GB2420512A

GB2420512A - An exhaust system in a tool driven by an internal combustion engine

Info

Publication number: GB2420512A
Application number: GB0523281A
Authority: GB
Inventors: Bernhard Huber
Original assignee: Andreas Stihl AG and Co KG
Current assignee: Andreas Stihl AG and Co KG
Priority date: 2004-11-26
Filing date: 2005-11-15
Publication date: 2006-05-31
Anticipated expiration: 2025-11-15
Also published as: CN100538038C; DE102004057110A1; US20060112683A1; DE102004057110B9; FR2878564B1; DE102004057110B4; GB0523281D0; CN1782338A; FR2878564A1; GB2420512B

Abstract

An exhaust system for a tool driven by an internal combustion engine, in particular hedge clippers or a similar device, comprises an exhaust pipe 1 for the exhaust gas stream 3, with an outlet end 4 of the exhaust pipe 2 discharging into the environment. Positioned on an internal wall 5 of the exhaust pipe 2 is a circumferentially running condensate guide element 6 which narrows the flow cross-section.

Description

1 2420512 An exhaust system in a tool driven by an internal combustion

engine The invention relates to an exhaust system in a manually operated tool driven by an internal combustion engine having an exhaust pipe for an exhaust gas stream, the outlet end of the exhaust pipe discharging into the environment.

Manually operated tools such as hedge clippers, strimmers, chain saws and similar devices are driven by internal combustion engines which are equipped in standard designs with total-loss lubrication systems In twostroke engine designs, in particular, the fuel/air mixture introduced into the engine contains a low lubricating oil content which, after passing through the engine, is discharged into the environment as a fine mist together with the exhaust emissions also created. In addition to the aforementioned oil mist, combustion residues, atmospheric moisture and other similar products may also lead to mist formation, a phenomenon also observed in four-stroke engines.

It has been shown that the aforementioned substances in the exhaust gas stream tend to form a film of condensate on the internal surfaces of the exhaust system. In certain designs the film of condensate can be observed being transported in the direction of the exhaust gas stream. Condensate is able to collect in the area of the outlet end of the exhaust pipe and then either drip from the outlet end or even run back along the outside of the exhaust pipe towards the engine. This leads to unpleasant fouling which requires the tool to be cleaned frequently.

The present invention seeks to develop an exhaust system in such a manner that the fouling effect is avoided or reduced.

According to the present invention there is provided an exhaust system for a tool driven by an internal combustion engine, having an exhaust pipe for an exhaust gas stream, the outlet end of the exhaust pipe discharging into the environment, wherein positioned on an internal wall of the exhaust pipe is a circumferentially running condensate guide element which narrows the flow cross-section.

In the proposed exhaust system for a tool driven by an internal combustion engine there is positioned on an internal wall of the exhaust pipe a circumferentially running condensate guide element which narrows the flow cross-section. The position of the condensate guide element running around the circumference of the pipe impedes the movement of the film of condensate in the direction of the exhaust gas stream. The condensate guide element extends radially inwards from the internal wall of the exhaust pipe. Condensate which has collected or been deposited on the walls of the exhaust pipe builds up in this area and is directed radially inwards. At the same time, because it extends radially inwards, the circumferential condensate guide element reduces the free flow crosssection. As a result, the speed of the exhaust stream is increased locally. Due to the increased flow speed, condensate which has collected and been directed inwards is carried along by the exhaust gas stream and discharged into the environment. The condensate is thus prevented from dripping out of the exhaust pipe.

In an advantageous development the exhaust pipe is connected downstream of a silencer with the condensate guide element being located in a section of the pipe outside the silencer. Hot exhaust emissions do not flow around the outer section of the exhaust pipe.

The comparatively cool environment favours the formation of condensate. The position of the condensate guide element in this area causes the condensate substance to be reintroduced into the exhaust gas flow almost directly at the place of condensate formation. The formation of large collections of condensate and the associated risk of droplet formation is reliably avoided.

