AU5975798A

AU5975798A - Multiple shaft engine

Info

Publication number: AU5975798A
Application number: AU59757/98A
Authority: AU
Inventors: Nigel Cameron Stokes; Warwick James Stokes
Original assignee: Nigel Stokes Pty Ltd
Current assignee: Nigel Stokes Pty Ltd
Priority date: 1998-02-20
Filing date: 1998-02-20
Publication date: 1998-09-09
Anticipated expiration: 2018-02-20
Also published as: AU741574B2

Description

TITLE: MULTIPLE SHAFT ENGINE

FIELD OF THE INVENTION

This invention relates to two mechanisms for application to reciprocating internal combustion engines.

The first mechanism concerns a piston/drive shaft coupling. The coupling

communicates energy from the piston to an engine's drive shaft in a manner that permits

the piston to move in a predetermined characteristic different from the conventional

sinusoidal characteristic resulting from a conventional piston coupling.

The second mechanism concerns a two-part exhaust system. This aspect allows

exhaust gases from a cylinder to leave firstly under the pressure difference between the

cylinder and atmosphere to a vent, then secondly the low-pressure remnants of the

exhaust gases are withdrawn through a chamber under suction to a vent.

These two mechanisms may be applied individually or in combination to either a

two-stroke or four-stroke reciprocating internal combustion engine.

BACKGROUND

It is known to provide a single crankshaft to which each piston is attached by

means of a connecting rod. When the crankshaft is rotating at constant speed, each

upstroke and downstroke of the piston has the same duration. The movement of the

piston in a conventional internal combustion engine is described approximately by a sine

wave which indicates the position of the piston in the cylinder as a function of the

degrees of rotation of the drive shaft.

It is known to provide a single exhaust system which operates at or above atmospheric pressure. The exhaust gases leave the cylinder either through valves at the top of the cylinder or ports at the side of the cylinder. The exhaust gases move from the cylinder to the exhaust system while the pressure in the cylinder remains above

atmospheric pressure and the exhaust passage to atmosphere remains open. Residual

exhaust gases cease to move from the cylinder when the pressure in the cylinder reaches

atmospheric pressure.

SUMMARY OF THE INVENTION

A first aspect of the invention provides an internal combustion reciprocating

engine including a drive shaft, at least one piston, and a piston coupling corresponding to

each piston wherein each coupling includes:

a crank rotatably coupled to the piston;

at least one toothed crank cam gear fixedly coupled to the crank; and

at least one complementary toothed drive cam gear fixedly coupled to the drive

shaft and drivingly engaged with the corresponding crank cam gear for at least a portion

of a complete revolution of the drive shaft so linear movement of the piston is converted

to rotation of the drive shaft.

Optionally the crank cam gear is integral with the crank.

Optionally the drive cam gear and crank cam gear are circular.

Preferably linear movement of the piston is converted to rotation of the drive

shaft in accordance with the selective shape of the crank cam gear and the drive cam

gear, the selective coupling location of the crank cam gear and drive cam gear to the

crank and drive shaft respectively, and the selective number of teeth on each of the crank

cam gear and the drive cam gear.

Preferably the drive cam gear and the crank cam gear have an equal number of

teeth so that one revolution of the drive shaft corresponds to one revolution of the crank gear.

Preferably each coupling includes:

a first toothed crank cam gear fixedly coupled to the crank and a first complementary

toothed drive cam gear fixedly coupled to the drive shaft and drivingly engaged with the

first crank cam gear for at least a first portion of a complete revolution of the drive shaft,

forming a first cam gear pair; and

a second toothed crank cam gear fixedly coupled to the crank and a second

complementary toothed drive cam gear fixedly coupled to the drive shaft and drivingly

engaged with the second crank cam gear for at least a second portion of a complete

revolution of the drive shaft, forming a second cam gear pair wherein linear movement

of the piston is converted to rotation of the drive shaft by at least the first cam gear pair

during the first portion and the second cam gear pair during the second portion.

Optionally the first toothed crank cam gear is integral with the crank.

Preferably, linear movement of the piston is converted to rotation of the drive

shaft in accordance with the selective shape of each of the crank cam gears and each of

the drive cam gears, the selective coupling location of each of the crank cam gears to the

crank and each of the drive cam gears to the drive shaft, and the selective number of

teeth on each of the crank cam gears and the drive cam gears.

