U.S. Pat. No. 6,571,756 discloses a membrane-controlled two-stroke engine which draws an air/fuel mixture into the crankcase via an inlet and inducts fuel-free fluid such as pure air into the transfer channel via a membrane-controlled fluid channel. Pure air passes from the transfer channel window into the crankcase at the crankcase end of the transfer channel whereby the mixture, which is stored in the crankcase, is made lean. A corresponding quantity of oil must be supplied to the crankcase with the fuel in order to ensure an adequate lubrication of the moving parts in the crankcase. This leads to a coking in the muffler as well as in the combustion chamber and causes poor exhaust-gas values.
European patent publication 0,302,045 discloses an internal combustion engine having crankcase scavenging wherein the necessary combustion air is drawn by suction via the crankcase and the fuel, which is needed for operation, is injected into the combustion chamber via an injection nozzle in the region of the inlet window. An operation of a two-stroke engine of this kind requires, however, a separate lubrication system in the crankcase which is complex and can lead to an increased entry of oil into the combustion chamber.
It is an object of the invention to provide a method for operating a two-stroke engine having scavenging advance storage wherein good exhaust-gas values are obtained with excellent lubrication of all moving parts.
The method of the invention is for operating a two-stroke engine including a two-stroke engine for a portable handheld work apparatus. The two-stroke engine includes: a crankcase; a cylinder connected to the crankcase; the cylinder having a cylinder wall defining a cylinder; a piston displaceably mounted in the cylinder for reciprocating movement therein and the piston and the cylinder conjointly defining a combustion chamber; a crankshaft rotatably mounted in the crankcase; a connecting rod connecting the piston to the crankshaft so as to permit the piston to drive the crankshaft as the piston reciprocates in the cylinder; the crankcase having an inlet through which an air/fuel mixture is drawn into the crankcase during an intake phase of the engine; a transfer channel for conducting the air/fuel mixture from the crankcase into the combustion chamber; and, a fluid channel communicating with the transfer channel. The method of the invention includes the steps of: drawing a fluid into the transfer channel through the fluid channel during the intake phase and storing the inducted fluid in the transfer channel with the fluid being a fuel-poor to fuel-free fluid; and, adjusting lambda (λ) of the air/fuel mixture stored in the crankcase in a range of approximately 0.2 to 0.6.
The mixture stored in the crankcase is adjusted to very rich in the part-load and full-load ranges of the two-stroke engine and the value of lambda lies in a range of approximately 0.2 to 0.6. The rich mixture deposits on the moving parts in the crankcase and vaporizes whereby heat is drawn away from the crankcase because of the vaporization process. An excellent cooling of the engine results. The problem of icing of the carburetor is reduced because of the vaporization of the fuel in the crankcase.
Furthermore, the depositing fuel/oil wall film in the crankcase leads to an improved thermal transfer because the thermal transport from a crankcase, which is, for example, made of aluminum, to a wall film is better than to a gaseous mixture.
The developing fuel/oil wall film also provides a significantly better lubrication so that a defective lubrication of the moving parts is avoided.
The improved preparation of the fuel in the crankcase in combination with the improved lubrication makes possible a lower metering of the total fuel and oil quantities so that a reduced coking is present in the muffler and in the combustion chamber.
Preferably, lambda is adjusted in the range of 0.3 to 0.5. At idle, lambda is greater than 0.6 and drops to a value of approximately 0.3 with increasing load. Lambda preferably drops approximately continuously as a function of load.
In a special embodiment of the invention, the inducted fluid volume (fuel poor to fuel free, for example, a pure air volume) is stored completely in the transfer channel or in the transfer channels in the case of a multi-channel engine. The volume of a transfer channel or the sum of the total volume of several such transfer channels lies between an inlet window in the combustion chamber and a transfer window to the crankcase. This volume is designed to be greater than the fluid volume (fuel poor to fuel free) under full load. In this way, an overflowing of the transfer channels into the crankcase is avoided so that the adjustment of a low lambda is easily possible via the carburetor. Preferably, the total volume of the transfer channels is approximately 15% to 35% of the piston displacement of the engine.
The invention will now be described with reference to the drawings wherein:
The portable handheld work apparatus shown in
The engine 1 shown schematically in
For operating the engine 1, an air/fuel mixture is inducted into the crankcase 4 through an inlet 11 which, in this embodiment, is a piston-port control inlet. The air/fuel mixture is prepared in a carburetor 8 which is connected to the inlet 11 via an inlet channel 9.
Referred to the longitudinal center axis 19 of the cylinder 2, an outlet 10 lies opposite the inlet 11 offset in elevation. Combustion gases are discharged from the combustion chamber 3 via the outlet 10.
The mixture metering from the crankcase 4 to the combustion chamber 3 takes place via at least one transfer channel (12, 15) which can be configured in the cylinder wall 14. The transfer channels (12, 15) can also be outer channels.
