WO1992021905A1

WO1992021905A1 - Crash proof solenoid controlled valve for natural gas powered vehicles

Info

Publication number: WO1992021905A1
Application number: PCT/US1992/004638
Authority: WO
Inventors: Lloyd G. Wass; Michael R. Baird; Peter R. Nelson
Original assignee: Wass Lloyd G; Baird Michael R
Priority date: 1991-05-30
Filing date: 1992-05-28
Publication date: 1992-12-10
Also published as: EP0586578A4; JPH06508422A; EP0586578A1

Abstract

A solenoid controlled valve (12) has a body (16) mounted within a compressed natural gas vessel (10), with a head (18) exposed outside the vessel (10). The solenoid controlled valve (12) includes a solenoid poppet (98) within a main bore (60) of the valve body (16). A valve seat (80) positioned between the solenoid poppet (98) and an outlet (74) has a flow passage (86) which communicates between the outlet (74) and, via an orifice (84), the main bore (60). A bias spring (104) biases the solenoid poppet (98) toward the orifice (84) to effect a seal (100) of the flow passage (86). A plunger (96) connected to the solenoid poppet (98) disengages the seal (100) when a solenoid coil (95) is energized.

Description

CRASHPROOFSOLENOID CONTROLLEDVALVE

FORNATURALGASPOWEREDVEHICLES

BACKGROUND OF THE INVENTION The present invention relates to gas valves. In particular, the present invention is a solenoid controlled valve for controlling the flow of a compressed gaseous fuel (such as compressed natural gas) from a pressure vessel that utilizes the pressure vessel as a damage shield or protective "cocoon" for the solenoid valve.

With the increasing concern over air pollution caused by vehicles using internal combustion engines, and with the prospect of increasingly strict emission standards for urban vehicles with internal combustion engines, attention has been directed to use of alternate fuels such as compressed natural gas (CNG) as a fuel for vehicles such as cars, trucks and buses. The compressed natural gas is stored in a pressure vessel, and flow of the gas from the pressure vessel to the engine is controlled by a gas shut off valve.

A gas valve used in a vehicular application can be exposed to a wide variation of operating temperatures. For example, if the compressed natural gas tank is filled in the early morning when the outdoor temperature is relatively low, and the vehicle is parked outside on a blacktop asphalt surface during the heat of the day, the gas pressure within the pressure vessel can rise dramatically (from, for example, a nominal working pressure of about 3,600 psi to close to 5,000 psi) . In the winter a vehicle may be fueled in frigid outdoor conditions and moved to a heated indoor garage. The gas valve must be capable of operating reliably over a wide temperature and pressure range. Another major concern is the vulnerability of the gas valve to crash damage. If the vehicle is involved in an accident, the gas valve must not fail in a unsafe or catastrophic manner. Also the valve should automatically return to a normally closed position upon any indication of a problem such as interruption of electric power or activation of a safety device such as an air bag.

SUMMARY OF THE INVENTION The gas valve of the present invention is a solenoid-controlled valve which includes a valve body having a neck which extends into the pressure vessel. The valve body also includes a head which is attached to the neck and which is located outside of the pressure vessel. A first bore extends from the inner end of the valve body^* through the neck to an outlet port in the head.

A valve seat is mounted in the first bore. The valve seat has an orifice at an inner end and a flow passage which extends between the orifice and the outlet port. In one embodiment, the flow passage includes a threaded portion capable of accepting a manually actuated sealing member. A solenoid poppet valve which is mounted to the inner end of the neck controls flow of gas from the pressure vessel through the orifice and the flow passage to the outlet port. The solenoid poppet valve includes a seal for closing the orifice, means for biasing the seal toward the seat to close the orifice, a plunger connected to the seal, and a solenoid coil for moving the plunger so tnat the seal moves out of engagement with the orifice.

A sealing member having a threaded exterior surface, a sealing surface and a hexagonal opening is provided to manually interrupt the flow of compressed natural gas from the flow passage to the outlet port. Normally in an open position, the sealing member can be manually actuated to position the sealing surface across the flow passage of the valve seat in the unlikely event of a valve failure or a leak of the poppet seal.

In preferred embodiments of the present invention, a "smart" control circuit for the solenoid valve is also mounted within the tank, so that it is not susceptible to crash damage. The control circuit for the solenoid is preferably mounted at the inner end of the solenoid with electrical leads extending out through the bore and a feed through passage to the exterior of the valve and the pressure vessel.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a top view of the valve of the present invention mounted in a pressure vessel.

