GB2396668A

GB2396668A - Extended cycle reciprocating gas expander

Info

Publication number: GB2396668A
Application number: GB0307488A
Authority: GB
Inventors: Thomas Tsoi Hei Ma
Original assignee: Individual
Current assignee: Individual
Priority date: 2002-12-24
Filing date: 2003-04-01
Publication date: 2004-06-30
Also published as: GB0307488D0

Abstract

A reciprocating gas expander 10,12 operates according to an extended cycle of four, six or more strokes, wherein the first two strokes are sequential expansion and exhaust strokes using a high pressure gas 30 as working fluid to produce power, and the remaining strokes are pairs of sequential filling and emptying strokes using ambient air as heat transfer fluid for transferring heat from outside the gas expander to inside the gas expander. The gas expander also contains a heat regenerator 14 for absorbing heat from the heat transfer fluid and releasing heat to the expanding gas. The invention achieves an expansion stroke which is near- isothermal and has significant advantages over the conventional gas expander by virtue of fewer components, smaller size, lighter weight, lower operating cost and higher efficiency.

Description

EXTENDED CYCLE RECIPROCATING GAS EXPANDER

Field of the invention

5 The present invention relates to a gas expander powered by expansion of a high pressure gas.

Background of the invention

lo There are two types of reciprocating gas expander powered by high pressure gas, the expander comprising at least one cylinder having a variable volume defined by a reciprocating piston which produces work cyclically when high pressure gas is admitted into the cylinder pushing 5 against the piston during the power stroke and then expelled from the cylinder displaced by the piston during the exhaust stroke. In a first type, the high pressure gas is admitted continuously into the cylinder according to an isobaric process in which the high gas pressure is applied constantly 20 along the entire power stroke of the piston and is released without doing expansion work on the piston. In a second type, a predetermined quantity of high pressure gas is admitted into the cylinder and allowed to expand to a lower pressure and temperature as it pushes against the piston 25 during the power stroke after which it is released when the gas pressure reaches near ambient pressure. In this latter case, more work could be extracted per unit mass of high pressure gas achieving high thermodynamic efficiency than in the isobaric case, but the torque produced during the power 30 stroke is lower than the isobaric case.

The first type of gas expander is typically driven by compressed air for powering work tools where torque is important. However, the second type driven by compressed 35 air or other high pressure gas is preferred for powering a road vehicle where thermodynamic efficiency, gas consumption and travelling range are the important parameters.

- 2 Depending on design, the thermodynamic efficiency of the second type of gas expander could lie between a lower limit where the expansion process is adiabatic and an upper limit where the expansion process is isothermal, the latter 5 being the ideal efficiency. The present invention is aimed at achieving a reciprocating gas expander in which the expansion process is as close to isothermal as possible.

Summary of the invention

According to the present invention, there is provided a reciprocating gas expander operating according to an extended cycle of 4, 6 or more strokes, wherein the first two strokes are sequential expansion and exhaust strokes using a high pressure gas as working fluid to produce power, and the remaining strokes are pairs of sequential filling and emptying strokes using ambient air as heat transfer fluid for transferring heat from outside the gas expander to inside the gas expander.

The reciprocating gas expander also contains a heat regenerator for absorbing heat from the heat transfer fluid and releasing heat to the expanding gas.

25 The reciprocating gas expander comprises at least one cylinder having a variable volume defined by a reciprocating piston which produces work when a predetermined quantity of high pressure gas at substantially ambient temperature serving as working fluid is admitted into the cylinder and 30 allowed to expand against the piston to produce power during the expansion stroke, and the expanded gas is subsequently expelled from the cylinder displaced by the piston during the exhaust stroke, characterized in that the reciprocating gas expander is operated according to an extended cycle 35 comprising after the said expansion and exhaust strokes, at least one pair of extra strokes each pair consisting of a filling stroke in which ambient air serving as heat transfer

- 3 fluid is drawn by the piston into the cylinder at substantially ambient pressure to fill the cylinder followed immediately by an emptying stroke in which the filled air is expelled by the piston at substantially ambient pressure out 5 of the cylinder, such that the filled ambient air warms the cylinder and piston and raises the gas expander temperature close to the temperature of the filled air during the extra strokes, before the extended cycle is repeated with the working fluid of fresh high pressure gas admitted into the lo cylinder during the next expansion stroke.

