WO2002084089A1

WO2002084089A1 - A positive displacement supercharger

Info

Publication number: WO2002084089A1
Application number: PCT/IE2002/000045
Authority: WO
Inventors: Derek Rynhart; Killian Kenny
Original assignee: Rynhart Research And Development Company Limited
Priority date: 2001-04-12
Filing date: 2002-04-12
Publication date: 2002-10-24

Abstract

A positive displacement supercharger (1) for an internal combustion engine (2) has a piston (15) in a supercharger cylinder (17) driven by a drive means (19) incorporating a piston rod (16), con rod (11), drive shaft (10) and, as illustrated, an energy storage line (20) incorporating a spring (21). Initially, on a compression stroke, the spring (21) is compressed, thus retarding the piston (15) so that supercharging occurs at the end of the stroke.

Description

"Δ Positive Displacement Supercharger"

Introduction

The present invention relates to a positive displacement supercharger for an internal combustion (IC) engine of the type comprising a supercharger piston housed within a supercharger cylinder and reciprocal therein, an engine air inlet manifold having a manifold inlet connecting the engine to the supercharger cylinder and a manifold outlet connecting it with the engine combustion chamber, the outlet being closed during part of the operating cycle of the IC engine.

Such positive displacement superchargers have been, for example, described and claimed in our co-pending PCT Patent Specification No. WO99\02829 which shows a particular construction of positive displacement supercharger. Other such superchargers are disclosed in US Patent Specification No. 5186137 (Salzmann) and DE 3725626 (Klaue). These latter superchargers have suffered from various disadvantages. To overcome these disadvantages, it has been proposed to use natural aspiration and then use a pulse of air from the positive displacement supercharger to top up the engine, as it were, immediately prior to closing the air inlet valve or simply the air inlet, where an inlet valve is not used as is common in 2-stroke engines. However, such a construction, for example, has been disclosed in US Patent Specification Nos. 4709683 (Schatz) and 4643156 (Schatz).

While the latter form of supercharger system gains over others by allowing natural aspiration of air to occur directly into the internal combustion engine and the supercharger is used only for top up, there is, of necessity, a pressure rise in the supercharger while the natural induction is occurring and this results in unacceptably large parasitic power loss to the engine. A further important disadvantage occurs as a result of supercharger pressure rise in that this supercharger pressure rise causes a substantial temperature rise due to increased compression pressure. There is a consequent reduction in mass flow rate of air and finally, unnecessary component stressing.

However, it has been found that such top up arrangements, where the positive displacement supercharger is used in conjunction with naturally aspirated engines, to deliver a sharp pulse of atr at arr elevated pressure immediately before^" the air inlet closes, are generally not successful. This is particularly so as the engine revolutions increase.

The present invention is directed towards overcoming these problems in positive displacement superchargers.

Statements of Invention

According to the invention there is provided a positive displacement supercharger for an internal combustion (IC) engine of the type comprising a supercharge piston housed within a supercharger cylinder and reciprocal therein, an engine air inlet manifold having a manifold inlet connecting the engine to the supercharger cylinder and a manifold outlet connecting it with the engine combustion chamber, the outlet being closed during part of the operating cycle of the IC engine characterised in that the supercharger comprises drive means for moving the supercharger piston at a variable speed, initially moving at a speed sufficient to cause the pressure within the supercharger cylinder to rise somewhat above atmospheric pressure and then shortly before closing of the manifold outlet moving at a greater speed to cause the pressure to rise to the desired supercharging pressure.

The positive displacement supercharger may have a manifold^* inlet which houses a non-return valve which opens when the pressure of the air in the supercharger cylinder exceeds the pressure in the air inlet manifold by a preset amount and may further include an air inlet for natural aspirating air and the supercharger acts in parallel with the natural aspiration.

In one embodiment the supercharger drive means are mechanically linked to the engine piston drive and incorporates an energy storage link whereby initial driving of the supercharger causes the supercharger piston to move minimally and the energy to be stored in the energy storage link.

