GB2230566A - Piston engine - Google Patents

Abstract

A piston engine has a cylinder 3 defining a bore closed at one end by a head 4, an inlet valve seat 23 cooperating with an inlet valve member 10, and a piston 24 reciprocable in the bore, the piston 24 having a pintle 32 upstanding from the centre of its crown to open the inlet valve member 10 as the piston 24 approaches the cylinder head 4. The cylinder head 4 includes, a metering chamber 6 having a bore axially aligned with the piston 24 and a metering piston 7 reciprocable in the motoring chamber 6. The inlet valve member 10 is resiliently supported in the crown of the motoring piston 7 so that during the forward (downward) stroke of the working piston 24 pressurised fluid from an inlet 18 flows from the metering chamber 6 into the cylinder 3 towards the end of the backward stroke the pintle lifts the inlet valve off the valve seat 33 to permit a charge of fluid to pass from the metering chamber into the cylinder bore to urge the piston forward again. At the end of the downward stroke an exhaust valve 26 in the working piston lifts off its seat 25 to allow a spring 33 to return the piston 24. A seal 16 carried by the inlet valve can engage a seat 17 on the motoring piston so that a metered quantity of pressurised fluid is displaced by the metering piston into the working cylinder 3 during each forward stroke. <IMAGE>

Description

PISTON ENGINE ne 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.

cle 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-t-med 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 condensaticn) 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 power cylinder, and usually comprised one or more push-rods or other mechanical linkages driven by a crank, shelve, 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 preferre 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 poppettype 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, glided 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 innovatons is described in US Patent No. 561169 (Lofiie) and discloses a s1jd0valve 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 cozmunication 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, wsild at the present time be rather expensive, especially spring 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 mazzner 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 ar.d inlet valves so as to be offset from the piston's centre-line adds significantly to manufacturing cost.

AE Patent No. 2007313 (Rilett) discloses an inlet valve in a 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 se nor means to continue exhausting during the piston's return stroke.

UN Patent No. 1092305 (Harvey)'is similar in its main principle to US Patent No. 2671434 (Schmiedeskamp) - see above - in that the process of exhausting is initiated when the stem of each exhaust valve strikes a guide and is thus opened at the end of the piston's forward stroke. In addition, this invention provides springs which are sensitive to the falling pressure of the working fluid during the forward stroke and which open exhaust valves when that pressure has fallen to a predetermined level. However, this invention again requires a plurality (typically three) of exhaust valves, offset from the piston's centre-line, and again would be expensive.

In none of the above-mentioned prior art is any provision made to meter and inject an accurately-known volume of the working fluid, through the inlet (or "admission") valve and into the power cylinder.

Instead, in such known art, admission of working fluid usually takes place through relatively slow-opening and slow-closing inlet valves - with consequent partial throttling of the working fluid leading to pressure loss and poor efficiency - and often leads to instability of timing and consequent variation of speed and working fluid economy.

These problems are particularly severe in the case of engines having no crank to define the limits of the piston's stroke (and sometimes called "linear oscillators") and often cause erratic running and very poor control of stroke length.

In contrast, an embodiment of the present invention provides, an inlet valve (called hereinafter a "charge transfer valve") which 5 biased away from an inlet valve seat in the cylinder head and which is light enough to be fully opened very much more rapidly than in previous practice when the power piston reaches the end of its return stroke, whereupon a metering piston moves towards the inlet valve seat so as to inject an accurately-known volume of working fluid into the power cylinder as the power piston moves away from the inlet valve seat and, when that known volume of working fluid has been so injected, the inlet valve then closes and the metering piston then moves away from the inlet valve seat so as to capture a further accurately-known volume of working fluid.This accurately-known volume of working fluid is defined by the stroke of the metering piston multiplied by its cross-sectional are, and is known as the "charge". Curving admission of this "charge", the metering piston effectively transfers the charge from the metering chamber to the power cylinder; hence the name "charge transfer valve".

An embodiment of the present invention also provides, 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 the charge transfer valve mounted in the cylinder head may be actuated when the piston is at the end of its return stroke.