Preferably the first and second toothed cam gears are different in circumferential

length and the first and second toothed drive cam gears are formed from a single toothed

cam gear having a first planer portion engaged with the first toothed cam gear for the

duration of one complete revolution of the crank, and a second planer portion engaged

with the second toothed cam gear for the duration of a subsequent complete revolution of the crank providing one drive shaft revolution per two crank revolutions.

Preferably, the engine includes a plurality of pistons wherein each crank

corresponding to a piston is rotatably coupled to the engine block by a dedicated crank

shaft and the toothed drive cam gears corresponding to each piston coupling are

selectively fixedly coupled to the drive shaft to provide selective phase or timing

operation of each piston in relation to the remaining pistons.

In one embodiment, each piston in the engine has its own crankshaft and each

crankshaft is connected by means of toothed cam gears to a single drive shaft.

Advantageously the gears connecting each crankshaft to the drive shaft may have

a variable radius allowing the piston to follow a non-sinusoidal path when the drive shaft

is rotating at constant speed.

A second aspect of the invention provides a two part exhaust system for an

internal combustion reciprocating engine wherein the exhaust system includes:

a switching means for selectively communicating the combusted gas in a cylinder

to an atmospheric vent means and an exhaust chamber that is maintained at a pressure

less than the pressure inside the cylinder at least while the chamber is in communication

with the cylinder.

Preferably the switching means includes a selectively actuated valve operably

coupled to a rotatable shaft.

Preferably the switching means includes a cam fixed on the rotatable shaft and a

cam follower operably coupled to the valve.

Preferably the rotatable shaft is the engine drive shaft.

Preferably the exhaust chamber is maintained at a pressure less than the pressure inside the cylinder by a pump means.

Preferably the pump means is operably coupled to a rotatable shaft.

Preferably the rotatable shaft is the engine drive shaft.

Preferably the pump means is a fan.

Preferably the fan is a centrifugal fan.

Preferably the engine is a two-stroke engine.

Optionally an adjustable choke mechanism such as a damper may be provided on

the fan discharge gas passage which selectively increases the back pressure in the

cylinder providing a means of regulating air inlet into the cylinder.

DRAWINGS

A preferred embodiment of the invention will now be described, by way of

example only, with reference to the following drawings in which:

Figure 1 illustrates a front view of a two stroke reciprocating internal combustion

engine generally showing the two part exhaust system components in accordance with

the invention;

Figure 2 illustrates a partial internal view of Figure 1 showing some detail of

both the piston coupling and two part exhaust system in accordance with the invention;

Figure 3 illustrates a partial internal rear view of Figures 1 and 2;

Figure 4 illustrates a partial internal side view of Figures 1 and 2;

Figure 5 illustrates a partial internal side view of Figures 1 and 2 opposite to that

shown in Figure 4;

Figure 6 illustrates a functional view of the operation of the piston coupling and

two-part exhaust in Figure 2 during various instances during a stroke; Figure 7 illustrates a conventional and a selective piston travel-drive shaft

characteristic in a two stroke reciprocating internal combustion engine;

Figure 8 illustrates an embodiment of some of the components of a piston

coupling in accordance with the invention;

Figure 9 illustrates the pitch circumference profile of the components in Figure 8;

Figure 10 illustrates a functional view of an embodiment of some of the

components of a piston coupling in accordance with the invention suitable for use in a

four stroke reciprocating internal combustion engine;

Figure 11 illustrates a functional view of the operation of the piston coupling in

Figure 10 during various instances during a complete four stroke cycle;

Figure 12 illustrates a conventional and a selective piston travel-drive shaft

characteristic in a 4 stroke reciprocating internal combustion engine;

Figure 13 illustrates a perspective view of an embodiment of some of the

components of a piston coupling in accordance with the invention;

Figure 14 illustrates an alternate perspective view to that shown in Figure 13

during a different instance during operation;

Figure 15 illustrates an embodiment of some of the components of a piston

coupling in accordance with the invention;

Figure 16 illustrates the pitch circumference profile of the components in Figure

15;

Figure 17 illustrates a perspective functional view of an arrangement of two

piston couplings in accordance with the invention suitable for use in a four stroke

reciprocating internal combustion engine; and Figure 18 illustrates the perspective view of Figure 17 view form the other side.

