GB2214574A

GB2214574A - Improved piston engine

Info

Publication number: GB2214574A
Application number: GB8801980A
Authority: GB
Inventors: John Walter Rilett
Original assignee: Metal Box PLC
Current assignee: Crown Packaging UK Ltd
Priority date: 1988-01-29
Filing date: 1988-01-29
Publication date: 1989-09-06
Also published as: GB8801980D0

Abstract

A piston engine powered by expanding fluid is characterised by the provision of internal valve gear. The engine comprises a hollow piston (7) for reciprocation within a working cylinder (20) for extracting work from a working fluid under pressure. The valve gear includes an inlet valve (4) mounted in the working cylinder head and a poppet-type exhaust valve (11) mounted substantially within and guided by and on the centre-line of the hollow piston. An inlet valve operating stem (13) attached to the exhaust valve opens the inlet valve as the piston approaches the cylinder head. A spring opens the exhaust valve when the piston is in the vicinity of the end of its expansion stroke thereby causing the working fluid to be exhausted through one or more outlets (15) in the piston throughout substantially the whole of its return stroke. The exhaust valve closes when the said piston is in the vicinity of the end of its return stroke. <IMAGE>

Description

IMPROVED PISTON ENGINE The present invention relates to piston engines having valve gear for controlling the flow of working fluid to and from the power cylinder of the engine. It is particularly concerned with such engines which are driven pneumatically, i.e. from a supply of compressible fluid such as compressed air, nitrogen, steam, carbon dioxide etc., but the invention is also applicable to other types of motor including hydraulic motors and internal combustion engines of, for example, the spark ignition and compression ignition types.

The field of valve gear for piston engines is certainly as old as the invention of the steam engine: From the start it was necessary to equip steam-operated piston engines with valve gear - which essentially comprises one or more mechanical linkages - so that the mechanical output of the steam engine would be linked with and made to operate any inlet, exhaust, water spray or other valves in a suitably-timed relationship with the cyclical motion of the engine. By means of such valve gear it was thus possible to control the admission and exhaust of steam (and, in atmospheric engines, its condensation) so as to occur at the desired instant, and to last for the desired duration, of each working cycle; in short, the valve gear controlled the "timing" of the engine.

At the beginning of steam engine development it was the practice to mount this valve gear so as to be external; that is to say, the valve gear was mounted outside the working (or "power") cylinder, and usually comprised one or more push-rods or other mechanical linkages, driven by a crank, sheave, cam or gearwheel on the main crankshaft, and arranged to actuate one or more rocker arms (to drive poppet-type inlet or exhaust valves) or oscillating or rotary or slide or sleeve or other types of valve. This choice of external valve gear is still the preferred one to this day, and led to the development and entrenchment of external valve gear in the internal combustion engine where - in the four-stroke cycle at least there is virtually no alternative to external valve gear comprising poppet-type valves actuated by a cam-shaft driven from the main crankshaft.All of such valve gear (barring the head of each poppet valve) lies outside the engine's power cylinders and, in consequence, has to be mounted, guided or borne by bearings, supports, brackets or guides provided for the purpose. The manufacturing cost of such external valve gear, including its means of support of whatever type, is typically several tens of pounds sterling.

In the long history of steam engine development there were, however, occasional departures from the rule of external valve gear, and certainly from the late nineteenth century onwards several innovations were made whereby at least some of the valve gear was mounted inside the power cylinder. One of the most notable of such innovations is described in US Patent No. 361169 (Lowrie) and discloses a slide-valve working within a hollow extension of the piston, which in turn slides in a hollow extension of the cylinder head having steam inlet parts therein. The slide valve is actuated by a cam on the small end of the connecting rod so as to control the admission of steam to the power cylinder on the working stroke and to put the cylinder space in communication with the exhaust during the return stroke, through ports in the piston communicating with ports in the side of the cylinder.

The construction of this engine, however, would be at the present time rather expensive, especially owing to the difficulty of maintaining concentricity between the piston's hollow extension and the mating extension in the cylinder head, which today would lead to a manufacturing cost (of the valve gear) measured in tens of pounds sterling.

