STRATIFIED CHARGED TWO-STROKE ENGINE COMPRISING AIR HEAD CHANNELS

TECHNICAL FIELD

The present disclosure relates to two-stroke combustion engines comprising reed valves in combination with stratified scavenging systems. The disclosure also relates to stratified scavenging systems in general, and to methods for scavenging in two-stroke combustion engines.

BACKGROUND

A two-stroke engine comprises a scavenging system. The scavenging system discharges exhaust gases from the combustion chamber via one or more exhaust ports and draws in fresh air/fuel mixture into the combustion chamber via one or more air/fuel mixture intake ports in preparation for the next engine cycle. The design and overall dimensioning of the scavenging system significantly impacts the performance of the engine with respect to, e.g., output power characteristics, fuel efficiency, and emission levels.

Stratified scavenging refers to scavenging systems where a portion of pure air, herein referred to as airhead, first enters the cylinder, followed by the air/fuel mixture. The portion of pure air can be configured to enter the combustion chamber via an airhead port in a way such that exhaust gases are discharged with a minimum of fresh air/fuel mixture escaping the combustion chamber during scavenging, thus lowering unwanted emission by the engine.

On two-stroke engines with stratified scavenging, there is often a compromise between the width of the intake port, the airhead ports, and the scavenging ports, due to the limited space available on the cylinder wall. Space restriction is especially severe on smaller two-stroke engines used for handheld power tools. Conventional pistons for stratified scavenging normally comprise airhead channels which require pistons that accommodate the extra material needed for airhead channels. This extra weight is a drawback in two-stroke engines used for handheld power tools, where vibration is always an issue.

W01999058829 A1 discloses a crankcase scavenged two-stroke engine comprising reed valves.

US6513464 B1 discloses a stratified two cycle engine which comprises a reed cage assembly arranged in intake manifolds of the combustion engine.

There is a need for further improved stratified charge two-stroke combustion engines, such as two-stroke engines suitable for handheld power tools where space in the housing is scarce and where a high power to weight ratio of the combustion engine is wanted.

SUMMARY

It is an object of the present disclosure to provide combustion engines, power tools, and methods which alleviate at least some of the problems mentioned above.

This object is at least in part obtained by a stratified charged two-stroke engine comprising a crankcase, a cylinder, a piston arranged to reciprocate in the cylinder, a first inlet channel suitable for an air/fuel (AF) mixture, and a second inlet channel adapted for air (AH). The first and second inlet channels are connected to a common reed valve cage that comprises a first reed valve assembly and a second reed valve assembly. The reed valve assemblies are separated by a partition creating a first reed channel and a second reed channel through the common reed valve cage. The first reed channel is connected to the first inlet channel and the second reed channel is connected to the second inlet channel, such that the first reed valve assembly is essentially connected to the first inlet channel only and the second reed valve assembly is essentially connected to the second inlet channel only. This way a single reed valve cage is used instead of two or more separate reed valve cages. This saves space which can be utilized for other purposes and provides for a slimmer overall engine design. It also provides for a more efficient manufacturing process, since only one reed valve cage needs to be assembled.

The separation partition separates resonance systems associated with air/fuel mixture intake and pure air intake, which simplifies optimization of the engine scavenging system.

The reed valve induction system provides for better engine performance, both at low and at high engine speeds.

The disclosed engine can also be equipped with an optional boost channel system, since a conventional air/fuel mixture intake port can be moved or removed, which provides improved acceleration performance and engine response.

The reed valve induction system significantly reduces spit back by a carburetor, which makes it possible to mount an air filter closer to the carburetor, which is an advantage since more space efficient designs can be achieved.

The disclosed engine also allows for an increased port duration due to the induction system, which again potentially increases engine performance.

According to aspects, the carburetor is a split-carburetor comprising a throttle valve, a first carburetor channel connected to the first reed channel, and a second carburetor channel separated from the first carburetor channel and connected to the second reed channel, wherein the throttle valve is arranged to regulate a flow of the air/fuel mixture in the first carburetor channel and a flow of the air in the second carburetor channel.

The split-carburetor is space and cost efficient in that the throttle valve regulates flows in the first and in the second carburetor channels.

According to aspects, the stratified charged two-stroke engine comprises one or more boost channels arranged as conduits for the air/fuel mixture from the first inlet channel and/or crankcase into the cylinder or arranged as conduits for the air/fuel mixture from the first inlet channel into the crankcase.

The boost channels provide increased engine performance, which is an advantage.

According to aspects, the piston comprises a first opening arranged to align with a corresponding first opening in the cylinder connected via a boost channel to a boost port of the cylinder, thereby allowing a gas flow through the piston to the boost port when the first opening of the piston is aligned with the first opening of the cylinder.

This flow of gas via the piston first opening provides for increased engine performance due to the improved scavenging operation, and provides for improved cooling of the piston, since the flow of gas via the piston transports heat away from the piston.

There is also disclosed herein a stratified charged two-stroke engine comprising a crankcase, a cylinder, a piston arranged to reciprocate in the cylinder, a first inlet channel adapted for an air/fuel mixture, and a second inlet channel adapted for air, wherein the first inlet channel comprises a reed valve and is connected to the crankcase and wherein the second inlet channel is arranged to be controlled by the piston. The reed valve is arranged to provide a first flow amount of air/fuel mixture measured relative to a first flow amount of pure air at a low engine speed, and a second flow amount of air/fuel mixture measured relative to a second flow amount of pure air at a high engine speed, wherein the first flow is larger than the second flow.

