WO2015159083A1

WO2015159083A1 - Opposed piston machine with rectilinear drive mechanisms

Info

Publication number: WO2015159083A1
Application number: PCT/GB2015/051149
Authority: WO
Inventors: Ali VESHAGH; Jean-Pierre Pirault
Original assignee: Osp Engines Limited
Priority date: 2014-04-16
Filing date: 2015-04-16
Publication date: 2015-10-22
Also published as: GB2525213B; GB2525213A8; GB2525213A; GB201406827D0

Abstract

A machine comprising at least a first and second opposed piston in a common cylinder, a first rectilinear drive mechanism with a first connecting rod yoke joined to said first opposed piston moving along a first axis, and with a third connecting rod yoke joined with a third piston element moving along a second axis which is orthogonal to the first axis, a second rectilinear drive mechanism with a second connecting rod yoke joined to said second opposed piston, moving substantially in- phase with the first opposed piston, but in an opposite direction, and along the same first axis and in said common cylinder with the first opposed piston, and said second rectilinear drive mechanism with a fourth connecting rod yoke joined to a fourth piston element moving along a third axis which is also orthogonal to the first axis and parallel to the second axis, and means for synchronously phasing the first and second rectilinear drive mechanisms. Used for compressors and engines.

Description

Title: Opposed Piston Machine with Rectilinear Drive Mechanisms Description of Invention

This invention relates to opposed piston machines that use versions of a disc drive type mechanism, also sometimes known as an eccentric drive, for the operation of the crankshaft and piston rods and for driving a multiplicity of pistons connected to each crankpin of each crankshaft.

At least one pair of these pistons are arranged to work in anti-phase with each other in a common cylinder and the other pistons may have individual cylinders. The cylinders may be arranged in one embodiment to provide compressed air, or in another embodiment they may arranged to be a 2-stroke internal combustion engine.

Opposed piston machines operating as compressors or internal combustion engines are well known. In the case of opposed piston engines, these need a source of scavenging air for exhausting the cylinder and providing fresh air for combustion. Opposed piston engines are also usually operated with a phase angle difference between the pistons as this offers some performance advantages. The use of a phase angle introduces out of balance forces on the engine and this can be a nuisance for certain cylinder configurations, particularly a single cylinder engine.

The proposed invention also enables opposed piston machines and their various embodiments to be operated with in double acting mode with dry piston skirts which is an advantage for some compressor applications and will be a friction and emission reducing advantage for opposed piston machines when used for internal combustion applications.

Definitions

A crankshaft is a shaft having a major axis of rotation, with some first journal elements of the shaft aligned along that axis of rotation, and having some second journal elements, sometimes known as crankpins, that are rigidly fixed by radial arms to said first elements, these second elements being offset from and parallel to the major axis of rotation of the shaft.

A main journal is a solid of revolution and usually an integral part of the crankshaft and is arranged concentrically on the main axis of a crankshaft and is supported by a bearing in a crankcase.

A crankpin is usually an integral part of a crankshaft which carries and is connected to the connecting rods that are in turn connected to the pistons via a slideable joint called the gudgeon pin. Each engine cylinder usually has a piston, subjected to combustion gas pressure and connected via the gudgeon pin to the "small end" of the connecting rod. The other end of the connecting rod, called the "big-end", connects rotatably with the crankpin.

A crankthrow of a crankshaft is usually an integral part of the crankshaft linking the main journal to the crankpin. There is usually at least one crankthrow connecting with each crankpin.

A crankshaft is usually a single part connecting all crankpins and main journals, the main journals.

A piston is the moving part of a positive displacement volumetric machine that acts on the fluid to displace, compress or expand the fluid. The piston is usually of a male shape which engages in a cylinder of a female shape, the motion of the piston moving the fluid to and from the cylinder.

A yoke is a substantially rigid link that joins two other parts, usually having some clearance with each of these other two parts so that there is some degree of rotational freedom between the link and the two parts.

A rectilinear drive mechanism is an assembly comprising a crankshaft fitted with paired discs (usually cylindrically shaped) which can rotate with clearance about the crankpin of the crankshaft, each disc of the pair being guided and constrained with clearance by at least one linear rail and the rail(s) of each disc being orthogonal to each other. For clarity, further description of rectilinear drive mechanisms is given with reference to diagrams, in the following sections.

Paired discs, sometimes referred to as paired eccentrics, are a component of a rectilinear drive mechanism which consist of two eccentric discs rigidly fixed together and which are able to rotate (as one body) about a point which is central between the two centres of the discs. In one form of rectilinear drive mechanism described below, the discs are of a small diameter and are in the form of pins which are joined by a rigid bar. The assembly of the bar and the two pins thus forms paired discs.

A reciprocating machine is a mechanism in which at least one component oscillates (travels along a single line with periodically reversing direction of motion) and which has a link between that reciprocating motion and the rotational motion of at least one shaft.

A linear guide is a substantially straight (but not necessarily flat) component fixed to ground and which has a groove or straight edge which engages with a moving component such that the moving component is constrained to move with linear motion. An example of a linear guide could be a cylinder in an engine or compressor.

A crosshead arrangement in a reciprocating machine such as an engine or compressor is one in which the active piston is supported on a carrier piston or slider through which all, or most, of the side loads are transmitted. This has the principal advantage that little lubrication is required on the side surfaces of the active piston.

An eccentric is a cylindrical solid of revolution, having its centre of geometry offset from the centre of rotation of the shaft to which it is connected, which rotates about the centre of rotation of the shaft .

Contra rotation of a body indicates rotation of that body in an opposite sense to the rotation of another body. A tooth sprocket is a cylindrical solid of revolution with evenly spaced and evenly sized radial teeth on the outer diameter of the solid, the major axis of said teeth being parallel with the centre axis of the solid of revolution and being of a tooth form that can engage with a corresponding continuous belt which has internal teeth to match the space between the teeth on the outer diameter of the solid of revolution.

A tooth belt is a continuous bendable strap with internal teeth that can rotatably link a first tooth sprocket to a second tooth sprocket in order to transmit torque from the first tooth sprocket to the second tooth sprocket.

A piston is the moving part of a positive displacement volumetric machine that acts on the fluid to displace, compress or expand the fluid. The piston is usually of a male cylindrical shape which engages closely in a corresponding cylinder of a female shape, the motion of the piston moving the fluid to and from the cylinder.

A power piston operates in the combustion cylinder and compresses and expands the gases in the combustion cylinder as part of the combustion process.