An open matrix heat regenerator constructed in fine mesh or thin wall cell structure of high heat capacity material is provided occupying the clearance space in the 5 cylinder and in intimate thermal contact with the gas or air inside the cylinder. The heat regenerator serves efficiently to absorb and store heat from the filled ambient air (heat transfer fluid) during the extra filling and emptying strokes of the extended cycle, and to release the JO stored heat to the expanding gas (working fluid) during the next expansion stroke.

In the invention, by using ambient air as heat transfer fluid to transfer external heat to the cylinder and piston :5 and the heat regenerator and raise the temperature of the cylinder and heat regenerator close to the temperature of the ambient air during the extra filling and emptying strokes, the admitted working fluid of high pressure gas expanding (and potentially cooling) during the next JO expansion stroke will be warmed progressively by the heat regenerator and stay at substantially the same temperature as the heat regenerator, thus enabling the admitted high pressure gas in the gas expander to achieve an expansion process which is near-isothermal at substantially ambient 35 temperature.

- 4 - The invention supports near-isothermal expansion by relaying heat to the expanding gas inside the cylinder very efficiently from outside the cylinder by virtue of the fact that the open matrix of the heat regenerator has a very 5 large surface area and is in intimate thermal contact with the gas and with ambient air at different times in the extended cycle, while the high heat capacity of the heat regenerator provides ample heat directly to the expanding gas without itself dropping significantly in temperature.

lo This has significant advantage compared with other known methods of attempting near-isothermal expansion by heating the cylinder from the outside but relying on the smooth inside surface of the cylinder and piston to transfer heat through to the gas. In these known methods, the heat transfer inside the cylinder is area-limited and rate-

limited because of poor internal mixing, making it inefficient and inadequate to support near-isothermal expansion. 20 In the present invention, because the filling and emptying of the heat transfer fluid during the extra strokes of the extended cycle take place at substantially ambient pressure, the pumping work associated with these two extra strokes will be small and does not significantly affect the 25 mechanical efficiency of the gas expander. Power produced by the gas expander comes entirely from the pressure energy stored in the high pressure gas and the expander operates at a mean temperature which is close to but below ambient temperature. If necessary, the sequential filling and emptying strokes may be repeated in pairs to allow the heat regenerator to soak up more ambient heat more thoroughly.

Thus the reciprocating gas expander could be operated 35 according to an extended cycle of 4, 6 or more strokes, where the first two strokes are the working strokes using the high pressure gas as working fluid to produce power and

- 5 the remaining pairs of strokes are the warming strokes using the ambient air as heat transfer fluid for transferring heat into the cylinder which also contains a heat regenerator acting as a heat storage.

The gas and air flows in and out of the cylinder during the various strokes of the extended cycle of the present invention may be programmed by appropriate timed valves driven by mechanical, electrical or hydraulic actuators and 1O controlling the flows through corresponding ports in the cylinder. Preferably, the same exhaust valve and port for the exhaust stroke is used as the filling and emptying valve and port for the extra strokes, thus bringing the gas or air in and out of the cylinder along a common passage with the exhaust valve timed to remain open during the exhaust and extra strokes, and to close only during the expansion stroke. Preferably, additional respective one-way valves are 20 provided in the inlet and outlet openings of the common passage to the ambient, arranged such that fresh ambient air is drawn into the passage only through the inlet one-way valve and the expanded gas and used ambient air are expelled out of the passage only through the outlet one-way valve.

In the invention, the admitted high pressure gas may be air or nitrogen gas supplied from a high pressure gas source at substantially ambient temperature such as a compressed gas storage cylinder.

During each cycle, the admission of a predetermined quantity of high pressure gas into the cylinder of the gas expander must be timed to take place rapidly at the beginning of the expansion stroke in order to allow the gas 35 to expand during most of the expansion stroke. This may be performed in two steps using a buffer chamber having a predetermined volume and connected between the high pressure

- 6 gas source and the cylinder by respective timed inlet and outlet shutoff valves synchronized with the piston strokes.

In the first step, high pressure gas is admitted into the buffer chamber with the outlet valve previously closed, by s briefly opening and then closing the inlet valve some time during the exhaust and extra strokes of the extended cycle.

