DE68913646T2 - Fuel injection device. - Google Patents

Description

This invention relates generally to a fuel injection system for an automotive internal combustion engine. More particularly, it relates to the construction of a fuel injection device in which fuel and air or other gas are premixed in a chamber of the device before being discharged into the combustion chamber of the engine, a dwell time being provided prior to injection to allow mixing and, in the case of a liquid fuel, vaporization of the fuel charge in combination with the gas, resulting in a premixed, rich fuel/gas charge which is injected with the fuel which is at least partially in a gaseous state.

A fuel injection device of this type is known from US-A-Patent No. 1,560,025, which discloses a fuel injection device comprising the following parts: a hollow body with a main fuel/gas mixing chamber which is open at both ends and which can be vented to ambient pressure and which initially contains a gas which is at substantially ambient pressure, a control valve which normally closes one end of the chamber and an outlet from the body and which can be operated to an open position to permit the outflow of a fuel/gas mixture from the chamber and the body, a first source of fuel and a second source of pressurized gas each separately connectable to the chamber via the body, and means movable between the open and closed positions to control the inflow of each of the sources into the chamber, the latter means being movable in a manner which first admits fuel into the chamber to mix it with gas therein and, after a time delay, to admit the pressurized gas into the chamber to further mix the fuel and gas in the chamber and to flow the mixture out from the chamber body. In this injection device, the fuel is introduced adjacent to the control valve and the opening and closing of the inlets and outlets is carried out mechanically by an externally operated piston valve which is arranged to connect the opening and closing of the respective openings.

Premixing of fuel and air or gas in an injection system is also known from other, earlier proposals. Igashira et al., US-A-4,465,050, discloses a multiple injection system that includes an air pump and a fuel pump that deliver their respective fluids to an injector with a single pulse solenoid that controls only the fuel flows, the air being controlled by a separate valve. However, the air and fuel pulses occur simultaneously and after the simultaneous introduction of fuel and air there is no dwell time to allow the fuel time to vaporize before it is injected into the engine.

McKay, US-A-4,554,945 shows a design in which fuel is first introduced into a metering chamber, and then air is supplied by a magnet and air pressure to to close the fuel inlet and outlet ports. However, again there is no mixing of air and fuel with a time delay sufficient to allow the fuel to evaporate and further mix before injection into the engine.

Tsummura et al., US-A-4,381,077 provides an injection device in which air is supplied simultaneously with the fuel and they are combined in a mixing chamber from where they are displaced by a piston. Although there is mixing and a residence time, the mixture is not displaced by the gas compressed by the engine or by air supplied to vaporize the fuel during the mixing process.

Sarich et al., US-A-4,462,760 first fills a metering chamber with fuel and then displaces the fuel by pressurized gas. However, there is no residence time for the fuel to vaporize before subsequent injection into the engine.

BE-A-738,280 discloses a mechanism for working materials in which the operating pressure is generated by the combustion of fuel in a cylinder, the fuel being first metered into an inlet chamber of a mixer containing residual air under pressure from a previous working cycle. An inlet valve from the mixer into the cylinder is opened and mixing of fuel with further air is forced into the cylinder by admitting further compressed air into the mixer. The inlet valve into the cylinder is then closed and the mixing chamber is left with compressed air.

It is an object of this invention to provide an improved mechanism and method for injecting premixed fuel/air or fuel/gas mixtures into internal combustion engines.

According to this invention, in a fuel injection device as defined in the preamble of claim 1, the fuel and gas sources are each separately connected through the body of the injection device to the end of the chamber remote from the control valve, the means for controlling the supply of fuel and gas can be selectively operated and the control valve is opened by the supply of the pressurized gas into the chamber to discharge the mixture from the chamber and the body.

These features, in conjunction with separating the delivery of a metered fuel charge into the chamber of the injector and the injection of the charge into the engine by a time interval during which the fuel charge is in contact with air or gas and liquid fuel can evaporate, contribute to the injection of a premixed, rich fuel/air or fuel/gas charge in which the fuel is at least partially in a gaseous state.