Figure 2 is a sectional view along section 2-2 of Figure 1.

Figure 3 is a partial sectional view along section 3-3 of Figure 1.

Figure 4 is an electrical schematic diagram of the control circuit for the solenoid valve of the valve of the present invention.

Figure 5 is an exploded sectional view of another embodiment of the valve seat used in the valve of the present invention.

Figure 6 is an exploded sectional view of a third embodiment of the valve seat used in the valve of the present invention. Fi rure 7 is a sectional view of the valve of the present invention assembled with the override stem in an open position. Figure 8 is a sectional view of the valve shown in Figure 7 with the override stem in a closed position.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Figure 1 shows a top view of pressure vessel

10 and solenoid controlled valve 12 of a compressed natural gas system for use in a motor vehicle. Pressure vessel 10 has a collar 14 at which valve 12 is connected. Valve 12 includes a main valve body 16 with a hexagonal head 18 located outside of collar 14. Attached to separate faces of head 18 are fill fitting 20, outlet fitting 22, and strain relief 24. Pressure vessel 10 is filled with compressed natural gas through fill fitting 20. The flow of compressed natural gas from pressure vessel 10 is through outlet fitting 22. The flow of compressed natural gas from the interior of pressure vessel 10 to outlet fitting 22 is controlled by a solenoid control signal supplied to two-pin connector 26 and multi-conductor wire 28.

The sectional view of Figure 2 shows pressure vessel 10 and solenoid valve 12 in further detail. In this embodiment, pressure vessel 10 is a composite pressure vessel having a metal inner lining 40 and a fiber glass/polymeric outer cover 42. Collar 14 is metal, and is an integral extension of inner lining 40. Collar 14 has a central bore 44 with internal threads for engaging valve 12. At the outer end of collar 14 is an annular groove 48 in which O-ring 50 is provided to make a seal between head 18 and collar 14. Valve body 16 includes neck 52, which extends into bore 44 of collar 14. Neck 52 has external threads which mate with the internal threads of collar 14 to hold valve 12 in place. Valve body 16 has a main bore 60 which extends from the inner end of neck 52 into head 18. Main bore 60 has an internally threaded lower end portion 62, an unthreaded central portion 64, a shoulder portion 66, and an internally threaded upper end portion 68. Fill port 70, feedthrough port 72, and outlet port 74 (Figure 3) are connected to main bore 60.

Mounted within bore 60 is seat 80. Upper end 80A of seat 80 has external threads which mate with the internal threads of upper end portion 68 of main bore 60. 0-ring 82 provides a seal between seat 80 and valve body 16. At its lower or inner end 80B, seat 80 has an orifice 84. Flow passage 86 extends from orifice 84 to the upper end 80A of seat 80. Attached to the lower or inner end of neck 52 is a solenoid poppet valve assembly which includes mount 90, jam nut 92, solenoid 94 (which includes coil 95 and plunger 96), poppet 98, seal 100, cap 102, return spring 104, and solenoid control circuit 105. Mount 90 has a threaded upper end which engages the internal threads of lower end portion 62 of bore 60. Mount 90 includes passages 106 through which gas can flow between main bore 60 and the interior of pressure vessel 10. The lower end of mount 90 has internal threads which engage external threads on neck 108 of solenoid 94. Jam nut 92 is also attached on threaded neck 108 between mount 90 and solenoid 94.

Poppet 98 has a threaded stud 110 which is threaded into the upper end of plunger 96. At its upper end, poppet 98 has a recess for holding seal 100, and has external threads onto which cap 102 is threaded. Cap 102 holds seal 100 in place, and also provides a bearing surface for the upper end of coil spring 104. The lower end of spring 104 engages the upper end of neck 108 of solenoid 94.

Solenoid control circuit 105 has a threaded stud 112 for attaching control circuit 105 to the lower end of solenoid 94. Multi-conductor wire 114 extends from control circuit 105 to solenoid 94 to provide the drive signal for energizing solenoid coil 95.

In the view shown in Figure 2, solenoid 94 is in an unenergized condition. In this condition, bias spring 104 has urged poppet 98, plunger 96, seal 100, and cap 102 upward so that seal 100 engages and closes orifice 84. In this position, compressed natural gas within pressure vessel 10 is not permitted to pass through orifice 84 and through flow passage 86 to outlet port 74. Because a gaseous fuel such as CNG is inherently slower refueling than conventional gasoline or diesel fuel, the ideal CNG valve should have a "fast fill" port that is independent of the valve outlet port and is sized significantly larger. This is also desirable from a safety standpoint since this allows the storage cylinder to be refueled with the control valve in the closed position.

