuel injection system includes a pressurized fuel storage tank, a pressure regulator for reducing the fuel from the relatively high storage pressure to a lower working pressure, a metering valve for controlling the gas supply to the engine and a gas/air mixer at the engine air intake. Some form of engine management system is also provided to control the metering valve and ensure proper engine operation. 
     Pressure regulators in existing gaseous fuel injection systems tend to be bulky and thus are difficult to locate in the often restricted space of a vehicle engine compartment. Accordingly, these are often only suitable for use on engines with spacious engine compartments. The bulk of existing regulators is due, in part at least, to the number of components which are present in a regulator: a fuel filter between the fuel line from the fuel storage tank; at least two regulating valve stages for reducing the pressure of the fuel as it passes through the regulator; a relief valve which opens in the event of a failure of the regulator valve to prevent high pressure fuel passing unchecked through the regulator, and a heater to warm the regulator and compensate for the cooling effect of expanding the fluid at the regulating valve. 
     Regulator fuel filters must be of rugged construction, since if they become blocked they may have to withstand high pressures (up to 4000 p.s.i. for natural gas), and a failure of the filter may result in considerable damage to the regulator and other components downstream of the valve. 
     Existing regulator valves permit relatively low flow rates, are prone to blocking and, in some cases, two or three stage regulators must be provided to accomplish a desired pressure drop and stability. Further, increasing the flow through the regulating valve tends to lead to pressure &#34;droop&#34;, that is the pressure drop at the valve at high flow rates is proportionally greater than the pressure drop at lower flows, leading to difficulties in calibration. 
     Tests on relief valves provided in existing fuel injection systems indicate that the valves are not particularly reliable and often will not open at the intended pressure. Further, some doubt has been expressed as to the ability of existing valves to accommodate the flow rates experienced on failure of the regulator valve: if a relief valve on a regulator should fail, the regulator may be destroyed, in explosive fashion, by the build-up of internal fuel pressure. 
     Heating of regulators is normally accomplished using the engine cooling fluid, and is particularly important in natural gas fuelled vehicles, where the drop in pressure and temperature produced by the regulator valve may result in the appearance of hydrates, a lattice of methane and water present in the fuel, which has the appearance of &#34;spongy&#34; ice and which will block most regulator valves. 
     The control of the flow of the fuel between the regulator and the engine is controlled by the metering valve which may take the form of a plurality of injectors which are operated to provide a desired fuel flow. The volume of gaseous fuel required for fuelling an engine normally necessitates provision of a number of injectors which must be capable of supplying fuel at the desired rates for idling up to maximum power, which may be a 1:40 range. 
     Conventional injection systems commonly utilize &#34;multi-point&#34; fuel injection systems in which at least one solenoid operated injector valve is provided for each engine cylinder. Sensors measure various engine operating parameters and an engine control system equates the inputs from the sensors to a desired fuel supply which is metered by, for example, operating the injectors for timed intervals, or varying the pressure of the fuel supplied to the injectors. 
     In &#34;single-point&#34; fuel injection a metered mass of fuel is supplied to a common inlet manifold. A form of such fuel injection for use with compressible fuels such as methane and propane is disclosed in U.S. Pat. No. 4,487,187 to Petro, entitled Electronically Controlled Fluid Flow Regulating System. The system is provided with a metering valve including a plurality of parallel lines, each of which contains a solenoid valve, operated in response to digital signals produced by an electronic digital processor. The valve orifice sizes, and the relative flow rates through the valves, are proportional to successive powers of two, and the fuel pressure differential in the system is maintained such that critical or choked flow is maintained through the orifices. 
     In common with other binary valves of this form, such as the valve described in U.S. Pat. No. RE 29,383 to Gallatin et al.,m the flow through the valve increases/decreases in small incremental steps, the relative size of the steps between minimum and maximum flow through the valve decreasing as the number of valves is increased. However, increasing the number of valves increases the bulk and expense of the metering device. 
     Further, binary systems such as those disclosed in the Petro and Gallatin et al. patents have poorest flow resolution at low flowrates, which tend to be the most critical for engine operation. Vehicle engines must operate over a wide dynamic range of fuel flows, typically around 35:1. At any point in the fuel delivery range, a fuelling system should be capable of adjusting the fuel flow by 0.25%. In, for example, a 12-bit binary valve for providing a flowrate of 4096 Standard Cubic Feet per Hour (SCFH), that is a binary valve having twelve valves which increase in flow capacities following a binary sequence, at the lowest point of the dynamic range, the minimum increment to the next flow point represents 0.85% of that flow. Thus, such systems would not be capable of controlling the fuel flow within the desired range (0.25%), and an engine equipped with this form of binary metering valve would operate inefficiently and have difficulty in conforming with proposed emission regulations which require precise control of the fuel supply. 
     SUMMARY OF THE INVENTION 
     According to a first aspect of the present invention there is provided apparatus for controlling the rate of flow of a fluid from a common inlet to a common outlet. The apparatus comprises a plurality of fluid conducting lines having a plurality of flow capacities, the lines being connected in parallel between the common inlet and the common outlet such that the total flow from the common inlet to the common outlet is equal to the sum of the separate flows through the individual lines. The apparatus further includes a pulsing fluid control valve in each of at least two of said lines, and a bi-stable fluid control valve in each of the remaining lines, sensors for measuring parameters of the fluid sufficient to determine the potential mass flow through each control valve when open and control means adapted to control selectively the opening and closing of the control valves and monitor the sensors to determine the potential mass flow through each control valve in accordance with the measured fluid parameters. The total fluid flow passing outwardly of the common outlet comprising a base flow component established by opening selected control valves to provide a continuous predetermined flow, discrete step changes in the base flow component being provided by opening and closing combinations of control valves, increments of fluid flow between the discrete steps being provided by opening at least one pulsing valve for a fraction of a predetermined time period to provide a desired average flow over the time period. 
     According to a second aspect of the present invention there is provided a method for controlling the rate of flow of a compressible fluid between a common inlet at a first pressure and a common outlet at a lower second pressure through the plurality of parallel fluid conducting lines adapted to provide a plurality of flowrates therethrough, each fluid conducting line being provided with a fluid flow control valve, at least two of the valves being pulsing valves, the remaining valves being bi-stable valves, The method comprises controlling selectively the opening and closing of the control valves to control the fluid flow passing outwardly of the common inlet by providing a base flow component established by opening selected bi-stable control valves to provide a continuous predetermined flow, discrete step changes in the base flow component being provided by opening and closing combinations of control valves, increments of fluid flow between the discrete steps being provided by opening at least one pulsing valve for a fraction of a predetermined time period to provide a desired average flow over the predetermined time period. 
     According to a third aspect of the present invention there is provided a fluid metering device for use in a compressible fuel injection system for an internal combustion engine comprising a body defining an inlet fluid manifold and a metered fluid manifold, and a fluid inlet for passage of fluid into the inlet fluid manifold. The device includes a plurality of valves for controlling fluid communication between the inlet fluid manifold and the metered fluid manifold, the body further defining a fluid outlet for passage of metered fluid from the metered fluid manifold. Sensors are provided for determining parameters of the inlet fluid and the metered fluid sufficient to determine actual mass flow of fluid through each of the valves. A device operating system includes valve control means for activating the valve actuators to open the valves in combinations and for time intervals to give a desired total mass flow of fluid through the valves. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other aspects of the present invention will now be described, by way of example, with reference to the accompanying drawings, in which: 
     FIG. 1 is a block diagram of a vehicle engine provided with a fuel supply system in accordance with a preferred embodiment of the present invention; 
     FIG. 2 is a block diagram of a fuel pressure regulator of the fuel supply of FIG. 1; 
     FIG. 3a is a sectional view of a fuel pressure regulator of the fuel supply system of FIG. 1, taken on line 3a-3a of FIG. 3b; 
     FIG. 3b is a bottom plan view of the fuel pressure regulator; 
     FIG. 3c is an end elevational view of the fuel pressure regulator; 
     FIG. 3d is a sectional view on line 3d-3d of FIG. 3b and illustrating details of a fuel inlet; 
     FIG. 3e is a sectional view on line 3e-3e of FIG. 3b and illustrating details of a relief valve assembly; 
     FIG. 4 is a block diagram of a fuel metering valve of the fuel supply system of FIG. 1; 
     FIG. 5a is a top plan view of the fuel metering valve of the fuel supply system of FIG. 1; 
     FIG. 5b is a side elevational view of the fuel metering valve; 
     FIG. 5c is an end elevational view of the fuel metering valve; 
     FIG. 5d is a pictorial view showing the top side of the metering valve electronic control board; 
     FIG. 6 is a sectional view on line 6-6 of FIG. 5a; 
     FIG. 7a is a top plan view of the valve block of the fuel metering valve showing the hidden detail of the inlet manifolding; 
     FIG. 7b is a top plan view of the valve block of the fuel metering valve showing the hidden detail of the outlet manifolding; 
     FIG. 7c is a top plan view of the valve block of the fuel metering valve showing the hidden detail of the coolant manifolding; 
     FIG. 7d is a sectional view on line 7d-7d of FIG. 7a. 
     FIG. 8 is a sectional view of an engine air intake showing one example of an air/fuel mixer for use with the fuel supply system of FIG. 1; 
     FIG. 9 is a sectional view of an engine air intake showing a further example of an air/fuel mixer, in the form of a nozzle, for use with the fuel supply system of FIG. 1; and 
     FIG. 10 is a block view of the controller of the supply system of FIG. 1. 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     Reference is first made to FIG. 1 of the drawings, which is a block diagram of the system of a preferred embodiment of the present invention. In this system, the fuel is stored under pressure in a fuel storage tank 10, the pressure varying with the type of fuel, for example natural gas being typically stored at around 3000 psi and propane at less than 312 psi, the amount of fuel in the tank 10 and the temperature of the fuel. 
     From the storage tank 10, a fuel line 12 leads to a fuel pressure regulator 14, where the pressure of the stored fuel is reduced to a pressure suitable for metering, and also to ensure that the fuel is in the gas phase for metering. From the regulator 14, the fuel passes through a metering device or valve 16, which supplies fuel to an air/fuel mixer 18, typically downstream of the engine air intake 17, where the fuel mixes with intake air and is drawn into the engine 19. 