Another invention, shown in US Patent No. 671394 (Curtis), discloses the use of an exhaust valve mounted to one side of the crown of the piston and actuated by a side projection on the small end of the connecting rod, but the inlet valve is actuated in the conventional manner by means of a push-rod driven by a cam on the crankshaft; this again is quite expensive.

The invention of US Patent No. 3910160 (Divine) likewise shows the use of side-projections on the small end of the connecting rod, but in this case one each is provided for the exhaust (scavenge) valve and for the inlet valve on opposite sides of the piston crown. Again good concentricity would be needed for the mating guides in the piston crown and cylinder head for the inlet valve, and this would contribute to rather high manufacturing costs.

US Patent No. 2671434 (Schmiedeskamp) discloses an exhaust (flap) valve to one side of the piston centre-line, opened by a stem which abuts a bearing when the piston nears the end of the forward stroke. The exhaust (flap) valve strikes the stem of an inlet valve (mounted in the cylinder head) in order to open it at the end of the piston's return stroke.

In this invention the positioning of exhaust and inlet valves so as to be offset from the piston's centre-line adds significantly to manufacturing cost.

UK Patent No. 2007313 (Rilett) discloses an inlet valve in the cylinder head opened by a valve operating member carried in the centre of the piston's crown and moved axially by a cam on the small end of the connecting rod. This invention does not provide an exhaust valve per nor means to continue exhausting during the piston's return stroke.

In contrast, the present invention has, as its primary object, the provision of totally internal valve gear adapted to continue exhausting the spent working fluid during substantially the whole of the piston's return stroke and comprising means whereby an inlet valve mounted in the cylinder head may be opened when the piston is at the end of its return stroke.

A further object of the present invention is to provide valve gear of substantially lower expense than heretofore, in particular by mounting such valve gear substantially on the piston's centre-line.

According to the present invention in its simplest form, there is provided a piston engine having a piston reciprocable in a bore in a cylinder, a cylinder head including a valve seat for an inlet valve disposed axially therein, an exhaust valve disposed axially in the piston, means carried by the piston to open the inlet valve when the piston approaches the cylinder head, and biasing means to hold the exhaust valve open during a substantial proportion of the return stroke of the piston to permit discharge through the piston of spent working fluid displaced from the cylinder, characterised in that the exhaust valve is of the poppet type and the means to open the inlet valve is a stem which projects axially from the piston to contact the inlet valve.

The crux of the present invention is the provision of a piston which acts to house, enclose and guide a poppet-type exhaust valve, in contrast to conventional external valve gear which must be mounted substantially outside the working cylinder. Furthermore, the exhaust valve of the present invention is actuated so as to be open during substantially the whole of the piston's return stroke, allowing the escape of the spent working fluid and preventing the undesirable compression of residual working fluid which prevents adequate "breathing" in conventional "uniflow" engines and so limits their expansion ratios and efficiencies.

It should be explained that, in conventional uniflow engines, exhaust ports are provided in the walls of the working cylinder and these ports are uncovered by the crown of the piston as it nears the bottom of its expansion stroke; clearly, to achieve a high expansion ratio (and therefore efficiency) these unit low exhaust ports should be positioned as low as possible; unfortunately, this means that the piston, on its upward return stroke, traps a substantial amount of residual spent working fluid above its crown as it covers the exhaust ports; this trapped working fluid is thus compressed by the rising piston with the result that, when the piston reaches the top of its return stroke and the inlet valve is opened, the pressure in the working cylinder is already close to that of the inlet supply pressure and, in consequence, "breathing" is difficult and little fresh working fluid will flow into the working cylinder. Thus, both the expansion ratio and the work output of the conventional unit low engine are limited undesirably.