A further aspect of embodiments 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, a metering chamber axially disposed within the cylinder head, a metering piston reciprocable for axial movement within the metering chamber and biased away from the valve seat, an inlet valve mounted co-axially inside the metering piston and biased away from the valve seat by first biasing means bearing upon the metering piston, a seal between the inlet valve and the metering piston biased by the first biasing means to seal against the metering piston, a passageway provided to allow the flow of working fluid from a supply to the valve seat when the said seal is broken, 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 second 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 metering chamber within the cylinder head adjacent the inlet valve, which metering chamber acts to house, enclose and guide a metering piston.

In operation, the metering piston can move between two positions: a first position wherein the metering piston is remote from the inlet valve seat; and a second position wherein it is adjacent the inlet valve seat. The metering piston's displacement from the first to the second position is named "the metering stroke" and the product of the metering stroke and the cross-sectional area of the metering piston is named "the metering volume".

Thus, in moving from the first to the second position, the metering piston takes working fluid supplied to the metering chamber and sweeps an amount of it - equal to the metering volume and also called "the charge'1 - to the inlet valve1 whereby thn charge is transferred from the metering chamber to the power cylinder to drive the piston engine. It is arranged that the inlet valve is opened at the tine and for the duration necessary to transfer the charge into the power cylinder; likewise the inlet valre is closed when the metering chamber is to be filled with fresh working fluid again; these processes are described more fully hereinafter.

A further feature of embodiments of the present invention is the provision of a piston (in the power cylinderj which acts to house, enclose and guide a poppet-type exhaust valve, in contrast to conventional external valve gear which first be mounted substantially outside the power cylinder. Fu-'hermore, 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 mould be explained that, in conventional uniflow engines, exhaust ports are provided in the walls of the power 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 uniflow 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 power 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 power cylinder. Thus, both the expansion ratio and the work output of the conventional uniflow engine are limited undesirably.

An embodiment of 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 nay 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 avid 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 expanse than heretofore is achieved in particular by mounting the inlet valve, its valve seat and ball valve (if used), the metering piston, the exhaust valve poppet, its seat, the two biasing means, and all associated parts on the common centre-line of the piston, power cylinder, cylinder head and metering chamber, and also by employing such techniques as injection-moulding, die-casting and sintering for mass-production of these parts. By these means the present invention may be embodied in a typical mass-produced artefact (such as a domestic lawn edge-trimmer, hedge clipper or garden spray) at a relatively low manufacturing cost.

It is sometimes 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 has been usefully expanded. To prevent such overlap, the present invention (nn one embodiment) provides for the longitudinal channels (through which the exhausting 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 centreline; 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 can-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 then 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 es to keep the inlet valv 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 i.e. does not open the exhaust valve at this time.

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 senstive 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 springs overcomes the force derived from the cylinder pressure acting on the exhaust valve pcppet and acts to lift the exhaust valve 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 nears 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 with fresh working fluid.When the piston nears the end of its downward expansion stoke, the exhaust valve poppet is moved from the lower, closed position and to the upper, open position under the action of the previously-described double-lobed cam or by the pressuresensitive means such as 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 longitudunal section, a first form of valve gear including n charge t-.efer valve according to the present invention and a pneumatic actuator in which they are embodied and which is termed a "linear oscillator of the single-acting type", showing the positions of the major components immediately before the pneumatic actuator's piston assembly has reached top dead centre, Figure 2 is similar to Figure 1, except that it shows the situation immedIately after the pneumatic actuator's piston assembly has reached top dead centre and the inlet valve has been opened, Figure 3 is similar to Figure 2, except that it shows the situation after the charge transfer valve has injected a metered quantity of working fluid into the cylinder of the pneumatic actuator, Figure 4 shows, in longitudinal section, a charge transfer valve and a second form of valve gear employing the double-lobed cam technique, according to the present invention, embodied in a rotary pneumatic motor and showing the positions of the major components at the instant when the motor's piston assembly has reached top dead centre.

Referring to Figure 1, this shows the main working components of a pneumatic actuator 1 of the type known as a "linear oscillator" bscause 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 2.

The pneumatic actuator has a power cylinder 3 with a cylinder head 4 which encloses the charge transfer valve 5 having a metering chamber 6, a me-terirrg piston 7 (provided with an upper lip seal 8 and a lower lip seal 9 to provide subs*ntially gas-tight seals against the bore cf the metering chamber) and a poppet-type inlet valve 10.