The condensate guide element is advantageously positioned in the immediate vicinity and upstream of the outlet end of the exhaust. The renewed condensing and depositing of the condensate substance back into the exhaust gas stream is effectively eliminated.

In a preferred version the condensate guide element forms a separation point for the exhaust gas stream in its cross-section. The term "separation point" used here is not limited to an angular edged shape. It covers any aerodynamic shape which narrows the flow cross-section of the exhaust pipe and which, in contrast to a venturi-type design, generates flow separation from the surface with subsequent swirl formation. The flow separation caused by the condensate guide element and the subsequent swirl formation support the effect of the exhaust gas stream as it carries the film of condensate on the walls along with it.

Swirled droplets of condensate are pushed into the inner area of the exhaust gas stream near the centre line which makes it harder for farther deposits to occur on the succeeding pipe walls.

In a useful development the condensate guide element runs in a spiral shape along the internal wall of the exhaust pipe. The spiral shape which extends axially permits a restricted degree of axial movement of the film of condensate deposited on the walls and thus its distribution over the axial length of the condensate guide element. Distributed over its length, the condensate guide element is able to return the deposited condensate to the exhaust gas stream with greater efficiency. Somewhere in the region of two to six spirals inclusive, and in particular a design featuring some four spirals, have proved useful in creating a sufficient flow direction effect at small volumes.

In an advantageous design the downstream end of the condensate guide element is located in the area of a the centre line of the exhaust pipe. Residual condensate which has not been returned the length of the condensate guide element into the exhaust gas stream, runs in the direction of flow of the exhaust gas along the condensate guide element to its downstream end. Here it is located in the area of the centre line where it can be better removed from the exhaust gas stream and discharged. The formation of condensate residues is avoided.

In a useful version the condensate guide element is designed as a spiralshaped wire spring. The circular shape of the wire cross-section has proved effective both in collecting the stream of deposited condensate and in directing the flow of the exhaust gas stream and forming swirls. These advantageous effects are matched by a simple and cost- effective design.

The upstream end of the wire spring is preferably bent radially inwards. The inwardly bent wire end can easily be used as an assembly aid. At the same time, the end of the wire which projects into the flow cross-section helps to form swirls and thus to improve the reabsorption of the condensate.

In an advantageous design the wire spring is held pre-tensioned radially in the exhaust pipe. In addition to reliable positioning, this radial pretensioning also ensures the application of a radial force biassing the spring and the internal wall together. A sealing effect can be observed in this area. The film of condensate is unable to creep between the wire spring and the pipe wall. In fact, it is diverted radially inwards around the wire cross-section of the wire spring where optimum conditions for reintroduction into the exhaust gas stream prevail.

Embodiments of the invention are explained in greater detail below with reference to the drawing.

Fig. I shows an overview of a manually operated tool based on the example of hedge clippers with an exhaust system illustrated in the form of a block diagram.

Fig. 2 shows a perspective, transparent view of the exhaust pipe of the arrangement illustrated in Fig. I with details of the condensate guide element positioned therein.

Fig. 3 shows a section through an enlarged schematic view of the flow conditions in the area of the wire cross-section of the condensate guide element in the arrangement illustrated in Fig. 2.

Fig. 4 shows a perspective, detailed view of the condensate guide element as illustrated in Fig. 2 in the form of a wire spring.

Fig. 5 shows a variant of the wire spring illustrated in Fig. 4 with one end drawn into the area of the centre line.

Fig. I shows a perspective view of a manually operated tool based on the example of hedge clippers which is driven by an internal combustion engine 1. Instead of hedge clippers, the tool in question might also be a strimmer, a parting-off grinder or a chain

saw, for example.

The internal combustion engine 1 which is not illustrated in greater detail is held in a housing 17 and is almost completely covered by the housing 17. All that can be seen of the internal combustion engine I in the view shown here is a spark plug connector 20 and a crank handle 19 for starting the internal combustion engine 1.

A front handle 15 and a rear handle 16 are provided for guiding the hedge clippers by means of which a shearing blade 18 driven by the internal combustion engine I can be guided along the foliage to be cut.