     The metering valve 16 supplies fuel in accordance with the fuel demands of the engine 19, which are determined from various sensors 20 for detecting engine operating parameters, and various further sensors 22 which detect parameters of the fuel and operating parameters of the metering valve 16. The readings obtained by the sensors 20, 22 are fed into a system controlling computer 24, which is programmed to operate the metering valve 16 in accordance with the engine fuel requirements, as determined by manipulation of the sensor readings. 
     The fuel storage tank 10, and the fuel lines 12 are conventional, and as mentioned above, supply fuel to the pressure regulator 14, which will now be described with reference to FIGS. 2 and 3 of the drawings. Reference is first made to FIG. 2, which is a block diagram indicating the main components of the regulator 14, and the path of fuel therethrough (indicated by arrows 15). The fuel line 12 is attached to the regulator by means of a conventional inlet fitting. The fuel is then passed through a filter 26 to remove any particulate matter from the fuel. Downstream of the filter 26 is a high pressure shut-off solenoid 28, which is used to shut off the fuel supply when the engine is turned off. 
     The pressure of the fuel is a function of the amount of fuel in the fuel storage tank 10, and also of the fuel temperature. Thus, by measuring the fuel temperature and pressure, the amount of fuel in the tank can be determined. A high pressure sensor 30 is provided in the regulator for this purpose, downstream of the shut-off solenoid 28. A signal from the sensor 30 is supplied to the system computer 24 which is programmed to combine the pressure reading with the reading from a temperature sensor (not shown) to provide a fuel level indication. 
     Downstream of the solenoid 28, the high pressure fuel passes through an adjustable pressure regulator valve 32 which is adjusted to lower the pressure of the fuel to a desired level, for example, natural gas stored at 3000 psi may be reduced to pressure of 100 psi. 
     The sudden expansion of the fuel results in a corresponding drop in fuel temperature. Particularly when using gaseous fuel, it is desirable to maintain the fuel temperature fairly close to ambient to maintain its state and maximize its energy density: with propane, a sufficiently low temperature may result in the propane returning to the liquid phase; and with natural gas less dramatic drops in temperature may result in the formation of &#34;hydrates&#34;, a spongy, ice-like material formed by water present in the fuel freezing and forming a lattice with fuel molecules which will obstruct, and block the flow of fuel. This cooling is overcome by heating the block, in this example by passing engine cooling fluid through a passage 34 in the regulator block. 
     In the event of a failure of the regulator valve 32, possibly combined with the failure of the solenoid 28, the high pressure of, for example, stored natural gas could result in the explosive destruction of the regulator. In order to minimize this possibility, a pressure relief valve 36 is provided downstream of the regulator 32, to allow for the release of pressure above a predetermined limit, before damage to the regulator 14 and metering valve 16 occur. 
     Reference is now made in particular to FIGS. 3a-3e of the drawings which illustrate the arrangement of the various components of the pressure regulator 14, mounted on a regulator block or body 38. As the regulator assembly must be accommodated in the restricted space of a vehicle engine compartment, the assembly is compact and is arranged such that the various connections between the assembly and other engine components can be made conveniently and in a space-efficient manner. 
     For ease of description and understanding, the regulator assembly will be described with reference to the particular orientation f the regulator seen in FIG. 3a, which illustrates the regulator body 38, a regulator valve assembly 32 mounted on an upper portion of the body, a high pressure solenoid 28 mounted on the side of the body, and a high pressure sensor 30 and relief valve assembly 36 mounted on a lower portion of the body. 
     An inlet fitting 42 (FIG. 3d) is provided for connection to the fuel line 12 leading from the fuel storage tank 10. The fitting 42 is attached to the main body at the mouth of an inlet passage 44 and is located in a stepped cylindrical passage 45 which accommodates a cylindrical section sintered metal filter 26, preferably fabricated of steel or brass. The filter 26 is relatively long (typically 2 cm), and has a correspondingly large surface area, such that the pressure drop over the filter 26 is minimal. Further, in the unlikely event of the filter 26 becoming blocked, the filter 26 is strong enough to withstand the pressure exerted by the fuel and will not collapse and damage the interior of the regulator. The filter 26 is held in place by a retaining spring 46 one end of which abuts a surface of the inlet fitting 42, and seats on an annular gasket 48 located around the mouth of the passage 44. 
     Directly downstream of the inlet passage 44 is the high-pressure solenoid 28 (FIG. 3a), which, when de-energised, isolates the regulator 14 from the fuel storage tank 10. The passage 44 intersects with a first inlet chamber 51 for communication with an inlet orifice 53 leading to a second main inlet chamber. The solenoid coil 50 will normally be activated on turning on the ignition of the vehicle to allow passage of fuel between the chamber 51 and the orifice 53, and due to the relatively high pressure of the fuel supply, a solenoid having magnetically latched pilot and primary pistons 52, 54 is provided, the operation of which will be described herein below. 
     The solenoid coil 50 is contained within a suitable casing 56, mounted on the body by means of a solenoid tube 58. The coil 50 is of course hollow and accommodates a smooth-walled, cylindrical portion 60 of the tube 58. A threaded, first end 62 of the tube 58 extends from a centrally located aperture 64 in the casing 56, and receives a complementary nut 66 which secures the casing 56 and coil 50 on the tube 58. The other end 68 of the tube is also threaded, but is of greater cross-section than the first end 62 and is adapted to engage a threaded bore 70 in the regulator body 38. 
     The tube 58 is solid over approximately half of the length from the first end 62. The remainder of the tube 58 is hollow and accommodates the pistons 52, 54. The primary piston 54 extends beyond the other end of the tube 58. When the solenoid is deactivated the inlet orifice 53 is closed, the primary piston 54 bearing against the surface of an annular raised portion 74 which surrounds the orifice 53. The proximal end of the piston is provided with an annular recess 78 to accommodate a resilient annular pad 80 to form a seal with the raised portion 74. A central bore 81 extends through the piston 54 and the pad 80 to the distal end of the primary piston 54, adjacent the pilot piston 52. The distal end of the primary piston 54 is provided with a raised nipple 84 with a relatively small diameter throughbore 90 (typically 0.016&#34; dia.). 
     The piston 54 is substantially cylindrical and is a fairly close fit in the tube. To provide for fluid communication between the first inlet chamber 51 and the gap 88 between the ends of the pistons, the proximal end of the primary piston is tapered, and four longitudinal slots 92 are provided in the outer surface of the piston, the purpose of the slots being described hereinbelow. 
     In the closed configuration, the end of the pilot piston 52 bears against the nipple 84 of the primary piston 54 to close the throughbore 90. The end of the piston 52 is provided with a circular recess 94 to accommodate a resilient cylindrical pad 96 which bears against the nipple 84. The distal end of the pilot piston features a frusto-conical portion 102, the end of this portion including an annular bearing surface 104 and a cylindrical stub 106 for mounting a coil spring 108, the opposite end of which bears against the end wall 110 of the tube. The spring 108 tends to push the pistons 52, 54 to close the inlet orifice 53. The spring 108 is located in a cylindrical recess 112 in the tube, and the end of tube 58 is provided with a 60° conical taper at 114, corresponding to the taper on the end of the pilot piston 52, the tapering ends of the piston and tube 58 allowing for a longer pull of the piston. 
     When the coil 50 is energised, the pilot piston 52 is pulled towards the end wall 110, against the spring 108. Initially, the primary piston 54 remains in place, as the pressure force acting on the distal end of the piston 54 is greater than the pressure force exerted by the gas in the main inlet chamber, exerted over the area of the orifice 53, and the pull on the piston 54 from the energised solenoid 50. When the pilot piston 52 is retracted, the end of the throughbore 90 is uncovered, and the high pressure gas from the inlet passes from the inlet chamber 51, through the slots 92 to the distal end of the piston 54, into the bore 80 and the orifice 53. After a short time, the pressure in the main inlet chamber rises to a level whereby the net pressure forces on the piston 54 are negligible, and the pull on the piston from the energised solenoid is sufficient to retract the piston 54 allowing gas to flow past the proximal end of the piston directly from the inlet and through the orifice 53. 
     The orifice 53 extends into a main inlet chamber, which is in the form of a chamber 116 formed in a central bore in the body. The pressure sensor 30 forms one end wall of the chamber 116, and has a threaded coupling portion 120 for engaging a complementary threaded portion 122 of the bore. The opposite end of the bore is provided with a metering orifice 127 which accommodates the metering pintle 126 of the adjustable regulating valve 32. The metering orifice leads into an outlet chamber 160. 
     The valve assembly 32 is mounted on an upper portion of the regulator body 38 and is contained within a spring tower or casing 128 in the form of a zinc casting with a threaded lower portion 130 for engaging a complementary threaded portion 132 on the regulator body, and having a breather hole 133 above the lower threaded portion 130. The upper end of the casing 128 accommodates a compression spring 134, the upper end of the spring bearing against a spring end plate 136, the position of the plate being controlled by adjusting a recessed adjusting screw 138 located in a threaded bore 140 in the upper end of the casing 128. 
     The metering pintle 126 includes an enlarged cylindrical head with a conical base, a resilient pintle seal 142 being positioned between the head and the metering orifice 127. An elongate portion 143 of the pintle 126 extends through the orifice 127 and engages a pintle retainer 144 which retains the metering pintle in the orifice 127. 
     The pintle retainer 144 abuts a diaphragm end cap 155 which is mounted to a spring plate 158. The lower end of the compression spring 134 bears on the spring plate 158, through a cup-shaped spring damper 156, and through the spring plate 158 and a diaphragm backing washer 152 on a diaphragm 150 which is mounted by means of a downturned edge in an annular groove 151 on the upper end of the body 38. The diaphragm is retained in position by the clamping action of the casing on the body 38. On the other side of the diaphragm 150 is a damper spring 146 which extends between the return spring washer 148 abutting the lower face of the diaphragm 150 and the regulator body 38. 
     Positioned between the sides of the pintle retainer 144 and the diaphragm end cap 155 is a bushing portion of a diaphragm stop 154, a flange portion extending from the bushing portion between the return spring washer 148 and the body 38, thus limiting the downward movement of the diaphragm 150. The flange portion opposes a raised cruciform surround above the metering orifice 127. 
     To adjust the regulator, the position of the spring end plate 136 is varied, which changes the downward spring force on the diaphragm 150. The spring force is balanced with the pressure of the regulated fuel in the chamber 160 acting on the other side of the diaphragm 150. When the pressure in the chamber 160 drops, the spring force will thus lift the pintle from the sealing position at the orifice 127 and allow fuel to flow past the pintle 126, the pressure of the fuel being reduced as the fuel is throttled on passing the pintle. 
