Patent Publication Number: US-6213066-B1

Title: Drive unit

Description:
TECHNICAL FIELD 
     The subject invention refers to a drive unit for a handheld working tool, such as a chainsaw, trimmer or hedge trimmer, said drive unit essentially comprises a housing part with a fuel tank and an engine unit with a fuel supply unit, such as a carburetor, a muffler and a crankcase scavenged combustion engine, with cylinder, piston and crankshaft. 
     BACKGROUND OF THE INVENTION 
     Due to requirements of low weight and compact design portable working tools are usually run by a crankcase scavenged two-stroke engine. However, a crankcase scavenged four-stroke engine is also conceivable. As a result of the crankcase scavenging, oil mixed with the fuel lubricates the engine. This means that the tool can be oriented in different directions while running, such as sideways or upside-down. The tools usually have a carburetor connected to an inlet duct, but also a low pressure injection system can be of interest. Since the tool can be used in a lot of positions there is a risk for fuel drops to accumulate inside the inlet duct and then be tipped into the cylinder when the tool&#39;s inclination is changed. This can bring about that the engine stops. The greatest risk for this is at lower engine speed. For this reason, the carburetor is usually placed near the inlet port resulting in a short inlet duct. Considering engine power at operating speed it would however be preferable to use a longer inlet duct. This could however be associated with said difficulties as well as pure space problems. Therefore the carburetor is usually adjacent the cylinder&#39;s top section, and provided With some kind of heat protecting baffle. This baffle implies a complication at the same time as temperature problems can still arise for the carburetor. Furthermore, the inlet duct usually turns obliquely downward towards the crankcase. Owing to this the inlet gases can not assist in cooling the critical area around the exhaust port. 
     PURPOSE OF THE INVENTION 
     The purpose of the subject invention is to substantially reduce the above outlined problems. 
     SUMMARY OF THE INVENTION 
     The above mentioned purpose is achieved in a drive unit, in accordance with the invention, having the characteristics appearing from the appended claims. 
     The drive unit according to, the invention is thus essentially characterized in that at least one inlet port is located in the engine cylinder wall and/or in an adjacent part of the engine crankcase, arid that the engine inlet duct from the inlet port up to the fuel supply unit has a length longer than 1.8 times the engine cylinder stroke, preferably longer than 2.0 times the cylinder stroke. The long inlet duct gives advantages but also implies a number of risks as mentioned above. It requires an elaborate drawing of the inlet duct so that the risk for fuel drops to accumulate in the inlet duct is reduced. Furthermore it is advantageous to have a long preheated section of the inlet duct, which vaporizes fuel drops and thus reduces said risks. By a suitable angling of the inlet duct a cooling effect can also be created around the cylinder&#39;s exhaust port and adjacent parts of the piston. These and other characteristic features and advantages of the invention will become apparent from the following detailed description of various embodiments with the support of the annexed drawing. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The invention will be described in closer detail in the following by way of various embodiments thereof with reference to the accompanying drawing, in which the same numbers in the different figures state one another&#39;s corresponding parts. Alternative parts are designated by a sign&#39; and are shown with phantom lines, e.g. handle  20 ′ in FIG. 1 is an alternative to handle  20 . 
     FIG. 1 shows a drive unit in accordance with the invention seen in a lateral cross-sectional view. In this case the drive unit is intended for a chainsaw, having a guide bar  23  shown in phantom. However, the drive unit can also be used for other applications, such as a hedge trimmer or trimmer. 
     FIG. 2 is a partial enlargement of FIG.  1 . 
    
    
     In the schematic FIG. 1, reference numeral  1  designates a drive unit in accordance with the invention. The drive unit  1  is essentially composed of a housing part  2  with a fuel tank  3  and an engine unit  4 . The engine unit  4  is composed of a crankcase scavenged combustion engine  7  with a fuel supply unit  5 , such as a carburetor and a muffler  6 . The drive unit  1  in the figure is intended for a chainsaw of so called “top-handle” model, where the handle is positioned on the top of the saw. This type of chainsaw is usually small and light-weight and is balanced to enable some cutting to be done while holding the saw with only one hand. The saw is held by the handle  20  which is provided with a throttle control  21  that can be controlled by the operator&#39;s finger. 
