Internal-combustion engines – Two-cycle – Rear compression
Reexamination Certificate
2000-08-24
2001-07-31
McMahon, Marguerite (Department: 3747)
Internal-combustion engines
Two-cycle
Rear compression
C123S0650PD
Reexamination Certificate
active
06267088
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to a two-stroke engine which is used especially as a drive motor in a portable handheld work apparatus such as a motor-driven chain saw, brushcutter, cutoff machine, blower apparatus or the like.
BACKGROUND OF THE INVENTION
A two-stroke engine of this kind is disclosed in international patent publication WO98/17901 and includes a combustion chamber defined by a cylinder and delimited by a reciprocating piston. The crankcase is connected to the combustion chamber via transfer channels. The first end of a transfer channel faces toward the cylinder and opens into the combustion chamber via an entry window lying in the cylinder wall and the lower second end of the transfer channel opens to the crankcase. The entry window of the transfer channel, which lies in the cylinder wall, is controlled by the piston in the manner of a slot control, that is, the entry window is opened or closed in dependence upon the stroke position of the piston.
The air/fuel mixture, which is necessary to operate the engine, is drawn in by suction through a mixture-preparation device and an inlet into the crankcase and, with a downward travel of the piston, is pushed into the combustion chamber via the transfer channels. To reduce the exhaust-gas emissions, fuel-free gas, especially air, is provided in the transfer channels arranged to the right and to the left of the outlet. This fuel-free gas is supplied to the transfer channels via respective gas channels.
In the induction stroke, and with the piston traveling upwards in the direction of top dead center, a mixture is drawn by suction into the crankcase, on the one hand, via the inlet from the mixture-preparation device; on the other hand, fuel-free air flows into the crankcase via the transfer channels from the gas channel. With the downwards travel of the piston in the direction of bottom dead center, the mixture is displaced from the crankcase via the transfer channels into the combustion chamber. For an operation as a scavenging engine, first, because of the charge of the transfer channels with air, fuel-free air flows into the combustion chamber ahead of the air/fuel mixture whereby the scavenging losses are reduced. In a subsequent upward stroke, residual amounts of the air/fuel mixture remain in the transfer channel from the previous stroke. These residual amounts are scavenged with fuel-free gas, especially air, in a next induction stroke. In practice, it has been shown that the inflowing gas flow of fuel-free air cannot always ensure a complete scavenging of the transfer channel so that residual amounts of the air/fuel mixture of a previous stroke enter the combustion chamber in a subsequent stroke together with the fuel-free air. For this reason, the scavenging losses increase. Because of the incomplete scavenging of the transfer channels with the fuel-free gas, the desired low exhaust-gas emissions often cannot be maintained.
SUMMARY OF THE INVENTION
It is an object of the invention to provide a two-stroke engine of the kind referred to above which is so improved that a complete scavenging of the transfer channels with fuel-free gas, especially air, is ensured.
The two-stroke engine of the invention includes a two-stroke engine in a portable handheld work apparatus. The two-stroke engine includes: a cylinder having a cylinder wall; a piston mounted in the cylinder to undergo a reciprocating movement along a stroke path between top dead center and bottom dead center during operation of the engine; the cylinder and the piston conjointly delimiting a combustion chamber; a crankcase connected to the cylinder; a crankshaft rotatably mounted in the crankcase; a connecting rod connecting the piston to the crankshaft to permit the piston to drive the crankshaft as the piston reciprocates in the cylinder; at least one transfer channel connecting the crankcase to the combustion chamber; the transfer channel having a first end defining an entry window opening into the combustion chamber; the entry window being formed in the cylinder wall and being controlled by the piston as the piston moves in the cylinder; the transfer channel having a second end opening into the crankcase; a gas channel for supplying essentially fuel-free gas flow to the engine; a check valve for connecting the gas channel to the transfer channel at a location thereon between the first and second ends so as to permit the fuel-free gas flow to flow from the gas channel into the transfer channel; a mixture-preparation device for supplying an air/fuel mixture; an intake channel for conducting the air/fuel mixture into the crankcase; the gas channel, the check valve and the transfer channel conjointly defining a flow path for the fuel-free gas flow; and, a flow element arranged along the flow path for fanning out the fuel-free gas flow.
The essentially fuel-free gas flow which flows from the gas channel into the transfer channel is broadly fanned out by the flow element provided according to the invention whereby the total cross section of the transfer channel is charged over its entire length with component flows flowing in various directions. In this way, a complete scavenging of the transfer channel with fuel-free gas is ensured within the shortest time. Even at high engine speeds, a complete scavenging of the transfer channel is ensured.
In a first embodiment of the invention, the flow element, which fans the gas flow, is provided as a recess configured in the roof of the transfer channel toward which the flow is directed. This recess is purposefully to be configured in dependence upon the dimensions of the transfer channel. The gas flow, which enters from the gas channel into the transfer channel, is directed into the recess and is there broken up and swirled by the base and the side walls of the recess so that swirled air masses moved with a high intensity flow from the transfer roof to the crankcase. Because of the air masses moved at high intensity, it is ensured that no penetration of the residual gases, which are present in the transfer channel, results; instead, these residual gases are acted upon over the entire cross section of the transfer passage and are scavenged.
A profiled channel segment of the transfer channel can be used as a flow element which fans out the gas flow. The profiled channel segment lies downstream of the valve in the region toward the crankcase. This profiled channel segment is purposefully approximately at the elevation of the valve and can be formed by a flow body which is mounted on the wall of the transfer channel lying opposite to the valve.
In a further embodiment, the flow element, which fans out the gas flow, can be provided by the configuration of the membrane of the valve configured as a membrane valve. For this purpose, it is provided to configure the portion of the membrane projecting in the longitudinal direction of the transfer channel to be greater than the portion of the membrane projecting transversely to the longitudinal direction of the transfer channel so that already when flowing over into the transfer channel, the gas flow is subdivided into a first component flow directed toward the roof of the transfer channel and second, third and additional component flows flowing laterally over the membrane. The projecting portion on the end of the membrane facing toward the roof of the transfer channel is preferably approximately twice as large as the portions of the membrane projecting to the sides and to the foot of the membrane.
REFERENCES:
patent: 5379732 (1995-01-01), Mavinahally et al.
patent: 5628295 (1997-05-01), Todero et al.
patent: 5899177 (1999-05-01), Binversie et al.
patent: 6101991 (2000-08-01), Glover
patent: 0 933 514 (1999-08-01), None
Bergmann Lars
Klimmek Axel
Rosskamp Heiko
Andreas Stihl AG & Co.
McMahon Marguerite
Ottesen Walter
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