     The lower pressure gas flows into the annular chamber 160, which is formed in a channel in the upper end of the regulator body., the channel being deepened over an arc-shaped portion 162, one end of which communicates with an outlet fitting 166 (see FIG. 3b). The fitting 166 is located in a bore which intersects one end of the arc-shaped portion 162. 
     The relief valve 36 (FIG. 3c) is located in a cylindrical bore 174 in the base of the body, which intersects a horizontal relief bore 177, the bore 174 extending into a smaller diameter bore extension 176, which intersects the other end of the arc-shaped portion 162. The relief valve normally seals the end of the bore extension 176, and when opened by a higher than normal pressure in the chamber 160 allows gas to flow from the regulator through one of two alternate outlets. In this example an outlet 179 at one end of the relief bore 177 is plugged, while an upright outlet fitting 181 is provided in a bore 183 which intersects the other end of the relief bore 177. 
     The relief valve 36 is shown in more detail in FIG. 3e, being illustrated in the open configuration, and comprises a valve housing 180, a relief valve piston 182 normally held to close the valve opening 188 at the base of the bore 176, and a compression spring 184 which acts between the piston 182 and the housing 180. The end 186 of the piston 182 that closes the relief valve opening 188 includes a relief valve seat 190, held in place by a plated washer 192 and plated machine screw 194. 
     The end of the spring 184 not acting on the piston is mounted on a spring end positioner 198, the location of which may be adjusted by an adjusting screw 200, accessible from the exterior of the regulator. 
     The end of the housing 180 includes a vent opening 202 provided with a vent filter 204. O-ring seals 206, 208 are provided between the housing 180 and body 38 and the relief valve piston and housing. 
     With the valve in the open configuration, as illustrated, fuel may flow from the chamber 160, into the bore 176, past the end of the piston 182 and into the relief bore 177. From the relief bore the fuel flows thorough the outlet fitting 181 (FIG. 3c), which may be provided with a hose to carry the fuel away from the engine. 
     In the event of a regulator valve failure, the flow capabilities of the relief valve and the associated outlets do not permit the build-up of dangerously high pressures in the regulator. This should prevent the destruction of the regulator and subsequent damage to the vehicle. 
     As was mentioned above, the expansion of the fuel in the regulator results in cooling of the fuel. It is desirable to minimise this net cooling, to prevent the temperature of the fuel falling below the critical temperature and returning to the liquid phase (propane only). Also hydrates may form in the cooled fuel, resulting in restriction and blocking of the regulator flow path. While these hydrate problems are primarily associated with fuels such as natural gas, rather than propane, it is also generally preferable to maintain the regulator at a sufficient temperature to prevent the build-up of ice on the exterior of the body. 
     The heating of the regulator is achieved by use of the engine coolant, typically water and &#34;anti-freeze&#34;, which is passed through the body 38. The heating fluid inlet 210 is located on a lower portion of the body (see FIG. 3b). An alternate inlet is provided 211 on the side of the body, the inlet not in use being plugged (in this example, inlet 211 is plugged). The heating fluid passage 34 extends in a U-shaped path through the body to an outlet 212 adjacent the inlet 210. The passage 34 is adjacent the fuel inlet 44, which is located in the &#34;deepest&#34; portion of the regulator, and thus the routing of the passage through this portion provides a long heat path and thus is more effective in heating the regulator. 
     From FIG. 3b is will also be noted that one side of the body is provided with a planar mounting surface 214 provided with threaded bores 216 to receive bolts extending from some suitable mounting structure. It will also be noted that the mounting side of the regulator is free of inlets and outlets which might make mounting more difficult. 
     From the regulator 32, the fuel, now at a suitable pressure for metering, passes through conduits to the metering valve 16, illustrated in block diagram form in FIG. 4 of the drawings, and further, in FIGS. 5-7 of the drawings, reference being made first to FIG. 4. The regulated fuel passes from the regulator through an inlet 220 provided with a filter 221 into an inlet manifold 222. The manifold communicates with, in this example, eight bi-stable fuel injectors and leads to an outlet 233 linked to the air/fuel mixer. Various sensors and control circuitry is illustrated in FIG. 4 and will be described hereinbelow following a physical description of the metering valve and its various elements. 
     For each of description, FIG. 5a is designated a top plan view, and FIGS. 5b and 5c represent front and side elevations of the metering valve. Further, the term longitudinal, when used herein with reference to the valve body, indicates perpendicular to the section line 6-6 of FIG. 5A, while the term transverse indicates a direction parallel to the section line. 
     As mentioned above with reference to FIG. 4, the fuel passes through the inlet 220 provided with a filter into a main inlet manifold and communicates with eight bi-stable fuel flow control valves on 224-231 with orifices of various sizes. Six &#34;high-flow&#34; injectors 224-229 are mounted in the same plane, and two &#34;low-flow&#34; or pulsing injectors 230, 231 are mounted parallel to one another perpendicular to the other injectors, between mounting posts 300, 302. A common outlet manifold 232 communicates with the outlets of the injectors and leads to a valve outlet 233. 
     The inlet manifolding is arranged such that all injector orifices operate at substantially the same stagnation pressure, regardless of which combination of orifices are flowing. Further, the outlet manifolding is arranged such that all orifices operate at a back pressure low enough to assure that each injector orifice remains sonic, regardless of which combination of orifices are flowing. In this description, the term sonic is used to indicate choked flow in the orifice, or that the pressure differential across the orifice is such that variations in downstream pressure do not affect the flow rate through the orifice. The manifolding will be described with reference primarily to FIGS. 7a-7d of the drawings, the description of the inlet manifolding being followed by a description of the injector configuration, as shown in FIG. 6 of the drawings and, subsequently, by a description of the outlet manifolding. 
     FIGS. 7a-7b illustrate the metering valve block 17, and each one of the Figures illustrate a different aspect of the valve body manifolding. Reference is first made to FIG. 7a, which illustrates the inlet manifolding. 
     The inlet 220 is located in an end face of the valve body. An inlet fitting 304 (FIG. 5a) is provided and comprises a hose connector portion 306, for fitting within the connecting hose (not shown) leading downstream from the regulator. The lower portion of the fitting engages within a collar 308 that extends from the valve body and accommodates a filter (not shown). Extending from the collar 308 and into the valve body is a cylindrical longitudinal bore 310, which intersects two parallel longitudinal bores 312, 314 extending from the other end of the valve body. These bores 312, 314 intersect the upright bores provided for accommodating the six high-flow injectors 224-229. A transverse bore 316 intersects the inlet bore 310 to supply fuel to the low-flow injectors 230, 231, and is in communication with an upright inlet bore in communication with injector inlets at the top end of the injectors 230, 231, in the mounting post 300. 
     For the low-flow injectors it is also possible to extend the inlet bore to provide traps for the collection of impurities in the fuel, this being particularly useful for use in propane metering, propane often containing an oily/powdery residue that might otherwise block the orifices. 
     As is evident from the Figures, and the inlet manifolding configuration, the gas feed location differs between the low-flow and high-flow injectors, that is the low injectors utilize top feed whereas the high-flow injectors utilize bottom feed. FIG. 6 is an enlarged sectional view through a portion of the metering valve and illustrate details of two high-flow injectors (mountings for the low-flow injectors not shown). The injector 224 illustrated in the right hand side of FIG. 6 is shown closed, the injector 225 shown on the left hand side of the Figure is shown in the open position, providing a flow path 234 for fluid from the inlet manifold 222 or bore 314 through the injector orifice 236, to the outlet manifold 233. The orifice 236 is formed in a non-magnetic insert 238 which is fitted into a larger opening 240 in the body of the metering valve or alternatively as a permanent part of a unit injector. However, providing a separate insert allows each injector to be constructed from standard parts, differing only in the insert, or more particularly in the orifice size in the insert. 
     The insert 238 includes a substantially cylindrical upper portion 242, having a flange 243 formed at the base thereof, of slightly greater diameter than the opening and which engages an upper edge of the opening. The orifice is located in the centre of the upper portion, surrounded by a raised portion 244 which extends beyond the planar upper surface of the portion 242. 
     Directly below the flange 243 is a portion 246 of slightly smaller diameter than the opening, and below this portion is a portion 248 of diameter corresponding to the opening 240, which creates an annular space 250 between the insert and the wall of the opening. A resilient o-ring 252 is located in the space 250. 
     The orifice extends through the insert and is preferably of the converging diverging type, having a short cylindrical portion 258 and a frusto-conical portion 260 which opens into the outlet manifold. With an orifice of this form, the pressure differential necessary to provide sonic flow through the orifice may be as low as 15%, whereas for a sharp-edged orifice the pressure differential may have to be as high as 53%. 
     To provide an immediately available reservoir of fuel when the injector opens, the upper portion 242 of the insert is located in a chamber 262 formed in the valve body, which provides a space surrounding the orifice opening and which one of the bores 312, 314 intersects. The upper portion of each chamber accommodates a lower portion of a magnetic injector plunger 264, which is movable to open and close the orifice. The lower end face 266 of the plunger is provided with a resilient washer seal 268 to form a seal with the raised portion 244 of the insert when the injector is closed. Alternatively, metal to metal sealing may be used. A non-magnetic injector core tube 284 slidably receives the plunger, the tube including a seating portion 286 which sits in an enlarged portion 288 of the chamber 262, the seating portion 286 itself forming an upper wall of the fuel receiving part of the chamber 262. The seating portion 286 is substantially annular and includes a circumferential groove 290 having a sloping base, to provide a space between the seating portion 286 and the valve body for accommodating a resilient o-ring 292. A compression spring 285 is mounted on the plunger 264 and seats against the core tube creating a force which causes the plunger to seal against the orifice. 
     The upper portion of the core tube fits within a magnetic coil case 294 adapted to accommodate the injector coil. The case is mounted on an upper surface of the core tube seating portion and the valve body. 
     As mentioned above, the low-flow injectors in this example of a metering valve utilize top flow and are, in fact, conventional gasoline injectors, such as a Lucas type FJ12. 
     Below the high-flow injectors are longitudinal bores 320, 321 for receiving the gas after it has passed through the injectors. Three of the injectors communicate with a main outlet bore 320 (see FIG. 7b) while the other three communicate with a smaller diameter bore 321 that is liked to the main outlet bore 320 by a transverse bore 322. The transverse bore 322 also intersects the outlets of the low-flow injectors in the post 302. The main outlet bore 320 extends to the end of the valve body and accommodates an outlet fitting 334, which includes a hose connector 326 (see FIG. 5c). 