The present invention achieves the object of opening the inlet valve when the piston approaches the top of its return stroke by the provision of an inlet valve operating member (hereinafter referred to as a "stem" or "pintle") which is preferably secured to the top of the exhaust valve. This stem or pintle acts to open the inlet valve by, for example, striking the ball of a non-return valve type of inlet valve so as to lift it from its seat and allow admission of fresh working fluid. The timing of this admission may be controlled, according to the present invention, by the motion of the exhaust valve and the stem or pintle when the piston is in the region of the top of its return stroke (hereinafter referred to as "top dead centre").

The object of exhausting the spent working fluid through the base of the piston so as to avoid the need for exhaust ports in the cylinder wall is accomplished in one embodiment by providing an exhaust valve seat (which may be sealed by the exhaust valve poppet) in the base of the hollow piston, such that lifting of the exhaust valve poppet allows spent working fluid to escape through a typically central hole in the exhaust valve seat; in another embodiment, the exhaust valve poppet is arranged to seal an exhaust valve seat at or near the top of the piston, and lifting of the exhaust valve poppet then allows escape of the spent working fluid along a plurality of longitudinal channels towards and to the base of the piston.

The object of providing valve gear of substantially lower expense than heretofore is achieved in particular by mounting the inlet valve, its valve seat and ball valve (if used), the exhaust valve poppet, its seat and all associated parts on the common centre-line of the piston and working cylinder, and also by employing such techniques as injection moulding, die-casting and sintering for mass-production of those parts. By these means the present invention may be embodied in a typical mass-produced artefact (such as a domestic lawn edge-trimmer or hedge clipper) at a manufacturing cost measured in pence rather than pounds sterling.

It is generally desirable to provide valve offset means so as to prevent overlap of exhaust and inlet timing, to prevent the inlet and exhaust valves from being simultaneously open, which would allow wasteful escape of admitted working fluid before it had been usefully expanded. To prevent such overlap, the present invention (in one embodiment) provides for the longitudinal channels (through which the exhausting working fluid escapes) to terminate some distance short of the exhaust valve seat, this distance being termed the "valve offset"; by this means the exhaust valve poppet can be lifted by an amount less than the valve offset - thereby to control or modify the opening of the inlet valve - without any great loss of working fluid.

To work usually in conjunction with the valve offset just described, a double-lobed cam may be provided to actuate both exhaust and inlet valves at differing times in the working cycle. Thus, for example, the profile of the "small end" of a connecting rod (for use in the present invention) may be in the shape of a first cam lobe disposed for example to the left side of the connecting rod's centre-line, and a second cam lobe disposed to the right side of the same centre-line; each of these cam lobes may strike a cam-follower attached to the exhaust valve poppet but, as the cam lobes are disposed left and right, they will lift the cam-follower and the exhaust valve poppet at differing times in the working cycle; thus the first cam lobe may be arranged to lift the exhaust valve poppet (by an amount greater than the valve offset) and open it at, for instance, 20 degrees after "bottom dead centre" and keep it open until, for instance, 20 degrees before "top dead centre", thus allowing exhaust throughout substantially the whole of the upward return stroke and thereby avoiding the undesirable compression described hereinbefore; and thus also the second cam lobe, being on the other hand of the small end, will not take effect until some few degrees after "top dead centre" and then may lift the exhaust valve poppet (by an amount less than the valve offset in order to prevent the working fluid from escaping) together with its stem or pintle so as to keep the inlet valve open for a longer time after "top dead centre" than could otherwise be achieved; it will be appreciated that the second cam lobe must have a profile giving less lift than that of the first cam lobe so that, when the second cam lobe is acting to control or modify the opening of the inlet valve, it lifts the exhaust valve poppet by an amount less than the valve offset and thereby avoids overlap of inlet and exhaust valve timings.

In other embodiments, the present invention is adapted to achieve pressure-sensitive initiation of the exhaust by means of an exhaust valve poppet which is sensitive to cylinder pressure by, for example, being biased by a compression spring acting against the cylinder pressure so that, when the cylinder pressure falls below a certain threshold pressure, the compression spring overcomes the force derived from the cylinder pressure acting on the exhaust poppet and acts to lift the exhaust poppet and to allow the spent working fluid to escape through the base of the piston.This adaptation of the present invention has the advantage of causing the exhaust process to be initiated at substantially the same pressure of the expanded working fluid whatever the degree of throttling or other condition of the working fluid, and also of reducing the noise emitted by the exhaust to a low and uniform level.

