WO2006032098A1

WO2006032098A1 - Gas injector

Info

Publication number: WO2006032098A1
Application number: PCT/AU2005/001453
Authority: WO
Inventors: Michael Leonard Mckay; Barry Neumann
Original assignee: Advanced Engine Components Ltd
Priority date: 2004-09-24
Filing date: 2005-09-23
Publication date: 2006-03-30
Also published as: CN100564863C; EP1805410A1; EP1805410A4; CN101031717A

Abstract

A gas injector (10) has a valve assembly (12) and a solenoid actuator (14). The valve assembly (14) includes a spherical valve element (26) which locates against a valve seat (24). The valve element (26) is biased in a closed position by a spring (30), and can be disengage from the valve seat (24) by the action of a putrid (86) controlled by the solenoid actuator (14). The pushrod (86) acts in the same direction as gas flow between a gas inlet (20) and a gas outlet (22), and thus the spring (30) acts against gas pressure when in the valve assembly (12) is in the closed position.

Description

TITLE

GAS INJECTOR

FIELD OF THE INVENTION The present invention relates to a gas injector, such as an injector for supplying compressed natural gas into an internal combustion engine.

BACKGROUND TO THE INVENTION

It is known to require injectors to supply atomised or gaseous fuel into the cylinders of combustion engines. It is important for the fuel to be supplied in a even, metered fashion, and for valves within the injectors to open and close at well defined and precise moments during a cylinder cycle.

One type of injector used for injecting fuel into internal combustion engines comprises a solenoid actuated, outwardly opening poppet valve. Fuel is supplied into the body of the poppet valve under pressure, and is released into an engine cylinder when an associated solenoid is energised. The poppet valve includes a biasing means such as a spring to hold the valve in a closed position against the fluid pressure when the solenoid is not energised.

Outwardly opening poppet valves have proved of limited value in accurately supplying gaseous fuel. In particular, a poppet valve head is rigidly connected to a valve stem and in turn to a solenoid armature. When the valve closes, there is a relatively large impact force between the valve head and the valve seat. This causes resonance within the valve, and a "bounce" phenomenon at the seal between the valve head and the valve seat. The degree of bounce or oscillation is dependent on several changeable, often uncontrollable factors such as mechanical friction and valve hysteresis. As a result, it is difficult to accurately control or define closure of the valve. This limits its capacity to function effectively.

Another type of valve used in gas injectors is shown in Figure 1. Figure 1 shows an injector 110 having a gas inlet 112 and a gas outlet 114. A substantially spherical valve element 116 is held against a valve seat 118 by the gas pressure supplied through the inlet 112 and by the action of a compressed return spring 120. A pushrod 122, arranged within a shroud 124, is located downstream of the valve element 116. The pushrod 122 is rigidly connected to the armature 126 of a solenoid

128. When the solenoid 128 is energised the pushrod 122 acts against the valve element 116, pushing the valve element 116 off the valve seat 118 and allowing gas flow between the inlet 112 and the outlet 114. This is known as "pull-in" of the solenoid. The stroke of the valve element 116 is limited by a stopping member 130. The type of gas injector shown in Figure 1 has several limitations and problems in use. The flow rate of gas within the injector, when the valve is open, is largely determined by the dimensions of the minimum flow path, hi this case, the minimum flow path is the annular space between the pushrod 122 and the shroud 124. It is difficult in practice to manufacture these items sufficiently accurately to provide a precise dimension. The large surface-to-volume ratio evident in such a path is also subject to fouling and reduced flow by even a modest build-up of contaminants on the surfaces. Further, the influence of viscosity on the flow, or extreme vorticity, may tend to make the flow dependent on the concentricity of the surfaces, this concentricity being difficult to manufacture in practice. Precise and quick closure of the valve of Figure 1 also presents difficulties. In the closed position there is a significant static pressure differential on either side of the valve element 116, maintaining it in the closed position. When in the open position, however, this static pressure differential is greatly reduced. The valve thus relies almost entirely on the action of the return spring 120 to close. The spring force must act against an inherent tendency of the armature 126 to "stick" in the active position due to magnetic remanence in the solenoid 128. There is therefore a large differential between the turn-on current, that is the minimum current in the solenoid 128 required to open the valve (typically about 4500ma), and the turn-off current, that is the minimum current in the solenoid required to maintain the valve in the open position

(typically about 250ma).

