WO2007027128A1

WO2007027128A1 - System for a two-stroke crankcase scavenged combustion engine

Info

Publication number: WO2007027128A1
Application number: PCT/SE2005/001277
Authority: WO
Inventors: Tore Jonsson
Original assignee: Husqvarna Ab
Priority date: 2005-09-02
Filing date: 2005-09-02
Publication date: 2007-03-08

Abstract

The present invention relates to a system (1) for supplying an air-fuel mixture to a two-stroke internal combustion engine cylinder (2), comprising a carburettor (3) having a throttle valve (4) and usually a choke valve. The system further has a transfer passage (6) between a crankcase chamber and a combustion chamber of said engine. A supply conduit for additional air (8) is having an air valve (5) for said additional air, and said supply conduit for additional air (8) is being adapted to discharge the additional air into the top portion of said transfer passage (6) during a portion of a cycle of said two-stroke internal combustion engine. Further the air valve (5) is controlled by the throttle valve (4) so as to affecting an air valve opening state when said throttle valve opening state is affected. Said air valve (5) is of a modified butterfly valve type having a control shaft (11) located within a duct section (12) of the supply conduit (8) but is located offset from the centre line of the duct section.

Description

SYSTEM FOR A TWO-STROKE CRANKCASE SCAVENGED COMBUSTION

ENGINE

Technical field

The present invention relates to a system for a two- stroke crankcase scavenged internal combustion engine, in which an air passage for additional air is arranged between an air inlet and the upper part of a transfer duct. Fresh air is added at the top of the transfer duct and is intended to serve as a buffer against the air/fuel mixture below. This buffer is mainly lost out into the exhaust outlet during the scavenging process. The fuel consumption and the exhaust emissions is thereby reduced.

Background of the invention

For conventional two-stroke combustion engines it is well known how to control the air/fuel mixture, but a high level of unburned hydrocarbon emissions is produced because in this engine the scavenging process is per- formed exclusively by a mixture of air and fuel. Some of the air/fuel mixture mixes with the exhaust gases and thereby some unburned fuel is lost out with the exhaust gases.

Two-stroke combustion engines using the additional air technique during the scavenging .are previously known in the art. These engines reduce fuel consumption and exhaust emissions. But for these prior art engines it is a problem to control the air/fuel mix during normal operation and during start. US 6,328,288 shows a system comprising two butterfly valves. An elongated slot is arranged in the throttle valve lever for fastening the link to the air valve lever to give a delayed opening of the air valve. There is a risk that the link will fall apart and a risk of icing in the slot in the winter. Apart from this the operator will have to overcome the action of two strong return springs when he is pressing the throttle control. Hence, there is a need for an improved system for a two-stroke engine using the additional air technique to achieve good properties for start and normal operation. Further it is an advantage to combine proper operation of such an engine with lean and environmentally friendly operation. Finally it would be advantageous to provide a robust, cost effective and high-quality system for this kind of engine.

Summary of the Invention

The object' of the present invention is to provide a system for supplying .an air/fuel mixture to a two-stroke internal combustion engine that overcomes at least some of the above-mentioned matters and present an improved control of the air/fuel mixture to the engine.

According to the invention there is provided a system for supplying an air-fuel mixture to a two-stroke crankcase scavenged internal combustion engine, comprising a carburettor . having a throttle valve. The system further has a transfer passage between a crankcase chamber and a combustion chamber of the engine, and a supply conduit for additional air having an air valve for said additional air. The supply conduit for additional air being adapted* to discharge the additional air into the top portion' of said transfer passage, either directly through a check valve or via a recess in an engine piston, during a portion of a cycle of said two- stroke internal combustion engine. The air valve is controlled by said, throttle valve so as to affecting a air valve opening state when said throttle valve opening state is affected, and in that said air valve is of a modified butterfly valve type having a control shaft located within a duct section of the supply conduit but is located offset from the centre line of the duct section.

Brief Description of the Drawings

By way of example, embodiments of the present invention will now be described with reference to the accompanying drawing figures in which:

Fig. 1 is a perspective side-view showing from a first side a two-stroke engine cylinder provided with a system for supplying air-fuel mixture and for supplying additional air during the scavenging process. Fig. 2 shows the same system in a perspective side- view from a second opposite side but without showing the engine cylinder.