In the second step, the high pressure gas stored in buffer chamber is released into the cylinder by opening the outlet valve at the beginning of the expansion stroke and closing lo it some time before the inlet valve is opened.

Preferably, the buffer chamber volume isolated by the timed shut-off valves is sufficiently small for it to be included with the expansion cylinder volume of the gas IS expander during the expansion stroke of the gas expander, such that the high pressure gas expands from the buffer chamber directly into the cylinder during substantially the full expansion stroke of the gas expander and achieves a high expansion ratio relative to and including the volume of 20 the buffer chamber sufficiently to bring the expanded air pressure to substantially ambient pressure at the end of the expansion stroke.

The above buffer chamber volume isolated by the timed 2s shut-off valves may be constructed in the form of at least one high pressure gas pipe forming part of an ambient heat exchanger. In this case during the expansion stroke, the expanding gas within this high pressure gas pipe would continue to absorb ambient heat from the heat exchanger 30 while expanding into the cylinder. This is additional to the heat absorbed within the cylinder from the heat regenerator thus achieving in the gas an expansion process which is near-isothermal at substantially ambient temperature. 3s The above arrangement of timed admission of the high pressure gas performed in two steps significantly relaxes

- 7 - the actuation design specification of the gas expander inlet

valve (the same valve as the buffer chamber timed outlet shut-off valve) which could have more than 180 crank angle opening period. This is to be contrasted with a 5 conventional reciprocating gas expander connected directly to a high pressure stock gas supply, where the gas expander inlet valve must be open and closed very quickly within a very short time period while the piston is still near TDC in order to limit the high pressure gas entering the cylinder lo and allow it to expand with a high expansion ratio after the inlet valve is closed. Such short valve opening period however poses severe problemsto the design of the valve actuation system which explains why most conventional gas expanders operate by isobaric (constant pressure) 5 displacement because of the necessarily long inlet valve opening period.

In a preferred embodiment, the above buffer chamber with its inlet and outlet shut-off valves may be designed as 20 a compact unit with the two shut-off valves combined into a multi-channel valve actuated by a single timed actuator.

This improves the timed control of gas admission into the gas expander, simplifies the installation and reduces cost.

25 The present invention is a companion invention with another one based on a parallel principle described in a co pending British Patent Application GB 0300136.9.0 by the same applicant for an extended cycle reciprocating gas compressor. The two inventions are particularly suitable 30 for use together in an extended cycle Ericsson engine (described in another co-pending patent application GB 0300134.4 by the same applicant) with heat supplied by an external fuel burner, and with air as working fluid undergoing near-isothermal compression and near- isothermal 35 expansion during the appropriate working strokes of the respective extended cycles, interspaced by extra heat transfer strokes with more air as heat transfer fluid.

- 8 Compared with other known Ericsson cycle engines, this extended cycle engine has higher efficiency, fewer components and less complicated start up procedure.

5 The present invention is suitable for application in a compressed gas driven vehicle by near-isothermal expansion with high efficiency. The compressed gas may be compressed air or compressed nitrogen gas. It may also be high pressure air or high pressure nitrogen gas produced by evaporating lo liquefied air or liquefied nitrogen prior to admission to the expander. When taking into consideration the production of the compressed gas or high pressure gas (treated as fuel for the vehicle) being carried out also by near-isothermal compression according to the above-mentioned co-pending IS invention, the economics of the energy life cycle for a transport system based on an infrastructure of compressed gas or liquefied gas could become very attractive especially for use in inner city areas with the added advantage of zero emission from the vehicles, plus quicker refuelling, longer 20 range and lower operating cost than battery driven vehicles.

The present invention is very effective in achieving near-isothermal expansion by internal heat transfer. This not only has superior performance compared with the 25 previously known methods by external heat transfer, but also eliminates the ancillary components necessary to provide the latter, such as fins, fan, fan drive eta disposed around the outside of the expansion cylinder in order to facilitate heat exchange through the walls to the gas inside. This 30 gives the extended cycle gas expander of the present invention very significant advantages over the conventional gas expander by virtue of fewer components, smaller size, lighter weight, lower operating cost and higher efficiency.

