WO2008111880A1

WO2008111880A1 - A carburettor for a two-stroke engine

Info

Publication number: WO2008111880A1
Application number: PCT/SE2007/000226
Authority: WO
Inventors: Gustaf DÖRAGRIP; Pär MARTINSSON
Original assignee: Husqvarna Aktiebolag
Priority date: 2007-03-09
Filing date: 2007-03-09
Publication date: 2008-09-18

Abstract

A carburettor (1) for a two-stroke internal combustion engine with scavenging air, comprising: a) a carburettor main body (2) including a main air passage (9) having at least a throttle valve (16) mounted therein, the main air passage (9) extends from an air intake side of the main body to an opposite air outtake side, b) a scavenging air component (3; 3 A, 3B) mounted on a fuel pump side (50) or a diaphragm side (51) of the main body (2), includes a scavenging air passage (2OA, 20B) having a scavenging air valve (6, 7; 6, 7') of butterfly type mounted therein, the scavenging air passage (2OA, 20B) extends substantially parallel to the main air passage (9) from an oblong air inlet (4) towards its air outlet(s) (5A, 5B), c) a coupling member (12) connecting the throttle valve (16) and the scavenging air valve (6, 7; 6, T).

Description

Applicant: HUSQVARNA AB, S-561 82 HUSKVARNA, Sweden Ref: P1690 PCT

A CARBURETTOR FOR A TWO-STROKE ENGINE

TECHNICAL FIELD

The invention is related to diaphragm carburettors for two-stroke internal combustion engine using scavenging air.

BACKGROUND OF THE INVENTION Two-stroke internal combustion engines having scavenging air system have typically been designed with special flow ducts through which fresh air is introduced into the combustion chamber ahead of the air-fuel mixture. In such a scavenging procedure, at the end of the engine power stroke, when the port of the exhaust duct is opened, fresh air is supplied behind the exhaust gases and before the fuel/air mixture. This supply of additional air to the combustion chamber is also called scavenging air. In an ideal state, all of the exhaust gases are pushed out, and this scavenging air acts as a buffer ahead of the air-fuel mixture subsequently coming into the chamber from the engine crankcase. Thus the scavenging air helps evacuating the exhaust gases as well as keeping the air-fuel mixture in the combustion engine. This provides a reduction of the fuel consumption and the achievement of cleaner exhaust gases.

In applications such as for instance chainsaws, space is limited, and it is therefore vital to keep the size of its components as small as possible. This is of course also valid for the carburettor of the engine and this invention addresses the total size of a carburettor using scavenging air.

JP 09268917 A shows a carburettor design utilizing scavenging air. Disposed between the air filter and the carburettor housing is an intermediate piece that extends the intake channel portion and from which branches a connecting line that opens into the transfer port between crank case and combustion chamber of the connected two-cycle engine. However, arranging an intermediate component between the air filter and the carburettor for branching off the air for combustion by itself increases the overall length, which is undesirable since space conditions are limited. Especially for portable, manually guided implements, the installation space provided for accommodating the carburettor and the air filter is very limited, for which reason the conversion of the layered scavenging is particularly problematic for such applications.

US6328288 shows a diaphragm carburettor for a two-stroke internal combustion engine having a scavenging air component mounted on top of the carburettor reducing the overall length. The bottom of the scavenging air component acts as the cover for the pump housing of the diaphragm carburettor, reducing the overall height, that is still however fairly big.

WO2004005692 shows a connective adapter utilized to accommodate the incorporation of a scavenging air supply to a two-stroke internal combustion engine. The adapter takes the form of a thin-body flange that is connected to an air-filter side of an engine, which gives a big total height.

FIG. 3 in US6928996 shows a scavenging air supply body carried by the carburettor, which has two circular shaped scavenging air passages arranged side by side. This reduces the overall height but since each air passage has a separate air control valve costs are increased. Further problems with tolerances may occur.

OBJECTS OF THE INVENTION

One object of the invention is to provide a carburettor for a two-stroke internal combustion engine that operates with scavenging air which overall height and length is minimised.

A further object is to provide a scavenging air component, which can be easily mounted to the main body of a carburettor.

Several further objects of the present invention, which may be achieved individually or in groups according to various aspects of the present invention, are:

- providing a scavenging air component that can be manufactured at comparably low costs.