     For ease of construction, the valve body is formed by drilling a solid block. The various bores are formed by drilling the block, the open ends of the bores being sealed by plugs. 
     The particular valve illustrated is intended for use in metering both natural gas and propane, though for a typical propane application it is likely that the valve would be provided with a greater number of injectors and the injectors would be provided with orifice inserts having larger area orifices (the propane is at lower pressure and, therefore, has a greater volume). In both natural gas and propane applications it is desired to keep the fuel from cooling to the degree that hydrates will form in the fuel or that the fuel will return to the liquid state, as discussed above. To minimize this possibility, the valve body is provided with a passage 450 (FIG. 7c) for conveying a heating fluid, typically the engine cooling fluid, in a manner similar to the regulator. The passage extends around the six high-flow injector bores. The passage begins at an inlet 452 (FIG. 5c) provided with a suitable inlet fitting which is received by a short, cylindrical longitudinal bore which intersects an upright bore 454 which leads to a solenoid chamber 456. A solenoid 458 (FIG. 5c) is mounted on the end of the valve block and is operated to control the flow of fluid through the passageway, which is formed by three intersecting bores 460, 462, 464. 
     The lower portion of the block is provided with a recess 466 which accommodates various sensors and the power transistor circuitry necessary to operate the injectors. Three pressure sensors are provided in the valve body: an outlet pressure sensor 340 (FIGS. 5d and 7b) which measures the pressure of the fuel downstream of the injectors by means of a plugged transverse bore 341 which intersects the main outlet bore 320 and which communicates with the sensor 340, mounted through an upright bore 342 which extends from the recess; an inlet or regulated pressure sensor 334 (FIGS. 5d, 7a and 7b) located in an upright bore 338 which communicates with a transverse bore 336, which intersects an inlet bore 312 (FIG. 7d); and an intake manifold absolute pressure (MAP) sensor 343 which measures the pressure in the engine intake manifold via a vacuum hose. By recording the pressure sensed by the outlet pressure sensor before starting the engine it is also possible to obtain a reading for barometric pressure. A regulated fuel temperature sensor may also be provided (FIG. 4). 
     As is seen most clearly in FIG. 7d, the inlet pressure sensor transverse bore 336 just &#34;nicks&#34; the inlet bore 312 and the communicating opening between bores is located in an upper wall of the bore 312. This is intended to minimize the ingress of any dirt or impurities in the fuel into the pressure sensor manifolding. The outlet pressure sensor manifolding is similarly arranged. 
     A temperature sensor may also be provided for measuring the temperature of the fuel in the outlet bore from the low-flow injectors. 
     The sensors are all connected to a metering valve electronic control board 344 (FIG. 6) carrying the power transistor circuitry which operates the injectors. During operation of the injectors, the circuitry generates heat which is dissipated through the valve body. FIG. 4 includes a representation of the various sensors and the associated circuitry. An external connector 346 (FIG. 5d) is provided at one end of the board 344 and extends through an opening 348 in the recess wall 350. 
     The open end of the recess may be covered with a rectangular cover 352 which is attached to the valve body using fastening screws. 
     As mentioned above, the system control computer selectively operates the injectors to provide a desired fuel flow to the engine. The control system includes sensors to detect various engine parameters and the fuel condition, and thus determine the fuelling needs of the engine. Various other operating parameters of the system and the engine ancillaries are also determined, and are utilized to provide accurate control of the metering valve and thus accurate fuelling of the engine. A description of the control system and the various associated sensors, will follow hereinafter. 
     The variation in fuel flow is provided by opening selected ones of the high-flow injectors to provide a base flow component, and preferably also by opening at least some of the low-flow injectors for predetermined timed intervals, or by &#34;pulsing&#34; the low-flow injectors. 
     The orifice sizes of the injectors are selected to provide a suitable series of flow areas which can be manipulated to provide accurate fuelling over a wide dynamic range, typically 1:40 or 1:35. The orifice sizing, or more precisely the different mass flows of fuel through each injector orifice, requires careful selection in order to minimise the number of injectors required, and reduce the number of injector closings occurring. 
     Sonic mass flow through an orifice is: ##EQU1## Where C=orifice discharge coefficient 
     A=orifice area 
     P=absolute supply stagnation pressure 
     g=a dimensional constant 
     k=specific heat ratio of the fluid (temp. dependant) 
     T=stagnation temperature of gas just before the orifice 
     R=specific gas constant 
     C, A, g are constant for each orifice, the combined effects of A and C being determined by testing each individual orifice during assembly of the metering valve, while P and T are determined by sensors to provide the actual flowrate for each orifice and k and R may be assumed for the known gas or be more accurately estimated from sensor data. For simplicity, the following description of the orifice flowrate sequences is in respect of conditions of constant temperature and pressure. 
     Each metering orifice M 0 , M 1 , M 2 , M 3  and M n  provides a respective gas flow Q 0 , Q 1 , Q 2  and Q n . 
     In one ideal sequence, the orifice sizes are selected such that the mass flow follows the sequence: 
     
         ______________________________________      Q.sub.0 = 2.sup.0 Q.sub.0 = 1 × Q.sub.0      Q.sub.1 = 2.sup.0 Q.sub.0 = 1 × Q.sub.0      Q.sub.2 = 2.sup.1 Q.sub.0 = 2 × Q.sub.0      Q.sub.3 = 2.sup.2 Q.sub.0 = 4 × Q.sub.0      .      .      .      Q.sub.n-1 = 2.sup.n-2 Q.sub.0      Q.sub.n = 2.sup.n-1 Q.sub.0______________________________________ 
    
     Note: in a non-ideal valve the mass flows for the first two units (Q 0  and Q 1  would each need to have a flowrate somewhat greater than 1×Q 0  (ideal). 
     In this sequence it will be noted that the flowrates, apart from the first two, vary according to natural binary. 
     In a further ideal sequence, the orifice sizes are selected such that the mass flow follows the sequence. 
     
         ______________________________________      Q.sub.0 = 2.sup.0 Q.sub.0 = 1 × Q.sub.0      Q.sub.1 = 2.sup.0 Q.sub.0 = 1 × Q.sub.0      Q.sub.2 = 2.sup.1 Q.sub.0 = 2 × Q.sub.0      Q.sub.3 = 2.sup.2 Q.sub.0 = 4 × Q.sub.0      .      .      .      Q.sub.n-1 = 2.sup.n-2 Q.sub.0      Q.sub.n = 2.sup.n-2 Q.sub.0______________________________________ 
    
     Note: in a non-ideal valve the mass flows for the first two units (Q 0  and Q 1  would each need to have a flowrate somewhat greater than 1×Q 0  (ideal). 
     It will be noted that the flowrates, apart from the first two, and the last flowrate, which is equal to second last flowrate, vary according to natural binary. 
     In the preferred ideal sequence, the orifice sizes are selected such that the mass flow follows the sequence. 
     
         ______________________________________Q.sub.0 = 2.sup.0Q.sub.0 = 1 × Q.sub.0Q.sub.1 = 2.sup.0 Q.sub.0 = 1 × Q.sub.0Q.sub.2 = 2.sup.1 Q.sub.0 = 2 × Q.sub.0Q.sub.3 = 2.sup.2 Q.sub.0 = 4 × Q.sub.0..Q.sub.n-3 = 2.sup.n-2 Q.sub.0Q.sub.n-2 = 2.sup.n-3 Q.sub.0Q.sub.n-1 = (Q.sub.n-3)+(Q.sub.n-2 =2.sup.n-4 Q.sub.0 +2.sup.n-3 Q.sub.0=2.sup.n-4 + 2.sup.n-3)Q.sub.0Q.sub.n = (Q.sub.n-3)+(Q.sub.n-2)=2.sup.n-4 Q.sub.0 +2.sup.n-3 Q.sub.0=(2.sup.n-4 + 2.sup.n-3)Q.sub.0______________________________________ 
    
     Note: in a non-ideal valve the mass flows for the first two units (Q 0  and Q 1  would each need to have a flowrate somewhat greater than 1×Q 0  (ideal). 
     It will be noted that the flowrates, apart from the first two, and the last two flowrates which are equal and also the sum of the third and fourth last flowrates, vary according to natural binary. 
     As the sequences are similar, apart from the last two orifice flow rates, each will be referred to by reference to the last two &#34;units&#34; of flow, that is, last two units use natural binary (LNB), last two units have equal output (L2E), and last two units equal the sum of the previous two units (L2S). 
     There follows tables of ideal and practical configuration with actual flowrates for orifices following such sequences, to provide a maximum flow of 3000 standard cubic feet per hour (SCFH). 
     
         ______________________________________InjectorNumber    LNB           L2E     L2S______________________________________1         0-47          0-63    0-752         0-47          0-63    0-753          94           125     1504         188           250     3005         375           500     6006         750           1000    9007         1500          1000    900Total     3001          3001    3000______________________________________ 
    
     In preferred systems, the total flow from the first two injectors will exceed the flow from the third injector, as mentioned in the notes above. This arrangement is preferred for two reasons: only a fraction of an injector&#39;s steady state flow capability can be used when the injector is &#34;pulsing&#34; to provide a continuously variable output flowrate, such that some &#34;reserve&#34; capacity is required; and if reserve of &#34;excess&#34; flow exists, it can be used to compensate for both transient and steady state flow errors from all of the injectors. 
     The use of only a fraction of an injector&#39;s steady state flowrate requires the pulsing injector to be shut off once each metering cycle. A typical (empirically supported) pulsed injector flow model can be expressed as: ##EQU2## Where Q[T]=Pulsing injector flow rate 
     Q s  =Steady state injector flow rate 
     T on  =Injector on time control signal duration 
     T open  =Time for injector to open 
     T close  =Time for injector to close 
     T on  +T off  =Time between spark plug firings 
     For example, a typical injector may require 1200 μs to open and 900 μs to close. If the metering cycle were on a 6 cylinder engine running at 5200 rpm, then one cycle would last 3846 μs (i.e. the time between spark plug firings). 