According to a further embodiment of the invention, there is provided an electrical control system comprising driver circuits arranged to actuate the respective intake valves for supplying fuel and gas/air to the injectors serving the respective combustion chambers of an engine to effect supply to the injectors Ln in a manner which provides a time interval between the introduction of the fuel and the discharge of the fuel/air or fuel/gas mixture into the combustion chambers.

As regards the method aspect of the invention, it provides a method for supplying and discharging a mixture of fuel and gas to and from a fuel injector for a motor vehicle which is biased in a closed position, comprising the steps of: firstly, filling a central chamber in the injector with gas at an ambient pressure level second, supplying fuel to the chamber at the end remote from the outlet thereof, mixing it with the gas to form an at least partially combustible mixture charge, third, maintaining the fuel/gas mixture charge in the chamber for a period of time sufficient to permit vaporisation of the fuel and further mixing of the fuel and gas, and fourth, supplying further gas to the chamber at a pressure level sufficient to enhance penetration of the gas into the fuel and vaporisation of the fuel, the further gas being supplied at a pressure sufficient to cause a normally closed outlet valve from the chamber to open and effect discharge of the fuel/gas mixture from the injector.

The gas used to exhaust the mixture may be compressed air or compressed gas coming from an engine cylinder during its compression stroke. Following the cessation of the supply of pressurized gas to the chamber and the closing of the exhaust valve, the chamber should be vented to an ambient pressure level and the device may for this purpose include means for connecting the gas inlet to air or to gas at ambient pressure.

Other features and forms of application of the invention are defined in the subclaims.

The invention will now be described in more detail by way of example with reference to the accompanying drawings, in which:

Figure 1 is a schematic cross-sectional view of a fuel injection device embodying the invention,

Figure is a cross-sectional view in a plane indicated and viewed in the direction of arrows II-II of Figure 1,

Figure 3 is an enlarged cross-sectional view of a section of Figure 1,

Figures 4, 5, 7 and 8 are schematic illustrations of fuel injection systems supplying both fuel and air to individual fuel injectors embodying the invention; and

Figure 6 is a cross-sectional view of a gas charging device for use with the construction shown in Figure 1.

The invention relates generally to air or gas controlled fuel injection systems for gasoline engines of the type in which electronically controlled devices such as solenoid valves are used as actuators for both air and gas control. However, other devices such as piezoelectric switches may be used instead of solenoids.

Although the fuel injection system to be described is primarily intended for direct fuel injection, it can also be used for injection through injection ports. Compressed air or, in another application, cylinder compression gas is used for fuel injection and atomization. One One of the key features of the system, however, is the fact that the metering of the fuel charge and fuel injection are separated by a time interval during which the fuel charge is in contact with air or gas and is allowed to vaporise, so that when injection occurs a premixed rich charge of fuel/air or fuel/gas is injected, the fuel being at least partly in the gaseous state. Essential features in the system are a set of injectors, one per cylinder, generally fitted directly into the cylinder head, and where air compressed by the cylinder is used, a set of charging valves, also one per cylinder, also normally fitted directly into the cylinder head. Fuel and compressed air or gas are supplied to each injector from separate fuel and air or gas supply systems.

More particularly, Figure 1 shows a fuel injector 10 having a housing or body 12 containing a central mixing chamber for air/gas and fuel. The chamber extends longitudinally along the axis of the injector and is enlarged at 16 at its lower end to enclose a fuel injection valve 18. The valve moves between open and closed positions in a valve body 20 and has a nozzle or tip 22 held against the body by a spring 24. Side ports 26 communicate the mixed fuel/air charge in the mixing chamber 14 with the tip of the injector along the channel or passage 28 containing the cone of the valve.

The upper part of the mixing chamber 14 is closed by a pair of normally closed solenoid operated poppet valves 30 and 32. Valve 30 is a fuel control valve. It normally closes a supply passage 34 which, as indicated, communicates at one end with the mixing chamber 14 and, at its other end, with a Fuel supply F. Valve 32 is an air control valve. It normally blocks the flow of compressed air A from a passage 35 (Figure 2) and chamber 36 into the mixing chamber 14 through a connecting passage 38. Figure 2 also illustrates an electrical input 40 to both the fuel and air solenoids.