A crankcase mounted air/fuel mixture induction system makes it possible to increase the width of airhead ports in the cylinder wall, since the conventional air/fuel mixture intake port is no longer needed. This increases the amount of airhead that can be supplied which lowers emissions and cools the engine since an increased amount of hot residual exhaust gas is pushed out by the relatively cool auxiliary air.

Also, the engine draws a larger amount of air/fuel mixture when beneficial for engine performance at low engine speeds, and gradually switches to a leaner mixture with larger airhead flow at higher engine speeds, which gives more power and at the same time lower emissions.

According to aspects, the stratified charged two-stroke engine comprises one or more boost channel ports arranged in-between airhead ports in the cylinder. The boost channels provide increased engine performance, which is an advantage especially in combustion engines for handheld power tools.

Some of the objects mentioned above are also obtained by an air head system for a stratified charged two-stroke engine. The engine comprises a piston with one or more second openings arranged to connect a scavenging port of the engine with a crankcase of the engine. The air head system comprises one or more air head channels connected to respective scavenging ducts, wherein each air head channel is connected to its respective scavenging duct at a connection point located between a crankcase connection of the scavenging duct and a scavenging port of the scavenging duct, whereby an airhead channel is connected to the crankcase via its respective scavenging duct and via the piston when a second opening is aligned with the scavenging port, thereby allowing air to fill the scavenging ducts in direction of the cylinder.

This way pure air is deployed in the scavenging ducts in a controlled manner to provide efficient stratified scavenging.

According to aspects, an air-head channel extends in a volume located between the cylinder wall and the one or more scavenging ducts.

This arrangement conserves space, which is an advantage especially when the air head system is used in handheld power tools where space is scarce.

Some objects are also obtained by a stratified charged two-stroke engine comprising a piston and first and second scavenging ducts. The piston comprises a recess in the piston skirt arranged to connect a second inlet channel adapted for air to the first and to the second scavenging ducts.

This way a flow of air from the second inlet channel is efficiently distributed between the first and second scavenging ducts. According to aspects, the second inlet channel is arranged at an incoming angle with respect to a radial line of the piston, wherein the angle is arranged to control relative portions of air entering the first and second scavenging ducts.

Thus, the relative amount of air flowing to the different scavenging ducts can be controlled. This provides for increased degrees of freedom in engine scavenging system design.

According to aspects, the recess comprises a ridge arranged to control relative portions of air entering the first and second scavenging ducts.

Thus, the relative amount of air flowing to the different scavenging ducts can be controlled. This provides for increased degrees of freedom in engine scavenging system design.

There are also disclosed herein construction equipment, power tools and methods associated with the above-mentioned advantages.

BRIEF DESCRIPTION OF THE DRAWINGS

The present disclosure will now be described in more detail with reference to the appended drawings, where

Figure 1 schematically illustrates a two-stroke combustion engine;

Figure 2 shows an example of a split carburetor;

Figures 3-5 schematically illustrate two-stroke combustion engines with fuel injectors;

Figures 6-7 show reed valve arrangements connected to throttle valves; Figures 8-9 schematically illustrate two-stroke combustion engines;

Figure 10 shows a piston arranged to reciprocate in a cylinder;

Figures 1 1 -12 show examples of stratified scavenging systems;

Figure 13 schematically illustrates a two-stroke combustion engine;

Figure 14 illustrates a stratified scavenging system; Figures 15-16 show example ducts in a two-stroke combustion engine;

Figure 17 schematically illustrates a stratified scavenging system;

Figure 18 shows ports in a cylinder wall of a two-stroke combustion engine; Figures 19-20 schematically illustrate details of stratified scavenging systems; Figure 21 schematically illustrates a power tool;

Figures 22-23 schematically illustrate an example reed valve assembly; and Figures 24-27 are flow charts illustrating methods.

DETAILED DESCRIPTION

Aspects of the present disclosure will now be described more fully hereinafter with reference to the accompanying drawings. The different devices and methods disclosed herein can, however, be realized in many different forms and should not be construed as being limited to the aspects set forth herein. Like numbers in the drawings refer to like elements throughout.

The terminology used herein is for describing aspects of the disclosure only and is not intended to limit the invention. As used herein, the singular forms "a", "an" and "the" are intended to include the plural forms as well, unless the context clearly indicates otherwise.

Reed valves are a type of check valve which restricts the flow of fluids to a single direction, opening and closing under changing pressure on each face reed valves are commonly used in high-performance versions of the two- stroke engine, where they control the fuel-air mixture admitted to the crankcase. As the piston rises in the cylinder a vacuum is created in the crankcase beneath the piston. The resulting pressure differential opens a reed valve and the fuel-air mixture flows into the crankcase. As the piston descends, it raises the crankcase pressure causing the valve to close to retain the mixture and pressurize it for its eventual transfer through to the combustion chamber.

The use of reed valves together with stratified scavenging in two-stroke engines for handheld power-tools is not common. There are several advantages associated with combining stratified scavenged two-stroke engines with reed valve induction in smaller two-stroke engines for handheld power tools, as will be detailed below.

Herein, the terms stratified scavenging and air head system will be used interchangeably. The air used for air head is referred to as air, pure air (as opposed to the air/fuel mixture), or auxiliary air, interchangeably.

Herein, an opening in the cylinder wall is referred to as a port. Ports used for scavenging are herein referred to as scavenging ports. Such scavenging ports are sometimes referred to as transfer ports in the literature. Sometimes a port is referred to by the channel which ends in the port, the port in question will then be clear from context.

Herein, space efficient, space is scarce, or to conserve space, refers to the often-limited space in a housing of a power tool in which a combustion engine according to the present teaching must fit.

