GAS COMPRESSOR

TECHNICAL FIELD

This invention relates to gas compressors, and more particularly to a compressor suitable for residential compression of natural gas with particular use in automotive applications.

BACKGROUND OF INVENTION

Natural gas powered vehicles are becoming more common, but the prevalence of commercial filling stations poses an issue to ensure reliable operation of the vehicle. In certain areas of the United States, nearly every home has access to a supply of natural gas from their local provider and this can be compressed for use in a natural gas vehicle. The typical supply of natural gas available to a home is around 200 millibar. A vehicle natural gas tank is generally pressurized to 248 bar at a temperature of 25° C.; this is a compression ratio of 1240 with respect to the home supply pressure—while adding nearly no temperature to the system. Due to the high degree that the force changes over a compression cycle, a hydraulically powered drive unit appears to offer a good tradeoff of cost, both initial and variable, and functionality, but mechanically driven systems also offer their own benefits of simplicity and efficiency. Achieving this level of compression while minimizing leaks, energy, power consumption, and maximizing fill rate poses a challenge while maintaining an affordable cost to the end user.

There are a few prior art commercial systems available. The fill rate on these products varies significantly from 0.5 gallon of gas equivalent (GGE) per hour up to a fill rate of 6 GGE per hour.

SUMMARY OF THE INVENTION

At least one embodiment of the present invention provides

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of an embodiment of a compressor system of the present invention which uses internally routed check valves and orifices to boost the pressure in the pressure boosted chamber of the first stage cylinder;

FIG. 2 is a schematic view of another embodiment of a compressor system of the present invention which utilizes a two stage filling process and a single pump;

FIG. 3 is a schematic view of an embodiment of a compressor system of the present invention which is similar to the embodiment shown in FIG. 1, except the passive hydraulically piloted unloading valve is replaced by a solenoid piloted unloading valve;

FIG. 4 is a schematic view of an embodiment of a compressor system of the present invention which is similar to the embodiment shown in FIG. 1, except the fixed relief valve is replaced with a variable relief valve;

FIG. 5 is a schematic view of a compressor section of an embodiment of a compressor system of the present invention wherein the compressor section which utilizes internally routed pressure boost in the second stage cylinder as well as the first stage cylinder;

FIG. 6 is a schematic view of another compressor section embodiment where the second stage pressure boosting chamber is connected to the pressure boosting chamber via check valves and orifices through a mutual end cap;

FIG. 7 is a schematic view of another compressor section embodiment which is a combination of FIGS. 5 and 6 which enables high pressure gas flow internally through the pistons and end caps;

FIG. 8 is a schematic view of a two cylinder pressure boost system utilizing internal orifices and check valves between the cylinders via the mutual end cap;

FIG. 9A is a cross-sectional view of a cylinder illustrating an embodiment of the invention that implements variable sized internally muted orifices and check valves that when the piston travels past an orifice a flow passage opens between the two chambers of the cylinder; and FIG. 9B is a detail cross-sectional view of this area of FIG. 9A;

FIG. 10 is a detail cross-sectional view of a cylinder of the present invention which utilizes inner diameter flow passages which selectively connect the two chambers of the cylinder based on the position of the piston;

FIG. 11 is a detail cross-section view of a piston used in an embodiment of the invention showing the check valves and internally routed orifice;

FIG. 12 is a top view of the piston of FIG. 11;

FIG. 13 is a schematic view of an embodiment of a compressor system of the present invention which utilizes a three stage filling circuit without pressure boost;

FIG. 14 is a schematic view of an embodiment of a three stage filling circuit without pressure boost similar to FIG. 13 but including a low pressure filling configuration;

FIG. 15 is a schematic view of an embodiment of a compressor system of the present invention which utilizes a five stage filling circuit;

FIG. 16 is a schematic view of an embodiment of a compressor system of the present invention which utilizes a three stage filling circuit with a high-low pump system;

FIG. 17 is a schematic view of an embodiment of a compressor system of the present invention which utilizes a two stage filling circuit with the drive system positioned between the cylinders;

FIG. 18 is a schematic view of an embodiment of a compressor system of the present invention which utilizes a two stage filling circuit with a mechanically driven system with cylinders arranged generally in parallel;

FIG. 19 is a schematic view of an embodiment of a compressor system of the present invention which utilizes a two stage filling circuit with a mechanically driven system with the cylinders arranged in series;

FIG. 20 is a schematic view of an embodiment of a compressor system of the present invention which utilizes a two stage filling circuit with a mechanically driven system driven by a connecting rod in a reciprocating fashion.

FIG. 21 is a schematic view of the embodiment shown in FIG. 3 depicting an initial stage of the compression cycle wherein the compression pistons are fully retracted;

FIG. 22 is a schematic view of the embodiment shown in FIG. 3 depicting a secondary stage of the compression cycle;

FIG. 23 is a schematic view of the embodiment shown in FIG. 3 depicting a third stage of the compression cycle;

FIG. 24 is a schematic view of the embodiment shown in FIG. 3 depicting a fourth stage of the compression cycle;

FIG. 25 is a schematic view of the embodiment shown in FIG. 3 depicting a fifth stage of the compression cycle;

FIG. 26 is a schematic view of the embodiment shown in FIG. 3 depicting a sixth stage of the compression cycle wherein the compression pistons are fully extended;

FIG. 27 is a schematic view of the embodiment shown in FIG. 3 depicting a seventh stage of the compression cycle where the compression pistons have started retracting;

FIG. 28 is a schematic view of the embodiment shown in FIG. 3 depicting a eighth stage of the compression cycle;

FIG. 29 is a schematic view of the embodiment shown in FIG. 3 depicting a ninth stage of the compression cycle;

FIG. 30 is a schematic view of the embodiment shown in FIG. 3 depicting a tenth stage of the compression cycle wherein the compression pistons are fully retracted and the gas is moving toward the state shown in FIG. 21;

FIG. 31 is a cross-sectional view of the compression piston within a cylinder depicting gas coming through the check valve and orifice in the compression piston to the pressure boosted side of the cylinder;

FIG. 32 is a cross-sectional view of the compression piston within a cylinder similar to FIG. 31 where the piston has extended further allowing gas to move around the piston through the cylinder inner diameter flow passages which connect to the two chambers of the cylinder;

FIG. 33 is a cross-sectional view of the compression piston within a cylinder similar to FIG. 32 where the piston has now fully extended;

FIG. 34 is a cross-sectional view of the compression piston within a cylinder similar to FIG. 33 where the piston begins to retract allowing gas to move around the piston through the cylinder inner diameter flow passages which connect to the two chambers of the cylinder;

FIG. 35 is a cross-sectional view of the compression piston within a cylinder similar to FIG. 31 where the piston has retracted to close the cylinder inner diameter flow passages;

FIG. 36 is a schematic view of another embodiment of a compressor system of the present invention which utilizes a three stage filling process and a high low pump system shown with the pistons beginning to extend within the cylinders;

FIG. 37 is a schematic view of the system of FIG. 36 shown with the pistons continuing to extend within the cylinders causing gas flow through the circuit;

FIG. 38 is a schematic view of the system of FIG. 37 shown with the pistons fully extended within the cylinders;

FIG. 39 is a schematic view of the system of FIG. 38 shown with the pistons beginning to retract within the cylinders;

FIG. 40 is a schematic view of the system of FIG. 39 shown with the pistons continuing to retract within the cylinders;

FIG. 41 is a schematic view of the system of FIG. 40 shown with the pistons fully retracted within the cylinders;

FIG. 42 is a schematic view of another embodiment of a compressor system of the present invention which utilizes a three stage filling process and a single pump system shown with the pistons extending within the cylinders;

FIG. 43 is a schematic view of the system of FIG. 42 shown with the pistons retracting within the cylinders;

FIG. 44A is a plan view of an end cap in accordance with an embodiment of the invention; and FIG. 44B is a perspective view of the end cap shown in FIG. 44A; and

FIG. 45 is a schematic view of a compressor system immersed in a liquid cooling tank in accordance with an embodiment of the invention.

DETAILED DESCRIPTION

A compressor system is provided comprising a drive section and a compression section. There are a number of different embodiments for each of the compressor and drive sections that can be can be combined into the full system resulting in additional system combinations. The compressor is discussed below primarily for use in compressing natural gas, however, the invention is not limited to use with a particular gas.