An opposed piston machine, which may be an opposed piston engine or an opposed piston compressor, is an engine or compressor in which at least two pistons slide in a common cylinder ..

An opposed stepped piston engine is an opposed piston engine or compressor that has at least one air transfer piston.

An air transfer piston is a piston used to transfer air from the air intake system to the power piston.

The air ports of a 2-stroke engine are those apertures or openings in the cylinder wall of the cylinder of the 2-stroke engine which control the admission of air to the cylinder that will be used for combustion. The exhaust ports of a 2-stroke engine are those apertures or openings in the cylinder wall of the cylinder of the 2-stroke engine which control the expulsion of exhaust gases from the cylinder after combustion.

The "air" piston is the power piston which controls the opening and closing of the air ports of the combustion cylinder.

The "exhaust" piston is the power piston which controls the opening and closing of the exhaust ports of the combustion cylinder.

The "phase" of a moving part of an engine relates the relative timing of that moving part to other moving parts. The phase angle is usually defined in terms of crankangle difference between the two moving parts. For example, the exhaust piston of an opposed piston engine usually moves with an advance of 20^Q crankangle versus the air piston; this means that the exhaust piston will reach its inner dead centre position before the air piston reaches its inner dead centre position, i.e. earlier in terms of the engine operating cycle. However, the term phase angle is also used to reference the phasing difference between adjacent cylinders, for instance the cylinders of a twin cylinder opposed stepped piston engine are phased by 180^Q crankangle, as well as the exhaust pistons in each cylinder being phased by an advance of 20^Q crankangle versus the air piston of the same cylinder.

"Inner dead centre" (IDC) refers to innermost position of a piston in its travel in the cylinder of an opposed piston engine, i.e. the closest position of the piston towards the centre of the cylinder. In engines with cylinder heads, IDC is normally referred to as "top dead centre".

"Outer dead centre" (ODC) refers to outermost position of a piston in its travel in the cylinder of an opposed piston engine, i.e. the furthest position of the piston from the centre of the cylinder. In engines with cylinder heads, ODC is normally referred to as "bottom dead centre". With opposed piston engines, the air and exhaust pistons approach inner dead centre simultaneously, separated in crankangle only by the phase angle between the air and exhaust pistons.

"Scavenging" air flow of a 2-stroke engine is the frequently used jargon to describe the air flow that passes into a 2-stroke engine, some of which is retained for combustion. The remainder of the air passes through to the exhaust system, removing or scavenging the burned products of combustion, also known as the exhaust products of combustion, from the cylinder.

Ports of 2-stroke engines are the apertures in the cylinder walls that enable the flow of gases from or into the cylinder. Ports are opened and closed by the motion of the power pistons .

An air duct or conduit, also known as a pipe, is a passageway or connecting route which allows air to be transferred from one point to another. Pipe and conduit are taken to have the same meaning in the following text.

A double diameter, also known as stepped, piston is a piston with two diameters each of which separately engages one of two female cylinders, the diameters of said cylinders lying on a common axis. The two piston diameters are usually rigidly connected, with the smaller diameter piston being the power piston and the larger diameter being the air transfer piston.

A stepped cylinder comprises a first cylinder, which has a first diameter for a first length and which is joined to a second cylinder which has a second diameter for a second length, the axes of first and second cylinders lying on the same axis.

The stepped piston and the stepped cylinder may be part of either a compressor or an engine.

A double acting piston is one which acts on a first volume of the working fluid with a first area on one side of the piston and which acts on a second volume of the working fluid with a second area on the other side of the piston. A check valve is a flow control mechanism that allows flow in one direction and prevents flow in the reverse direction. The mechanism is usually a simple leaf-spring flap, located in a conduit, that opens in one direction and closes against an abutment in the reverse direction.

The opening pressure of a check valve is the flow pressure required to enable flow in one direction.

The compression ratio of a cylinder volume with a piston that moves from an innermost to outer most position within the cylinder volume is the ratio of total cylinder volume with the piston at its outermost position divided by the cylinder volume with the piston at its innermost position.

Summary of Invention

The broadest aspect of this invention is a machine comprising at least a first and second opposed piston in a common cylinder, a first rectilinear drive mechanism with a first connecting rod yoke joined to said first opposed piston moving along a first axis, and with a third connecting rod yoke joined with a third piston element moving along a second axis which is orthogonal to the first axis, a second rectilinear drive mechanism with a second connecting rod yoke joined to said second opposed piston, moving substantially in-phase with the first opposed piston, but in an opposite direction, and along the same first axis and in said common cylinder with the first opposed piston, and said second rectilinear drive mechanism with a fourth connecting rod yoke joined to a fourth piston element moving along a third axis which is also orthogonal to the first axis and parallel to the second axis, and means for synchronously phasing the first and second rectilinear drive mechanisms.

In one embodiment, the invention, with only one pair of single throw crankshafts rotatably linked to each other, is applied with appropriate airway connections and check valves to provide up to seven compressor displacement volumes or a compressor with up to seven stages of compression. In another embodiment, the invention, with only one pair of single throw crankshafts rotatably linked to each other, is used with appropriate airway connections, fuelling systems, ignition and exhaust systems to provide an opposed piston internal combustion engine with its own scavenge pump which can be arrange to be either 90° or 180° phasing, or both, to the power cylinders.

Embodiments of the invention will now be described, by way of example only, with reference to the following figures, of which:

Figure 1 is an exploded isometric view of a typical rectilinear drive with two pistons and one crankshaft;

Figure 2 is a sectional view through the plane of the power piston and its guides;

Figure 3 is an isometric view of an assembly of a pair of orthogonal pistons attached to one crankshaft and operating within the guides of a rectilinear drive;

Figure 4 is an isometric view of an assembly of two pairs of orthogonal pistons attached to one crankshaft and operating within the guides of two rectilinear drives;

Figure 5 is an isometric view of an assembly of one orthogonal pair of double ended connecting rod yokes with pistons at the ends of each yoke attached to one crankshaft and operating within the guides of two rectilinear drives which are not shown;

Figure 6 is an isometric view of an assembly of two pairs of orthogonal pistons attached to one crankshaft and operating within the guides of two rectilinear drives which are located within two crankcase halves;

Figure 7 is a diagrammatic front elevation of an arrangement showing two rectilinear drives which each have a piston engaging in a common cylinder of an opposed piston machine arranged to work as a compressor; and Figure 8 is a diagrammatic arrangement of one embodiment showing two rectilinear drives which each have a piston engaging in a common cylinder of an opposed piston machine arranged to work as an internal combustion engine.