Figure 1 schematically illustrates a two-stroke engine 100. A piston 130 is arranged to reciprocate in a cylinder 120 comprising a combustion chamber. A fuel/air mixture having a composition decided by a fuel setting is drawn or injected into the combustion chamber as the piston moves in an upwards direction. Just before the piston reaches its topmost position, often referred to as top dead center, the fuel/air mixture is ignited by an ignition device. The fuel/air mixture then burns, causing gas expansion, which forces the piston in a downwards direction. The linear motion of the piston 130 is transferred in a known manner to a circular motion of a crankshaft 105 arranged in a crankcase 1 1 0.

A tool, such as a cutting disc or saw chain, can be powered by the combustion engine 100 via the crankshaft 105. Combustion engines suitable for handheld power tools are preferably designed for a high power to weight ratio, which means that high power is to be delivered from a light weight engine. Space is often scarce in handheld power tool housings. This means that the engine cannot comprise any bulky components. Rather, a slim design is often targeted. The combustion engine 100 shown in Figure 1 comprises a first inlet channel 140 adapted for an air/fuel AF mixture, and a second inlet channel 145 adapted for air head AH. The air/fuel mixture can, as will be discussed below, be obtained from a carburetor 180 or from an injector which will be discussed below. The air is pure air for a stratified scavenging system in the combustion engine 100. Stratified scavenging in general was discussed above.

Notably, the first and second inlet channels are connected to a common reed valve cage 150 that comprises a first reed valve assembly 151 and a second reed valve assembly 152.

Herein, a reed valve assembly is a reed valve system comprising at least one reed membrane and a seat together configured as a one-directional pressure- controlled valve.

Reed valves comprising a reed cage configured to house more than one reed valve membrane are known. However, the reed valve assemblies shown in Figure 1 are separated by a partition 160 creating a first reed channel 153 and a second reed channel 154 through the common reed valve cage 150. The first reed channel 153 is connected to the first inlet channel 140 and the second reed channel 154 is connected to the second inlet channel 145, such that the first reed valve assembly 151 is essentially connected to the first inlet channel 140 only and the second reed valve assembly 152 is essentially connected to the second inlet channel 145 only.

The first reed valve assembly 151 is mounted on the common reed valve cage 150 such that it controls the flow through the first reed valve channel 153, and the second reed valve assembly 152 is mounted on the common reed valve cage 150 such that it controls the flow through the second reed valve channel 154.

It is appreciated that the connection between the first reed valve assembly 151 and the first inlet channel 140 need not necessarily be a hermetically sealed connection. It is furthermore appreciated that the connection between the second reed valve assembly 152 and the second inlet channel 145 need not necessarily be a hermetically sealed connection. The skilled person realizes that some small passages between the first and second inlet channels can be tolerated without significant loss of performance. Herein, essentially connected to a given channel only can, according to some aspects, be interpreted as there being some small passage elsewhere.

It is appreciated that the configuration of the first reed valve assembly 151 being essentially connected to the first inlet channel 140 only is relevant downstream 190 from the common reed valve cage 150, and not necessarily upstream 191 from the common reed valve cage where there may be a single air channel configured to feed both first and second inlet channel with air. Likewise, it is appreciated that the configuration of the second reed valve assembly 152 being essentially connected to the second inlet channel 140 only is relevant downstream 190 from the common reed valve cage 150, and not necessarily upstream 191 from the common reed valve cage.

However, in a preferred implementation, there is no passage between the first and second inlet channels, i.e., the first and second inlet channels are totally separated from each other. Thus, according to some aspects, the first reed valve assembly (151 ) is connected to the first inlet channel (140) only and the second reed valve assembly (152) is connected to the second inlet channel (145) only.

Herein, a reed valve assembly such as the first and second reed valve assemblies 151 , 152, comprises an opening arranged to be covered by a membrane. The membrane reacts to changes in pressure upstream and downstream from the reed valve assembly. Thus, reed valves are a type of check valve which restrict the flow of fluids or gases to a single direction, opening and closing under changing pressure on each face.

Instead of two or more separate reed valves for the pure air intake and one or more for the air/fuel mixture, the combustion engine 100 illustrated in Figure 1 only uses a single reed valve module with a common reed valve cage 150. As mentioned above, this type of reed valve induction system enables improved performance for both low and high engine speeds. The design also allows for conserving space, which is an issue especially important in handheld power tools where space is always scarce.

The partition 160 that separates the first and the second reed valve assembly effectively decouples the resonance system associated with the first inlet channel from the resonance system associated with the second inlet channel. This simplifies engine optimization, since one intake system can be designed with a reduced dependency on the other intake system, at least with respect to matching of the two intake systems.

Manufacturing of the combustion engine 100 is simplified since the common reed valve cage is mounted as a single component, as opposed to having a plurality of separate reed valve cages for the different purposes of fuel induction and auxiliary air induction to the engine.

According to some aspects, the reed valve membrane mounted in the first reed valve assembly has different characteristics compared to the reed valve membrane mounted in the second reed valve assembly. Thus, for example, the different membranes may have different stiffness yielding different valve properties for fuel/air mixture and for auxiliary air. The different membranes may also have different dimensions, i.e., surface areas, yielding different valve properties for fuel/air mixture and for auxiliary air.

According to some aspects, the stratified charged two-stroke engine 100 comprises a carburetor arranged to provide the air/fuel mixture AF into the first inlet channel 140. This carburetor can, optionally, be a conventional known carburetor 180. However, the carburetor is preferably a split carburetor such as the carburetor 200 illustrated in Figure 2.

