Crankshaft, power unit, two stroke piston engine, and hand-held power tool

A crankshaft (1) for a crankcase scavenged two stroke piston engine (2) is disclosed. The crankshaft (1) is configured to rotate around a rotation axis (ax) during operation. The crankshaft (1) comprises a crankpin (3) configured to move in a crank plane (cp) during operation, and a first and a second counterweight (5, 7) arranged on either side (S1, S2) of the crank plane (cp). The first counterweight (5) has a first outer radius (r1) measured from the rotation axis (ax) and the second counterweight (7) has a second outer radius (r2) measured from the rotation axis (ax). The first outer radius (r1) is smaller than the second outer radius (r2). The present disclosure further relates to a power unit (10), a crankcase scavenged two stroke piston engine (2), and a hand-held power tool (20).

TECHNICAL FIELD

The present disclosure relates to a crankshaft for a crankcase scavenged two stroke piston engine. The present disclosure further relates to a power unit for a crankcase scavenged two stroke piston engine, a crankcase scavenged two stroke piston engine, and a hand-held power tool comprising a crankcase scavenged two stroke piston engine.

BACKGROUND

A two-stroke engine is a type of internal combustion engine which completes a power cycle with two strokes of the piston during only one crankshaft revolution. The uppermost position of a piston in a cylinder is usually referred to as the top dead centre and the lowermost position of the piston in the cylinder is usually referred to as the bottom dead centre. Compared to four-stroke engines, two-stroke engines have a smaller number of moving parts, and consequently can be made more compact and significantly lighter. Therefore, two-stroke petrol engines are used in applications where mechanical simplicity, light weight, and high power-to-weight ratio are main concerns. Typical applications are hand-held tools such as chainsaws.

Most small sized two-stroke engines are crankcase-scavenged engines meaning that these engines use the area below the piston as a charging pump to build up pressure in the crankcase during the power stroke of the piston. Two-stroke engines are usually provided with a carburettor arranged to supply an air/fuel mixture to the crankcase. In the power stroke of a two-stroke engine, the increased pressure and temperature in the cylinder obtained by the combustion of fuel is partially converted into mechanical work supplied to a crankshaft of the engine. At the same time, the pressure in the crankcase increases as a result of the movement of the piston towards the bottom dead centre. An exhaust port arranged in the cylinder wall is opened to allow exhaust gases to flow out from the cylinder when the piston reaches a first position relative the cylinder in its movement towards the bottom dead centre. The piston continues the movement towards the bottom dead centre and when it reaches a second position, below the first position, an inlet port arranged in the cylinder wall is opened. The inlet port is fluidly connected to the crankcase via a scavenging channel. The air/fuel mixture in the crankcase is forced to flow into the cylinder via the inlet port by the overpressure in the crankcase.

The volume of the crankcase affects the scavenging efficiency of the engine, i.e. the efficiency of the transfer of the air/fuel mixture from the crankcase into the cylinder via the inlet port. A smaller volume of the crankcase increases the scavenging efficiency and a greater volume reduces the scavenging efficiency. In turn, the scavenging efficiency affects the operation and the power output of the engine. For this reason, some crankcase scavenged two stroke piston engines comprise fillers arranged on the crankshaft of the engine. The fillers reduce the free volume of the crankcase and thereby increases the scavenging efficiency of the engine. However, fillers may reduce lubrication of bearings of the engine, such as a crank bearing of the crankshaft of the engine. In additions, the fillers add costs, weight, and complexity to two-stroke engines, and in general, on today's consumer market, it is an advantage if products, such as two-stroke engines and associated components, have conditions and/or characteristics suitable for being manufactured and assembled in a cost-efficient manner.

SUMMARY

It is an object of the present invention to overcome, or at least alleviate, at least some of the above-mentioned problems and drawbacks.