The metering piston is biased upwards by the meterg spring 11 and the inlet valve is biased upwards by the inlet valve spring 12 which bears upon the metering piston and also pushes upwards against the head 13 of the inlet valve. The lower extremity of the inlet valve is provided with a first valve seal 14 (advantageously made of an elastomeric material such as nltrile rubber, pol=yurethane rubber or PEBAX (Registered Trade I k)) clamped to the inlet valve by the inlet valve cup 15.The inlet valve is also fitted with a second valve seal 16 (advantageously of similar elastomeric material) which, when the inlet valve spring is extended, bears against the upper sealing surface 17 of the metering piston so as to make a deliberatelyimperfect seal against it.

The charge transfer valve is fed with compressed carbon dioxide (hereinafter referred to as "working fluid") supplied to the inlet 18, and thence via the grooves 19 in the inlet valve head 13, gap 20, radial clearance 21 and passageways 22,to the inlet valve seat 23.

The diameter of the inlet valve seat and the force of the inlet valve spring are chosen so that the pressure of the working fluid acting on the outer sealing diameter of the inlet valve seat is more than adequate to produce a gas-tight seal between the first valve seal and the inlet valve seat, so that, when in its "at rest" condition, the inlet valve and the metering piston of the charge transfer valve are as depicted in Figure.1.

The working piston 24 of the pneumatic actuator is provided with an exhaust valve seat 25, upon which bears the exhaust valve 26 into which is crimped the exhaust valve seal 27 (again advantageously made of elastomeric material as for the first valve seal 14). The lower end of the exhaust valve is provided with a foot 28 upon which bears (in an upward direction) the exhaust valve spring 29.The upward force of the exhaust valve spring is chosen so that, when divided by the cross-sectio.al area of the exhaust valve seat, it is equal to the desired exhausting pressure cf the working fluid, so that, widen the pressure of the working fluid above the working piston falls to this exhausting pressure, the exhaust valve spring lifts the exhaust valve and its exhaust valve seal away from the exhaust valve seat, allowing the spent working fluid to be exhausted through the cross-drilled holes 30 and the centrally-drilled hole 31 to atmosphere.

The upper end of the exhaust valve terminates in a stem or pintle 32 which passes through the inlet valve seat so that, if the working piston were raised slightly (from its position in Figure 1) to the position known as "two? dead centre", the stem or pintle would strike the first valve seal and lift it away from the inlet valve seat, breaking the g s-tight teal between the inlet valve seat and the first valve seal, and allowing the poppet-type inlet valve to be lifted by the inlet valve spring. This is the situation depicted in Figure 2. The inlet valve 10 has been lifted very rapidly (typically in 10 nilliseconds) by the inlet valve spring to the upper,position shown in Figure 2 such that the second valve seal 16 has risen to bear upwards against the upper sealing surface 17, making a deliberately-imperfect seal against it, by means of slight grooves or scratches across the second valve seal's upper surface.

The stem or pintle 32 has a diameter slightly less than the bore of the inlet valve seat 23 so that working fluid can flow from the metering chamber 6 to the space above the exhaust valve 26 at a sufficient rate; this inrushing working fluid exerts a rising pressure on the exhaust valve, causing it to close sharply, and applies a rising pressure upon the working piston 24, forcing it downwards to provide the desired mechanical output, and compressing the piston return spring 33 against a lower abutment (not shown).

It will be seen from Figure 2 that, when the inlet valve has been very rapidly lifted as shown there, the metering piston 7 together with the inlet valve combine as a plunger which, fed by working fluid entering through the inlet 18, will be forced downwards through a distance equal to the metering stroke 34; this accurately-known metering stroke, multipied by the cross-scctional area of the metering chamber 6 (which is also accurately known), gives the metering volume called the "charge" 35 and which is also therefore accurately known.