Figure 2 shows a split-carburetor 200 comprising the throttle valve 170. A first carburetor channel 210 is connected to the first reed channel 153, and a second carburetor channel 220 separated from the first carburetor channel, is connected to the second reed channel 154. The throttle valve 170 is arranged to regulate a flow of the air/fuel AF mixture in the first carburetor channel 210 and a flow of the air AH in the second carburetor channel 220. Thus, an AH flow from an air filter (not shown in Figure 2) towards the cylinder 120 passes the second carburetor channel 220, the second reed channel 154, and the second inlet channel 145. An AF flow from the air filter (not shown in Figure 2) towards the cylinder 120 passes the first carburetor channel 210, the first reed channel 153, and the first inlet channel 140.

The split-carburetor 200 comprises a partition 240 which separates the first carburetor channel 210 from the second carburetor channel 220. This partition is, according to some aspects, configured to align with, and to form a seal against, the partition 160 which separates the reed valve assemblies 151 , 152 shown in Figure 1 . Thus, when the two partitions 240, 160 are aligned, the first carburetor channel 210 is connected to the first inlet channel 140 only and the second carburetor channel 220 is connected to the second inlet channel 145 only. It is, however, appreciated that the connection between partition 160 and partition 240 need not necessarily be hermetically sealed. Rather, some leakage in this connection point between the two partitions 240, 160 is, according to some aspects, acceptable. It is furthermore appreciated this leakage is more acceptable in case pure air flows through a channel compared to if an air/fuel mixture flows through the channel.

The air/fuel mixture is generated by means of a carburetor 180 or in general by a fuel disposing device 230, which may comprise a fuel injector arranged to inject a controlled amount of fuel into the first carburetor channel.

It is an advantage to combine the split carburetor device 200 shown in Figure 2 with the reed valve arrangement comprising the common reed valve cage 150 and the partition 160 as shown in Figure 1 . This way a single throttle valve is configured to control both the air flow for the air/fuel mixture, and the air flow for the stratified scavenging, i.e., the auxiliary air flow. During manufacturing, the split-carburetor 200 can be manufactured as a single module configured to interface directly with the common reed valve cage 150, which provides for an efficient manufacturing process. The combination of split-carburetor 200 and the reed valve arrangement shown in Figure 1 is also more space efficient compared to many known designs. Figure 3 illustrates some aspects of the combustion engine 300 where the combustion engine comprises a fuel injector 310 arranged to inject fuel into the first inlet channel 140 upstream from the first reed valve assembly 151 . This way the air/fuel mixture is generated in connection to the reed valve assembly. The first and second inlet channels are fed by a common pure air intake 320. The fuel injection device 310 is arranged to be controlled by a control unit 330.

Figure 4 illustrates some other aspects of the combustion engine 400 where a fuel injector 410 is arranged to inject fuel into the first inlet channel 140 downstream from the first reed valve assembly 151. This design is similar to the design schematically illustrated in Figure 3, with the difference that the air/fuel mixture is generated downstream from the reed valve 151 .

Figure 5 illustrates some further aspects of the combustion engine 500 where a fuel injector 510 is arranged to inject fuel into the crankcase 1 10. This configuration is also fed by a common pure air intake which feeds both the air/fuel mixture intake and the auxiliary air intake used for the air head system.

Figure 6 and Figure 7 illustrate throttle valve configurations, which may be used separately or in combination. Figure 6 illustrates a throttle valve 610 arranged to regulate a flow of the air/fuel AF mixture and air-head flow. The throttle valve is arranged upstream 620 from the common reed valve cage 150 that comprises the first reed valve assembly 151 and the second reed valve assembly 152.

Figure 7 illustrates a throttle valve 710 arranged to regulate a flow of the air/fuel AF mixture and air-head flow, wherein the throttle valve is arranged downstream 720 from the common reed valve cage 150 that comprises the first reed valve assembly 151 and the second reed valve assembly 152.

The upstream throttle valve 610 and the downstream throttle valve 710 may be used in combination. For instance, one of the upstream and the downstream throttle valve may be arranged to be controlled by a manual throttle control operated by a user, while the other of the upstream and the downstream throttle valve may be arranged to be controlled by a control unit 330.

Figure 8 illustrates aspects of a combustion engine 800 comprising one or more boost channels 810, 830 arranged as conduits for the air/fuel mixture from the first inlet channel 140 into the cylinder 120 and/or into the crankcase 1 1 0.

The boost channel 810 is connected to one or more boost ports 820 in the cylinder wall. This boost port may be located at the rear of the cylinder, i.e., opposite the exhaust port. A first purpose of a boost port is to allow an extra amount of air/fuel mixture to enter the combustion chamber directly from the first inlet channel 140, which enables increased power output from the engine. A second purpose of a boost port is to improve cooling of the combustion engine. The boost flow from the boost port 820 may also be designed to improve the scavenging operation.

According to some aspects, the first inlet channel 140 is arranged as conduit for the air/fuel mixture into the cylinder 120. However, as illustrated in Figure 9, the first inlet channel 140’ is, according to some other aspects, connected to the crankcase 1 10. A boost channel 910 may be configured to feed air/fuel mixture from the first inlet channel 140’ to the one or more boost ports 820.

