GB2512651A

GB2512651A - Opposed piston engine with double sided stepped piston scavenging

Info

Publication number: GB2512651A
Application number: GB201306179A
Authority: GB
Inventors: Jean-Pierre Pirault; Alireza Veshagh
Original assignee: OSP ENGINES Ltd
Current assignee: OSP ENGINES Ltd
Priority date: 2013-04-05
Filing date: 2013-04-05
Publication date: 2014-10-08
Also published as: GB201306179D0; WO2014162141A1

Abstract

An opposed piston engine 100 with at least one combustion cylinder volume 1000 having at least one stepped double sided piston having three areas that can be used to displace, compress or expand fluids in the engaging cylinder. The stepped double sided air transfer piston 3a and 3b is located in a stepped cylinder lb in which the inner side 302 of the air transfer piston 3b acts in-phase with the corresponding power pistons 3 and 2 and the outer side 301 of the air transfer piston 3b acts in anti-phase with the power pistons 3 and 2. In which the inner side 302 and outer side 301 of the piston 3b are integral with either the exhaust power piston, which controls the opening and closing of the exhaust ports, or the air power piston, which controls the opening and closing of the air ports of the combustion chamber. The staggered scavenging of the stepped piston design provides a lower work requirement than required for a conventional constant pressure system.

Description

g s crt1 p Opposed Piston Engine with Double Sided Stepped Piston Scavenging Keywords 2 stroke engines, Stepped pistons, double diameter pistons, double sided pistons, double acting pistons, air transfer piston, opposed piston engine, staged scavenging

Background

This invention relates to opposed piston engines that use stepped double sided pistons to provide the air flow used for combustion. The invention applies to two stroke (2-stroke) engines.

The 2-stroke cycle does not, excepting certain examples, have a uniqUe stroke for fresh air induction into the cylinder. The air induction takes places in parallel with the power stroke and exhaust gas exchange.

The proposed invention provides scavenging air for 2-stroke engines without external compressors or scavenge blowers.

Some definitions are provided before describing the invention Definitions The following descriptions are provided with reference to Figure 1, Figure 2 and Figure 3 to help interpretation of this text. The descriptions are not intended as universal definitions.

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

An opposed piston engine or compressor is an engine or compressor in which two power pistons slide in a common cylinder compressing and expanding a common volume of air.

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

A double sided piston is a piston having two areas that can be used to displace, compress or expand fluids in engaging cylinders.

A double sided stepped piston is a piston having three areas that can be used to displace, compress or expand fluids in engaging cylinders.

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 air 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 200 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.

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

"Outer dead centre" refers to outermost position of a piston in its travel in the cylinder of an opposed piston engine, i.e. the furthest position the centre of the cylinder. In engines with cylinder heads, this is normally referred to as "bottom dead centre".

With opposed piston engines, the air and exhaust pistons approach inner dead centre simultaneously, separated only by the phase angle between the air and exhaust pistons.

The forward side of an air transfer piston is the side of the larger diameter of the stepped double sided piston which acts in-phase with the air piston or an exhaust piston.

The reverse side of an air transfer piston is the side of the larger diameter of the stepped double s sided piston which acts in anti-phase with the air piston or an exhaust piston.

An air duct or conduit is a passageway or connecting route which allows air to be transferred from one point to another.

"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.

Scavenging efficiency is a measure of the effectiveness of filling the combustion cylinder volume (1000 in Figure 1) with clean air.

A scavenge pump or scavenge blower is a compressor or pump that provides clean air to purge and fill the combustion volume 1000.

Ports of 2-stroke engines are the apertures in the cylinder walls that enable the flow of gases from or into the cylinder. For example, reference Figure 1, 10 are the exhaust ports that allow the exhaust to flow from the cylinder, when uncovered by the power piston 3, to the exhaust pipe 11. Air ports 7a (Figure 1) allow fresh air from the engine scavenge pumps to enter the combustion cylinder volume 1000; the ports are opened and closed by the motion of the power piston 2. The air ports are sometimes referred to as "transfer" ports in that they allow air to be moved from the air transfer pistons to the working cylinder 1000.

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.

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.