Fill fitting 20 is mounted within fill port 70. In the embodiment shown in Figure 2, fill fitting 70 includes an internal fill check valve 120. In Figure 2, fill check valve 120 is shown in its inner most position, which it assumes when the pressure at the inlet of fill fitting 20 exceeds the pressure within main bore 60. In that case, fill check valve 120 moves inward until it is stopped by seat 80, and gas is permitted to flow from the inlet of fill fitting 20 into bore 60, and then through openings 106 into the interior of pressure vessel 10. Openings 106 may also contain a porous metal filter to prevent contamination of seal 100.

In normal operation, when fill fitting 20 is not connected to a source of high pressure gas, fill check valve 120 moves outward to block the flow of gas out of fill fitting 20. Fill check valve 120 also acts as a safety device during filling in the event of fill hose rupture.

Multi-connector wire 28 (which is connected to two-pin connector 26) passes through strain relief 24 and through feedthrough port 72 into main bore 60. Feedthrough seal 122 is located within feedthrough port 72 and surrounds wire 28. Feedthrough seal 122, which preferably utilizes components of an engineering plastic such as Delrin or an eguivalent, and an elastomer such as SBR or an equivalent, prevents any leakage of gas out of valve 12 through feedthrough port 72.

Wire 28 passes through main bore 60 and through one of the openings 106 in mount 90. The end of wire 28 is connected to control circuit 105, as shown in Figure 2.

As shown in Figure 3, outlet fitting 22 is threaded into outlet bore 74. Passage 130 connects outlet bore 74 with the upper end 68 of main bore 60. When solenoid 94 is de-energized, orifice 84 is closed (as shown in Figure 2 ) , and no gas can flow through orifice 84 and passage 86 to passage 130 and outlet bore 74. On the other hand, when solenoid 94 is energized, seal 100 is moved out of engagement with orifice 84. That permits natural gas within pressure vessel 10 to flow from main bore 60 through orifice 84 and passage 86 to the upper end 68 of bore 60. The gas can then flow through passage 130 to outlet bore 74, and through outlet fitting 22 to the outlet hose or tubing (not shown) which carries the natural gas to the internal combustion engine (not shown) .

Valve 12 is actuated to permit gas flow from pressure vessel 10 to outlet fitting 22 by a low voltage (6-24 volt) DC control signal supplied to two-pin connector 26. When the control signal is supplied through wire 28 to control circuit 105, a drive signal is supplied by control circuit 105 through multi- conductor wire 114 to solenoid coil 95. This drive signal causes solenoid coil 95 to pull solenoid plunger 96 into coil 95. In other words, plunger 96 moves downward from its position shown in Figure 2. When plunger 96 moves downward, it pulls poppet 98 and seal 100 away from orifice 84 of seat 80. This allows gas to flow through orifice 84 and passage 86 to outlet fitting 22.

In a preferred embodiment of the present invention, maximum power is initially supplied to solenoid coil 95 to insure that adequate force is available to pull solenoid plunger 96 into coil 95 and move seal 100 away from orifice 84.

After the solenoid poppet valve has been energized (by the energization of coil 95) and seal 100 has been moved away from seat 80, no pressure drop is present at orifice 84 (as there is when seal 100 is in engagement with seat 80) . As a result, less force is required to hold the valve open than is required to open the valve in the first place. Control circuit 105 is a "smart" circuit that supplies a "stepped down" power level to coil 95 on a continuing basis once the valve is opened.

This multi-level energization of solenoid coil 95 is important, because solenoid coil 95 is made as small as possible in order to minimize the size cf bore -9-

44 in collar 14. If the maximum power level were supplied on a continuous basis to solenoid coil 95, a dangerous temperature rise could occur in solenoid coil 95 in its insulated environment inside the cylinder. The stepped down power level assures that power can be dissipated by a relatively small solenoid coil, while still obtaining the initial force which is required to open the valve. Because vehicle weight is directly related to fuel economy, the ideal CNG valve must be compatible with light weight, high strength "space age" composite, high pressure gas storage cylinders typically constructed with a relatively thin aluminum liner over wrapped (wound) with a fiberglass/epoxy resin matrix. Vehicular space limitations availability (usually under carriage) on smaller vehicles such as vans and pickup trucks limit pressure vessel diameters to about 9" diameters, which in turn limit the vessel port openings to about 1 ¹/4" diameter so as to allow for fiber glass wind angle optimization to reduce/minimize vessel cost and weight. This relatively small cylinder opening coupled with the relatively high operating pressure requirements dictate that a small diameter, high powered solenoid be used. A solenoid of this type would rapidly over heat and burn up when installed in the "insulated" environment inside the pressure vessel unless protected by a "smart" electronic circuit that limits current flow after the valve has been opened.

