Method for operating a two-stroke engine

Abstract

Method for operating a two-stroke engine, wherein the two-stroke engine (1) has a cylinder (2) in which a combustion chamber (3) is formed, which is bounded by a piston (5), wherein the piston (5) drives a crankshaft (7) rotatably mounted in a crankcase (4), and wherein the crankcase (4) is connected to the combustion chamber (3) in at least one position of the piston (5) via at least one transfer port (17), with an inlet (11) into the crankcase (4) and with an outlet (19) from the combustion chamber (3), with a device for supplying fuel, with a control (20), and with a means for sensing the crankcase pressure (p). KGH ), where the crankcase pressure (p KGH ) is determined during each engine cycle, characterized in that the fluctuation of the determined crankcase pressure (p KGH ) determined and used to determine whether combustion takes place in each engine cycle, with a limit value (Δp) grenz) is compared, whereby the fluctuation of the crankcase pressure (p KGH ) as the difference in crankcase pressure (p KGH ) to a mean value (p M ) of the crankcase pressure (p KGH ) is determined.

Description

[0001] The invention relates to a method for operating a two-stroke engine of the type specified in the preamble of claim 1.

[0002] It is known that two-stroke engines can enter four-stroke mode. In this operating state, combustion only occurs every other crankshaft revolution. This can result in increased emissions from the two-stroke engine. Furthermore, incorrect adjustments to the fuel delivery rate can occur during four-stroke operation. Therefore, it is desirable to determine whether combustion takes place during every engine cycle.

[0003] It is known from DE 10 2005 002 273 A1 that four-stroke operation has an effect on the pressure level in the crankcase. However, the pressure level in the crankcase alone does not reliably indicate four-stroke operation, as the pressure level is also influenced by other factors such as engine speed and other engine parameters.

[0004] From DE 10 2006 060 617 A1 a method for operating a two-stroke engine is known in which the air mass flow through the combustion chamber is determined from the crankcase pressure.

[0005] German patent DE 699 38 564 T2 describes a method for detecting engine faults by measuring pressure in the crankcase. For this purpose, the crankcase pressure is compared with a limit value that corresponds to the current engine operating conditions. If the engine limit value is exceeded, an engine malfunction, such as a cylinder or piston fault, is detected.

[0006] US Patent 5,284,118 A discloses a method for determining the amount of fuel supplied to a two-stroke engine.

[0007] The invention is based on the objective of creating a method for operating a two-stroke engine that can reliably determine, without the need for complex sensors, whether combustion takes place in each engine cycle.

[0008] This problem is solved by a method having the features of claim 1.

[0009] It has been shown that fluctuations in crankcase pressure provide a reliable indication of whether combustion occurs in every engine cycle, or whether engine cycles occur without combustion. To detect these fluctuations, only pressure measurement devices, such as a simple pressure sensor in the crankcase, are required. These are often already present, so no additional sensors are needed.

[0010] It has been shown that the crankcase pressure remains relatively constant at a given point in an engine cycle if combustion occurs in every cycle. However, if combustion does not occur in some engine cycles, the pressure level in the crankcase fluctuates considerably. This pressure fluctuation can be used to determine not only whether combustion occurs with every crankshaft revolution, but also whether the engine cycle regularly operates without combustion every other revolution (i.e., in four-stroke mode), or whether there is a different number of combustion and non-combustion cycles, for example, combustion occurring every third, fourth, or fifth crankshaft revolution.

[0011] The engine can then be controlled based on the recognized pattern of engine cycles with and without combustion.

[0012] Advantageously, the crankcase pressure fluctuation is determined as the difference between the crankcase pressure and a mean value. The mean value can, for example, be the average of several consecutively measured crankcase pressure values. Advantageously, the crankcase pressure is measured at the same crankshaft angle during each engine cycle. Specifically, the crankcase pressure is measured at a crankshaft angle where the crankcase is closed. In particular, the crankcase pressure is measured during the piston's upward stroke after the transfer port closes and before the intake opens. It has been shown that the pressure fluctuation in the crankcase results from the pressure fluctuation in the combustion chamber, which is transmitted to the crankcase via the transfer port.If the pressure in the crankcase is measured after the transfer port has closed and before the intake has opened, the pressure fluctuation in the crankcase is most pronounced, since the combustion chamber pressure has been transferred to the crankcase via the transfer port and the intake is still closed, meaning no fresh combustion air has yet been drawn in.