Main Claim An opposed piston engine with at least one cylinder in which at least one piston is arranged as a stepped double sided piston in a stepped cylinder to provide some or all of the engine airflow s requirements for combustion during both the expansion and compression phases of the power pistons.

This and other embodiments are outlined in the following description.

List of Figures Figure 1 shows an end view of the general diagrammatic arrangement of a single cylinder opposed piston engine with an external compressor.

Figure 2 shows an end view of the general diagrammatic arrangement of a first embodiment of a single cylinder opposed piston engine 100 with a stepped double sided piston.

Figure 3 shows a side view of the general diagrammatic arrangement of the first embodiment of a single cylinder opposed piston engine 100 with a stepped double sided piston.

Figure 4 is a diagram showing the approximate relative phasings of the volume changes in the air transfer cylinder volumes and in the combustion cylinder volume of the engine depicted in Figure 2 and Figure 3.

Figure S is a graph comparing the theoretical gas exchange work for a constant pressure compressor scavenge pump versus the 2-stage stepped double sided piston scavenge systent Figure 6 shows an end view of the general diagrammatic arrangement of a second embodiment of a single cylinder opposed piston engine 200 with a stepped double sided piston.

Figure 7 shows an end view of the general diagrammatic arrangement of a third embodiment of a single cylinder opposed piston engine 400 with one stepped piston and one stepped double sided piston.

Figure 8 is a diagram showing the approximate relative phasings of the volume changes in the air transfer cylinder volumes and in the combustion cylinder volume of the engine depicted in Figure 7.

Description

With reference to Figure 2 and Figure 3 and a first embodiment of the invention, the outer crankpins 11 9a and 11 9b of a crankshaft 13 are rotatably connected respectively to the long connecting rods 1 iSa and 1 lSb which in turn are rotatably connected to a transverse beam 116 of the exhaust piston 3 of the opposed piston engine 100. As shown in Figure 3, the transverse beam 116 may also have a centre pivot bearing 121 which prevents side loading of the piston 3.

The exhaust ports 10 in the cylinder liner la are controlled by of the displacement of the exhaust power piston 3, as defined by the crankthrow 1 Sb of the crankshaft 13, such that the exhaust ports are fully open when the piston 3 is at its outer dead centre position, and are filly closed when the piston skirt 3c fully covers the exhaust ports 10.

The air transfer ports 7a in the cylinder liner Ia are controlled by of the displacement of the air power piston 2, as defined by the crankthrow 15a of the crankshaft 13, such that the air transfer ports are fully open when the piston 2 is at its outer dead centre position, and are fully closed when the piston skirt 2a fully covers the air transfer ports 7a.

Piston 3 is a stepped double sided piston with a larger diameter 3b that is a first air transfer piston which acts in phase with the smaller diameter exhaust piston 3b. The piston elements 3a and 3b of the piston 3 may be rigidly linked or articulated relative to each other. The skirt of piston 3a slides in the cylinder bore Ia whilst the skirt of piston 3b slides in the cylinder bore lb. The stepped double sided piston 3b may be either circular, elliptical or rectangular in shape, and cylinder bore lb would be a corresponding shape.

The air induction streams in to the engine 100 enter via two complementary routes 8 and 80.

Firstly, considering the air stream 8 The "reverse" or outer side of the stepped double sided piston 3b (hereafter simply referred to as "piston 3b") carries a closing diaphragm 301 so that as piston 3a and 3b move, the volume 2000, enclosed by diaphragm 301, the cylinder walls lb and the cylinder cap 2001, increases or decreases according to the direction of motion of piston 3b. Check valve 8a allows air 8 into cylinder 2000 as the piston 3b moves towards inner dead centre, and check valve 9a allows the same air 9 to leave cylinder 2000 in to the air transfer pipe ób as the piston 3b moves towards its outer dead centre position. The check valve 8a ensures that air flow can only enter the cylinder 2000 and cannot leave the cylinder volume 2000, whilst the check valves 9a ensures that air flow can only leave the cylinder volume 2000 and cannot enter the cylinder volume 2000. In this way, the piston 3b, its closing diaphragm 301, the cylinder walls Ib, the cylinder cap 2001 and the check valves 8a and 9a act as a first compressor or air pump P1, transferring air from the surrounding atmosphere or engine air induction system into the compressor volume 2000 and then to the engine combustion cylinder 1000.

