WO2014111142A1

WO2014111142A1 - A combined decompression and priming valve for an internal combustion engine, and an engine provided with the valve

Info

Publication number: WO2014111142A1
Application number: PCT/EP2013/050744
Authority: WO
Inventors: Joel Berneklev; Mikael Bergman
Original assignee: Husqvarna Ab
Priority date: 2013-01-16
Filing date: 2013-01-16
Publication date: 2014-07-24

Abstract

To make a small two-stroke engine of the kind used primarily in handheld working tools, such as chainsaws, for example, easier to start, it is provided with a combined decompression and priming valve (30), which can be actuated manually, or by an electronic control unit or by pressure differences arising upon rotation of the crankshaft (16) of the engine and acting on a piston (34) in a piston and cylinder assembly included in the valve (30). The combined decompression and priming valve (30) has an inlet (323) connected to an induction conduit (20 for an air-fuel mixture by a conduit (201), so as to prime the engine (1) when the downwards moving piston (7) of the engine creates an underpressure in the engine cylinder (9).

Description

A COMBINED DECOMPRESSION AND PRIMING VALVE FOR AN INTERNAL COMBUSTION ENGINE, AND AN ENGINE PROVIDED WITH THE VALVE

DESCRIPTION

TECHNICAL FIELD

The present invention relates to a combined decompression and priming valve adapted to be connected to a combustion chamber of a two- stroke internal combustion engine for reducing compression and priming the engine in order to facilitate the starting thereof.

It also relates to a two- stroke internal combustion engine having a cylinder, a reciprocating piston movable back and forth in the cylinder, a crankcase, a combustion chamber in the cylinder that has a wall and an intake port provided in the cylinder wall, an induction conduit for passing an air- fuel mixture to the intake port, and a throttle valve for controlling the amount of air- fuel mixture passing through the induction conduit.

BACKGROUND ART

Hand-held working tools usually are driven by two-stroke internal combustion engines. 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.

In this context, the term "underpressure" is used to designate a pressure below that of the surrounding atmosphere. Further, to make two- stroke internal combustion engines easier to start, they have hitherto also been primed by a start pump, usually a bulb-type pump made of rubber, which can be actuated manually to draw or purge a gaseous phase, usually referred to as fuel vapor, and stale liquid fuel from the carburetor via a fuel passage upstream of the start pump and to direct the fuel vapor and liquid fuel into a induction conduit downstream of the throttle valve of the carburetor. No priming systems passing an air-fuel fixture from an induction conduit through a separate conduit directly into the combustion chamber of a two-stroke internal combustion engine are known. Two-stroke engines with automatic decompression valves are also disclosed in US

4,619,228, US 6,892,688 B2, US 7,228,843 B2 and JP9112394A, for example. However, in no case where two-stroke engines are equipped with automatic decompression valves, a combined decompression and priming valve has been used. In fact, such a combined valve appears to be novel.

SUMMARY OF THE INVENTION

One object of the present invention is to provide a novel type of valve, namely a combined decompression and priming valve, which as is obvious from its denomination cannot just temporarily reduce the compression of the gases in the combustion chamber of an internal combustion engine by discharging a portion of said gases, but which also can prime the engine by supplying an air-fuel mixture directly into the combustion chamber in order to facilitate the starting of the engine.

This object is achieved in that such a novel combined decompression and priming valve in accordance with the present invention comprises:

a) a valve housing;

b) a valve member movable in said housing between a first position, where the valve is fully open, and a second position, where the valve is closed;

c) first means for holding the valve member in the fully open first position, a pressure created by combustion of a fuel-air mixture in the combustion chamber overcoming the holding force of the first means and moving the valve member to its second position, where the valve is closed;

d) second means for holding the valve member in the closed second position when the engine is running;

e) an outlet from the housing for discharging gases passing from the combustion chamber past the valve member when the valve is open; and

f) an inlet for a priming air-fuel mixture into the housing and past the valve member into the combustion chamber when the valve is open. In a preferred embodiment of the invention, the valve member includes a valve head and a valve stem, the valve housing has a seat for the valve head, at least one of said means includes a recess in the valve stem and a spring biased locking member in the valve housing, and the valve member (is snap locked in the fully open first position and the closed position. Such a valve is simple and reliable.

Then, the two means include together either a common spring biased locking member and two recesses that are axially spaced along the valve stem, or a common recess and two axially spaced spring biased locking members.

Preferably, the valve stem has a circular cross-section and the recess is a peripheral groove around the periphery of the stem. Then, the spring biased locking member is supplemented with two additional spring biased locking members, all three being equiangularly spaced from one another for simultaneous engagement of them in the peripheral groove.