When air is used as the mixed and pressurized gas, the mixing chamber 14 always contains air. For this purpose, the mixing chamber 14 is vented to the outside so that its residual pressure after the end of the injection always falls to a low level approximately equal to atmospheric pressure. Referring to Figure 3 and in more detail, the air control valve 32 has an axial channel 42 extending along its length with a calibrated orifice 44 at its tip which communicates with the channel 38 and the chamber 14. Between injections, the channel 42 connects the mixing chamber to the interior of the magnet which is vented through the space 46 between the top of the valve and the magnet housing and around the valve cone and outward via a channel 48 to the intake port of the air compressor or to the intake port of the engine.

In this case, the solenoids would be controlled by an electrical control system which provides voltage signals of variable width and duration to the solenoid, the signals being fed through the connector 40 shown in Figure 2. When the fuel valve 30 opens, fuel is metered into the mixing chamber 14. The amount of fuel metered is determined by the duration of the opening of the fuel control valve, the size of the opening and the supply of fuel pressure. It is normally controlled by controlling the solenoid pulse width. After fuel is fed into the mixing chamber 14, the fuel remains in the air-filled chamber for a substantial portion of the engine cycle. This creates a time interval during which the fuel is exposed to the air. and evaporate before the mixture is injected into the engine. This gives the fuel time to penetrate the air in the chamber.

Therefore, when the air control valve 32 opens, a charge of compressed air fills the mixing chamber 14 to further mix the air with the fuel by infiltrating the air into the fuel and also opens the normally closed injector 22 to expel the premixed fuel/air charge past the valve tip. This is the injection process or stroke. The timing of fuel injection can be controlled by controlling the timing of the air control solenoid pulse. Varying the compressed air pressure can also vary the amount of fuel injected and the penetration of fuel. Injection ends when the air control solenoid or actuator is deactivated and the air control valve 32 closes. As already stated, after the injection is completed, the mixing chamber 14 is vented outwardly to the intake port of the air compressor or to the intake port of the engine through the calibrated opening 44 in the air control valve 32 and from there through the vent channel 48. This in turn causes the mixing chamber 14 to fill with air at ambient or atmospheric pressure level.

Figure 4 shows a schematic overall view of a compressed air and fuel supply arrangement for a system with three injectors 50. A fuel pump 52 draws fuel from a tank 54 and feeds it under pressure through a pressure regulator 56 to a common fuel supply rail 58 to which all three injectors 50 are connected. Excess fuel is returned to the tank 54 via line 57. In the upper part of the diagram or drawing, a compressor 60 draws atmospheric air A through an inlet 62 and pumps it under pressure into a compressed air tank or accumulator 64. A control valve 66 at the inlet and a solenoid valve 68 at the Outlet, which is only open when the engine is running, maintains the air pressure in the reservoir 64 when the engine is not running. During engine operation, compressed air is discharged from the reservoir 64 through the open solenoid valve 68 and conveyed under pressure by a controllable pressure regulator 70 to a common air supply rail 72 to which all three injectors 50 are connected in parallel by connecting lines 74. A change in the air pressure in the common rail 72 changes the fuel injection quantity and the penetration of the fuel jet, as already explained above in connection with the application shown in Figures 1-3. The vented air in the air control valve 32 is returned to the inlet of the compressor via the line 48.

The three injectors are in this case controlled by an electronic control system which would receive the operator's command signal and provide the required magneto voltage pulse width and duration needed to supply the engine with the required amount of fuel at the correct time in the power cycle. A voltage pulse of the correct duration and time is sent to each magneto and in principle each magneto would require a separate drive circuit. However, it is possible to reduce the number of drive circuits by employing a single magneto driver to drive two magnets simultaneously. An example of such an arrangement is shown schematically in Figure 5 in which three magneto drivers 80, 82, 84 are employed to drive six magnets 1F, 2F, 3F, 1A, 2A, 3A associated with the three injectors numbered I₁ to I₃. Each of the drivers 80, 82, 84 drives an air control solenoid, e.g. 1A in one injector and a fuel control solenoid, e.g. 2F in another. Assuming that the operating order of the injectors is 1-2-3, activation of the driver 84 will energize solenoids 2A and 3F, causing injection of injector No. 2 and fuel metering in injector No. 3. Activation of driver 82 therefore results in injection from injector No. 3 and metering into injector No. 1. It can be seen that the dwell time between the injection of fuel and the opening of the air control valve has been undeniably created.