In the embodiment shown, there are a total of four transfer channels (12, 15) of which each two are arranged on one side of a plane containing the longitudinal center axis 19 and running through the inlet 11 and the outlet 10. In
In the downward movement of the piston shown in
According to the invention, it is provided that the air/fuel mixture, which is supplied to the crankcase 4, is adjusted in such a manner that, in the crankcase 4, a value of lambda results in a range of approximately 0.2 to 0.6 as a function of load. Preferably, lambda is adjusted in a range of 0.3 to 0.5. At idle, lambda is preferably greater than 0.6 and falls with increasing load to a value of approximately 0.3 at full load 51. This drop is especially approximately continuous. In a part-load range 50 which follows idle, lambda is held approximately constant.
In the combustion chamber 3, in contrast, and preferably after the outlet is closed and before the transfer channels are opened, lambda is adjusted at approximately 0.7 to 0.95 over the entire load range. For this purpose, a fuel-poor to fuel-free fluid, especially fresh air, is conducted into the transfer channels (12, 15) via a fluid channel 17. In
With an upward movement of the piston 5 in the longitudinal direction of the longitudinal center axis 19, an underpressure results in the crankcase 4 which is not only present at the inlet 11 but also at the transfer windows 22 and 23 of the transfer channels 12 and 15. Because of the underpressure, the membrane valve 26 opens the fluid window 18 and fuel-poor to fuel-free fluid (especially pure air) flows according to arrow 28 through the fluid window 18 into the transfer channel 15 and displaces an air/fuel mixture of a previous transfer cycle which may possibly still be disposed therein.
The transfer channel 15 is so configured that the inducted fluid air volume or pure air volume is stored essentially completely in the transfer channel 15. For this reason, the total volume of the transfer channel 15, which lies between the entry window 16 into the combustion chamber 3 and the transfer window 23 to the crankcase 4, is designed to be equal, preferably greater, than the fluid volume or pure air volume inducted by the engine 1 under full load. The configuration in the embodiment of
The inducted fuel-poor to fuel-free fluid volume is stored only in the transfer channel 15 and therefore little or no fluid enters into the crankcase 4 from the transfer window 23. For this reason, the rich air/fuel mixture, which is inducted via the inlet 11, remains essentially unchanged in its composition so that the adjustment of the lambda of 0.2 to 0.6 in the crankcase is easily possible via the carburetor 8.
If an overflow of fuel-poor or fuel-free fluid (especially pure air) is permitted into the crankcase 4 from the transfer channels (12, 15), then this would not be adjusted to more than 20% to 30% of the channel volume of the transfer channels (12, 15). With an adjustment of the overflow volume of this kind, the adjustment of lambda of approximately 0.2 to 0.6 can be ensured in the crankcase as a function of the load.
The course of lambda under load is shown in FIG. 4. Lambda is plotted along the y-axis and the throttle flap angle (°DK) of a throttle flap mounted in the carburetor 8 is plotted on the x-axis (see FIG. 2). In a first part-load range 50, which follows idle, the lambda remains relatively large and corresponds approximately to the lambda value of about 0.75 which adjusts in the combustion chamber. Beyond the part-load range 50, lambda (λ) in the crankcase 4 drops with increasing load or throttle flap angle continuously to a value of about 0.2 at full load for a fully opened throttle flap (90°) at the end of the full-load range 51.
If one plots lambda, which adjusts in the crankcase, as a function of rpm (1/min), then, at low rpms under load, a value lambda of about 0.3 results which increases at high rpm under load to approximately 0.6. This behavior is significant for a membrane-controlled fluid window 18.
In contrast to the membrane-controlled scavenging engine shown in
As shown in
The operation of the two-stroke engine of
The trace of lambda as a function of load (degree of opening of the throttle flap angle—°DK) corresponds approximately to the trace shown in
The plot of lambda as a function of rpm remains approximately constant at 0.3 as shown by the solid curve in FIG. 4.
The adjustment of a rich air/fuel mixture having a value lambda of 0.2 to 0.6 leads to an improved cooling of the engine because the heat-draining vaporization process of the fuel no longer takes place only in the carburetor but also in the crankcase. The problem of an icing of the carburetor is reduced.
In total, less fuel and oil is supplied to the crankcase and a better cooling is nonetheless obtained because an air/oil wall film can form in the crankcase because of the low lambda. The wall film leads to an improved heat transfer from the material of the crankcase to the mixture and corresponds to an injection-oil cooling known per se. The forming fuel/oil wall film leads also to an improved lubrication of the moving parts because a thicker lubricant film is obtained. The reduced quantities of fuel and oil needed reduce a coking in the muffler and in the combustion chamber.
In the embodiments, the inlet 11 to the crankcase 4 is piston-port controlled. In lieu of a piston-port controlled inlet 11, a membrane-controlled crankcase inlet or even a rotating-disc controlled inlet can be practical. A valve can be used as a membrane valve of a membrane-controlled crankcase inlet and this valve can correspond to the membrane valve 26 with respect to its configuration.
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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102 41 213 | Sep 2002 | DE | national |
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6234456 | Gerhardy | May 2001 | B1 |
6298811 | Sawada et al. | Oct 2001 | B1 |
6571756 | Rosskamp et al. | Jun 2003 | B1 |
6595169 | Raffenberg et al. | Jul 2003 | B2 |
20020100438 | Raffenberg et al. | Aug 2002 | A1 |
Number | Date | Country |
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0 302 045 | Feb 1989 | EP |
Number | Date | Country | |
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20040045517 A1 | Mar 2004 | US |