     However, the drive unit  1  can also be used for other applications. If the drive unit  1  is used for a so-called trimmer, the drive unit  1  is not mounted to a handle  20 , but is instead mounted on a rig tube. In this case the control cables and/or electric cables extend from the handle to the drive unit  1 , for control of engine speed and stop function. The rig tube extends parallel to a centerline  17  of the engine&#39;s crankshaft and perpendicular to the illustrated plane of the figure. 
     When starting the tool, usually with a starter rope, the tool is placed on the ground  25 . The drive unit  1  can also be used for a chainsaw with conventional handles. Such kind of chainsaw has a rear handle  20 ′ with throttle control  21 ′ and a side handle (not shown). In that case the handle  20  is omitted. This kind of chainsaw should always be used, with two hands, which is recommendable from a safety point of view. In both cases the guide bar  23  with saw chain are arranged in a conventional way. 
     FIG. 2 shows the drive unit  1  enlarged. The engine unit  4  is attached to a housing part  2 , which also comprise; handle  20  with throttle control  21 . The engine unit  4  in question is a one-cylinder two-stroke engine, usually with a displacement between 25 and 100 cc. The engine unit  4  is air-cooled and therefore provided with a large number of cooling fins  26  and with a cooling fan (not shown). The engine unit  4  has a cylinder  8  with a piston  15  with a piston rod  29 , which rotates the engine&#39;s crankshaft about its center  17 . The engine unit  4  has an inlet port  9  with an associated inlet duct  13 . Furthermore the engine has an exhaust port  16  with a directly mounted muffler  6 . A spark plug  24  connects to the  30  for the cylinder  8  extends down through the crankshaft center  17 . In this case crank pin center  31  is also located on the cylinder centerline  30 . The distance between crankshaft center  17  and crank pin center  31  represents half of the engine&#39;s cylinder stroke  18 , which is marked in the figure. 
     The engine has an extremely long inlet duct  13 . This creates opportunities for increasing the engine&#39;s traction power at normal operating speed, and can give higher power and/or cleaner exhaust gases at operating speed. Due to increased throttling on the exhaust side, less exhaust emissions can be achieved while retaining power. The reason for this improvement is that a higher engine charging efficiency can be obtained at the present engine speed. This is achieved due to a more favorable Helmholz resonance frequency. The resonance frequency is determined essentially by the relation between the length of the inlet duct  13  and the crankcase volume, which is relatively well reflected by the cylinder stroke  18 . From a calculation point of view the distance from an air filter  32  to the inlet port  9  is mostly representative as the length of the “inlet duct”. However, it is also advantageous to place the air filter  32  adjacent the fuel supply unit  5  in order to reduce disturbances from pressure variations, so called “quarter-waves”. Long inlet ducts are not being used for these drive units for several reasons. Obviously, a long inlet duct takes up a great deal of space which is critical for a handheld working tool considering its demand for being compact and low-weight. However, a more serious problem, when handling the tool, is the risk of engine stop, especially in connection with idling. This is because fuel drops accumulate in a long inlet duct. When handling the tool the result can be that fuel is being poured into the engine which then stops. However, it has turned out that these risk factors can be reduced by using a very elaborate shaping of the inlet duct&#39;s geometry and by using a long preheated section of the inlet duct. Normally the engine has an inlet port  9  located in the engine cylinder wall  10  and/or an adjacent part of the engine crankcase  11 . This location creates conditions for an advantageous drawing of the inlet duct  13 . Preferably the opening and closing of the inlet port  9  is controlled by the movement of the piston  15 , but the inlet duct  13  can also be provided with a so called reed valve. Preferably the inlet duct  13  from the inlet port  9  on to the fuel supply unit  5 , has a length longer than 1.8 times the engine cylinder stroke  18 , preferably longer than 2.0 times the cylinder stroke  18 . 