The air 9 is displaced along the air transfer pipe 6b by a combination of the motion of the piston 3b and the momentum of the air column 9. The air 9 enters the cylinder volume 1000 via the air transfer pipe connection 7, joined to air conduit ób, and then via the air ports la. As the diaphragm 301 moves substantially in anti-phase with the air piston 2, the air in the cylinder volume 2000 is transferred into the cylinder volume 1000 during the opening period of the air transfer ports. The engine 100 is therefore suppliedflrstly with fresh air for combustion from the compressor cylinder volume 2000 to the combustion cylinder volume 1000 during the motion of the pistons 2 and 3 from their inner dead centre positions to their outer dead centre positions.

To re-emphasize, the relative phasing of the volume changes for the cylinder volume 1000 and cylinder volume 2000 are shown in Figure 4, from which it can be seen that volume 2000 moves in anti-phase with volume 1000, and hence volume 2000 is being displaced in to volume 1000 as the pistons 2 and 3 move towards their outer dead centre positions.

Secondly, considering the air stream 80: The "forward" or "inner " side of the piston 3b has a diaphragm 302 so that as piston 3a and 3b move, the volume 3000, enclosed by diaphragm 302, the cylinder walls lb and the upper rim of the cylinder liner or wall 1 a, increases or decreases according to the direction of motion of piston 3a, Check valve 80a allows air 80 into cylinder 3000 as the piston 3b moves towards.

outer dead centre, and check valve 90a allows the same air 90 to leave cylinder 3000 in to the air transfer pipe 6b from the air pipe 90b as the piston 3b moves towards its inner dead centre position. The check valve 80a ensures that air flow can only enter the cylinder 3000 and cannot leave the cylinder volume 3000, whilst the check valves 90a ensures that air flow can only leave the cylinder volume 3000 and cannot enter the cylinder volume 3000. In this way, the piston 3b, its diaphragm 302, the cylinder walls ib, the upper rim of the cylinder liner I a and the check valves 80a and 90a act as a second or complementary compressor or air pump P2, transferring air from the surrounding atmosphere or engine air induction system into the compressor volume 3000 and then to the engine combustion cylinder 1000.

The air 90 is displaced along the air transfer pipe 9Db and thence 6b by a combination of the motion of the piston 3a and the momentum of the air column 90. The air 90 enters the cylinder volume 1000 via the air transfer pipe connection 70, joined to air conduit 6b, and then via the air ports 7a. As the diaphragm 302 moves substantially in-phase with the air piston 2, the air in the cylinder volume 3000 is transferred into the cylinder volume 1000 during the closing period of the air transfer ports. The engine 100 is therefore supplied secondly with fresh air for combustion from the compressor cylinder volume 3000 to the combustion cylinder volume 1000 during the motion of the pistons 2 and 3 from their outer dead centre positions to their inner dead centre positions.

With reference to Figure 4, the relative phasing of the volume changes for the cylinder volume 1000, the cylinder volume 2000 and cylinder volume 3000 are shown versus the cranicangle position of exhaust piston 3, which is phased notionally 30° crankangle in advance of the air piston 2. The exhaust port open period corresponds to the crankangle between EO-EC, i.e. approximately 160° cranlcangle duration. The air port open period corresponds to the crankangle between 10-IC, i.e. approximately 1000 crankangle duration. The asynmietry of the port timings is an optional beneficial feature of opposed piston engines, and opposed stepped piston engines, and arises from the phasing of the exhaust and air pistons which in this example is notionally 30° crankangle, as previously stated. These curves show volume 2000 moves in anti-phase with volume 1000, and hence volume 2000 is being displaced in to volume 1000 as the pistons 2 and 3 move towards their outer dead centre (ODC in Figure 4) positions. This air transfer from the volume 2000 to the main cylinder 1000 occurs during the expansion stroke of the engine (Ti in Figure 4) as the air ports open (JO in Figure 4) after the opening of the exhaust ports (E0 in Figure 4), and continues to outer dead centre of pistons 2 and 3. Figure 4 also shows that volume 3000 moves in-phase with volume 1000, and hence volume 3000 is being displaced in to volume 1000 as the pistons 2 and 3 move towards their inner dead centre positions (T2 in Figure 4). This air transfer provides a continuing scavenging or clearing of the exhaust gases from the cylinder 1000 via the exhaust ports as the pistons 2 and 3 move away from their outer dead centre positions towards their inner dead centre location. In this way, the cylinder 1000 is continuously positively scavenged with fresh air from the opening to closing of the air ports. It should be explained that in Figure 4 the volume displacements 1000, 2000 and 3000 are all shown as having maximums of 96-100% notionaily for simplicity and clarity.