- 9 Brief description of the drawing

The invention will now be described further, by way of example, with reference to the accompany drawings in which 5 Figure 1 shows a schematic view of a reciprocating gas expander operating according to the present invention at substantially ambient temperature, Figure 2 shows a schematic plan view of a preferred embodiment of a heat regenerator mounted lo inside the gas expander, Figure 3 shows a schematic plan view of an alternative embodiment of a heat regenerator mounted inside the gas expander, and Figure 4 shows a preferred embodiment of a buffer 5 chamber and shut-off valves for use in substitution in Figure 1.

Detailed description of the preferred embodiment

JO Figure 1 shows a reciprocating gas expander comprising a cylinder 10 having a variable volume defined by a reciprocating piston 12 which produces work when a predetermined quantity of high pressure gas serving as working fluid supplied from a high pressure gas tank 30 at 25 substantially ambient temperature is admitted into the cylinder 10 and allowed to expand against the piston 12 to produce power during the expansion stroke, and the expanded gas is subsequently expelled from the cylinder 10 displaced by the piston 12 during the exhaust stroke. The 30 reciprocating gas expander is further equipped to operate according to an extended cycle comprising after the said expansion and exhaust strokes, at least one pair of extra strokes each pair consisting of a filling stroke in which ambient air serving as heat transfer fluid is drawn by the 35 piston 12 (as shown by the ingoing arrow) into the cylinder 10 at substantially ambient pressure to fill the cylinder 10 followed immediately by an emptying stroke in which the

- 10 filled air is expelled by the piston 12 (as shown by the outgoing arrow) at substantially ambient pressure out of the cylinder 10 back to the ambient. In use, the filled ambient air warms the cylinder 10 and piston 12 and raises the gas 5 expander temperature close to the temperature of the filled air during the extra strokes, before the extended cycle is repeated with the working fluid of fresh high pressure gas admitted into the cylinder 10 during the next expansion stroke. An open matrix heat regenerator 14 constructed in fine mesh or thin wall cell structure of high heat capacity material is also provided occupying the clearance space in the cylinder 10 and in intimate thermal contact with the gas 5 or air inside the cylinder 10. The heat regenerator 14 serves efficiently to absorb and store heat from the filled ambient air (heat transfer fluid) during the extra filling and emptying strokes of the extended cycle, and to release the stored heat to the expanding gas (working fluid) during 20 the next expansion stroke.

Figure 1 shows the piston position during a filling stroke of the extended cycle when ambient air is drawn into the cylinder 10 through a one-way valve 22 along a filling 25 port 20 controlled by an opened filling valve 18. The filling air passes through the open matrix of the heat regenerator 14 and rapidly attains equilibrium temperature with the heat regenerator 14.

30 Figure 2 shows an enlarged plan view of the preferred embodiment of the heat regenerator comprising an open matrix 14 of thin wall cell structure secured across the cylinder 10 with an unobstructed space above the matrix 14. Figure 3 shows an enlarged plan view of an alternative embodiment of 35 the heat regenerator comprising a dense array of fins 141 extending from the roof of the cylinder 10 and arranged radially around the filling and emptying valve 18. Another

- 11 set of dense array of fins may be provided extending from the crown of the piston 12 (not shown) to serve as an additional heat regenerator overlapping into the spaces between the fins 141 in the roof of the cylinder 10 when the 5 piston 12 approaches the top of its stroke.

The function of the heat regenerator 14 is to absorb or release heat to the air or gas passing through it depending on the initial temperature of the air or gas being hotter or lo colder than the heat regenerator 14. Because the heat regenerator 14 has a high heat capacity, it can maintain a stable mean temperature with only a small temperature variation up or down depending on the direction of heat transfer with the air or gas passing through it, and because 5 it has a very large heat transfer surface area, it can rapidly bring the air or gas temperature close to the matrix mean temperature as the air or gas exchanges heat with the matrix whatever is the initial temperature of the air or gas. In Figure 1, ambient air is used as heat transfer fluid to transfer external heat to the cylinder 10 and piston 12 and the heat regenerator 14 and raise the temperature of the cylinder 10 and heat regenerator 14 close to the temperature 25 of the ambient air during the extra filling and emptying strokes of the extended cycle. In the following expansion stroke of the cycle, the working fluid of high pressure gas is admitted into the cylinder 10 and allowed to expand (and potentially cool) while producing work, but the gas will be JO warmed progressively by the heat regenerator 14 and stay at substantially the same temperature as the heat regenerator 14, thus achieving an expansion process which is near-

isothermal at substantially ambient temperature.