     For such an injector the nominal minimum pulsed on time would be T on  =T open , and the nominal maximum pulsed on time would be T on  =T on  +T off  -T close . Under these conditions, a reasonable injector flow model predicts the minimum useable injector pulsed flow rate to be 900/3846ths (or 23.4%) of its steady state flow rate. As well, the injector&#39;s maximum predictable pulsed flow rate would be (3846-1200)/3846ths (or 68.8%) of its steady flow rate. 
     In the following sequence, a series of injector TRUTH TABLES are provided which illustrate examples of achieving, by various valve designs, a range of fuel flows. In each case, a range of flowrates is given for the injectors which are pulsed (once per combustion event) for the length of time required to provide the required average flow rate. Examples are provided where the first two or three injectors are pulsed. In all cases, the sum of the steady state flow of the pulsing injectors is greater than the output of the next injector. Notably, the first injector is never allowed to output its full steady state output for the reason noted above where only 68.8% availability of the steady flow was available at 5200 rpm. However, all other pulsing injectors may be operated at 100% of their output, if required, by merely leaving them on all of the time. 
     The first TRUTH TABLE includes a column headed `Overlap`, which indicates the variation of flow at particular lines which may be obtained using the same injector or value configuration by varying the pulse width or the open time of the first injector. 
     
         __________________________________________________________________________                             OUTPUT - % F.S.LINE    TOT FLOW      #1  #2  #3 #4 #5 #6 #7 OVER                                 No of InjectorsNO  min  max      88  88  110                 220                    440                       900                          900                             LAP   7                                      6                                         5__________________________________________________________________________ 12345678 0-61 62-116117-168169-171172 226227-278279 281282-336       0-6131-5829-8059-6131-5829-8059-6131-58          31-5888-8831-5888-8831-58              110110110                 220220                     ##STR1##                              34.9%21.1%                                  6%6%8%10%10%                                     9% 9%12%15%15%                                       18%18%24%29%30% 9   337-388      29-80          88-88  220             12%                                    18%                                       36%10   389-391      59-61   110                 220         15.2%                                 14%                                    21%                                       41%11   392-446      31-58          31-58              110                 220             14%                                    21%                                       41%12   447-498      29-80          88-88              110                 220             16%                                    24%                                       47%13   499-501      59-61         440      11.8%                                 18%                                    27%                                       53%14   502-556      31-58          31-58     440          18%                                    27%                                       53%15   557-608      29-80          88-88     440          20%                                    30%                                       59%16   609-611      59-61   110   440       9.7%                                 22%                                    33%                                       64%17   612-666      31-58          31-58              110   440          22%                                    33%                                       65%18   667-718      29-80          88-88              110   440          24%                                    36%                                       71%19   719-721      59-61      220                    440       8.2%                                 26%                                    39%                                       76%20   722-776      31-58          31-58  220                    440          26%                                    39%                                       76%21   777-828      29-80          88-88  220                    440          28%                                    42%                                       82%22   829-831      59-61   110                 220                    440       7.1%                                 30%                                    45%                                       88%23   832-886      31-58          31-58              110                 220                    440          30%                                    45%24   887-938      29-80          88-88              110                 220                    440          32%                                    48%25   939-961      39-61            900    4.2%                                 34%                                    51%26   962-1016      31-58          31-58        900       35%                                    52%27  1017-1068      29-80          88-88        900       37%                                    55%28  1069-1071      59-61   110      900    5.5%                                 39%                                    58%29  1072-1126      31-58          31-58              110      900       39%                                    58%30  1127-1178      29-80          88-88              110      900       41%                                    61%31  1179-1181      59-61      220   900    5.0%                                 43%                                    64%32  1182-1236      31-58          31-58  220   900       43%                                    64%33  1237-1288      29-80          88-88  220   900       45%                                    67%34  1289-1291      59-61   110                 220   900    4.6%                                 47%                                    70%35  1292-1346      31-58          31-58              110                 220   900       47%                                    70%36  1347-1398      29-80          88-88              110                 220   900       49%                                    73%37  1399-1401      59-61         440                       900    4.2%                                 51%                                    76%38  1402-1456      31-58          31-58     440                       900       51%                                    76%39  1457-1508      29-80          88-88     440                       900       53%                                    79%40  1509-1511      59-61   110   440                       900    3.9%                                 55%                                    82%41  1512-1566      31-58          31-58              110   440                       900       55%                                    82%42  1567-1618      29-80          88-88              110   440                       900       57%                                    85%43  1619-1621      59-61      220                    440                       900    3.6%                                 59%                                    88%44  1622-1676      31-58          31-58  220                    440                       900       59%                                    88%45  1677-1728      29-80          88-88  220                    440                       900       61%                                    91%46  1729-1731      59-61   110                 220                    440                       900    3.4%                                 63%                                    94%47  1732-1786      31-58          31-58              110                 220                    440                       900       63%                                    94%48  1787-1838      29-80          88-88              110                 220                    440                       900       65%                                    97%49  1839-1861      39-61            900                          900                              2.1%                                 67%50  1862-1916      31-58          31-58        900                          900    68%51  1917-1968      29-80          88-88        900                          900    70%52  1969-1971      59-61   110      900                          900                              3.0%                                 72%53  1972-2026      31-58          31-58              110      900                          900    72%54  2027-2078      29-80          88-88              110      900                          900    74%55  2079-2081      59-61      220   900                          900                              2.8%                                 76%56  2082-2136      31-58          31-58  220   900                          900    76%57  2137-2188      29-80          88-88  220   900                          900    78%58  2189-2191      59-61   110                 220   900                          900                              2.7%                                 80%59  2192-2246      31-58          31-58              110                 220   900                          900    80%60  2247-2298      29-80          88-88              110                 220   900                          900    82%61  2299-2301      59-61         440                       900                          900                              2.6%                                 84%62  2302-2356      31-58          31-58     440                       900                          900    84%63  2357-2408      29-80          88-88     440                       900                          900    86%64  2409-2411      59-61   110   440                       900                          900                              2.4%                                 88%65  2412-2466      31-58          31-58              110   440                       900                          900    88%66  2467-2518      29-80          88-88              110   440                       900                          900    90%67  2519-2521      59-61      220                    440                       900                          900                              2.3%                                 92%68  2522-2576      31-58          31-58  220                    440                       900                          900    92%69  2577-2628      29-80          88-88  220                    440                       900                          900    94%70  2629-2631      59-61   110                 220                    440                       900                          900                              2.2%                                 96%71  2632-2686      31-58          31-58              110                 220                    440                       900                          900    96%72  2687-2746      29-88          88-88              110                 220                    440                       900                          900    98%__________________________________________________________________________ 
    
     
                                           INJECTOR TRUTH TABLE__________________________________________________________________________ ##STR2##TOTAL FLOW   OUTPUT FLOW USED FROM EACH INJECTORLINE    (SCFH)   #1  #2 #3  #4 #5  #6 #7  #8NO  min  max 59  64 64  150                      300 600                             900 900__________________________________________________________________________ 1   0-59     0-59 2  60-91    28-59            32 3   92-123  28-59            64 4  124-155  28-59            64 32 5  156-187  28-59            64 64 6  138-209  38-59      150 7  210-241  28-59            32     150 8  242-273  28-59            64     150 9  274-305  28-59            64 32  15010  306-337  28-59            64 64  15011  338-359  38-59         30012  360-391  28-59            32        30013  392-423  28-59            64        30014  424-455  28-59            64 32     30015  456-487  28-59            64 64     30016  488-509  38-59      150                      30017  510-541  28-59            32     150                      30018  542-573  28-59            64     150                      30019  574-605  28-59            64 32  150                      30020  606-637  28-59            64 64  150                      30021  638-659  38-59             60022  660-691  28-59            32            60023  692-723  28-59            64            60024  724-755  28-59            64 32         60025  756-787  28-59            64 64         60026  788-809  38-59      150    60027  810-841  28-59            32     150    60028  842-873  28-59            64     150    60029  874-905  28-59            64 32  150    60030  906-937  28-59            64 64  150    60031  938-959  38-59                90032  960-991  28-59            32               90033   992-1023        28-59            64               90034  1024-1055        28-59            64 32            90035  1056- 1087        28-59            64 64            90036  1088-1109        38-59      150       90037  1110-1141        28-59            32     150       90038  1142-1173        28-59            64     150       90039  1174-1205        28-59            64 32  150       90040  1206-1237        28-59            64 64  150       90041  1238-1259        38-59         300    90042  1260-1291        28-59            32        300    90043  1292-1323        28-59            64        300    90044  1324-1355        28-59            64 32     300    90045  1356-1387        28-59            64 64     300    90046  1388-1409        38-59      150                      300    90047  1410-1441        28-59            32     150                      300    90048  1442-1473        28-59            64     150                      300    90049  1474-1505        28-59            64 32  150                      300    90050  1506-1537        28-59            64 64  150                      300    90051  1538-1559        38-59             600                             90052  1560-1591        28-59            32            600                             90053  1592-1623        28- 59            64            600                             90054  1624-1655        28-59            64 32         600                             90055  1656-1687        28-59            64 64         600                             90056  1688-1709        38-59      150    600                             90057  1710-1741        28-59            32     150    600                             90058  1742-1773        28-59            64     150    600                             90059  1774-1805        28-59            64 32  150    600                             90060  1806-1837        28-59            64 64  150    600                             90061  1838-1859        38-59                900 90062  1860-1891        28-59            32               900 90063  1892-1923        28-59            64               900 90064  1924-1955        28-59            64 32            900 90065  1956-1987        28-59            64 64            900 90066  1988-2009        38-59      150       900 90067  2010-2041        28-59            32     150       