Figure 4 shows a diagram of control circuit 105. In this embodiment, circuit 105 includes a pair of input terminals 150 and 152 (which are connected to conductors 28A and 28B of wire 28) , a pair of output terminals 154 and 156 (which are connected through conductors 114A and 114B of wire 114 to solenoid coil 95), drive circuit 157, FET switch 158, and diode 160. Control circuit 105 provides the two level energization of coil 95 by controlling the current through FET switch 158. Drive circuit 157 initially powers coil 95 by turning on FET 158 for a period which is long enough to shift plunger 96 and open the valve. Thereafter, drive circuit 157 applies control pulses to FET 158 to apply a pulsed energization to solenoid coil 95. Diode 160 allows current flowing in solenoid coil 95 to continue to flow when FET 158 shuts off. The pulses are at a predetermined pulse width and rate so that a lower power level is supplied to coil 95. The stepped down (pulse width modulated) power level can be adjusted by changing either the pulse width or the rate of the pulses (or both) . In vehicular systems that utilize a short fuel line (small volume) the relatively fast opening of a solenoid control valve can result in a violent shock to the first stage, downstream regulator under conditions encountered with a fully charged cylinder at high temperature.

This shock wave (which can significantly shorten the life expectancy of the regulator) can be essentially eliminated with the inclusion of an internal surge protection poppet valve, as illustrated in Figure 5. This surge protection valve will also act as a flow control device in the event of a severed fuel line or catastrophic regulator failure.

Figure 5 shows an alternative embodiment of the valve seat in a sectional, exploded view. Valve seat 180 of Figure 5 replaces valve seat 80 in valve 12, and provides pressure surge protection and a flow control option. Valve seat 180 includes seat body 182, poppet housing 184, poppet 186 and spring 188. Flow passage 190 extends through seat body 182 and forms an orifice 192 at the lower end of seat body 182. The upper end of seat body 182 has external threads 194 which mate with internal threads 196 of poppet housing 184. Poppet 186 is located within cavity 198 of poppet housing 184, and is normally urged by spring 188 toward seat body 182 so that there is substantially unrestricted flow from passage 190 through cavity 198 and out through passage 200 in the upper end of poppet housing 184.

When force on poppet 186 exceeds the bias force of spring 188, poppet seats against shoulder 202.

This reduces the instantaneous pressure surge by limiting flow while the poppet valve is closed to flow through passage 204 in poppet 186.

To maximize crash resistance, a "crash proof" CNG solenoid control valve must have a absolute minimum amount of surface exposure outside the tank - preferably no more than is required to accommodate an inlet fitting, an outlet fitting, and an electrical connector.

The present invention provides a solenoid controlled valve for compressed natural gas operated vehicles with a well-protected valve package. The solenoid and the controlled circuitry for the solenoid are "buried", and are preferably located within the interior of the pressure vessel 10. As a result, the possibility of crash damage causing a malfunction of valve 12 is greatly reduced.

The solenoid operated poppet valve action of valve 12 permits a two level energization of solenoid coil 95. A maximum level is required for initially opening the valve, and a lower power level can be used for maintaining the valve in an open condition. As a result, power dissipation in solenoid coil 95 is reduced, which in turn allows the size of solenoid coil 95 to be maximized in relation to the available cylinder opening. That in turn makes it practical to mount solenoid coil 95 (and solenoid control circuit 105) at the inner end of valve body 16, rather than on the exterior of the valve 12.

Figure 6 shows a third embodiment of the valve seat used in the valve of the present invention. Figure

6 shows valve body assembly 300, which includes valve body 302, valve seat 304, override stem 306, override stem retainer 308 and threaded cap 310.