35 Because the filling and emptying of the heat transfer fluid during the extra strokes of the extended cycle take place at substantially ambient pressure, the pumping work

- 12 associated with these two extra strokes will be small and does not significantly affect the mechanical efficiency of the gas expander. Power produced by the gas expander comes entirely from the pressure energy stored in the high s pressure gas and the expander operates at a mean temperature which is close to but below ambient temperature.

If necessary, the sequential filling and emptying strokes may be repeated in pairs to allow the heat lo regenerator to soak up more ambient heat more thoroughly.

Thus the reciprocating gas expander could be operated according to an extended cycle of 4, 6 or more strokes, where the first two strokes are the working strokes using the high pressure gas as working fluid to produce power and Is the remaining pairs of strokes are the warming strokes using the ambient air as heat transfer fluid for transferring heat into the cylinder 10 which also contains the heat regenerator 14 acting as a heat storage.

20 The gas and air flows in and out of the cylinder 10 during the various strokes of the extended cycle may be programmed by appropriate timed valves driven by mechanical, electrical or hydraulic actuators and controlling the flows through corresponding ports in the cylinder 10.

In Figure 1, the same exhaust valve 18 and port 20 for the exhaust stroke is used as the filling and emptying valve 18 and port 20 for the extra strokes, thus bringing the gas or air in and out of the cylinder 10 along a common passage 30 20 with the exhaust valve 18 timed to remain open during the exhaust and extra strokes, and to close only during the expansion stroke. In this case, the exhaust valve 18 may conveniently be actuated by a cam (not shown) driven at 1/2 expander speed if the extended cycle is 4 strokes, or 1/3 3s expander speed if the extended cycle is 6 strokes.

- 13 In Figure 1, additional respective one-way valves 22, 24 are provided in the inlet and outlet openings of the common passage 20 to the ambient, arranged such that fresh ambient air is drawn into the passage 20 only through the 5 inlet one-way valve 22 and the expanded gas and used ambient air are expelled out of the passage 20 only through the outlet one-way valve 24.

In Figure 1, the admitted high pressure gas (working lo fluid) may be air or nitrogen gas supplied from a high pressure gas tank 30 at substantially ambient temperature.

During each cycle, the admission of a predetermined quantity of high pressure gas into the cylinder 10 of the gas expander must be timed to take place rapidly at the 5 beginning of the expansion stroke in order to allow the gas to expand during most of the expansion stroke. This may be performed in two steps using a buffer chamber comprising at least one high pressure gas pipe 32 having a predetermined volume and connected between the high pressure gas tank 30 20 and the cylinder 10 by respective timed inlet and outlet shut-off valves 26, 28 synchronized with the piston strokes.

In the first step, high pressure gas is admitted into the high pressure gas pipe 32 with the outlet valve 28 previously closed, by briefly opening and then closing the 2s inlet valve 26 some time during the exhaust and extra strokes of the extended cycle. In the second step, the high pressure gas stored in high pressure gas pipe 32 is released into the cylinder 10 by opening the outlet valve 28 at the beginning of the expansion stroke and closing it some time 30 before the inlet valve 26 is opened.

The volume of the high pressure gas pipe 32 isolated by the timed shutoff valves 26, 28 should be sufficiently small for it to be included with the expansion cylinder 35 volume 10 of the gas expander during the expansion stroke of the gas expander, such that the high pressure gas expands from the high pressure gas pipe 32 directly into the

- 14 cylinder 10 during the full expansion stroke of the gas expander and achieves a high expansion ratio relative to and including the volume of the high pressure gas pipe 32 sufficiently to bring the expanded air pressure to 5 substantially ambient pressure at the end of the expansion stroke. In Figure 1, the high pressure gas pipe 32 isolated by the timed shut-off valves 26, 28 also forms part of an lo ambient heat exchanger 48. In this case during the expansion stroke, the expanding gas within the high pressure gas pipe 32 would continue to absorb ambient heat from the heat exchanger 48 while expanding into the cylinder 10.

This is additional to the heat absorbed within the cylinder; IS 10 from the heat regenerator 14 thus achieving in the gas an expansion process which is near-isothermal at substantially ambient temperature.