     The inlet duct  13  has a long preheated end section  12 . Fuel drops can hereby be vaporized in this section, so that a homogeneity of the fuel mixture can be achieved. The end section  12  is arranged to form a fixed and heat conducting part of the cylinder  8  and has preferably a length longer than 0.8 times the engine cylinder stroke  18 , preferably longer than 1.0 times the cylinder stroke  18 . Preferably the end section  12  is cast in one piece together with the cylinder  8 , but the end section  12  can also comprise a firmly mounted tube. 
     As appears from the figure inlet duct  13  is directed towards the cylinder&#39;s top section with the combustion chamber  14 . Otherwise these tools&#39; inlet ducts normally have an opposite inclination, sloping down towards the crankcase  11 . This can provide a somewhat lower inflow resistance. The advantage of having the direction towards the cylinder&#39;s top section is that the inflowing gases are cooling the piston  15  and the cylinder wall  10  at the engine exhaust port  16 . Hereby the risk of engine seizure is reduced. In the present application the cylinder  8  is almost completely lying down and the inlet duct  13  has a strong inclination in relation to the cylinder&#39;s centerline  30 . However, the cooling effect can also be achieved at less inclination. The determining fact is that the end section  12  of the engine inlet duct  13  up to the inlet port  9  differs from a perpendicular direction in relation to the cylinder&#39;s axial extension and is directed more towards the exhaust port  16  and combustion chamber  14  of the cylinder  8 . Then the inlet gases will be deflected, either towards the piston  15  or towards the cylinder wall  10 , and thus create the desired cooling effect. 
     As mentioned, the drive unit  1  in accordance with the invention, can be used both for a working tool with a vertical crankshaft, such as a hedge trimmer, or for a tool with a horizontal crankshaft. The orientation is then related to when the tool is placed on horizontal ground  25 . In the shown embodiment crankshaft  17  is essentially horizontal and the cylinder  8  extends axially in an essentially horizontal direction, where the crankshaft centerline  17  inclines more than 45 degrees in relation to a vertical line, and the inlet duct  13  and the fuel supply unit  5  are essentially located on top of the cylinder  8 , being above a plane which extends through the crankshaft center  17  and the cylinder center  30 . The location of the fuel supply unit  5  and the drawing of the inlet duct  13  are of very great importance. In the present case the inlet duct  13  runs with an even inclination down towards the cylinder  8  where the inlet duct  13  bends and then slopes more steeply into the inlet port  9 . When the tool is standing on plane ground  25  and at the same time is idling, some fuel drops inside the inlet duct  13  can thus flow down into the cylinder  8  all the time. This continuous supply is much more propitious than if a larger quantity would suddenly flow down. This could happen if the inlet duct  13  had a horizontal part where fuel could accumulate into a pool. When the user is lifting the tool up the pool can be released by another inclination and flow down into the tool causing engine stop. FIG. 2 shows a connection  38  in the end section  12 . For the sake of clarity the connection  38  is only shown in the enlarged FIG.  2 . Connection  38  communicates the inlet duct  13  with the crankcase  11 , so that fuel drops can run from the inlet duct  13  to the crankcase  11 . Hereby a certain improvement of the idling is achieved. Preferably the connection  38  has a collecting portion, which opens into the wall of the end section  12  and has a diameter of approximately 3 mm, and another portion, which opens into the crankcase  11  and has a smaller diameter of approximately 1 mm. Placing the connection  38  adjacent the outer mouth of the end section  12  will make it possible and easy to drill the holes through the mouth of the end section  12 . 