For reference, the most basic form of the compressor system (110) of this invention comprises two pneumatic cylinders (7), (8) used to compress the natural gas, the cylinders driven by a hydraulic cylinder (4) connected to an electrically driven hydraulic power unit as shown in FIG. 2. One or more of the sides of the pneumatic cylinders are connected to the home supply of natural gas (6) and connections between the pneumatic cylinders in the form of a gas circuit are plumbed differently and varying components to achieve varying functionality and efficiency. The compressor system has an inlet (60) which connects the gas conduit to the supply of gas (6) and an outlet (61) which connects the gas conduit to the gas storage tank (9). For clarity, the left side of the first pneumatic cylinder (7) is generally referred to as the first pneumatic cylinder chamber and the right side of the first pneumatic cylinder (7) is generally referred to as the second pneumatic cylinder chamber while the left side of the second pneumatic cylinder (8) is generally referred to as the third pneumatic cylinder chamber and the right side of the second pneumatic cylinder (8) is generally referred to as the fourth pneumatic cylinder chamber. In specific embodiments, the compressor systems compress the gas in stages and these cylinder chambers may be designated by the stage sequence. Check valve (5), (8) are utilized in the circuit to help control the flow of the gas. By plumbing the system differently, varying numbers of stages of compression can be obtained to increase the flow rate and decrease the energy and power consumption.

One aspect that is illustrated in FIG. 2, but could be present in several other embodiments are the cylinder standoffs (51) which are used to separate the hydraulic cylinder (2) from the CNG Cylinders (7), (8). This feature helps prevent contamination of the natural gas which harmful to the vehicles that will be filled. Besides separating the natural gas, the cylinder standoffs (51) are used as structural support and cost reduction as the same function could be achieved by some sort of empty cylinder body. The open nature of the standoffs (51) will allow technicians to service the device more easily as it is more accessible and the standoffs allow for easily assembly and disassembly. From an integration perspective the cylinder standoffs (51) can also be used to transport hydraulic fluid (which could be used as coolant) from the hydraulic pump or reservoir to where it is needed such as on the line connecting the First Stage CNG Cylinder (7) and the Second Stage CNG Cylinder or the final compression chamber, among other locations.

Also illustrated in FIG. 2 is an inlet capacitance (52), which could be included on any of the embodiments, and is used as volume chamber before the first stage cylinder (7) to ensure there is ample gas available when filling any of the chambers with gas from the natural gas supply (6). The inlet capacitance (52) present is expected to maintain the pressure of the natural gas supply at a more consistent level as well as preventing the functioning of this invention from affect the natural gas service of others in the area due to a rapid demand in gas. The inlet capacitance (52) could take a number of forms including a volume chamber, a large or long pipe, a small low pressure accumulator, or any other device that increases the effective line volume and/or mass of gas near the inlet (60) to the compressor system (110).

Another embodiment of the compressor (111) shown schematically in FIG. 1, compresses gas in a pseudo three stage process in a method to increase the number of moles of gas in a constant volume chamber or dwelling cylinder chamber, above that allowed by the ideal gas law when said chamber is connected to a constant pressure. The double rod first stage pneumatic cylinder (7) is connected to the second stage pneumatic cylinder (8) where the two chambers of the first stage cylinder (7) are both connected through inlet (60) to the natural gas supply (6) via CNG supply check valves (5) and connected to each other through the first stage piston (30) via internal check valve(s) (32) and orifice(s) (31); the drive cylinder (4) is connected to the first stage pneumatic cylinder (7) and the second stage pneumatic cylinder (8) and is actuated via the hi-low hydraulic pump circuit. The first stage pneumatic cylinder utilizes the second pneumatic cylinder chamber as the pressure boosting chamber (28) to increase the mass of natural gas in the first pneumatic cylinder chamber as the pressure boosted chamber (29), which then compresses the gas and passes it to the second stage pneumatic cylinder (8) via a series of high pressure CNG check valves (34) where it is passed to the fourth pneumatic cylinder chamber as the final compression chamber (43), compressed again, and then passed through outlet (61) to the CNG storage tank (9). The hi-low circuit is composed of an electric motor (23) to drive both the high flow pump (14) and low flow pump (13), which are constant volume units, where a hydraulically piloted unloading valve (17) is used to control the power when both pumps are in use and the relief valve (35) is used to control the maximum power when only the low flow pump (13) is in use. The high flow pump (14) is connected to the low the flow pump (13) via a hydraulic check valve (15) and the two pumps are connected to the drive cylinder (4) via another hydraulic check valve (15) and the directional control valve (3); the directional control valve (3) is used to determine the direction of motion for the drive cylinder (4). The hydraulically piloted unloading valve (17) is set to open at some pressure below that of the relief valve (35). The hydraulically piloted unloading valve (17) is used to ensure that the flow power from the hydraulic pumps do not exceed the maximum rating of the electric motor (23) by reducing the pressure, and therefore power, of the high flow pump (14) to nearly zero and only allowing the low flow pump (13) to generate pressure. The relief valve (35) is used to limit the pressure, and therefore flow power, from the low flow pump (13) so as to ensure the maximum rated power of the electric motor (23) is not exceeded.

In the embodiment of FIG. 1, the high flow pump (14) actually produces less flow than the low flow pump (13), but to maintain the assumption that the high flow device is unloaded the present nomenclature is selected. Further, the hydraulically piloted unloading valve (17) is set to the open at the pressure when the flow power (pressure×flow of both pumps) exceeds the electric motor (23) maximum rated power. The maximum size of the low flow pump (13) is determined by solving for the flow that will equal the maximum rated power of the electric motor (23) at the pressure setting of the relief valve (35). The size of the high flow pump (14) is simply determined by taking the difference between the total flow required and the low flow pump (13) flow. The total flow required and the relief valve setting is a function of the size of the pneumatic cylinders and the desired fill rate.

Another embodiment of the compressor system (112), illustrated in FIG. 3, differs from the system shown in FIG. 1 in that it replaces the hydraulically piloted unloading valve (17) with a solenoid operated pilot valve (36) which is actuated by the signal from the hydraulic pressure transducer (37) connected to the hydraulic line directly preceding the directional control valve (3). The benefit of this embodiment is that the pressure required to actuate the solenoid operated pilot valve (36) can be changed over the course of the filling cycle. Another embodiment of the compressor system (113) is shown in FIG. 4, which is also a derivative of the circuit in FIG. 1, replaces the relief valve (35) with a variable relief valve (38) where the relief pressure is set based on the direction of motion and the pressure in the CNG storage tank (9) as reported by the pneumatic pressure transducer (39). The pressure setting on the variable relief valve (38) can be changed over the filling cycle as the pressure required to dwell at either end is a function of the pressure in the CNG storage tank (9) and therefore increases during the filling cycle. The purpose of the embodiment depicted in FIG. 4 is to reduce the flow power that is wasted over the variable relief valve (38) when the system is dwelling at either end; this reduces the average power required from the electric motor (23) and the energy required over a fill cycle. Another embodiment, not shown in this document, combines the additional components utilized in FIGS. 3 and 4 to obtain functionality where both the relief valve and unloading valve pressures can be adjusted depending on the current pressure in the CNG tank or other parameters such as environmental temperature, CNG storage tank (9) temperature, etc. which leads to reducing the average power required and the cost of energy to complete a fill cycle. An embodiment exists for driving the compression section using a variable displacement pump, or a horsepower limiting variable displacement pump. Another option for providing flow to the drive cylinder (4) is using a fixed displacement pump connected to a variable frequency drive. The variable frequency drive can be used to limit the power of the electric motor and the hydraulic pump, so as the pressure increases the speed can be decreased. This is an advantage over the hi-lo pump circuit as there is a discreet step change in the power output when the high flow pump (14) is unloaded to tank. Other benefits include reducing the wasted power when dwelling as the required pressure can continue to be provided by the flow can be minimized by slowing the electric motor to near its stall speed. There are a number of different methods not mentioned in this document to achieve the desired result and are considered within the scope of the present invention.

Another embodiment of the compression system (114), shown in FIG. 5, uses a pressure boost concept described in both the first stage pneumatic cylinder (7) and the second stage pneumatic cylinder (8). This will increase the mass of gas in the final compression chamber (43) before the gas is propagated from the pressure boosted chamber (29) as gas is also delivered from the natural gas supply (6) through a CNG supply check valve (5) and the second stage pressure boosting chamber (40). Notice that in FIG. 5 the orientation of the check valve (32) in the second stage piston (33) is reversed compared to the internal check valve (32) in the first stage piston (30). The internal check valve (32) is reversed as the stroke direction when the gas should propagate from the boosting chamber (28) to the boosted chamber (29) is the opposite between the first stage pneumatic cylinder (7) and the second stage pneumatic cylinder (8). If using the system (114) described in FIG. 5 the mass of gas delivered to the CNG storage tank (9) per cycle and therefore the fill rate is increased when compared to an equally sized system using the compression system (112) depicted in FIG. 3. It is possible also to use a system where the final compression chamber (43) is not connected to the natural gas supply (6).