The exhaust valve poppet may be arranged in or on the piston so that it can move between an upper and a lower limit such that, at the upper limit, the exhaust valve is in an open position and, at the lower limit, the exhaust valve is in a closed position. By means of a stem or pintle fixed to the exhaust valve poppet, the inlet valve (in the head of the power cylinder) is then opened when the piston reaches the upward extreme of its return stroke (because the stem or pintle then strikes the inlet valve) and, at the same time, the reactive downward force of the inlet valve moves the exhaust valve poppet to its lower limit of movement i.e. its closed position, thus preventing wasteful escape of working fluid whilst the power cylinder is charged.When the piston nears the end of its downward expansion stroke, the exhaust valve poppet is moved from the lower, closed position and to the upper, open position under the action of the previouslydescribed double-lobed cam or by the pressure-sensitive means such as a compression spring described above. Thereby the desired object of providing a bistable exhaust valve is achieved.

A number of embodiments of the present invention will now be more particularly described, by way of example, and with reference to the accompanying drawings, in which: Figure 1 shows, in longitudinal section, a first form of valve gear according to the present invention and a pneumatic actuator in which it is embodied and which is termed a "linear oscillator of the single-acting type", Figure 2 shows, in longitudinal section, a second form of duplicated valve gear according to the present invention and a pneumatic actuator in which it is embodied and which is termed a "linear oscillator of the double-acting type", Figure 3 shows, a third form of valve gear according to the present invention and a rotary pneumatic motor in which it is embodied and which employs the double-lobed cam technique, and Figure 4 shows, largely in longitudinal section, a variation of the linear oscillator of the double-acting type as is described in the Figure 2 embodiment herein and incorporating a force pump for pumping a fluid such as water.

Referring to Figure 1, this shows a pneumatic actuator of the type known as a "linear oscillator" because its mechanical output is in the form of an oscillating or reciprocating motion along the line of its central axis, as shown by the double-headed arrow 1.

The linear oscillator is fed with compressed carbon dioxide supplied to the inlet connection 2 and thence to the inlet valve seat 3 which is normally closed by the ball 4 under the action of the supply gas pressure and the ball retaining spring 5. These last three items comprise the inlet valve 6.

Below the inlet valve 6 there is provided a hollow piston 7 and a retaining ring 8 through which axial passageways 9 are provided to allow flow of working fluid into and through the piston 7. Inside the hollow piston 7 (and guided thereby) is the exhaust valve 10 which comprises an exhaust valve poppet 11 urged upward (as in Figure 1) by the exhaust valve spring 12, and a stem or pintle 13 secured to the poppet 11 and projecting through the retaining ring 8 so as to be able to strike the ball 4 of the inlet valve 6 when in the position shown in Figure 1.

To allow escape of working fluid during the exhausting stage of operation, an exhaust valve seat 14 and exhaust orifice 15 are provided, leading to exhaust ports 16 (in the partlyhollow piston rod 17) and finally to exhaust outlets 18. A piston return spring 19 is provided to urge the piston to return to the position shown in Figure 1, and the whole assembly is contained in the working cylinder 20.

In operation and in the position shown in Figure 1, the stem 13 has struck the ball 4 and, in doing so, has been forced downwards (as in Figure 1), moving the exhaust valve poppet 11 downwards also, causing it to abut and seal the exhaust valve seat 14. Thus, as shown in Figure 1, working fluid supplied at the inlet connection 2 can flow past the ball 4, through the inlet valve seat 3 and into both the void above the retaining ring 8 and (via the axial passageways 9) the void above the exhaust valve poppet 11. This process induces a charge of working fluid into these spaces, where it is momentarily contained, and causes the pressure to rise therein.

