A gas compressor to which the invention is particularly suitable is disclosed in WO 00/34655.

This document relates to a gas compressor for compressing gas from a domestic power supply and supplying the compressed gas to a tank on a vehicle.

The compressor comprises a pair of hydraulic rams which are directly linked to one another with each ram reciprocating in its own cylinder to alternately draw gas into the cylinder on a downstroke and compress and expel gas on the upstroke. While one of the rams is compressing the gas in the cylinder, the other is expanding each cylinder to draw gas into the cylinder.

The compressor may either be a symmetrical arrangement in which both rams have the same diameter, or may be an asymmetrical arrangement in which gas is compressed to an intermediate stage in a first ram, and is fed to the second ram for further compression.

Once the filling of the vehicle based tank is complete, the fuel hose coupling the compressor to the vehicle tank has to be uncoupled. According to WO 00/34655, a bypass and relief circuit is provided to reduce the gas pressure in the delivery hose after filling of the cylinder is complete. The present invention is directed to providing such a circuit which provides improved operation over a conventional bypass and relief circuit.

According to the present invention there is provided a gas compressor comprising a chamber in which a compression member is movable to compress the

gas; a fluid supply arranged to move the compression member; a compressed gas supply line to supply compressed gas from the chamber to a vessel; and a relief valve which is normally closed and which is connected to the fluid supply line such that it opens to relieve pressure in the compressed gas supply line when the fluid supply stops moving the compression member.

By arranging for the relief valve to be opened when the fluid stops moving the compression member the relief circuit is made failsafe. Thus, once compression is complete the fluid supply to the compressor is withdrawn, and this will trigger the opening of the relief valve to relieve pressure in the compressed gas supply line. This mechanism guarantees that there is no fuel-line delivery pressure in the fuel hose once refuelling is stopped. Further, as the mechanism can operate without electrical control, it can be made intrinsically safe.

Preferably, the relief valve comprises an inlet connected to the compressed gas supply line, a relief outlet communicating with the inlet, a valve member selectively movable between an open position in which the inlet and outlet are in communication with one another and a closed position in which it blocks communication between the inlet and outlet and in which the compressed gas in the supply line provides a net pressure force to maintain the valve member closed; a valve actuator being movable under the action of the fluid supplied to the compression member to move the valve member between the open and closed positions, the valve actuator having means to equalise the pressure exerted on the valve actuator by the compressed gas.

This provides a relief valve in which the actuator is pressure balanced in terms of the compressed gas, so that pressure fluctuations in the compressed gas will not affect the operation of the actuator. Further, as the compressed gas provides a net pressure force to maintain the valve member closed, any unexpected increase in compressed gas pressure will only serve to hold the valve element more firmly on its seat.

Preferably, the valve member is slidably retained by the valve actuator, and a spring is provided between the valve member and valve actuator to urge the valve member towards its seat.

Preferably, a shoulder is provided to engage with the actuator to prevent further travel of the actuator once the valve member has seated.

A return force on the actuator may be provided by known means, such as a further fluid or an electromechanical actuator. However, preferably, the return force is provided by a resilient member. This provides a simple mechanism and a consistent returning force.

The actuator is preferably a piston, and a passage is preferably provided linking the two sides of the piston to equalise the pressure exerted on the valve actuator. Preferably, this passage is a bore through the piston.

The compressed gas in the compressed gas supply line is preferably relieved back to another part of the gas supply circuit. One possibility is to vent it back to the low pressure gas supply source.

Alternatively, if the compressor is of the type which

compresses the gas in two stages, the pressure may be vented back to the intermediate stage of compression.

Preferably, a relief line through which fluid is bled from the relief valve comprises a first branch through which fluid may flow up to a certain rate, and a second branch, parallel to the first branch, which is closed by a valve which is arranged only to allow flow once a certain pressure is reached. With such an arrangement, if the pressure in the relief line is particularly high, the valve will open, allowing more rapid relief of the pressure in the relief line, and hence quick operation of the relief valve and more rapid relief of the gas. Such an arrangement can be particularly beneficial in a two-stage compressor, where the return hydraulic flow can be different between the two stages of compression.

Examples of a compressor constructed in accordance with the present invention will now be described with reference to the accompanying drawings, in which: Fig. 1 is a schematic diagram of the compressor showing the gas and hydraulic circuits incorporating the relief valve; Fig. 2 is a cross-section through the relief valve ; Fig. 3 is a cross-section similar to Fig. 2 showing an alternative relief valve; and Fig. 4 is a cross-section through a supplementary valve as used in Fig. 1.

The system consists of a hydraulic power circuit

linked directly and integrally with a gas compression circuit. A flexible fuel hose 1 with a quick release nozzle 2 is provided to deliver compressed gas to an external storage cylinder 3.