However, the absolute volumes 1000, 2000 and 3000 can all be different and adjusted as previously mentioned by design of the selected diameters of the pistons 2 and 3, and crankshaft strokes l5a and 15b, and the entry port Sb, 80b arid outlet port 9b positions relative to the moving surfaces of the piston 3, and the pressure settings of the check valves 8a, 80a and 9a, 90a.

With reference to Figure 5, the functional advantage of the previously described scavenging system invention ("Staged Scavenging" in Figure 5) versus a constant pressure compressor or scavenge blower system ("Scavenge Blower" in Figure 5) is compared graphically in terms of so arbitrary instantaneous power units versus the crankangle position of exhaust piston 3, which is phased notionally 30° crankangle in advance of the air piston 2. The "Scavenge Blower" curve in Figure 5 is the theoretical instantaneous power required to deliver the air from the external scavenge blower which generally produces a constant airflow rate versus the engine crankangle. The work required to drive the external compressor is constant as the air flowrate is constant and backpressure, against the airflow through the cylinder volume 1000 and the exhaust pipe and receiver 11, is also constant. The required compressor work is directly proportional to the air floiate and backpressure acting against the air flowrate; as the air flowrate and the backpressure are constant, the instantaneous work is constant, as depicted by the flat portion of the Scavenge Blower curve during the scavenge period Ti plus T2 (T1-i-T2).

The "Staged Scavenging" curve in Figure 5 is the theoretical instantaneous power required to deliver the air from the stepped piston scavenging system which produces a variable airflow rate versus engine crankangle during the scavenge period Ti plus T2 (Tl-i-T2), and as a result there is also a proportionally variable backpressure opposing the airflow in the cylinder 1000 and the exhaust pipe and receiver Ii. At the start of the stepped piston staged scavenging period, i.e. the start of Ti, the scavenge airflow rate from volume 2000 to volume 1000 is designed to be equal to the flowrate from an external compressor and this scavenge air tlowrate will have to work against the same backpressure as the externally driven compressor. The power required by the engine to produce the stepped piston scavenging from the air pump P1 is therefore a product of two quantities that vary with engine crankange, starting with similar values of instantaneous power required during the early phase of air port opening (i.e. 10 in Figure 5) for both the compressor and stepped piston staged scavenging system. As the power pistons move towards their outer dead centre positions ("ODC" in Figure 5), the flowrate from the P1 scavenge pump reduces whereas the scavenge blower compressor flowrate remains constant, the same relative differences also applying to the backpressures acting on the the "Staged Scavenging" and "Scavenge Blower" systems. Afler the power pistons 2 and 3 start returning towards inner dead centre position (]DC in Figure 5), the air pump P2 starts ("e" in Figure 5) to deliver air from the cylinder volume 3000 to the cylinder volume 1000 and continues the scavenging process, again with a variable flowrate and backpressure versus the constant pressure and flowrate of the compressor scavenging system. The flowrate from volume 3000 gradually diminishes as the volume 3000 diminishes, and likewise the instantaneous power reduces towards zero at the point of air port closure ( end of period "T2" in Figure 5), whereas the compressor based scavenge system still pushes a substantial air flowrate through the open exhaust ports against a substantial exhaust back pressure until the exhaust ports close ("g" in Figure 5). This last phase of the compressor airflow delivery is particularly wasteful in terms of work and unused air, and does not significantly increase the scavcnge efficiency of the engine.