DETAILED DESCRIPTION

The numerical references in any one of the Figures 1 to 19 generally relate to the

same item in the remainder of the Figures. The invention will now be described by way

of example only with general reference to Figures 1 to 9 inclusive. These figures

illustrate an embodiment of a piston coupling 34 and a two-part exhaust system 10 in

accordance with the invention as applied to a single cylinder two-stroke reciprocating

engine 12.

The internal combustion reciprocating engine 12 includes a drive shaft 7. at least

one piston 27, and a piston coupling 34 corresponding to each piston 27. Each piston

coupling 34 includes, a crank 26 rotatably coupled to the piston 27, at least one toothed

crank cam gear 21 fixedly coupled to the crank 26, at least one complementary toothed

drive cam gear 23 fixedly coupled to the drive shaft 7, and meshingly engaged with the

corresponding crank cam gear 21 for at least a portion of a complete revolution of the

drive shaft 7 so linear movement of the piston 27 is converted to rotation of the drive

shaft 7. Optionally crank cam gear 21 is integrally formed (not shown) with crank 26.

In operation, linear movement of piston 27 is converted to rotation of drive shaft

7 in accordance with:

the selective shape of crank cam gear 21 and drive cam gear 23,

the selective coupling location of crank cam gear 21 and drive cam gear 23 to crank 26

and drive shaft 7 respectively; and

the selective number of teeth 33 on each of crank cam gear 21 and drive cam gear

23. This way a selective piston travel drive shaft characteristic is obtainable as

illustrated generally by 71 in Figure 7.

When drive cam gear 23 and crank cam gear 21 have an equal number of teeth 33

as in Figure 8, one complete revolution of drive shaft 7 corresponds to one revolution of

crank cam gear 21.

Preferably crank cam gear 21 is elliptical in shape and is mounted on crankshaft

22. Crankshaft 22 passes through crank cam gear 21 off the centre of the minor axis of

crank cam gear 21. Crank 26 is mounted on crankshaft 22 so that its longitudinal centre

line is parallel to the minor axis of crank cam gear 21.

In operation, crank cam gear 21 meshes with complementary drive cam gear 23

on drive shaft 7 that has the same pitch circumference as seen in Figure 9 and the same

number of teeth 33 as the crank cam gear 21. Consequently one complete revolution of

drive shaft 7 corresponds to one revolution of crank cam gear 21.

The shape of drive cam gear 23 and the distance between crankshaft 22 and drive

shaft 7 is selectively determined so that gears 21 and 23 complete one rotation together

even though at different stages during the rotation crankshaft 22 will be rotating more

quickly or more slowly than drive shaft 7.

Advantageously, piston 27 will move more slowly in the Top Dead Centre

(TDC) and Bottom Dead Centre(BDC) regions and more quickly during a compression

stroke as generally indicated by characteristic 71 in Figure 7 when compared to a piston

operably coupled to a drive shaft rotating at the same speed with a conventional piston

coupling as generally indicated by characteristic 70.

The illustrated crank cam gear 21 and the drive cam gear 23 have the specifications below.

CRANK CAM GEAR 21 SPECIFICATIONS:

Shape of Pitch Elliptical

Semimajor axis 50mm

Semiminor axis 30mm

Pitch Circumference 360mm

Crankshaft Position 12.68mm off-centre along the

semiminor axis

Number of Teeth 36

Gear Type American Standard Involute System

- Stub Tooth

Circular Pitch 10mm

Pressure Angle 20 degrees

Addendum 2.546mm

Dedendum 3.183mm

Backlash 0.5mm.

DRIVE CAM GEAR 23 SPECIFICATIONS:

Shape Complement to Crank cam gear 21

Pitch Circumference 360mm

Number of Teeth 36

Distance between Crankshaft and Drive shaft 113.045

The internal combustion reciprocating engine 12 also includes a two part exhaust

system 10 in accordance with the invention. Exhaust system 10 includes exhaust valve 29 for selectively communicating the combusted gas in the cylinder 1 to high pressure

exhaust outlet 10 and low pressure exhaust chamber 40. Chamber 40 is maintained at a

pressure less than the pressure inside cylinder 1 at least while chamber 40 is in

communication with cylinder 1.

Valve 29 is operably coupled to drive shaft 7 and is selectively actuated by

exhaust valve cam 28 fixed on drive shaft 7 and cam follower 35 operably coupled to

valve 29. The timing of the operation of valve 29 in relation to piston 27 is determined

by the selective shape of the valve cam 28 and the orientation of cam 28 on drive shaft 7.

