GB2437742A

GB2437742A - Free piston engine

Info

Publication number: GB2437742A
Application number: GB0608559A
Authority: GB
Inventors: Nigel Paul Schofield
Original assignee: BOC Group Ltd; Edwards Ltd
Current assignee: Edwards Ltd
Priority date: 2006-05-02
Filing date: 2006-05-02
Publication date: 2007-11-07
Anticipated expiration: 2026-05-02
Also published as: GB2437742B; GB0608559D0

Abstract

A free piston gas engine, eg a gas-fuelled two-stroke, comprises a piston 18 reciprocating in a combustion cylinder 16. The piston 18 has a piston head 20 connected to a piston rod 22 and an elongate skirt 24 connected to the head 20 and surrounding the rod 22. The length of the skirt 24 is preferably twice the diameter of the piston head 20. A sealing element 33, eg a dry seal element formed from PTFE, extends around the external periphery of the piston skirt 24 at a location at or towards the end of the skirt remote from the piston head 20. The piston may be urged toward TDC by a return spring 68 and may define a pre-compression chamber 34 connected to the combustion chamber 36 by a scavenging passage 38 controlled by an intake valve 40. The exhaust valve 44 may be located centrally. The movement of the valves 40 and 44 may be controlled electronically by control unit (50,fig.2) or by movement of the piston 18. Electrical energy from a linear generator 70,74 is conveyed to a battery (200,fig.2) via the control unit 50.

Description

FREE PISTON ENGINE

This invention relates to a free piston engine.

One known free piston engine for providing a small motor for charging batteries or the like is a two-stroke engine in which a single piston is located within a combustion cylinder. A mixture of fuel and an oxidant, usually air, is compressed within the combustion cylinder by the piston, and then ignited to drive the piston against the force of a return spring so that the piston oscillates within the combustion cylinder. The piston is attached to a piston rod, which carries a coil assembly in close proximity to a stationary magnet to provide a linear power generator for producing electrical energy at the oscillatory frequency of the piston.

::::. One or more piston rings ar provided on the outer peripheral surface of the piston *::::5 to provide a seal with the inner cylindrical surface of the combustion cylinder to * prevent combustion gases from leaking around the sides of the piston. In order to prevent excessive wear between the piston ring and the combustion cylinder, *** lubricating oil is supplied to the piston ring. However, the requirement to supply *::: : lubricating oil to the piston ring increases the cost and complexity of the piston engine.

The present invention provides a free piston gas engine comprising a combustion cylinder and a piston reciprocally moveable within the cylinder, the piston comprising a piston head, a piston rod connected to the piston head, a skirt connected to the piston head and extending about the piston rod, and a sealing element extending about the external periphery of the skirt and located at or towards the end of the skirt remote from the piston head, for engaging an inner surface of the cylinder.

Preferred features of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 is a schematic cross-sectional view of a free piston engine; and Figure 2 is a schematic block diagram of a control system for controlling the engine of Figure 1.

S

Figure 1 illustrates a two-stroke, free piston engine. The engine 8 comprises a housing 10 having an inlet 12 for receiving a fuel-air mixture, and an exhaust 14 from which combustion gases are exhaust from the housing 10. The fuel may be any gaseous fuel, such as hydrogen, methane, butane or propane. A non-return valve may be provided within or upstream from the inlet 12.

The fuel-air mixture is conveyed by the inlet 12 into a combustion cylinder 16, having a substantially circular cross-section. A cylindrical piston 18 is located :.:::. within the combustion cylinder 16, substantially co-axial therewith. The piston 18 *....5 is reciprocally moveable within the combustion cylinder 16 between a TDC (top : dead centre) position and a BDC (bottom dead centre) position. *sss 5.

* The piston 18 comprises a piston head 20 attached to one end of a piston rod 22.

: An elongate cylindrical skirt 24 is attached to the piston head 20, the skirt 24 *:::: surrounding the piston rod 22 and having an outer cylindrical surface 26 lying adjacent to the inner cylindrical surface 28 of the combustion cylinder 16 to define a narrow annular channel 30 therebetween. The channel 30 preferably has a thickness in the range from 0.1 to 0.3 mm. The skirt 24 preferably has a length that is larger than the diameter of the piston head 20, preferably at least twice the diameter of the piston head 20. The piston head 20 and the skirt 24 are preferably formed from a material having a relatively low thermal conductivity, such as stainless steel, and may be integral with each other. A dry lubricated PTFE bearing 32 mounted on the housing 10 surrounds the piston rod 22 to guide the movement of the piston rod 22 during the reciprocal movement of the piston 18 and inhibit radial movement of the piston rod 22 relative to the housing 10.