- providing a scavenging air component which can be assembled time efficiently.

- providing a scavenging air valve with good sealing efficiency. SUMMARY OF THE INVENTION

At least one of the above mentioned objects are solved by providing a carburettor for a two-stroke internal combustion engine that operates with scavenging air, comprising: a) a carburettor main body including a main air passage having at least a throttle valve mounted therein, the main air passage extends from an air intake side of the main body to an opposite air outtake side of the main body, the main body further including a fuel pump side comprising a fuel pump and an opposite diaphragm side comprising a diaphragm, both being transversal to the intake and outtake sides, b) a scavenging air component mounted on the fuel pump side or the diaphragm side, which scavenging air component includes a scavenging air passage having a scavenging air valve of butterfly type mounted therein, the scavenging air passage extends substantially parallel to the main air passage from an air inlet towards at least one air outlet, the air inlet and the air intake side of the main body forming an air filter facing side of the carburettor, c) a coupling member connecting the throttle valve and the scavenging air valve; where the air inlet is oblong providing an air inlet width along the main body and an air inlet height extending from the main body.

The oblong air inlet 4 is preferably rectangular having rounded corners, but may have any shape ranging from oval to rectangular.

Such a carburettor provides the benefit of a reduced overall all height, while maintaining a sufficient scavenging air flow.

According to a further aspect of the invention the scavenging air valve is spring loaded, biasing the scavenging air valve towards a closed position and the width of air inlet is at least 1,5 times longer than the height of air inlet, preferably twice as long.. The scavenging air component can be mounted on the fuel pump side or on the diaphragm side preferably acting as a cover for the fuel pump or the diaphragm.

According to a preferred embodiment of the invention; the scavenging air passage includes a first portion having a oblong cross section, which first section extends from the air inlet towards the air outlet and a second portion extending from the first portion to the air outlet. The second portion is divided into two apertures whereby the at least one air outlet being two. A first valve shaft is rotatably mounted in the first portion and a oblong valve plate is centrally secured to the first valve shaft, which oblong valve plate is adapted in its shape to seal against the inner walls of the first portion when the scavenging air valve is in a closed position. Further, the height of the oblong valve plate is preferably somewhat larger than the height of the first portion so that the oblong valve plate will be somewhat tilted when in the closed position forming an angle βl to the bottom and top of the scavenging air passage. The angle βl is preferably less than 90° and larger than 70°. Preferably the oblong valve plate has grooves along at least the width part of the margin of the oblong valve plate, which valve plate is preferably made of a rigid polymer material e.g. a thermo plastic such as POM. The mouths of the grooves are preferably arranged to point in the same direction as a rotational closing direction of the scavenging air valve.

According to a second embodiment of the invention the scavenging air component comprises a circular valve seat, which valve seat's plane is tilted at an angle α in relation to the plane of the main body side on which the scavenging air component is mounted, the angle α being less than 45° and larger than 10°, preferably less than 35° and preferably larger than 15°. The valve seat separating an air outtake portion of the scavenging air passage connected to the air outlets and an air intake portion of the scavenging air passage connected to the air inlet. The scavenging air valve of the second embodiment includes a second valve shaft rotatably mounted in the scavenging air component and a circular valve plate centrally secured to the second valve shaft.

The circular valve plate has a diameter which is larger than that of the circular valve seat thereby forming an angle β2 to the circular valve seat at the two opposite positions of the margin of the circular valve plate being farthest away from the second valve shaft, when the valve is closed. The angle β2 is less than 90° and larger than 70°. The circular valve plate has a chamfered margin adapted as to seal against the circular valve seat.

Preferably the scavenging air component of the second embodiment consists of a two body parts; 1) a first body part including the valve seat and having the second valve shaft with the centrally mounted circular valve plate rotatably mounted therein, and 2) a second body part forming the air intake portion together with the first body part. The first body part also includes the air outtake portion. BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a perspective view of the carburettor according to a preferred embodiment of the invention,

FIG. 2 shows the side of the carburettor of the preferred embodiment of the invention facing the air filter,

FIG. 3 shows the side of the carburettor of the preferred embodiment of the invention facing the engine,

FIG. 4 shows a cross sectional view of the scavenging air component taken along the line

IV-IV in FIG. 3, FIG. 5 shows a cross sectional view of the carburettor taken along the line V-V in FIG. 2,

FIG. 5a is a zooming of the dotted circle in FIG. 5,

FIG. 5b shows the arrangement of valve plate grooves according to conventional valves,

FIG. 6 shows an exploded view of the carburettor according to a second embodiment of the invention, FIG. 7 shows a perspective view of the carburettor of the second embodiment, where the cover, the valve shaft, and the valve plate of the scavenging sir component have been removed,

FIG. 8 shows the a perspective view of the scavenging air component according to the second embodiment, and FIG. 9 shows a cross sectional view of the carburettor according to the second embodiment.