One issue with two-stroke combustion engines designed for a high power to weight ratio is heat generation and cooling. The piston is one part of the combustion engine which tends to become very hot, especially when the combustion engine is operating under heavy load. Figure 10 schematically illustrates details of a stratified charged two-stroke engine 1000, where the piston 130’ comprises a first opening 1010 arranged to align with a corresponding first opening 1020 in the cylinder 120’. The first opening 1010 is connected via a boost channel 1030 to one or more boost ports 1040 of the cylinder 120’. This channel is associated with the advantage that it provides some lubrication of, e.g., the connecting rod small end bearing. This configuration of the piston and cylinder allows gas, such as an air/fuel mixture AF, to flow 1050 through the piston 130’ to the boost port 1040 via the boost channel 1030 when the first opening 1010 of the piston 130’ is aligned with the first opening of the cylinder. Advantageously, this arrangement provides increased engine output power due to the boost function, and at the same time transports heat away from the piston 130’, which cools the piston. The gas which flows through the first opening 1010 originates from the crankcase 1 10, which has a lower temperature than the piston 130’.

Figure 22 schematically illustrates an example air head configuration where the stratified charged two-stroke engine 2200 comprises a second inlet channel 145 which branches into a plurality of second inlet channels 145A, 145B. Here, the common reed valve cage 150 comprises a second reed valve assembly 152 with two reed valves 152A, 152B. The two reed valves face in opposite directions, such that a first air-head flow is directed to one side of the cylinder and another airhead flow is directed to the other side of the cylinder. This way, a space efficient implementation is achieved, since a distance 2210 from the common reed valve cage to the cylinder can be minimized.

The first reed channel 153, not shown in Figure 22, is as before connected to the first inlet channel 140, not shown in Figure 22, while the second reed channel 154 branches into first 145A and second 145B second inlet channels, such that the first reed valve assembly 151 is essentially connected to the first inlet channel 140 only and the second reed valve assembly 152 is essentially connected to the second inlet channels 145 only.

Thus, according to some aspects, the second reed valve assembly 152 comprises a first air-head reed valve 152A and a second air-head reed valve 152B, The second inlet channel 145 is divided into first 145A and second 145B second inlet channels, wherein the first second inlet channel 145A is directed to a left side (LS) of the cylinder 130, and wherein the second second inlet channel 145B is directed to a right side (RS) of the cylinder 130 opposite the left side (LS).

According to some further aspects, the first reed valve assembly 151 comprises a plurality of reed valve membranes arranged to face in different directions. Figure 23 shows a side view of an example common reed cage 150 suitable for the air head configuration in Figure 22. Here, the first air-head reed valve 152A is shown, arranged in the common reed valve cage 150, and separated from the first reed valve assembly 151 by the partition 160. The first reed valve assembly 152 may comprise one or more reed valves, although Figure 23 only shows one reed valve membrane.

It is appreciated that the common reed valve cage 150 may be designed with different geometries, and that the illustrated common reed valve cages are just examples of the disclosed common reed valve cage 150.

Figure 1 1 and Figure 12 schematically illustrate an air head system for a stratified charged two-stroke engine such as the engines discussed above in connection to Figures 1 -10 and 22. The stratified charged combustion engine comprises scavenging ducts 1 1 10 extending from the crankcase 1 10 to scavenging ports 1 160 in the cylinder wall. These scavenging ducts may optionally be so-called cup-handle scavenging ducts having an arcuate form and running between the crankcase 1 10 and the cylinder 120. Figure 1 1 exemplifies such cup-handle scavenging ducts having the arcuate form. Notably, as illustrated in Figures 1 1 and 12, one or more air-head channels 1 120, or auxiliary air channels, extend between the cylinder wall 1 180 and respective scavenging ducts 1 1 10. According to some aspects, the air-head channels 1 120 extend in a volume 1 190 located between the cylinder wall 1 180 and the one or more scavenging ducts 1 1 10.

Each air head channel 1 120 is connected to its respective scavenging duct 1 1 10 at a connection point 1 170 located between a crankcase connection 1 150 of the scavenging duct 1 1 10 and a scavenging port 1 160 of the scavenging duct 1 1 10.

Of course, the air-head channels 1 120 may optionally extend in a volume 1 191 outside of the one or more scavenging ducts 1 1 10, i.e., on the opposite side of the scavenging ducts 1 1 10 from the cylinder wall 1 180. It is therefore appreciated that the connection point can be located on an inside or on an outside of the scavenging duct 1 1 10, although only inside connection points are shown in Figure 1 1 .

The piston 130” comprises one or more second openings 1 140, whereby the airhead channels 1 120 connect to the crankcase 1 10 via the one or more second openings in the piston 130” when a second opening 1 140 is aligned with a scavenging port 1 160, thereby allowing air to fill the scavenging ducts 1 1 10. Thus, as a connection is provided between the auxiliary air channel 1 120 and the crankcase via the hole in the piston, pure air is drawn into the scavenging ducts, at least partially filling the scavenging ducts with pure air. The air flow from the auxiliary air channel 1 120 is divided into two flows in the respective scavenging duct 1 1 10, a first flow is in direction of the crankcase 1 130A, while a second flow is in direction of the cylinder 1 130B. This pure air then enters the cylinder ahead of the air/fuel mixture during the scavenging operation, i.e., provides a stratified scavenging operation.

Any piston 130” arranged to provide the connection between auxiliary air channel 1 120 and crankcase can be used with the combustion engine illustrated in Figure 1 100. However, advantageously, the piston may be a skeleton-type piston which is associated with a low weight.

It is appreciated that the arrangement illustrated in Figure 1 1 can be used with the type of reed valve arrangement shown, e.g., in Figure 1 , which comprises a common reed valve cage 150 to control air/fuel intake and air-head intake.

It is furthermore appreciated that the arrangement illustrated in Figure 1 1 can also be used with separate reed valves arranged to control air/fuel intake and air-head intake.