The piston 18 includes least one peripheral seal element 33 located on the outer cylindrical surface 26 of the skirt 24, and positioned towards or at the end of the skirt 24 that is remote from the piston head 20. The seal element 33 contacts the inner surface 28 of the combustion cylinder 16, and therefore slides along the inner surface 28 of the combustion cylinder 16 as the piston 18 moves reciprocally within the combustion cylinder 16. The seal element is preferably a dry seal element, and is preferably formed from PTFE, preferably having one or more of a range of fillers.

io The piston 18 defines within the combustion cylinder 16 a pre-compression chamber 34 located to one side of the piston head 20 and extending about the piston rod 22, and a combustion chamber 36 located on the other side of the piston head 20. The combustion chamber 36 is connected to the pre-compression : * : :* chamber 34 by a scavenging passage 38. The charging of the combustion **** . . . * s chamber 36 with the fuel-air mixture is controlled by an intake valve 40 moveable * *1U * within an intake port 42 located to one side of the combustion chamber 36 and in * : fluid communication with the scavenging passage 38. The exhaust of combustion gases from the combustion chamber 36 is controlled by an exhaust valve 44 : * moveable within a central exhaust port 46 in fluid communication with the exhaust * o 14 from which combustion gases are exhaust from the housing 10. I...

A spark plug (not shown) is provided within the combustion chamber 36. With reference to Figure 2, a control unit 50 outputs a spark control signal 52 to the spark plug at a given timing, in response to which the spark plug produces a spark to ignite the fuel-air mixture within the combustion chamber 36.

The movement of the intake valve 40 and the exhaust valve 44 may be controlled electronically by the control unit 50. For example, the valves 40, 44 may be electromagnetically actuable valves, the opening and closing of which is controlled by signals output from the control unit 50. However, in this example the movement of these valves 40, 44 is actuated by the movement of the piston 18 within the combustion cylinder 16. As mentioned above, one end of the piston rod 22 is attached to the piston head 20. The other end of the piston rod 22 is connected to a cross member 54, which may be in the form of a disc. The disc 54 is housed within a generator chamber 56 located within the housing 10, the generator chamber 56 being separated from the pre-compression chamber 34 by the bearing 32 supporting the piston rod 22. The disc 54 is connected to a valve striker plate 58 by a plurality of rigid push rods or other connection members 60 extending about the housing 10 and each passing through a respective push rod aperture of the housing 10. Valve striker pins 62, 64 are attached to the striker plate 58, each for engaging a respective valve 40, 44.

The generator chamber 56 also houses a return spring 68 or other resilient means for applying a force to the disc 54 to urge the piston 18 towards the TDC position.

The return spring 68 may be a simple coil spring as illustrated in Figure 1 having :.. one end connected to or otherwise engaging the housing 10 and another end *:::: connected to or otherwise engaging the disc 54 so that with reciprocal movement * of the piston 18, the length of the spring 68 changes. Alternatively, the resilient means may be provided by a gas spring, or by one or more diaphragm springs *** fastened to the piston rod 22 and supported by the housing 10 for both urging the * * piston 18 towards the TDC position and restricting the radial movement of the *:..o piston rod 22 relative to the housing 10. In this case, the central guide bearing 32 may be omitted.

In use, when the piston is at the BDC position, the piston 18 is pushed towards the TDC position by the return spring 68 due to the elastic energy stored therein, and a compression stroke is initiated. As the piston 18 moves towards the TDC position, the volume of the combustion chamber 36 decreases, and so a charge of fuel-air mixture located within the combustion chamber 36 is compressed. In addition, as the piston 18 moves towards the TDC position the volume of the pre-compression chamber 34 increases, decreasing the pressure within that chamber.

This reduced pressure may draw a fresh charge of the fuel-air mixture into the pre-compression chamber 34 through the inlet 12 from a fuel-air mixture supply 150 (illustrated in Figure 2). Alternatively, this fresh charge may be injected into the pre-compression chamber 34 by the fuel-air mixture supply 150 upon receipt of an actuating signal from the control unit 50. The fresh charge of the fuel-air mixture is directed by the portion of the housing 10 supporting the bearing 32 along the inner surface 69 of the skirt 24 as it enters the pre-compression chamber 34 so as to take heat from the skirt 24 and prevent the sealing element 33 from over-heating.

When the piston 18 reach the TDC position, the control unit 50 supplies an ignition control signal 52 to the spark plug, in response to which the spark plug produces a spark to ignite the compressed fuel-air mixture within the combustion chamber 36.

The pressure within the combustion chamber 36 rises rapidly, with the combustion gases generated by the ignition of the fuel-air mixture pushing the piston 18 back towards the BDC position against the force of the spring 68 and initiating a return ::::. stroke. The annular channel 30 provides a thermal barrier to shield the sealing * 5 element 33 from the heat generated by the combustion of the fuel-air mixture *** within the combustion chamber 36.