The solenoid valves are designed so that the minimum pulse width required for the air controlled solenoid is never less than the required fuel controlled solenoid pulse. In the arrangement just described above, the time interval between fuel metering or entry into chamber 14 and the fuel injection process in each injector is one third of the cycle. However, it will be appreciated that in a slightly different arrangement this time interval could be, for example, two thirds of the cycle, thus allowing more time for the fuel to vaporize in the mixing chamber 14. To this end, the pairs of simultaneously energized solenoid actuators should be: 1A and 3F, 2A and 1F, 3A and 2F. It will also be appreciated that each pair of simultaneously energized solenoids should be connected in series and not in parallel as shown.

In a multi-cylinder engine, it is desirable to supply the same amount of fuel to all cylinders. The amount of fuel metered by the fuel-controlled solenoid valve is determined by the flow rate of the fuel through the valve opening and the duration of the valve opening.

Q = qt (1)

where:

Q = fuel quantity in mg

q = fuel flow velocity in mg/ms

t = duration of valve opening in ms.

The flow velocity q depends on the opening area and the pressure differential through the opening. If the openings in all solenoid valves are manufactured with very high precision, their areas are the same. In a given system, the pressure differentials for all openings are also the same. In such a system, q can be considered a system constant. The duration of the valve opening depends on the solenoid control pulse width.

t = tc - ta + td (2)

where:

t = the duration of the valve opening in ms

tc = the solenoid control pulse in ms

ta = the valve opening delay in ms

td = the valve closing delay in ms.

The algorithm for the control pulse is derived from equation (2):

tc = t + ta - td (3)

If the valve opening and closing delays were the same in all magnets, equal control pulse widths in all magnets would result in equal fuel deliveries to all cylinders. However, due to unavoidable variations in manufacturing tolerances, the values of the delays ta and td vary from magnet to magnet. The result of this is that equal control pulses in all magnets produce different amounts of fuel in different engine cylinders. This situation can be improved if the control pulse width tc in each magnet is individually adjusted to provide the required fuel delivery. instead of the magnet-to-magnet dispersion in the valve opening and closing delays. To do this, the algorithm for the magnet control pulse is modified as follows.

tc = t + ts + tx (4)

where:

tc = the magnetic control pulse

t = the required duration of the valve opening

ts = (ta-td)min = the minimum value of the pure opening and closing delays, which is constant

tx = the correction factor.

The values of t and tc are the same for all fuel control valves for a given fuel quantity requirement. The value of the correction factor tx is in principle different for each solenoid and is chosen to ensure an equal fuel delivery to all cylinders. The value of the correction factor tx for each individual injector can be determined experimentally during the injector test on the test bench and can be encoded on the injector in the form of a number which can be called the injector identification number. The control reads the value of the correction factor tx and determines the control pulse required for a given value of the required fuel quantity Q using the following algorithm:

tc = Q/q + ts + tx

In many cases, the precision with which the control orifices in the solenoid valves are manufactured is not high enough, and the differences in orifice areas from magnet to magnet cannot be ignored. In this case, the fuel flow velocity g in equation (1) cannot be considered a system constant. As a direct function of the orifice area, it varies from Injector to injector. In such a case, the single point calibration of the injectors described above is not appropriate because the injector characteristic coefficient expressed by equation (1) varies from injector to injector, and calibration for equal fuel delivery to one point cannot guarantee equal deliveries to other points. To achieve equal deliveries to all injectors at all points of the fuel flow versus valve opening time characteristic, equation (1) can be modified as follows:

Q = Kqt (6)

where:

Q = the fuel quantity in mg,

q = the individual flow velocity of the injection device in ms

t = the duration of the valve opening in ms

K = the correction factor.