Skeleton-type pistons are sometimes referred to as partial skirted pistons or slipper skirt pistons in the literature.

The arrangement illustrated in Figures 1 1 and 12 provides a reduced weight stratified scavenging system which also reduces vibration due to the reduced weight piston. To summarize, there is disclosed herein an air head system for a stratified charged two-stroke engine 1 100, 1200. The engine comprises a piston 130” with one or more second openings 1 140 arranged to connect a scavenging port 1 160 of the engine with a crankcase 1 10 of the engine. The air head system comprises one or more air head channels 1 120 connected to respective scavenging ducts 1 1 10. Each air head channel 1 120 is connected to its respective scavenging duct 1 1 10 at a connection point 1 170 located between a crankcase connection 1 150 of the scavenging duct 1 1 10 and a scavenging port 1 160 of the scavenging duct 1 1 10, whereby an airhead channel 1 120 is connected to the crankcase 1 10 via its respective scavenging duct and via the piston 130” when a second opening 1 140 is aligned with the scavenging port 1 160, thereby allowing air to fill the scavenging ducts 1 1 10 in direction of the cylinder 1 130B. Air also fills the scavenging ducts 1 1 10 in direction of the crankcase 1 130A since there is an opening into the crankcase from the scavenging duct, i.e., a crankcase connection 1 150.

The connection location 1 170 is an important design parameter. An air head channel 1 120 is, according to some aspects, connected to the respective scavenging duct 1 1 10 at a connection location 1 170, wherein the connection location 1 170 is determined such that air flowing from the air head channel 1 120 fills the scavenging duct 1 1 10 up to the cylinder wall 1 180, and such that air is prevented from entering the crankcase 1 10.

According to some aspects, an air head channel 1 120 is connected to the respective scavenging duct 1 1 10 at a distance x1 , x2 larger than 10mm measured from the respective scavenging port 1 160.

It is appreciated that the measures x1 and x2 may be the same or may be different measures, depending on engine design.

The measures x1 and x2 are defined as lengths in mm measured along a centrum of a cross-section of the respective scavenging duct 1 1 10. The measures x1 and x2 therefore quantify a length of a scavenging duct portion between the cylinder wall 1 180 and the connection point 1 170, as indicated in Figure 1 1. It is appreciated that other measurement definitions may be equally valid, as realized by the skilled person.

It is appreciated that the connection location 1 170 may also be located in a vicinity of the scavenging port 1 160. In this case the second openings are optional since the pure air will fill the scavenging duct in direction of the crankcase 1 130A. Consequently, there is disclosed herein an air head system for a stratified charged two-stroke engine 1 100, 1200. The air head system comprises one or more air head channels 1 120 connected to respective scavenging ducts 1 1 10, wherein each air head channel 1 120 is connected to its respective scavenging duct 1 1 10 at a connection point 1 170 located next to a scavenging port 1 160 of the scavenging duct 1 1 10, whereby an airhead channel 1 120 is connected to the crankcase 1 10 via its respective scavenging duct, thereby allowing air to fill the scavenging ducts 1 1 10 in direction of the crankcase 1 130A.

In other words, according to a further example, the air head channel 1 120 is connected to the respective scavenging duct 1 1 10 at a distance x1 , x2 close to 0 mm measured from the respective scavenging port 1 160. According to this example the second openings 1 140 are optional.

Figure 13 illustrates a stratified charged two-stroke engine 1300 comprising a crankcase 1 10, a cylinder 120, a piston 130 arranged to reciprocate in the cylinder, a first inlet channel 1330 adapted for an air/fuel AF mixture, and a second inlet channel 1320 adapted for air AH. The first inlet channel comprises a reed valve 1340 and is connected to the crankcase 1 10. The second inlet channel 1320 is arranged to be controlled by the piston 130, or by an optional reed valve 1321 . The reed valve 1340 is arranged to provide a first flow amount of AF mixture measured relative to a first flow amount of AH at a low engine speed, and a second flow amount of AF mixture measured relative to a second flow amount of AH at a high engine speed, wherein the first flow is larger than the second flow.

This way the combustion engine is provided with more air/fuel mixture in relation to auxiliary air at lower engine speeds where larger quantities of air/fuel mixture is beneficial for, e.g., acceleration. At higher engine speeds, the larger quantities of auxiliary air compared to fuel/air mixture provides increased output power and reduced emissions, which is an advantage

The reed valve 1340 may be configured with different types of membranes, e.g., with different stiffness, and may be designed with different geometries, e.g., different membrane surface areas, in order to obtain the effect of providing a first flow amount of AF mixture measured relative to a first flow amount of AH at a low engine speed, and a second flow amount of AF mixture measured relative to a second flow amount of AH at a high engine speed, wherein the first flow is larger than the second flow. Thus, the specification for the reed valve 1340 schematically illustrated in Figure 13 is, according to some aspects, chosen according to how the combustion engine is to be operated.

An advantage associated with the combustion engine shown in Figure 13 is that the air/fuel delivery system is mounted to the crankcase 1 10 which is much cooler than the hot cylinder 120. A carburetor will therefore not need as much cooling, which is an advantage.

Since the piston only is designed to support air head and need not convey air/fuel mixture for the main air/fuel intake, there is room for additional boost channels, which is an advantage.

Figure 14 schematically illustrates a stratified charged two-stroke engine 1400 comprising one or more boost channels or scavenging channels 1410 arranged in-between airhead ports 1420 in the cylinder 120.

In-between here means that a boost channel or scavenging channel 1410 is located with an airhead port to each side along a lateral direction on the cylinder wall.