In addition to the lower work required for the airflow delivery with the stepped piston staged scavenging versus compressor scavenging, the mechanical and compression efficiencies of the stepped piston staged scavenging system are significantly higher than those of an externally driven compressor of whatever type, e.g. rotary positive displacement compressor, aerodynamic compressor, and this efficiency advantage further reduces the work requirement of the stepped piston staged scavenging compared to externally driven compressors.

Further advantages of the stepped piston staged scavenging in comparison to other scavenging systems are that it can be well matched to the engine combustion airflow requirements over the engine speed range, and its compactness, simplicity and low cost.

With reference again to Figure 2 and Figure 3, the volumetric displacements 2000 and 3000 of the scavenge air pumps P1 and P2 can be controlled independently of the engine combustion air volumetric displacement 1000 by appropriate sizing of the of the outer piston diaphragm 301 and the inner piston diaphragm 302 of the stepped double sided piston 3b, and also by appropriate sizing of the stroke of exhaust power piston 3, as controlled by the crankthrow 15b.

By appropriate selection of the diameter of the outer piston diaphragm 301 and the inner piston

B

diaphragm 302, and the crankshaft stroke 1 Sb, differing air flowrates can be arranged for the motion of the power pistons 2 and 3 towards outer dead centre and for the motion of the power pistons 2 and 3 towards inner dead centre.

With further reference to Figure 2, Figure 3 and Figure 5, the timing of the start of air delivery from the volume 2000 of scavenge pump P1 is governed primarily by the compression ratio of the compressor P1 and the opening pressure of the check valve 9a; the compression ratio of the compressor P1 and the opening pressure of the check valve 9a are adjusted so that the pressure of the air flowrate 9 is in excess of the pressure in cylinder 1000 as the air ports open ("c" in Figure 5).

The timing of the start of air delivery from the volume 3000 of scavenge pump P2 is governed primarily by the compression ratio of the compressor P2 and the opening pressure of the check valve 90a; the compression ratio of the compressor P2 and the opening pressure of the check is valve 90a are adjusted so that the pressure of the air flowrate 90 is in excess of the pressure in cylinder 1000 as the power pistons 2 and 3 are near their outer dead centre positions.

To summarise, the first embodiment of the invention, is an opposed piston engine 100 with at least one combustion cylinder volume 1000 having one stepped double sided air transfer piston 3a and 3b in a stepped cylinder lb in which the inner or forward side 302 of the stepped air transfer piston 3b acts in-phase with the corresponding power pistons 3 and 2 and the outer or reverse side 301 of the stepped double sided air transfer piston 3b acts in anti-phase with the power pistons 3 and 2, and in which the inner or forward side 302 and outer or reverse side 301 of the stepped double sided air transfer piston 3b are integral with the exhaust power piston.

Furthermore, the inner or forward side 302 of the stepped double sided air transfer piston 3b discharges air into a first air delivery pipe 90b which is in connection with the air ports 7a and the outer or reverse side 301 of the stepped double sided air transfer piston 3b discharges air into a second air delivery pipe 9b which is in connection with the air ports 7a. Alternatively, the inner or forward side 302 of the stepped double sided air transfer piston 3b discharges air via a check valve 90a into a first air delivery pipe 90b which is in connection with the air ports 7a and the outer or reverse side 301 of the stepped double sided air transfer piston 3b discharges air via a check valve 9a into a second air delivery pipe 9b which is in connection with the air ports 7a. The airflow from the inner or forward side 302 of the stepped double sided air transfer piston 3b and the outer or reverse side 301 of the stepped double sided air transfer piston 3b discharge air into a third pipe 7 which is in connection with the transfer ports 7a.

In a variation of the aforementioned flow arrangements, the inner or forward side 302 of the air transfer piston and the outer or reverse side 301 of the stepped double sided air transfer piston discharge 3b air via a check valve 70a into third pipe which is in connection with the transfer ports 7a The crankshaft of the engine 100 is arranged so that the stepped double sided air transfer piston 3b, which is an integral part of the exhaust power piston 3, is connected via the first connecting rod 11 8a and third connecting rod 11 8b to the first crankpin 1 19a and third crankpin 1 l9b of the a crankshaft 13, and the air power piston 2 is connected via a second connecting rod 17 to a second crankpin 117 of the crankshaft 13. The second crankpin 117 of said crankshaft may be disposed between 150-210° relative to the first crankpin 1 19a and third crankpin 11 9b of the crankshaft 13, depending on the direction or rotation and speed of the engine.