According to a first aspect of the invention, the object is achieved by a crankshaft for a crankcase scavenged two stroke piston engine. The crankshaft is configured to rotate around a rotation axis during operation. The crankshaft comprises a crankpin configured to move in a crank plane upon rotation of the crankshaft, and a first and a second counterweight arranged on either side of the crank plane. The first counterweight has a first outer radius measured from the rotation axis and the second counterweight has a second outer radius measured from the rotation axis. The first outer radius is smaller than the second outer radius.

Since the first counterweight has a smaller outer radius than the second counterweight, the crankcase of an engine comprising the crankshaft can be designed to have a reduced free volume thereby increasing the scavenging efficiency of the engine. This because the first counterweight can be surrounded by a crankcase enclosure section with a reduced radius.

The crankpin of a crankshaft is arranged at a distance from the rotation axis of the crankshaft so as to provide a crank movement in the crank plane upon rotation of the crankshaft. A crankshaft further comprises a crankpin adjoining portion on either side of the crank plane. The radial distances between the rotation axis and radial outer portions of the crankpin adjoining portions puts limitations on the design of enclosure sections of the crankcase. In some designs, one of the crankpin adjoining portions needs to be larger in size than the other crankpin adjoining portion.

Thus, since the first and second counterweights have different outer radiuses, one of the crankpin adjoining portions is allowed to have a larger outer radius than the other crankpin adjoining portion. Thereby, the crankshaft according to the embodiments herein provides conditions for optimizing the measurements of a crankcase so as to provide a small free volume thereof. In this manner, the free volume of the crankcase can be reduced and the scavenging efficiency of an engine comprising the crankshaft can thereby be increased.

Accordingly, since the free volume of the crankcase can be reduced in this manner, the need for using fillers arranged on the crankshaft is reduced which potentially saves costs, weight, and complexity of the crankshaft. In addition, by not arranging fillers on the crankshaft, the lubrication of bearings of the engine can be increased, such as a crank bearing attached to the crankpin and main bearings supporting the crankshaft relative to a crankcase of the engine.

Accordingly, a crankshaft is provided overcoming, or at least alleviating, at least some of the above-mentioned problems and drawbacks. As a result, the above-mentioned object is achieved.

Optionally, the first outer radius is within the range of 0.8-0.99 of the second outer radius or is within the range of 0.91-0.97 of the second outer radius. Thereby, the first counterweight can be surrounded by a crankcase enclosure section with a reduced radius while allowing different sizes of the crankpin adjoining portions of the crankshaft. In this manner, the free volume of the crankcase can be reduced and the scavenging efficiency of an engine comprising the crankshaft can thereby be increased without using fillers on the crankshaft.

Optionally, the crankshaft comprises a first crankshaft portion comprising the first counterweight and a first crankpin adjoining portion arranged opposite to the first counterweight relative to the rotation axis, and wherein the first crankpin adjoining portion has an outer radius smaller than, or equal to, the first outer radius. Thereby, conditions are provided for optimizing the measurements of a crankcase so as to provide a small free volume thereof. In this manner, the free volume of the crankcase can be reduced and the scavenging efficiency of an engine comprising the crankshaft can thereby be increased without using fillers on the crankshaft.

Optionally, the first crankshaft portion comprises the crankpin, and wherein the first crankshaft portion constitutes a first one-piece crankshaft portion of the crankshaft. Thereby, the first crankshaft portion can be provided with a first crankpin adjoining portion having a small outer radius. This because first crankshaft portion comprises the crankpin and constitutes a first one-piece crankshaft portion of the crankshaft. Thereby, conditions are provided for optimizing the measurements of a crankcase so as to provide a small free volume thereof without using fillers on the crankshaft.

Optionally, the crankshaft comprises a second one-piece crankshaft portion comprising the second counterweight and a second crankpin adjoining portion arranged opposite to the second counterweight relative to the rotation axis. Thereby, a crankshaft is provided which may comprise a two-piece set of crankshaft portions. Since the second counterweight has a larger outer radius than the first counterweight, the second crankpin adjoining portion is allowed to be greater in size than the first crankpin adjoining portion. In this manner, conditions are provided for optimizing the measurements of a crankcase so as to provide a small free volume thereof without using fillers on the crankshaft.