Valves of the type shown in Figure 1 use an electrical driver having a current sensor. This sensor senses the current being delivered during actuation of the solenoid. The electrical driver is designed for a peak current, typically 4 amperes. This means that once the peak current is reached the driver switches to a lower "holding" current, typically about 1 amperes. This reflects the fact that more current is required to open the solenoid than to maintain it in the open position. Electrical drivers of this sort will continue to operate successfully despite variations in system voltage, for instance during engine start-up. The period in which current is supplied during actuation before reaching the peak current is known as the peak duration.

Problems with this type of driver include a susceptibility to variations in the reluctance of the solenoid, and an inability to adjust for variations in differential pressure across the valve. Use of such a driver can also lead to relatively high turn-off delays during low fuel delivery. The present invention attempts to overcome at least in part some of the aforementioned disadvantages of previous gas injectors.

SUMMARY OF THE INVENTION In accordance with one aspect of the present invention there is provided a gas injector comprising a valve assembly and an associated solenoid actuator, the gas injector having a gas inlet and a gas outlet, the valve assembly comprising a substantially spherical valve element arranged to seal against a valve seat and prevent flow of gas between the gas inlet and the gas outlet, characterised in that the valve assembly includes a first resilient biasing means supplying a force against the valve element to urge the valve element into sealing engagement with the valve seat, and the actuator includes a pushrod arranged to overcome the force supplied by the biasing means and selectively disengage the valve element from the valve seat in order to allow the flow of gas between the gas inlet and the gas outlet, a pressure differential between the gas inlet and the gas outlet acting against the valve element in substantially the same direction as the pushrod and against the force supplied by the biasing means. Advantageously, this provides for a relatively low pull-in force for a given size of valve element, and a relatively small differential between the turn-on and turn-off currents in the solenoid required to activate the valve. Preferably the valve includes a metering orifice located between the gas inlet and the valve element. Advantageously, this provides a fixed minimum flow path when the valve is in an open position which is not subject to the fit between two elements, and which has a relatively low surface-to-area ratio. As a further advantage, this provides static pressure differentials across the valve which promote efficient operation. In accordance with a second aspect of the present invention there is provided a gas injector comprising a valve assembly and an associated solenoid actuator, the gas injector having a gas inlet and a gas outlet, the valve assembly including a valve element and a valve seat, wherein the gas injector includes a sealable diagnostic port, the diagnostic port being fluidly connected to a pressurised space between the gas inlet and the valve element.

In accordance with a third aspect of the present invention there is provided a gas injector comprising a valve assembly and an associated solenoid actuator, the solenoid actuator being associated with an electrical driver which during actuation is arranged to switch from an initial current to a holding current after a defined peak duration, the peak duration being a function of a supply voltage.

BRIEF DESCRIPTION OF THE DRAWINGS The present invention will now be described, by way of example, with reference to the accompanying drawings, in which:

Figure 1 is a schematic cross sectional view of a prior art gas injector as described herein above under the heading "Background to the Invention";

Figure 2 is a schematic cross sectional view of a gas injector in accordance with a first embodiment of the present invention;

Figure 3 is a schematic cross sectional view of a gas injector in accordance with a second embodiment of the present invention;

Figure 4 is a graphic representation of a first correction function used in an electrical driver of the present invention; and Figure 5 is a graphic representation of a second correction function used in an electrical driver of the present invention.

DESCRIPTION OF PREFERRED EMBODIMENTS Referring to Figure 2, there is shown a gas injector 10, comprising a valve assembly

12 and a solenoid actuator 14. The valve assembly 12 includes a substantially cylindrical outer casing 16 having a central axis 18. The outer casing 16 includes at least one gas inlet 20 at a first axial location 21, and at least one gas outlet 22 at a second axial location 23. It will be appreciated that the outer casing 16 may include a plurality of circumferentially spaced gas inlets 20 at the first axial location 21, and a plurality of circumferentially spaced gas outlets 22 at the second axial location 23. The particular cross section shown in Figure 2 shows one such gas inlet 20 and two such gas outlets 22. A valve seat 24 is located within the valve assembly 12 at a third axial location 25, between the first axial location 21 and the second axial location 23. The valve seat 24 is substantially perpendicular to the central axis 18.

A substantially spherical valve element 26 is arranged to seal against the valve seat 24. The valve element 26 is located between the second axial location 23 and the third axial location 25. In a preferred embodiment of the invention the valve seat 24 includes a guide ring 28 to restrict lateral movement of the valve element 26 in use.

