Internal-combustion engines – Two-cycle – Rear compression
Reexamination Certificate
2000-08-24
2001-07-10
McMahon, Marguerite (Department: 3747)
Internal-combustion engines
Two-cycle
Rear compression
C123S0730PP, C123S0650PD
Reexamination Certificate
active
06257181
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 a transfer channel. The first end of the 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. The transfer channels advantageously lie opposite each other with respect to the cylinder axis. To reduce the exhaust-gas emissions, a fuel-free gas, especially air, is provided in the transfer channels and is supplied via a gas channel to each transfer channel. For this purpose, each transfer channel, between its ends, communicates with an air-conducting gas channel and a check valve configured as a membrane valve.
During the induction stroke, an air/fuel mixture is, on the one hand, inducted into the crankcase via the inlet from the mixture-preparation device when there is an upward travel of the piston in the direction of top dead center. On the other hand, fuel-free air flows in from the gas channel via the transfer channels. For a piston traveling downward in the direction of bottom dead center, the air/fuel mixture is displaced from the crankcase via the transfer channels into the combustion chamber. Because the transfer channels are filled with air, fuel-free air first flows into the combustion chamber ahead of the air/fuel mixture whereby the scavenging losses are reduced. In the following upward stroke, residual amounts of the air/fuel mixture are in the transfer channel from the previous stroke. Because of the position of the check valve between the ends of the transfer channel, a region results in the channel section to the entry window into the combustion chamber which is not scavenged by the air flowing in during the induction stroke. In a following stroke, the residual amounts of the air/fuel mixture therefore flow out of the transfer channels first and only thereafter the air, which is introduced into the transfer channel, and then the air/fuel mixture from the crankcase. The residual portions of the air/fuel mixture, which remain in the transfer channels because of the scavenging dead volume, deteriorate the exhaust-gas emissions.
SUMMARY OF THE INVENTION
It is an object of the invention to provide a two-stroke engine which ensures a complete filling of the transfer channel with an advance quantity of fuel-free gas.
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 to the engine; a valve for connecting the gas channel to the transfer channel at a location thereon between the first and second ends; a mixture-preparation device for supplying an air/fuel mixture; an intake channel for conducting the air/fuel mixture into the crankcase; and, means for opening the entry window to the crankcase through an opening cross section at a stroke position of the piston along the stroke path in the region of the top dead center.
The valve can, for example, be a piston-ported control device or a check valve.
The entry window of the transfer channel is open to the crankcase in a stroke position of the piston in the region of top dead center. For this reason, a portion of the fuel-free gas, which flows via the check valve into the transfer channel, flows via the entry window in the cylinder wall to the crankcase and in this way completely scavenges possible residual quantities of an air/fuel mixture from a previous stroke. The transfer channel is completely filled with fuel-free gas, especially air, from its entry window in the cylinder wall to its end open to the crankcase so that, in a subsequent stroke ahead of an inflow of an air/fuel mixture, only fuel-free gas flows into the combustion chamber and scavenges the exhaust gases.
In order to ensure that the fuel-free gas also intensively scavenges the segment of the transfer channel to the crankcase, the opening cross section corresponds only to a portion of the area of the entry window whereby the main flow of the inducted fuel-free gas, namely air, leads from the check valve downwards to the open end of the transfer channel in the crankcase and only a component flow, which is lesser by volume, flows over through the opened entry window into the crankcase. It has been shown to be advantageous to provide an opening cross section of approximately 5% to 45% of the total area of the entry window, and especially 10% to 30% of the total area of the entry window.
In a further embodiment of the invention, a cutout is formed in the piston wall and the cutout lies approximately opposite the entry window in a stroke position of the piston in the region of top dead center. The upper edge of the cutout faces toward the base of the piston and is purposefully approximately at the elevation of the upper edge of the entry window facing toward the cylinder head so that (while considering the elevation of the entry window measured in the stroke direction of the piston) a connection of the transfer channel via the entry window and the cutout in the piston wall to the crankcase is given over a crankshaft angular region ahead of the upper top dead center. Advantageously, the edges lie precisely opposite each other at top dead center of the piston.
In a simple configuration, the cutout is a venting window in the piston wall and is open to the crankcase. The venting window is configured in elevation and/or in width less than the elevation and/or the width of the entry window. Advantageously, the venting window can also be configured as one or several through bores in the piston wall.
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
Rosskamp Heiko
Andreas Stihl AG & Co.
McMahon Marguerite
Ottesen Walter
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