DESCRIPTION

TECHNICAL FIELD

The present invention relates to a crankcase-scavenged two-stroke internal combustion engine having a cylinder, a reciprocating piston movable back and forth in the cylinder, and a conduit ported by the piston for supply of additional air into the interior of the cylinder for the scavenging. The engine is primarily intended for a hand-held working tool.

BACKGROUND ART

US 7,634,980 B2 discloses a crankcase-scavenged two-stroke internal combustion engine having an additional air supply arranged to its transfer ducts, connecting a crankcase volume and a transfer port. There is at least one recess in a piston arranged below a piston ring, and further there is a flow channel arranged in the piston or in a cylinder wall of the engine cylinder, and the recess is arranged to register with the transfer port and the flow channel for certain first piston positions, i.e. to create a communication between the transfer port/s and the crankcase volume. The scavenging prevents air-fuel mixture from the crankcase to disappear out through the exhaust port or at least reduces such losses of fuel heavily. Thereby also the pollution of the environment is reduced.

Further examples of crankcase-scavenged two-stroke internal combustion engines having an additional air supply and various designs of ducts or conduits for flow of the additional air and/or the air-fuel mixture are disclosed in US 2002/0043227 Al and US 2003/

0029398 Al.

WO 2004/005692 Al , WO 2008/111880 Al and US 6,928,996 B2 disclose carburetors for crankcase-scavenged two-stroke internal combustion engines having an additional air supply through at least one conduit wherein an additional air valve controls the flow of the additional air. A coupling member provides a lost motion connection between the additional air valve and a throttle valve provided in the carburetor's conduit for the air- fuel mixture. The lost motion connection enables a certain motion of the throttle valve for the air-fuel mixture before throttle valve for the additional air starts moving, whereby no additional air will be supplied at lower end speeds, including idle speed. To make two-stroke engines easier to start, they can be equipped with automatic decompression valves. US 6,253,723 Bl discloses an arrangement including an automatic valve for reducing compression in order to facilitate starting of a two-stroke internal combustion engine includes a movable valve adapted to control a gas flow through an opening provided in a wall of the combustion chamber of the engine. A spring is adapted to move the movable valve to an open position, and a drive actuated by underpressure is adapted to move the movable valve to a closed position against the action of the spring. The driver includes a cylinder, a piston movable in the cylinder and connected to the movable valve, and a conduit connecting the cylinder to a source of underpressure, viz. a duct for transferring the air-fuel mixture from the crankcase to the combustion chamber. A check valve is provided in the conduit for allowing an air flow in a direction away from the driver cylinder only, and a leak passage is adapted to allow a small flow of air into the for facilitating the opening of the movable valve when operation of the engine has been stopped.

The term "underpressure" is used to mean a pressure below that of the surrounding atmosphere.

Two-stroke engines with automatic decompression valves are also disclosed in US

4,619,228, US 7,228,843 and JP9112394A, for example. However, in all known cases where two-stroke engines are equipped with automatic decompression valves, foreign matter, such as oil from the fuel, e.g. from the crankcase or the carburetor, will eventually clog the check valve, so that the check valve will not work properly. Probably, there will also be an undesirable reduction of the compression at some engine speeds.

SUMMARY OF THE INVENTION

One object of the present invention is to install an automatic decompression arrangement on a crankcase-scavenged two-stroke internal combustion engine in a manner such that the arrangement will have a longer service life and a better function.

This object is achieved in that the crankcase-scavenged two -stroke internal combustion engine referred to in the first paragraph above comprises an automatic decompression valve for reducing compression in order to facilitate starting of the engine, said decompression valve being connected to a combustion chamber of the engine and being closed by underpressure periodically created in a section of the engine when the engine piston is moving back and forth, said section including the additional air conduit, said

decompression valve being connected to the additional air conduit by an underpressure conduit having a check valve for preservation of the underpressure so as to keep the decompression valve closed when the engine is running, and a bleed conduit for eliminating the underpressure within a few seconds after the engine has stopped so as to open the decompression valve. Thereby, the check valve will be exposed to no or very small amounts of oil, whereby the arrangement will have a longer service life and a better function.

Preferably, the additional air conduit includes a additional air valve, and the underpressure conduit is connected to the additional air conduit at a position downstream of the additional air valve, whereby there will be less undesirable reduction of the compression in the engine, since the additional air valve will be open at high engine speeds.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following, the invention will be described in more detail with reference to preferred embodiments and the appended drawings.