Optionally valve 29 may be operably coupled to any other rotating shaft (not shown) in

engine 12 to time the operation of valve 29.

The low pressure inside exhaust chamber 40 is maintained by centrifugal exhaust

fan 30 operably coupled to the drive shaft 7 by fan pulley 6 on drive shaft 7 driving fan

pulley 4 on the fan's shaft 30. Exhaust fan 30 vents gases in exhaust chamber 40 to

atmosphere via low pressure exhaust outlet 31.

Optionally a cooling means (not shown) may be provided for exhaust chamber 40

that contributes to the pressure differential between cylinder 1 and exhaust chamber 40.

Discharge of the combusted gases in cylinder 1 occurs through cylinder exhaust

port 20 located in the wall of cylinder. The port 20 is exposed by piston 27 towards the

end of a power stroke as shown in drawing (c) in Figure 6.

The exhaust gas passage from cylinder exhaust port 20 divides into two exhaust

passages. The passages are separated by exhaust valve 29 into a high-pressure exhaust

passage venting to high pressure exhaust outlet 9 and a low-pressure exhaust passage

venting to low pressure exhaust chamber 40 via low pressure exhaust duct 24. When exhaust port 20 is first exposed, high-pressure exhaust outlet 9 is in

communication with exhaust port 20 allowing combusted gases in cylinder 1 to be

vented as a result of pressure inside the cylinder.

In operation valve 29 will be actuated by exhaust valve cam 28 fixed on drive

shaft 7 and cam follower 35 as piston 27 moves towards BDC. Then valve 29 will close

the exhaust passage to the high-pressure exhaust outlet 9 and open the exhaust passage to

low-pressure exhaust chamber 40 as shown in drawing (d) in Figure 6.

Shortly thereafter fuel/air inlet valve 41 at the top of the cylinder opens allowing

fresh fuel charge to enter cylinder 1 as also seen in drawing (d) in Figure 6. Optionally,

an adjustable choke mechanism (not shown) such as a damper may be provided in low

pressure exhaust outlet 1 which selectively increases the back pressure in cylinder 1

regulating air entering through valve 41 in cylinder 1.

In the meantime the remnants of the combusted gases flow from cylinder 1 into

low-pressure exhaust chamber 40 as a result of the of the pressure differential between

the cylinder 1 and exhaust chamber 40.

When piston 27 moves beyond BDC, fuel/air inlet valve 41 closes and exhaust

port 20 is closed by piston 27 allowing the fresh charge to be compressed during the

compression or upstroke. Drawing (a) in Figure 6 illustrates the piston 27 in the TDC

position at the end of the compression stroke.

During the upstroke valve 24 opens the exhaust gas passage to the high pressure

exhaust outlet 9 ready to receive exhaust gas once exhaust port 20 is next opened at the

end of the power stroke, as shown in drawings (a) and (b) in Figure 6.

In the case of an engine having a plurality of cylinders (not shown) a common low pressure exhaust chamber 40 may serve two or more cylinders.

The invention will now be further described by way of example only with

general reference to Figures 10 to 16 inclusive. These figures illustrate an embodiment

of a piston coupling 34' in accordance with the invention suitable for use with each

piston 27 in a four-stroke reciprocating engine (not shown).

Coupling 34' includes a first toothed crank cam gear 21a fixedly coupled to

crank 26 and a first complementary toothed drive cam gear 23a fixedly coupled to drive

shaft 7 forming a first cam gear pair. In the embodiment illustrated in the Figures, first

toothed crank cam gear 21a is also crank 26. The first cam gear 21a is meshingly

engaged with first drive cam gear 23a for at least a first portion of a complete revolution

of drive shaft 7 as illustrated in drawings (a) to (d), (h) and (i) in Figure 11, and Figure

13.

Coupling 34' further includes a second toothed crank cam gear 21b fixedly

coupled to crank 26, and a second complementary toothed drive cam gear 23 b fixedly

coupled to drive shaft 7 forming a second cam gear pair. The second cam gear 21b is

meshingly engaged with second drive cam gear 23b for at least a second portion of a

complete revolution of drive shaft 7 as shown in (d) to (h) in Figure 11, and Figure 14.

Coupling 34' is arranged so that linear movement of piston 27 is converted to

rotation of drive shaft 7 by at least the first cam gear pair during the first portion, and the

second cam gear pair during the second portion.