In an inexpensive first design, the valve member is displaceable manually.

In a second design, the valve member is displaceable in response to a signal from an electronic control unit, the valve may be an automatic piezoelectric or solenoid valve.

In a third design, the valve member is displaceable in response to pressure differences between the combustion chamber and a gas conduit upstream of the combustion chamber, and the valve is an automatic pneumatically operated valve.

Usually, it is recommendable that at least one of the outlet and the inlet is provided with a check valve to permit flow in a desired direction only.

Another object of the present invention is to provide an engine that is easy to start.

This object is achieved in that the engine specified in the second paragraph above in accordance with the present invention is equipped with a combined decompression and priming valve that is connected to the combustion chamber for reducing compression and priming the engine in order to facilitate starting thereof, said valve having an inlet for a priming air-fuel mixture, a conduit being interconnected between the induction conduit and the inlet for passing an air-fuel mixture directly from the induction conduit through the valve into the combustion chamber to prime the engine. Thereby, that starting of the engine will be so facilitated that in some cases, no choke will be needed. Preferably, the combined decompression and priming valve comprises:

a) a valve housing having said inlet;

d) second means for holding the valve member in the closed second position when the engine is running; and

e) an outlet from the housing for discharging gases passing from the combustion chamber past the valve member when the valve is open. In an inexpensive first design, the valve member is displaceable manually.

In a second design, the valve member is displaceable in response to a signal from an electronic control unit, the valve may be an automatic piezoelectric or solenoid valve. In a third design, the valve member is displaceable in response to pressure differences between the combustion chamber and a gas conduit upstream of the combustion chamber, and the valve may be an automatic pneumatically operated valve.

Then, the valve preferably includes a pressure transmitting member operatively connected to the valve member, and a conduit, e.g. a hose, for transferring pneumatic pressure from an underpressure source to the pressure transmitting member to displace the valve member, a check valve is provided for preserving the underpressure so as to keep the decompression valve closed when the engine is running, and a bleed conduit and a vent are provided for eliminating the underpressure within a few seconds after the engine has stopped so as to open the valve. Thereby, the valve can operate automatically.

The pressure transmitting member is one of a piston in a piston and cylinder assembly or a membrane. A compression spring may be provided for biasing the valve member toward its open first position, and the size of the vent and the force from the compression spring are matched against the underpressure in the conduit. While a piston requires the compression spring, the resilience of a membrane may make the compression spring unnecessary. The two- stroke engine has a transfer duct that leads from the crankcase to a transfer port in the cylinder wall, and optionally a conduit ported by the engine piston for supply of additional air into the interior of the cylinder for scavenging., Then, the underpressure source suitably is one of the crankcase, the transfer duct, the air-fuel mixture conduit, and the scavenging air conduit.

To facilitate achievement of desired flow conditions, the outlet may be larger than the inlet and suitably have a diameter of 2-15 mm, preferably 5-10 mm. The gases from the outlet are preferably discharged either through a conduit to an exhaust conduit from the engine cylinder or directly to surrounding atmosphere or to a fresh air conduit downstream of an air filter but upstream of a carburetor and a possible branching of the fresh air conduit into a conduit through the carburetor and another conduit for supply of additional fresh air to the engine cylinder.

If desired, a check valve is provided for preventing gases from passing in a wrong direction through the outlet.

In a special embodiment, the outlet may also form the inlet. Then, during a first portion of the period of time that the valve is open, gases from the combustion chamber are discharged through the valve and the conduit into the induction conduit, while during a second portion of the period of time the flow through the conduit is reversed and air-fuel mixture from the induction conduit is passed through the valve directly into the combustion chamber.

Suitably, the volume of the gases leaving the combustion chamber through the combined automatic decompression and priming valve is at least twice the size of the internal volume of the conduit connecting said inlet with the induction conduit at a position downstream of the throttle valve. Preferably, ratio between the volumes is at least 7: 1.

Further, to ensure optimum conditions during the start of the engine, the combined automatic decompression and priming valve is mounted to direct the flow of priming air- fuel mixture toward the vicinity of the spark. When the conduit connecting the inlet with the induction conduit is a hose, the hose suitably has an internal diameter of 1-10 mm, preferably 6 mm, and suitably a length of 5- 25 cm, preferably 10 cm. To get a desired mixing ratio in the air- fuel mixture to be passed directly from the induction conduit through the valve into the combustion chamber, the conduit from the induction conduit to the inlet of the valve preferably starts at a position downstream of the throttle valve.