In another embodiment the supercharger drive means may comprise a drive shaft and a con rod connecting the piston rod and the piston to the drive shaft, the piston rod incorporating the energy storage link between it and the con rod and in which the energy storage link comprises a spring mounted on a proximal end of the piston rod con rod and engaging, at its distal end intermediate the ends of the supercharger piston rod, a spring support slidable relative to the piston rod and fast relative to the con rod.

In this latter embodiment the spring support comprises a spring embracing sleeve also embracing the piston rod, the sleeve having a distal bored spring contacting plate and a proximal base plate connected to the con rod, the proximal end of the spring engaging a collar fast on the piston rod over which the sleeve reciprocates. The spring may be pre-compressed prior to mounting between the spring support and the con rod. In many instances the spring is pre-compressed to a level such that further compression of the spring requires the pressure in the supercharger to exceed the desired engine supercharging pressure by a preset small amount.

In another embodiment the drive means comprises a profiled cam driven directly by the engine.

In a still further embodiment the drive means comprises a lost motion device driven directly by the engine.

In some, embodiments the supercharger piston incorporates a non-return valve which may be a flap valve.

In another embodiment the supercharger piston mounts a one-way peripheral seal assembly between itself and the supercharger cylinder wall whereby the seal assembly operates to prevent the passage of air therebetween on the pressure within the supercharger exceeding atmospheric pressure and allows the passage of air when the pressure on both sides of the piston is approximately the same.

A suitable construction of the seal assembly comprises:-

a seal housing in the piston side wall in the form of a peripheral groove having a base and side walls;

an air passageway for delivering pressurised air from the supercharger to the groove adjacent the base thereof; and

an annular sea having a cylinder wall contacting outer surface, a pair of groove side wall contacting surfaces and a rear surface spaced- apart from the outer surface, the seal being so configured as to be movable under air pressure away from the base of the groove into contact with the supercharger cylinder wall.

The annular seal may be a continuous seal of the one length of material having end mating surfaces inclined to a radius defined by the seal.

A positive displacement supercharger of the invention may be fitted with a decompressor comprising a pressure release valve mounted in the engine air inlet manifold for pressure reduction during starting. The pressure release valve is operatively connected to the engine starter motor for operation when the starter motor is being used.

In many superchargers a buffer stop of a resilient material is provided between some or all of the moving parts which engage and disengage during operation of the supercharger.

When a positive displacement supercharger of the invention is used with a fan assisted air cooled engine there is provided means for delivering some of the air from the fan to the outside of the supercharger piston to allow the induction of this air at an elevated pressure and also to reduce the work required by the piston during its compression stroke.

When the positive displacement supercharger of the invention is used with a four stroke engine the supercharger operates at half engine speed. Dfitattmt Ωffsffliptiππ Ωf 1tre tnventirm

The invention will be more clearly understood from the following description of some embodiments thereof, given by way of example only, with reference to the accompanying drawings, in which:-

Fig. 1 is a partially schematic view of a positive displacement supercharger according to the invention, connected to an internal combustion engine at the start of its cycle;

Fig. 2 shows the supercharger further on in its cycle;

Fig. 3 shows it still further on in its cycle;

Fig. 4 shows the supercharger at the end of its cycle;

Fig. 5 is a detailed view of portion of the supercharger;

Fig. 6 is a detailed enlarged view of the circled portion of Fig. 5;

Fig. 7 is a partial plan view of a seal forming part of the supercharger as illustrated in Fig. 6,

Fig. 8 is a front view of the seal;

Fig. 9 is a side sectional view, with sectional lines omitted for clarity, of a piston forming part of the supercharger of the previous Figs;

Fig. 10 is an underneath plan view of the piston;

Fig 11 is a top plan view of the piston;

Figs. 12 to 15 are views similar to Figs. 1 to 4, of another construction of supercharger;

Fig. 16 is a detailed side view of the supercharger of Figs. 12 to 15 inclusive;

Fig. 17 is a sectional view of portion of another supercharger substantially similar to the supercharger of Fig. 10;

Fig. 18 is a sectional view along the line A-A of Fig. 17;

Fig. 19 is a view of portion of Fig. 18 showing a decompressor of Fig. 18 in an open position;