900 90068  2042-2073        28-59            64     150       900 90069  2074-2105        28-59            64 32  150       900 90070  2106-2137        28-59            64 64  150       900 90071  2138-2159        38-59         300    900 90072  2160-2191        28-59            32        300    900 90073  2192-2223        28-59            64        300    900 90074  2224-2255        28-59            64 32     300    900 90075  2256-2287        28-59            64 64     300    900 90076  2288-2309        38-59      150                      300    900 90077  2310-2341        28-59            32     150                      300    900 90078  2342-2373        28-59            64     150                      300    900 90079  2374-2405        28-59            64 32  150                      300    900 90080  2406-2437        28-59            64 64  150                      300    900 90081  2438-2459        38-59             600                             900 90082  2460-2491        28-59            32            600                             900 90083  2492-2523        28-59            64            600                             900 90084  2524-2555        28-59            64 32         600                             900 90085  2556-2587        28-59            64 64         600                             900 90086  2588-2609        38-59      150    600                             900 90087  2610-2641        28- 59            32     150    600                             900 90088  2642-2673        28-59            64     150    600                             900 90089  2674-2705        28-59            64 32  150    600                             900 90090  2706-2737        28-59            64 64  150    600                             900 90091  2738-2759        38-59         300 600                             900 90092  2760-2791        28-59            32        300 600                             900 90093  2792-2823        28-59            64        300 600                             900 90094  2824-2855        28-59            64 32     300 600                             900 90095  2856-2887        28-59            64 64     300 600                             900 90096  2888-2909        38-59      150                      300 600                             900 90097  2910-2941        28-59            32     150                      300 600                             900 90098  2942-2973        28-59            64     150                      300 600                             900 90099  2974-3005        28-59            64 32  150                      300 600                             900 900100 3006-3037        28-59            64 64  150                      300 600                             900 900__________________________________________________________________________ ##STR3##TOTAL FLOW   OUTPUT FLOW USED FROM EACH INJECTORLINE    (SCFH)   #1  #2 #3  #4 #5  #6 #7NO  min  max 90  97 150 300                      600 900                             900__________________________________________________________________________ 1   0-90     0-90 2   91-138  43-90            48 3  139-187  42-90            97 4  188-240  38-90  150 5  241-288  43-90            48 150 6  289-337  42-90            97 150 7  338-390  38-90      300 8  391-438  43-90            48     300 9  439-487  42-90            97     30010  488-540  38-90  150 30011  541-588  43-90            48 150 30012  589-637  42-90            97 150 30013  638-690  38-90         60014  691-738  43-90            48        60015  739-787  42-90            97        60016  788-840  38-90  150    60017  841-888  43-90            48 150    60018  889-937  42-90            97 150    60019  938-990  38-90             90020   991-1038        43-90            48            90021  1039-1087        42-90            97            90022  1088-1140        38-90  150        90023  1141-1188        43-90            48 150        90024  1189-1237        42-90            97 150        90025  1238-1290        38-90      300    90026  1291-1338        43-90            48     300    90027  1339-1387        42-90            97     300    90028  1388-1440        38-90  150 300    90029  1441-1488        43-90            48 150 300    90030  1489-1537        42-90            97 150 300    90031  1538-1590        38-90         600 90032  1591-1638        43-90            48        600 90033  1639-1687        42-90            97        600 90034  1688-1740        38-90  150    600 90035  1741-1788        43-90            48 150    600 90036  1789-1837        42-90            97 150    600 90037  1838-1890        38-90             900                             90038  1891-1938        43-90            48            900                             90039  1939-1987        42- 90            97            900                             90040  1988-2040        38-90  150        900                             90041  2041-2088        43-90            48 150        900                             90042  2089-2137        42-90            97 150        900                             90043  2138-2190        38-90      300    900                             90044  2191-2238        43-90            48     300    900                             90045  2239-2287        42-90            97     300    900                             90046  2288-2340        38-90  150 300    900                             90047  2341-2388        43-90            48 150 300    900                             90048  2389-2437        42-90            97 150 300    900                             90049  2438-2490        38-90         600 900                             90050  2491-2538        43-90            48        600 900                             90051  2539-2587        42-90            97        600 900                             90052  2588-2640        38-90  150    600 900                             90053  2641-2688        43-90            48 150    600 900                             90054  2689-2737        42-90            97 150    600 900                             90055  2738-2790        38-90      300                      600 900                             90056  2791-2838        43-90            48     300                      600 900                             90057  2839-2887        42-90            97     300                      600 900                             90058  2888-2940        38-90  150 300                      600 900                             90059  2941-2988        43-90            48 150 300                      600 900                             90060  2989-3037        42-90            97 150 300                      600 900                             900__________________________________________________________________________ ##STR4##TOTAL FLOW   OUTPUT FLOW USED FROM EACH INJECTORLINE    (SCFH)   #1  #2 #3  #4 #5  #6 #7 #8NO  min  max 53  58 75  150                      300 600                             900                                900__________________________________________________________________________ 1   0-53     0-53 2  54-82    25-53            29 3   83-111  25-53            58 4  112-128  37-53  75 5  129-157  25-53            29 75 6  158-186  25-53            58 75 7  187-203  37-53      150 8  204-232  25-53            29     150 9  233-261  25-53            58     15010  262-278  37-53  75  15011  279- 307 25-53            29 75  15012  308-336  25-53            58 75  15013  337-353  37-53         30014  354-382  25-53            20        30015  383-411  25-53            58        30016  412-428  37-53  75     30017  429-457  25-53            29 75     30018  458-486  25-53            58 75     30019  487-503  37-53      150                      30020  504-532  25-53            29     150                      30021  533-561  25-53            58     150                      30022  562-578  37-53  75  150                      30023  579-607  25-53            29 75  150                      30024  608-636  25-53            58 75  150                      30025  637-653  37-53             60026  654-682  25-53            29            60027  683-711  25-53            58            60028  712-728  37-53  75         60029  729-757  25-53            29 75         60030  758-786  25-53            58 75         60031  787-803  37-53      150    60032  804-832  25-53            29     150    60033  833-861  25-53            58     150    60034  862-878  37-53  75  150    60035  879-907  25-53            29 75  150    60036  908-936  25-53            58 75  150    60037  937-953  37-53                90038  954-982  25-53            29               90039   983-1011        25-53            58               90040  1012-1028        37-53  75            90041  1029-1057        25-53            29 75            90042  1058-1086        25-53            58 75            90043  1087-1103        37-53      150       90044  1104-1132        25-53            29     150       90045  1133-1161        25-53            58     150       90046  1162-1178        37-53  75  150       90047  1179-1207        25-53            29 75  150       90048  1208-1236        25-53            58 75  150       90049  1237-1253        37-53         300    90050  1254-1282        25-53            29        300    90051  1283-1311        25-53            58        300    90052  1312-1328        37-53  75     300    90053  1329-1357        25-53            29 75     300    90054  1358-1386        25-53            58 75     300    90055  1387-1403        37-53      150                      300    90056  1404-1432        25-53            29     150                      300    90057  1433-1461        25-53            58     150                      300    90058  1462-1478        37-53  75  150                      300    90059  1479-1507        25-53            29 75  150                      300    90060  1508-1536        25-53            58 75  150                      300    90061  1537-1553        37-53             600                             90062  1554-1582        25-53            29            600                             90063  1583-1611        25-53            58            600                             90064  1612-1628        37-53  75         600                             90065  1629-1657        25-53            29 75         600                             90066  1658-1686        25-53            58 75         600                             90067  1687-1703        37-53      150    600                             90068  1704-1732        25-53            29     150    600                             90069  1733-1761        25-53            58     150    600                             90070  1762-1778        37-53  75  150    600                             90071  1779-1807        25-53            29 75  150    600                             90072  1808-1836        25-53            58 75  150    600                             90073  1837-1853        37-53                900                                90074  1854-1882        25-53            29               900                                90075  1883-1911        25-53            58               900                                90076  1912-1928        37-53  75            900                                90077  1929-1957        25-53            29 75            900                                90078  1958-1986        25-53            58 75            900                                90079  1987-2003        37-53      150       900                                90080  2004-2032        25-53            29     150       900                                90081  2033-2061        25-53            58     150       900                                90082  2062-2078        37-53  75  150       900                                90083  2079-2107        25-53            29 75  150       900                                90084  2108-2136        25-53            58 75  150       900                                90085  2137-2153        37-53         300    900                                90086  2154-2182        25-53            29        300    900                                90087  2183-2211        25-53            58        300    900                                90088  2212-2228        37-53  75     300    900                                90089  2229-2257        25-53            29 75     300    900                                90090  2258-2286        25-53            58 75     300    900                                90091  2287-2303        37-53      150                      300    900                                90092  2304-2332        25-53            29     150                      300    900                                90093  2333-2361        25-53            58     150                      300    900                                90094  2362-2378        37-53  75  150                      300    900                                90095  2379-2407        25-53            29 75  150                      300    900                                90096  2408-2436        25-53            58 75  150                      300    900                                90097  2437-2453        37-53             600                             900                                90098  2454-2482        25-53            29            600                             900                                90099  2483-2511        25-53            58            600                             900                                900100 2512-2528        37-53  75         600                             900                                900101 2529-2557        25-53            29 75         600                             900                                900102 2558-2586        25-53            58 75         600                             900                                900103 2587-2603        37-53      150    600                             900                                900104 2604-2632        25-53            29     150    600                             900                                900105 2633-2661        25-53            58     150    600                             900                                900106 2662-2678        37-53  75  150    600                             900                                900107 2679-2707        25-53            29 75  150    600                             900                                900108 2708-2736        25-53            58 75  150    600                             900                                900109 2737-2753        37-53         300 600                             900                                900110 2754-2782        25-53            29        300 600                             900                                900111 2783-2811        25-53            58        300 600                             900                                900112 2812-2828        37-53  75     300 600                             900                                900113 2829-2857        25-53            29 75     300 600                             900                                900114 2858-2886        25-53            58 75     300 600                             900                                900115 2887-2903        37-53      150                      300 600                             900                                900116 2904-2932        25-53            29     150                      300 600                             