In operation linear movement of piston 27 is converted to rotation of drive shaft 7

in accordance with: the selective shape of each of crank cam gears 21a and 21b and each of drive cam gears 23a and 23b, the selective coupling location of each of the crank cam gears 21a

and 21b to crank 26 and each of drive cam gears 23a and 23b to drive shaft 7, and

the selective number of teeth 33 on each of crank cam gears 21a and 21b and drive cam

gears 23a and 23b.

First and second toothed cam gears 21a and 21b are selectively of different

circumferential length. First and second toothed drive cam gears 23a and 23b are formed

from a single toothed cam gear having a first planer portion engaged with the first

toothed cam gear 21a for the duration of one complete revolution of the crank, and a

second planer portion engaged with the second toothed cam gear 23b for the duration of

a subsequent complete revolution of the crank providing one drive shaft revolution per

two crank revolutions. This is shown generally in Figurers 1 1, 13 and 14.

The first and second crank cam gears 21a and 21b illustrated are circular and the

second crank cam gear 21b is centrally mounted on crankshaft 22.

The first crank cam gear 21a is mounted eccentrically on crank shaft 22 so that its

pitch circle 181 touches the pitch circle 180 of the second crank cam gear 21 b at one

point as illustrated in Figure 16 and the teeth 33 on each of the first and second crank

cam gear 21a and 21b is operably in alignment as shown generally in Figure 10. The

pitch circumference 181 and 180 of crank cam gears 21a and 21b form a continuous and

closed curve through a rotation of 720 degrees.

The two circular crank cam gears 21a and 21b on crank shaft 22 mesh with a

drive cam gear 23a and 23b having a circumference of pitch circle 182 equal to the sum

of the circumferences of pitch circles 181 and 180 corresponding to first and second

crank cam gears 21a and 21b respectively. The shape of the drive cam gear 23a and 23b and the distance between the crankshaft 22 and the drive shaft 7 are selected by the

requirement that the crankshaft 22 completes two rotations as the drive shaft 7 completes

one rotation as generally shown in Figure 1 1.

The drive cam gears 23a and 23b illustrated are formed by a single cam gear

having two planer portions corresponding to drive cam gears 23a and 23b. Each planer

portion provides meshing engagement operation in a plane respectively between drive

cam gears 23a and 23b with the corresponding crank cam gear 21a and 21b as generally

shown in Figures 1 1 and 13. to 15. In one plane, the teeth 33 on the drive cam gear 23a

mesh with teeth 33 on crank cam gear 21a. In the other plane teeth 33 on the drive cam

gear 23b mesh with teeth 33 on crank cam gear 21b.

The illustrated crank cam gears 21a and 21b, drive cam gears 23a and 23b, and

some other aspects of coupling 34' have the specifications as set out below.

Crank cam gear 21a

Shape Circular

Pitch Radius 45.84mm

Pitch Circumference 288mm

Crankshaft Position 22.92mm off centre

Number of Teeth 36

Crankshaft Length 48mm (Distance between the centres

of crankshaft 22 and the connecting

rod journal) Crank cam gear 21b

Shape Circular

Pitch Radius 22.92mm

Pitch Circumference 144mm

Crankshaft Position centre

Number of Teeth 18

Drive cam gear 23 a

Part 1 6:

Shape Open ovate

Pitch Length 288mm

Number of Teeth 36

Drive shaft Radius 19.2mm

Drive cam gear 23 b

Shape Circular arc

Radius 88.02mm

Centre Centre of drive shaft 22

Number of Teeth 18

Pitch length 144mm

Centre Distance (Between crank shaft 22 and drive shaft 7) - 97.01mm

Gear Teeth 33

Type American Standard Involute System

- Stub Tooth

Circular Pitch 8mm Pressure Angle 20 degrees

Addendum 2.0368

Dedendum 2.5464mm

Backlash 0.4mm

The effect of coupling 34' is to allow selective duration of the power stroke in

relation to the duration of the other three strokes in a four-stroke cycle.

Figures 17 and 18 illustrate the application of coupling 34' in a two piston

arrangement suitable for use in a two cylinder engine. In this arrangement the toothed

drive cam gears 23a, 23b and 23a', 23b' corresponding to each piston coupling 34^" are

selectively fixedly coupled to drive shaft 7 to provide selective phase or timing operation

of piston 27 in relation to piston 27'. Similarly, coupling 34' may be applied to more

than two piston arrangements (not shown).