[0013] Advantageously, the fluctuation in crankcase pressure can be used to determine whether the engine is operating in four-stroke mode. If four-stroke operation is detected, the fuel supply is reduced until four-stroke operation ceases. To rule out the possibility that the pressure fluctuation in the crankcase is caused by other factors, such as a change in engine speed, in addition to the crankcase pressure, the engine speed and / or the engine's air consumption are monitored and compared to a limit value. The engine's air consumption can also be easily determined from the crankcase pressure signal at two predefined crankshaft angles: before and after the transfer ports open.

[0014] An embodiment of the invention is explained below with reference to the drawing. The drawing shows: Fig. 1 a perspective partially cutaway view of a two-stroke engine, Fig. 2 a diagram of the timing of the two-stroke engine from Fig. 1, Fig. 3 a diagram showing the crankcase pressure over time, Fig. 4 a diagram showing the amount of fuel supplied over time and Fig. 5 a flowchart of the method according to the invention.

[0015] Fig. Figure 1 shows a two-stroke engine 1, designed as a single-cylinder engine, which can, for example, be the drive motor in a handheld power tool such as a chainsaw, an angle grinder, a brush cutter, a lawnmower, or the like. The two-stroke engine 1 has a cylinder 2 in which a combustion chamber 3 is formed. The combustion chamber 3 is bounded by a piston 5 mounted to reciprocate within the cylinder 2. The piston 5 drives a crankshaft 7, which is rotatably mounted in a crankcase 4, via a connecting rod 6. In the area of ​​the Fig. At the bottom dead center of piston 5 shown in Figure 1, the interior of the crankcase 4 is via a total of four transfer ports 17, of which in Fig. The two ports shown in Figure 1 are connected to the combustion chamber 3. The transfer ports 17 open into the combustion chamber 3 via transfer windows 18. An exhaust port 19 leads from the combustion chamber 3 for exhaust gases. An inlet 11, controlled by the piston 5, opens into the crankcase 4. An intake port 12 opens into the inlet 11, through which combustion air is supplied to the two-stroke engine 1. It can also be provided that a fuel / air mixture is supplied to the crankcase 4 via the intake port 12. A throttle valve 13 is pivotally mounted in the intake port 12, which serves to control the amount of air supplied. A throttle position sensor 14 is arranged on the throttle valve 13, which can be used to determine the position of the throttle valve 13. However, the throttle position sensor 14 can also be omitted.

[0016] A fuel valve 15 is provided for the supply of fuel, which in the exemplary embodiment opens into a bypass channel 17. However, the fuel valve 15 can also open into the crankcase 4 or into the intake manifold 12. A temperature sensor 21 and a pressure sensor 22 are arranged on the crankcase 4. The temperature sensor 21, the pressure sensor 22 and the fuel valve 15 are connected to a control unit 20.

[0017] A generator 9 is arranged on the crankshaft 7, which supplies a speed signal to the control unit 20. The generator 9 can also supply energy to operate other electrical components and a spark plug 16. The spark plug 16 protrudes into the combustion chamber 3 and serves to ignite the mixture in the combustion chamber 3. A fan wheel 8 is also fixedly mounted on the crankshaft 7. An ignition module 10 is provided on the outer circumference of the fan wheel 8, in which the energy to operate the spark plug 16 is induced when the generator 9 is not used for this purpose. The ignition module 10 can also supply a speed signal. The ignition module 10, like the generator 9, is connected to the control unit 20.