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 side view of a manual decompression and priming valve shown partly in cross- section and having a two-groove snap lock with a spring biased locking member for locking the valve in a fully open and a closed position.

Fig. 2 is a side view of a manual decompression and priming valve shown partly in cross- section and having a single-groove snap lock with axially spaced spring biased locking members for locking the valve in a fully open and a closed position.

Fig. 3 is a cross-sectional view of the manual decompression and priming valve shown either of Figs. 1 and 2 and showing the spring biased locking members.

Fig. 4 is a side view of a solenoid-type decompression and priming valve shown partly in cross-section.

Fig. 5 is a side view of a pneumatically operated decompression and priming valve shown partly in cross-section and having a compression spring trying to keep the valve open and a piston and cylinder assembly trying to close the valve against the force of the spring.

Fig. 6 is a is a side view of a pneumatically operated decompression and priming valve shown partly in cross-section and having a compression spring trying to keep the valve open and a membrane trying to close the valve against the force of the spring.

Fig. 7 is a simplified principle sketch of a crankcase-scavenged two-stroke internal

combustion engine having an automatic decompression and priming valve arranged in the cylinder head of the engine in accordance with one preferred embodiment of the present invention. Fig. 8 is a simplified principle sketch of a crankcase- scavenged two-stroke internal combustion engine having an automatic decompression and priming valve arranged in the cylinder wall in a position close to the cylinder head of the engine in accordance with another preferred embodiment of the present invention.

Fig. 9 is a partial section of a two-stroke internal combustion engine incorporating an

automatic decompression and priming valve of the type shown in Fig. 5 but having a separate outlet for exhaust gas and a separate inlet for priming air- fuel mixture according to a preferred embodiment of the present invention.

Fig. 10 is a partial section similar to Fig. 9 but having a first check valve provided in the outlet for exhaust gas.

Fig. 11 is a partial section similar to Fig. 10 but having a second check valve provided in the inlet for exhaust priming air-fuel mixture.

MODE(S) FOR CARRYING OUT THE INVENTION

Fig. 1 is a side view of a decompression and priming valve 30 in accordance with a first preferred embodiment of the present invention and shown partly in cross-section. The valve 30 comprises a valve housing 32 and a valve member 40 that is axially movable in the housing 32 between a first position, where the valve 30 is fully open, and a second position, where the valve 30 is closed. The valve member 40 has a valve head 401 and a valve stem 39 having a first end carrying the valve head 401. The housing 32 has a seat 41, against which the head 401 seals when the valve 30 is closed. Further, the housing 32 has a tubular first end, which radially outside the seat 41 has an external thread 321 for threaded engagement with a mating threaded through bore in a cylinder head or cylinder wall of a two-stroke internal combustion engine. In addition, the housing 32 has an outlet 44 for exhaust gas from the combustion chamber of the engine and an inlet 323 for a priming air- fuel mixture from a suitable source, such as an induction conduit downstream of a throttle valve of a carburetor, for example, to be passed to the combustion chamber. In both directions, the gases are passed through a radial space between the valve stem 39 and an inner wall of the housing 32. If desired and as shown, the outlet 44 may be provided with a check valve 441 to prevent gas from being sucked back through the outlet into the valve 32. Similarly, the inlet 323 may be provided with a check valve 325 to prevent exhaust gas from trying to leave the valve housing 32 through the inlet 323. To facilitate achievement of desired flow conditions, the outlet 44 may be larger than the inlet 323 and suitably have a diameter of 2-15 mm, preferably 5-10 mm. The valve shown in Fig. 1 is designed to be operated manually. The valve stem 39 has a second end, which extends out of the housing 32 and is provided with a knob 403 or the like that offers a grip for the fingers of an operator. Further, the valve 30 is provided with first means 326, 405 for holding the valve member 40 in the fully open first position. A pressure created by combustion of a fuel-air mixture in the combustion chamber 13 will overcome the holding force of the first means and move the valve member to its second position, where the valve 30 is closed. Further, the valve 30 is provided with second means 326, 404 for holding the valve member 40 in the closed second position when the engine is running. In the embodiment shown in Fig. 1 the first and second means are formed by a conventional snap lock mechanism that includes two spaced circumferential grooves 404 and 405 provided on the valve stem 39 and a spring biased locking member 326, e.g. a ball, located in the wall of the housing 32 for engaging in either of the grooves 404 and 405 and thereby locking the valve in a fully open and a closed position. In the preferred embodiment shown in the cross-sectional view of Fig. 3, the snap lock mechanism includes three equiangularly spaced spring biased balls 326. A tubular thin sheet metal cover 327 around the area, where the bores in which the spring biased locking members are located, retains the springs in their intended places and maintains the intended biasing force.