The valve assembly 12 further includes a resilient biasing means in the form of a main spring 30. The main spring 30 is a helical compression spring having a lower end 32 and an upper end 34. The lower end 32 is maintained in a desired axial location by a spring stop 36. A spring pad 38 is fixed to the upper end 34. The spring pad 38 acts against the valve element 26, and provides a force to the valve element 26 urging the valve element 26 into a sealing engagement with the valve seat 24.

The valve assembly 12 includes a substantially cylindrical inner wall 40 extending from the valve seat 24 in an axial direction away from the main spring 30. The inner wall 40 has a circumference such that a cylindrical air gap 42 is present between the outer casing 16 and the inner wall 40. The cylindrical air gap 42 is fluidly connected to the gas inlet 20, and represents a first pressurised space.

The inner wall 40 defines a substantially cylindrical internal space 44. The internal space 44 is fluidly connected to the first pressurised space 42 by a metering orifice 46. The metering orifice 46 is substantially circular. It will be appreciated that static pressure within the internal space 44 will thus be substantially equal to that in the first pressurised space 42. The internal space 44 thus represents a second pressurised space.

The second pressurised space 44 is bordered at a lower end thereof by the valve element 26. The second pressurised space 44 is bordered at an upper end thereof by an inward radially extending portion 48 of the inner wall 40. The inwardly radially extending portion 48 has a centrally located aperture 50.

The solenoid actuator 14 includes a coil winding 52 wound about a bobbin 54. The bobbin 54 has an internal diameter similar to that of the outer casing 16 of the valve assembly 12. The coil 52 is enclosed on an outer side thereof by an outer case 56. A major pole 58 extends internally of the bobbin 54 at a lower end thereof. The major pole extends from a lower end 60 adjacent the radially extending portion 48 of the inner wall 40, below the bobbin 54, to an upper end 62 within the bobbin 54. The major pole 58 extends along approximately 40% of the length of the coil 52. It will be understood that the major pole extends along as far as 50% of the length of the coil 52, and will be particularly efficacious if the extent is more than about 30%. In the embodiment shown in Figure 2 the major pole 58 provides a seal between the inner wall 40 and the outer casing 16 at an upper end of the first pressurised space 42.

The major pole 58 has an internal bore 64 which is aligned with the central axis 18. A thin, substantially cylindrical sleeve 66 is located internally of the bobbin 54, and has a lower portion external of an upper portion of the major pole 58. The sleeve 66 extends downwardly from a substantially cylindrical covering portion 68, having a lower radial surface 70. A cylindrical space 72 is located within the sleeve 66, bordered at an upper edge by the lower radial surface 70 and at a lower edge by the upper end 62 of the major pole 58.

The sleeve 66 is preferably constructed of a weakly ferromagnetic material having high electrical resistivity. Preferably the material has a relative magnetic permeability of about 1.003 to 1.005, assessed at a magnetic field strength of around 16 kiloamperes per metre. Preferably also the material has a resistivity of about 70 micro-ohm centimetres.

An inner pole 74 comprises an annular portion 76 located immediately above the bobbin 54, and a cylindrical portion 78 extending within the bobbin, towards the major pole 58. The cylindrical portion 78 extends along approximately 25% of the length of the coil 52, although it is envisaged that an extent of between 20% and 30% would produce a particularly useful effect. The cylindrical portion 78 is outside the sleeve 66. The annular portion 76 has a lower edge substantially axially level with the lower radial surface 70. The solenoid actuator 14 includes a cylindrical armature 80 located within the cylindrical space 72. The armature 80 has an outer diameter slightly smaller than an inner diameter of the sleeve 66. A relatively small axial clerestory space 82 exists between an upper edge of the armature 80 and the lower radial surface 70.