As soon as the pressure on the piston 7 creates a downward force greater than the upward force of the piston return spring 19, the moving assembly (comprising the piston 7, retaining ring 8, exhaust valve 10, piston rod 17 and return spring 19) will be urged downwards and soon starts to move downwards, allowing the ball 4 to come to rest on - and to seal - the inlet valve seat 3; this cuts off any further inward flow of working fluid.

The said moving assembly thus moves downwards and, as it does so, the pressure of the carbon dioxide working fluid in the space above the piston and the exhaust valve falls progressively as it becomes more and more expanded. As this downward movement and expansion proceed, a point is reached when the said pressure is overcome by the exhaust valve spring 12, whereupon the exhaust valve poppet is urged upward (with respect to the piston 7) and away from the exhaust valve seat 14, allowing the contained working fluid to escape through the exhaust orifice 15. The exhaust valve spring 12 is designed so that this exhaust process is initiated at a desirably low pressure level, thereby to maximise the energy extracted from the working fluid and to limit the noise level of the escaping working fluid.

The escape of working fluid causes a rapid fall in its pressure above the piston and exhaust valve and this soon allows the return spring 19 to overcome the downward momentum of the said moving assembly and to urge it upwards towards the cylinder head. During such upward motion the exhaust valve remains open, allowing residual working fluid to be swept out of the spaces above the piston and exhaust valve and to escape through the exhaust orifice 15. Radial passageways 21 and longitudinal passageways 22 are provided in the exhaust valve poppet 11 to facilitate such escape of spent working fluid.

When the said moving assembly approaches the vicinity of the cylinder head, the stem 13 strikes the ball 4 and the downward force of gas pressure above the ball 4 is sufficient to cause the stem 13 and the exhaust valve poppet 11 to be moved downwards (with respect to the piston 7) so as to abut and seal the exhaust valve seat 14, thus preventing further escape of working fluid. A moment later, the stem 13 (which is now moving upward with the piston 7) lifts the ball 4 from the inlet valve seat, thus permitting inward flow of fresh working fluid which quickly arrests further upward motion of the moving assembly and brings it to the position shown in Figure 1.The cycle of operation is now complete and, once a fresh charge of working fluid has been admitted to the spaces above and within the piston 7, another cycle can begin and, thereby, the said moving assembly oscillates continuously as long as working fluid is upplied to the inlet connection 2.

The invention so far described would have one major disadvantage, namely, a marked tendency for the said moving assembly to strike the cylinder head on the upward stroke, which would result in noisy operation, possible damage and rough and irregular running. Accordingly, the present invention specifically provides that this major disadvantage is overcome by the provision of an exhaust restrictor 23, advantageously mounted in the piston rod 17 downstream of the exhaust orifice 15, and having a small restricting orifice hole 24 which, in this embodiment, may have a diameter in the region of 0.3 mm to 1.0 mm typically.The effect of this exhaust restrictor 23 is to restrict the flow of exhausted working fluid and, thereby, to cause a back-pressure to build up above the piston 7; this has the desirable effects of cushioning the upward stroke of the piston so as to prevent it from striking the cylinder head, and of stabilising the upward stroke limit of the said moving assembly. Of course, an exhaust restrictor could alternatively be provided in one or more of several alternative locations (for instance in each of the exhaust ports 16), and so the practice of an exhaust restrictor (as taught by the present invention) is not limited to the described location.

Referring to the second embodiment shown in Figure 2, this shows a pneumatic actuator employing duplicated valve gear according to the present invention and known as a linear oscillator of the double-acting type. As shown in Figure 2, this embodiment is built into a symmetrical working cylinder 25 which has an upper inlet valve assembly 26 and a lower inlet valve assembly 27, each such inlet valve assembly being similar to that of the Figure 1 embodiment. Likewise, an upper piston assembly 28 and a lower piston assembly 29 are mounted back-to-back (so as to oppose each other) on a common piston rod 30, each such piston assembly being similar to that in the Figure 1 embodiment and comprising a piston, retaining ring, exhaust valve poppet, exhaust valve spring, exhaust restrictor and stem.By means of this arrangement, one piston assembly will deliver its power (expansion) stroke while the other piston assembly is performing its return (exhaust) stroke, and vice versa. Therefore, the previous need for a piston return spring (as in the Figure 1 embodiment) is eliminated, as each piston assembly acts to return the other piston assembly towards and to its respective inlet valve.