The effect of the metering piston and inlet valve being forced downward as just described is to force the working piston downwards so as to sweep out a volume equal to the charge 35, bringing about the situation depicted in Figure 3: the working piston has descended off the page so that the charge 35 has a volume equal to the metering volume and, in the case of the power cylinder 3, equal to the crosssectional area of the bore of the power cylinder multiplied by the stroke of the working piston downwards from top dead centre. ow, as the metering volume and the bore of the power cylinder are both accurately known, it follows that the stroke of the working piston will be accurately determined.

Once the working piston has been forced downwards by an amount sufficient for it to sweep out the metering volume, the pressure of the working fluid above the working piston starts to fall rapidly as the charge is expanded; this pressure fall is detected by the exhaust valve spring so that, at a desired threshold level, it lifts the exhaust valve seal away from the exhaust valve seat and allows escape of the spent working fluid. In addition, a plurality of pins 36 (as depicted in Figures 1 and 2) may be provided with their lower ends projecting beneath the base of the working piston such that, when the working piston has moved downwards through a desired length of stroke, the lower ends of these pins strike an abutment (not shown) in the lower part of the power cylinder, causing the pin-heads 37 to strike the foot 28 of the exhaust valve, moving it up and so opening the exhaust valve.This additional method of opting the exhaust valve can be valuable for accurate deterinnation of the working piston's stroke length and essential if the linear oscillator is to be operated over a range of supply pressures.

Referring to Figure 3, the second valve seal 16 is shown slightly separated from the upper sealing surface 17 so as to provide a slight gap between them. In actual practice it may be preferable to provide one or more scratches or grooves in the upper surface of the second valve seal or in the mating surface of the upper sealing surface, so as to provide a deliberately-imperfect seal between the two (as described hereinbefore). 'Pile purpose of this is to allow a small flow of working fluid between the two said surfaces and down the passageways 22 to the base of the metering piston 7; this allows a tendency towards equalisation of the working fluid pressures above and below the metering piston, such that the metering spring 11 can force the metering piston upwards: after the metering piston has moved upwards z fraction, the second valve seal separates from the upper sealing surface, allowing substantial flow of working fluid through the gap between them and along the passageways 22 and allowing the metering piston to be raised rapidly to the position shown in Figure 1. Thus the process is completed and the charge transfer valve is returned to its original state (as in Figure 1), ready to start a new cycle. It will be appreciated that the working piston return spring 33 will act, after the spent working fluid has been exhausted, to return the working piston and its exhaust valve to the position shown in Figure 1.

Referring now to another embodiment of the present invention as shown in Figure 4, this illustrates a rotary pneumatic actuator with reciprocating piston, employing firstly a charge transfer valve and, secondly, a different form of exhaust valve gear according to the present invention using what is termed the "double-lobed cam technique", as will be explained.

As bofore, compressed carbon dioxide is supplied as the working fluid to the inlet 18 and thence to a charge transfer valve 5 which is substantially similar to the charge transfer valve depicted in the previous Figures except that the metering stroke 38 is considerably shorter than the previous metering stroke 34.The reason for this is that, whereas in the previous embodiment the working fluid was employed substantially non-expansively" i.e. was not expanded to a volume sustantially greater than its volume at inlet, in this FIgure 4 embodiment, the rotary actuator is designed to expand the working fluid to 2 volume six times greater th,n its volume at inlet i.e. the rotary motor has an expansion ratio of 6:1. This allows a given mass of worizirg fluid to produce a much greater work output.

Within the working cylinder 39 and beneath the inlet valve seat 40 is provied a working piston assembly 41 comprising a working piston 42 having an upper surface which defines an exhaust valve seat 43, an elastomeric valve sell 44, an exhaust valve member 45 and a cam follower 46. As shown in Figure 4, the cam follower has an upper screw-threaded projection which engages with a screw thread 5n the eXhaust valve member 45 (whereby the two are assembled together) and terminates at its upper end in a stem 47 adapted to strike and actuate the charge transfer valve 5.

The exhaust valve member 45, valve seal 44 and cam follower 46 constitute a movable exhaust valve assembly 48 which can be displaced relative to the piston 42 by guidably sliding in the central bore 4s of the piston 42 so as to move upward (as in Figure 4) relative to the piston 42, whereupon the valve seal 44 separates from the exhaust valve seat 43. However, working fluid will not immediately flow between the separating faces of the valve seat and the exhaust valve seal because the closely-fitting cylindrical surfaces of the bore of the piston and the mating stem--asher 50 prevent substantial flow therebetween, until the relative displacement between the piston and the exhaust valve assembly exceeds a predetermined amount, equal to the thickness of the stem-washer 50 and called the "valve offset".