DETAILED DESCRIPTION OF THE INVENTION

The invention concerns a carburettor 1 of a two-stroke internal combustion engine that operates with scavenging air. The internal combustion engine is a two-cycle engine that operates with layered scavenging and is preferably used in manually guided implements. Such manually guided, portable implements can be embodied as power chainsaws, cut-off machines, brush cutters, blowers or the like. FIG. 1-5B shows the preferred embodiment of the present invention and FIG. 6-9 shows a second embodiment of the invention. In the drawings, parts that are identical or close to identical have been given the same numerals.

In FIG.l the carburettor 1 is shown comprising a carburettor main body 2 including a main air passage 9 extending from an air intake side of the main body to an opposite air outtake side (se FIG. 3) of the main body. When the carburettor 1 is mounted in e.g. a chainsaw the air intake side faces the air filter and the air outtake side faces the engine. A choke valve 11 is mounted in the main air passage 9 near the air intake side and a throttle valve 16 (see FIG. 3 and FIG. 5) is mounted near the air outtake side.

A fuel pump is located at a fuel pump side 50 of the main body 2 and a diaphragm is located at the opposite side, which is referred to as the diaphragm side 51.

A scavenging air component 3 is mounted on the fuel pump side 50 of the main body 2. We refer the side of the scavenging air component 3 mating the fuel pump side, to as the bottom side of the scavenging air component 3, consequently the opposite side of the scavenging air component 3 is refereed to as its top side. However, it should be noted that when mounted in e.g. a chainsaw the top side of the scavenging air component 3 could very well face the bottom of the chainsaw as well as any other direction.

The scavenging air component 3 is secured to the main body 2 by a centrally placed screw 15 and two aligning pins (not shown but the same as for the second embodiment, seen in FIG. 6). The bottom side of scavenging air component 3 acts as a cover for the fuel pump in order to reduce the overall height of the carburettor 1.

The scavenging air component 3 includes a scavenging air passage 2OA, 2OB having a scavenging air valve 6, 7 of butterfly type mounted therein. The scavenging air passage 2OA, 20B extends substantially parallel to the main air passage 9, from its air inlet 4 towards its two air outlets 5A, 5B. The air inlet 4 and the air intake side of the main body 2 forms a common side, which we refer to as the air filter facing side of the carburettor.

The shape of the air inlet 4 is rectangular oval, i.e. rectangular shaped but with rounded corners, and having an opening width wl and an opening height hi (see FIG. 4 and FIG. 5). However, according to the invention the air inlet 4 could preferably have any oblong shape in the range from oval to rectangular.

The width wl of the air inlet 4 is longer than the height hi thereof, preferably at least 1,5 times as long and even more preferred at least twice as long. The reason for this is to reduce the overall height of the carburettor while maintaining a sufficient air flow through the scavenging air component 3.

The scavenging air passage 2OA, 2OB comprises a first portion having a first length Ll and a rectangular cross section having close to or the same height and width as those of the air inlet 4. The first section extends from the air inlet 4 towards a second portion, which follows the first portion. The second portion is split in two apertures which end at the two air outlets 5A, 5B, receptively. The second portion has a second length L2.

A scavenging air valve is mounted in the first portion. The scavenging air valve includes a first valve shaft 6 rotatably mounted in the first portion at the shaft bearings labelled 33. A rectangular valve plate 7 is centrally secured to the first valve shaft 6. The rectangular valve plate is adapted in its shape to seal against the inner walls of the first portion when the scavenging air valve is in a closed position, as seen in e.g. FIG. 5. The height h2 of the rectangular valve plate 7 is somewhat larger than the height hi of the first portion so that the rectangular valve plate 7 will be somewhat tilted when in its closed position. Thus the rectangular valve plate 7 forms an angle βl to the bottom of the scavenging air passage 2OA, 2OB, which angle βl is less than 90° and larger than 70°. The reason for this is to create a natural stop for the valve plate 7, i.e. a valve seat.