A working air gap 84 exists between a lower edge of the armature 80 and the upper end 62 of the major pole 58. A pushrod 86 extends downwardly from the armature 80, and is rigidly connected to the armature 80. The pushrod 86 extends through the internal bore 64 of the major pole 58, and through the centrally located aperture 50 of the radially extending portion 48 of the inner wall 40. The pushrod 86 terminates at a lower end of the second pressurised space 44 adjacent the valve element 26. The pushrod 86 is supported within the internal bore 64 by a bearing 88 The armature 80 has a centrally located recess 90 in the upper edge thereof, adjacent to and corresponding to a centrally located recess 92 in the lower radial surface 70 of the covering portion 68. A secondary resilient biasing means in the form of an auxiliary compression spring 94 is located within the recesses 90, 92. The major pole 58 further includes a fluid path between the centrally located aperture 50 of the inner wall 40, and hence the second pressurised space 44, and the cylindrical space 72. In the embodiment of Figure 2 the fluid path comprises an axial vent path 96 within the major pole 58 and a radial transfer duct 98 within the major pole 58 adjacent the upper end 62. In this way the static pressure within the cylindrical space 72 is substantially equal to that of the first and second pressurised spaces 42, 44. The cylindrical space 72, including the clerestory space 82, thus represents a third pressurised space.

The gas injector further includes a sealable diagnostic port 100 located within the covering portion 68. The diagnostic port 100 comprises a channel 102 which is fluidly connected to the clerestory space 82 and a removable capping member 104 which seals the channel 102.

In use, the valve assembly 12 is moveable between a closed configuration shown in Figure 1 in which the valve element 26 is sealed against the valve seat 24, and an open configuration in which the valve element 26 is disengaged from the valve seat 24.

When in the closed configuration, gas is supplied to the gas injector at pressure through the gas inlet 20. The gas is freely moveable between the first, second and third pressurised spaces and thus the gas pressure in each of these spaces is substantially equal. In the closed configuration the valve element is being acted on by a spring force supplied by the main spring 30 through the spring pads 38, a spring force supplied by the auxiliary spring 94 through the pushrod 86, and the pressure differential between the second pressurised space 44 and the gas outlet 22. In the closed configuration these forces achieve an equilibrium with the valve element 26 urged against the valve seat 24.

When the coil 52 of the solenoid assembly 14 is energised, a magnetic field is created between the inner pole 74 and the major pole 58 and the armature 80 is moved through the working air gap 84 to the upper end 62 of the major pole 58. The geometry of the solenoid assembly 14, with the cylindrical portion 78 of the inner pole 74 extending outside the sleeve 66 for at least half the axial length of the armature 80, reduces the reluctance of the solenoid and means that the turn-on current required to move the armature 80 is lower than in prior art solenoids such as that of Figure 1. The provision of magnetic flux through the weakly ferromagnetic sleeve 66 assists in reducing the reluctance, and the resistivity of the sleeve 66 reduces eddy currents, thereby promoting efficiency of the solenoid and decreasing delays in both developing and collapsing a magnetic field within the solenoid. The pushrod 86 is urged against the valve element 26, providing a downward force sufficient to overcome the force provided by the main spring 30 and moving the valve assembly 12 into the open configuration. It will be appreciated that the force provided by the pushrod is augmented by the pressure differential between the gas inlet 20 and the gas outlet 22.

When the valve assembly 12 is in the open configuration, gas can flow freely along a path comprising the gas inlet 20, the first pressurised space 42, the metering orifice 46, the second pressurised space 44, the gap between the valve seat 24 and valve element 26, and the gas outlet 22. The minimum flow path along the path is the metering orifice 46. As such, accurate sizing of the metering orifice 46 provides a primary control of the rate of gas flow through the open valve. It is considered a significant advantage of the present invention that the sizing of the metering orifice 46 does not rely on the fit between two parts as in the prior art of Figure 1. It will be appreciated also that the circular nature of the metering orifice 46 provides an optimum surface-to-volume ratio to reduce the impact of fouling over time. A further advantage of the metering orifice of the present invention is that variations in the stroke of the armature do not significantly affect the gas flow rate. It will be appreciated that the opening of the valve assembly 12 will cause a pressure reduction in the second pressurised space 44. This will in turn promote the flow of a quantity of gas from the third pressurised space 72 to the second pressurised space 44. This flow will be reversed upon closure of the valve. The reversing gas flow within the third pressurised space 72 creates a fluid purging action in the clerestory space 82, thus preventing the build up of contaminants.

When the coil 52 is de-energised, the main spring 30 will act on the valve element 26 to re-seal the valve element 26 against the valve seat 24. As the gas pressure acting on the up-stream side of the valve element 26 is slightly smaller than the equilibrium pressure obtained once shut, the geometry of the present invention results in a more positive closing action on the valve assembly 12 than in the prior art. This effect is enhanced by the presence of the metering orifice up-stream of the valve seat 24. As a result, the turn-off current for the solenoid assembly 14 is considerably higher than in the prior art shown in Figure 1 , thus allowing use of a larger valve element 26. The turn-off current for the preferred embodiment of the invention described is in the order of 1250ma.