Referring to Figure 4, it will be seen that, below the stem-washer 50, an exhaust guide 51 is provided 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 (i.e. by the thickness of the stem-washer 50) 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 46 (it being suitably relieved at its side edges) and downwards (as in

Figure 4) into the crarcase 54 of the rotary motor(/viaaplurality of exhaust flutes 55 provided in the lower bore of the working cylinder 39) whence it may escape to atmosphere through a suitable port (not shown) in the crankcase wail.

The rotary motor has a conventional crankshaft 56, crankweb 57 and crankpin 58 which carries the "big" end 59 of the connecting rod 60 and is driven thereby in the direction of the arrow 61 (in this embodiment).

The upper (conventionally the "small") end 62 of the connecting rod is lirked by the gudgeon pin 63 to the working piston 42. The upper extremity of the upper end of the connecting rod is configured to its lef; srd side (in the Figure 4 embodiment) to form a larger exhaust cam lobe 64 and, to its rightward side, a smaller inlet cam lobe 65.The upper surfaces of these cam lobes may be,conventionally, close to the under surface of the cam follower or, unconventionally, may be arranged to be some distance short of the under surface of the cam follower (as depicted in Figure 4) in which the gap therebetween will produce subsbanr;al'y the same effect as the "valve offset" otherwise produced by the stem-washer 50. In either case, if the connecting rod were rocked by a suitable amount from side to side, 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 working piston 42.

Firstly, to consider the situation depicted in Figure 4, this shows the rotary motor at the instant when the connecting rod and working piston assembly are at the position known as "Top Dead Centre" (TDC), when the stem 47 has actuated the charge transfer valve 5 (so that its metering piston and inlet valve are just beginning their downward stroke, as expel ine hereinbefore) and a fresh charge of working fluid is beginning to flow into the space above the working piston; this is termed "inletting". Now it is desirable that inltting should start when the crankpin 58 is only a few degrees (say 150) before TDC.

However, this would normally mean that inletting would terminate symmetrically at 15 after TDC and this would lead to disadvantages; firstly there say then be insufficient time for the fresh working fluid to flow ^ to the working cylinder from the charge transfer valve; and secondly 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 disadvantages are overcome by the action of the smaller inlet cam lobe 65. as follows. EEncl the crankpin 58 passes through TDC in the direction of the arrow 61, the upper end 62 will rotate in the direction of the arrow 66, causing the smaller inlet cam lobe 65 to rise; this lifts the cam follower 46 and, with it, the exhaust valve assembly 48 including the stem 47; finally, Whis keeps the charge transfer valve opened later than would otherwise have been the case without the smaller inlet cam lobe 65. The net effect is to allcw the charge transfer valve to continue inletting for a longer period after TDC than it was open before TIC.For example, if th3 inlet valve opened at 150 before TIC, its closure may be delayed to 250 after TDC.

Secondly to consider the situation when the crankpin is at "Bottom Dead Centre" (BDC) - i.e. advanced 180 from the position shown in Figure 4 - it will be appreciated that the crankpin will then be moving right to left (as in Figure 4). This will cause the upper end of the connecting rod to rotate in a direction opposite to that of the arrow 66 and will cause the larger exhaust cam lobe 64 to rise, lifting the cam follower and the exhaust valve assembly.

It will be appreciated that, although this lifting of the exhaust valve assembly also lifts the stem 47, this now has no effect upon the charge transfer valve because the piston assembly 41 is now at SDC and remote from the charge transfer valve. Instead, the exhaust valve assembly is now lifted dative to the working piston 42, firstly by an amount equal to the valve offset (which might typically occur at 400 after Sic), 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 working piston. tnximmm 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 e:ample occur typically at 400 before TDC) whereupon the exhaust passages 52 will again become closed. By this means e=hac,ting is terminated somewhat before ~letting 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 BIC and TIC which define the timing of inletting and exhausting may be varied by suitable choice of the size of the two cam lobes 64 and 55, the amount of valve offset (or gap between the foot of the cam follower and the two cam lobes), the length of the connecting rod 60, the throw of the crankpin 58, 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 64 and 65: for example if the larger cam lobe is angled as shown by the dashed line 67, this will delay and reduce the effect of the exhaust cam lobe, causing exhausting to start later and to terminate earlier (i.e. further before TC); correspondingly similar effects may be achieved by anglir.g the top surface of the smaller inlet cam lobe 65.