In the embodiment shown, the internal combustion engine 1 is a two-stroke internal combustion engine with total-loss lubrication in which a portion of lubricating oil is mixed with the fuel for operation. It is also possible to provide separate lubrication with a separate lubricant reservoir and an at least approximately loss-free lubrication system, in a four-stroke engine, for example.

The exhaust emissions created when the internal combustion engine is in operation are discharged into the environment by an exhaust system which is covered by the housing 17 and indicated in the illustrated by means of a block diagram. For this purpose, the exhaust system comprises a silencer 7 downstream of which is connected an exhaust pipe 2. A stream of exhaust gas indicated by means of an arrow 3 passes through first the silencer 7 and then the exhaust pipe 2 until it discharges into the environment at the free outlet end 4 of the exhaust pipe 2. In order to create the flow-conducting connection, an inner pipe section 9 of the exhaust pipe 2 projects into the silencer 7. A pipe section 8 of the exhaust pipe 2 positioned downstream of this runs outside the housing of the silencer 7. Positioned in the outer pipe section 8 in the immediately vicinity and upstream of the outlet end 4 is a condensate guide element 6 which is described in greater detail below.

The exhaust pipe 2 shown in Fig. 1 is illustrated in an enlarged and transparent form in Fig. 2. The exhaust gas pipe 2 has a circular crosssection and runs along a centre line 12. Positioned at the outlet end 4 is the condensate guide element 6 which is designed as a spiral or helical shaped wire spring 14 in the embodiment shown. The spiral-shaped wire spring 14 screw spring is inserted radially pre-tensioned into the free end of the exhaust pipe 2 near the outlet end 4. Due to the radial pre-tensioning force the wire spring is fixed in position in both a frictional and positive fit and conforms to the internal wall 5 of the exhaust pipe 2 on the inside. The exhaust pipe 2 is also provided with an axial retainer 21 at the outlet end 4 to fix the position of the wire spring 14 axially.

Upstream of the condensate guide element 6, the exhaust pipe 2 provides the exhaust gas stream 3 with an undisturbed, circular flow cross-section. This flow cross-section is narrowed several times circumferentially and radially inwards by the wire spring 14 which extends inwarsly from the internal wall 5.

The flow conditions at the condensate guide element are shown in an enlarged, schematic view in Fig. 3. The circular cross-section of a wire 22 of the wire spring 14 [Fig. 2] is pressed against the internal wall 5 of the exhaust pipe 2. The course of the exhaust gas stream 3 is shown by means of flow lines. In the interests of clarity only two 27 of these flow lines, located near the wall, are shown in the drawing. A film of condensate 23 has formed on the internal wall 5 upstream of the wire 22. Due to the interaction with the exhaust gas stream 3 and in some cases under the effect of gravity, the film of condensate 23 endeavours to migrate in the direction of the exhaust stream 2 towards the outlet end 4 [Fig. 2]. The view shown in Fig. 3 illustrates how as a result the film of condensate 23 builds up upstream of the wire 22 and is directed away from the internal wall 5 towards the centre line 12 by the condensate guide element 6 [Fig. 2].

The aerodynamic shape selected for the circular cross-section of the wire 22 causes a break in the laminar flow of the exhaust gas stream 3. Downstream of the maximum distance to the internal wall 5 forms a separation point 26 after which the flow separates off from the surface of the condensate guide element 6 and the subsequent section of the internal wall 5. Thus the swirls 25 indicated are formed. Once a significant portion at least of the exhaust gas stream 3 close to the wall downstream of the condensate guide element 6 has formed swirls, the separation point 26 formed in relation to the cross- section of the wire 22 manifests itself as a separation line along the length of the wire material. Instead of the circular cross-section of the wire 22 shown, other, for example angular cross-section fonns may be useful for the formation of an aerodynamic separation point 26.