[0018] During operation, combustion air is supplied to the two-stroke engine 1 in the crankcase 4. As the piston 5 descends, the combustion air is compressed in the crankcase 4 and flows into the combustion chamber 3 via the transfer ports 17 at the piston 5's bottom dead center. Fuel can be added to the combustion air via the fuel valve 15 during the transfer or compression phases. In the combustion chamber 3, the fuel / air mixture is compressed during the piston 5's upward stroke and ignited by the spark plug 16 at the piston 5's top dead center. The combustion of the mixture in the combustion chamber 3 accelerates the piston 5 towards the crankcase 4. Once the exhaust port 19 is opened by the piston 5, the exhaust gases escape from the combustion chamber 3.It may be provided that the two-stroke engine 1 additionally has an air channel through which largely fuel-free combustion air is supplied in advance in the transfer channels 17 in order to separate the exhaust gases from the incoming fresh mixture.

[0019] The amount of fuel supplied to the two-stroke engine 1 is controlled by the control unit 20. For this purpose, the control unit 20 evaluates the rotational speed n of the two-stroke engine 1. In order to better determine the amount of fuel x to be supplied, it is advantageous if the control unit 20 detects when combustion does not occur in the combustion chamber 3 during every engine cycle.

[0020] In Fig. Figure 2 shows the control sides of the two-stroke engine 1. During the downward stroke of the piston 5 from top dead center (TDC), the intake 11 opens first at time ES. Subsequently, the exhaust 19 opens at time AÖ. With further downward stroke of the piston 5, the transfer ports 17 open at time ÜÖ. During the upward stroke of the piston 5, the ports close and open in reverse order. First, the transfer ports 17 close at time ÜS. Then, the exhaust 19 closes at time AS. Finally, the intake 11 opens at time EÖ. To determine whether combustion occurs in the combustion chamber 3 with each revolution of the crankshaft 7, the crankcase pressure p is measured. KGH The pressure is measured at a crankshaft angle KW1 where the crankcase 4 is completely closed. This is the case when the transfer ports 17 are closed and the intake 11 is not yet open. Advantageously, the pressure p KGHmeasured by pressure sensor 22 at a crankshaft angle KW1 shortly before the EÖ time, at which the intake 11 opens.

[0021] Fig. Figure 3 shows the individual measured pressure values ​​for the crankcase pressure p KGH The pressure values ​​for crankcase pressure p fluctuate over time at the crankshaft angle KW1. As the figure shows, the pressure values ​​for crankcase pressure p vary. KGH Initially very high. From time t3 onwards, the pressure values ​​are at an almost constant level. Until time t3, the two-stroke engine 1 operates in four-stroke mode, i.e., combustion takes place in the combustion chamber 3 only every second revolution of the crankshaft 7. The pressure value p1 represents the crankcase pressure p KGH at time t1, after combustion has taken place in combustion chamber 3. The pressure value p2 represents the pressure p KGH in the crankcase 4 at time t2 after an engine cycle in which no combustion took place in the combustion chamber 3.

[0022] In Fig. 3 is also a mean value p M for crankcase pressure p KGH shown. To easily determine whether the two-stroke engine 1 is operating in four-stroke mode, the pressure difference Δp1 between the pressure value p1 and the mean value p is calculated. M The pressure difference Δp2 is also determined for the pressure value p2 relative to the mean value p. M determined. The mean p M This is the average value over several pressure values ​​p1, p2, for example, over pressure values ​​from eight consecutive engine cycles at crankshaft angle KW1. How Fig. Figure 3 shows that the pressure differences Δp1, Δp2 are comparatively large. The pressure differences Δp1, Δp2 are defined by one or more limit values ​​Δp grenz The comparison is made. From this, the control unit 20 recognizes that the two-stroke engine 1 is running in four-stroke mode. The pressure value p1 prevails at time t1 and the pressure value p2 at time t2. At these times, as described in Fig. As shown in section 4, the amount of fuel supplied, x, is reduced in each case. Since large pressure fluctuations in the crankcase pressure p also occur subsequently. KGH consist of, as in Fig. As shown in Figure 3, the fuel quantity x continues to decrease. At time t3, with crankshaft angle KW1, there is a pressure p3 in the crankcase 4, which corresponds to the mean value p M exhibits only a very small pressure difference Δp3. The subsequent pressure values ​​are at approximately the same level, the crankcase pressure p KGH The crankshaft angle KW1 is therefore approximately constant from time t3 onwards. From time t3 onwards, combustion takes place in combustion chamber 3 with each revolution of the crankshaft 7. The amount of fuel supplied x is therefore no longer reduced. From time t3 onwards, the amount of fuel supplied x can again be determined by the control unit 20 in the usual manner.