Fig. 2 shows an alternative embodiment, which differs from that of Fig. 1 in that the valve stem 39 is provided with a single circumferential groove 404, and instead there are two axially spaced sets of spring biased locking balls 326 or the like. The single circumferential groove 404 and the left-hand set of spring biased locking members 326 form the first means for holding the valve member 40 in the fully open first position, while the second means is formed by the single groove 404 and the right-hand set of spring biased locking members 326. As before, the valve 30 will be locked in its fully closed and its fully open positions.

When an operator wants to start a two-stroke internal combustion engine provided with the manual decompression and priming valve, he presses the knob 403 to move the valve member 40 from its closed to its open position, where it is locked by the snap lock mechanism. Then he pulls the starting cord of the engine. As the combined decompression and priming valve 30 is open, exhaust gas from the combustion chamber will pass through the valve 30 and be discharged through its outlet 44. When the engine piston has moved so far that suction is created in the engine cylinder, a priming air-fuel mixture from a suitable source will be sucked through the inlet 323 into the valve 30 and further into the cylinder. When the spark from the spark plug ignites the air-fuel mixture in the combustion chamber, the created pressure wave will overcome the holding force of the snap lock mechanism and close the valve 32. Then, the valve will be closed until the operator will start the engine a second time and once more presses the knob 403 to move the valve member 40 from its closed to its open position.

Instead of being manually operated, the decompression and priming valve may be a solenoid valve or a piezoelectric valve operated by signals from an electronic control unit, not shown. Fig. 4 is a simplified principle sketch of a solenoid-type decompression and priming valve 30 shown partly in cross-section. Here, the combination of a solenoid 46 and a compression spring 42 supported by an end wall 35 of the housing 32 and acting on the end surface of the second end of the valve stem 39 is substituted for the push knob 403 and the snap lock mechanism, i.e. grooves 404, 405 and spring biased locking member 326. The signals from the electronic control unit are generated only when the crankshaft of the engine rotates, and they decide whether the solenoid 46 will keep the valve member 40 in its closed position by pulling against the force of the compression spring 42, or the compression spring 42 will keep the valve member 40 in its open position. Thus, in this embodiment the first means is formed by the compression spring 42, and the second means by the solenoid 46 (actuated by the electronic control unit).

In another preferred embodiment, the decompression and priming valve is operated pneumatically, i.e. by differences in gas pressure. Fig. 5 is a side view of a pneumatically operated decompression and priming valve 30 shown partly in cross-section and having a compression spring 42 trying to keep the valve 30 open and a piston and cylinder assembly trying to close the valve 30 against the force of the spring 42. The piston and cylinder assembly includes a piston 34 fixed to the second end of the valve stem 39, and the piston 34 is axially movable in a matching cylinder 33 formed by the inside of a wall of the housing 32. Further, the housing 32 has an end wall 35, and the compression spring 42 is placed between the piston 34 and the end wall 35 to keep the distance between them as large as possible. The end wall 35 is provided with a nipple 36 or other suitable connector for a conduit, e.g. a hose, not shown, connecting the piston and cylinder assembly to a suitable underpressure source, not shown. To ensure that atmospheric pressure will always be present on the left-hand side of the piston 34 in Fig. 5, the left-hand end of the cylinder 33 is provided with a small bore 43 to surrounding atmosphere. Consequently, in this embodiment the first means is formed by the compression spring 42, and the second means includes the piston and cylinder assembly 33, 34 (actuated by the underpressure source (generally designated 50, Figs. 9-11) via the interconnecting conduit 37 as shown in Figs. 9-11).

In Fig. 5, the valve member 40 is shown in its open position, in which gases from the combustion chamber of the engine may pass the valve head 401 and be discharged to atmosphere via the outlet (generally designated 44). The opening 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 piston has moved so far that suction is created in the engine cylinder, a priming air-fuel mixture from a suitable source will be sucked through the inlet 323 into the valve 30 and further into the engine cylinder. When a spark from the spark plug ignites the air-fuel mixture in the combustion chamber and starts the engine, the created pressure wave in combination with the underpressure acting on the right-hand side of the piston 34 will overcome the force of the compression spring 42 and close the valve 32 by moving the valve member 40 to make the valve head 401 seal against the valve seat 41.