Figures 1 - 5 generally provide an introduction to the rectilinear drive mechanism which forms the basis of the embodiments of the invention as described with reference to Figures 6 - 9.

With reference to Figures 1 - 3, crankshaft journals 20a and 20 are part of a crankshaft, rotating about axis 21 , said crankshaft having crankpin 22 onto which is rotatably mounted paired discs 23. Paired discs 23 comprise two cylindrical discs, disc 31 with central axis 29 and disc 32 with central axis 30, said discs being rigidly joined to each other. Of course, it should be understood that shapes other than cylindrical discs may also be suitable, and the term 'disc' is not intended to be limited to cylindrical discs. Central axes 29 and 30 are both the same distance from axis 33, which is the axis through a cylindrical hole that is a sliding fit with the crankpin, as axis 33 is from axis 21 , which is the axis of rotation of the crankshaft. The centre of disc 31 may rotate with clearance within the cylindrical bore 4c of the connecting rod yoke 4, said yoke being slideably constrained to move along the axis 34 of the connecting rod yoke 4 by means of linear guides 26 and 27 in the crankcase 80, said guides engaging with contact pads 4a and 4b of the connecting rod yoke 4. In a similar fashion, the centre of disc 32 may rotate with clearance within the cylindrical bore 3c of the connecting rod yoke 3, said yoke being slideably constrained to move along the axis 35 of the connecting rod yoke 3 by means of linear guides 36 and 37 in the crankcase 80, said guides engaging with contact pads 3a and 3b of the connecting rod yoke 3. The axis 35 of the connecting rod yoke 3 is substantially orthogonal to second axis 34. Guides 26, 27, 36, and 37 are connected together in the form of a rigid frame 80 which may be rigidly joined to a crankcase, shown later with reference to Figure 5, or the guides can be part of the crankcase, as discussed with reference to Figure 5. For clarity in Figures 1 - 4, the frame 80 is shown as a unique part with the guides 36 and 37, parallel to each other and the axis 35, and parallel to the pads 3a and 3b of the connecting rod yoke 3, and with the guides 26 and 27 parallel to each other and the axis 34, and parallel to the pads 4a and 4b of the connecting rod yoke 4. The guides 26 and 27 are therefore orthogonal to the guides 36 and 37.

Again with reference to Figures 1 - 3, the combination of connecting rod yokes 3 and 4, paired discs 23, frame 80 with guides 26, 27, 36, 37 and crankshaft 20 comprise a rectilinear drive 500, this latter terminology being used to distinguish the assembly from other assemblies in which the orthogonal guiding is achieved via surfaces at the other ends of the connecting rod yokes 3 and 4, for example piston skirts 1 and 2 which would be arranged to slide in cylindrical bores, which are not shown in Figures 1 - 3. An advantage of the proposed rectilinear drive arrangement is that the guidance of the yokes in the guides of the frame can be conducted in a fully lubricated condition without the need to lubricate the pistons in their cylindrical bores. In fact, all the side thrusts from the crankshaft, yoke and disc elements is taken by the guides of the frame 80, so are on side loads between the pistons and their cylinders which can therefore run unlubricated. This is important in terms of preventing lubricating oil ingress from the crankcase elements in to the working fluids above the pistons, as the oil would be potentially a contaminent to the working fluid, which for example might be the air delivery of a compressor, or the combustion air of an internal combustion engine. The fully lubricated side thrust condition between the connecting rod yokes 3 and 4 and the frame guides 26, 27, 36, 37 will result in lower friction and less wear than the mixed or partial lubricated condition that normally exists between piston skirts and their cylinders. In this sense, the rectilinear drive has lower friction than an equivalent crankshaft, connecting rod and piston assembly with two cylinders.

With continued reference to Figures 1 - 3, the combination of connecting rod yokes 3 and 4, paired discs 23, frame 80 with guides 26, 27, 36, 37 and crankshaft 20, comprising a rectilinear drive 500, results in pure sinusoidal and orthogonol motion of the connecting rod yokes 3 and 4 and any masses attached to them. As the sinusoidal motion of the two connecting rod yokes and any attached masses is phased by 90° , it is well known that the vectorally combined out-of-balance forces from these two connecting rod yokes and any attached masses will comprise a single constant force vector acting along the radius of the crankthrow to which the crankpin 22 is attached, said vector rotating at crankshaft speed. Such a constant force engine speed vector is completely and readily balanced by a balance mass located diametrically opposite the crankpin relative to the crankshaft axis 21 , said balance mass being configured and sited to have the same mass times radial value as the mass of a single piston, and the reciprocating portion of a connecting rod yoke, with an effective centre of mass equivalent to twice the crank throw, which is twice the distance between the axis 33 of the crankpin and the axis of rotation 21 of the crankshaft. The pure sinusoidal motion of the mechanism described with reference to Figures 1 - 3 also means that there are no secondary or higher order accelerations as exist with conventional crankshaft, connecting rod and piston systems. It should also be clear that the connecting rod yokes do not each need to be linked to a piston which moves in a cylinder. The balance integrity of the mechanism is maintained if either the first or second piston acts as a "piston element", that is to say as a mass, equivalent to the mass of the other piston and the other connecting rod yoke, said piston element mass being reciprocated by its connecting rod yoke without having to be in a cylinder bore. So, in effect, one of the piston elements may be a balancing piston mass, for the purpose of balancing the other piston element, rather than specifically working a volume within a cylinder as a second working piston head. In this case, the connecting rod yoke attached to the piston element can be shortened to reduce the envelope of the engine.

With reference to Figure 2 this shows more clearly the disc 32 which rotates within the bore 3c of the connecting rod yoke 3, the orbiting of the disc 32 being forced by the sliding motion of the connecting rod yoke 3 within the guides 36 and 37. The linear motion of the connecting rod yoke 3 also enables a monolithic combination of the piston 1 with the connecting rod yoke 3, which is useful in terms of structural integrity, simplicity and cost reduction. In the embodiment of Figure 2, the monolithic piston and connecting rod yoke features a twin strut connection between the piston 1 and the cylindrical portion of the connecting rod yoke that encompasses the disc 32. A circumferential groove 951 at the lower edge of the piston is used to contain an oil control ring which prevents any oil reaching the cylinder bore in the volume above the upper crown of the piston.