Fig. 20 is a sectional view of a still further construction of supercharger;

Fig. 21 is another sectional view of the supercharger of Fig. 20, in a different position of use;

Fig. 22 is a perspective view of a still further construction of top up supercharger;

Fig. 23 is a sectional view of the supercharger of Fig. 22;

Fig:^" 24 is another sectional view of the supercharger of Fig. 22 during its stroke;

Fig. 25 is a still further sectional view of the supercharger of Fig. 22 at the end of its working stroke;

Fig. 26 is a sectional view along the lines A-A of Fig. 23; and

Fig. 27 is a sectional view along the lines A-A of Fig. 25.

Referring to the drawings and initially to Figs. 1 to 5 thereof, there is illustrated a positive displacement supercharger, indicated generally by the reference numeral 1 , for an internal combustion engine, indicated generally by the reference numeral 2. Tiισinternatτraτπbustiϋπ^" n ine (1CE) 2 comprises a^" cylinder 3, piston 4,^" exhaust valve 5 and air inlet valve 6 in a manifold outlet 9 of an engine air inlet manifold 7. The manifold 7 has a manifold inlet 8 connecting it to the supercharger 1 and this ICE 2 is a conventional 4-stroke compression ignition engine.

The positive displacement supercharger 1 comprises a drive shaft 10 driving a con rod 11 which is driven at a two-to-one reduction directly off the ICE 2. The drive is by a belt shown by interrupted lines 12. The positive displacement supercharger comprises a piston 15 connected to a piston rod 16 which, in turn, is connected by an energy storage link, indicated generally by the reference numeral 20, to the drive shaft 10. The term "energy storage link" is used in this specification as an identification of the function of the link which is to absorb the power generated by the piston 15, reduce the travel of the piston 15 and then to subsequently restore it. The piston 15 reciprocates within a supercharger cylinder 17 having a cylinder wall 18. The supercharger drive shaft 10, con rod 11 , energy storage link 20 and piston rod 16 provide a drive means, identified by the reference numeral 19 for moving the supercharger piston 15 at a variable speed within the supercharger cylinder 17.

The energy storage link 20 comprises a spring 21 engaging a collar 22 fast on a free end of the piston rod 16. The spring 21 embraces the con rod 16 and, at its distal end, engages intermediate the ends of the supercharger piston rod 16, a spring support slidable relative to the piston rod, the spring support comprising a sleeve 24 having a distal bored spring contacting plate 25 mounting a bearing 26. The sleeve 24 has a proximal base plate 27 connected to the con rod 11. When the spring 21 is placed in position, it is preloaded to a pressure which will equate to the required supercharging pressure and thus, in the absence of any piston pressure, exerts a force itself between the collar 22 and the spring contacting plate 25.

Referring now specifically to Figs. 5 to 9 inclusive, the supercharger piston 15 mounts a one-way peripheral seal assembly, indicated generally by the reference numeral 30 and shown circled in Fig. 5 and in greater detail in Figs. 6 to 9 inclusive. The seal assembly 30 comprises a seal housing 31 in the form of a peripheral groove around the piston having a base 32 and side walls 33 (see also Fig. 9). An air passageway 34 connects the seal housing 31 with the underneath of the piston 15 for delivering pressttrtsed-atrlromlhe-sttpercharger 1^" tσth ^" groove, namely, ^"the seat roτjslng 3 ^' adjacent the base 32. An annular seal 35 of essentially rectangular cross section is mounted -in the groove 31 and has a supercharger cylinder wall contacting outer surface 36 and a pair of groove side wall contacting surfaces 37 and a rear surface 38 spaced-apart from and substantially parallel to the outer surface 36. It will be noted that the seal 35 is so configured that it is a relatively tight-fit against the side walls 33 of the groove 31 but is a loose fit with respect to the base 32 and has end mating surfaces 38 cut at a bias, namely, having end mating surfaces 38 inclined to a radius defined by the seal itself.