A particular characteristic of the present invention is that the charge transfer valve disclosed herein can be "tuned" to run at a specific speed and, indeed, it will not run very effectively and operate properly at speeds very much higher than that specific speed. It is therefore specifically disclosed hereby that this quality of the subject charge transfer valve may be adapted so as to make it work as a speed governor. Thus, for example in the Figure 4 embodiment, the metering piston of the charge transfer valve may be adapted (by choice of the metering spring force, metering piston mass and the various flow passages) so as to operate most effect vely at, for example, 10 cycles per second; it then follows that the rotary motor will run most effectively at 10 revolutions per second (i.e. 600 rpm) and, if the load on the motor is reduced or removed, it will be governed to a maximum speed of, for example, 800 rpm.

Claims (13)

Claims

1. A piston engine having a piston reiprocable in a bore in a cylinder, a cylinder head including a valve seat for an inlet valve disposed axially therein, a metering chamber axially disposed ;within the cylinder head, a metering piston reciprocable for axial movement within the metering chamber and biased away from the valve seat, an inlet valve mounted co-axially inside the metering piston and biased away from the valve seat by first biasing means bearing upon the metering pistc::l, a seal between the inlet valve and the nete~ing piston biased by the first biasing means to coal against re meuerin, p zWe--, a passageway provided to allow the flow of working fluid from a supply to the valve seat when the said seal is broken, 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 second 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 2 stem which projects axially from the piston to contact the inlet valve.

2. A piston engine according to claim 1, wherein the seal between the inlet valve and the metering piston is imperfect.

3. A piston engine according to claim 2, wherein the seal is made imperfect by means of scratches or grooves across its surface.

4. A piston engine according to any preceding claim, wherein the second 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.

5. A piston engine according to any preceding claim, wherein the means carried by the piston to open the inlet valve comprises a stem extending from the poppet of the exhaust valve.

6. A piston engine according to any preceding claim, wherein the exhaust valve contains a passageway which conducts exhausted working fluid from the cylinder for disposal.

7. A piston engine according to claim 6, wherein the exhaust valve carries a foot which, for the purpose of exhausting the working fluid, is urged by a pin so as to open the exhaust valve.

8. A piston engine according to claim 7, wherein the pin strikes an abutment fixed to the cylinder so as to urge the foot.

9. A piston engine according to any one of claims 1, 2, 3, 5 or 6, 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.

10. A piston engine according to claim 9, wherein said one end of the connecting rod has a second cam lobe to control the extent of lift of the stem of the exhaust valve to control the timing of opening of the inlet valve via controlled lift of the exhaust valve poppet.

11. A piston engine according to claim 10, wherein the lift of the first cam lobe to control exhaust is greater than that of the second cam lobe to control the inlet valve.

12. A piston engine of a kind having a cylinder 3 defining a bore closed at one end by a head and having an inlet valve seat 23 cooperating with an inlet valve member 10, and a piston 24 reciprocable in the bore having a pintle 32 upstanding from the centre of the crown of the piston to open the inlet valve 10 as the piston 24 approaches the cylinder head 4 wherein the cylinder head 4 includes, a metering chamber 6 having a bore axially aligned with the piston 24, a metering piston 7 reciprocable in the metering chamber 6; and an inlet valve 5 resiliently supported in the crown of the metering piston 7 so that during the forward stroke of the working piston the pintle is clear of the inlet valve and the inlet valve admits pressurised fluid through the metering piston into the chamber; and towards the end of the backward stroke the pintle pushes the inlet valve off the valve seat to permit a charge of fluid to pass from the measuring chamber into the bore to urge the piston forward again.

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