Advantageously, other selective shapes of the crank cam gears and drive cam

gears to those illustrated herein will vary the piston travel-drive shaft characteristic

allowing a desired characteristic suitable for selective applications. For example, it may

be required to slow the piston down when the piston is at the top of the drive stroke to

allow more time for combustion under high pressure and at the bottom of the intake

stroke to allow more time for intake of air into the cylinder.

Although the invention has been described with reference to particular examples,

it will be appreciated by those skilled in the art that the invention may be embodied in

many other forms. For example, the crank and drive cam gears may not directly mesh,

but could alternatively be drivingly connected by a chain, a pulley, a toothed belt, or an

intermediate gear train.

Claims

1. An internal combustion reciprocating engine including a drive shaft, at least one

piston, and a piston coupling corresponding to each piston wherein each coupling

includes:

a crank rotatably coupled to the piston,

at least one toothed crank cam gear fixedly coupled to the crank.

at least one complementary toothed drive cam gear fixedly coupled to the drive

to rotation of the drive shaft.

2. An engine in accordance with claim 1 wherein the crank cam gear is integral with

the crank.

3 An engine in accordance with claim 1 or 2 wherein the drive cam gear and crank

cam gear are circular.

4. An engine in accordance with any of claims 1 to 3 wherein linear movement of

the piston is converted to rotation of the drive shaft in accordance with the selective

shape of the crank cam gear and the drive cam gear, the selective coupling location of

the crank cam gear and drive cam gear to the crank and drive shaft respectively, and the

selective number of teeth on each of the crank cam gear and the drive cam gear.

5. An engine in accordance with claim 4 wherein the drive cam gear and the crank

cam gear have an equal number of teeth so that one revolution of the drive shaft

corresponds to one revolution of the crank cam gear.

6. An engine in accordance with claim 1 wherein each coupling includes:

a first toothed crank cam gear fixedly coupled to the crank and a first

engaged with the first crank cam gear for at least a first portion of a complete revolution

of the drive shaft, forming a first cam gear pair; and

a second toothed crank cam gear fixedly coupled to the crank and a second

7. An engine in accordance with claim 6 wherein the first toothed crank cam gear is

integral with the crank.

8. An engine in accordance with claim 6 or 7 wherein linear movement of the piston

is converted to rotation of the drive shaft in accordance with the selective shape of each

of the crank cam gears and each of the drive cam gears, the selective coupling location of

each of the crank cam gears to the crank and each of the drive cam gears to the drive

shaft, and the selective number of teeth on each of the crank cam gears and the drive cam

gears.

9. An engine in accordance with any of claims 6 to 8 wherein the first and second

toothed cam gears are different in circumferential length, and the first and second

toothed drive cam gears are formed from a single toothed cam gear having a first portion

engaged with the first toothed cam gear for the duration of one complete revolution of the crank, and a second portion engaged with the second toothed cam gear for the

duration of a subsequent complete revolution of the crank providing one drive shaft

revolution per two crank revolutions.

10. An engine in accordance with any preceding claim including a plurality of

pistons wherein each crank corresponding to a piston is rotatablv coupled to the engine

block by a dedicated crank shaft and the toothed drive cam gears corresponding to each

piston coupling are selectively fixedly coupled to the drive shaft to provide selective

phase or timing operation of each piston in relation to the remaining pistons.

1 1. A two-part exhaust system for an internal combustion reciprocating engine

wherein the exhaust system includes:

a switching means for selectively communicating the combusted gas in a cylinder

with the cylinder.

12. An exhaust system in accordance with claim 11 wherein the switching means

includes a selectively actuated valve operably coupled to a rotatable shaft.

13. An exhaust system in accordance with claim 12 wherein the switching means

includes a cam fixed on the shaft and a cam follower operably coupled to the valve.

14. An exhaust system in accordance with any of claims 1 1 to 13 wherein the

exhaust chamber is maintained at a pressure less than the pressure inside the cylinder by

a pump means.

15. An exhaust system in accordance with claim 14 wherein the pump means is

operably coupled to a rotatable shaft.

16 An exhaust system in accordance with claim 14 or 15 wherein the pump means is

a fan.

17. An exhaust system in accordance with any one of claims 10 to 16 wherein the

engine is a two-stroke engine.

18. An exhaust system in accordance with any one of claims 12 to 17 wherein the

rotatable shaft is the engine drive shaft.

19. An exhaust system in accordance with any one of claims 15 to 18 wherein the

rotatable shaft is the engine drive shaft.