When the engine has stopped, the combined decompression and priming 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 suitably 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. Preferably, the combined decompression and priming valve 30, which in this case includes the source of underpressure 50 and the conduit 37, is arranged such that the valve 30 opens already at an idle speed of the engine or close to idle speed. This is favorable as the combustion chamber will then be primed through the valve 30, i.e. added with fuel, when running on idle, which e.g. implies a smoother run on idle. Said idle speed is typically chosen about 1000 rpm, but can of course be chosen differently such as a speed in the range 1000 to 1500 rpm or in the range 800 to 1000 rpm. Instead of the gap 45, a corresponding bleed opening allowing a controlled entry of atmospheric air may be provided in the conduit between the valve cylinder 33 and the underpressure source (50 Figs. 9-11).

Fig. 5 also shows an embodiment where the valve housing 32 is modified in that the outlet 44 for exhaust gas from the combustion chamber and the inlet 323 for a priming air- fuel mixture to be passed to the combustion chamber are combined. Consequently, with such a valve, compressed exhaust gas from the combustion chamber will pass through the valve 30 and be discharged through its combined inlet-outlet through a conduit to a source of air-fuel mixture. When the engine piston has moved so far that suction is created in the engine cylinder, a priming air- fuel mixture from a suitable source will be sucked through the conduit, the combined inlet-outlet into the valve 30 and further into the engine cylinder. Such a design is possible when the total internal volume of the gas passage between the engine cylinder and the source of air-fuel mixture is small in comparison to the volumes of exhaust gas and priming air-fuel mixture passing through the valve 32.

Fig. 6 shows an embodiment that basically is similar to that of Fig. 5, but a membrane 47 is substituted for the piston and cylinder assembly. The membrane 47 has a center that is attached to the second end of the valve stem 39 and a periphery that is clamped in the cylinder 33 formed by the inside of the wall of the housing 32. Like in the other

embodiments, a compression spring 42 may be provided to keep to valve 32 open, but if desired, the spring may be omitted and the membrane 47 made so rigid, that a deformation of the membrane 47 by underpressure on its right-hand side in Fig. 6 will bias the valve member 40 toward its open position and eliminate the need for the compression spring 42. To ensure that underpressure will not remain on the right-hand side of the membrane 47 in Fig. 6 when the engine has stopped, the right-hand end of the cylinder 33 is provided with a small bore 43 to surrounding atmosphere. The bore 43 is so small, that it does not affect the operation of the valve 32, when the engine is running, and air entering through the bore 43 is immediately sucked away to the underpressure source (50, Figs. 9-11). Further, in the embodiment shown in Fig. 6, the inlet 323 for a priming air-fuel mixture and the outlet 44 for exhaust gas from the combustion chamber are separated from each other, and no check valves are provided therein.

Fig. 7 is a simplified principle sketch of a crankcase-scavenged two-stroke internal combustion engine 1 having an automatic decompression and priming valve 30 arranged in the cylinder head of the engine 1 in accordance with one preferred embodiment of the present invention. The automatic decompression and priming valve 30 shown in Fig. 7 could be either one of the two of Figs. 5 and 6, and with an obvious, slight modification of the maneuvering of the valve 30, also any one of the valves of Figs. 1-4 could be used.

The engine 1 shown in Fig. 7 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 induction 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 underpressure 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. The carburetor 12 and the additional air conduit 21 receive clean air from a single air filter 48 via an air manifold 49.

To facilitate starting, the engine 1 is provided with an automatic decompression and priming valve 30 that in Fig. 7 is actuated by underpressure from the crankcase volume 4, but underpressure from any other suitable underpressure source (50, Figs. 9-11), such as the transfer duct 3, the air-fuel mixture conduit 20, and the scavenging air conduit 21, for example, could be used. In Fig. 7, a conduit 37 connecting the underpressure source 50 (3; 4; 20; or 21) to the decompression and priming valve 30 is shown as being a hose or a pipe, which has a check valve (38, Figs. 9-11) permitting flow toward the underpressure source 50 (3; 4; 20; or 21) but blocking flow in the opposite direction. This will be described more in detail in connection with Figs. 9-11.

To enable the decompression task of the valve 30, the pressurized exhaust gases from the combustion chamber 13 usually leave the valve either through the outlet 44 directly to the surrounding atmosphere or via a conduit (not shown) to the exhaust conduit 6 from the engine cylinder 9, or to the clean air manifold 49 downstream of the air filter 48.

Similarly, to enable the priming task of the valve 30, the inlet 323 for priming air- fuel mixture into the valve 30 is connected to the induction conduit 20 downstream of the throttle valve 14 via a conduit 201. As described above with reference to Fig. 5, the outlet 44 of the valve 30 for exhaust gas from the combustion chamber and the inlet 323 for a priming air-fuel mixture to be passed to the combustion chamber may be combined.