Fig. 3 shows the carburettor and air-valve from the side. The air valve is shown in a partly crossectioned view.

Fig. 4 is an axial-view of the air valve and carburettor, as ,seen from the engine cylinder.

Fig. 5 is a perspective side-view from the first side showing only the carburettor and air valve in enlarged scale.

Detailed Description of Preferred Embodiments

A first embodiment of the present invention will be described in more detail in the following, with reference to the accompanying drawings .

Referring to Fig 1 and 2, a two-stroke engine cylinder 2 with carburettor 3 and a system for additional air 9 is shown. The carburettor 3 is provided at an intake of the cylinder. Further is the piston provided for letting air/fuel mix into the crankcase and fresh air, for the scavenging process, into the upper part of a transfer passage 6 during a portion of an engine cycle. During a stage of the engine cycle the air/fuel mixture is compressed in the crankcase and thereafter the exhaust gases is pressed out through the exhaust port by the compressed air/fuel mixture. The buffer of fresh air in the transfer passage 6, provided ^' from the additional air system, enter the combustion chamber before the air/fuel mixture. Consequently the risk for uncombusted fuel to accompany the exhaust gases out from the combustion chamber is reduced. In the different figures no air filter assembly is shown for clarity reasons. Instead a supply conduit 8 has been indicated in figure 1. It will connect to the air filter assembly and lead the fresh air to the cylinder 2. The additional air 9 is provided into the transfer passage 6 through a channel 8 and an aperture in the cylinder. Which aperture, during a portion of the engine cycle, leads into a recess in the piston fluidly connected to "the transfer passage 6. The channel 8 for additional air could also- connect directly to a transfer passage 6 via a check valve or so called Reed valve. The flow of additional air 9 into the transfer passage 6 is substantially simultaneous with the flow of air/fuel mixture into the crankcase.

The additional air channel 8 is provided with an air valve 5 for controlling the additional air to the engine 2. The air valve 5 for additional air is interconnected with the throttle valve 4 of the carburettor 3, this is to give synchronous control of the opening state of the throttle and the supply of additional air via the air valve 5. ..:„. •

A shielding .baffle .13 is fastened to the cylinder by a number of screws. Some of these screws go through apertures (15, 16, 17, 18) in the¹, carburettor and in the air valve housing. ^■ L, .-. .-, . Figure 1-5.schematically^■ show. how sthis system works, i.e. how the throttle valve 4 and ,the air valve 5 work together, in figure 2-4 both the.-throttle' valve and the air valve 5 are fully closed while they are fully open in figures 1 and 5.

As explained the supply conduit 8 for additional air 9 has a first end in a filter housing and a second end in an additional air port in a cylinder wall.

In figure 1 air 18 is fed to the carburettor 3 and additional air 9 is fed to the air valve 5 and further through a shielding baffle 13 and into engine ports for additional air and air-fuel mixture. Gaskets seal between the shielding baffle and the engine ports. As explained the additional air will be fed to the top part of each transfer passage 6.

In figure 1 a choke valve is apparent in the end of the carburettor. It consists of a valve plate 19 on a choke shaft 20 having a choke lever 21, supplied with a finger 22. This finger is pushed when controlling the choke valve. In figure 2 the choke valve plate 19 has been removed for clarity reasons. Behind the shaft 20 you can see the throttle valve 4. It also has a shaft 30, that is only visible in figure 4. The throttle shaft 30 has a throttle lever 23 supplied with a finger 24. The throttle lever 23 is turned by the wire 25 leading to the throttle control.