The value of the correction factor K is generally different for each injector and is chosen so that the product Kg is the same for all injectors. This guarantees equal coefficients for all injector characteristics and enables subsequent single-point calibration. For practical reasons, the values of K should always be greater than 1 (or always less than 1). The value of the correction factor K can be determined experimentally for each individual injector during the injector test on the test bench and encoded on the injector as part of the same code that contains the information on the correction factor tx. The control reads both values, the correction factor tx and the correction factor K, and determines the required control pulse tc for a given value of the required fuel quantity Q using the following algorithm.

tc = Q/Kq + ts + tx (7)

The way in which the identification number is encoded should allow the information in the number to be easily transferred to the electronic control. To this end, the injector may be equipped with a memory device in which the value of the identification number is retained and can be "read" by the control. A microchip memory would be suitable for this purpose, but since only one number needs to be stored, much simpler devices can be used.

Since each fuel control solenoid is normally driven by a separate power transistor driver, misallocation of fuel from cylinder to cylinder can also be the result of differences between the individual solenoid drivers. This deficiency can be corrected in the same way as the differences between the solenoids. An additional correction factor representing the driver characteristic deviation is inserted into the fuel delivery algorithm, an individual code is encoded on each driver, and the controller reads the codes of all drivers and all solenoids and makes the correct control pulse width settings.

The embodiment of Figures 1-5 show the use of compressed air to effect the injection of the fuel/air mixture into the engine. Figure 6 shows a charging valve arrangement in which engine gas compressed in the engine cylinders during the compression strokes is used instead of the compressed air in the embodiment of Figures 1-5 to effect the fuel injection for improved atomization of the fuel or both. More specifically, each charging valve has the purpose of tapping the pressure generated in the cylinder during a compression stroke and to pressurize the compressed gas supply system to to charge to the specified maximum pressure. In the most basic case, the pressurized gas delivery system would simply consist of lines connecting each charging valve to one of the injectors.

Figure 6 shows an example of a charge valve device. The device actually includes two valves, a normally open pressure relief valve 90 and a normally closed check valve 92 leading to a line 94 connecting the valve to the pressurized gas supply system for the injectors. The pressure relief valve is a poppet valve having an upper surface 96 sealingly secured to a spring seat 98 for a spring 100. The spring would hold the valve 90 normally open below a given cylinder compression gas pressure level. The check valve 92 could be of known construction and operation, with a spring-loaded ball 102 closing the passage 104 communicating with the passage or chamber 106.

During the compression stroke, the pressure relief valve 90 is initially open and when the pressure in the engine cylinder exceeds the residual pressure in the compressed gas supply system, the cylinder pressure opens the control valve 92 and the gas supply system is charged by the engine compressed gas to a maximum pressure determined by the preload of the spring 100. When the maximum pressure in the system is reached, the compressive force acting on the piston or face 96 of the pressure relief valve 90 would overcome the force of the spring 100 and close the valve 90. It would reopen in the later part of the compression stroke when the compressive force acting on the pressure relief valve falls below the preload of the spring 100. The space above the face 96, that is, the chamber 110 containing the spring, is vented to the outside through a vent port 112. In some cases this space must be connected to the intake port of the engine. It should be noted that the loading valve can in principle be without the pressure relief valve 90. In this case However, the gas supply system would be charged with hot combustion gas.

