Internal-combustion engines – Charge forming device – Crankcase vapor used with combustible mixture
Utility Patent
1999-12-07
2001-01-02
McMahon, Marguerite (Department: 3747)
Internal-combustion engines
Charge forming device
Crankcase vapor used with combustible mixture
Utility Patent
active
06167874
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to a portable handheld work apparatus such as a motor-driven chain saw, cutoff machine, hedge trimmer, blower apparatus, brushcutter or the like.
BACKGROUND OF THE INVENTION
U.S. Pat. No. 4,962,617 discloses a portable handheld work apparatus in the form of a cutoff machine having a drive motor configured as a two-stroke engine. The combustion chamber is delimited by the piston and is connected via transfer channels to the crankcase. The air/fuel lubricating oil mixture, which is needed for the operation, is drawn by suction via the crankcase and is conducted into the combustion chamber. Engines of this kind are especially suitable for use in portable handheld work apparatus because they combine low weight for the power generated with minimum complexity with respect to maintenance.
U.S. Pat. No. 6,819,418 discloses an overhead tree trimmer which includes a telescopic guide tube having a first end at which a drive motor is arranged and a second end on which a cutting device is mounted. The cutting device is a saw chain. Here too, a two-stroke engine is preferred as the drive motor and guarantees excellent manipulability of the overhead branch trimmer because of its low weight for the power generated.
In the same manner, U.S. Pat. No. 5,361,783 discloses a blower which utilizes a two-stroke engine for driving a blower wheel. The two-stroke engine has a minimum structural elevation which is measured in the direction of the cylinder axis and which is caused by the arrangement of the transfer channels and the inlet and outlet windows. This minimum structural elevation cannot be made any less.
European patent publication 0,615,576 discloses a brushcutter having a four-stroke engine. A separate lubricating-oil loop is provided for lubricating the moving parts of the four-stroke engine. On the one hand, a four-stroke engine of this kind is advantageous with respect to the exhaust-gas performance but the arrangement of the separate lubricating-oil loop leads to considerable technical complexity whereby the four-stroke engine is built large, is heavy and must have a corresponding quantity of lubricating oil ready for use in addition to a fuel tank. The engine is only reliable to a limited extent in all positions and requires a regular and complex maintenance because of the technically complex construction. This known separately lubricated four-stroke engine is built larger and heavier than a two-stroke engine. For this reason, the housing must be newly constructed for use in portable handheld work apparatus while also considering the center of gravity which changes because of the increased weight.
SUMMARY OF THE INVENTION
It is an object of the invention to provide a work apparatus of the kind described above which is improved so that the exhaust-gas emissions are significantly reduced compared to a two-stroke engine. It is still another object of the invention to provide such a work apparatus which provides greater convenience as to maintenance and for which the mounting space and the weight of the engine remains low.
The portable handheld work apparatus of the invention includes a motor-driven chain saw, cutoff machine, hedge trimmer, blower apparatus, brushcutter or the like. The portable handheld work apparatus includes a work tool and an internal combustion engine for driving the work tool. The engine includes: a cylinder; a piston movably mounted in the cylinder; the cylinder and the piston conjointly delimiting a combustion chamber; the cylinder having an intake opening and an intake valve for opening and closing the intake opening; the cylinder having an exhaust opening and an exhaust valve for opening and closing the exhaust opening; a valve housing connected to the cylinder; the intake valve and the exhaust valve movably mounted in the valve housing; a crankcase connected to the cylinder and communicating with the valve housing; a crankshaft rotatably journalled in the crankcase; a connecting rod interconnecting the piston and the crankshaft; the piston reciprocating in the cylinder to rotatingly drive the crankshaft via the connecting rod and alternately generate an overpressure and an underpressure in the crankcase; a valve drive assembly driven by the crankshaft for actuating the intake valve and the exhaust valve; a mixture-preparation device for supplying an air/fuel lubricant mixture; an intake channel conducting the mixture to the intake valve and the combustion chamber; means for charging the valve drive assembly and the valve housing with at least a portion of the air/fuel lubricant mixture in response to the overpressure and underpressure; and, the engine being a short stroke engine having a stroke/bore ratio of less than 0.8.
The elevation, which is measured in the direction of the cylinder axis, is significantly reduced because of the short-stroke configuration of the engine with a stroke-bore ratio of less than 0.8. For this reason, the dimensions in the direction of the vertical axis of the cylinder do not become greater than for a two-stroke engine because of the arrangement of the inlet and outlet valves and their cam drive notwithstanding the valve housing. By eliminating the transfer channels, the cylinder bore can be configured larger so that, in the end result, the stroke volume of the engine remains essentially unchanged when the piston stroke is short.
The lubrication of all movable parts in each position of the engine is guaranteed by the selected mixture supply to the valve drive, the valve housing, and the crankcase. A separate oil loop is not required because the pumping of the mixture via the crankcase pressure (crankcase pump) ensures an adequate lubrication in every operational position of the engine. In this way, an excellent all-position reliability at low weight results and the lubrication remains reliable even at extreme temperatures. The engine according to the invention is therefore especially suitable for motor-driven chain saws.
Because of the mixture lubrication, only one fuel tank is needed for the engine so that the fuel/lubricating oil mixture can still be used in a manner known per se as is conventional in the case of a two-stroke engine. In this way, the four-stroke engine according to the invention can be exchanged for and mounted in lieu of a two-stroke engine. Larger changes on the housing become unnecessary because of the approximately same mounting space as needed for a two-stroke engine. The structural complexity of the four-stroke engine is low because of the mixture lubrication which is provided since oil loops, oil wiper rings or like components are unnecessary.
Advantageously, the valve drive assembly, the valve housing and the crankcase are connected to the intake channel via a common connecting channel. In a first embodiment, the crankcase is connected to the intake channel exclusively via the valve drive assembly, the valve housing and the connecting channel. It has been surprisingly shown that a connection of this kind to the intake channel results in a lubrication which operates in all positions.
In another embodiment of the invention, the valve drive assembly, the valve housing, the crankcase and the first connecting channel together with a second connecting channel are configured as an annular line so that the intake channel is connected via the second connecting channel to the crankcase, the crankcase is connected via the valve drive assembly to the valve housing and the valve housing is again connected via the first connecting channel to the intake channel. In a configuration of this kind, the mixture guidance takes place essentially in a flow direction.
REFERENCES:
patent: 4068636 (1978-01-01), Nau et al.
patent: 4162662 (1979-07-01), Melchior
patent: 4962617 (1990-10-01), Tilders et al.
patent: 5014663 (1991-05-01), Melchior
patent: 5361738 (1994-11-01), Iida
patent: 5582145 (1996-12-01), Aizawa et al.
patent: 5819418 (1998-10-01), Uhl
patent: 5947068 (1999-09-01), Araki
patent: 0615576 (1994-09-01), None
Becker Georg
H{umlaut over (a)}berlein J{umlaut over (u)}rgen
Andreas Stihl AG & Co.
McMahon Marguerite
Ottesen Walter
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