Figure 2 shows a situation where both the throttle valve and the air valve 5 are fully closed, i.e. no air will pass through the two ducts. When the throttle wire 25 is tensioned the throttle lever 23 turns and its finger will contact an air valve lever 27 mounted to an air valve shaft 26 that holds an air valve plate 29. When the wire 25 is tensioned first the throttle valve will open somewhat before the air valve starts to open. But as soon as the finger 24 contacts the lever 27 it will push the lever and turn the air valve. In figure 1 you can see the air valve in a fully open state. The air valve lever 27 has a return spring 28 that turns the air valve back when throttle is decreased, so the valve can shut and seal in its end position. Also the shaft for the throttle and for the choke valve has return springs. This is however conventional and will not be described further. This means that when pressing the throttle control you will have to overcome not only the return spring for the throttle lever but also the return spring 31 for the air valve lever. This tends to increase the necessary force of the throttle control experienced by the operator. In order to reduce this force the air valve has been arranged in a completely unconventional and novel way. This is best seen in figure 4 where a distance, or offset, E has been marked between the centre line of the air valve shaft 26 and the centreline of the duct section 12. Thereby the underpressure that is created when the valve is shutting will help to close the valve and keep it shut. This action is created by the difference in area of the air valve plate 29 on the two sides of its shaft. This pressure action makes it possible to reduce the spring force of the air valve return spring 27, giving a reduced force to operate the throttle control during most of its travel. This duct section has a diameter D. The ratio of said offset E divided with said diameter D, i.e. E/D, is bigger than" 0,05 but smaller than 0,2 and is preferably bigger than 0,08 but smaller than 0,2 and is most preferably bigger than 0,1 but smaller than 0,15. Figure 1-5 show the air valve 5 of modified butterfly valve type and the air valve housing 10. The housing could form an integrated part of the shielding baffle (13).

Further the finger 24 that turns the air valve lever 27 provides a reliable solution. To further reduce friction it can be supplied with a rolling element that turns around the axis of finger 24.

As seen in fig.3 there is a grove 32 in side wall of the duct section 12 to house the edges of the air valve plate 29. Preferably the air valve shaft 26 has a diameter that is greater than the width of the grove 32, and preferably the shaft 26 covers the two groves, as seen axially. This provides for good and simple sealing.

As discussed the shielding baffle is firmly connected to at least the cylinder and usually also to a crankcase. It could however also be resiliently connected to the cylinder and thereby enable an antivibrating attachment of the carburettor and air valve.

Claims

1. A system (1) for supplying an air-fuel mixture to a cylinder (2) of a two-stroke crankcase scavenged internal combustion engine, comprising a carburettor (3) having a throttle valve (4) and feeding an air-fuel mixture to an intake port of the cylinder, said system further has a transfer passage (6) between a crankcase chamber and a combustion chamber of the engine, and a supply conduit (8) for additional air (9) having an air valve (5) for the additional air, and said supply conduit for additional air (9) being adapted to discharge the additional air into the top portion of the transfer passage (6), either directly through a check-valve or via a recess in an engine piston, during a portion of a cycle of said two-stroke internal combustion .engine, c h a r a c t e r i s e d in that said air valve (5) is controlled by said throttle valve (4) so as to affecting an air valve opening state when said throttle valve opening state is- affected, and in that said air valve (5) is of a modified butterfly valve type having a control shaft (11) located within a duct section (12) of the supply conduit (8) but is located offset from the centre line of the duct section (12). of the supply conduit (8).

2. A system (1) according to claim 1, wherein in said throttle valve (4) of the carburettor (3) is of a butterfly valve type..'

3. A system (1) according to claim 1 or 2, wherein said air valve (5) comprises an air valve housing (10).

4. A system (1) according to claim 3, wherein said air valve housing (10) and the carburettor is attached to a shielding baffle (13) located downstream of the system and attached to the cylinder (2).

5. A system (1) according to claim 3, wherein said carburettor is attached to the shielding baffle (13) while the air valve housing (10) forms an integrated part of the shielding baffle.

6. A system (1) according to claim 4 or 5, wherein said shielding baffle is firmly attached to at least the cylinder (2 ) .

7. A system (1) according to claim 4 or 5 , wherein said shielding baffle is resiliently connected to the cylinder (2) enabling movement between the two and therefore enabling an antivibrating attachment of the carburettor (3) and air valve (5).

8. A system (1) according to any of the preceding claims, wherein the ratio of said offset (E) divided with said diameter (D), i.e. E/D is bigger than 0.05 but smaller than 0.2 and is preferably bigger than 0.08 but smaller than 0.2 and is most preferably bigger than 0.1 but smaller than 0.15.

9. A system (1) according to. any of the preceding claims, wherein the carburettor is provided with a choke valve (14) upstream of the throttle valve.

10. A two-stroke internal combustion engine provided with a system ( 1 ) for supplying an air-fuel mixture according to any one of- the claims 1-9.

11. A hand-held working tool provided with a two- stroke internal combustion engine having a system (1) for supplying an air-fuel mixture according to any one of the claims 1-10.