Consequently, with such a valve, compressed exhaust gas from the combustion chamber will pass through the valve 30 and be discharged through its combined inlet-outlet through the conduit 201 to the induction conduit 20. When the engine piston 7 has moved so far that suction is created in the engine cylinder 9, a priming air-fuel mixture from the induction conduit 20 will be sucked through the conduit 201, the combined inlet-outlet, into the valve 30 and further into the engine cylinder 9. Such a design is possible when the total internal volume of the gas passage between the engine cylinder 9 and the induction conduit 20 is small in comparison to the volumes of exhaust gas and priming air-fuel mixture passing through the valve 32. Suitably, the volume of the gases leaving the combustion chamber 13 through the combined automatic decompression and priming valve 30 is at least twice the size of the internal volume of the conduit 201 connecting the inlet 323 with the induction conduit 20 at a position downstream of the throttle valve 14. Preferably, the ratio between the volumes is at least 7: 1. In the embodiment shown in Fig. 7, the conduit connecting the inlet 323 with the induction conduit 20 is a hose 201, and then the hose 201 suitably has an internal diameter of 1-10 mm, preferably 6 mm, and suitably a length of 5-25 cm, preferably 10 cm. However, if desired, the conduit 201 may be embedded in the material of the engine cylinder 9. To facilitate achievement of desired flow conditions when a separate outlet and a separate inlet are used, the outlet 44 may be larger than the inlet 323 and suitably have a diameter of 2-15 mm, preferably 5-10 mm.

To achieve an efficient use of the priming air-fuel mixture supplied through the combined decompression and priming valve 30, the valve should be designed and placed in manner that the flow of priming air-fuel mixture from the valve 30 into the engine cylinder 9 will be directed toward the vicinity of the electrodes of the spark plug 26. If desired, the valve head 401 may be of a shape that is more like a plug than like a disk. The embodiment shown in Fig. 8 differs from that of Fig. 7 only in that the combined decompression and priming valve 30 is not mounted in the cylinder head but in a top portion of the cylinder 9 just below the cylinder head.

Figs. 9-11 are enlarged and more detailed of a section of Fig. 8 and show that the combined decompression and priming 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 combined decompression and priming valve 30 is basically of the type shown in Fig. 5, but is also equipped with a separate outlet 44 for exhaust gases from the engine cylinder 9. The 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 via a conduit 37 to an underpressure source (50, Figs. 9-11), which may be any one of the crankcase volume 4, the transfer duct 3, the induction conduit 20, or the conduit 21 for scavenging fresh air. The conduit 37 has a check valve 38 therein. The check valve 38 permits air flow in a direction from the combined decompression and priming valve 30 toward the underpressure source (50, Figs. 9-1 l)and prevents air flow from the underpressure source toward the combined decompression and priming valve 30. If the check valve 38 in conduit 37 risks becoming exposed to small amounts of oil in the passing flow of gas so that the proper functioning of the valve will be impaired, it is

recommendable to select or the conduit 21 for scavenging fresh air as underpressure source 50. The piston 34 is connected by means of a valve stem 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.

While in Fig. 9 the outlet 44 and the inlet 323 are free from flow blocking or restricting components, Fig. 10 shows an embodiment where the outlet 44 is provided with a check valve 441 to prevent gas from being sucked back through the outlet into the valve 32. In the embodiment of Fig. 11, also the inlet 323 is provided with a check valve 325 to prevent exhaust gas from trying to leave the valve housing 32 through the inlet 323. Of course, the embodiment that uses a combined inlet and outlet (Fig. 5) cannot use any one of the check valves 325 and 441.

In Figs. 9-11, 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 at least indirectly to atmosphere via at least one radial bore 44 in the valve housing 32. The outlet 44 may be connected via a passage (not shown) to the left-hand end of the valve 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 underpressure source 50 (3; 4; 20; or 21), 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 combined decompression and priming 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 combined decompression and priming 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 embodiments 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. Also, although the present invention above is described with reference to a carburetor engine, it is applicable to an injection engine.

INDUSTRIAL APPLICABILITY

The invention is applicable for providing small two-stroke engines used primarily in handheld working tools, such as chainsaws, for example, with a combined decompression and priming valve to make them easier to start.

Claims

A combined decompression and priming valve (30) adapted to be connected to a combustion chamber (13) of a two-stroke internal combustion engine (1) for reducing compression and priming the engine (1) in order to facilitate the starting thereof, comprising:

a) a valve housing (32);

b) a valve member (40) movable in said housing (32) between a first position, where the valve (30) is fully open, and a second position, where the valve (30) is closed; c) first means (326, 405; 326, 404; 42) for holding the valve member (40) in the fully open first position,

d) an outlet (44) from the interior of the housing (32) for discharging gases passing from the combustion chamber (13) past the valve member (40) when the valve (30) is open; and

e) an inlet (323) for leading priming air-fuel mixture into the interior of the housing (32) such that the air-fuel mixture can continue past the valve member (40) and into the combustion chamber (13) when the valve (30) is open.