Another embodiment of the compressor system (115), shown in FIG. 6, uses the pressure boost concept, but a slightly modified version as gas is propagated from the second stage boosting chamber (40) to the pressure boosting chamber (28) in the first stage pneumatic cylinder (7). Pressure boost is not used in the piston (33) of the second stage pneumatic cylinder (8). This will increase the mass of gas in the pressure boosting chamber (28) so more mass of gas can be passed to the pressure boosted chamber (29) and therefore more gas pumped per cycle. One benefit of this modification is that the gas is passed internal to the cylinders (7), (8), so as to minimize the leak points. This embodiment alternatively can be used to the decrease the cycle time as the same mass of gas can be filled in the pressure boosted chamber (29) in a shorter amount of time.

Another embodiment of the compressor system (116), shown in FIG. 7, illustrates a configuration where the high pressure compressed gas can be entirely routed internally until it must be passed to the CNG storage tank (9); this embodiment minimizes leak points in comparison to previous embodiments. The embodiment is quite similar to that described in FIG. 6, except that the high pressure CNG check valves (34) have been removed between the first stage pneumatic cylinder (7) and the second stage pneumatic cylinder (8), and the check valves (32, 44) in the first stage piston (30) and the mutual end cap (42) have reversed their orientation. Further, internal high pressure check valves (44) have been added to the second stage piston (33) so as to be able to transfer gas from the second stage boosting chamber (40) to the final compression chamber (43). It is key to recognize that the internally routed passage through the second stage piston does not include an internal orifice, but only a high pressure check valve (44) because the second stage pressure boosting chamber is predominantly used as a pass through, and therefore the flow resistance should be minimized to pass gas as quickly as possible to the final compression chamber (43) when the system is moving to the left. It is contemplated that a system could use internal orifices (31) in series with the internal high pressure check valves (44) muted through the second stage piston (33) if the second stage pressure boosting chamber (40) were to be used for increasing the mass of gas (or pressure). Please note that the location of the pressure boosting chamber (28) and the pressure boosted chamber (29) have also been reversed. Also note that although the second stage pneumatic cylinder (8) is depicted as being larger in diameter than the first stage pneumatic cylinder (7), but it is not a requirement for this invention. Another benefit of this embodiment is that the line diameter between the chambers can be made very large as the line length will be very short and this will create a very small dead volume. Previously the line had to extend from the bottom of the first stage piston (7) to the top of the second stage piston (8) which is about twice the stroke length of a single piston; if all the lines are routed internally the line lengths will simply be the height of the pistons or end caps. This will reduce the energy consumption of the system as the pressure build up due to choked flow will be minimized and potentially the system can operate at a higher frequency as the time for gas to propagate between chambers will be reduced.

The invention can be expanded to a plurality of stages, as demonstrated by the system (117) in FIG. 8, by combining the cylinders back to back and routing internal orifices (31) and check valves (32) internal to the mutual end cap (42). The embodiment shown in FIG. 8 is composed of two cylinders separated by a mutual end cap (42) and where the pistons (30, 33) in each cylinder and mutual end cap (42) have internally routed orifices (31) and check valves (32). Each of the chambers is connected through inlet (60) to the constant pressure source (6) through the low pressure check valves (5). In this configuration, the pneumatic chambers are generally reversed in comparison to the previous embodiments. The right most chamber is referred to as the 1^stpressure boosting chamber (45) the next right most chamber is referred to as the 2^ndpressure boosting chamber (46), the right most chamber in the left cylinder is referred to as the 3^rdpressure boosting chamber (47), and finally, the left most chamber in FIG. 8 is referred to as the pressure boosted chamber (29). At steady state, as one examines the chambers from right to left, the mass of gas will be increasing. It is straightforward to recognize that this concept can be expanded to a plurality of pressure boosting stages by combing additional cylinder back to back with the mutual end cap. Also note that this invention can be driven from either end and the end that is not driven can be modified to be a single ended cylinder to achieve more surface area and chamber volume.

Besides routing the flow internally it is also possible to route the flow externally where a check valve and orifice continue to separate the flow path between the two chambers. The added volume of the line segment between the two chambers is detrimental to the process as more moles will remain in the pressure boosting stage (28) and the flow path. Further, by routing the flow externally additional leak points are introduced. Another embodiment, as shown in FIG. 9, where the externally routed flow connects via a single path to the pressure boosting chamber (28) but to multiple locations on the pressure boosted chamber (29) spaced along the stroke length. A check valve (32) and orifice (31) can either be placed in each of the flow passages or a single check valve (32) and orifice (31) in the path out of the pressure boosting chamber (28). This embodiment results in a variable orifice size between the chambers depending on the position of the piston (30). The size of the orifices (31) do not necessarily require the same flow area.

To obtain the variable orifice functionality while maintaining the high pressure fluid internal to the cylinder (7), low resistance flow passages can connect the chambers near the end of stroke or route the flow through the end cap. One embodiment to achieve the described functionality utilizes notches or cylinder ID flow passages (48) created in the cylinder wall to allow flow to pass from one chamber to the other, with minimal restriction, once the piston (2) has passed a certain point, as illustrated in FIG. 10. One issue with this arrangement is that when the piston (30) begins to move in the other direction there will be free flow between the two chambers until the piston (30) passes the point where the cylinder ID flow passages (48) no longer connect the two chambers. This will be detrimental to the performance as the two chambers will be attempting to equalize in pressure, but depending on where the cylinder ID flow passages (48) end, only a minimal amount of gas will exit the pressure boosted chamber (29). This embodiment allows gas to propagate very quickly from the pressure boosting chamber (28) to the pressure boosted chamber (29) once the piston (30) passes a certain point in its travel, and therefore will reduce the dwell time required for the fluid to pass through the internal restriction (31). This concept also reduces the energy consumption per stroke, because once the two chambers are connected with large orifices the pressure will balance quickly. On one hand, the end position of the cylinder ID flow passages (48) should be located in a position where they will connect the two chambers once the pressure in the boosted chamber (29) has exceeded the constant pressure source's (6) pressure (the purpose of the internal orifices (31) in the piston (30) is to slow down the propagation of gas between the two chambers to maximize the gas filled from the constant pressure source (6)). On the other hand, opening the notches or cylinder ID flow passages (48) too early in the stroke would result in the notches or cylinder ID flow passages (48) closing too late when moving in the opposite direction meaning too much mass may propagate back to the pressure boosting chamber (28) from the pressure boosted chamber (29). A plurality of cylinder ID flow passages (48) can also be added around the circumference of the inner diameter of the cylinder with the same or varying lengths. If varying lengths of cylinder ID flow passages (48) are used they will change the flow area based on the position of the piston (30). This, however, could create the issue of excessive “leakage” between the two chambers when moving the piston (30) in the direction that reduces the volume of the pressure boosted chamber (29).

In an alternate embodiment, the pressure boosting flow can be routed through a check valve in the end cap (32) and then to flow passages into the cylinder (7). In this case, the flow passages could be machined directly into the ID, but would need to be in the cylinder wall and then enter the pressure boosted chamber (29) via a hole to connect the flow passage and the chamber. This will achieve the same functionality as FIG. 10, but the flow will not be able to propagate from the pressure boosted chamber (29) back to the pressure boosting chamber (28) due to the check valve in the end cap. Both of the concepts above and others similar to these can be combined with or without the concept of internally routing an orifice (32) and check valve (31) through the piston (30).

The construction of the piston and internally routed orifice can vary depending on the needs of the system. In one embodiment, a single internally muted orifice (31) with check valve (32) may be considered. In a single orifice and check valve design, the orifice and check valve are located close to the center of the piston and preferably directly in the center of the piston so as to minimize any moments applied to the piston due to flow forces or variations in local static pressure due to the gas flowing between the chambers (28), (29). In another embodiment, a plurality of orifices (31) with check valves (32) may be used such as in FIG. 11 showing two internally routed orifices (31) and check valves (32) placed circumferentially at 1800 spacing. An alternative embodiment, shown in FIG. 12, uses three sets of check valves (4) and internally routed resistances (31) arranged circumferentially around the piston (30) and spaced 1200 apart. This will achieve the same benefit as inserting a single set of an internally routed orifice (31) and a check valve (32) in series through the rod as the flow forces and variations in local pressure would be distributed equally around the piston resulting a minimal net moment. It is further contemplated that other embodiments of the piston design encompass a plurality of check valves inserted into the piston at regular or non-regular spacing. The shape of the orifices (31) affects the flow characteristics between the two chambers. FIGS. 10 and 11 illustrate orifices (32) that resemble a converging nozzle, but it is also permissible to utilize an orifice (32) with a constant flow area. The purpose of utilizing an orifice (32) shaped like a converging nozzle is that it will permit sonic flow at the throat without shrinking the effective flow area due to the vena contracta phenomenon. Use of a converging nozzle with the same throat area as the area of an orifice will result in a higher mass flow rate; it will also be easier to calculate the mass flow rate as the flow area will not be reduced by the boundary layer separation and recirculation.