900                                900117 2933-2961        25-53            58     150                      300 600                             900                                900118 2962- 2978        37-53  75  150                      300 600                             900                                900119 2979-3007        25-53            29 75  150                      300 600                             900                                900120 3008-3036        25-53            58 75  150                      300 600                             900                                900__________________________________________________________________________ ##STR5##TOTAL FLOW   OUTPUT FLOW USED FROM EACH INJECTORLINE    (SCFH)   #1  #2 #3  #4 #5  #6     #7NO  min  max 80  86 125 250                      500 1000   1000__________________________________________________________________________ 1   0-80     0-80 2   81-123  38-80            43 3  124-166  38-80            86 4  167-205  42-80  125 5  206-248  38-80            43 125 6  249-291  38-80            86 125 7  292-330  42-80      250 8  331-373  38-80            43     250 9  374-416  38-80            86     25010  417-455  42-80  125 25011  456-498  38-80            43 125 25012  499-541  38-80            86 125 25013  542-580  42-80         50014  581-623  38-80            43        50015  624-666  38-80            86        50016  667-705  42-80  125    50017  706-748  38-80            43 125    50018  749-791  38-80            86 125    50019  792-830  42-80      250                      50020  831-873  38-80            43     250                      50021  874-916  38-80            86     250                      50022  917-955  42-80  125 250                      50023  956-998  38-80            43 125 250                      50024   999-1041        38-80            86 125 250                      50025  1042-1080        42-80             100026  1081-1123        38-80            43            100027  1124-1166        38-80            86            100028  1167-1205        42-80  125        100029  1206-1248        38-80            43 125        100030  1249-1291        38-80            86 125        100031  1292-1330        42-80      250    100032  1331-1373        38-80            43     250    100033  1374-1416        38-80            86     250    100034  1417-1455        42-80  125 250    100035  1456-1498        38-80            43 125 250    100036  1499-1541        38-80            86 125 250    100037  1542-1580        42-80         500 100038  1581-1623        38-80            43        500 100039  1624-1666        38-80            86        500 100040  1667-1705        42-80  125    500 100041  1706-1748        38-80            43 125    500 100042  1749-1791        38-80            86 125    500 100043  1792-1830        42-80      250                      500 100044  1831-1873        38-80            43     250                      500 100045  1874-1916        38-80            86     250                      500 100046  1917-1955        42-80  125 250                      500 100047  1956-1998        38-80            43 125 250                      500 100048  1999-2041        38-80            86 125 250                      500 100049  2042-2080        42-80             1000   100050  2081-2123        38-80            43            1000   100051  2124-2166        38-80            86            1000   100052  2167-2205        42-80  125        1000   100053  2206-2248        38-80            43 125        1000   100054  2249-2291        38-80            86 125        1000   100055  2292-2330        42-80      250    1000   100056  2331-2373        38-80            43     250    1000   100057  2374-2416        38-80            86     250    1000   100058  2417-2455        42-80  125 250    1000   100059  2456-2498        38-80            43 125 250    1000   100060  2499-2541        38-80            86 125 250    1000   100061  2542-2580        42-80         500 1000   100062  2581-2623        38-80            43        500 1000   100063  2624-2666        38-80            86        500 1000   100064  2667-2705        42-80  125    500 1000   100065  2706-2748        38-80            43 125    500 1000   100066  2749-2791        38-80            86 125    500 1000   100067  2792-2830        42-80      250                      500 1000   100068  2831-2873        38-80            43     250                      500 1000   100069  2874-2916        38-80            86     250                      500 1000   100070  2917-2955        42-80  125 250                      500 1000   100071  2956-2998        38-80            43 125 250                      500 1000   100072  2999-3041        38-80            86 125 250                      500 1000   1000__________________________________________________________________________ ##STR6##TOTAL FLOW   OUTPUT FLOW USED FROM EACH INJECTORLINE    (SCFH)   #1  #2 #3  #4 #5  #6 #7NO  min  max 74  80 94  188                      375 750                             1500__________________________________________________________________________ 1   0-74     0-74 2   75-114  35-74            40 3  115-168  21-74  94 4  169-208  35-74            40 94 5  209-262  21-74      188 6  263-302  35-74            40     188 7  303-356  21-74  94  188 8  357-396  35-74            40 94  188 9  397-449  22-74         37510  450-489  35-74            40        37511  490-543  21-74  94     37512  544-583  35-74            40 94     37513  584- 637 21-74      188                      37514  638-677  35-74            40     188                      37515  678-731  21-74  94  188                      37516  732-771  35-74            40 94  188                      37517  772-824  22-74             75018  825-864  35-74            40            75019  865-918  21-74  94         75020  919-958  35-74            40 94         75021   959-1012        21-74      188    75022  1013-1052        35-74            40     188    75023  1053-1106        21-74  94  188    75024  1107-1146        35-74            40 94  188    75025  1147-1199        22-74         375 75026  1200-1239        35-74            40        375 75027  1240-1293        21-74  94     375 75028  1294-1333        35-74            40 94     375 75029  1334-1387        21-74      188                      375 75030  1388-1427        35-74            40     188                      375 75031  1428-1481        21-74  94  188                      375 75032  1482-1521        35-74            40 94  188                      375 75033  1522-1574        22-74                150034  1575-1614        35-74            40               150035  1615-1668        21-74  94            150036  1669-1708        35-74            40 94            150037  1709-1762        21-74      188       150038  1763-1802        35-74            40     188       150039  1803-1856        21-74  94  188       150040  1857-1896        35-74            40 94  188       150041  1897-1949        22-74         375    150042  1950-1989        35-74            40        375    150043  1990-2043        21-74  94     375    150044  2044-2083        35-74            40 94     375    150045  2084-2137        21-74      188                      375    150046  2138-2177        35-74            40     188                      375    150047  2178-2231        21-74  94  188                      375    150048  2232-2271        35-74            40 90  188                      375    150049  2272-2324        22-74             750                             150050  2325-2364        35-74            40            750                             150051  2365-2418        21-74  94         750                             150052  2419-2458        35-74            40 94         750                             150053  2459-2512        21-74      188    750                             150054  2513-2552        35-74            40     188    750                             150055  2553-2606        21-74  94  188    750                             150056  2607-2646        35-74            40 94  188    750                             150057  2647-2699        22-74         375 750                             150058  2700-2739        35-74            40        375 750                             150059  2740-2793        21-74  94     375 750                             150060  2794-2833        35-74            40 94     375 750                             150061  2834-2887        21-74      188                      375 750                             150062  2888-2927        35-74            40     188                      375 750                             150063  2928-2981        21-74  94  188                      375 750                             150064  2982-3021        35-74            40 94  188                      375 750                             1500__________________________________________________________________________ 
    
     To accurately control the flow of fuel it is necessary to have the variables included in equation (1) above to calculate the mass flow of fuel through each orifice of the metering valve, and also to be able to predict the response times of individual injectors to &#34;open&#34; and &#34;close&#34; signals, and also the amount of fuel flowing through each orifice while the injector is changing states. These transient flowrates are affected by the operating voltage of the injector coil and the fuel pressure and temperature. The operating voltage and fuel pressure and temperature are readily obtained using appropriate sensors, while the injector coil temperature may be approximated by a control computer using fuel temperature, valve ambient temperature, injector on time, mass flowrate through the valve, and nominal heat rise and heat transfer characteristics of the injectors. 
     This information allows the fuel flow during the transient period to be predicted, such that the injectors are opened and closed at the correct times, and for the correct time periods. 
     The control system includes sensors 22a to detect the vehicle battery voltage (typically between 6 and 24 volts), as the response time of the injector will vary with the coil energising voltage. The control system maintains the energising current at, for example, 2 Amps., which is required to quickly open the injector and due to the low coil impedance assures that the injector will open even when the vehicle battery is operating at unusually low voltages. However, once the injector is open, the energising current is reduced to 1/2 Amp. This feature, known as current foldback, in this example minimises the generation of heat by the energised coil, prolongs coil life, and reduces the power consumption of the system and most importantly, reduces the time required for the magnetic field in the coil to collapse, which minimises the injector closing period. 
     While injectors and orifices will be manufactured for specific orifice areas and subsequently specific flowrates, there will still be small production variations in the orifice areas, and thus in the predicted flowrates, when compared to the &#34;perfect&#34; metering valve. To minimise fuelling errors and to avoid discontinuities in the fuel flow, each injector is tested, as mentioned above, such that the characteristics of each individual injector are known and can be introduced into the system control. The testing information may be provided in the form of a bar code, adhered to the injector or metering valve, which bar code may be read when the system is being fitted to a vehicle. 
     Below are flow figures for an exemplary seven injector, L2S metering valve, for producing a maximum flow of 3000 SCFH. 
     
         ______________________________________Injector All @   All @    All @  CombinationsNumber   -2%     Nominal  +2%    Min/Max                                   Random______________________________________1         73.5    75      76.5   76.5   73.52         73.5    75      76.5   73.5   73.53         147.0  150      153.0  150    146.04         294.0  300      306.0  294    306.05         588.9  600      612.0  612    588.06         882.0  900      918.0  882    918.07         882.0  900      918.0  918    918.0Total    2940    3000     3060   3006   3023takingthecombinations______________________________________                       Min/              Nominal  Max     Random______________________________________Output of Injectors 1 + 2 + 3              300      300     293Output of Injector 4              300      294     306Error re I + 2 + 3 vs 4               0       -2.0%   4.4%Output of Injectors 1 + 2 + 3 + 5              900      912     881Output of Injector 6              900      882     918Error re 1 + 2 + 3 + 5 vs 6               0       -3.3%   4.2%______________________________________ 
    
     It may be seen that if it was simply assumed that all injectors were nominal, or &#34;perfect&#34; when the metering valve was increasing the flow from, for example 900 SCFH to just over 900 SCFH, when the valve changed from using injectors 1+2+3+5 to using injector 6, there would possibly be an error in fuelling of 3-4%. This might not necessarily noticeably affect the performance of some engines, but for proper operation of, for example, three-way catalytic convertors, it is desirable to have the fuel supply controlled with 0.25%. 
     To compensate for such errors, one of the pulsing injectors may be used to bridge the gap between the two flow rates. The use of pulsing injectors further provides for greater definition of flow using a limited number of injectors. Clearly, the pulsing cannot be used to provide infinite flow definition, and to provide more accurate prediction of flow a minimum on-time is set for each injector, such that proper control of the valve is maintained and the less predictable pull-in and drop out flowrates have proportionally less effect on the total predicted flow. Typically, for the desired degree of control, a pulsing injector requires a minimum off time of 900 μs and has a pull in time of 1200 μs. 
     Downstream of the metering valve, the metered fuel is mixed with air before being supplied to the engine. The fuel and air are conveniently mixed in or after the engine air cleaner. A fuel mixer 360 and a fuel nozzle 362 suitable for use in this environment is illustrated in FIGS. 8 and 9 of the drawings. Both the fuel mixer 360 and the fuel nozzle 362 are adapted for providing dispersion of the fuel in the air stream. In this system, the fuel is supplied from the metering valve at a positive pressure, preferably at a pressure of between 1 and 2 p.s.i. 
     FIG. 8 illustrates a mixer 360 suitable for location in the intake air duct 364 between the air cleaner and throttle plate. The nozzle 360 includes a right-angle nozzle inlet 366 for connection to a hose (not shown) leading from the metering valve outlet, the inlet 366 leading into a diffuser tube 368 provided with a series of openings 370 through which the fuel diffuses into the air being drawn into the engine. The ends of the diffuser tube extend through openings 372, 374 in the intake tube 364 and are located, at one end by the nozzle inlet 366, and at the other end by a diffuser tube support 376. Sealing washers 378, 380 are located between the nozzle inlet 366 and tube support 376 and flats 382, 384 formed on the exterior of the intake air duct 364, while retaining nuts 386, 388 are provided on the interior of the duct 364 to hold the tube 368 in the nozzle inlet 366 and support 376. 