With reference to Figure 3, this shows an assembly of the items shown in Figure 1 and also a second crank throw with balance crankshaft balance weights and webs 41 a and 41 b, said second crankthrow in this example being arranged at 180° crankangle to the first crankpin 22, which is masked in this figure by the crankweb 20e.

With reference to Figure 4, this shows the same assembly as Figure 3 with the addition of a second rectilinear drive 600 comprising a second set of paired discs, which are masked by the crankweb 41 , another pair of connecting rod yokes 130 and 140 which engage slideably with two pairs of orthogonal guides in the frame 180, and pistons 1 10 and 120 joined to the connecting rod yokes 130 and 140. In the case of a machine containing more than one rectilinear drive mechanism, as shown in this figure, all the frames containing the orthogonal guides are preferably separate components which are rigidly fixed to the surrounding crankcase. This eases machining and assembly of the machine with multiple rectilinear drives.

With reference to Figure 5, the connecting rod yoke 4, which engages with the disc 32, has rigidly connected piston rods 444a and 444b which are arranged along a common axis 34 but in opposite senses, said piston rods being connected respectively to pistons 442 and 2. Similarly, the connecting rod yoke 3, which engages with the disc 31 , has rigidly connected piston rods 333a and 333b which are arranged along a common axis 35 but in opposite senses, said piston rods being connected respectively to pistons 331 and 1 . Although not shown, the connecting rod yoke 3 is arranged to operate slideably within a first set of parallel guides which are rigidly connected to the crankcase, and the connecting rod yoke 4 is arranged to operate slideably within a second set of parallel guides which are rigidly connected to the crankcase and arranged orthogonally with respect to the first set of guides. This double ended piston arrangement has the same advantageous balancing characteristics as the rectilinear drive machines shown with respect to Figures 1 -4, but of course requires larger balancing masses to compensate for the added reciprocating mass of each yoke and piston.

With continued reference to Figure 5, the piston rods 444a and 444b of the connecting rod yoke 4 may be arranged to be cylindrical, as shown, and either pistons 2 and 442 may be detachable from their respective piston rods, or the pistons detachable from the main body of the connecting rod yoke 4. It is therefore possible to arrange for each piston rod, such as 444a, to pass through a seal in a static plate (not shown) in the crankcase 80 so that the volume contained between the underside 442a of the piston 442 and the static plate is alternately increased and reduced. With the fitment of a check valve to control the fluid into said volume and the fitment of a check valve to control the fluid from the said volume, the piston 442 and its cylinder (not shown) may act as a double acting compressor. Similarly, pistons 1 , 2, and 331 and their respective piston rods may each be arranged to pass through a seal in a static plate (not shown) in the crankcase 80 so that the volumes contained between the undersides of said pistons and their corresponding static plates are alternately increased and reduced, these systems therefore being double acting compressors. The presence of the seal in each static plate also excludes any crankcase liquid lubricant, such as oil from entering the fluid volumes above and below the pistons so that the fluid being pumped or compressed is not contaminated with lubricant.

With reference to Figure 6, crankcase halves 280 and 281 encompass the two rectilinear drive mechanisms 500 and 600 which are largely masked by the crankcase halves. The assembly 700 is a complete rectilinear drive unit which may be connected to at least one or several cylinders, and associated fluid conduits which can function as at least one or several compressors or as at least one or several internal combustion engines. The guide frames, also masked by the crankcase halves, are located relative to the crankshaft, disks and connecting rod yokes by dowels between the frame and the crankcase halves, and retained rigidly to the crankcase halves via fixings such as bolts that arranged peripherally around the crankcase halves at location such as 280a, 280b, 280c, 281 a and 281 c, the other fixing location being hidden in this view. The pistons or piston elements emerge from the crankcase halves via apertures that allow the piston skirts or peripheries to be assembled and pass into the crankcase halves if required. In the case of machines used as compressors or engines, cylinders would be fitted to or would be part of the shown surfaces of the crankcase halves.

With reference to Figure 7, a first rectilinear drive unit 700a is connected to a first piston 1 and a third piston 2, which engage respectively with cylinders 199 and 299 arranged respectively along axes 35 and 34, the cylinders being assembled on the crankcase 80. A piston rod 318 passes through seal 316 which is attached to the crankcase 80. This arrangement, in combination with inlet air check valve 191 and air delivery check valve 1 92, comprises a first air pump or compressor which delivers air 193 to a receiver 302. As described with reference to Figure 5, the connecting rod yoke which is rigidly fitted to piston rod 318 can be fitted with a similar piston rod and piston, also known as a counterpart piston, operating in the diametrally opposite direction and said piston rod and piston engaging with a seal (not shown) which is attached to the crankcase 80, and this arrangement in combination with inlet an air check valve (not shown) and a delivery air check valve (not shown) comprises a second air pump or compressor which delivers air via conduits (not shown) to a receiver 302. The third piston 2 can also be double-acting when used in conjunction with the volume above it, said volume having a cylinder and cylinder head (not shown) with inlet and delivery check valves (not shown) and connected with an airway conduit (not shown) to the receiver 30. Similarly, the diametrally opposed piston fitted to piston rod 318 can operate in a double-acting mode if fitted with appropriate cylinder, cylinder head, valves and airway conduits. In this way, pistons acting along the axis 34 of the rectilinear drive mechanism 700a can deliver up to four volumes of air per rotation of the mechanism 700a, one volume of air delivery at each 90° of rotation of the crankshaft of the mechanism 700a. The air delivery volume will depend on the stroke of the mechanism 700a, which is four times the crankthrow of said mechanism, and the diameter of the piston 2 and the diameter of its counterpart piston connected to the connecting rod yoke in the diametrally opposite direction which may be smaller or larger than the piston 2. It is possible to achieve four stages of compression by appropriate sizing of the diameters of piston and its counterpart piston

With continued reference to Figure 7, piston rod 3, also connected to the rectilinear drive mechanism 700a, is linked to piston 1 which moves along axis 35 in an opposed cylinder 199, which also engages with piston 1 1 1 . The motion of piston 1 , in combination with an appropriately phased motion of piston 1 1 1 , operating in the cylinder 199 fitted with inlet airway check valve 303a, and a delivery airway check valve 198a, allows air 303 to be induced via the inlet ports 304 into the volume between the first and second pistons 1 and 1 1 1 in the cylinder 199, and to be subsequently displaced via the delivery ports 198 and delivery air check valve 198a to the receiver 302, supplementing the air delivered from the third piston 2 and also from any counterpart piston attached to the connecting rod yoke that drives the third piston 2, as described earlier in this paragraph, and from any double acting arrangements of the third piston 2 and its counterpart.