Mechanical power output from the double-acting embodiment of Figure 2 is derived from a driving pin 31 affixed to the common piston rod 30 and engaging with a slotted lever 32 pivoted on a fulcrum pin 33 which is supported on a mounting lug 34 affixed to the wall of the working cylinder 25. Thus the outer tip 35 of the slotted lever 32 oscillates as indicated by the double-headed arrow 36, providing a source of useful mechanical power.

Referring to the third embodiment shown in Figure 3, this illustrates a rotary pneumatic motor with a reciprocating piston, using another form of valve gear according to the present invention and employing what is termed the "double-lobed cam technique", as will be explained.

As before, compressed carbon dioxide is supplied to the inlet connection 37 and thence to the inlet valve 38 which is substantially similar to the inlet valves in the previous embodiments and which is secured and sealed to the top end (as in Figure 3) of the working cylinder 39. Within the working cylinder 39 and beneath the inlet valve 38 is provided a piston assembly 40 comprising a piston 41 having an upper surface which defines an exhaust valve seat 42, an= elastomeric valve seal 43, an exhaust valve member 44 and a cam follower 45. As shown in Figure 3, the cam follower has an upper screw-threaded projection which engages with a screw thread in the exhaust valve member 44 (whereby the two are assembled together) and terminates at its upper end in a stem 46 adapted to strike and lift the valve ball 47.

The exhaust valve member 44, valve seal 43 and cam follower 45 constitute the three elements of a movable exhaust valve assembly 48 which can be displaced relative to the piston 41 by guidably sliding in the central bore 49 of the piston 41 so as to move upward (as in Figure 3) relative to the piston 41, whereupon the valve seal 43 separates from the exhaust valve seat 42. However, working fluid will not immediately flow between the separating faces of the valve seat and the exhaust valve seal 43 because the closelyfitting cylindrical surfaces of the bore of the piston and the mating stem 50 of the exhaust valve member prevent substantial flow therebetween, until the relative displacement between the piston and the exhaust valve assembly exceeds a predetermined amount known as the "valve offset".Referring to Figure 3, it will be seen that the stem 50 of the exhaust valve member is "plain" for a short distance of approximately 2 mm which is termed the valve offset 51, after which the stem of the exhaust valve member is grooved with a plurality of exhaust passages 52. Thus, only when the exhaust valve assembly is displaced upwardly relative to the piston by an amount greater than the valve offset can there be any substantial flow of working fluid through the exhaust valve assembly.

The exhaust passages 52 lead to an annular chamber 53 whence the exhausting working fluid can flow round the foot of the cam follower 45 (it being suitably relieved at its side edges) and downwards (as in Figure 3) into the crankcase (not shown in Figure 3, for clarity) of the rotary motor, whence it may escape to atmosphere through a suitable port in the crankcase, for example.

The rotary motor has a conventional crankshaft 54, crankweb 55 and crankpin 56 which carries the "big" end 57 of the connecting rod 58 and is driven thereby in the direction of the arrow 59 (in this embodiment).

The upper (conventionally the "small") end 63 of the connecting rod 58 is linked by the gudgeon pin 60 to the piston 41. The upper extremity of the upper end of the connecting rod is configured to form to its leftward side (in the Figure 3 embodiment) a larger exhaust cam lobe 61 and, to its rightward side, a smaller inlet cam lobe 62. The upper surfaces of these cam lobes are mounted close to the under surface of the cam follower 45 so that, if the connecting rod 58 were rocked from side to side by a desired small amount, the cam lobes would come into contact with the cam follower and cause it and the movable exhaust valve assembly 48 to be displaced upwardly relative to the piston 41.