The narrowing of the free flow cross-section due to the condensate guide element 6 causes a local acceleration of the exhaust gas stream 3 which is shown in the illustration by means of the decreasing distance between the flow lines 27 and which reaches its maximum value somewhere in the area of the maximum radial lifting of the condensate guide element 6 from the internal wall 5. The exhaust gas flow 3 and/or the weight direct the fihn of condensate 23 around the cross-section of the wire 22 into the aforementioned area. Here the high flow speed in conjunction with the separated swirls lead to the break-up of the film of condensate 23 into individual drops 24 which are carried along by the exhaust gas flow 3 before they have a chance to precipitate out on the internal wall 5 again. The separation point 26 and the drops 24 are positioned a certain radial distance from the internal wall 5 which helps to avoid renewed deposit formation.

Fig. 4 shows an individual, perspective view of the wire spring 14 illustrated in Fig. 2.

The wire spring 14 usefully comprises some two to six screw-like spirals 10. In the embodiment shown illustrates a spiral 14 with slightly more than four turns. The wire spring 14 is also curved radially inwards at its upstream end 13 as a result of which the end 13 projects inwards into the flow cross-section of the exhaust gas flow 3 [Fig. 2].

A variant of the wire spring 14 shown in Fig. 4 is illustrated in Fig. 5. Its downstream end 11, seen in the direction of flow of the exhaust gas stream 3 [Fig. 3], runs from the section of the spiral 10 at the circumference radially inwards as far as a longitudinal axis 28 of the wire spring 14. There the downstream end 11 is bent in the direction of the longitudinal axis 18. When fitted, the spirals 10 run in a spiral around the internal wall 5 of the exhaust pipe 2, as is also the case in the embodiment illustrated in Figs. 2 and 4.

Here the longitudinal axis 28 coincides with the centre line 12 of the exhaust pipe 2 [Fig.

2]. The downstream end 11 of the wire spring 14 thus lies at least approximately on the centre line 12 of the exhaust pipe 2 or in the centre of the flow cross-section of the exhaust gas stream 3. The embodiment illustrated in Fig. 5 corresponds to that shown in Fig. 4 in so far as all the other features and reference numerals are concerned.

In the embodiments illustrated here the condensate guide element 6 is designed as a spiral-shaped wire spring 14. A metal strip, a contoured exhaust pipe 2 wall or comparable other designs may also be advantageous. Instead of a spiral-shaped course, an annular aperture-like design may also be useful, it also being possible to positioned several circular disks one after another in a cascade.

Claims

Claims 1. An exhaust system for a tool driven by an internal combustion

engine, having an exhaust pipe for an exhaust gas stream, the outlet end of the exhaust pipe discharging into the environment, wherein positioned on an internal wall of the exhaust pipe is a circumferentially running condensate guide element which narrows the flow cross-section.
2. An exhaust system in accordance with claim 1, wherein the exhaust pipe is connected downstream of a silencer, the condensate guide element being positioned in a section of the pipe outside the silencer.
3. An exhaust system in accordance with claim I or 2, wherein the condensate guide element is positioned in the immediate vicinity and upstream of the outlet end.
4. An exhaust system in accordance with any one of claims 1 to 3, wherein the condensate guide element forms a separation point for the exhaust gas stream in its cross-section.
5. An exhaust system in accordance with any one of claims I to 4, wherein the condensate guide element runs along the internal wall of the exhaust pipe in a spiral or helical shape.
6. An exhaust system in accordance with claim 5, wherein the condensate guide element has from two to six spirals.
7. An exhaust system in accordance with claim 6, wherein the condensate guide element has four spirals.
8. An exhaust system in accordance with claim 5, 6 or 7, wherein the downstream end of the condensate guide element is positioned in the region of a centre line of the exhaust pipe.
9. An exhaust system in accordance with any one of claims 5 to 8, wherein the condensate guide element comprises a spiral-shaped wire spring.
10. An exhaust system in accordance with claim 9, wherein the upstream end of the wire spring is shaped to extend radially inwards.
11. An exhaust system in accordance with claim 9 or 10, wherein the wire spring is held radially pre-tensioned in the exhaust pipe.
12. An exhaust system for a tool driven by an internal combustion engine, substantially as described herein with reference to, and as illustrated in, the accompanying drawings.