Optionally, the second crankpin adjoining portion has an outer radius smaller than, or equal to, the second outer radius. Thereby, the crankshaft provides conditions for optimizing the measurements of a crankcase so as to provide a small free volume thereof without using fillers on the crankshaft.

Optionally, the second crankpin adjoining portion comprises a hole, and wherein the crankpin is received in the hole. Thereby, a rigid and reliable crankshaft is provided while conditions are provided for reducing the free volume of the crankcase without using fillers arranged on the crankshaft.

Optionally, the crankpin is interference fitted into the hole. Thereby, a rigid and reliable crankshaft is provided while conditions are provided for reducing the free volume of the crankcase without using fillers arranged on the crankshaft.

Optionally, the second counterweight comprises a recess at a radially outer portion thereof, and wherein the recess extends in a first angular sector of the second counterweight measured from the rotation axis. Thereby, the free area formed by the recess can be utilized for allowing a piston of the engine to move closer to the rotation axis of the crankshaft. In this manner, the free volume of the crankcase can be further reduced.

Optionally, the first angular sector is smaller than a total angular sector occupied by the second counterweight measured from the rotation axis. Thereby, conditions are provided for allowing a piston of the engine to move closer to the rotation axis of the crankshaft while not significantly reducing the weight and/or the volume of the second counterweight.

Optionally, the first angular sector is within the range of 0.15 to 0.93 of the total angular sector or is within the range of 0.60 to 0.80 of the total angular sector. Thereby, conditions are provided for allowing a piston of the engine to move closer to the rotation axis of the crankshaft while not significantly reducing the weight and/or the volume of the second counterweight.

Optionally, the first angular sector is within the range of 23 degrees to 142 degrees or is within the range of 92 degrees to 122 degrees, measured from the rotation axis. Thereby, conditions are provided for allowing a piston of the engine to move closer to the rotation axis of the crankshaft while not significantly reducing the weight and/or the volume of the second counterweight.

Optionally, the second counterweight comprises an axial end portion facing away from the crank plane, and wherein the recess is provided at the axial end portion of the second counterweight. Thereby, conditions are provided for allowing a piston of the engine to move closer to the rotation axis of the crankshaft while not significantly reducing the weight and/or the volume of the second counterweight.

According to a second aspect of the invention, the object is achieved by a power unit for a crankcase scavenged two stroke piston engine, wherein the power unit comprises a piston, a connecting rod, and a crankshaft according to some embodiments of the present disclosure, and wherein the piston is connected to the crankpin via the connecting rod.

Thereby, a power unit is provided which provides conditions for optimizing the measurements of a crankcase so as to provide a small free volume thereof without using fillers arranged on the crankshaft. In this manner, the free volume of the crankcase can be reduced and the scavenging efficiency of an engine comprising the power unit can thereby be increased without using fillers arranged on the crankshaft.

Accordingly, since the need for using fillers arranged on the crankshaft is reduced, conditions are provided for a power unit saving costs, weight, and complexity of an engine. In addition, by not arranging fillers on the crankshaft, the lubrication of bearings of the engine can be increased, such as a crank bearing attached to the crankpin and main bearings supporting the crankshaft relative to a crankcase of the engine.

Accordingly, a power unit is provided overcoming, or at least alleviating, at least some of the above-mentioned problems and drawbacks. As a result, the above-mentioned object is achieved.

Optionally, the piston comprises a portion cyclically assuming a first position relative to the rotation axis of the crankshaft in which the radial distance between the portion of the piston and the rotation axis is shorter than the second outer radius of the second counterweight. Thereby, a power unit is provided allowing the piston to move to a position close to the rotation axis of the crankshaft. In this manner, the free volume of a crankcase of an engine comprising the power unit can be further reduced.