The above arrangement of timed admission of the high So pressure gas performed in two steps significantly relaxes the actuation design specification of the gas expander inlet

valve 28 (the same valve as the buffer chamber timed outlet shut-off valve 28) which could have more than 180 crank angle opening period. This is to be contrasted with a Is conventional reciprocating gas expander connected directly to a high pressure stock gas supply, where the gas expander inlet valve must be open and closed very quickly within a very short time period while the piston is still near TDC in order to limit the high pressure gas entering the cylinder So and allow it to expand with a high expansion ratio after the inlet valve is closed. Such short valve opening period however poses severe problems to the design of the valve i actuation system.

as In a preferred embodiment shown in Figure 4, the above high pressure gas pipe 32 with its inlet and outlet shut-off valves 26, 28 is designed as a compact unit 50 with the two

- 15 shut-off valves combined into a multi-channel valve 26/28 actuated by a single timed actuator. This improves the timed control of gas admission into the gas expander, simplifies the installation and reduces cost.

The gas expander shown in Figures 1 is very effective in achieving nearisothermal expansion by internal heat transfer. This not only has superior performance compared with the previously known methods by external heat transfer, lo but also eliminates the ancillary components necessary to provide the latter, such as fins, fan, fan drive etc disposed around the outside of the expansion cylinder in order to facilitate heat exchange through the walls to the gas inside. This gives the extended cycle gas expander of 5 the present invention very significant advantages over the conventional gas expander by virtue of fewer components, smaller size, lighter weight, lower operating cost and higher efficiency.

20 The present invention is suitable for application in a compressed gas driven vehicle by near-isothermal expansion with high efficiency, for use in inner city areas with the advantage of zero emission from the vehicle, plus quicker refuelling, longer range and lower operating cost than a 25 battery driven vehicle.

Claims

- 16 CLAIMS

1. A reciprocating gas expander operating according to an extended cycle of 4, 6 or more strokes, wherein the 5 first two strokes are sequential expansion and exhaust strokes using a high pressure gas as working fluid to produce power, and the remaining strokes are pairs of sequential filling and emptying strokes using ambient air as heat transfer fluid for transferring heat from outside the lo expander to inside the gas expander.

2. A reciprocating gas expander as claimed in claim 1 wherein a heat regenerator is provided inside the gas expander for absorbing heat from the heat transfer fluid and 5 releasing heat to the expanding gas.

3. A reciprocating gas expander as claimed in claim 1 comprising at least one cylinder having a variable volume defined by a reciprocating piston which produces work when a 20 predetermined quantity of high pressure gas at substantially ambient temperature serving as working fluid is admitted into the cylinder and allowed to expand against the piston to produce power during the expansion stroke, and the expanded gas is subsequently expelled from the cylinder 25 displaced by the piston during the exhaust stroke, characterized in that the reciprocating gas expander is operated according to an extended cycle comprising after the said expansion and exhaust strokes, at least one pair of extra strokes each pair consisting of a filling stroke in 30 which ambient air serving as heat transfer fluid is drawn by the piston into the cylinder at substantially ambient pressure to fill the cylinder followed immediately by an emptying stroke in which the filled air is expelled by the piston at substantially ambient pressure out of the 3s cylinder, such that the filled ambient air warms the cylinder and piston and raises the gas expander temperature close to the temperature of the filled air during the extra

- 17 strokes, before the extended cycle is repeated with the working fluid of fresh high pressure gas admitted into the cylinder during the next expansion stroke.

5

4. A reciprocating gas expander as claimed in claim 2 and 3, wherein an open matrix heat regenerator of high heat capacity is provided occupying the clearance space in the cylinder and in intimate thermal contact with the gas or air inside the cylinder, the heat regenerator serving lo efficiently to absorb and store heat from the filled ambient air (heat transfer fluid) during the extra filling and emptying strokes of the extended cycle, and to release the stored heat to the expanding gas (working fluid) during the next expansion stroke.

5. A reciprocating gas expander as claimed in any one of claims 3 or 4, wherein the gas and air flows in and out of the cylinder during the various strokes of the extended cycle are programmed by appropriate timed valves driven by 20 mechanical, electrical or hydraulic actuators and controlling the flows through corresponding ports in the cylinder.