Referring now specifically to Figs. 9 to 11 inclusive, the piston 15 has a plurality of valve holes 40 against each of which is mounted, in a recess 41 , a non-return valve, namely, a flap valve 42 pivoted at 43 on the piston 15. A buffer stop 45 is mounted on the underneath of the piston 15 for engagement with the spring contacting plate 25 of the sleeve 24. Further, buffer stops 47 for engagement with the underneath of the piston 15 and a buffer stop 48 for engagement between the collar 22 and base plate 27 are also provided (see Fig. 5).

In operation, and referring to Figs. 1 to 4 inclusive, the operation of the supercharger is as follows. The piston 15 is moved from the position illustrated in Fig. 4 and described in more detail later, to the position in Fig. 1 , in its half cycle immediately after the inlet valve 6 of the engine closes. This is its induction cycle. Then, with the inlet valve still closed, the piston 15 starts to move slowly downwards and at top dead centre, for the piston 3 of the ICE2 after ignition and exhaust, the piston 15 which is almost midway through its stroke since it is running at half engine cycle speed, has not moved halfway down its cylinder because once the pressure under the piston 15 exceeds the force by the spring 21 between the collar 22 and the plate 25, the spring 21 starts compressing. Thus, as the pressure builds up, the spring 21 is pushed downwards against the collar 22 thus compressing it and gradually opening a gap (identified by the arrow X in Fig. 2), between the collar 22 and the base plate 27. This can be seen clearly from Fig. 2. At this position, the IC piston is at TDC and travelling at minimum speed, while the supercharger piston 15 is travelling at maximum speed. The air inlet valve 6 is open and the pressure is building up and the spring 21 is being further compressed until it can compress no more and further air pτeBSuτe^"1srgerτeτated1ιτthe^" engine 2Ηnd supercharger; This pressure Is^'aBdve the^" final supercharger pressure. Then the position of Fig. 3 is reached where maximum engine piston 4 speed, with consequent increase in engine volume, is happening as the supercharger piston 15 is slowing down. The spring 21 will start to expand until at the end of the supercharger stroke in the position illustrated in Fig. 4, the piston 15 is at BDC and the supercharging pressure in the engine 2 and the air inlet manifold 7 is at the required pressure with the spring 21 fully expanded. When the piston 15 contacts the bottom of the supercharger cylinder 17, the inlet valve 6 closes and the piston 15 starts to rise. The piston 15 rises a distance equal to the linear compression of the spring and when the pressure beneath the piston 15 is less than atmospheric, the flap valves 42 will open and air will be induced into the supercharger cylinder 17. After one half cycle of the supercharger 1, the piston 15 will be again in the position illustrated in Fig. 1.

The preloading of the spring may be greater or less than the desired supercharging pressure. This will depend entirely on the degree of supercharging and the ICE characteristic required. Generally it will be as described above, namely such that it is further compressed at about supercharging pressure.

Referring now to Figs. 12 to 19 inclusive, there is illustrated a supercharger, again identified .generally by the reference numeral 1 since it is of substantially the same construction as the supercharger of the previous drawings and parts similar to those described with reference to the previous drawings are identified by the same reference numerals. In this embodiment, the supercharger 1 incorporates a nonreturn valve 50 in the manifold inlet 8 which is held in position by a spring 51. Within the engine air inlet manifold 7, there is mounted a decompressor, indicated generally by the reference numeral 55, comprising a solenoid valve, shown in the closed position in Fig. 18 and in the open position in Fig. 19. The decompressor, namely, the solenoid valve 55 is operatively connected to the starter motor whereby, on the starter motor being operated, the solenoid valve 55 operates to connect the air inlet manifold to atmosphere and thus negate the operation of the supercharger.

In operation, the supercharger operates the same as heretofore, except that the non- return valve closes immediately the inlet valve 6 closes and thus there is a volume of

-air a -elevafec supercharger pressure stored ^" ithin the engine^"' arlnler^"manifold^"T which effectively forms an air charging plenum chamber. When the engine is in the position illustrated in Fig. 13, the air inlet valve 6 is about to open. When the air inlet valve 6 opens, the volume of air trapped in the engine air inlet manifold 7 will be at a higher pressure than the air within the ICE cylinder 3. Thus, the air will reduce in pressure in the engine air inlet manifold 7 above the engine piston 4, namely, in the combustion chamber above the piston 4. When the pressure beneath the piston 15 rises sufficiently to open the non-return valve 50, it will open and then the operation of the engine is as heretofore. This is illustrated in Figs. 14 and 15.