The stratified changed two-stroke engine comprises a reed valve induction system to the crankcase, which frees up some room in the cylinder wall that was previously used for air/fuel mixture intake. This space can now be used by boost channels 1410 instead of the air/fuel mixture ports. It is appreciated that the AH channel can be controlled by a common reed valve 1421 or by separate reed valves arranged in each channel (not shown in Figure 14). Figure 15 and Figure 16 show some example boost channel arrangements for use with the stratified charged combustion engine shown in Figures 13 and 14. The channel 1510 may for example be separated from the cylinder wall or the channel 1610 may be machined as a recess in the cylinder wall. Both channel configurations 1510, 1610 provide a connection between the crankcase 1 10 and the combustion chamber in the cylinder 120.

The channel 1510 illustrated in Figure 15 is a separate or closed channel. This channel allows for better control of the direction of flow exiting the port 1410, and provides some support for the piston 130 during the reciprocating motion.

The channel 1610 illustrated in Figure 16 is an open channel machined into the cylinder wall. This channel is associated with the advantage that it provides some lubrication of the con rod little end bearing.

Figure 17 is a top view of a combustion engine illustrating some example aspects of a connection system between auxiliary air channel, or inlet channel adapted for air 1755, and scavenging ducts, i.e., an air head system for stratified scavenging. It is appreciated that Figure 17 is not a cross-section view of the combustion engine. Here the piston 130 comprises a recess 1710 in the piston skirt arranged to connect the inlet channel 1755 adapted for air AH to first and second scavenging ducts 1720, 1721 .

The piston further comprises a symmetrically arranged optional further recess 1710’ in the piston skirt arranged to connect a further inlet channel 1755’ adapted for air AH to further first and second scavenging ducts 1720’, 1721 '.

Advantageously, this arrangement cools the piston 130 since relatively cool auxiliary air contacts the piston during filling of the scavenging channels with auxiliary air.

Consequently, there is disclosed herein a stratified charged two-stroke engine 1700 comprising a piston 130 and first and second scavenging ducts 1720, 1721 , wherein the piston 130 comprises a recess 1710 in the piston skirt arranged to connect an inlet channel adapted for air 1755 AH to the first and to the second scavenging ducts 1720, 1721 . It is appreciated that the system for distributing auxiliary air to scavenging ducts shown in Figure 17 is a feature which does not require support from any of the other features discussed above but can be used on its own.

It is appreciated that the stratified charged two-stroke engine 1700 may comprise more than the illustrated number of scavenging ports.

Figure 18 schematically shows approximate port locations in a combustion engine comprising a cylinder wall 1880 for a stratified charged two-stroke engine 1800. A 360-degree view of the cylinder wall 1880 is shown. Thus, the exhaust port 1810 is shown with half the port to the left and half the port to the right. The cylinder wall 1880 between exhaust port halves therefore represents 360 degrees around the circular cylinder wall 1880.

Ports 1840 are air/fuel mixture inlet ports connected to the crankcase, i.e., not openings in the cylinder wall 1880 per se, but openings into the crankcase 1 10. Port 1730, also shown in figure 17, is an air/fuel mixture inlet port which is connected to the crankcase via a hole in the piston 130, similar to the arrangement illustrated in Figure 10, where the port is labelled as 1040.

Ports corresponding to the first and second scavenging ducts 1720, 1721 ; 1720’, 1721’ in Figure 17 are also shown.

Ports 1755” and 1755’” are air head ports AH connected to channels 1755 and 1755’, respectively.

A crankcase mounted air/fuel delivery system makes it possible to have wider airhead ports at the rear of the cylinder, close to the boost port. This increases the possibility to use large amounts of auxiliary air, which increases engine output power, reduces emissions, and improves engine cooling.

According to aspects, the stratified charged two-stroke engine 1700, 1800 comprises an optional further inlet channel adapted for air AH, 1755’. At least one scavenging duct port 1721 , 1721’ is arranged in-between the inlet channel adapted for air AH, 1755 and the further inlet channel adapted for air AH, 1755’. In other words, a distance measured along the cylinder wall between a scavenging duct port 1721 and the exhaust port 1810 is longer than a distance measured along the cylinder wall between the inlet channel adapted for air AH, 1755 and the exhaust port 1810. This placement of the scavenging duct port 1721 is enabled by the reed valve arrangements disclosed herein.

According to aspects, at least one boost port 1730, 1820 is arranged in- between the inlet channel adapted for air AH, 1755 and the further inlet channel adapted for air AH, 1755’. In other words, a distance measured along the cylinder wall between a boost port 1730 and the exhaust port 1810 is longer than a distance measured along the cylinder wall between the inlet channel adapted for air AH, 1755 and the exhaust port 1810. This placement of the boost port 1730 is enabled by the reed valve arrangements disclosed herein.

According to aspects, the inlet channel adapted for air AH, 1755 is arranged in-between the first and the second scavenging ducts 1720, 1721 .

Here, in-between means that there is a first 1720 or a second 1721 scavenging duct port in each lateral direction from the inlet channel adapted for air AH, 1755. It is also appreciated that in-between two ports, in the context of Figure 18, refers to in-between and opposite the exhaust port 1810.

Figure 18 schematically illustrates aspects where a scavenging port 1721 is arranged on an AF intake side of an airhead port 1755”.

Figures 17 and 18 also schematically illustrate aspects where an airhead channel 1755 is connected to at least two scavenging ports 1720, 1721 .