With continued reference to Figure 7, piston rod 1 13, connected to the rectilinear drive mechanism 700b, is linked to piston 1 1 1 which moves along axis 35 in an opposed cylinder 199, which also engages with piston 1 , as previously mentioned. The motion of the second piston 1 1 1 , in combination with an appropriately phased motion of the first piston 1 , operating in the cylinder 199 fitted with inlet airway check valve 303a, and a delivery airway check valve 198a, allows air 303 to be induced via the inlet ports 304 into the volume between the first and second pistons 1 and 1 1 1 in the cylinder 199, and to be subsequently displaced via the delivery ports 198 and delivery air check valve 198a to the receiver 302, supplementing the air delivered from the third piston 2 and also from any counterpart piston attached to the connecting rod yoke that drives the third piston 2, as described earlier in this paragraph, and from any double acting arrangements of the third piston 2, cylinder and its counterpart piston and cylinder.

In the same way as the first rectilinear drive unit 700a, a second rectilinear drive unit 700b is connected to the second piston 1 1 1 and a fourth piston 1 13 which engage respectively with cylinders 399 and 199 arranged respectively along axes 134 and 35, the cylinders being assembled on the crankcase 180. A piston rod 319 passes through a seal 317 which is attached to the crankcase 180, and this arrangement in combination with inlet air check valve 195 and air delivery check valve 196 comprises a first air pump or compressor which delivers air 197 to a receiver 302. As described with reference to Figure 5, the connecting rod yoke which is rigidly fitted to the piston rod 319 can be fitted with a similar piston rod and piston, also known as a counterpart piston, operating in the diametrically opposite direction and said piston rod and piston engaging with a seal (not shown) which is attached to the crankcase 180, and this arrangement in combination with inlet an air check valve (not shown) and a delivery air check valve (not shown) comprises another second air pump or compressor which delivers air via conduits (not shown) to a receiver 302. Fourth piston 1 12 can also be double acting when used in conjunction with the volume above it, said volume having a cylinder and cylinder head (not shown) with inlet and delivery check valves (not shown) and connected with an airway conduit (not shown) to the receiver 30, in the same manner as described previously in relation to the third piston 2. Similarly, another diametrally opposed piston fitted to piston rod 319 can operate in a double acting mode if fitted with appropriate cylinder, cylinder head, valves and airway conduits. In this way, pistons acting along the axis 134 of the rectilinear drive mechanism 700b can deliver up to another four volumes of air per rotation of the mechanism 700b, one volume of air delivery at each 90° of rotation of the crankshaft of the mechanism 700b. The air delivery volume will depend on the stroke of the mechanism 700b, which is four times the crankthrow of said mechanism, and the diameter of the piston 1 12 and the diameter of its counterpart piston connected to the connecting rod yoke which may be smaller or larger than the piston 1 12. It is also possible to achieve another four stages of compression by appropriate sizing of the diameters of piston, cylinder and its counterpart piston and cylinder.

As described previously, in embodiments the third piston 2 and/or the fourth piston 1 12 may instead be a mass, equivalent to the mass of the other piston (i.e. the first 1 and second 1 1 1 pistons, respectively) and the other connecting rod yoke, said piston balance mass being reciprocated by its connecting rod yoke without having to be in a cylinder bore. So in this way, the third piston 2 and fourth piston 1 12 may be balancing piston masses, for the purpose of balancing the first 1 and second 1 1 1 piston elements, rather than specifically working volumes within respective cylinders as working piston heads.

The two rectilinear drives may be synchronously linked by various state of art methods. For example a first toothed sprocket 701 may be rigidly attached to the first crankshaft of rectilinear drive 700a, and a second toothed sprocketmay be rigidly attached to the second crankshaftof rectilinear drive 700b, said first and second toothed sprockets being synchronously connected by a continuous toothed belt, which may have guides and tensioners. The toothed belt enables torque to be transmitted between the crankshafts to wherever the torque is required, and ensures the as assembled phasing of the first and second crankshafts, which controls the phasing of the two rectilinear drives and therefore the phasing of all the pistons and in particular the pistons 1 and 1 13 that operate in the cylinder 199. Other methods, such as gear drives, chain drives, eccentric rods and shafts may be used to synchronously connect the first and second crankshafts. It is also possible to use a phase change element (not shown) in the connecting drive between the first and second crankshafts to dynamically change the relative phasing of the crankshafts during machine operation.

With further reference to Figure 7, connecting rod yokes 3 and 1 13 moving on axis 35 may also each be fitted with counterpart pistons driven respectively by rectilinear drive mechanisms 700a and 700b, and said counterpart pistons may be double acting, as previously described with reference to the pistons moving along axes 34 and 134. The pistons moving along axis 35 can therefore deliver another eight volumes of air per rotation of the mechanisms 700a and 700b. The phasing of the air delivery from the pistons operating along axis 35 and driven by rectilinear drive mechanisms 700a and 700b depend on the phasing between the crankshaft in mechanism 700a and the crankshaft in mechanism 700b.

In embodiments, the crankshafts of mechanisms 700a and 700b may be phased so that pistons 1 and 1 1 1 moving on axis 35 arrive simultaneously at the inner dead centre of cylinder 199 in which case the receiver 302 will have compressed air delivery from mechanisms 700a and 700b at each 90° of rotatbn of the combined crankshafts. Such embodiments of the invention are, with the appropriate balance masses on each crankshaft, in complete rotary and reciprocating balance. In such embodiments, the "third piston" and "fourth piston" are balancing piston masses 2, 1 12.

In embodiments of the compressor system, the crankshafts of mechanisms 700a and 700b may be phased so that pistons 1 and 1 1 1 moving on axis 35 are arranged to arrive 45° before or after each other at the innerdead centre of cylinder 199 in which case the receiver 302 will have compressed air delivery from mechanisms 700a and 700b at each 45° of rotation of the combined crankshafts. This compressor embodiment of the invention is, with the appropriate balancing piston masses on each crankshaft, in complete rotary and reciprocating balance.

In a further embodiment of the compressor system described in earlier paragraphs, the various pistons and their cylinders, in combination with double acting operation and use of counterpart pistons and cylinders, and appropriate check valves and airway conduit connections between cylinders, and appropriate sizing of pistons and cylinders, and intercooling between stages, can be configured to provide 15 stages of compression with a fully balanced machine.