Optionally, the portion of the piston extends into the recess when the portion of the piston is in the first position. Thereby, a power unit is provided allowing the piston to move to a position close to the rotation axis of the crankshaft. In this manner, the free volume of a crankcase of an engine comprising the power unit can be further reduced.

Optionally, the piston comprises an aperture configured to cyclically superimpose an intake of a stratified scavenging intake arrangement of an engine, and wherein the portion of the piston is a portion of a section forming part of delimiting surfaces of the aperture. Thereby, the piston is allowed to move closer to the rotation axis of the crankshaft, while having the section forming part of the delimiting surfaces of the aperture.

According to a third aspect of the invention, the object is achieved by a crankcase scavenged two stroke piston engine comprising a crankcase, a cylinder, and a power unit according to some embodiments of the present disclosure, wherein the piston of the power unit is configured to reciprocate in the cylinder during operation of the engine.

Thereby, a crankcase scavenged two stroke piston engine is provided having conditions for a reduced free volume of the crankcase thereof. In this manner, the engine has conditions for operating with higher scavenging efficiency and thereby also higher power output.

Moreover, a crankcase scavenged two stroke piston engine is provided having a reduced need for filers arranged on the crankshaft of the engine. Thereby, an engine is provided having conditions and characteristics suitable for being manufactured and assembled in a cost-efficient manner. Moreover, an engine is provided having conditions for having a lower weight and complexity. In addition, by not arranging fillers on the crankshaft, the lubrication of bearings of the engine can be increased, such as a crank bearing attached to the crankpin and main bearings supporting the crankshaft relative to the crankcase of the engine.

Accordingly, a crankcase scavenged two stroke piston engine is provided overcoming, or at least alleviating, at least some of the above-mentioned problems and drawbacks. As a result, the above-mentioned object is achieved.

Optionally, the crankcase comprises a first enclosure section and a second enclosure section, the first counterweight being configured to rotate in the first enclosure section and the second counterweight being configured to rotate in the second enclosure section during operation of the engine, and wherein the first enclosure section has a smaller outer radius than the second enclosure section measured from the rotation axis of the crankshaft.

Thereby, an engine is provided having conditions for a reduced free volume of the crankcase of the engine. Moreover, an engine is provided capable of utilizing the fact that, in some designs, one of the crankpin adjoining portions needs to be larger in size than the other crankpin adjoining portion, for reducing the free volume of the crankcase of the engine.

Optionally, the crankcase scavenged two stroke piston engine is a single-cylinder engine.

According to a fourth aspect of the invention, the object is achieved by a hand-held power tool comprising a tool and an engine according to some embodiments of the present disclosure, wherein the engine is configured to power the tool during operation of the hand-held power tool.

Thereby, a hand-held power tool is provided comprising an engine having conditions for operating with higher scavenging efficiency and thereby also higher power output without using fillers arranged on the crankshaft of the engine.

Thereby, a hand-held power tool is provided having conditions and characteristics suitable for being manufactured and assembled in a cost-efficient manner. Moreover, a hand-held power tool is provided having conditions for having a lower weight and complexity.

Accordingly, a hand-held power tool is provided overcoming, or at least alleviating, at least some of the above-mentioned problems and drawbacks. As a result, the above-mentioned object is achieved.

Optionally, the hand-held power tool is a chainsaw.

Thereby, a chainsaw is provided comprising an engine having conditions for operating with higher scavenging efficiency and thereby also at higher power output levels without using fillers arranged on the crankshaft of the engine.

Thereby, a chainsaw is provided having conditions and characteristics suitable for being manufactured and assembled in a cost-efficient manner. Moreover, a chainsaw is provided having conditions for having a lower weight and complexity.

Accordingly, a chainsaw is provided overcoming, or at least alleviating, at least some of the above-mentioned problems and drawbacks. As a result, the above-mentioned object is achieved.

Further features of, and advantages with, the present invention will become apparent when studying the appended claims and the following detailed description.