[0023] Fig.Figure 5 schematically illustrates the process. In process step 26, the crankcase pressure p is measured. KGH determined and the pressure difference Δp between the current crankcase pressure p KGH and the mean p M formed. In process step 27, the pressure difference Δp is measured with a limit value Δp. grenz compared. In process step 28, it is also determined whether the change in rotational speed n is less than a limit value Δn. grenz for the change in rotational speed n and whether the change in air consumption LA is less than a limit value ΔLA grenz The change in air volume LA is relevant. If this is the case, i.e., if the engine speed n and air volume LA are approximately constant, then the amount of fuel supplied x is reduced. Otherwise, the amount of fuel supplied x remains unchanged and the process is repeated in the next engine cycle.

[0024] The exemplary embodiment describes the detection of four-stroke operation. However, the method according to the invention can also be used to determine other combustion patterns. The method can also be used to verify whether a desired combustion pattern, for example, combustion every 3, 4, 5, or 6 engine cycles, is actually present. Advantageously, the method is carried out under full load of the two-stroke engine 1. However, the method can also be advantageously applied in other operating conditions of the two-stroke engine 1.

Claims

[1] Method for operating a two-stroke engine, wherein the two-stroke engine (1) has a cylinder (2) in which a combustion chamber (3) is formed, which is bounded by a piston (5), wherein the piston (5) drives a crankshaft (7) rotatably mounted in a crankcase (4), and wherein the crankcase (4) is connected to the combustion chamber (3) in at least one position of the piston (5) via at least one transfer port (17), with an inlet (11) into the crankcase (4) and with an outlet (19) from the combustion chamber (3), with a device for supplying fuel, with a control (20), and with a means for sensing the crankcase pressure (p). KGH ), where the crankcase pressure (p KGH ) is determined during each engine cycle, characterized by , that the fluctuation of the determined crankcase pressure (p KGH ) determined and used to determine whether combustion takes place in each engine cycle, with a limit value (Δp) grenz) is compared, whereby the fluctuation of the crankcase pressure (p KGH ) as the difference in crankcase pressure (p KGH ) to a mean value (p M ) of the crankcase pressure (p KGH ) is determined. [2] Method according to claim 1, characterized by , that the crankcase pressure (p KGH ) is measured at the same crankshaft angle (KW1) during each engine cycle. [3] Method according to claim 1 or 2, characterized by , that the crankcase pressure (p KGH ) is measured at a crankshaft angle (KW1) at which the crankcase (4) is closed. [4] Method according to claim 3, characterized by , that the crankcase pressure (p KGH ) is measured during the upward stroke of the piston (5) after closing the overflow channel (17) and before opening the inlet (11). [5] Method according to any one of claims 1 to 4, characterized by , that from the fluctuation of the crankcase pressure (p KGH) determines whether the two-stroke engine (1) is running in four-stroke mode. [6] Method according to claim 5, characterized by , that when four-stroke operation is detected, the amount of fuel (x) supplied to the two-stroke engine (1) is reduced until four-stroke operation is no longer present. [7] Method according to any one of claims 1 to 6, characterized by , that in addition to the crankcase pressure (p KGH ) the rotational speed (n) and / or the air consumption (LA) of the two-stroke engine is monitored and measured with a limit value (Δn) grenz , ΔLA grenz ) is compared.

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