The inventive step with respect to the system described in Figures 7is the use of two rectilinear drives, with appropriate balance masses on their crankshafts, having at least one opposed piston from each rectilinear drive engaged in a common opposed cylinder and with a synchronous connection between the crankshafts of the rectilinear drives so that the machine can operate with any phasing between the rectilinear drives without any out-of-balance forces acting on the machine as a whole.

With reference to Figure 8, this machine, which is configured as a 2-stroke internal combustion engine, has many similarities to the compressor machine explained with reference to Figure 7. The arrangement of the rectilinear drives 700a and 700b and their interconnection through a synchronous drive by toothed belt and sprockets, gears, chains and sprockets, eccentric rods and shafts with bevel gears is the same for the compressor machine of Figure 7 and for the 2-stroke internal combustion engine of Figure 8. However, with reference to Figure 8, the purpose of any pistons which are not engaged in the common opposed cylinder 199, such as piston 2 and its counterpart (not shown), and piston 1 12 and its counterpart (not shown), and of any counterpart pistons (not shown) to pistons 1 and 1 1 1 , and of any double acting versions (not shown) of any of these pistons, is to provide scavenge air for the combustion volume controlled by the opposed pistons 1 and 1 1 1 in the opposed cylinder 199.

In one embodiment of the invention, when applied to a 2-stroke internal combustion engine, the scavenge air for the combustion volume controlled by the opposed pistons 1 and 1 1 1 in the opposed cylinder 199 is supplied from a counter piston (not shown), attached on the other side of the connecting rod yoke 3 to piston 1 , said counter piston operating in a cylinder fitted with check valves (not shown) to control the inflow and delivery of air which is routed into conduit 193 to the air ports 198. The counter piston is phased at 180° to the piston 1 so that the counter piston reaches its top dead centre (TDC) position and completes its air delivery to the volume between pistons 1 and 1 1 1 when said volume is a maximum, i.e. pistons 1 and 1 1 1 are at their outer dead centre (ODC) positions.. The displaced volume from said counter piston may be less than, equal to, or more than the combined volumetric displacements of pistons 1 and 1 1 1 in cylinder 199 depending on the desired performance of the engine.

In a similar manner, the scavenge air for the combustion volume controlled by the opposed pistons 1 and 1 1 1 in the opposed cylinder 199 is supplied from a counter piston (not shown), attached on the other side of the connecting rod yoke 1 13 to piston 1 1 1 , said counter piston operating in a cylinder fitted with check valves (not shown) to control the inflow and delivery of air which is routed into conduit 197 to the air ports 198. The counter piston is phased at 180° to the piston 1 1 1 so that the counter piston reaches its top dead centre (TDC) position and completes its air delivery to the volume between pistons 1 and 1 1 1 when said volume is a maximum, i.e. pistons 1 and 1 1 1 are at their outer dead centre (ODC) positions. The displaced volume from said counter piston may be less than, equal to, or more than the combined volumetric displacements of pistons 1 and 1 1 1 in cylinder 199 depending on the desired performance of the engine.

In embodiments of the invention, when applied to a 2-stroke internal combustion engine, the scavenge air for the combustion volume controlled by the opposed pistons 1 and 1 1 1 in the opposed cylinder 199 is supplied from double acting pistons (not shown) associated with the pistons 1 and 1 1 1 (not shown), said double acting pistons operating in both ends of the cylinder 199 fitted with check valves at both ends (not shown) to control the inflow and delivery of air which is routed along conduits 193 and 197 to the air ports 198. The double acting pistons are phased at 180° to the pistons 1 and 1 1 1 so that said double acting pistons reach their top dead centre (TDC) position and complete their air delivery to the volume between pistons 1 and 1 1 1 when said volume is a maximum, i.e. pistons 1 and 1 1 1 are at their outer dead centre (ODC) positions. The combined displaced volume from said double acting pistons associated with pistons 1 and 1 1 1 will necessarily be less than the combined volumetric displacements of pistons 1 and 1 1 1 in cylinder 199 because of the presence of the connecting rod yokes 3 and 1 13, but the desired performance of the engine may be adequate for some applications.

In a further embodiment of the invention, the scavenge air for the displaced volume controlled by pistons 1 and 1 1 1 may be provided by a combination of air delivery from counter pistons and double acting pistons. In one arrangement, the double acting and counter pistons associated with connecting rod yoke 3 and piston 1 would provide the air required for combustion between pistons 1 and 1 1 1 . This would obviate the need to have any scavenge air supply from scavenge pumps associated with piston 1 1 1 . In another arrangement, the double acting and counter pistons associated with connecting rod yoke 1 13 and piston 1 1 1 would provide the air required for combustion between pistons 1 and 1 1 1 . This would obviate the need to have any scavenge air supply from scavenge pumps associated with piston 1 .

With continued reference to Figure 8, in embodiments of the invention, the piston(s), counter-piston(s) and double acting piston(s) operating along either axis 34 or 134 may be used singularly or in combination to provide the scavenge air for the displaced volume controlled by pistons 1 and 1 1 1 . In one arrangement, the displaced volume above piston 2 and contained by the cylinder 299 fitted with a closing plate (not shown) and check valves (not shown) to control the inflow and delivery of air, is routed into conduit 193 to the air ports 198. This air delivery is 90° retarded from the motions of the pistons 1 and 1 1 1 and this is advantageous for improved scavenging and air supply for the pistons 1 and 1 1 1 .

Similarly, in another arrangement, the displaced volume above piston 1 12 and contained by the cylinder 399 fitted with a closing plate (not shown) and check valves (not shown) to control the inflow and delivery of air, is routed into conduit 197 to the air ports 198. This air delivery is 90° retarded from the motions of the pistons 1 and 1 1 1 and this is advantageous for improved scavenging and air supply for the pistons 1 and 1 1 1 .

In a further embodiment of the invention, the scavenge air for the displaced volume controlled by pistons 1 and 1 1 1 may be provided by the air delivery from the double acting portion of the counter piston associated with the connecting rod yoke 318. The volume swept by the underside of the counter piston on connecting rod yoke 318 moves in phase with the volume above the piston 2 and therefore this volume on the underside of the counter piston of connecting rod yoke 318, when fitted with check valves to control the inflow and delivery of air from the volume, may be used to supply air to the conduit 193 and then to the air ports 198. Further scavenge air delivery arrangements are possible using combinations of the pistons, counter pistons and double acting pistons. For instance, the double acting and counter pistons associated with connecting rod yoke 3 and piston 1 would provide the air required for combustion between pistons 1 and 1 1 1 . This would obviate the need to have any scavenge air supply from scavenge pumps associated with piston 1 1 1 . In another arrangement, the double acting and counter pistons associated with connecting rod yoke 1 13 and piston 1 1 1 would provide the air required for combustion between pistons 1 and 1 1 1 . This would obviate the need to have any scavenge air supply from scavenge pumps associated with piston 1 .