Another embodiment of the compressor system (118) is shown in FIG. 13. The compressor comprises a first pneumatic cylinder (7) having a piston separating a first cylinder chamber and a second cylinder chamber. The compressor further comprises a second pneumatic cylinder (8) having a piston separating a third cylinder chamber and a fourth cylinder chamber. The compressor includes a means for moving the piston of each cylinder in this configuration shown as pump (1) in this case a fixed displacement pump (1) driven by an electrical motor (not shown) at a constant speed, that provides flow to the drive cylinder (4). The directional control valve (3) is used to control the direction of the drive cylinder (4). When the drive cylinder (4) is dwelling at either end of the stroke the flow from the pump (1) is dumped over the pump relief valve (2) to the tank. In this embodiment the position of the directional control valve (3) is determined by a timer, but it can also be controlled by an electric signal. The first stage pneumatic cylinder or CNG cylinder (7) and the second stage pneumatic cylinder or CNG cylinder (8) are coupled via mechanical shaft to the drive cylinder (4) and therefore move with the same cadence. All four chambers of the two CNG cylinders are used to compress the natural gas; the left chamber of the first stage CNG cylinder (7) is considered to be the second stage chamber (which is also designated the first pneumatic cylinder chamber), the right side of the first stage CNG cylinder (7) and the left side of the second stage CNG cylinder (8) are considered to be the first stage chambers (which are also designated the second and third pneumatic cylinder chambers, respectively), and the right side of the second stage CNG cylinder (8) is considered to be the third stage (which is also designated the fourth pneumatic cylinder chamber). As with all the embodiments, the compressor system (118) has an inlet (60) which connects the gas conduit to the supply of gas (6) and an outlet (61) which connects the gas conduit to the gas storage tank (9). The gas conduit circuit connects the inlet (60) to the first cylinder chamber, the second cylinder chamber, the third cylinder chamber, and the fourth cylinder chamber; the gas conduit circuit connects the first cylinder chamber and the third cylinder chamber to the outlet (61) when the pistons are retracting and the gas conduit circuit connecting the second cylinder chamber and the fourth cylinder chamber to the outlet (61) when the pistons are extending. The compressed gas moves from the outlet to the gas storage tank (9).

All chambers composing the first and second stage compression are connected through inlet (60) to the natural gas supply (6) through CNG check valves (5). The two first pneumatic cylinder chambers are connected in parallel to the CNG reservoir (11), but through individual CNG check valves (5) connected to each of the chambers; the CNG reservoir (11) is used to store the compressed gas from the first stage before passing it to the second stage. The shutoff valve (10) is used to separate the first stage chambers and the CNG reservoir (11) from the second stage chamber; the position of the shutoff valve (10) is controlled via an electronic signal. The second stage chamber (left side of the first stage pneumatic cylinder (7)) is connected to the third stage chamber (right side of second stage pneumatic cylinder (8)) via a pipe and a series of CNG check valves (5) to control the flow direction.

In another embodiment of the compressor system (119), shown in FIG. 14, the three stage filling process adds low pressure filling functionality by altering the plumbing as well as adding a number of CNG check valves (5), a CNG directional valve (12), and connecting the natural gas supply (6) to all cylinder chambers. This embodiment has two distinct modes of operation: one when the CNG storage tank pressure is below a certain valve and a second when it is above a certain value. When the CNG storage tank pressure is above a certain value, the functionality of this embodiment is precisely the same as that described for FIG. 13. This embodiment is plumbed so that when the pressure in the CNG storage tank is low, all four chambers have direct access to the CNG storage tank and this is accomplished by connecting the previously referenced first and second stages to the CNG directional valve (12). The CNG directional valve (12) merges the flow from the first and second stage chambers and plumbs it directly to the CNG storage tank (9) via a CNG check valve (5). The CNG directional valve (12) is shown to be an on/off valve that is pilot operated by the pressure in the CNG storage tank (9), however it can also be an electronically controlled on/off valve. When the CNG directional valve (12) is actuated the connections from the previous embodiment, FIG. 13, are reestablished, the low pressure filling mode ends, and the system reverts to the previously described actuation.

Although the two stage and three stage compression processes have been highlighted to this point, it is also possible to expand the invention to any number of compression stages. FIG. 15 highlights an embodiment of the compressor system (120) where there are three pneumatic cylinders and five compression stages; there also exists an embodiment composed of three pneumatic cylinders that utilizes only four or less compression stages. The requirement for adding additional compression stages is that the maximum volume of the previous stage must be greater than that of the next stage; this can be accomplished via either a reduction in bore diameter or stroke length if the pneumatic cylinders' positions can be controlled independently. To fulfill the previously stated requirement, the system (120) illustrated in FIG. 15 must compress gas in the piston side of the third stage pneumatic cylinder (28) before it is passed to the rod side of the third CNG stage cylinder (28). Further, to move the gas from the third stage compression chamber to the fourth stage compression chamber an additional shutoff valve (10) and CNG reservoir (11) are needed as the third stage and the fourth stage compression chambers both compress when the system is extending. Adding additional compression stages may increase the total gallons of gas equivalent pumped per hour by increasing the amount of gas pumped per stroke.

Besides altering the compression section, the drive section can also be modified. FIG. 16 illustrates an embodiment of the compressor system (121) where the basic drive section from FIG. 13 is replaced with a more complex section comprised of a permutation of a hi-lo pump circuit. The compression section is the same as that described for FIG. 13, except that there is a pressure sensor (20) added to the CNG storage tank (9). A typical hi-lo circuit is generally designed where one pump provides high flow and low pressure and the other pump provides low flow and high pressure where the unloading valve is operated by a pressure setting. Like normal, this hi-lo circuit utilizes a high flow pump (13), a low flow pump (14) and a hydraulic check valve (16), however, in this embodiment the hi-lo circuit's unloading valve (17) is controlled via a signal from the unloading valve controller (18) determined by the position measured from the position sensor (21). In the description accompanying FIG. 8 it was noted that when the drive cylinder (4) dwells at either end the entire flow from the pump would be dumped over a relief valve, however in this case only a small amount of flow, that from the low flow pump (14), is dumped over the variable pump RV (16). Further, the variable pump RV (16) uses a signal from the relief valve controller (19) based on the measure pressured from the pressure sensor (20) in the CNG storage tank (9). The relief valve controller (19) computes the amount of force, and therefore hydraulic pressure, required to hold the drive cylinder (4) in place while dwelling and therefore minimizing the energy wasted over the variable pump RV (16). Although not explicitly shown, there is also an embodiment where the variable pump RV (16) can be set to fixed value. The drive cylinder (4) can also be mounted in the middle of the two pneumatic cylinders as opposed to at one end as illustrated in FIG. 17. Note that the drive cylinder is separated from the pneumatic cylinder, and as such the hydraulic fluid cannot mix with the natural gas and contaminate the mixture. Further, the drive cylinder (4) is connected to the first stage pneumatic cylinder (7) in such a way that the portion of the rod that comes in contact with the hydraulic fluid will not contact the natural gas; one possible embodiment of this is where the length of the rod between the drive cylinder (4) and the first stage cylinder (7) is greater than twice the stroke length.

The previous figures have illustrated an arrangement where the drive cylinder (4) is in series with the first and second stage pneumatic cylinders, however it is also possible to arrange it in a manner where the pneumatic cylinders are instead arranged in a parallel or stacked manner. Further, all of the drive sections illustrated to this point are hydraulically powered, however it is also possible to drive the pneumatic cylinders in a purely mechanical manner in either a series or stacked manner. For example, FIG. 18 illustrates a design where the pneumatic cylinders are oriented horizontally and operate parallel to one another and are driven by an electric motor (23) connected to a gear box (26); this can also be referred to as a stacked arrangement. The pneumatic cylinder rods (25) have gear teeth cut into them so they act as a rack and the gear attached to the output of the gear box (26) as the pinion (24). Very similar to FIG. 18 there also exists an embodiment where instead of orienting the system horizontally it can be oriented vertically. Further, another embodiment, shown in FIG. 19 can be used where the pneumatic cylinders are connected in series and are driven from a central electric motor (23) connected to the previously described gear box (26) and rack (25) and pinion (24) concept. On both embodiments described in FIGS. 18 and 19, ample support will need to be applied to the rods with the rack machined into them. Securing a journal bearing, or other type of bearing, on each side of the rod where the pinion interacts with the rack should provide the support required to minimize stress and deflections. The geared solutions also offers the benefit of being able to use different stroke lengths for the stacked cylinder concepts as the gear ratio of the gear box can be altered between its two outputs to each pinion gear. A more compact embodiment, as shown in FIG. 20 is composed of vertically or horizontally oriented pneumatic cylinders where the rods are attached to a centrally located electric motor (23) via a connecting rod (27) that the electric motor (23), attached a to a gear box potentially (26), is able to rotate. The attachment point between the connecting rod (27) and rods of the cylinders must be able to rotate to facilitate the required motion. The inventions in FIGS. 18-20 will need a variable speed electric motor so it can change direction.