The rectangular valve plate 7 further comprises grooves 21, 22, as seen in FIG. 5 A and FTG 2, around at least the width part of its margin. These grooves 21, 22 provides for the margin of the valve plate 7 to flex somewhat when the valve closes, thereby enhancing the sealing efficiency against the inner walls of the first portion. Of course these grooves are mainly suitable when the valve plate 7 is made of a material which is not too stiff, and it is therefore preferred that the rectangular valve plate 7 is made in a rigid polymer material e.g. a thermo plastic such as POM. Conventionally such grooves has been arranged as shown in FIG. 5B, i.e. having the mouths of the grooves (21', 22') pointing in the same direction as the rotational opening direction of the scavenging air valve. Such an arrangement of the grooves provides for a very good sealing, but the valve plate 7 may unfortunately get wedged in the closed position. An inventive feature of the present invention is therefore to arranging the mouths of the grooves 21, 22 in an opposite manner, i.e. having the groove openings of the grooves (21, 22) pointing in the same direction as the rotational closing direction of the scavenging air valve. It has been discovered that the sealing efficiency is maintained, in particular when the valve is spring loaded, while the wedging problems have been minimised.

The scavenging air valve is spring loaded, biasing the scavenging air valve towards a closed position. In the Figures relating to the first embodiment this spring has been omitted, however, it is the same as the spring labelled 30 in the second embodiment.

A throttle valve lever 13 controls the throttle valve 16 and a coupling rod 12 connects the throttle valve 16 with the scavenging air valve 6, 7, so that when the throttle lever 13 is actuated, the throttle valve 16 and the scavenging air valve 6, 7 will both be affected. Thus the turning of the scavenging air valve takes place in synchronism with the turning of the throttle valve 16 via a coupling rod 12. They are so arranged that when the throttle valve 16 is in the idling position, the scavenging air valve is completely closed. Then, when the engine speed is increasing as the throttle's movement rotates the valve shaft of the throttle valve, the scavenging air valve will first be opened slowly, thereafter proportionally more rapidly.

In FIG. 6-9 a second embodiment of the present invention is shown. The carburettor main body 2 is the same as in FIG. 1-5. FIG. 6 shows an exploded view of the carburettor 1. The scavenging air component 3 A, 3B acts a cover for the fuel pump side 51 and is secured to the main body 2 by a single centrally placed screw 40 in cooperation with the alignment pins 37, which are adapted to fit in the corresponding holes 38 of the main body 2.

The scavenging air component according to the second embodiment consists of a two body parts; 1) a first body part 3A which includes a circular valve seat 35, 36, an air outtake portion 2OB of the scavenging air passage 2OA, 2OB, below the valve seat 35, 36 and two air outlets 5A, 5B, and 2) a second body part 3B forming an air intake portion 2OA of the scavenging air passage together with the first part 3A which has an rectangular oval shaped air inlet 4. A scavenging air passage 20A, 2OB is hence formed by the intake portion 2OA and the outtake portion 2OB.

The circular valve seat 35, 36 is tilted at an angle α in relation to the plane of the fuel pump side. The angle α is less than 45° and larger than 10°, preferably less than 35° and preferably larger than 15°. The circular valve seat 35, 36 separates the air outtake portion 2OB of the scavenging air passage 2OA, 2OB connected to the air outlets 5A, 5B from the air intake portion 2OA connected to the air inlet 4. The diameter of the circular valve seat 35, 36 is indicated by dl in FIG.9.

The valve seat 35, 36 consists of two flange portions, a first flange portion 35 and a second flange portion 36, each forming a half circle separated from each other by two opposite located bearings 33 in which a valve shaft 6 is rotatably secured. Hence, the valve shaft 6 divides the valve seat area in two equal semi circles. A circular valve plate 7' is centrally secured to the valve shaft 6 separating the outtake portion 20 B and the intake portion 2OA of the scavenging air passage 2OA, 2OB when the valve is closed. The fully open position of the valve is indicated by the dotted lined circular valve plate 7' and the closed position by the full lined circular valve plate T .