Similarly, the double acting and counter pistons associated with connecting rod yoke 318 and piston 2 would provide the air required for combustion between pistons 1 and 1 1 1 . This would obviate the need to have any scavenge air supply from scavenge pumps associated with piston 1 1 1 . In another arrangement, the double acting and counter pistons associated with connecting rod yoke 319 and piston 1 12 would provide the air required for combustion between pistons 1 and 1 1 1 . This would obviate the need to have any scavenge air supply from scavenge pumps associated with piston 1 .

To summarise, embodiments of the invention comprise at least a first and second opposed piston in a cylinder, a first rectilinear drive mechanism linked with a first connecting rod yoke to said first opposed piston moving along a first axis, and with a third connecting rod yoke linked to a third piston element (which may be a balancing piston mass) moving along a second axis which is substantially orthogonal to the first axis, a second rectilinear drive mechanism linked with a second connecting rod yoke to said second opposed piston, moving substantially in anti-phase with the first opposed piston along the same first axis in a common cylinder, and said second rectilinear drive mechanism linked with a fourth connecting rod yoke to a fourth piston element (which may be a balancing piston mass) moving along a third axis which is also orthogonal to the first axis and parallel to the second axis, and means for rotatably connecting the first and second rectilinear drive mechanisms. In this arrangement, the third piston element may be a compression and expansion piston and moving in a second cylinder whose major axis is coaxial with the second axis and the fourth piston element is a compression and expansion piston and moves in a third cylinder whose major axis is coaxial with the third axis.

It should be understood that where balancing piston masses 2, 1 12 are employed, the term "rod yoke" used in the context of a connecting rod yoke linked to a balancing piston mass simply means that the yoke provides a rigid link that connects the balancing piston mass to the driving mechanism.

The invention as described may be used with appropriate valves, connecting conduits and intercoolers for single and multi-stage compressors and may be used other appropriate valves, connecting conduits, fuelling and ignition systems for internal combustion engines which may be either of compression ignition or spark ignition combustion types.

It is intended that features of the various embodiments described may be used with features of other embodiments, in any combination, unless specifically stated otherwise.

When used in this specification and claims, the terms "comprises" and "comprising" and variations thereof mean that the specified features, steps or integers are included. The terms are not to be interpreted to exclude the presence of other features, steps or components.

The features disclosed in the foregoing description, or the following claims, or the accompanying drawings, expressed in their specific forms or in terms of a means for performing the disclosed function, or a method or process for attaining the disclosed result, as appropriate, may, separately, or in any combination of such features, be utilised for realising the invention in diverse forms thereof.

Claims

1 A machine comprising at least a first and second opposed piston in a common cylinder, a first rectilinear drive mechanism with a first connecting rod yoke joined to said first opposed piston moving along a first axis, and with a third connecting rod yoke joined with a third piston element moving along a second axis which is substantially orthogonal to the first axis, a second rectilinear drive mechanism with a second connecting rod yoke joined to said second opposed piston, moving substantially in-phase with the first opposed piston, but in an opposite direction, and along the same first axis and in said common cylinder with the first opposed piston, and said second rectilinear drive mechanism with a fourth connecting rod yoke joined to a fourth piston element moving along a third axis which is also substantially orthogonal to the first axis and substantially parallel to the second axis, and configured such that the first and second rectilinear drive mechanisms are synchronously phased.

2 A machine according to Claim 1 , in which the third piston element is a compression and expansion piston and moves in a second cylinder whose major axis is coaxial with the second axis.

3 A machine according to Claim 1 , in which the fourth piston element is a compression and expansion piston and moves in a third cylinder whose major axis is coaxial with the third axis and parallel with the second axis.

4 A machine according to claim 1 or claim 2, in which the fourth piston element is a compression and expansion piston and moves in a cylinder whose major axis is coaxial with the third axis.