Firstly, to consider the situation depicted in Figure 3, this shows the rotary motor at the instant when the connecting rod 58 and piston assembly 40 are at the position known as "Top Dead Centre" (TDC), when the stem 46 has lifted the valve ball 47 and is allowing a fresh charge of working fluid to flow into the space above the piston; this is termed "inletting". Now it is desirable that inletting should start when the crankpin 56 is only a few degrees (say 150) before TDC.However, this would normally mean thatinletting would terminate symmetrically at 15 0C after TDC and this would lead to a number of disadvantages; firstly there may then be insufficient time for fresh working fluid to flow into the working cylinder; secondly the inflowing working fluid may be excessively throttled by the inlet valve (owing to insufficent lift of the valve ball 47); and thirdly there may be a tendency for the rotary motor to "backfire", that is, to reverse its direction of rotation, especially at low rotational speeds.

These and other disadvantages are overcome by the action of the smaller inlet cam lobe 62, as follows. When the crankpin 56 passes through TDC in the direction of the arrow 59, the upper end 63 will rotate in the direction of the arrow 64, causing the smaller inlet cam lobe 62 to rise; this lifts the cam follower 45 and, with it, the exhaust valve assembly 48 including the stem 46; finally, this tends to lift the valve ball 47. The net effect is to allow an increase in lift of the valve ball and to allow the inlet valve 38 to continue inletting for a longer period after TDC than it was open before TDC. For example, if the inlet valve opened at 150 before TDC, its closure may be delayed to 250 after TDC.

A fuller disclosure of the effects of the smaller inlet cam lobe is given in UK Patent No. GB 2007313. However, not disclosed in that specification is the invention of the exhaust valve offset 51, and the present invention specifically discloses that the valve offset 51 is provided so as to be greater than the maximum amount of lift caused by the smaller inlet cam lobe 62; by this means it is ensured that no substantial loss of working fluid will occur due to wasteful flow through the exhaust valve, during the period of inletting.

Secondly to consider the situation when the crankpin is at "Bottom Dead Centre" (BDC) - i.e. advanced 1800 from the position shown in Figure 3 - it will be appreciated that the crankpin will then be moving right to left (as in Figure 3).

This will cause the upper end 63 of the connecting rod to rotate in a direction opposite to that of the arrow 64, and cause the larger exhaust cam lobe 61 to rise, lifting the cam follower and exhaust valve assembly 48.

It will be appreciated that, although this lifting of the exhaust valve assembly also lifts the stem 46, this now has no effect upon the inlet valve because the piston assembly 40 is now at BDC and remote from the inlet valve 38. Instead, the exhaust valve assembly is now lifted relative to the piston 41, firstly by an amount equal to the valve offset 51 (which might typically occur at 200 after BDC), and then onwards from that point so that the exhaust passages 52 are opened to flow of spent working fluid from the space above the piston.Maximum opening of the exhaust passages will, in this embodiment, occur at 900 after BDC, after which the exhaust valve assembly will fall relative to the piston until the relative lift between the two again equates to the valve offset (which might in the same example typically occur at 200 before TDC) whereupon the exhaust passages 52 will again become closed. By this means exhausting is terminated a little before inletting starts and which, in this example, might occur at 150 before TDC. Wasteful loss of working fluid is thus avoided.

Of course the choice of the angles with respect to BDC and TDC which define the timing of inletting and exhausting may be varied by suitable choice of the size of the two cam lobes 61 and 62, the size of the valve offset 51, the length of the connecting rod 58, the throw of the crankpin 56, etc., whilst remaining within the scope of the present invention.The present invention also discloses that in some embodiments it may be advantageous to angle the top surface of one or both of the cam lobes 61 and 62: For example if the larger exhaust cam lobe is angled as shown by the dashed line 65, this will delay and reduce the effect of the exhaust cam lobe (producing an effect similar to that of reducing the valve offset 51), causing exhausting to start a longer time after BDC and to terminate earlier (i.e. further before TDC); correspondingly similar effects may be achieved by angling the top surface of the smaller inlet cam lobe 62.