The circular valve plate 7' has a circular valve plate diameter d2 which is larger than the circular valve seat diameter dland when the scavenging air valve is closed the circular valve plate T forms an angle β2 to the valve seat 35, 36, at the two opposite positions of margin 45 of the circular valve plate T being farthest away from the valve shaft 6. The angle β2 is less than 90° and larger than 70°. The margin 45 of the circular valve plate T is chamfered so as to mate the valve seat 35, 36 when the valve is closed. Of course, the slope of the chamfered margin 45 will vary around the circular valve plate T in order to mate the valve seat 35, 36, from a slight inclination the two opposite positions of margin 45 being farthest away from the valve shaft 6 corresponding to the angle β2 towards a right angled margin 45 at the bearings 33.

The valve seat flange portions 35, 36 have their lateral extensions at least as long as the chamfered margin 45 of the circular valve plate 7'. Due to manufacturing conditions the second flange portion 36 has a longer lateral extension than the first flange portion 35.

Whereas the invention has been shown and described in connection with the preferred embodiments thereof it will be understood that many modifications, substitutions, and additions may be made which are within the intended broad scope of the following claims. From the foregoing, it can be seen that the present invention accomplishes at least one of the stated objectives. For certain engines it might be justified to use additional air even at idling to support proper scavenging. In any event, in order to enable more exact amounts of scavenging air to flow past and be controlled by the valve, a small air hole (or a number of small air holes) can be optionally provided in the valve plate 7, 7' that permits any vacuum created at the valve plate 1, 1' to be more easily broken, and the valve opened and controlled with precision.

Further, even though the scavenging air component 3 has been shown as mounted as a cover for the fuel pump, it would of course be possible to have a separated cover for the fuel pump and the scavenging component mounted on top of that cover.

Further it would also be feasible to have the scavenging air component 3 mounted on the opposite side of the main body 2, i.e. the side having the diaphragm. The scavenging air component 3 could then be mounted as a cover for the diaphragm, but also here it would be possible to have a separate cover for the diaphragm upon which the scavenging air component 3 is mounted.

The valve plates 7, 7' of the first and second embodiments could be e.g. be made of metallic materials such as e.g. brass, aluminium or a rigid polymer material e.g. a thermo plastic such as POM. However for the first embodiment it is preferred that the valve plate is made of a rigid polymer material, preferably a thermo plastic, even more preferred POM, since in this embodiment it desired to have a valve plate with flexible margins, which is enabled by having a valve plate made of a rigid polymer material having grooves around at least a portion of the margin. For the second embodiment, however, it is preferred that the valve plate is made of metallic material such as brass or aluminium, since a valve plate made of such materials can be manufactured with lower tolerances (for the first embodiment the tolerances could be higher due to compensation of the flexible margin).

The body of the scavenging air component is preferably made of a metallic material such as e.g. brass or aluminium or a rigid polymer material.

Claims

1. A carburettor (1) for a two stroke internal combustion engine that operates with scavenging air, comprising: a) a carburettor main body (2) including a main air passage (9) having at least a throttle valve (16) mounted therein, the main air passage (9) extends from an air intake side of the main body to an opposite air outtake side of the main body, the main body further including a fuel pump side (50) comprising a fuel pump and an opposite diaphragm side (51) comprising a diaphragm, both being transversal to the intake and outtake sides, b) a scavenging air component (3; 3 A, 3B)) mounted on the fuel pump side (50) or the diaphragm side (51), which scavenging air component includes a scavenging air passage (2OA, 20B) having a scavenging air valve (6, 7;6, 7') of butterfly type mounted therein, the scavenging air passage (2OA, 20B) extends substantially parallel to the main air passage (9) from an air inlet (4) towards at least one air outlet (5 A, 5B), the air inlet (4) and the air intake side of the main body (2) forming an air filter facing side of the carburettor, c) a coupling member (12) connecting the throttle valve (16) and the scavenging air valve (6, 7;6, 7'); characterised in that, the air inlet (4) is oblong providing an air inlet width

(wl) along the main body (2) and an air inlet height (hi) extending from the main body (2).

2. A carburettor according to claim 1 wherein the scavenging air valve is spring loaded (30), biasing the scavenging air valve towards a closed position.

3. A carburettor according to claim 1 or 2 wherein the air inlet width (wl) is at least 1,5 times longer than the air inlet height (hi), preferably twice as long.