DETAILED DESCRIPTION

Aspects of the present invention will now be described more fully. Like numbers refer to like elements throughout. Well-known functions or constructions will not necessarily be described in detail for brevity and/or clarity.

FIG.1illustrates a crankshaft/connecting rod assembly4according to some embodiments. The crankshaft/connecting rod assembly4comprises a crankshaft1and a connecting rod33. The crankshaft1comprises a crankpin3and the connecting rod33is connected to the crankpin3of the crankshaft1. The connecting rod33may be connected to the crankpin3via a crank bearing, such as a needle bearing, or the like.

The crankshaft1is a crankshaft for a crankcase scavenged two stroke piston engine, as is further explained herein. The crankshaft1is configured to rotate around a rotation axis ax during operation of an engine comprising the crankshaft1. InFIG.1, the crankshaft/connecting rod assembly4is illustrated in a viewing direction in which the rotation axis ax of the crankshaft1is perpendicular to the viewing direction.

The crankpin3of the crankshaft1is configured to move in a crank plane cp upon rotation of the crankshaft1around the rotation axis ax. That is, the crankpin3is arranged at a distance from the rotation axis ax and is configured to circulate, i.e. move in a circular path, around the rotation axis ax upon rotation of the crankshaft1. The crank plane cp is perpendicular to the rotation axis ax. Due to these features, a back and forth movement can be provided of a piston connected to a piston connection portion34of the connecting rod33.

The crankshaft1further comprises a first and a second counterweight5,7arranged on either side S1, S2of the crank plane cp. Each of the first and the second counterweights5,7is arranged opposite to the crankpin3relative to the rotation axis ax and each of the first and the second counterweights5,7has a centre of mass at a distance from the rotation axis ax. The first and the second counterweights5,7are designed to counter the inertia effect of the movement of the crankpin3, the connecting rod33, and a piston connected to the connecting rod33upon rotation of the crankshaft1. The first counterweight5has a first outer radius r1measured from the rotation axis ax and the second counterweight7has a second outer radius r2measured from the rotation axis ax. As can be seen inFIG.1, the first outer radius r1is smaller than the second outer radius r2. Thereby, the crankshaft1provides conditions for optimizing the measurements of a crankcase so as to provide a small free volume of the crankcase, as is further explained herein. According to the illustrated embodiments, the first outer radius r1is approximately 0.943, i.e. approximately 94.3%, of the second outer radius r2. According to further embodiments, the first outer radius r1may be within the range of 0.8-0.99 of the second outer radius r2or may be within the range of 0.91-0.97 of the second outer radius r2.

FIG.2illustrates a first cross section of a crankcase51and a power unit10of a crankcase scavenged two stroke piston engine2. The power unit10comprises a crankshaft/connecting rod assembly4according to the embodiments illustrated inFIG.1, i.e. a crankshaft1and a connecting rod33, according to the embodiments illustrated inFIG.1, and a piston31connected to the piston connection portion34of the connecting rod33. The crankshaft1is arranged in the crankcase51and is rotationally arranged around a rotation axis ax relative to the crankcase51via main bearings54. Main bearings54are sometimes referred to as crank shaft bearings54. The piston31is thus connected to the crankpin3of the crankshaft1via the connecting rod33. InFIG.2, a cylinder53of the engine2is schematically indicated. The piston31is configured to reciprocate in the cylinder53upon rotation of the crankshaft1. InFIG.2, the piston31and the crankshaft1are illustrated in positions corresponding to a top dead centre of the piston31relative to the cylinder53.

As seen inFIG.2, the crankcase51comprises a first enclosure section55and a second enclosure section57. The first counterweight5is configured to rotate in the first enclosure section55and the second counterweight7is configured to rotate in the second enclosure section57upon rotation of the crankshaft1around the rotation axis ax. Each of the first and second enclosure sections55,57comprises arc-shaped delimiting surfaces arranged at a small distance from the respective first and second counterweights5,7. As seen inFIG.2, the first enclosure section55has a smaller outer radius r5than the second enclosure section57measured from the rotation axis ax of the crankshaft1. Thus, the smaller outer radius r1of first counterweight5is utilized to reduce the free volume inside the crankcase51by arranging the first enclosure section55with a reduced outer radius r5.