With reference to Figure 6 which shows a second embodiment 200 of the invention, piston 331 is a stepped double sided air power piston controlling air ports 171, and piston 221 is an exhaust power piston controlling the exhaust ports 410. In this arrangement, the air delivery 9 from scavenge pump P1 and the air delivery 90 from scavenge pump P2 would have a shorter routing to the air ports 171 than is shown in Figure 2, as the air ports 171 are located closer to the scavenge pumps P1 and P2. The engine 200 may have some performance advantages over the arrangement in Figure 2 which has longer conduits from the scavenge pumps P1 and P2 to the airports 7a (of Figure 2 and Figure 3). In this embodiment, the stepped double sided air power piston 331 comprises the smaller diameter piston 331b and the larger double diameter piston 331a.

The arrangement of the crankshaft 13 and connecting rods, tranverse beam and centre pivot bearing for engine 200 in Figure 6 is the same as described for engine 100 with reference to Figure 2 and Figure 3 with the exceptions that the connecting rod 117 is connected to the exhaust piston 221 and connecting rods 1 18a and 1 18b are connected to the stepped double sided air piston 331 via the tranverse beam 116 and the centre pivot bearing 121.

To summarise, the second embodiment of the invention, is an opposed piston engine 200 with at least one combustion cylinder volume 1000 having one stepped double sided air transfer piston 331a and 331b in a stepped cylinder 33k in which the inner or forward side 302 of the stepped double sided air transfer piston 331 acts in-phase with the corresponding power pistons 331 and 221 and the outer or reverse side 301 of the stepped double sided air transfer piston 331 acts in anti-phase with the power pistons 331 and 221, and in which the inner or forward side 302 and outer or reverse side 301 of the stepped double sided air transfer piston 33 la are integral with the air power piston 331, With reference to Figure 7, this embodiment 400 derives from that shown in Figure 2 with the addition of a third scavenge pump P3 which is fonned from the stepped piston 20a attached to the exhaust power piston 20b. Piston 3 and scavenge pumps P1 and P2 of this engine embodiment 400 operate in the same fashion as is described with reference to the scavenge pumps P1 and P2 of engine 100 on Figure 2 and Figure 3.

In the arrangement shown in Figure 7, the "forward" or "inner" side of the piston 20a has a diaphragm 304 so that as piston 20a and 20b move, the volume 4000, enclosed by diaphragm 304, the cylinder walls 20c and the lower rim of the cylinder liner or wall Id, increases or decreases according to the direction of motion of piston 20a. Check valve 98a allows air 98 into cylinder volume 4000 as the piston 20a moves towards outer dead centre, and check valve 99a allows the same air 99 to leave cylinder 4000 to the air transfer pipe 7 from.the air pipe 99b as the piston 20a moves towards its inner dead centre position. The check valve 98a ensures that air flow can only enter the cylinder 4000 and cannot leave the cylinder volume 4000, whilst the check valves 99a ensures that air flow can only leave the cylinder volume 4000 and cannot enter the cylinder volume 4000. In this way, the piston 20a, its diaphragm 304, the cylinder walls 20c, the lower rim of the cylinder liner id and the check valves 98a and 99a act as a third or complementary compressor or air pump P3, transferring air from the surrounding atmosphere or engine air induction system into the compressor volume 4000 and then to the engine combustion cylinder 1000.

The air 99 is displaced along the air transfer pipe 99b and thence 70'a by a combination of the motion of the piston 20a and the momentum of the air column 99. The air 99 enters the cylinder volume 1000 via the air transfer pipe connection 7, joined to air conduit 6b from the other scavenge pumps P1 and P2, and then via the air ports 7a. As the diaphragm 304 moves in-phase with the air piston 20a, the air in the cylinder volume 4000 is transferred into the cylinder volume 1000 during the closing period of the air ports 7a. The engine 400 is therefore supplied thirdly with fresh air for combustion from the pump cylinder volume 4000 to the combustion cylinder volume 1000 during the motion of the pistons 20 and 3 from their outer dead centre positions to their inner dead centre positions.