4. A carburettor according to anyone of claim 1 to 3 wherein the air inlet (4) is rectangular.

5. A carburettor according to claim 4 wherein the corners of the rectangular air inlet (4) are rounded.

6. A carburettor according to anyone of claim 1 to 3 wherein the air inlet (4) is oval.

7. A carburettor according to anyone of claim 1 to 6 wherein the scavenging air passage includes a first portion (Ll) which has a oblong cross section, the first section extending from the air inlet (4) towards the air outlet (5 A, 5B).

8. A carburettor according to claim 7 wherein the scavenging air passage includes a second portion (L2) extending from the first portion (Ll) to the air outlet

(5A, 5B), the second portion is divided into two apertures whereby the at least one air outlet being two.

9. A carburettor according to claim 7 or 8 wherein the scavenging air valve includes a first valve shaft (6) rotatably mounted in the first portion (Ll) and a oblong valve plate (7) centrally secured to the first valve shaft, which oblong valve plate is adapted in its shape to seal against the inner walls of the first portion (Ll) when the scavenging air valve is in a closed position.

10. A carburettor according to claim 9 wherein the height (h2) of the oblong valve plate (7) is somewhat larger than the height (hi) of the first portion so that the oblong valve plate (7) will be somewhat tilted when in the closed position forming an angle βl to the plane of the main body side on which the scavenging air component (3) is mounted, the angle βl is less than 90° and larger than 70°.

11. A carburettor according to claim 9 or 10 wherein the oblong valve plate (7) is made of a rigid polymer material, preferably a thermo plastic.

12. A carburettor according to anyone of claim 9 to 11 wherein the oblong valve plate (7) has grooves (21, 22) along at least the width part of the margin of the oblong valve plate (7).

13. A carburettor according to claim 12 wherein the mouths of the grooves (21, 22) points in the same direction as a rotational closing direction of the scavenging air valve.

14. A carburettor according to anyone of claim 1 to 6 wherein the scavenging air component (3A, 3B) comprises a circular valve seat (35, 36) having a valve seat diameter dl, which valve seat's plane is tilted at an angle α in relation to the plane of the main body side (50, 51) on which the scavenging air component (3 A, 3B) is mounted, the angle α being less than 45° and larger than 10°, preferably less than 35° and preferably larger than 15°, the valve seat separating an air outtake portion

(20B) of the scavenging air passage (2OA, 20B) connected to the air outlets (5 A, 5B) and an air intake portion (20A) of the scavenging air passage (2OA, 20B) connected to the air inlet (4).

15. A carburettor according to claim 14 wherein the scavenging air valve includes a second valve shaft (6) rotatably mounted in the scavenging air component (3A, 3B) and a circular valve plate (7') centrally secured to the second valve shaft (6), the circular valve plate (V) having a circular valve plate diameter d2 which is larger than the valve seat diameter dl.

16. A carburettor according to claim 15 wherein the circular valve plate (7') forms an angle β2 to the circular valve seat (35, 36) at the two opposite positions of the margin (45) of the circular valve plate (7') being farthest away from the second valve shaft (6), which angle β2 is less than 90° and larger than 70°.

17. A carburettor according to claim 12 or 13 wherein circular valve plate (7') has a chamfered margin (45) adapted as to seal against the circular valve seat (35, 36).

18. A carburettor according to anyone of claims 15 to 17 wherein the scavenging air component consists of a two body parts; 1) a first body part (3A) including the valve seat (35, 36) and having the second valve shaft (6) with the centrally mounted circular valve plate (7') rotatably mounted therein, the first body part (3A) also including the air outtake portion (20B), 2) a second body part (3B) forming the air intake portion (20A) together with the first body part (3A).

19. A carburettor according to anyone of the claims above wherein the scavenging air component (3; 3A, 3B) is mounted on the fuel pump side (50).

20. A carburettor according to claim 19 wherein the side of the scavenging air component (3; 3 A, 3B), which faces the fuel pump side (50), also is a cover for the fuel pump.

21. A carburettor according to anyone of claims 1- 18 wherein the scavenging air component (3; 3A, 3B) is mounted on the diaphragm side (51).

22. A carburettor according to claim 21 wherein the side of the scavenging air component (3; 3 A, 3B), which faces the diaphragm side (51, also is a cover for the diaphragm.