According to the illustrated embodiments, the crankshaft1is a two-piece crankshaft comprising a first one-piece crankshaft portion11and a second one-piece crankshaft portion12. According to the illustrated embodiments, each of the first and second one-piece crankshaft portions11,12is formed in one piece of continuous material, such as a steel-material. The first one-piece crankshaft portion11is in some places herein referred to as “the first crankshaft portion11” and the second one-piece crankshaft portion12is in some places herein referred to as “the second crankshaft portion12”. The first crankshaft portion11comprises the first counterweight5, the crankpin3, and a first crankpin adjoining portion15adjacent to the crankpin3. The first crankpin adjoining portion15is arranged opposite to the first counterweight5relative to the rotation axis ax.

The second crankshaft portion12comprises the second counterweight7and a second crankpin adjoining portion17arranged adjacent to the crankpin3. The second crankpin adjoining portion17is arranged opposite to the second counterweight7relative to the rotation axis ax. Moreover, first crankshaft portion11comprises a first shaft portion11′ and the second crankshaft portion12comprises a second shaft portion12′. Each of the first and second shaft portions11′,12′ is accommodated in a main bearing54. Moreover, according to the illustrated embodiments, the first shaft portion11′ is configured to provide power output from the engine2via a clutch arranged in a clutch housing56. The first shaft portion11′ may therefore be referred to as an output shaft of the engine2. Moreover, according to the illustrated embodiments, the second shaft portion12′ is configured to be connected to a flywheel arranged in a flywheel housing58.

The second crankpin adjoining portion17comprises a hole19. As seen inFIG.2, the crankpin3is received in the hole19. According to the illustrated embodiments, the crankpin3is interference fitted into the hole19. The wording interference fit, as used herein, is intended to encompass a press fit, a shrink fit, or a friction fit which all are forms of fastening between two tight fitting mating parts that produces a joint which is held together by friction after the parts are assembled. Since the crankpin3forms part of the first crankshaft portion11, the first crankpin adjoining portion15can be provided with a smaller outer radius r1′ than the second crankpin adjoining portion17. The outer radius r1′ of the first crankpin adjoining portion15is indicated inFIG.1.

FIG.3illustrates a second cross section of the crankcase51and the power unit10illustrated inFIG.2. InFIG.3, the piston31and the crankshaft1are illustrated in positions corresponding to a bottom dead centre of the piston31relative to the cylinder53. As seen inFIG.3, the first crankpin adjoining portion15is located in the first enclosure section55and the second crankpin adjoining portion17is located in the second enclosure section57when the piston31is in a region of the bottom dead centre, whereas the first counterweight5is located in the first enclosure section55and the counterweight7is located in the second enclosure section57when the piston31is in a region of the top dead centre, as seen inFIG.2.

As is indicated inFIG.1, the first crankpin adjoining portion15has an outer radius r1′ smaller than, or equal to, the first outer radius r1of the first counterweight5. Likewise, the second crankpin adjoining portion17has an outer radius r2′ smaller than, or equal to, the second outer radius r2of the second counterweight7. In this manner, the crankshaft1can rotate in the crankcase51without hitting any delimiting surfaces of the crankcase51. Moreover, the fact that the first crankpin adjoining portion15can be provided with a smaller outer radius r1′ than the outer radius r2′ of the second crankpin adjoining portion17is utilized to minimize the free volume of the crankcase51. In this manner, a crankcase51having a small free volume can be obtained without using fillers arranged on the crankshaft1. According to the illustrated embodiments, the crankshaft1comprises no fillers. Fillers are profiles attached to a crankshaft to reduce the free volume of a crankcase. Fillers are effective in reducing the free volume of a crankshaft. However, they add costs, weight, and complexity to two-stroke engines. Moreover, fillers may impair the lubrication of a crank bearing60arranged between the crankpin3and the connecting rod33and may impair lubrication of main bearings54supporting the crankshaft1relative to the crankcase51. Most crankcase scavenged two-stroke piston engines are lubricated by adding a lubrication to the fuel, wherein an air/fuel mixture comprising the lubricant is led through the inner volume of the crankcase. Thus, by arranging fillers onto the crankshaft, these fillers may limit the lubrication obtained by the crank bearing arranged between the crankpin and the connecting rod as well as of main bearings supporting the crankshaft relative to the crankcase.