* : The volume of the pre-compression chamber 34 gradually decreases during the * : * return stroke, and so the pressure of the fresh charge of fuel-air mixture within the *::::0 pre-compression chamber 34 gradually increases. Simultaneously, the pressure of the combustion gases within the combustion chamber 36 gradually decreases as the volume of the combustion chamber 36 increases.

As the piston 18 moves towards the BDC position, the valve striker pins 62, 64 come into contact with the intake valve 40 and the exhaust valve 44 respectively, and push the valves 40, 44 towards the combustion chamber 36, thereby opening the intake port 42 and the exhaust port 46. The combustion gases present within the combustion chamber 36 are exhaust from the combustion chamber 36 through the open exhaust port 46, and leave the engine through the exhaust 14.

Simultaneously, due to the pressure difference between the pre-compression chamber 34 and the combustion chamber 36 the fresh charge of the fuel-air mixture within the pre-compression chamber 34 flows into the combustion chamber 36 through the scavenging passage 38 and the open intake port 42.

The location of the intake port 42 to one side of the combustion chamber 36, and/or the design of at least one of the intake valve 40 and the intake port 42, causes the fresh charge of the fuel-air mixture to swirl around the inner surface 28 of the combustion cylinder 16 upon entering the combustion chamber 36. For example, guide vanes may be located on one or both of the intake valve 40 and the intake port 42 to direct the fresh charge of the fuel-air mixture towards the sides of the combustion chamber 36. This inhibits mixing between the fresh charge of the fuel-air mixture entering the combustion chamber 36 with the combustion gases leaving the combustion chamber 36 through the central exhaust port 46, and thereby prevents any significant loss of unburned fuel from the combustion chamber 36. *1S* * .15 *.*

: Furthermore, as the piston 18 moves towards the BDC position against the force of the return spring 68, the return spring 68 is compressed, thereby storing elastic energy. When the piston reaches the BDC position, the piston 18 is pushed back towards the combustion chamber 36 by the return spring 68, thereby initiating another compression stroke. As the piston 18 moves towards the TDC position, the valve striker pins 62, 64 move away from the intake valve 40 and the exhaust valve 44 respectively. Valve springs of the valves 40, 44 cause the valves 40, 44 to move to closed positions within the intake and exhaust ports 42, 46 respectively as the piston 18 moves towards the TDC position.

As mentioned above, the volume of the pre-compression chamber 34 increases as the piston 18 moves towards the TDC position, and a fresh charge of the fuel-air mixture is supplied to the pre-compression chamber 34 from the fuel-air supply 150. When the piston 18 reaches the TDC position, the control unit 50 outputs an ignition control signal 52 to the spark plug, which produces a spark to ignite the compressed fuel-air mixture within the combustion chamber 36.

The above-described strokes are repeated, so that the free piston engine continues to operate until the supply of the fuel-air mixture and ignition control signals 52 to the engine are stopped by the control unit 50.

The generator chamber 56 further houses a generator assembly for generating electrical energy during the reciprocal movement of the piston 18. The generator assembly is electrically connected to the control unit 50. The generator assembly is in the form of a linear generator, which in this example comprises a coil 70 wound around or otherwise located on the outer surface of a cylindrical skirt 72 io attached to the disc 54 so that the coil 70 is substantially co-axial with the piston rod 22. A stationary permanent magnet 74 surrounded by the coil 70 so that movement of the coil 70 within the magnetic field of the magnet 74 induces a current in the coil 70, thereby generating electrical energy. Alternatively, a magnet ::::. may be attached to the piston rod 22 or the skirt 72, with a stationary coil *::::.15 surrounding the magnet so that movement of the magnet relative to the coil : induces a current in the coil. I...

* : The energy generated by the generator assembly is conveyed to the control unit * 50, which stores the energy in a battery 200. As the generator assembly can : :.. operate as a motor when supplied with electrical energy, energy stored within the battery 200 can be supplied to the coil 70 to start up the engine. Alternatively, if available an external current source may be used to start up the engine.

As described above, one or more of the start up of the engine, the supply of the fuel-air mixture and the supply of the ignition control signals 52 to the engine may be controlled by the control unit 50. The control unit 50 may detect phases in the electrical energy generated by the generator assembly. The magnitude and phase of the current generated by the movement of the coil 70 relative to the magnet 74 can be detected by the control unit 50, and from these the control unit 50 can determine the position of the piston 18 relative to the combustion cylinder 16 and the direction of motion of the piston 18. The control unit 50 can then output an ignition control signal 52 to the spark plug when the piston 18 has reached the TDC position, and (if required) a control signal to the fuel-air mixture supply 150 to supply the fuel-air mixture to the inlet 12 of the engine as the piston 18 moves towards the TDC position. Alternatively, the output of the ignition control signal 52 to the spark plug and the control signal to the fuel-air mixture supply 150 may be performed on a predetermined timing basis. As another alternative, a position sensor may be used to monitor the movement of the piston 18 within the combustion cylinder 16, and for outputting signals to the control unit 50 which are indicative thereof. The control unit 50 can then issue the signals to the spark plug and the fuel-air mixture supply in dependence on the signals received from this io sensor. The control unit 50 may also control the flow of current in the coil 70 so that the piston 18 comes to rest in the TDC position. This can prevent the piston 18 overshooting the TDC position, and thereby control the compression ratio achieved by the engine. * S. * S S *S..