The hydraulic power circuit consists of a small electric motor 4 coupled to a hydraulic gear or piston pump 5 which pumps hydraulic fluid from the tank 6.

The hydraulic pump intake is connected via a filter (not shown) to a point on the tank which is gravitationally well below the fluid level. High pressure fluid output from the pump is connected to a spool type reciprocating shuttle valve 7, pressure relief valves 8,9 and two hydraulically opposed rams 10,11 having pistons 10A, 1lA which move within respective cylinders 10B, 11B. Each ram has a respective fluid connection 12,13 for flow/discharge to/from the shuttle valve. A low pressure discharge 14 from the shuttle valve 7 is connected to a tank 6.

A spill back valve 15 is provided between the high and low pressure parts of the hydraulic circuit to guard against over pressurising the high pressure part of the circuit.

The gas compression circuit consists of two cylinders, 20,21 at the opposite ends of the ram 10, 11 to the hydraulic circuit. These cylinders provide an intermediate compression cylinder 20 and a high pressure compression cylinder 21. The intermediate compression cylinder has an inlet 22 for gas from low pressure gas supply 23. In practice, this represents a connection to the domestic gas supply. A pressure relief valve 24 and vent 25 are provided on the low pressure gas supply. An intermediate gas outlet 26 leads from the intermediate compression cylinder 20 to the high pressure compression cylinder 21 where it forms an intermediate gas inlet 27 the high pressure

gas outlet 28 leads from the high pressure compression cylinder 21.

In practice, when high pressure hydraulic fluid is supplied to ram 11, the piston 11A is displaced downwardly causing gas to be sucked into intermediate compression cylinder 20 through the inlet 22. When the shuttle valve 7 switches to its second configuration, high pressure hydraulic fluid is supplied to the ram 10 and is relieved from ram 11 thereby reversing the direction of motion of the piston causing gas to initially be compressed in the intermediate compression cylinders 20 and subsequently forced out of this cylinder along intermediate discharge line 26. As the pistons 10A, 11A are performing the downstroke, the piston 11A compresses the gas in the cylinder 21 and expels it from the cylinder, and while the pistons 10A, 11A are performing an upstroke, the piston 11A sucks gas into the high pressure compression cylinder 21. Suitable non-return valves (not shown) are provided to the inlet and outlet connections of each cylinder.

As described thus far, the arrangement is similar to that shown in Fig. 2 of WO 00/34655.

The high-pressure gas delivery pipe is of a small-bore flexible type fitted with an in-line quick release breakaway coupling. For motor vehicle applications, the storage cylinder is usually mounted under the vehicle body. To facilitate easy uncoupling of the quick release nozzle 2, a pressure offloading mechanism has been developed to automatically offload gas in the fuel hose 1 to enable the quick release nozzle 2 to be safely removed at a low fuel pressure.

This offloading mechanism will now be described

with particular reference to Fig. 2 which shows a relief valve 30. The relief valve has three connections, a high pressure gas inlet 31 which branches off from the fuel hose 1, a low pressure gas outlet 32 which is connected either back to the intermediate compression cylinder 20 along vent line 33, or alternatively back to the low pressure gas supply 23 along vent line 34. The third connection to the relief valve 30 is a hydraulic supply line 35 which is in communication with low pressure discharge line 14.

The relief valve 30 is shown in Fig. 2. The moving parts comprise: a pressure balanced piston 40, a valve member 41, a first spring 42, a gas valve sleeve 43 and a second spring 44, whilst the stationary parts comprise: a valve seat 45, a valve seat housing 46 screwed into a body 47 and sealed with a seal 48.

The valve member 41, which is shown in the open/rest position, is free to move within its chamber and is held against the valve sleeve 43 by first spring 42. A shaft seal 49 prevents gas at fuel line delivery pressure from line 31 escaping along the piston shaft outer annulus whilst the whole assembly of valve member 41, first spring 42 and shaft seal 49 is held in place by the valve sleeve 43, which is screwed to the piston 40.

Hydraulic fluid applied along line 35 moves the piston against the second spring 44 until it rests on a shoulder 50 of the valve seat housing 46. A piston seal 51 and shaft seal 52 retain hydraulic fluid behind the piston 40. The movement of the piston is such that the valve sleeve 43 moves to just clear the valve seat 45 whilst the valve member 41 rests on the

valve seat 45 urged by first spring 42 to cover a port 53 in the valve seat 35. Both the valve member 41 and the valve seat 45 have ground flat faces to ensure an adequate seal between the high pressure gas port line 51 and port 53, whilst a seal 45A prevents gas escaping around the seat 45.