Accordingly, since the crankshaft1according to the embodiments herein is capable of reducing the free volume of the crankcase51without using fillers, the crankshaft1provides conditions for saving costs, weight, and complexity of a two-stroke engine2, while conditions are provided for obtaining a high degree of lubrication of the crank bearing60arranged between the crankpin3and the connecting rod33as well as of main bearings54supporting the crankshaft1relative to the crankcase51.

FIG.4illustrates a side view of the crankshaft/connecting rod assembly4according to the embodiments illustrated inFIG.1. InFIG.4, the crankshaft/connecting rod assembly4is illustrated in a viewing direction coinciding with the rotation axis ax of the crankshaft1. Moreover, inFIG.4, the crankshaft/connecting rod assembly4is illustrated from a second side S2of the assembly4. The second side S2of the assembly4is indicated inFIG.1. As clearly seen inFIG.1, the second side S2comprises the second counterweight7.

As is indicated inFIG.1andFIG.4, according to the illustrated embodiments, the second counterweight7comprises a recess21at a radially outer portion23thereof. Moreover, as is indicated inFIG.1, the recess21is provided at an axial end portion25of the second counterweight7, wherein the axial end portion25faces away from the crank plane cp. The recess21is made such that the radius r3measured from the rotation axis ax of the crankshaft1to a delimiting surface of the recess21at the axial end portion25of the second counterweight7is smaller than the second outer radius r2of the second counterweight7. Thereby, a piston connected to the piston connection portion34of the connecting rod33is allowed to move closer to the rotation axis ax of the crankshaft1when the piston is in a region of the bottom dead centre. In this manner, the free volume in a crankcase comprising the crankshaft can be further minimized, as is further explained herein.

As indicated inFIG.4, the recess21extends in a first angular sector A1of the second counterweight7measured from the rotation axis ax of the crankshaft1. According to the illustrated embodiments, the first angular sector A1is smaller than a total angular sector A2occupied by the second counterweight7measured from the rotation axis ax of the crankshaft1. Moreover, according to the illustrated embodiments, the first angular sector A1is approximately 0.69, i.e. approximately 69%, of the total angular sector A2. According to further embodiments, the first angular sector A1may be within the range of 0.15 to 0.93 of the total angular sector A2or may be within the range of 0.60 to 0.80 of the total angular sector A2.

According to the illustrated embodiments, the first angular sector A1is approximately 105 degrees and the total angular sector A2occupied by the second counterweight7is approximately 154 degrees measured from the rotation axis ax. According to further embodiments, the first angular sector A1may be within the range of 23 degrees to 142 degrees or may be within the range of 92 degrees to 122 degrees, measured from the rotation axis ax of the crankshaft1. Due to these features, it is ensured that a piston connected to the piston connection portion34of the connecting rod33is allowed to move closer to the rotation axis ax of the crankshaft1when the piston moves past the region of the bottom dead centre.

FIG.5illustrates a third cross section of the crankcase51and the power unit10illustrated inFIG.2andFIG.3. InFIG.5, the viewing direction coincides with the rotation axis ax of the crankshaft1. Moreover, the viewing direction is in a direction towards the second side of the crankshaft1comprising the second counterweight7, as referred to above. InFIG.5, the piston31and the crankshaft1are illustrated in positions corresponding to a bottom dead centre of the piston31relative to the cylinder53.