*:::: The use of dry elements for the bearing 32 and seal element 33 can enable the * engine to operate for long periods of time without any maintenance. In order to * .: : reduce wear, the piston stroke is relatively short so that the surface speed of the * : seal element 33 is relatively low, and so the piston head 20 has a relatively large *": * diameter to achieve sufficient volume of the fuel-air mixture within the combustion *.::20 chamber. The frequency of the reciprocation of the piston 18 within the combustion cylinder is relatively low, around 50 Hz in the preferred embodiment.

The positioning of the seal element 33 towards the end of the skirt 24 remote from the combustion chamber 36, and the relatively long length of the skirt 24 in comparison to the diameter of the piston head 20, serves to shield the seal element 33 from the high temperatures generated within the combustion chamber 36 upon ignition of the fuel-air mixture, and can enable the seal element 33 to operate at temperatures less than 100 C, that is, lower than the upper limit normally set for the operational temperature range for such dry elements. The use of a gaseous fuel is preferred as it allows the engine to be run at lower temperatures. A cooling jacket may be provided around the housing 10 to cool the engine during use to maintain the temperature of the dry elements 32, 33, below their upper limit.

The lack of any lubricating oil can also enable the overall size of the engine to be reduced. * *, * **S. **** * * **** * ** * S S S...

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Claims

CLAIMS

1. A free piston gas engine comprising a combustion cylinder and a piston reciprocally moveable within the cylinder, the piston comprising a piston head, a piston rod connected to the piston head, a skirt connected to the piston head and extending about the piston rod, and a sealing element extending about the external periphery of the skirt and located at or towards the end of the skirt remote from the piston head for engaging an inner surface of the cylinder.

2. An engine according to Claim 1, wherein the sealing element is a dry lubricated sealing element. * I. * I S

* SS S *::::* 3. An engine according to Claim I or Claim 2, wherein the skirt has a : length that is at least twice the diameter of the piston head. ****

: An engine according to any preceding claim, comprising a : combustion chamber located within the cylinder, the combustion *...s20 chamber comprising an intake port and an exhaust port each having I...

a valve moveably located therein.

5. An engine according to Claim 4, wherein the exhaust port is substantially co-axial with the piston rod.

6. An engine according to Claim 4 or Claim 5, wherein the valve moveably located within the intake port is shaped to direct a flow of gas towards the periphery of the combustion chamber.

7. An engine according to any of Claims 4 to 6, wherein the valves are electromagnetically actuable valves. -11 -

8. An engine according to any of Claims 4 to 6, wherein the piston has a valve actuating member connected thereto for reciprocal movement therewith and for engaging the valves to open the ports.

9. An engine according to Claim 8, wherein the valve actuating member carries a plurality of pins each for engaging a respective valve as the piston moves away from the combustion chamber.

10. An engine according to Claim 8 or Claim 9, comprising a plurality of connecting members connecting the valve actuating member to a cross member located on the piston rod, the rods being external of the combustion cylinder.

: * : * An engine according to any of Claims 5 to 10, comprising a spring for urging the piston towards the combustion chamber.

12. An engine according to Claim 11 when dependent from Claim 10, wherein the spring is coupled to the cross member. *** * S *

*...20 13. An engine according to any preceding claim, wherein the piston rod S...

is supported by a bearing for inhibiting radial movement of the piston relative to the cylinder.

14. An engine according to Claim 13, wherein the bearing is a dry lubricated bearing.

15. An engine according to Claim 11, wherein said spring is a diaphragm spring for inhibiting radial movement of the piston relative to the cylinder.

16. An engine according to any preceding claim, comprising a coil reciprocally moveable with the piston within a magnetic field to generate electrical energy.

17. An engine according to Claim 16 when dependent from Claim 10, wherein the coil is carried by a skirt connected to the cross member and substantially co-axial with the piston rod.

18. An engine according to any of Claims 1 to 15, comprising a magnet reciprocally moveable with the piston relative to a coil to generate electrical energy.

19. An engine according to any preceding claim, wherein the piston head : *. and the skirt are formed from stainless steel. I.. * .15 * .. * S * S...

S S.. 55.

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