     The nozzle 362 illustrated in FIG. 9 includes a simple diffuser cone 390 fitted to a right angle nozzle inlet 396 and is suited for fitting to an air cleaner, possibly in the air cleaner lid or the inlet snorkel. The illustrated example is shown fitted to an air cleaner lid 392, a sealing washer 394 being provided between the lid and the inlet 396. The cone 390 provides a cone-shaped outlet which diffuses the fuel in the intake air. 
     When installed in the air-cleaner lid, a nozzle is located over each throttle bore. 
     The air cleaner lid is the preferred location on direct mounted air cleaners as a means to minimise transient delays and to minimise the volume of &#34;stored&#34; combustible charge above the throttles. 
     CONTROL OF METERING VALVE 
     To control the operation of the metering valve to provide the correct quantity of fuel to the engine requires the theoretical modelling of the fuel requirements of the engine, and this is calculated from the calculated mass air flow through the engine. For sonic operation, the mass flow through the injectors of the metering valve has a linear dependency on absolute inlet pressure (assuming temperature constant). Thus the different fuel requirements of different size engines may be broadly accommodated by adjusting the metering valve inlet pressure, using the following equation to assure adequate flow capacity exists. ##EQU3## K R  =constant depending on regulator characteristics P p  =peak engine power [kW] 
     LHV=lower heating value of fuel [kJ/g] 
     m max  =maximum fuel flow capacity at 100 psig [g/s] 
     This adjustment is achieved by adjustment of the pressure regulator. As mentioned above, an engine s instantaneous fuel flowrates are calculated based on the mass air flow to the engine. The mass air flow (MAF) is calculated as follows: ##EQU4## Where K=conversion constant 
     n air  =air portion of intake charge 
     D=engine displacement (liters) 
     RPM=engine speed (rev./min.) 
     MAP=manifold absolute pressure (&#34;Hg) 
     η vol  =volumetric efficiency, function of MAP, RPM 
     T mix  =air charge obsolute temperature near intake valve (° K) 
     λ=mass air/fuel ratio 
     As can be seen above, equation (3) includes the volumetric efficiency (η vol ) as a function of MAP and RPM. One implementation of the control programme for the system includes a universal 208-point volumetric efficiency table (16 RPM cells×13 MAP cells). An acceptable choice for the points are: 
     MAP : 29.6, 27.5, 25.6, 23.3, 21.2, 19.2, 17.5, 15.9, 14.5, 13.1, 11.7, 9.9, 7,.4 
     R.P.M. : I, I(T/I) 2/9 , . . . I(T/I) 8/9 , T, T(P/T) 1/2 , T(P/T) 3/4 , P, 1.15.P, (1.15) 2 .P 
      where: 
     =peak power RPM 
     T=peak torque RPM 
     I=idle RPM 
     Volumetric efficiency can be empirically determined on each engine. Alternatively, a mathematical model may be used to develop the volumetric efficiency of each engine. The overall volumetric efficiency is defined as the actual mass of fresh air which passes into an engine cylinder during one intake stroke divided by the ideal air mass (M air ) which would fill the piston displacement under the same conditions. ##EQU5## where ηpeak=estimated peak volumetric efficiency 
     ε=compression ration 
     RPM P .T. =engine speed at peak torque (rev./min.) 
     R.P.M. p .p. =engine speed at peak torque ##EQU6## a 0 , a 1 , a 2 , b 0 , b 1 , b 2 , c 0 , c 1 , c 2  =empirical constants 
     RPM i  =engine speed at idle (rev./min.) 
     η m  =mechanical efficiency 
     T c  =thermostat opening temperature [K] 
     T b  =base temperature [K] 
     z=number of cylinders 
     b=cylinder bore [mm] 
     As the estimation of peak volumetric efficiency ηpeak) for each engine is somewhat cumbersome and it has been found that ηpeak=0.85 is a reasonable approximation for the majority of naturally aspirated spark ignition (S.I.) engines. However, the figure is typically higher when tuned intake or exhaust manifolds are used, or when an engine is turbocharged or supercharged. Appropriate values must be calculated or approximated for these less common configurations. 
     As noted from equation (3), mass flow determination requires measurement of air temperature near the intake valve (T mix ). For existing vehicles, the inconvenience of an aftermarket installation of a temperature sensor near the intake valve requires the prediction of the air charge temperature increase from the air temperature sensor location, typically in the air cleaner to the intake valve. 
     The modelling of the mixture temperature (T mix )° K near the intake valve involves calculation using calibrated and measured data: ##EQU7## T air  =air inlet temperature [K] T fuel  =fuel temperature [K] 
     Z=∩ of cylinders 
     L=intake runner length [m] 
     RPM pp  =engine speed at peak power [RPM] 
     D=engine displacement [liter] 
     A=stoichiometric air/fuel ratio 
     LAMBDA=nominal excess air ratio; for real time metering 
     MAF=mass air flow [g/s](calculated using equation (1) with T mix  =T air ) 
     K 1 ,K 2  =empirically determined constants 
     FIG. 10 is a block diagram of a suitable computer and sensor set up for controlling the metering valve to provide desired fuel flows, as described above, and illustrates the various sensors which are utilised to determine the settings of the metering valve. 
     The system power inputs 400 include an input from the battery and a ground line. The analog inputs 402 include engine data 404, fuel system data 406, miscellaneous data 408 and a number of spare lines for future requirements 410. The engine data 404 includes engine coolant temperature, intake air temperature, intake manifold absolute pressure, and exhaust gas oxygen. The fuel system data includes fuel storage pressure (gauge), fuel regulated pressure (absolute) and fuel regulated temperature. The miscellaneous inputs include barometric pressure, battery voltage and a reference voltage (5 volts dc). 
     The analog inputs are converted to digital signals for processing. 
     The digital inputs 412 relate primarily to status, and include inputs from the starter solenoid, gasoline mode select, natural gas or propane mode select, the exhaust gas recirculator (EGR), and ignition key. 
     Inputs 414 relating to engine speed are classified as high speed inputs, and comprise the coil negative, that is the engine R.P.M., and possibly also an input relative to actual vehicle speed. In this particular example, for use in fitting to a conventional gasoline fuelled vehicle a number of high speed inputs may also be obtained by intercepting the original engine manufacturer&#39;s (OEMs) data lines 416, such as the top dead centre sensor, the knock sensor and exhaust gas oxygen sensor. 
     An input may also be provided from the vehicle dashboard fuel gauge. 
     Calibration inputs 418 are provided for communication with a synchronous serial port 419 for use in calibration of the sensors for detecting top dead centre, knock, exhaust gas oxygen and the dashboard fuel gauge. 
     The inputs are fed to the computing section 420 which includes an analogue to digital converter (for the analogue inputs), a clock, a microprocessor, an EPROM, bus drivers, and battery (for back-up power). 
     An output from the computing section 420 passes through an output buffer 422 to provide the reference voltage output 423. 
     Other outputs from the computing section pass through saturated 1 Amp drivers 424 to an EGR override solenoid and an optional idle compensator outputs 425. 
     2 Amp drivers 426 are also provided for outputs 427 for control of a gasoline solenoid or relay, for twin-fuelled vehicles, the solenoid in the pressure regulator, and a coolant supply solenoid. 
     A plurality of metering valve data lines 428 are also provided, one line for each solenoid of the valve. For a typical natural gas application approximately seven or eight outputs 429 will be provided, and on propane or LPG applications nine or ten injectors and outputs may be required. As mentioned above with reference to FIG. 5d, due to the heat they produce, the hi-current foldback drivers are located in the metering valve body. 
     CLOSED LOOP SYSTEM 
     To further compensate for fuelling errors, an active oxygen sensor feedback system and a keep alive adapted memory are included in the control system. 
     Fuelling errors may be caused by various deviations, including: 
     sensor deviation from the respective calibration curve 
     inaccuracy of metering valve calibration curve 
     deviation of actual vs. calculated volumetric efficiency 
     deviation of actual vs. calculated air charge temperature near intake valve 
     deviation of actual vs. calculated fuel demand 
     changes in natural gas composition 
     In order to assist the oxygen sensor in quickly finding stoichiometry, an adaptive memory strategy is provided and structured in a similar manner as the volumetric efficiency table, that is, a table provided with cells, for example 16 RPM cells×13 MAP cells, each cell holding a value for a volumetric efficiency correction matrix, VC (RPM,MAP). 
     Initially, the table holds values of VC=1, and as the vehicle is operated within closed loop within a cell, the particular value of VC (RPM, MAP) is altered until stoichiometry is reached. The table holds the altered values and can thereafter reach stoichiometry more quickly, as the starting value is likely to be closer to the desired value. 
     As a further refinement, a series of separate VC (RPM, MAP) factors could be created and stored in the computers memory, to uniquely compensate for metering errors identified by the closed loop during readily identifiable distinct operating modes such as for EGR active, EGR inactive, air temperature within specific limits, barometric pressure within specific limits, rate of change of engine speed greater than a specific value, MAP value less than minimum standard point by more than a specific value, and the value in the standard matrix. 
     The above described embodiment has been described with reference to sonic flow through the orifices of the metering valve. It is also possible to operate the system using sub-sonic flow, though this complicates control as the outlet pressure in the metering valve must be taken into account in order to calculate the mass flow rate through each orifice. Sub-sonic flow may be considered in circumstances where the fuel, typically propane, will be operating at or below the critical pressure for the orifice geometry used (that is, the sonic pressure ratio could not predictably be maintained across the orifice). In propane systems this would be most likely to occur at temperatures below -20° C. where there was inadequate heat from the engine and/or environment to vaporize the fuel and maintain it at a sufficiently high pressure. 
     Reference has primarily been made to spark ignition engines, though the system may also have utility in conversion of compression ignition or diesel engines. Typically natural gas will not ignite due to compression at compression ratios below 20:1, such that an ignition using either pilot injected diesel fuel, or a separate spark plug arrangement will be necessary. Nonetheless, the system described herein is viable to such applications. 
     Further, while the above described embodiments are adapted for use with internal combustion engines, it is of course possible that the system, and the components thereof, could be adapted for use in other applications.