Although there are several ways in which the gas driven injection system could be arranged, one of these is shown schematically in Figure 7 for a 3-cylinder engine 4. Fuel and pressurized gas are supplied to the three injectors 120, 122 and 124 associated with the respective cylinders C1, C2, C3 via two separate systems. In the fuel supply system, a fuel pump 126 draws fuel from a tank 128 and delivers it under pressure via a pressure regulator 127 to a common fuel rail 130 to which all three injectors are connected, excess fuel being returned via a return line 129. In the pressurized gas supply system, a gas line 132 from one of the charging valve devices 5 described in connection with Figure 6 connects each injector to one of the charging valves. The volume should be sufficient to store enough pressurized gas to perform at least one injection event. For a given cylinder, the injector would be connected to the charging valve installed in a cylinder preceding the given cylinder in the firing order. In an example illustrated in Figure 5, the firing order of the cylinder is 1-2-3; and therefore the charging valve in cylinder one would supply pressurized gas to the injector in cylinder two, while the charging valves in cylinders two and three would supply gas to cylinders three and one, respectively. The gas vented from all the injectors would be supplied via a line 133 to the intake port of the engine, as indicated.

Another example of a gas driven injection system is shown in Figure 8, which is essentially the same as the embodiment in Figure 4, except for the gas supply to the tank 64'. In this case, all three injectors receive pressurized gas from a gas rail supply system, rather than air. The charging valves in all engine cylinders would supply pressurized gas to a common pressurized gas tank 64' where it could be discharged via a solenoid valve 68' and a controllable gas pressure regulator 70' into the common pressurized gas supply rail 72'. The solenoid valve would only be open during engine operation and therefore the gas pressure in the tank would be maintained even when the engine was not running. The system otherwise operates as in connection with the application shown in Figure 4.

A similar system to that described above could also be used to improve fuel atomization. Fuel injection systems in which improved fuel atomization is achieved by injecting compressed air into the exiting fuel stream are well known and used. Their application in reciprocating internal combustion engines is limited because of the additional cost resulting from the need for an air compressor. The use of engine-compressed gas supplied to fuel injectors in one of the ways described above allows gas-assisted fuel atomization to be achieved without a compressor.

Claims (11)