Valve body 302 is configured similar to main valve body 16 shown in Figure 2, with one exception: valve body 302 includes threaded opening 312, which is configured to accept override stem retainer 308. Threaded opening 312 communicates with main bore 314 and transverse flow passage 316 of valve body 302. Transverse flow passage 316 further communicates with outlet port 318, which has internal threads to accept an outlet fitting similar to outlet fitting 22 shown in Figure 3. Main bore 314 includes internally threaded upper portion 320 and internally threaded lower end portion 322 similar to the embodiment shown in Figure 2. Radial shoulder 323 is provided at the intersection of main bore 314 and transverse flow passage 316. Radial shoulder 323 serves as a sealing surface against which a seal may be formed when override stem 306 is properly inserted within internal threaded portion 332 of valve seat 304. Valve seat 304 is configured similar to seat

80 shown in Figure 2. Valve seat 304 includes an externally threaded upper end 324, lower end 326, orifice 328, and flow passage 330. Like seat 80 shown in Figure 2, flow passage 330 extends from orifice 328 to upper end 324; however, unlike flow passage 86 of seat 80, flow passage 330 includes internal threaded portion 332. Internal threaded portion 332 is configured to accept override stem 306. Override stem 306 is a generally cylindrical- shaped member having externally threaded lower portion 336 and upper portion 338. Upper end portion 338 includes notch 340, which holds O-ring 342 (shown in Figure 7) . Upper end portion 338 also includes hexagonal-shaped opening 344, which is fashioned to accept an Allen wrench for rotating override stem 306 in a clockwise or counter-clockwise direction within valve seat 304. Lower end portion 336 includes opening 346, which extends the length of lower end portion 336 and terminates at upper end portion 338. Opening 346 communicates with flow passage 330 when override stem 306 is inserted within valve seat 304. Side holes 348 are positioned adjacent radially tapered shoulder 350 of upper end portion 338. Side holes 348 communicate between opening 346 and transverse flow passage 316 when override stem 306 is inserted within valve seat 304 in an open condition (shown in Figure 7) . Radially tapered shoulder 350 provides a surface on override stem 306 which is capable of forming a seal against radial shoulder 323 when override stem 306 is in a closed position (shown in Figure 8) .

Override stem retainer 308 serves as a cover for override stem 306 and ensures proper alignment of holes 348 with transverse flow passage 316 when override stem 306 is in the open position (as shown in Figure 7) . Override stem retainer 308 includes externally threaded end 352, which permits override stem retainer 308 to be threaded within internally threaded opening 312 of valve body 302. Override stem retainer 308 includes a longitudinal bore 354, which extends from threaded end 352 to hexagonal-shaped cap end 356. Bore 354 includes smooth cylindrical bore 356 and internally threaded portion 358. Smooth cylindrical bore 356 is separated from internally threaded portion 358 by bearing surface 360. Smooth cylindrical bore 356 is sized to allow upper end portion 338 of override stem 306 a free range of longitudinal and axial movement when upper end portion 338 is inserted within override stem retainer 308. As shown in Figures 7 and 8, O-ring 342 provides a movable seal of bore 354 regardless of the position of override stem 306 within override stem retainer 308. As shown in Figure 7, when override stem 306 is in an open position, upper end portion 338 of override stem 306 engages bearing surface 360 to ensure that holes 348 are aligned with transverse flow passage 316.

Cap 310 includes threaded stud 362 and hexagonal-shaped head 364. Threaded stud 362 is sized to be threaded within threaded portion 358 of bore 354. O-ring 366 provides a secondary seal of bore 354 when cap 310 threaded within threaded portion 358 of bore 354 and override stem 306 is in an open position.

Figure 7 is a sectional view of the third embodiment of the valve seat showing the override stem in an open position. Figure 7 represents the normal position of override stem 306 during normal operation of the solenoid valve. Externally threaded upper end 324 of valve seat 304 is threaded into internally threaded upper portion of main bore 314, with O-ring 367 providing a seal between valve seat 304 and valve body 302. As shown in Figure 7, a flow path exists from flow passage 330, through opening 346, out openings 348, and through transverse flow passage 316 to outlet port 318. The addition of override stem 306 to valve body assembly 302 enhances the flexibility and safety of the solenoid valve of the present invention. For instance, in the unlikely event of a failure of the valve (i.e, a leak or seat failure) , fluid flow through valve seat 304 to outlet 318 can be halted by removing cap 310, inserting an Allen wrench into hexagonal opening 344 of override stem 306, and rotating override stem 306 in a clockwise fashion to effect a seal between radially tapered surface 350 and radial shoulder 323 (shown in Figure 8) . As override stem 306 is rotated in a clock-wise fashion, holes 348 become disaligned with transverse flow passage 316. O-ring 342 ensures that no fluid passes through bore 354 of override stem retainer 308.