At the moment the valve element 26 impacts that valve seat 24, the pushrod 86 will disengage from the valve element 26, thus greatly reducing the mass impacting the valve seat 24 when compared to the poppet valve typical of prior art, and reducing any propensity for resonance or bounce.

It will be appreciated that in the preferred embodiment of the invention the force supplied by the main spring 30 can be calibrated by adjustment of the location of the spring stop 36. Under normal operation the maximum gap between the valve element 26 and valve seat 24 is determined by the size of the working air gap 84. In other words, the stroke ends when the armature 80 impacts the upper end 62 of the major pole 58. Any resultant "sticking" force is not sufficiently high to significantly effect the positive closing force subsequently applied. This is in contrast to the prior art of Figure 1 in which such a sticking force would interfere with closing of the valve. The prior art requires the maximum gap to be determined by the location of the stopping member 130. In the prior art both opening and closing of the valve can result in relatively high impact forces, causing recoils, "bounce", and resonance effects, hi the present invention kinetic energy is largely dissipated by springs 30 and 94, reducing the wear damage on rigid elements. A further advantage of the present invention is that the auxiliary spring 94 acts to precisely locate the armature 80 and pushrod 86 whilst the valve assembly 12 is closed, thus allowing any overshooting of the armature 80 on closure to be corrected before the next opening of the valve assembly 12. In an alternative embodiment of the present invention, shown in Figure 3, a stopping member 106 is fixed in position relative to the main spring 30 by a locknut 108. The stopping member 106 is fixed in a position such that the valve element 26 impacts the stopping member 106 substantially simultaneously with the impact of armature 80 and the major pole 58. Wear on the armature 80 and major pole 58 is thus reduced.

The diagnostic port 100 provides access, in use, to the clerestory space 82. This provides for measurement of the injector function, for instance measurement of the stroke. It also provides an opportunity to supply cleaning fluids and/or lubrication agents into the clerestory space 82 and thence through the third and second pressurised spaces, 72, 44 through to the gas outlet 22.

In an additional aspect of the present invention, the electrical driver is adapted so as to switch from a variable peak current to a holding current following a set peak duration.

The peak duration at which the switch occurs is a predetermined correction function of the supply voltage, as shown in Figure 4. Further, this duration may be controlled as correction functions of other variables, for instance the differential pressure measured between inlet and outlet of the injector. An example of a correction function for the peak duration is shown in Figure 5. Each of the correction functions may be combined in a linear manner to determine the correct peak duration. In this aspect of the invention, the peak duration is selected so as to provide the optimum amount of stored energy in the coil's magnetic field. This has the effect of reducing the turn-off delay in the valve, that is the period of time between the end of the supplied electrical current to the solenoid and the moment the valve fully closes.

This advantage is particularly evident in cases where the valve is open for a sufficiently small time that the hold current is not properly established by the time valve closure is required.

Modifications and variations as would be apparent to a skilled addressee are deemed to be within the scope of the present invention.

Claims

1. A gas injector comprising a valve assembly and an associated solenoid actuator, the gas injector having a gas inlet and a gas outlet, the valve assembly comprising a substantially spherical valve element arranged to seal against a valve seat and prevent flow of gas between the gas inlet and the gas outlet, characterised in that the valve assembly includes a first resilient biasing means supplying a force against the valve element to urge the valve element into sealing engagement with the valve seat, and the actuator includes a pushrod arranged to overcome the force supplied by the biasing means and selectively disengage the valve element from the valve seat in order to allow the flow of gas between the gas inlet and the gas outlet, a pressure differential between the gas inlet and the gas outlet acting against the valve element in substantially the same direction as the pushrod and against the force supplied by the biasing means.

2. A gas injector as claimed in claim 1, characterised in that the valve includes a metering orifice located between the gas inlet and the gas outlet.

3. A gas injector as claimed in claim 2, characterised in that the metering orifice is located between the gas inlet and the valve element.

4. A gas injector as claimed in claim 2 or claim 3, characterised in that the metering orifice represents a minimum flow path between the gas inlet and the gas outlet when the valve element is disengaged from the valve seat.