Fig. 1 is a simplified principle sketch of a crankcase- scavenged two -stroke internal

combustion engine having an automatic decompression arrangement in accordance with a preferred embodiment of the present invention.

Fig. 2 is a partial section of a two-stroke internal combustion engine incorporating a

decompression valve according to a preferred embodiment of the present invention.

MODE(S) FOR CARRYING OUT THE INVENTION

Fig. 1 is a simplified principle sketch of a crankcase-scavenged two-stroke internal combustion engine having an automatic decompression arrangement in accordance with a preferred embodiment of the present invention. The engine 1 has a cylinder 9 with cylinder bore having a cylinder wall 29. A piston 7 is intended to be movable back and forth in the cylinder bore. The piston is connected to a crankshaft 16 via a piston rod 18. The cylinder is attached to a crankcase 17. The underside of the piston 7 and the crankcase 17 forms a crankcase volume 4 that will vary when the piston moves up and down. The upper side of the piston 7 and the cylinder wall 29 defines a combustion chamber 13 that will vary when the piston moves up and down. The cylinder 9 includes an exhaust port 6 ported by the piston 7 for exiting exhaust gases at defined piston positions. A spark plug 26 is provided in the cylinder head for igniting the air-fuel mixture in the combustion chamber 13.

At least one transfer duct 3 connects the crankcase volume 4 with a transfer port 5 which at defined piston positions connects to the combustion chamber 13; here the transfer duct 3 starts in a first part 23 in the crankcase. For clarity reasons the cylinder 9 and crankcase 17 is shown in a longitudinal cross-section, but the piston 7 is shown in a side view. This makes it easier to see the recess 24 in the piston. Also the piston is partially cut away to make all ports in the cylinder wall visible. This engine has two transfer ducts 3, but only one is visible, but could also have three, four or five or possibly one. This means that the recess 24 shown in the piston cooperate with ports above the plane of the paper while the recess on the not visible backside of the piston cooperate with the shown ports 5, 22.

In addition, the engine has an air- fuel mixture conduit 20 that leads from a fuel supply unit, here shown as a carburetor 12, and to an intake port 27 in the cylinder wall 29. The air-fuel inlet port 27 is ported by the piston 7 and a mixture of air and fuel will be sucked down into the crankcase volume 4 through the intake port 27 when the piston has risen above the intake port 27.

Two additional air conduits 21 each connects to a corresponding air supply port 22 of the cylinder wall 29 for supply of additional air to the transfer ducts 3. The air supply port 22 is connected to the transfer port 5 via a recess 24 in the piston 7 at certain piston positions. In the described embodiment there are two air ducts 21 each leading to an air supply port 22. But there could also be a single air duct 21 and a branch in the cylinder wall so that the air branches off to the two different air supply ports 22. When the first recess 24 in the piston will register with air supply port 22 and transfer port 5, air will be sucked down into the transfer ducts 3. Air will fill the transfer ducts 3 almost completely. This reduces fuel losses through the exhaust port 6, when the piston 7 moves toward a bottom position and is a normal operation for a piston-ported crankcase scavenged two-stroke engine with additional air.

The carburetor 12 includes a choke valve 15 and a throttle valve 14. The additional air conduit 21 has an additional air valve 19, preferably a butterfly valve. Suitably the additional air valve 19 is connected to the throttle valve 14 by a coupling member (not shown) that provides a lost motion connection between the additional air valve 19 and the throttle valve 14. The lost motion connection enables a certain motion of the throttle valve 14 for the air-fuel mixture before throttle valve for the additional air starts moving, whereby no additional air will be supplied at lower end speeds, including idle speed. Since the additional air valve 19 is closed, at least partly, during the start of the engine; the under pressure created downstream of the additional air valve 19 in the additional air conduit 21 is increased. Example of a suitable lost motion couplings are described in WO2004005692A1 and WO 2008/111880 Al which hereby are incorporated by reference. To facilitate starting, the engine 1 is provided with an automatic decompression valve 30 that is actuated by under pressure from the additional air duct 21 by means of a hose or other conduit 37 having a check valve 38 permitting flow toward the additional air duct 21 but blocking flow in the opposite direction. Thereby, the check valve 30 will be exposed to no or very small amounts of oil, whereby the arrangement will have a longer service life and a better function. This will be described more in detail in connection with Fig. 2.