A particular characteristic (and often an advantage) of the present invention as shown embodied in Figure 3 is that the rotary motor will run in only one direction - clockwise when as in the Figure 3 embodiment. This characteristic facilitates another feature of the present invention which is particularly disclosed as follows:A piston return spring 66 is provided as shown with its upper extremity urging the piston 41 upwards (its lower extremity, though not shown in Figure 3, abuts a ledge in the working cylinder 39 or in the crankcase) so that, when the supply of working fluid is disconnected, the piston return spring 66 is sufficiently powerful to lift the piston assembly 40 substantially to the TDC position as exemplified in Figure 3; consequently the inlet valve 38 is then open, and if the supply of working fluid is then reconnected, will admit fresh working fluid and cause the rotary motor to run - and then in the correct direction, since reverse rotation is not possible. Thus the rotary motor embodying the valve gear and piston return spring of the present invention is self-starting; manual turning-over is unnecessary and mere connection of a supply of working fluid is sufficient.

Referring to the fourth embodiment shown in Figure 4, this shows how the double-acting linear oscillator of Figure 2 can be adapted to pump a fluid such as water. In Figure 4 a pump piston 70 is affixed to the piston rod 30 between the two piston assemblies 28 and 29 and a wall 71 is fixed inside the cylinder to define a pump cylinder 72. An inlet valve 73 admits fluid as the pump piston 70 moves away from the wall 71 and closes as the pump piston approaches the wall. An outlet valve 74 works in reciprocal cycle to the inlet valve to release fluid from the pump cylinder 72 to an optional accumulator 75, the volume of which is chosen so as to even out the pulsed delivery of pumped fluid so that a substantially continuous stream is delivered from the outlet pipe 76.

Exhaust outlets 77 and 78 are provided in the cylindrical wall of the working cylinder to allow venting of the spent working fluid from the piston assemblies 28 and 29 respectively.

Claims

1. A piston engine having a piston reciprocable in a bore in a cylinder, a cylinder head including a valve seat for an inlet valve disposed axially therein, an exhaust valve disposed axially in the piston, means carried by the piston to open the inlet valve when the piston approaches the cylinder head, and biasing means to hold the exhaust valve open during a substantial proportion of the return stroke of the piston to permit discharge through the piston of spent working fluid displaced from the cylinder, wherein the exhaust valve is of the poppet type and the means to open the inlet valve is a stem which projects axially from the piston to contact the inlet valve.

2. A piston engine according to claim 1, wherein the inlet valve comprises a ball which is resiliently urged into sealing engagement with the valve seat in the cylinder head.

3. A piston engine according to claim 1 or claim 2, wherein a restrictor orifice restricts the flow of spent working fluid from the exhaust valve seat.

4. A piston engine according to any preceding claim, wherein a compressed spring is provided to urge the piston along the return stroke.

5. A piston engine according to any preceding claim, wherein the biasing means is a spring compressed between a portion of the piston and the poppet of the exhaust valve, the force of said spring being chosen to overcome the pressure of spent working fluid, at the end of the forward stroke of the piston, to open said exhaust valve.

6. A piston engine according to any preceding claim, wherein one end of a piston rod which is fixed to the piston contains a passageway which conducts exhausted fluid from the exhaust valve for disposal.

7. A piston engine according to claim 6, wherein a second like piston is fixed to the other end of the piston rod, which also contains a passageway for exhausted fluid, and the second piston is accommodated in a second cylinder axially aligned with the first cylinder, so as to work in reciprocal cooperation with the first piston and cylinder to move the piston rod.

8. A piston engine according to any one of claims 1 to 4, wherein the piston is pivotally connected to one end of a connecting rod, the other end of which is pivotally connected to a flywheel or crank, said one end of the connecting rod having a first cam lobe engageable with the exhaust valve poppet to hold the exhaust valve open during the return stroke of the piston and a second cam lobe to control the extent of lift of the stem to control the extent of opening of the inlet valve via controlled lift of the exhaust valve poppet.

9. A piston engine according to claim 8, wherein the lift of the first cam lobe to control exhaust is greater than that of the second cam lobe to open the inlet valve.

10. A piston engine substantially as described and as shown in Fig. 1, Fig. 2 or Fig. 3, of the accompanying drawings.