A valve as claimed in claim 1, wherein at least one of the outlet (44) and the inlet (323) is provided with a check valve (325).

A valve as claimed in any one of the claims 1 or 2, wherein said outlet (44) is larger than said inlet (323) and suitably has a diameter of 2-15 mm, preferably 5-10 mm.

A valve as claimed in any one of the claims 1 or 2, wherein said outlet (44) also forms said inlet (323).

A valve as claimed in any one of the preceding claims, wherein the first means (42) is a spring (42), preferably a compression spring, biasing the valve member (40) towards its first open position.

A valve as claimed in any one of the preceding claims including a pressure transmitting member (34; 47) operatively connected to the valve member (40), said member (34; 47) defining a decreasable volume together with interior walls of the housing (32), wherein said volume is adapted to be connected to a source of underpressure (50) via a conduit (37), and the pressure transmitting member (34; 47) being displaceable in response to a pressure drop inside the source of underpressure (50).

7. A valve as claimed in claim 6, wherein said source of underpressure (50) is a gas conduit (3; 4; 20; 21) upstream of the combustion chamber (13), preferably a crank case volume (4). 8. A valve as claimed in claim 6 or 7 comprising a bleed conduit (43) connected to the decreasable volume for connecting said volume with the atmosphere.

9. A valve as claimed in any one of the claims 6-8, wherein the pressure transmitting member (34; 47) comprises a membrane (47) which forms a seal between the decreasable volume and the combustion chamber (13).

10. A valve as claimed in any one of the claims 6-8, wherein the pressure transmitting member (34; 47) comprises a piston (34) of a piston and cylinder assembly. 11. A valve as claimed in any of the claims 1-4 including second means (326, 404) for holding the valve member (40) in the closed second position when the engine is running, wherein the valve member (40) includes a valve head (401) and a valve stem (39), the valve housing (32) has a seat (41) for the valve head (401), at least one of said first- or second means (326, 404; 326, 405) includes a recess (404) in the valve stem (39) and a spring biased locking member (326) in the valve housing (32), and the valve member (40) is snap locked in the fully open first position and the closed position.

12. A valve as claimed in claim 11, wherein the two means (326, 404; 326, 405) include together either a common spring biased locking member (326) and two recesses (404, 405) that are axially spaced along the valve stem (39), or a common recess (404) and two axially spaced spring biased locking members (326).

13. A valve as claimed in claim 11 or 12, wherein the valve stem (39) has a circular cross- section and the recess is a peripheral groove (404, 405) around the periphery of the stem (39).

14. A valve as claimed in claim 13, wherein the spring biased locking member (326) is supplemented with two additional spring biased locking members (326), all three being substantially equiangularly spaced from one another for simultaneous engagement of them in the peripheral groove (404, 405).

15. A valve as claimed in any one of claims 11-14, wherein the valve member (40) is

manually displaceable.

16. A valve as claimed in any one of the claims 1-5, wherein a solenoid valve (46) is arranged to displace the valve member (40) to the second closed position in response to a signal from an electronic control unit.

17. An engine including the valve (30) according to any one of the preceding claims.

18. A two-stroke internal combustion engine (1) having a cylinder (9), a crankcase (17), and a reciprocating piston (7) movable back and forth in the cylinder (9) and defining a combustion chamber (13) in a top portion of the cylinder (9), , the cylinder (9) having a cylinder wall (29) and an intake port (27) provided in the cylinder wall (29), the engine further having an induction conduit (20) for passing an air-fuel mixture to the intake port (27), and a throttle valve (14) for controlling the amount of air-fuel mixture passing through the induction conduit (20), c h a r a c t e r i z e d i n t h a t the engine comprises a combined decompression and priming valve (30) connected to the combustion chamber (13) for reducing compression and priming the engine (1) in order to facilitate starting thereof, said valve (30) having an inlet (323) for a priming air-fuel mixture, and a conduit (201) being interconnected between the induction conduit (20) and the inlet (323) for passing an air-fuel mixture directly from the induction conduit (20) through the valve (30) into the combustion chamber (13) to prime the engine (1) when the valve (30) is open.