In general, in a preferred implementation, the different airhead flows are controlled by reed valves, which may comprise a common reed valve or separate reed valves per airhead channel. The air/fuel intake can be reed valve controlled or piston controlled, or a combination of reed and piston control. It is furthermore appreciated that different injection systems can be used for injecting fuel into, e.g., the crankcase. In such cases the air/fuel intake only carries pure air.

Figure 19 illustrates aspects of the auxiliary air transfer from the inlet channel adapted for air 1755 to the scavenging ducts 1720, 1721. Here, the inlet channel adapted for air 1755 is arranged at an incoming angle a with respect to a radial line 1920 of the piston 130. The angle a is arranged to control relative portions of air entering the first and second scavenging ducts 1720, 1721 . Thus, by using a small value of a, the flow of pure air is more equally distributed between the first and the second scavenging ducts. By using a large positive value of a, the flow into the first scavenging duct is increased compared to the flow into the second scavenging duct. By using a large negative value of a, the flow into the second scavenging duct is increased compared to the flow into the first scavenging duct.

According to some aspects, the value of a ranges between -90 to +120 degrees.

According to some such aspects, the value of a is smaller than 45 degrees.

Figure 20 illustrates further aspects of the auxiliary air transfer from the inlet channel adapted for air 1755 to the scavenging ducts 1720, 1721 . Here the recess 1710 comprises a ridge 2010 arranged to control relative portions of air entering the first and second scavenging ducts 1720, 1721 . The relative location w2 of the ridge together with the diameter of the auxiliary air channel determines respective flows of pure air into the first and second scavenging ducts.

According to some aspects, the relative value of w1/w2 ranges between w1/w2=0.1 to w1/w2=0.9.

It is appreciated that the ridge 2010 can be combined with the incoming angle a for further control of the relative portions.

Figure 21 illustrates a handheld power tool comprising a combustion engine 100, 200, 300, 400, 500, 600, 700, 800, 900, 1000, 1400, 1500, 1600, 1700, 1800, 1900, 2000, 2200, or air head system 1 100, 1200 as discussed herein.

Figure 24 is a flowchart illustrating methods. The methods are example operations which can be performed by the combustion engines discussed above. There is illustrated a method in a stratified charged two-stroke engine 100, 200, 300, 400, 500, 600, 700, 800, 900, 1000, 1 100, 1200, 1400, 1500, 1600, 1700, 1800, 1900, 2000, 2200 comprising a crankcase 1 10, a cylinder 120, a piston 130 arranged to reciprocate in the cylinder. The method comprises configuring Sa1 a first inlet channel 140 adapted for an air/fuel AF mixture, configuring Sa2 a second inlet channel 145 adapted for air AH, connecting Sa3 the first and second inlet channels to a common reed valve cage 150 that comprises a first reed valve assembly 151 and a second reed valve assembly 152, wherein the reed valve assemblies are separated by a partition 160 such that the first reed valve assembly 151 is connected to the first inlet channel 140 only and the second reed valve assembly 152 is connected to second inlet channel 145 only, inputting Sa4 an air/fuel mixture via the first inlet channel 140 adapted for the air/fuel AF mixture, and inputting Sa5 air via the second inlet channel 145 adapted for the air AH.

Figure 24Figure 25 is a flowchart illustrating methods. The methods are example operations which can be performed by the combustion engines discussed above. There is illustrated a method in a stratified charged two- stroke engine 1300 comprising a crankcase 1 10, a cylinder 120, and a piston 130 arranged to reciprocate in the cylinder. The method comprises configuring Sb1 a first inlet channel 140 adapted for an air/fuel AF mixture, wherein the first inlet channel comprises a reed valve 1340 and is connected to the crankcase 1 10, configuring Sb2 a second inlet channel 145 adapted for air AH, wherein the second inlet channel is arranged to be controlled by the piston 130, providing Sb3 a variable ratio between an AF flow amount and an AH flow amount by the reed valve 1340, wherein the ratio of AF flow amount and AH flow amount is larger at low engine speed compared to at high engine speed.

Figure 25Figure 26 is a flowchart illustrating methods. The methods are example operations which can be performed by the combustion engines discussed above. There is illustrated a method of providing air head in a stratified charged two-stroke engine 1 100, 1200. The method comprises providing Sc1 the engine with a piston 130” having one or more second openings 1 140 arranged to connect a scavenging port 1 160 of the engine with a crankcase 1 10 of the engine, configure Sc2 one or more air head channels 1 120 connected to respective scavenging ducts 1 1 10, wherein each air head channel 1 120 is connected to its respective scavenging duct 1 1 10 at a connection point 1 170 located between a crankcase connection 1 150 of the scavenging duct 1 1 10 and a scavenging port 1 160 of the scavenging duct 1 1 10, connecting Sc3 an airhead channel 1 120 to the crankcase 1 10 via its respective scavenging duct and via the piston 120” when a second opening 1 140 is aligned with the scavenging port 1 160, thereby allowing air to fill the scavenging ducts 1 1 10 in direction of the cylinder 1 130B.

Figure 26Figure 27 is a flowchart illustrating methods. The methods are example operations which can be performed by the combustion engines discussed above. There is illustrated a method in a stratified charged two- stroke engine 1700 comprising a piston 130 and first and second scavenging ducts 1720, 1721. The method comprises configuring Sd1 a recess 1710 in the piston skirt, wherein the recess is arranged to connect a second inlet channel adapted for air AH to the first and to the second scavenging ducts 1720, 1721 , and transferring Sd2 air from the second inlet channel to the first and second scavenging ducts 1720, 1721 via the recess when the recess aligns with the second inlet channel.