5. A gas injector as claimed in claim 4, characterised in that the metering orifice comprises a substantially circular hole within an inner wall of the valve assembly.

6. A gas injector as claimed in any preceding claim, characterised in that the gas inlet is fluidly connected to a pressurised space about a portion of the pushrod adjacent the valve element.

7. A gas injector as claimed in claim 6, characterised in that the solenoid actuator includes a working air gap, the working air gap being fluidly connected to the pressurised space.

8. A gas injector as claimed in claim 7, characterised in that the solenoid actuator includes a clerestory space, the clerestory space being fluidly connected to the working air gap.

9. A gas injector as claimed in any preceding claim, characterised in that the solenoid actuator comprises a coil winding about a bobbin, the solenoid actuator including a major pole extending internally of the bobbin.

10. A gas injector as claimed in claim 9, characterised in that the major pole extends along 30% to 50% of the length of the coil.

11. A gas injector as claimed in claim 10, characterised in that the major pole extends along about 40% of the length of the coil.

12. A gas injector as claimed in any one of claims 9 to 11, characterised in that the solenoid actuator includes an inner pole opposed to the major pole, the inner pole extending internally of the bobbin.

13. A gas injector as claimed in claim 12, characterised in that the inner pole extends along 20% to 30% of the length of the coil.

14. A gas injector as claimed in claim 13, characterised in that the inner pole extends along about 25% of the length of the coil.

15. A gas injector as claimed in any one of claims 12 to 14, characterised in that the solenoid actuator includes a sleeve located substantially internally of the bobbin, the major pole having a portion within the sleeve and the inner pole having a portion outside the sleeve.

16. A gas injector as claimed in claim 15, characterised in that the sleeve is constructed of a weakly ferromagnetic material having high electrical resistivity.

17. A gas injector as claimed in claim 15 or 16, characterised in that the solenoid actuator includes an armature located within the bobbin, the armature having an axial length, the solenoid actuator being arranged such that the inner pole extends outside the sleeve for at least half the axial length of the armature.

18. A gas injector as claimed in any preceding claim, characterised in that the gas injector includes a secondary resilient biasing means arranged to provide a force to the pushrod in opposition to that of the first resilient biasing means, such that when the valve element is in sealing engagement with the valve seat the valve element is acted on by forces supplied by the first resilient biasing means in one direction and the second resilient biasing means and the pressure differential in a second direction.

19. A gas injector as claimed in any preceding claim, characterised in that the valve seat includes a guide ring to restrict lateral movement of the valve element.

20. A gas injector as claimed in any preceding claim, characterised in that the gas injector includes a plurality of gas inlets circumferentially spaced about a first axial location on the valve assembly.

21. A gas injector as claimed in any preceding claim, characterised in that the gas injector includes a plurality of gas outlets circumferentially spaced about a second axial location on the valve assembly.

22. A gas injector as claimed in any preceding claim, characterised in that the first resilient biasing means is maintained in a desired axial location by a spring stop.

23. A gas injector as claimed in any preceding claim, characterised in that the pushrod is arranged to disengage from the valve element at the moment the valve element contacts the valve seat.

24. A gas injector as claimed in any preceding claim, characterised in that the gas injector includes a stopping member fixed in position in relation to the first resilient biasing means, the stopping member acting to prevent movement of the valve element further than a working distance of the solenoid actuator.

25. A gas injector comprising a valve assembly and an associated solenoid actuator, the gas injector having a gas inlet and a gas outlet, the valve assembly including a valve element and a valve seat, characterised in that the gas injector includes a sealable diagnostic port, the diagnostic port being fluidly connected to a pressurised space between the gas inlet and the valve element.

26. A gas injector comprising a valve assembly and an associated solenoid actuator, characterised in that the solenoid actuator is associated with an electrical driver which during actuation is arranged to switch from an initial current to a holding current after a defined peak duration, the peak duration being a function of a supply voltage.

27. A gas injector as claimed in claim 26, characterised in that the peak duration is further controlled by correction functions of other variables.

28. A gas injector as claimed in claim 27, characterised in that one correction function is a function of the differential pressure measured between a gas inlet and a gas outlet.

29. A gas injector as claimed in claim 27 or claim 28, characterised in that in the correction functions are combined linearly.

30. A gas injector as claimed in any one of claims 26 to 30, characterised in that the peak duration is selected so as to provide the optimum amount of stored energy in the solenoid actuator.