Other embodiments not shown are also contemplated. For examples one can implement either of the drive sections with any of the circuit configurations described previously. Further, a two stage low pressure filling circuit or a three stage filling circuit where only a single chamber is used as the first stage are examples of permutation only presented to demonstrate other obvious solutions but not intended to limit the scope of this document. Besides the two and three stage compression concepts shown in this document it is quite obvious how this can be expanded to a plurality of compression stages. The invention can have a plurality of stages and is not limited to the compressor sections described herein.

Another aspect of the invention is the cooling of the compressed natural gas as compressing the gas will undoubtedly lead to a rise in temperature meaning higher pressures and more input work required for compression. Therefore, ample cooling is required and there are a number of locations where the cooling can be performed such as line sections between compression chambers, the CNG reservoir, or potentially directing the flow through a heat exchanger between any stage or before entering the CNG storage tank. Alternatively, cooling can also be accomplished inside the compression chambers with an in-situ cooler such as using the end cap as a cooler or a more advanced shape. Besides cooling the fluid internal to the cylinders it is also feasible to submerge the cylinders in the coolant via a cooling jacket. The coolant will be pumped through the cooling jacket removing heat from the gas which has convected into the cylinder wall and then conducted through the wall. The cooling flow can be generated by a standalone pump, can be tapped from the main hydraulic pump, or it can be used in series from the hydraulic pump. Additionally, the gas leaving the final stage cylinder can be cooled by an additional heat exchanger before it travels to the CNG storage tank (9).

The small volume of gas trapped at very high pressure at the end of each stroke in either the extend or retract direction can be used to propel the assembly in the opposite direction for a small distance. As the pressure in the storage tank increases there would be more energy available at the end of each stroke to accomplish this task. Using this high pressure yet low volume gas should not be an issue because as the volume of the chamber gets larger the pressure will decrease according to the ideal gas law. To implement this idea, the directional control valve (3) would need a position where both the rod side and the drive side of the drive cylinder (4) are connected to tank. To some extent this phenomenon already occurs when the directional control valve (3) is changing direction as the pump pressure decreases rapidly and therefore the pressure that was holding the drive cylinder (4) is removed and the high pressure natural gas at the end of stroke rapidly moves the drive cylinder (4) at a rate which is higher than that provided by the flow from the pump. This is of benefit as it increases the volumes of the chambers more rapidly so they begin filling with natural gas sooner and also lessens the negative impact of the time required for the directional valve to shift.

Other hydraulic drive or CNG compression setups can be utilized. Other drive and compression systems based on similar principles exist and further that each of the drive systems and CNG compression systems can be combined with each other to create a number of permutations of the full invention even though they may not all be noted in this document.

The present invention provides a number of advantages over current systems used for the same purpose by both modifying the current art while also applying novel concepts resulting in better performance, reliability, and other key metrics. The present inventions allow an increase the amount of compressed natural gas pumped, measured in gallons of gas equivalent (GGE) per hour, as compared to a similar sized unit using the prior art for linear compressors, especially by taking advantage of all chambers of a pumping device. The compression section and drive section are separated and therefore the natural gas will not be contaminated by other medium. This is especially a benefit when the drive section is operated by a hydraulic cylinder so the hydraulic fluid and natural gas cannot mix. The invention can reduce the number of leak points by internally routing the high pressure natural gas. The invention can increase reliability of a residential system by operating at a lower frequency and decrease the amount of energy required to pump each GGE of natural gas.

The benefits of using the pressure boost concept with the invention include: Increasing the pressure of a chamber connected to a constant pressure source above the pressure of the constant pressure source. Increasing the mass of gas in a constant volume chamber beyond that specified by the ideal gas law when it is connected to a constant pressure source and result in a required lower energy output for the linear actuator to move a given mass of gas.

FIGS. 21-30 illustrate the operation of the invention, however a slightly modified system from the preferred embodiment is used to ease the description of the operation of the invention; the description mostly closely resembles that of the system configuration depicted in FIG. 3. Throughout the following description it is assumed that the temperature in the compressor section remains constant unless otherwise noted. Realistically this assumption is not true, but the general principals and operation described below will still remain true. Further, please note that the color of the particles and the number of the particles relates to the pressure and mass or moles of gas in a chamber respectively. From lowest to highest pressure the colors are light blue, blue, green, light purple, light orange, and red; the color blue indicates a pressure close to or at the supply pressure. Further, the colors red and blue are used in the drive section to indicate the relative pressure between the hydraulic lines. These colors have no relation to the colors used in the compressor section.

FIG. 21 illustrates the system when it is dwelling in the fully retracted position. To ensure force on the pistons (30, 33), due to the compressed gas in the pressured boosted chamber (29), does not move the drive cylinder (4), it is held in place by pressure from the low flow pump (13). To limit the holding force, and therefore the low flow pump pressure, a relief valve (35) is used to direct the flow back to tank. The high flow pump (14) is sending its flow through an unloading valve (17) at a very low pressure back to tank. After a long time dwelling the compressor section has come to steady state where the natural gas supply (6) has equalized in pressure with the pressure boosting chamber (28) and the pressure boosted chamber (29) has ceased propagating gas to the final compression chamber (43). The pressure in the pressure boosted chamber (29) is quite high as there is some mass of gas left in the chamber but a very small volume. It is important to note that dwelling in this position until steady state has been achieved is not vital to the operation of the invention.

FIG. 22 illustrates the situation where the system has begun to extend due to the shifting of the directional control valve (3). When the directional control valve (3) shifts, the high pressure in the pressure boosted chamber (29) will rapidly move the system and this is a benefit as the volume in the chamber increases more rapidly. Concurrently, the pressure in the hydraulic system drops so both the unloading valve (17) and the pressure relief valve (35) close so both pumps are providing flow to the drive cylinder (4) for the extend operation. The volume of the pressure boosted chamber (29) increases resulting in a large pressure drop, below that of the natural gas supply (6), so the natural gas supply (6) begins filling the chamber through the CNG supply check valve (5). At the same time the final compression chamber (43) and pressure boosting chamber (29) are reducing in volume which raises their pressures; in the depicted state the pressure in the chambers have not gone high enough to begin propagating gas to its next destination so at this point the chambers undergo a constant mass process.

FIG. 23 depicts a state in which the system is continuing to extend, but both the final compression chamber (43) and the pressure boosting chamber (28) are sending gas to the CNG storage tank (9) and the pressure boosted chamber (28) respectively. The pressure in the pressure boosted chamber (29) continues to be below that of the natural gas supply (6), so both the natural gas supply (6) and the pressure boosting chamber (28) are filling the pressure boosted chamber (29) with gas. The hydraulic circuit continues to operate as described in FIG. 22's state.

The pressure boosting concept uses the properties of compressible fluids and the tendency for gas to move from a higher pressure volume to a lower pressure chamber. The effect of the pressure boost concept increases the mass of gas beyond that specified by the ideal gas law for a given volume connected to a constant pressure source at a given temperature. It accomplishes this by connecting both chambers of a cylinder (first stage pneumatic cylinder (7) in the current example) to a constant pressure source (6) and connecting the two chambers via an internal orifice (31) and check valve (32) to restrict the flow rate and direction of flow. As the piston is extended quickly, for example, the chamber that is getting larger will begin to fill with gas from the constant pressure source and the chamber that is getting smaller will rise in pressure. However, due to the orifice (31) between the two chambers, gas will propagate from the higher pressure to lower pressure side at a rate highly dependent on the orifice size; the smaller the orifice the longer the time it will take for the two sides to balance and conversely the larger the orifice the shorter the time it will tank for the pressure to balance in pressure. Gas will continue flowing from the higher pressure side to the lower pressure side until the pressures have balanced. The key to this concept is that the low pressure volume is continuing to fill with gas from the constant pressure source while also receiving gas from the high pressure chamber. Selecting the proper sized internal orifice (31) and proper closing pressure on the internal check valve (32) is the key to optimizing this process as those two factors determine the boost in mass, but also the time constant for the pressure to balance, and also the required energy to perform the operation. If the cylinder moves in the other direction, the check valves prevent the same operation from occurring, but instead allow the pressure boosting chamber to fill with gas from the constant pressure source while the pressure boosted chamber is compressed and potentially passes its gas to the next element in the work path.

The system is continuing to extend in FIG. 24, but the pressure in the pressure boosted chamber (29) has increased to or above the natural gas supply pressure (6) and therefore the CNG supply check valve (5) closes so the chamber is only being supplied gas from the pressure boosting chamber (28). The mass of gas in the final compression chamber (43) and the pressure boosting chamber (28) continue to decrease as the system extends. At some point, as depicted in FIG. 25, the hydraulic system pressure will exceed that of the setting on the unloading valve (17) and it will open which dumps the flow from the high flow pump (14) at a very small pressure back to the tank. Please note that the unloading valve (17) may open before the pressure boosted chamber (29) reaches the pressure of the natural gas supply (6) or potentially not until the system is dwelling in the fully extended position. The point in the stroke at which the unloading valve opens is highly dependent on the location in the fill cycle which relates to the pressure in the CNG storage tank (9). The effect of unloading the high flow pump (14) reduces the flow rate and therefore the extend speed of the drive cylinder (4), but it also reduces the power demand on the electric motor (23). This will eventually allow the low flow pump (13) to generate a higher pressure to finish the extend cycle as the total power is limited by the maximum rated power of the electric motor (23).