6. A reciprocating gas expander as claimed in claim 5 25 wherein the same exhaust valve and port for the exhaust stroke is used as the filling and emptying valve and port for the extra strokes, thus bringing the gas or air in and out of the cylinder along a common passage with the exhaust valve timed to remain open during the exhaust and extra 30 strokes, and to close only during the expansion stroke.

7. A reciprocating gas expander as claimed in claim 6 wherein additional respective one-way valves are provided in the inlet and outlet openings of the common passage to the ambient, arranged such that fresh ambient air is drawn into the passage only through the inlet one-way valve and the

- 18 -

expanded gas and used ambient air are expelled out of the passage only through the outlet one-way valve.

8. A reciprocating gas expander as claimed in any one 5 of claims 3 to 7, wherein the admitted high pressure gas includes air or nitrogen gas, supplied from a high pressure gas source at substantially ambient temperature such as a compressed gas storage cylinder.

lo

9. A reciprocating gas expander as claimed in claim 8 wherein during each cycle, timed admission of a predetermined quantity of high pressure gas into the cylinder of the gas expander is performed in two steps using a buffer chamber having a predetermined volume and connected 5 between the high pressure gas source and the cylinder by respective timed inlet and outlet shut-off valves synchronized with the piston strokes, wherein during the first step, high pressure gas is admitted into the buffer chamber with the outlet valve previously closed, by briefly 20 opening and then closing the inlet valve some time during the exhaust and extra strokes of the extended cycle, and during the second step, the high pressure gas stored in buffer chamber is released into the cylinder by opening the outlet valve at the beginning of the expansion stroke and closing it some time before the inlet valve is opened.

1O. A reciprocating gas expander as claimed in claim 9 wherein the buffer chamber volume isolated by the timed shut-off valves is sufficiently small for it to be included 30 with the expansion cylinder volume of the gas expander during the expansion stroke of the gas expander, such that the high pressure gas expands from the buffer chamber directly into the cylinder during substantially the full expansion stroke of the gas expander and achieves a high expansion ratio relative to and including the volume of the buffer chamber sufficiently to bring the expanded air

- 19 pressure to substantially ambient pressure at the end of the expansion stroke.

11. A reciprocating gas expander as claimed in claim 5 10, wherein the buffer chamber is constructed in the form of at least one high pressure gas pipe forming part of a heat exchanger.

12. A reciprocating gas expander as claimed in any one lo of claims 9 to 11, wherein the buffer chamber with its inlet and outlet shut-off valves is designed as a compact unit with the two shut-off valves combined into a multi-channel valve actuated by a single timed actuator.

5

13. A reciprocating gas expander as claimed in any one of claims 3 to 12, operating according to an extended cycle of 4, 6 or more strokes, where the first two strokes are the working strokes using the high pressure gas as working fluid to produce power and the remaining pairs of strokes are the 20 warming strokes using the ambient air as heat transfer fluid for transferring heat into the cylinder which also contains a heat regenerator acting as a heat storage.

14. A reciprocating gas expander as claimed in claim 13, operating as a gas expander comprising at least one cylinder having a variable volume defined by a reciprocating piston which produces work when a predetermined quantity of high pressure gas at substantially ambient temperature serving as working fluid is admitted into the cylinder and JO allowed to expand against the piston to produce power during the expansion stroke, and the expanded gas is subsequently expelled from the cylinder displaced by the piston during the exhaust stroke, characterized in that an open matrix heat regenerator of high heat capacity is provided occupying the clearance space in the cylinder and the reciprocating gas expander is operated according to an extended cycle comprising after the said expansion and exhaust strokes, at

- 20 least one pair of extra strokes each pair consisting of a filling stroke in which ambient air serving as heat transfer fluid is drawn by the piston into the cylinder at substantially ambient pressure to fill the cylinder followed 5 immediately by an emptying stroke in which the filled air is expelled by the piston at substantially ambient pressure out of the cylinder, such that the filled ambient air warms the heat regenerator inside the cylinder and raises the heat regenerator temperature close to the temperature of the lo filled air during the extra strokes, before the extended cycle is repeated with the working fluid of fresh high pressure gas admitted into the cylinder, warmed by the heat regenerator while expanding to produce power during the next expansion stroke.

15. A reciprocating gas expander as claimed in any preceding claim, used to power a compressed gas driven vehicle.