The piston31comprises a portion35cyclically assuming a first position relative to the rotation axis ax of the crankshaft1in which the radial distance r4between the portion35of the piston31and the rotation axis ax is shorter than the second outer radius r2of the second counterweight7. According to the illustrated embodiments, the portion35of the piston31is in the first position when the piston31is in the region of the bottom dead centre relative to the cylinder53. As can be seen inFIG.5, the portion35of the piston31extends into the recess21when the portion35of the piston31is in the first position. According to the illustrated embodiments, the portion35of the piston31forms part of a stratified scavenging intake arrangement43of an engine2. That is, according to the illustrated embodiments, the piston31comprises an aperture37. The aperture37is configured to cyclically superimpose an intake41of a stratified scavenging intake arrangement43of an engine2, so as to transfer air from the intake41to an intake port of the engine2during operation of the engine2. In this manner, emissions of unburnt hydrocarbons from the engine2can be reduced. The portion35of the piston31, which extends into the recess21when the portion35of the piston31is in the first position, is a portion35of a section36forming part of delimiting surfaces38of the aperture37.

Thus, due to the recess21, the piston31is allowed to move closer to the rotation axis ax of the crankshaft1when the piston31is in a region of the bottom dead centre, while having the section36forming part of the delimiting surfaces38of the aperture37.

According to embodiments herein, the crankshaft1explained above may comprise a first and a second counterweight5,7arranged on either side S1, S2of the crank plane cp, wherein the first and second counterweights5,7have the same outer radiuses r1, r2measured from the rotation axis ax of the crankshaft1, and wherein one of the first and second counterweights5,7has a recess21as explained with reference toFIG.1,FIG.4andFIG.5. In this manner, conditions are provided for reducing the free volume of the crankcase51of an engine2because conditions are provided for letting a piston31of the engine2to move closer to the rotation axis ax of the crankshaft1.

FIG.6illustrates a hand-held power tool20according to some embodiments. The hand-held power tool20comprises a tool61and a crankcase scavenged single-cylinder two stroke piston engine2, according to the embodiments explained with reference toFIG.1-FIG.5. That is, the two stroke piston engine2of the power tool20, may comprise a crankshaft/connecting rod assembly4according to the embodiments explained with reference toFIG.1andFIG.4and/or a crankcase51and a power unit10, according to the embodiments explained with reference toFIG.2,FIG.3, andFIG.5.

The crankcase scavenged two stroke piston engine2is configured to power the tool61during operation of the hand-held power tool20. According to the illustrated embodiments, the hand-held power tool20is a chainsaw, i.e. a power tool20comprising a tool61in the form of a cutting chain61. According to further embodiments, the hand-held power tool20, as referred to herein, may be another type of hand-held power tool, such as a trimmer, a brush cutter, a debris blower, a circular saw, a multi-tool, a hedge trimmer, or the like.

The wording “hand-held” indicates that the power tool referred to is portable and is configured to be supported by one or two hands of a user during operation of the power tool.

An outer radius, as referred to herein, is a radius measured from the rotation axis ax of the crankshaft1to a radially outer portion of the object referred to. Moreover, an outer radius, as referred to herein, may be measured in a plane parallel to the crank plane cp. Likewise, an angular sector, as referred to herein, may be located in a plane being parallel to the crank plane cp.

It is to be understood that the foregoing is illustrative of various example embodiments and that the invention is defined only by the appended claims. A person skilled in the art will realize that the example embodiments may be modified, and that different features of the example embodiments may be combined to create embodiments other than those described herein, without departing from the scope of the present invention, as defined by the appended claims.

As used herein, the term “comprising” or “comprises” is open-ended, and includes one or more stated features, elements, steps, components, or functions but does not preclude the presence or addition of one or more other features, elements, steps, components, functions, or groups thereof.