1. A fuel injection device comprising a hollow body (12) having a main fuel/gas mixing chamber (14) open at both ends and capable of being vented to ambient pressure so as to initially contain gas at substantially ambient pressure, a control valve (18) normally closing one end of the chamber and an outlet (22) from the body and operable to an open position to permit the discharge of a fuel/gas mixture from the chamber and the body, a fuel source and a pressurized gas source both separately connectable to the chamber (14) via the body (12), and means (30, 32) movable between open and closed positions for controlling the supply of fuel and gas from their respective sources to the chamber (14), the latter means movable in a manner which it first allows fuel to be supplied to the chamber to be mixed with gas therein and after a time delay to supply pressurized gas to the chamber to further mix fuel and gas in the chamber and expel the mixture from the chamber and the body, characterized in that the fuel and gas sources can each be connected to the end of the chamber (14) remote from the control valve (18), that the means (30, 32) for controlling the supply of fuel and gas can be selectively operated, and that the control valve (18) is opened by the supply of pressurized gas to the chamber (14) to expel the mixture from the chamber and the body. 2. A fuel injection device according to claim 1 for injecting a jet of partially vaporized fuel mixed with air into the engine, comprising a fuel injection body (12) with an air inlet, a fuel inlet, an operable, normally closed fuel/air outlet (22), means (42, 46, 48) connecting the air inlet to air at ambient pressure, means (30) connecting the fuel inlet to a pressurized fuel source adapted to supply fuel to the body, mix it with and penetrate the ambient air in the body and partially vaporize the fuel, and means (32) for supplying pressurized air to the air inlet after supplying fuel to the body and after an intervening time delay for further air to penetrate the fuel and vaporize it and for operating the valve to open to allow the fuel/air to flow out through the outlet (22). 3. A fuel injection device according to claim 1 or 2, wherein the selectively operable means includes an electromagnetically operated gas flow control valve (32) movable in a gas passage (38) between open and closed positions to control flow therethrough, the passage opening (38) being connected at one end to the chamber (14) and its other end to the pressurized gas source, a second passage (42) connected at one end to a vent opening (48) at substantially ambient pressure level and at its other end to the valve (32), the valve (32) having a tapered opening (44) through which communication between the chamber and the second passage and the vent opening is possible at all times, regardless of the closed position of the valve (32), thereby venting the chamber (14) to ambient pressure level, when the selectively operable valve means (30, 32) and the control valve (18) are closed. 4. Apparatus according to claim 1, wherein the means for supplying pressurized gas to the chamber comprises means connecting the pressurized gas from an engine cylinder to storage means (64') for storing the gases at a predetermined pressure level and channel means containing at least one of the selectively operable valve means (32) which connect the stored gases to the chamber depending on the operation of the valve means. 5. Apparatus according to claim 4, wherein the pressurized gas source includes a loading valve device (5) for loading the storage means (64') with pressurized gas coming from the engine cylinder, the loading valve device (5) including a first gas channel (106) connecting the engine cylinder containing the pressurized gas to the storage means (64'), a normally open pressure relief valve (90) in the first gas channel (106) closed by gas pressure and movable from the open position to a closed position in response to the reaching of a predetermined pressure level in the first gas channel (106) to thereby limit the pressure level in the first gas channel, and control valve means (92) in the first gas channel between the storage means (64') and the pressure relief valve (90) for maintaining a predetermined pressure level in the storage means (64') for subsequent introduction into the mixing chamber of the Injection device included. 6. Device according to claim 5, wherein the loading valve device (5) includes spring means (100) biasing the pressure relief valve (90) in an open position, the latter valve having surface means (96) thereon which can be actuated by the gas pressure cylinder in a direction opposite to the force of the spring means (100) to close the pressure relief valve (90). 7. Device according to claim 6, wherein the surface means (96) of the pressure relief valve (90) and the control valve (92) in the first gas channel (106) have a parallel flow relationship, whereby the control valve (92) is opened before the pressure relief valve (90) is closed as soon as the pressure acting against the control valve of the storage means (64') is smaller than the charge gas pressure coming from the engine cylinder. 8. A fuel injection system comprising, in combination, a plurality of individually sequentially operable fuel injectors (50) according to any preceding claim, each injector comprising a fuel/air mixing chamber (14) having a respective fuel inlet and gas inlet leading thereto and a mixing outlet leading therefrom which is normally blocked by a spring-open, pressure-closed valve (18), a source of pressurized fuel, a source of pressurized gas, means for venting the chamber (14) to ambient pressure prior to the admission of fuel, first solenoid control means connecting the fuel source to each fuel inlet of the injectors, second solenoid control means connecting the gas source to each gas inlet of the injectors, and electrically operated means controlling the energization of the solenoid means to selectively supply fuel and gas to each injector in a manner, in which a time delay is created between the introduction of fuel into the injector and the introduction of the pressurized gas to allow the penetration of the fuel into the gas in the chamber to vaporize the fuel prior to the discharge of the fuel/gas mixture past the valve (18) after the introduction of the pressurized gas into the chamber (14), the electrically operated means including a plurality of individually selectively energizable solenoid drivers (80, 82, 84) and switching means, each of which connects each driver to at least one of the first solenoid control means for the fuel source for one injector and simultaneously to the second solenoid control means for the gas source for another of the injectors, the drivers being individually operable in sequence to produce the desired time delay. 9. A method for supplying and discharging a fuel/gas mixture into and from a fuel injection device (10) for a motor vehicle which is biased in a closed position, comprising the following steps: - firstly, to connect a main chamber (14) in the injection device with gas at ambient pressure level, - secondly, supplying fuel to the chamber at the end remote from its outlet for mixing with gas to form an at least partially combustible mixture charge, - thirdly, to maintain the fuel/gas mixture in the chamber for a period of time sufficient to evaporate the fuel and further mix the fuel and gas, and - fourth, to supply additional gas to the chamber at a pressure level sufficient to enhance the penetration of gas into the fuel and vaporisation of the fuel and to cause a normally closed valve to open and effect the discharge of the fuel/gas mixture charge from the injector. 10. A method according to claim 9, wherein the second step includes supplying pressurized fuel into the chamber (14) in an amount metered in accordance with the engine operating characteristics for gas penetration and fuel vaporization. 11. A method according to claim 9 or claim 10, comprising the step of: - fifth, to vent the chamber to an ambient pressure level after ceasing the supply of pressurized gas to the chamber and closing the outlet valve (18).