As shown in Fig. 2, a decompression valve 30 is provided in the wall of the engine cylinder 9 slightly below the top dead center of the piston 7 and is connected to the interior of the cylinder 9 via a bore 31 in the cylinder wall 29. The decompression valve 30 comprises a housing 32 that defines a valve cylinder 33 having a valve piston 34 movable therein.

Together they form a piston and cylinder assembly. The housing 32 has an end wall 35 provided with a nipple 36, which is connected to the additional air duct 21 of the engine via a conduit 37. The conduit 37 has a check valve 38 therein. The check valve 38 permits airflow in a direction from the decompression valve 30 toward the additional air duct 21 and prevents air flow from the additional air duct 21 toward the decompression valve 30.

The piston 34 is connected by means of a piston rod 39 to a valve member 40 cooperating with a valve seat 41. The valve cylinder 33 has a compression spring 42 therein resiliently actuating the piston 34 to be moved to the left-hand side in the drawings so as to bring the valve member 40 to assume an open position. The left-hand end of the cylinder 33 is connected to the atmosphere via a bore 43 in the cylinder wall, thereby ensuring that atmospheric pressure will always be present on the left-hand side of the piston 34.

In Fig. 2, the valve member 40 is shown in its open position, in which gases from the engine cylinder 9 may pass the valve member 40 and be discharged to atmosphere via a plurality of bores 44 annularly spaced around the valve housing 32. The openings 44 may be connected via a passage (not shown) to the left-hand end of the cylinder 33, whereby the bore 43 may be omitted. The valve member 40 is held in the open position by the spring 42, whereby when the crankshaft is rotated in order to start the engine, the compression in the engine cylinder 9 will be substantially lower than its normal value, and this essentially reduces the power required for rotating the crankshaft to start the engine. When the engine starts, an underpressure is created in the additional air duct 21, especially downstream of the at least partially closed additional air valve 19 (Fig. 1) and this underpressure actuates the piston 34 via the conduit 37 so as to move the piston 34 to the right-hand side in the drawing against the action of the spring 42, whereby the decompression valve 30 will be closed by the valve member 40 sealing against the valve seat 41. In operation of the engine, an overpressure will be created alternately in the crankcase 17, namely during the phase in which the air/fuel mixture is compressed by the downward movement of the piston 7. As the check valve 38 is closed in this phase, actuation of the decompression valve 30 by the overpressure is prevented, and the valve member 40 remains in the closed position. When the engine is running, the combustion pressure in the engine cylinder 9 also contributes to maintaining the valve member 40 in its closed position. When the engine is running, the degree of opening/closing of the additional air valve 19 is controlled by a linkage between the throttle valve of the carburetor and the additional air valve 19. Since the additional air valve 19 will be open at high engine speeds, there will be less undesirable reduction of the compression in the engine.

When the engine has stopped, the decompression valve 30 opens in that the spring 42 displaces the valve member 40 from its closed to its open position. It is important that the opening takes place without any essential delay in order to facilitate immediate restart of the engine, if required. Therefore, the underpressure in the valve cylinder 33 must be restored rapidly to atmospheric pressure, and to this end the valve piston 34 has a small gap 45 allowing a controlled flow of atmospheric air to pass the piston 34. The air flow is preferably selected so as to open the valve member 40 within a preferred, short period of time, for example 1-2 seconds after the engine has stopped. Instead of the gap 45, a corresponding bleed opening allowing a controlled entry of atmospheric air may be provided in the conduit 37 between the valve cylinder 33 and the check valve 38.

The present invention is not restricted to the preferred embodiment shown in the drawings but can be varied within the scope of the appended claims. As an example, the number, position and orientation of the conduits or ducts and their associated ports may be as disclosed in anyone of US 2002/0043227 Al, US 2003/ 0029398 Al, WO 2004/005692 Al, WO 2008/111880 Al and US 6,928,996 B2, if desired. For instance, the additional air valve 19 could be controlled independently from the throttle valve 14 of the carburetor.

For instance, the shown embodiment is a so called piston-ported engine considering the supply of additional air. However, the engines could have their additional air supplied directly to their transfer ducts 3 through check valves, also called Reed valves. Also in this case, the feed of air would be improved by giving improved conditions at different speeds for feeding of the additional air. INDUSTRIAL APPLICABILITY

The invention is applicable for providing a longer service life and a better function to automatic decompression valve arrangements in small crankcase-scavenged two-stroke engines used primarily in handheld working tools, such as chains aws, for example.