Finally, the drive cylinder (4) reaches the fully extended position and may or may not dwell in this position as shown in FIG. 26. To hold the drive cylinder (4) in place a large force/pressure is required and therefore the directional control valve (3) remains shifted to its current position and the low flow pump (13) provides the necessary pressure to hold the drive cylinder (4) in place while the balance of the flow is dumped over the relief valve (35). While the system is dwelling gas will continue propagating from the pressure boosting chamber (28) to the pressure boosted chamber (29) until the pressures equalize. At this point the mass of gas will have exceeded that specified by the ideal gas law for a chamber of a given volume at some temperature and connected to a constant pressure source, such as the natural gas supply (6). The final compression chamber (43) may continue to propagate gas to the CNG storage tank (9), but it will more than likely have equalized in pressure already as the high pressure in the final compression chamber (43) results in a high density and therefore a higher mass flow rate for a given volumetric flow rate.

Eventually, the directional control valve (3) will shift which allows the compressed gas in the final compression chamber (43) and the pressure boosting chamber (28) to expand and rapidly begin retracting the system as illustrated in FIG. 29. Further, due to the hydraulic fluid pressure drop during the directional control valve shift the unloading valve (17) and the pressure relief valve (35) close so both pumps can supply flow to the drive cylinder (4) for the retract operation. The pressure in the pressure boosting chamber (28) has decreased below that of the natural gas source (6), so the natural gas source (6) begins filling the pressure boosting chamber (28) with gas through the CNG supply check valve (5). The pressure in the pressure boosted chamber (29) is increasing and the pressure in the final compression chamber (43) is decreasing as their volumes are decreasing and increasing respectively, but the high pressure CNG check valves (34) have not opened yet so it is a constant mass process for those chambers during this state. It is important to note that gas will not propagate from the pressure boosted chamber (29) to the pressure boosting chamber (28) due to the orientation of the internal check valves (32) in the first stage piston (30).

As the system continues to retract, as shown in FIG. 28, gas will begin to propagate from the pressure boosted chamber (29) to the final compression chamber (43) while the pressure boosting chamber (28) continues to receive gas from the natural gas supply (6). At some point during the retracting process, depending on the point in the fill cycle and the pressure in the CNG storage tank (9), the pressure of the hydraulic circuit will exceed the specified pressure for the unloading valve (17). At this point the state depicted in FIG. 29 will occur where the high flow pump (14) is directing its flow to tank through the shifted unloading valve (17) at a very low pressure while the low flow pump (17) continues to provide flow to the drive cylinder (4). The pressure of the gas in the final compression chamber (43) may or may not be rising depending on the mass flow rate and the rate at which the system is retracting; a similar relationship exists for the gas in the pressure boosted chamber (29) due to the unknown parameterization of the system.

Eventually the cylinders will be fully retracted as shown in the state depicted by FIG. 30 and the system may or may not dwell in this position. While dwelling in the fully retracted position, the low flow pump (13) will provide pressure to the drive cylinder (4) to hold the system in this position, but the balance of the flow will be directed through the pressure relief valve (35). Gas will continue to flow into the pressure boosting chamber (28) from the natural gas supply (6) until they become equal in pressure when the CNG supply check valve (5) will close. Depending on the parameterization of the system, gas may continue to propagate from the pressure boosted chamber (29) to the final compression chamber (43) until the pressure has equalized, but it is likely this process is complete or will finish quickly due to the increased density discussed earlier. If the system dwells in this state long enough it will be same as the state depicted in FIG. 21.

Up to this point it has simply been noted that the directional valve (3) shifts position, but there are a number of methodologies to determine when the valve should shift. One method is simply based on a constant time before the valve shifts, such as 2.5 seconds retracting and 1.7 seconds extending, or potentially equal times. Another method may use a position sensor that when the cylinder reaches the full extended or retracted position it dwells for a certain amount of time and then shifts the valve. Another means to decide when to actuate the directional control valve could be a measure of the pressure in the hydraulic circuit, the CNG Storage tank, or some combination dependent on the current state both. Alternatively some means of measuring the mass of gas in a cylinder chamber may be invented and that could be used to determine when to shift the valve. The point trying to be illustrated is that there are an infinite number of methods to determine when to shift the directional control valve (3) and should not be limited to the methods described above.

The operation described above is very closely related to the operation of the systems depicted in FIGS. 1, 3, and 4; however the difference in operation of the system depicted in FIG. 4 will be highlighted. In the described operation above it was assumed that the relief valve (35) was set to a constant relief pressure, however it is possible to reduce the power and energy consumption by utilizing a variable relief valve (38) as depicted in FIG. 4. The required pressure to hold the drive cylinder in either dwell position can be computed by knowing the area for the rod and piston sides of the drive cylinder (4) and each of the compression cylinders along with the current pressure in the CNG storage tank. The pressures in the pressure boosted chamber (29) and the pressure boosting chamber (28) will positively vary with the current pressure in the CNG storage tank (9) and therefore the required variable relief valve pressure (38) can start low and increase as the required holding pressure at the dwell locations increases with a rising pressure in the CNG storage tank (9). It is interesting to note that depending on the parameterization of the system the required dwell pressure can vary drastically.

The system depicted in FIG. 5 operates similarly to the system described via FIGS. 21-30, but it differs in the fact that it increases the mass of gas in the final compression chamber (43) by using the pressure boosting concept on the left most chamber of the second stage cylinder (8), which is referred to as the second stage pressure boosting chamber.

FIG. 6 also operates in a similar fashion to the system described via FIGS. 21-30. While it is retracting it supplies gas to the pressure boosting chamber (28) via the natural gas supply (6) as well as the second stage pressure boosting chamber (40). The second stage pressure boosting chamber is connected via internal check valves (32) and orifices (31) routed through the mutual end cap (42) of the first stage pneumatic cylinder (7) and the second stage pneumatic cylinder (8).

FIG. 10 illustrates a device where the restricted fluid passage is external and where there are multiple connection points along the stroke length of the pressure boosted chamber (28). If the piston (30) is dwelling in a position where the pressure boosted chamber (28) volume is very small the pressure boosting chamber (29) will equalize with the constant pressure source (6) after a long time. As the piston (30) begins to extend, which reduces the volume of the pressure boosting chamber (29), the pressure of that gas will increase, but gas cannot travel between the two chambers until the piston (30) passes the first orifice (31) and check valve (32). FIG. 10 illustrates an instance when the piston (30) has extended far enough so as to connect the boosted chamber (28) to both the constant pressure source (6) and a single connection to the boosting chamber (29). At this point both the constant pressure source (6) and the pressure boosting chamber (29) will both provide mass flow to pressure boosted chamber (28) as long as its pressure remains lower than the constant pressure source (6). Eventually, the piston (30) will pass the next combination of a check valve (32) and orifice (31) along the stroke length which will increase the flow area and therefore the mass flow rate between the chambers.

FIG. 8 illustrates an embodiment where two cylinders are connected back to back and there is a fluid passage composed of check valves (32) and internal orifices (31) routed through the mutual end cap (42). This embodiment operates in a very similar fashion to other embodiments using pressure boost, except that there are additional pressure boost chambers. When the pistons (30,33) are moving to the left the 1st pressure boosting chamber (45) will be filling with gas from the constant pressure source (46) and the 3rd pressure boosting chamber (47) will be increasing the mass of gas in it via the 2nd pressure boosting chamber (46) and the constant pressure source (6); the principle of moving gas from the 2nd pressure boosting chamber to the 3rd pressure boosting chamber is the same as described in FIGS. 21-30, except instead of being routed through a piston it is routed through a mutual end cap. At the same time the pressure in the pressure boosted chamber (28) is rising and will eventually pass its compressed gas to the next element through the check valve (32). When the pistons (30, 33) are moving to the right the constant pressure source (6) will be increasing the mass of gas in the pressure boosted chamber (29) and the 2nd pressure boosting chamber (46). The pressure boosted chamber will also be receiving gas from the 3rd pressure boosted chamber (47) via the check valves (32) and internally routed orifices (31) through the first stage piston (30) while at the same time the 2nd pressure boosting chamber (46) will be receiving gas also from the 1st pressure boosting chamber (45) with the orifices (31) and check valves (32) routed through its respective piston (33).

