This application claims priority of German patent application no. 103 46 048.9, filed Oct. 2, 2003, the entire content of which is incorporated herein by reference.
The invention relates to a two-stroke engine including a two-stroke engine in a portable handheld work apparatus such as a motor-driven chain saw, cutoff machine or the like.
A two-stroke engine having transfer channels is disclosed in German patent publication 2,624,249. An ancillary outlet opening is provided in this two-stroke engine to reduce the development of noise during idle. The outlet is connected to the combustion chamber via this ancillary outlet opening during the downward stroke of the piston already in advance of the opening of the main outlet.
With an ancillary outlet opening of this kind, the running performance of the two-stroke engine can be improved when starting and during idle. However, the exhaust-gas values at full load of the two-stroke engine are thereby deteriorated.
It is an object of the invention to provide a two-stroke engine of the kind described above wherein an excellent running performance is provided during operation and wherein there are low exhaust-gas values at full load.
The internal combustion engine of the invention is for a portable handheld work apparatus including a motor-driven chain saw, cutoff machine, brushcutter or the like. The internal combustion engine includes: a cylinder; a piston mounted in the cylinder to undergo a reciprocating movement along a stroke path 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; an outlet for conducting exhaust gases away from the combustion chamber; at least one transfer channel for connecting the crankcase to the combustion chamber at pregiven positions of the piston; the transfer channel defining a transfer window opening into the combustion chamber; an ancillary channel having a first end opening into the combustion chamber and a second end opening into the transfer channel; the ancillary channel being open to the combustion chamber in a pregiven operating state during a downward stroke of the piston while the transfer window to the combustion chamber is still closed; and, means for opening the ancillary channel during idle and part load and for closing the ancillary channel during full load.
A decompression of the combustion chamber can take place via the ancillary channel in a pregiven operating state of the two-stroke engine, especially, during idle or part load. Because the ancillary channel opens into a transfer channel and not into the outlet, the situation is avoided that uncombusted fuel escapes from the combustion chamber into the outlet. The pregiven operating state is advantageously idle operation and part load operation. In this way, a quiet running of the engine can be achieved at idle. The idle rpm can be reduced and the idle emissions drop.
The ancillary channel opens into the combustion chamber at at least one inflow window. The upper edge of the inflow window lies approximately at the same elevation as the upper edge of the outlet or is offset relative to the upper edge of the outlet in a direction toward the combustion chamber. In each case, the upper edge of a window is the edge facing toward the combustion chamber and away from the crankcase. In this way, when opening the ancillary channel, the exhaust gas from the combustion chamber can be prestored in the transfer channel. The flow cross section of the ancillary channel is less than the flow cross section of the transfer channel. At low rpms and when starting the two-stroke engine, the small flow cross section of the ancillary channel is sufficient in order to effect a decompression in the combustion chamber and therewith make the rpm of the engine uniform and obtain an ignitable mixture in the combustion chamber. At high rpms of the two-stroke engine, the flow resistance of the ancillary channel is very high because of the high flow speeds so that the influence of the ancillary channel is minimal.
A switching device is advantageously provided for opening or closing the ancillary channel. The switching device can be manually actuated. However, it can also be practical that the switching device automatically switches the ancillary channel. The operation of the two-stroke engine is hereby simplified because the operator need not be troubled with the switching of the ancillary channel. A simple configuration of the two-stroke engine results when the switching device mechanically switches the ancillary channel. The switching device especially switches the ancillary channel in dependence upon the position of a throttle lever of the two-stroke engine. It can, however, be advantageous that the switching device controls the ancillary channel in dependence upon the rpm of the crankshaft. Here, especially a centrifugal-force controller is provided for determining the rpm.
Advantageously, the switching device pressure-control switches the ancillary channel. Especially, the switching device switches the ancillary channel in dependence upon the pressure level in different operating states of the two-stroke engine. It can, however, be practical that the two-stroke engine has a fan wheel which is fixed on the crankshaft and the switching device switches the ancillary channel in dependence upon the pressure generated by the fan wheel. A simple configuration of the switching device results when the switching device includes a valve or a flap. In order to obtain a good running performance of the two-stroke engine in all operating states, the ancillary channel is opened at idle and during starting of the engine and is closed at full load of the engine.
The invention will now be described with reference to the drawings wherein:
The two-stroke engine 1 shown in
In
An outlet 10 leads out of the combustion chamber 3 for discharging exhaust gases from the combustion chamber. The upper edge 22 of the outlet 10 is arranged approximately at the elevation of the upper edge 23 of the inflow window 17. In each case, the upper edge is the control edge of the inflow window 17 and the outlet 10 with these control edges facing toward the combustion chamber 3 and away from the crankcase 4. The outlet 10 and the inflow window 17 are thereby opened by the piston 5 approximately at the same time during the downward stroke of the piston 5. The inflow window 17 can, however, also be arranged above the outlet 10 and be opened before the outlet 10 during the downward stroke of the piston 5.
The two-stroke engine 1 is mounted especially in a portable handheld work apparatus such as a cutoff machine or the like. A throttle lever 32 is provided to facilitate operator control of the two-stroke engine 1. The throttle lever 32 is pivotally journalled at a rotation point 33, for example, on the housing of the work apparatus. An actuator 34 is fixed on the throttle lever 32 and acts on the carburetor 8. The actuator 34 can, for example, adjust a throttle flap mounted in the carburetor 8. Furthermore, an actuator 35 is fixed on the throttle lever 32 which acts on a switching device (not shown in
During operation of the two-stroke engine 1, an air/fuel mixture is drawn from the carburetor 8 into the crankcase 4 via the inlet 11 with the upward stroke of the piston 5 from the position shown in
The two-stroke engine 1 has a switching device 30 which includes a flap 18. The flap 18 is pivotally journalled on a joint 21 and closes the port 26 of the ancillary channel 14 in the closing position 19 shown in
The switching device 30 can be actuated manually by an operator via an operator-controlled element 38. The switching device 30 can, however, also be actuated automatically. For this purpose, a pressure transducer 27 is provided in the region of the fan wheel 38 of the two-stroke engine 1 with the fan wheel being fixed to the crankshaft 7. The flap 18 is opened or closed via the pressure on the pressure transducer 27. The pressure transducer 27 can, however, also determine the different pressure levels for different operating states of the two-stroke engine 1 within the engine, for example, in the crankcase 4 or in the intake channel 9.
The switching device 30 can, however, also be actuated mechanically. For this purpose, a centrifugal-force controller 29 is mounted on the crankshaft 7 and this controller causes the flap 18 to assume the closed position 19 or the open position 20 in dependence upon the rpm of the crankshaft 7. The ancillary channel 14 is open at idle and at part load and is closed at full load.
In
The ancillary channel 14 can be connected to one or to several and especially to all transfer channels (12, 15). When the ancillary channel 14 is connected to a transfer channel 15, it can be practical that the other transfer channels are closed at least during idle operation.
It is understood that the foregoing description is that of the preferred embodiments of the invention and that various changes and modifications may be made thereto without departing from the spirit and scope of the invention as defined in the appended claims.
Number | Date | Country | Kind |
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103 46 048.9 | Oct 2003 | DE | national |