Referring now to Figs. 20 and 21, there is illustrated a positive displacement supercharger, again indicated generally by the reference numeral 1. Parts similar to those described with reference to the previous drawings are identified by the same reference numerals. In this embodiment, the supercharger drive means, again identified by the reference numeral 19, comprises a profiled cam 61 mounted on a drive shaft 62, again driven at half engine speed, as before, from the engine drive shaft. Mpunted on the extremity of the piston rod 16 is a cam follower 63 which is urged by a spring 64 into contact with the profiled cam 61. As can be seen, particularly from Fig. 21 , the profiled characteristics of the cam 61 are such as to provide a dwell time such that the piston 15 is held back for a considerable amount of the compression stroke and then it is rapidly forced down the supercharger cylinder 17, as before. Rapid return at the end of the stroke is achieved by suitable profiling.

Referring to Figs. 22 to 27 inclusive, there is illustrated an alternative construction of supercharger, again identified by the reference numeral 1 , in which parts similar to those described with reference to the previous drawings, are identified by the same reference numerals. In this embodiment, the .supercharger 1 is partly incorporated within a conventional air inlet manifold casing 71 having a flange 72 for connection to an engine and including air inlets 73 open to the atmosphere. The manifold casing 71 is mounted above a crankshaft casing 74 by an extension housing 75. A drive shaft 76 is mounted within a drive casing 77 and is driven through a drive pulley 78 off the ICE. Referring now specifically to Figs. 23 to 25 inclusive, a spool valve body 78 is mδϋ^ήTedbh h^'e sTeeve^"2 adfacent We ^"spring contacting plate 25. The spool valve body 78 has a recess 79 and in this embodiment, mounts the bearing 26. An air discharge duct 80 is mounted in the supercharger and connects between the interior of the supercharger and the air inlet manifold 7 at 81.

Nitrile flaps 85, the extremity of which form tapering wedges 86 are mounted in the air discharge duct 80 over the air inlets 73 used for natural aspiration. Mounted on the exposed faces of the flanges 72 are thermal barriers 87.

In operation and referring initially to Figs. 23, with the piston 15 at top dead centre, the con rod 11 , on rotating, starts to pull the piston 15 downwards by pulling the sleeve 24 downwards. As before, initially the piston 15 moves downwards until the pressure of the trapped air in the supercharger cylinder 17 causes the spring 21 to compress, rather than the piston 15 to move. This is determined by the preloading of the spring 21. The spring 21 will not compress until the predetermined pressure is achieved - Then the sleeve 24 starts to move downwards and pull the valve spool body 78 downwards with the sleeve 24 continually compressing the spring 21. This can be seen starting in Fig. 24. When the recess 79 in the spool valve body 78 coincides with the duct 80, some pressurised air escapes into the air discharge duct 80. Suddenly, there is no pressure exerted on the spring 21. Then, the spring 21 recovers rapidly, pushing the collar 22 down causing rapid discharge of the pressurised air into the air inlet manifold 7. Prior to that, as can be seen from Fig. 26, the engine will normally be aspirating air being drawn into the engine. When the spool valve body 78 reaches the position illustrated in Fig. 25, the remainder of the air will have been discharged into the air inlet manifold 7. This will close the nitrile flaps. Needless to say, the configuration of the spool valve body 78 and the recess 79 will determine the charging time which can be a very rapid pulse or a more extended compression and filling operation.