The piston 40 is pressure balanced against changes in fuel line pressure 31. To achieve this, gas at fuel line delivery pressure is delivered to both ends of the piston 40 simultaneously via a gas port 54 through the centre of the piston. Port 54 ensures that the piston 40 can move under hydraulic forces to close the valve member 41 and under forces exerted by the second spring 44 to open the valve member without influence from forces exerted on the piston 40 by gas at fuel line delivery pressure. The action of the pressure balanced piston 40 therefore allows the high pressure gas inlet 31 to provide a pressure activated closing force on the valve member 41 whilst pressure remains neutral on the hydraulically activated piston 40.

In normal operation when fuel is being delivered, the hydraulic pressure generated by the action of the compressor 10 and supplied along the line 35 moves the piston 40 against the second spring 44 to close the valve member 41 and thereby to seal the high pressure gas inlet 31 from the low pressure gas outlet 32. At the end of the fill period, when the hydraulic pump 5 is stopped, the piston 40 moves back towards the rest position, under the action of the second spring 44.

As the piston 40 moves back, the valve sleeve 43 picks up the valve member 41, which remains pressure energized by the action of the high pressure gas at fuel line delivery pressure from line 31. However, the force exerted by the second spring 44 on the

piston is sufficient to lift the valve member 41 off the valve seat 45 against the action of the gas pressure acting on the seat. Gas at fuel line delivery pressure trapped in the fuel delivery hose 1 can then escape through the low pressure outlet 32, thus off loading the compressor.

The relief valve 30 has been designed to operate from either the line supplying the spool valve 7 from the hydraulic pump 5 or the return line to tank 6. In both cases, the shoulder 50 limits the piston 40 movement to prevent the hydraulic force applied to the piston through line 35 from crushing the valve sleeve 43 against the valve seat 45. The piston 40, therefore, has been designed to withstand the full hydraulic fluid through the spool valve 7. The piston 40 is sized to compress the second spring 44 to the shoulder 50 with a minimum hydraulic pressure of typically 3 Bar (residual hydraulic pressure measured typically at lOBar)-thus ensuring that the mechanism operates over the whole hydraulic pressures in the hydraulic supply line 35. With some hydraulic pump 5 and spool valve 7 combinations the hydraulic fluid was slow to dissipate back to the hydraulic tank 6 once refuelling was stopped-thereby delaying the compressor off loading.

In order to provide a more positive offloading action, a T-line relief valve 60 is provided as shown in Fig. 1, and as shown in greater detail in Fig. 4.

This valve has an inlet 61 connected to the low pressure discharge 14 hydraulic supply line 35, and two hydraulic fluid return lines 62,63 connected in parallel, both of which lead back to the hydraulic tank 6. The first return line 62 provides an uninterrupted return to the tank 6, but is a flow limiting orifice sized to generate sufficient

hydraulic back pressure with the minimum hydraulic flow. The second return line 63 has a larger diameter, but is closed by a spring and ball valve 64 which is designed to open at a pressure of typically 5 Bar to provide a more substantial return to tank 6.

This limits the hydraulic back pressure and system losses to a maximum pressure typically of 5 Bar.

An alternative relief valve is shown in Fig. 3.

This relief valve is particularly applicable to a two- stage pressure of the type shown in Fig. 1.

This relief valve has similar components to those described with respect to Fig. 2 including the pressure balanced piston 40, valve member 41, first spring 42, gas valve sleeve 43 and second spring 44.

Further, the stationary part comprises similar components in the valve seat 45, housing 46, body 47 and seal 48. However, the operation of this relief valve is different as described below.

Hydraulic pressure is provided along hydraulic supply line 35 as before. However, instead of simply supplying the full gas pressure to the relief valve, the alternative relief valve in Fig. 3 has a high pressure gas line 31A receiving high pressure gas from the high pressure cylinder 21, and an intermediate gas line 31B receiving gas from the intermediate compression cylinder 20. Again, the piston 40 has a port 54 balancing the pressure across the piston. A low pressure gas outlet 32 leads from the chamber containing the second piston 44. In use, when refuelling is taking place, hydraulic pressure through line 35 causes the piston 40 to move to the left from the position shown in Fig. 3. This causes the valve element 41 to engage with valve seat 45 thereby sealing port 53. Also, the opposite end of valve

element 41 engages with a shoulder 70 on the piston 40 thereby closing high pressure line 31A. When the hydraulic pressure is removed, the piston 40 is moved to the position shown in Fig. 3 under the action of the second spring 44. The valve element 41 no longer closes either passage 31A or 31B, such that the pressure in these lines is vented along gas outlet 32.

This arrangement therefore provides a way of venting the gas pressure in both the high pressure line 31A and intermediate line 31B.