FIG. 31 illustrates the embodiment shown in FIG. 9 when the cylinder (30) is moving in the upward position which is reducing the volume of the pressure boosting chamber (28); this motion will be referred to as extending. At this point the gas is only traveling to the pressure boosted chamber (29) through the check valves (32) and internally routed orifices (31) as the pressure boosted chamber (29) has surpassed the pressure of the constant pressure source (6) and the piston (30) has not extended enough where the cylinder ID flow passages (48) connect the two chambers. The pressure of the gas in the pressure boosted chamber (29) is at or above the supply pressure (6) and the pressure in the pressure boosting chamber (28) is at a pressure well above that of the constant pressure source as the volume has been reduced and the internally routed orifices (31) are restricting the mass flow and therefore the rate at which the pressures can balance.

As the piston (30) continues to extend the cylinder ID flow passages (48) will eventually connect the pressure boosting stage (28) and the pressure boosted stage (29) as shown in FIG. 32. Because the cylinder ID flow passages (48) are large, a larger mass of gas can quickly transfer from the pressure boosting chamber (28) to the pressure boosted chamber (29) through the cylinder ID flow passes (48) than if solely flowing through the internal orifices (31). Once the pressure between the two chambers has equalized, gas will cease propagating through the internally routed orifice (31) and check valve (32) as there is a path of less resistance, namely the cylinder ID flow passage (48). Also, at this point the amount of force required to continue extending the piston (30) will dramatically decrease, because the pressure on the pressure boosting chamber (28) side of the piston (30).

As the piston (30) continues extending the volume of the boosting chamber (28) will continue decreasing and therefore the gas will propagate over to the pressure boosted chamber (29). When at end of stroke, as shown in FIG. 33, if the piston (30) can “bottom out” all of the gas will be in the pressure boosted chamber (29) as there is zero volume in the pressure boosting chamber (28). Having zero volume at the end of stroke is not necessary for the operation of this invention. One of the benefits of this invention is that it will take less time for the gas to propagate from the boosting chamber (28) to the boosted chamber (29).

As the piston (30) begins to retract, as shown in FIG. 34, gas from the pressure boosted chamber (29) will actually propagate back to the pressure boosting chamber (28) as there are not check valves to prevent the back flow. However, the theoretical distribution of gas is equal to the percent volume of each chamber immediately after the piston (30) travels far enough as to disconnect the chambers via the cylinder ID flow passage (48). So for example, if the piston (30) has traveled far enough that the pressure boosting chamber (28) takes up 10% of the volume it is expected 10% of the mass of gas is in the pressure boosting chamber (28) and 90% in the pressure boosted chamber (29). Even though some gas propagated back to the pressure boosting chamber (29), there is still more gas in the pressure boosted chamber (8) than if it had only been connected to the constant pressure source (6).

Finally, as mentioned before, once the piston (30) retracts far enough so the chambers are no longer connected the gas will not propagate from chamber to chamber due to the flow direction restriction of the check valve (32) in the piston (30); FIG. 35 illustrates this state. However, as the volume of the pressure boosting chamber (28) is increasing, the pressure will decrease rapidly and the constant pressure source (6) will begin filling it with gas. At the same time, the volume of the pressure boosted chamber (29) is shrinking and therefore the pressure will increase.

The principles of operation for the system depicted in FIG. 8 are very similar to those depicted in the systems discussed so far, but it differs in that all the high pressure flow is routed internally which requires the rearrangement of the chambers. As the system retracts from the fully extended position the natural gas supply (6) and the pressure boosting chamber (28) will supply gas to the pressure boosted chamber (29) which is now the right most chamber of the first stage pneumatic cylinder (7). At the same time the second stage pressure boosting chamber (40) will pass gas from it to the final compression chamber (43) through check valves (44) internally routed through the second stage piston (33). The pressure build up in the second stage pressure boosting chamber (40) is expected to be very low as the orifice through the second stage piston is either non-existent or very large; the second stage pressure boosting chamber (40) is primarily used as a holding chamber to pass the gas to the final compression chamber (43), but is necessary due to the desire to internally route the flow; the second stage pressure boosting chamber (40) can potentially be used to increase the mass of gas depending of the parameterization of the system and the components. After the system has attained the fully retracted position there should be gas in the pressure boosted chamber (29) above that allowed by the ideal gas law based on the pressure of the natural gas supply (6). Further the gas will have fully propagated to final compression chamber from the second stage pressure boosting chamber (40) especially if the dead volume in the chambers is minimized. As the system extends from the fully retracted position, gas in the final compression chamber (43) will travel to the CNG storage tank (9) and gas from the pressure boosted chamber (29) will travel to the second stage pressure boosting chamber (40) via the check valves (44) and orifices (31) internally routed through the mutual end cap (42). Maximizing the orifice (31) size will minimize the required energy expenditure to extend the cylinder, but it will also minimize and entirely cancel out any chance of using the pressure boost concept in the second stage pressure boosting chamber (40) as gas will propagate between the two chambers very quickly.

FIGS. 36-41 describe the operation of the hi-low pump circuit using a three stage compression as shown schematically in FIG. 13. FIG. 36 illustrates one embodiment of the invention when the pistons are beginning to move to the right, i.e. the drive cylinder (4) is extending. As the embodiment begins to extend the piston side chamber of the first stage pneumatic cylinder (7), referred to as the second stage compression chamber, and the left side of the second stage pneumatic cylinder (8), part of the first stage compression chamber, have some, but very little working fluid in them at a relatively high pressure; this high pressure fluid is left over from the last cycle and is the part that could not be extracted. As such more gas cannot enter these chambers until the pressure decreases by expanding their volume. The right side of the first stage cylinder (7), the other piece of the first stage compression chamber, has been pressurized by the natural gas supply (6) and achieved or nearly achieved the supply pressure. The piston side of the second stage pneumatic cylinder (8), referred to as the third stage compression chamber, is at a pressure significantly above that of the natural gas supply (6) as it has already undergone compression by the first and second stage chambers. Finally the CNG reservoir (11) is at some pressure elevated from that of the natural gas supply (6) because pressurized gas is present from the previous cycle where the first stage chamber of the second stage pneumatic cylinder (8) compressed gas into it and it is contained there due to the closed shutoff valve (10) and a CNG check valve (5).

As the cylinders are extending the pressure is decreasing in the second stage compression chamber and the first stage compression chamber on the second stage pneumatic cylinder (8), because the volume is increasing. Eventually the natural gas supply (6) pressure will be greater than that in either of the previously mentioned chambers and at this point the CNG check valves (5) will open and the previously mentioned chambers will begin filling with gas; the number of moles of gas in the chambers will be increasing. At the same time, pressure in the third stage chamber will rise as the volume of the chamber will be decreasing. Eventually the pressure inside the third stage compression chamber will surpass that of the pressure in the CNG storage tank (9) and the gas will flow into the tank. The number of moles in the third stage compressions chamber will be decreasing and the moles in the CNG storage tank (9) will be increasing. The volume of the CNG storage tank (9) is constant so as more moles of gas enter both the pressure and the temperature will increase according to the ideal gas law. Concurrently to the previously described events, the first stage compression chamber of the first stage pneumatic cylinder (7) will see a pressure rise and eventually the gas inside the chamber will rise beyond that of the CNG reservoir (11) causing the CNG check valve (5) to open. The first stage chamber volume will continue to decrease while storing gas at increasingly higher pressures in the CNG reservoir (11). FIG. 37 is illustrating a time when the previously described events have occurred, but the shutoff valve (10), has not opened.

Sometime later, the shutoff valve (10) will open and connect the CNG reservoir (11) to the second stage chamber which is filled with gas at a pressure equal to the supply pressure; the shutoff valve (10) then closes sometime later before the cylinders begins to retract. The pressure between the CNG reservoir (11) and the second stage chamber will nearly equalize; the moles of gas present in each chamber will be proportional to approximately the inverse of their volume ratios. Eventually, the cylinders will reach their fully extended position where they will dwell for a moment to allow the chambers open to the natural gas supply (6) to equilibrate to its pressure as is shown in FIG. 38.

Just before the system begins to retract, there are very few moles of gas, at a high pressure, in the third stage compression chamber and the first stage compression chamber on the first stage pneumatic cylinder (7). The first stage compression chamber on the second stage pneumatic cylinder (8) has gas present in it equal to or less than that of the supply pressure. The second stage compression chamber will have pressurized gas in it at a pressure above that of the natural gas supply (6), but less than the CNG storage tank (9). The compression chambers are still pressurized from the previous cycle and they will not begin filling with gas from the natural gas supply (6) even though it is retracting as shown in FIG. 39.