The adva^'htage of the use of a non-return valve such as the non-return valve 50 of Figs. 12 to 19, is that the supercharged air in the engine air manifold is not lost and can be used immediately the air inlet valve is opened. It has been found that the use of normally aspirated air, in conjunction with the supercharger, was unsuitable for speeds in excess of about 2000 RPM, as the time interval in which supercharging teok-plaee-was-insuffieienti a-puise-O airwas-injected-over a short time-period. —

In the case of many small utility engines, fan cooling is employed and at greater engine speeds in the range of 3000 RPM or so, these usually give a pressure of the order of 2 psi. It is envisaged that by taking some of the air and supplying this to the induction side of the supercharger, two advantages are provided. Firstly, there is pressurised air induced into the supercharger which reduces the work of the supercharger and secondly, if the pressurised air is applied to the back face of the supercharger, it reduces the work required by the supercharger piston.

It is envisaged that many other forms of drive for the supercharger piston may be provided such as overhead cams or indeed lost motion devices.

In one compression ignition 4-stroke engine fitted with a supercharger according to the invention and providing a supercharging pressure of 0.6 bar for the engine which normally peaked at 17.5 N.M. at 2600 RPM, a torque of 31 N.M. was achieved. Even when the engine had been running for some considerable time and had got quite hot, the torque did not fall below 28.5 N.M. It would appear that the reason why the present invention works successfully is that by running at half engine speed, there is again a mechanical advantage in the drive, there is improved cylinder breathing in the cylinder at this 2:1 speed reduction, there is a low temperature generated in the supercharger due to the low speed of compression and there appears to be a high mass density of air filling the engine cylinder due to the invention. Further, there appears to be low mechanical losses and the particular method of supercharging appears to give high swirl rates at filling giving good fuel air mixing. Further, because the actual initial part of the compression stroke does not take up a major proportion of the cycle, sealing of the supercharger piston is not as critical as might be expected and thus excessive heat is not generated.

By way of comparison, instead of using the energy storage link or the delayed variable speed operation, the maximum supercharging that could be achieved with the supercharger was 23 N.M. and further, the supercharger became excessively hot. Further, as mentioned, while in the embodiments described above, sealing between the-superchargerpistσπ" arιd^~1rιe^~5αperctτaτg©r cytiπd^^'wallsTϊas Been ^' Used^", it^' s envisaged that this sealing does not have to be particularly efficient. The point is that when the supercharger is operating to deliver the pressurised air to the internal combustion engine, it is only operating at this elevated pressure over a relatively short period. Thus, even if a certain amount of air were to escape, it would be irrelevant.

In the specification the terms "comprise, comprises, comprised and comprising" or any variation thereof and the terms "include, includes, included and including" or any variation thereof are considered to be totally interchangeable and they should all be afforded the widest possible interpretation and vice versa.

The invention is not limited to the embodiment hereinbefore described, but may be varied in both construction and detail within the scope of the appended claims.

Claims

£LΔ S

1. A positive displacement supercharger (1) for an internal combustion (IC) engine (2) of the type comprising a supercharge piston (15) housed within a supercharger cylinder (17) and reciprocal therein, an engine air inlet manifold

(7) having a manifold inlet connecting the engine to the supercharger cylinder and a manifold outlet (9) connecting it with the engine combustion chamber, the outlet (9) being closed during part of the operating cycle of the IC engine characterised in that the supercharger (1) comprises drive means (19) for moving the supercharger piston (15) at a variable speed, initially moving at a speed sufficient to cause the pressure within the supercharger cylinder to rise somewhat above atmospheric pressure and then shortly before closing of the manifold outlet (9) moving at a greater speed to cause the pressure to rise to the desired supercharging pressure.

2. A_,positive displacement supercharger (1) as claimed in claim 1, in which the manifold inlet (7) houses a non-return valve (50) which opens when the pressure of the air in the supercharger cylinder (17) exceeds the pressure in the air inlet manifold (7) by a preset amount.

3. A positive displacement supercharger (1) as claimed in claim 1 or 2, in which the air inlet manifold (71) includes an air inlet (73) for natural aspirating air and the supercharger (1) acts in parallel with the natural aspiration.

4. A positive displacement supercharger (1) as claimed in any preceding claim, in which the supercharger drive means (19) is mechanically linked to the engine piston (4) drive and incorporates an energy storage link (20) whereby initial driving of the supercharger (1) causes the supercharger piston (15) to

. move minimally and the energy to be stored in the energy storage link (20).