19. An engine as claimed in claim 18, wherein the combined decompression and priming valve (30) comprises:

a) a valve housing (32) having said inlet (323);

b) a valve member (40) movable in said housing (32) between a first position, where the valve (30) is fully open, and a second position, where the valve (30) is closed; c) first means (326, 405; 326, 404; 42) for holding the valve member (40) in the fully open first position, a pressure created by combustion of a fuel-air mixture in the combustion chamber (13) overcoming the holding force of the first means (326, 405) and moving the valve member (40) to its second position, where the valve (30) is closed;

d) second means (326, 404; 37, 50) for holding the valve member (40) in the closed second position when the engine is running; and

e) an outlet (44) from the housing (32) for discharging gases passing from the

combustion chamber (13) past the valve member (40) when the valve (30) is open.

20. An engine as claimed in claim 19, wherein the valve member (40) is manually displaceable.

21. An engine as claimed in claim 19, wherein the valve member (40) is displaceable in response to a signal from an electronic control unit.

22. An engine claimed in claim 19, wherein the valve member (40) is displaceable in

response to pressure differences between the combustion chamber (13) and a gas conduit (3; 4; 20; or 21) upstream of the combustion chamber (13).

23. An engine as claimed in claim 22, wherein the valve (30) includes

-a pressure transmitting member (34; 47) operatively connected to the valve member (40), -a conduit (37) for transferring pneumatic pressure from an underpressure source (50) to the pressure transmitting member (34; 47) to displace the valve member (40),

-a check valve (38) for preserving the underpressure so as to keep the decompression valve

(30) closed when the engine (1) is running, and

-a bleed conduit (45) and a vent (43) for eliminating the underpressure within a few seconds after the engine (1) has stopped so as to open the valve (30). 24. An engine as claimed in claim 23, wherein the pressure transmitting member (34; 47) is one of a piston (34) in a piston and cylinder assembly or a membrane (47).

25. An engine as claimed in claim 24, wherein a compression spring (42) is provided for biasing the valve member (40) toward its open first position, and the size of the vent (43) and the force from the compression spring (42) are matched against the

underpressure in the conduit (37).

26. An engine as claimed in any one of claims 23-25, wherein the conduit (37) is a hose. 27. An engine as claimed in any one of claims 23-26, wherein the engine has a transfer duct (3) that leads from the crankcase (17) to a transfer port (5) in the cylinder wall (29), and optionally a conduit (21) ported by the engine piston (7) for supply of additional air into the interior of the cylinder (9) for scavenging, and wherein the underpressure source (50) is one of the crankcase volume (4), the transfer duct (3), the air-fuel mixture conduit (20), or the scavenging air conduit (21).

28. An engine as claimed in any one of claims 23-27, wherein said outlet (44) is larg

said inlet (323) and suitably has a diameter of 2-15 mm, preferably 5-10 mm.

29. An engine as claimed in any one of claims 19-28, wherein gases from the outlet (44) are discharged either through a conduit to an exhaust conduit (6) from the engine cylinder (9) or directly to surrounding atmosphere or to a fresh air conduit (49) downstream of an air filter (48) but upstream of a carburetor (12) and possibly upstream of a possible branching of the fresh air conduit into a conduit (20) through the carburetor (12) and another conduit (21) for supply of additional fresh air to the engine cylinder (9).

30. An engine as claimed in any one of claims 19-29, wherein a check valve (441) is

provided for preventing gases from passing in a wrong direction through the outlet (44).

31. An engine as claimed in any one of claims 19-27, wherein said outlet (44) also forms said inlet (323).

32. An engine as claimed in claim 31, wherein the volume of the gases leaving the

combustion chamber (13) through the combined automatic decompression and priming valve (30) is at least twice the size of the internal volume of the conduit (201)

connecting said inlet (323) with the induction conduit (20) at a position downstream of the throttle valve (14).

33. An engine as claimed in claim 32, wherein the ratio between the volumes is at least 7: 1.

34. An engine as claimed in any one of claims 18-33, wherein the engine has a spark plug (26) for forming an igniting spark, and the combined automatic decompression and priming valve (30) is mounted to direct the flow of priming air-fuel mixture toward the vicinity of the spark.

35. An engine as claimed in any one of claims 18-34, wherein the conduit connecting said inlet (323) with the induction conduit (20) is a hose (201), suitably having an internal diameter of 1-10 mm, preferably 6 mm, and suitably a length of 5-25 cm, preferably 10 cm.

36. An engine as claimed in any one of claims 18-35, wherein a throttle valve (14) is

provided in the induction conduit (20), and the conduit (201) for the priming air-fuel mixture is connected to the induction conduit (20) at a point downstream of the throttle valve (14).