     In the present case inlet duct  13  consists of both the propitious preheated end section  12  and an upstream part  19 , which is associated with the end section  12  and continues on to the fuel supply unit  5 . The upstream part  19  has an essentially smooth inner side. Otherwise it is more common to use folded rubber constructions for the upstream part  19  of the inlet duct  13 . This is because it is more common to use an anti-vibration carburetor. In that case the engine unit  4  is elastically suspended in the housing part  2  and the fuel supply unit  5  is mounted into the housing part  2  and the upstream part  19  of the inlet duct  13  is made of a compliant material, such as rubber. Hereby the engine itself moves while the carburetor and the drive unit  1  with handle  20  is vibrating much less. In the present case three elastic attachments  37  are used, usually steel springs for mounting of the combustion engine proper. The upstream part  19  is located adjacent the drive unit&#39;s movement center, which results in small movements in the part  19  and allows using a flat tube. The upstream part  19  thus continues essentially in the same direction as the end section  12  on to the fuel supply unit  5 . 
     Hereby an even inclination down towards the cylinder  8  is created, and the cylinder  8  is lying substantially horizontal. The inlet duct  13  runs away from the top section of the cylinder  8  where commonly a spark plug  24  is located. The inlet port  9  is preferably located in the cylinder wall  10 . This arrangement creates conditions, on the one hand for a long inlet duct  13  with a long preheated section  12 , and on the other hand for an advantageous placing of the fuel supply unit  5 . 
     Said fuel supply unit  5  will thus be situated far away from the warmest parts of the engine, which is an obvious advantage. An air filter  32  is connected to the fuel supply unit  5  with a some kind of quick fastener. In this case there is a screw handle  33  which fastens the air filter  32 . The air filter  32  is located at one side of the drive unit  1 , said side is essentially opposite to that side where the cylinder&#39;s top section is located. This makes it easy to change air filter  32  by loosing the screw handle  33  and a protective cover  34 . 
     In the shown embodiment of the top handle saw, the cylinder  8  is almost lying down, such that the cylinder centerline  30  inclines less than 15 degrees in relation to the horizontal ground  25 , which goes along the tool&#39;s under edge. Hereby the handle  20  with the throttle control  21  can be placed very close to and above the engine unit  4 . The handle&#39;s longitudinal direction is essentially parallel with the centerline  30  of the cylinder  8 . This enables a saw with good balance characteristics and a low total height. The handle  20  is located so far forward that the throttle control  21 , where the user grips the handle  20 , is essentially just above the crankshaft center  17 , which contributes to the good balance. The fuel supply unit  5  is located in front of a recess  22  in the handle  20  further away from the cylinder&#39;s top section. This means that the long inlet duct  13  is not only contributing to a powerful engine, but also to that the handle  20  can be placed low down to give the tool a low total height. To be able to give throttle with the throttle control  21  a conventional safety catch  35  in the handle  20  must be pushed in. A lifting hole  36  is located in the rear part of the handle  20 . By connecting a lifting strap to this lifting hole  36  the user can carry the chainsaw and still have both hands free. The guide bar  23  of the saw, with the saw chain, is directed essentially in the same direction as the fuel supply unit  5 , away from the cylinder&#39;s top section. This also results in a special advantage when removing a saw which has got stuck in a tree. The elastically suspended engine unit  4  is then moving in a direction towards the fuel supply unit  5  and the rubber tube will be compressed instead of being pulled out or folded. This is advantageous as regards durability. In this case the housing part  2  has three tanks, one fuel tank  3  with an adherent expansion tank  27 , and one chain oil tank  28 . Naturally, the chain oil tank  28  is specific to chainsaw applications. One problem with the cylinder  8  being horizontal is that the muffler  6  ends up under the cylinder  8 . This problem is reduced by the exhaust port  16  and the scavenging ducts (not shown) being tilted by 15 degrees from a vertical position. As a result, the muffler  6  is also tilted, giving the muffler  6  more space. In this embodiment, the inlet port  9  is also tilted by the same angle of 15 degrees. However it is not necessary for the inlet port  9  to be tilted at the same angle as the exhaust port  16 , the scavenging ducts, and the muffler  6 . The angle of tilt for the inlet port  9  can be larger or smaller. As best shown in FIG. 2, one of the crankcase  11  halves is integrated into the cylinder  8 , and the end section  12  of the inlet duct  13  extends to the plane dividing between the crankcase  11  halves.