A positive displacement supercharger (1) as claimed in claim 4, in which the supercharger drive means (19) comprises a drive shaft (10) and a con rod (11) connecting the piston rod (16) and the piston to the drive shaft (10), the piston rod (16) incorporating the energy storage link (20) between it and the con rod (11) and in which the energy storage link (20) comprises a spring (21)

at Its distal end intermediate the ends of the supercharger piston rod (16), a spring support slidable relative to the piston rod (16) and fast relative to the con rod (11).

6. A^'positive displacement supercharger (1) as claimed in claim 5, in which the spring support comprises a spring embracing sleeve (24) also embracing the piston rod (16), the sleeve (24) having a distal bored spring contacting plate (25) and a proximal base plate (27) connected to the con rod (11), the proximal end of the spring (21) engaging a collar (22) fast on the piston rod (16) over which the sleeve (24) reciprocates.

7. A positive displacement supercharger (1) as claimed in claim 5 or 6 in which the spring (21) is pre-compressed prior to mounting between the spring support and the con rod (11).

8. A positive displacement supercharger (1) as claimed in claim 7 in which the spring (21) is pre-compressed to a level such that further compression of the spring (21) requires the pressure in the supercharger to exceed the desired engine supercharging pressure by a preset small amount.

9. A positive displacement supercharger (1) as claimed in claims 1 to 3, in which the drive means (19) comprises a profiled cam (61) driven directly by the engine (2).

10. A positive displacement supercharger (1) as claimed in claims 1 to 3, in which the drive means comprises a lost motion device driven directly by the engine

^' (2).

11. A positive displacement supercharger (1) as claimed in any preceding claim, in which the supercharger piston (15) incorporates a non-return valve.

12. A positive displacement supercharger (1) as claimed in claim 11 , in which the non-return valve is a flap valve (42).

13. A positive displacement supercharger (1) as claimed in any preceding claim, in which the supercharger piston (15) mounts a one-way peripheral seal assembly (3) between itself and the supercharger cylinder wall (18) whereby the seal assembly (3) operates to prevent the passage of air therebetween on the pressure within the supercharger (1) exceeding atmospheric pressure and allows the passage of air when the pressure on both sides of the piston (15) is approximately the same.

14. A-positive displacement supercharger (1) as claimed in claim 13, in which the seal assembly (3) comprises:-

a seal housing (31) in the piston side wall in the form of a peripheral groove having a base (32) and side walls (33);

an air passageway (34) for delivering pressurised air from the supercharger to the groove (31) adjacent the base (32) thereof; and

an annular seal (35) having a cylinder wall contacting outer surface

(36), a pair of groove side wall contacting surfaces (37) and a rear surface (38) spaced-apart from the outer surface, the seal (35) being so configured as to be movable under air pressure away from the base (32) of the groove (31) into contact with the supercharger ^"" cylinder wall (18).

15. A positive displacement supercharger (1) as claimed in claim 14, in which the annular seal (35) is a continuous seal of the one length of material having end mating surfaces (38) inclined to a radius defined by the seal.

16. A positive displacement supercharger (1) as claimed in any preceding claim, in which there is provided a decompressor (55) comprising a pressure release valve mounted in the engine air inlet manifold (7) for pressure reduction during starting.

T7. A^ sItTve^{^} displacement supercharger (1) as claimed in claim 16, in which the pressure release valve is operatively connected to the engine starter motor for operation when the starter motor is being used.

18. A positive displacement supercharger (1) as claimed in any preceding claim, in which a buffer stop (45, 47, 48) of a resilient material is provided between some or all of the moving parts which engage and disengage during operation of the supercharger (1 ) .

19. A positive displacement supercharger (1) as claimed in any preceding claim, for use with a fan assisted air cooled engine in which there is provided means for delivering some of the air from the fan to the outside of the supercharger piston (15) to allow the induction of this air at an elevated pressure and also to reduce the work required by the piston during its compression stroke.

20. A positive displacement supercharger (1) as claimed in any preceding claim for a four stroke engine in which the supercharger (1) operates at half engine speed.