5 A machine according to any one of claims 1 to 3, in which the third piston element is a balancing piston mass.

6 A machine according to claims 1 or claim 2, in which the fourth piston element is a balancing piston mass. A machine according to any preceding claim, in which the first connecting rod yoke has two contact faces, which are parallel to the major axis of said first connecting rod yoke, the first contact face engaging slidably and with clearance with a first fixed yoke guide, and the second contact face engaging slidably and with clearance with a second fixed yoke guide, said yoke guides being parallel to each other and therefore parallel to the major axis of said first connecting rod yoke, and in which the third connecting rod yoke has two contact faces, which are orthogonal to the major axis of said first connecting rod yoke, the first contact face engaging slidably and with clearance with another first fixed yoke guide, and the second contact face engaging slidably and with clearance with another second fixed yoke guide, said yoke guides being parallel to each other and therefore orthogonal to the major axis of said first connecting rod yoke, and in which the second connecting rod yoke has two contact faces, which are parallel to the major axis of said first connecting rod yoke, the first contact face engaging slidably and with clearance with the another first fixed yoke guide, and the second contact face engaging slidably and with clearance with the another second fixed yoke guide, said yoke guides being parallel to each other and therefore parallel to the major axis of said first connecting rod yoke, and in which the fourth connecting rod yoke has two contact faces, which are orthogonal to the major axis of said first connecting rod yoke, the first contact face engaging slidably and with clearance with the another first fixed yoke guide, and the second contact face engaging slidably and with clearance with the another second fixed yoke guide, said yoke guides being parallel to each other and therefore orthogonal to the major axis of said first connecting rod yoke, A machine according to any preceding claim, in which the yoke guides are monolithic with the machine crankcase. A machine according to any one of claims 1 to 7, in which the yoke guides are a separate component. A machine according to claim 9, in which the yoke guides component is rigidly linked to the crankcase. A machine according to claim 10, in which the yoke guides component is rigidly linked to the crankcase by dowels and fixings. A machine according to claim 1 1 , in which the fixings are bolts. A machine according to any one of claims 1 to 7 or claims 9 to 12, comprising at least two or more sets of a first and second opposed pistons in two or more cylinders. A machine according to claim 13, in which the two or more sets of rectilinear drives at each end of the opposed cylinders are each driven by a single crankshaft. A machine according to any one of the preceding claims, in which the cylinders of the at least first and second opposed pistons are fitted with inlet airway check valves allowing air into the cylinders and outlet airway check valves allowing air to leave the cylinders and so act as a compressor. A machine according to claim 15, in which at least the second cylinders are fitted with inlet airway check valves allowing air into the cylinders and outlet airway check valves allowing air to leave the cylinders and so act as a compressors. A machine according to claim 15, in which at least the third cylinders are fitted with inlet airway check valves allowing air into the cylinders and outlet airway check valves allowing air to leave the cylinders and so as to act as compressors. A machine according to any one of the preceding claims, in which the at least first connecting rod yokes are also fitted with another piston which moves in anti-phase in a fourth cylinder which is co-axial with the first cylinder and disposed spatially at 180 degrees to the first cylinder. A machine according to any one of the preceding claims, in which the at least second connecting rod yokes are also fitted with another piston which moves in anti-phase in a fifth cylinder which is co-axial with the first cylinder and disposed spatially at 180 degrees to the first cylinder. A machine according to any one of the preceding claims, in which the at least third connecting rod yokes are also fitted with another piston which moves in anti-phase in a sixth cylinder which is co-axial with the second cylinder and disposed spatially at 180 degrees to the second cylinder. A machine according to any one of the preceding claims, in which the at least fourth connecting rod yokes are also fitted with another piston which moves in anti-phase in a seventh cylinder which is co-axial with the third cylinder and disposed spatially at 180 degrees to the third cylinder. A machine according to any one of the preceding claims, which operate as a single stage compressor. A machine according to any one of the preceding claims, and with appropriate sizing of cylinders, appropriate connections and intercooling between cylinders which operates as a multiple stage compressor. A machine according to claim 23, and with appropriate balancing masses on the crankshafts of each rectilinear drive, which operates as a fully balanced multiple stage compressor. A machine according to claim 23, in which the at least a first cylinder has approximately three to six times the displacement of at least a second cylinder, said second cylinder having itself approximately three to six times the displacement of the at least a fourth cylinder, said fourth cylinder having itself approximately three to six times the displacement of the at least a sixth cylinder and in which the outlet from the first cylinder is connected via an intercooler to the inlet of the at least a second cylinder, the outlet from the said at least a second cylinder being connected via an intercooler to the inlet of the at least a fourth cylinder, the outlet from the at least a fourth cylinder being connected via an intercooler to the inlet of the at least a sixth cylinder, said machine being a five stage compressor. A machine according to claim 24, in which the at least a first cylinder has approximately three to six times the displacement of at least a second cylinder, said second cylinder having itself approximately three to six times the displacement of the at least a fourth cylinder, said fourth cylinder having itself approximately three to six times the displacement of the at least a sixth cylinder and in which the outlet from the first cylinder is connected via an intercooler to the inlet of the at least a second cylinder, the outlet from the said at least a second cylinder being connected via an intercooler to the inlet of the at least a fourth cylinder, the outlet from the at least a fourth cylinder being connected via an intercooler to the inlet of the at least a sixth cylinder, said machine being a balanced five stage compressor. A machine according to any one of the preceding claims, in which at least one piston is double acting. A machine according to claim 27, in which the at least one piston uses its underside to induce and displace air in the volume of the cylinder beneath it which is fitted with at least one check valve to control the air flow into the underside cylinder and at least one check valve to control the air flow from the underside cylinder, said piston, cylinder and check valves being arranged as a double acting compressor. A machine according to claim 27, in which a multiplicity of pistons use their undersides to induce and displace air in the volume of the cylinders beneath each piston, each of said cylinders being fitted with at least one check valve to control the air flow into the underside cylinders and at least one check valve to control the air flow from the underside cylinders, said pistons, cylinders and check valves being arranged as a double acting compressors. A machine according to any one of claims 1 to 14, in which at least a first opposed cylinder is equipped with two opposed pistons, at least one ignition system, at least one fuelling system, exhaust ports and air ports that receive air during open period of its air ports from the at least a second cylinder and second piston, said second cylinder and second piston having a displacement which is at least eighty percent of the displacement of the first cylinder and said second cylinder having a check valve in its inlet system, said machine operating as a spark ignition two stroke engine. A machine according to any one of claims 1 to 14, in which at least a first opposed cylinder is equipped with two opposed pistons with at least one in- cylinder injection fuelling system, exhaust ports and air ports that receive air during open period of its air ports from the at least a second cylinder and second piston, said second cylinder and second piston having a displacement which is at least eighty percent of the displacement of the first cylinder and said second cylinder having a check valve in its inlet system, said machine operating as a compression ignition two stroke engine. A machine according to claim 30 in which the second piston in the second cylinder is 180° crankangle retarded from at least one of the two opposed pistons in the first opposed cylinder. A machine according to claim 31 , in which the second piston in the second cylinder is 180° crankangle retarded from at least one of the two opposed pistons in the first opposed cylinder. A machine according to claim 30, in which the second piston in the second cylinder is 90° crankangle retarded from at least one of the two opposed pistons in the first opposed cylinder. A machine according to claim 31 , in which the second piston in the second cylinder is 90 "crankangle retarded from at least ore of the two opposed pistons in the first opposed cylinder. A machine according to any one of claims 30 to 35, in which the crankshafts of the at least two sets of rectilinear drive mechanisms are arranged to have a phase angle between them such that the motion of the opposed pistons to and from the dead centres are asynchronous, that is to say the opposed pistons do not reach their dead centres simultaneously. A machine according to claim 36 in which the phase angle is at least 5 degrees crankangle advance for the first opposed piston, arranged to be the opposed piston controlling the exhaust ports, versus the second opposed piston which controls the air ports. A machine according to any one of claims 30 to 37, in which at least one of the connecting rod yokes and one of the attached pistons is a double acting piston in a cylinder fitted with check valves to control air inflow and air delivery via conduits to the volume between the opposed pistons in the opposed cylinder. A machine according to any one of claims 30 to 38, in which at least one double acting piston supplements the air delivery of the second piston in the second cylinder via conduits to the volume between the opposed pistons in the opposed cylinder. A machine according to any one of claims 30 to 39, in which at least one of the another pistons on the connecting rod yoke is arranged to have a different diameter to the other piston fitted to the connecting rod yoke, said arrangement constituting a stepped piston. A machine substantially as hereinbefore described with reference to and as shown in the accompanying drawings. Any novel feature or novel combination of features described herein and/or in the accompanying drawings.