As the system is retracting the pressure in the first stage compression chamber on the first stage pneumatic cylinder (7) will be decreasing as the volume increases and eventually the supply pressure will surpass the pressure in the chamber and it will begin filling with gas. At the same time, the volume of the first stage compression chamber on the second stage pneumatic cylinder (8) will begin decreasing and eventually its pressure will surpass that of the CNG reservoir (11) and gas will begin flowing through the CNG check valve (5) from the first stage compression chamber to the CNG reservoir (11) and remain there as the shutoff valve (10) is closed. Concurrently to all of this the volume of the third stage compression chamber is increasing, hence a decrease in pressure, and the volume of the second stage compression chamber is decreasing, hence a rise in pressure. Eventually the second stage cylinder compression chamber will open the CNG check valve (5) between it and the pipe connecting to the CNG check valve (5) directly preceding the third stage compression chamber. The rising number of moles of gas in the intermediate line will increase the pressure according to the ideal gas law where eventually the CNG check valve (5) directly proceeding the third stage compression chamber will open and connect the second stage compression chamber to the third stage compression chamber. FIG. 40 illustrates the previously described situation, but the invention has reached such a position that the check valves have opened. Finally, when the piston is fully retracted and dwelled for a short time the same conditions will exist as described just as the system begins to extend as shown in FIG. 41.

FIG. 14 illustrated a compressor system 119 that had a circuit using both three stage filling and low pressure filling. This circuit operates in the same manner as the one just described when it is above the pressure required to actuate the CNG directional valve (12). However, when it is below the pressure required to actuate the CNG directional valve (12) all of the chambers have access to the supply pressure and the CNG storage tank (9). So therefore when the cylinder is extending the second stage chamber will be filling with gas at the natural gas supplies' (6) pressure while the third stage cylinder and the first stage chamber part of the first stage pneumatic cylinder (7) will be directing pressurized gas directly to the CNG storage tank (9) as illustrated in FIG. 42. Although not shown, when the cylinder is retracting, the second stage compression chamber and the first stage compression chamber part of the second stage pneumatic cylinder (8) will be directing pressurized gas directly to the CNG storage tank (9) while the natural gas supply is pressurizing the third stage compression chamber and the first stage compression chamber attached to the first stage pneumatic cylinder (4). This cycle will continue until the CNG storage tank (9) reaches a certain pressure and the CNG directional control valve (12) will shift connecting the two first stage compression chambers to the CNG reservoir (11) and the second stage chamber, the second stage compression chamber to the third stage compressions chamber, and solely connecting the third stage compression chamber to the CNG storage tank (9).

The drive section as illustrated in FIG. 17 uses a fixed displacement pump to control cylinder speed and a directional control valve to control the direction. The pump (1) is turned at a constant speed by an electric motor which is not illustrated in any of the figures. In an alternate embodiment the pump (1) can be turned by a variable displacement electric motor. The directional control valve (3) is electronically actuated and controlled by a timer where it will extend plus dwell for some time and then dwell plus retract for some time; this pattern is repeated until the CNG storage tank (9) is full. When the system is dwelling at either the retract or extend position all of the flow is directed over the pump RV (2) at a high pressure; drive cylinder (4) is held in the dwell position by this high pressure fluid.

The drive section illustrated in FIG. 5 uses a modified hi-lo circuit where the unloading valve (17) is controlled by the position of the drive cylinder (4). When the drive cylinder (4) moving either extending or retracting but is not at or near the dwell position both the unloading valve (17) will be closed and both the high flow (13) and low flow pump (14) will be contributing flow to move the drive cylinder (4) as illustrated in FIG. 10 when the cylinder is extending and FIG. 13 when retracting. However, when the drive cylinder (4) is nearing or at the dwell position, as reported by the position sensor (21), the unloading valve controller (18) will signal the unloading valve (17) to open so the high flow pump (13) can direct its flow at a low pressure to tank as illustrated in FIG. 12 when dwelling in the extend position and FIG. 15 when dwelling in the retracted position. The low flow pump (14) will continue to provide flow to either hold the drive cylinder (4) at the dwell position or finish the stroke. If the low flow pump (14) is holding the drive cylinder (4) at either of the dwell position then its flow will go to tank through the variable pump relief valve (16). The hydraulic check valve (15) ensures the low flow pump's (14) flow directed to the variable pump RV (16) or the drive cylinder (4) as opposed to traveling to tank through the unloading valve (17). The relief setting on the variable pump RV (16) can be adjusted by using the signal from the pressure sensor attached to the CNG storage tank (9) to compute, via the relief valve controller (19), the required force and therefore pressure to hold the drive cylinder (4) at the dwell position. The present invention provides a number of advantages over current systems used for the same purpose by both modifying the current art while also applying novel concepts resulting in better performance, reliability, and other key metrics namely: (i) Increase the amount of compressed natural gas pumped, measured in gallons of gas equivalent (GGE) per hour, as compared to a similar sized unit using the prior art for linear compressors; (ii) the compression section and drive section are separated and therefore the natural gas will not be contaminated by other medium. This is especially a benefit when the drive section is operated by a hydraulic cylinder so the hydraulic fluid and natural gas cannot mix; (iii) increase reliability of a residential system by operating at a lower frequency; (iv) decrease the amount of energy required to pump each GGE of natural gas.

This invention—a multi stage natural gas compression unit is composed of two distinct pieces: the drive section and the compression section. There are a number of different embodiments for each of the compressor and drive sections that can be can be combined into the full system resulting in additional system combinations. For reference, the most basic form of the invention is composed of two pneumatic cylinders used to compress the natural gas driven by a hydraulic cylinder connected to an electrically driven hydraulic power unit as shown in FIG. 17. One or more of the sides of the pneumatic cylinders are connected to the home supply of natural gas and connections between the pneumatic cylinders are plumbed differently and varying components to achieve varying functionality and efficiency. By plumbing the system differently, varying numbers of stages of compression can be obtained to increase the flow rate and decrease the energy and power consumption.

The construction and arrangement of cylinders in this device is quite novel for this application as previous products have utilized small compression chamber arranged in a circular fashion that operate at a high frequency. This device operates in a linear fashion, generally at a low frequency, where the drive cylinder (4) moves the pistons of the two attached cylinders to move the gas between chambers, progressively compressing the natural gas, and eventually storing it in the CNG storage tank (9). As opposed to small volume chambers on a rotary compressor the linear nature of this present invention allows the use of large cylinders with large volume allowing more mass of gas per stroke. More gas per stroke means to compress the same amount of gas less strokes are required. This reduces the component fatigue and heat generated from friction due to the lower speeds.

The construction of the preferred embodiment is also quite novel as tie rod cylinders are used both for the drive cylinder (4) as well as the first and second stage pneumatic cylinders (7, 8). Using tie rod cylinders allows for higher pressures inside the chambers as well as connecting, securing, and using a mutual end cap for the first stage pneumatic cylinder (7) and the second stage pneumatic cylinder (8). Tie rod cylinder construction is used in the design because it has a significant pressure cycle fatigue life advantage over welded construction. The inlets and outlets for natural gas are integrated in the end caps of the cylinder and then channeled into the cylinder itself. In the end cap (63) of final compression chamber (43) a single internal orifice (64), which communicates with both the inlet port (65) and outlet port (66), improves efficiency by minimizing the volume of high pressure gas that does not flow to CNG Storage Tank (9) at end of stroke as shown in FIG. 44A and FIG. 44B. The pistons in the pneumatic cylinders use internally lubricated seals and therefore will not discharge particulate into the natural gas. This is a benefit as the lubrication does not need to be replenished, particulate will not clog or damage the cylinders or orifices, but most importantly it will ensure clean and uncontaminated gas being provided to the CNG storage tank (9). Clean gas supply to the vehicle will increase the service life of on board CNG filters.

Cooling the compressor system may be accomplished by any of a variety of methods. One possible way to cool the required components is to submerge part or the entire device in a liquid as shown in FIG. 45 where system 111 is shown submerged in a cooling tank 50. It is contemplated that the cooling liquid could be the same fluid used through the pumps and to drive the hydraulic cylinder. By submerging the device in liquid all of the device would be convecting its heat to a liquid rather than the air and this is beneficial due to the higher convective heat transfer coefficient of liquids than air. Further, by submerging the entire device there would be a large amount of volume and therefore a large heat sink where a lot of heat can be rejected. A further benefit of submerging the pump, electric motor, and other components in the fluid is the fluid will dampen and lessen the sound from these components.

Alternatively only part of the system need be submerged in the enclosure/tank (200). For example, cooling the 1st and 2nd stage cylinders (7), (8) is very important as the temperature of their output gas is important and therefore could be submerged in the liquid. However, a component such as the relief valve or the unloading valve will generate a lot of heat and instead of adding it to the cooling fluid it could be rejected to the environment.

Although the principles, embodiments and operation of the present invention have been described in detail herein, this is not to be construed as being limited to the particular illustrative forms disclosed. They will thus become apparent to those skilled in the art that various modifications of the embodiments herein can be made without departing from the spirit or scope of the invention.

Number	Date	Country
61760390	Feb 2013	US
61820688	May 2013	US
61820670	May 2013	US

GAS COMPRESSOR

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CPC

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