The addition of override stem 306 to valve seat 304 also enhances the flexibility of valve body assembly 300. For instance, where pressure testing of fuel lines downstream from the solenoid valve is necessary, override stem 306 can be advanced to the closed position (shown in Figure 8) thereby isolating the solenoid valve and compressed natural gas from the lines. Override stem 306 provides a simple, inexpensive, and redundant means for addressing safety concerns of the solenoid control valve of the present invention while improving the versatility of the valve. Although the present invention has been described with reference to preferred embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention.

Claims

WHAT IS CLAIMED IS:

1. A valve for controlling flow of compressed gas from a pressure vessel, the valve comprising: a valve body having a head and a neck; the head having an outlet port; the neck having external threads for engagement with the pressure vessel, having an inner end, and having a bore which extends from the inner end to the outlet port; a valve seat mounted in the bore and having an orifice at an inner end and a flow passage extending between the orifice and the outlet port; a solenoid valve mounted to the inner end of the neck, the solenoid valve including a seal for closing the orifice, means for biasing the seal toward the seat to close the orifice, a plunger connected to the seal, and a solenoid coil for moving the plunger so that the seal moves out of engagement with the orifice.

2. The valve of claim 1 and further comprising: a control circuit for controlling operation of the solenoid valve.

3. The valve of claim 2 wherein the control circuit is mounted to an inner end of the solenoid valve.

4. The valve of claim 3 and further comprising: a feedthrough port in the head which communicates with the bore; and a wire extending from outside the valve through the feedthrough port and the bore to the control circuit.

5. The valve of claim 2 wherein the control circuit applies a control signal at a first power level to the solenoid valve during a first time period sufficient to permit the seal to be moved out of engagement with the orifice.

6. The valve of claim 5 wherein the control circuit apples the control signal of a second, reduced power level to maintain the valve open after the first time period.

7. The valve of claim 1 and further comprising: a fill port in the head which communicates with the bore; and a fill fitting mounted in the fill port.

8. The valve of claim 1 wherein the valve seat includes a surge protection poppet valve in the flow passage.

9. A valve for controlling flow of compressed gas from a pressure vessel, the valve comprising: a valve body having a head with an outlet port and having a neck which is inserted into and is connected to an opening in the pressure vessel; and a solenoid poppet valve mounted to the neck and having an outer dimension which permits the solenoid poppet valve to be located within the pressure vessel, the solenoid poppet valve being normally closed and opening in response to a control signal to permit gas flow from the pressure vessel to the outlet port.

10. The valve of claim 9 wherein the solenoid poppet valve comprises: a valve seat having an orifice and a flow passage leading from the orifice to the outlet port; a seal for closing the orifice; means for biasing the seal toward the seat; and a solenoid for moving the seal away from the seat in response to the control signal.

11. The valve of claim 10 wherein the valve seat includes a surge protection poppet valve in the flow passage.

12. The valve of claim 9 and further comprising: a control circuit for providing the control signal to the solenoid poppet valve.

13. The valve of claim 12 wherein the control circuit is mounted to an inner end of the solenoid poppet valve.

14. The valve of claim 12 wherein the control circuit applies the control signal at a first power level to open the solenoid poppet valve and at a second, reduced power level to maintain the solenoid poppet valve open.

15. A valve for controlling flow of compressed gas from a pressure vessel, the valve comprising: a valve body having a head and a neck; the head having an outlet port; the neck having external threads for engagement with the pressure vessel, having an inner end, and having a bore which extends from the inner end to the outlet port; a valve seat mounted in the bore and having an orifice at an inner end and a flow passage extending between the orifice and the outlet port; a manual override valve between the orifice and the outlet port for selectively sealing the flow passage; and a solenoid valve mounted to the inner end of the neck, the solenoid valve including a seal for closing the orifice.

16. A valve for controlling flow of compressed gas from a pressure vessel, the valve comprising: a valve body having a head with an outlet port and having a neck which is inserted into and is connected to an opening in the pressure vessel; a solenoid poppet valve mounted to the neck and having an outer dimension which permits the solenoid poppet valve to be located within the pressure vessel, the solenoid poppet valve being normally closed and opening in response to a control signal to permit gas flow from the pressure vessel to the outlet port; and a manual override valve, positioned between the solenoid poppet valve and the outlet port, for selectively blocking gas flow between the pressure vessel and the outlet port.

17. The valve of claim 16 wherein the solenoid poppet valve comprises: a valve seat having an orifice and a flow passage leading from the orifice to the outlet port; a seal for closing the orifice; means for biasing the seal toward the seat; and a solenoid for moving the seal away from the seat in response to the control signal.