With reference to Figure 8, this shows the relative phasing of the volume changes for the cylinder volume 1000, the cylinder volume 2000, the cylinder volume 3000 and the cylinder volume 4000. These curves show that volume 2000 moves in anti-phase with volume 1000, and hence volume 2000 is being displaced in to volume 1000 as the pistons 20 and 3 move towards their outer dead centre positions (ODC in Figure 8), This air transfer to the main cylinder 1000 occurs during the expansion stroke of the engine as the air ports open after the opening of the exhaust ports 10, and continues to outer dead centre of pistons 20 and 3. Figure 8 also shows that volume 3000 and volume 4000 move in-phase with volume 1000, and hence volumes 3000 and 4000 are being displaced in to volume 1000 as the pistons 20 and 3 move towards their inner dead centre positions (DC in Figure 8). This air transfer provides a continuing scavenging or clearing of the exhaust gases from the cylinder 1000 via the exhaust ports 10 as the pistons 20 and 3 move from their outer dead centre positions towards their inner dead centre location. In this way, the cylinder 1000 is continuously positively scavenged with fresh air from the opening to closing of the air ports 7a.

The added advantage of the staged scavenge system of engine 400 (Figures 7) over the staged scavenge system of engine 100 (Figures 2 and 3) and engine 200 (Figure 6) is that the pistons and 3 may be arranged to have a phase angle between themselves ("Phase Angle" in Figure 8), so that pistons 20 and 3 arrive at their respective inner dead centre positions by a crankangle difference corresponding to the phase angle, and arrive at their outer dead centre positions by a crankangle difference corresponding to the phase angle, as is commonly the case with opposed piston engines, and opposed stepped piston engines. The effect of this phase angle between pistons 20 and 3 is to extend or contract the notional overall 180° crankangle scavenging period of pistons 20 and 3, for example to 210° crankangle or for example to 1500 crankangle for a phase angle of 30° cranlcangle advance or retard. For example, in Figure 8, the volume 4000 extends the higher scavenge pressure further into the T2 period, or further towards the point of air port (7a) closure (IC in Figure 8), and this will increase the trapped charge density in volume 1000.

A further feature of the three stage scavenge system of engine 400 (Figure 7) is that the diameter of the piston diaphragm 304 can be made different to the diameter of the piston diaphragm 302 so that scavenge pumps P3 can deliver a different quantity of air 4000 to cylinder volume 1000 in comparison to the volumetric delivery 3000 of scavenge pump P2, and phasing of this volumetric delivery 4000 can be different to that of the volumetric delivery 3000, as explained in the preceding paragraph.

To summarise, the third embodiment of the invention, is an opposed piston engine 400, with at least one combustion cylinder volume 1000 having a first stepped double sided air transfer piston 3a and 3b in a stepped cylinder lb and a second air transfer piston 20a and 20b in a stepped cylinder 20c, in which the inner or forward diaphragm 304 of the stepped air transfer piston 20a acts in-phase with the corresponding power pistons 3 and 20, and in which the inner or forward side 304 of the stepped air transfer piston 20a is either integral with the exhaust power piston or integral with the air power piston. Furthermore, in the opposed piston engine 400, the inner or forward diaphragm 304 of the air transfer piston 20a discharges air into a first so air delivery pipe 99 which is in connection with the air ports 7a. In an alternative arrangement, the inner or forward diaphragm 304 of the air transfer piston 20a discharges air via a check valve 99a into an air delivery pipe 99 which is in connection with the air ports 7a. In both these arrangements, the air deliveiy pipe 99 is in connection with air delivery pipe 7 which is in connection with the air ports 7a The engine embodiment 400 may be arranged either with the air transfer piston 20a as part of an exhaust power piston, or with the aft transfer piston 20a as part of an air power piston.

S

The engine embodiments 100 shown in Figure 2 and Figure 3 may be applied to any number of cylinders to make a multi-cylinder engine. Likewise, the engine embodiments 200 shown in Figure 6 may be applied to any number of cylinders to make a multi-cylinder engine, and the engine embodiments 400 shown in Figure 7 may be applied to any number of cylinders to make a multi-cylinder engine.