JPH0550573B2 - - Google Patents

Description

Detailed Description of the Invention [Industrial Application Field] This invention has an exhaust port on the inner peripheral wall of a cylinder that is opened and closed by a piston, and a control valve that adjusts the opening area of the exhaust port. This invention relates to a blow-through prevention device for two-stroke engines.

[Prior Art] A two-stroke engine has an exhaust port and a scavenging port formed on the inner circumferential wall of the cylinder, which are opened and closed by a piston, and burnt gas is discharged from the exhaust port using fresh air sent through the scavenging port. There is something I tried to do.

In this type of two-stroke engine, a blow-by phenomenon occurs in which part of the fresh air is discharged from the exhaust port during scavenging, so conventionally, the edges of the exhaust port are
A control valve is installed to make the opening area of the exhaust port variable, reducing the amount of fresh air that blows through (for example,
56-54336). When the engine speed is low at the upper edge of the exhaust port, the control valve adjusts the opening area of the exhaust port by protruding the tip of the valve toward the lower edge.

[Problems to be Solved by the Invention] Although the conventional example described above performs the function of adjusting the opening area of the exhaust port, the compression stroke length of the engine also changes accordingly, and as a result, the engine The compression ratio increases when the engine rotates at low speeds, and decreases when the engine rotates at high speeds where high output is desired.

By the way, in general, one way to improve the maximum output of an engine is to set a high compression ratio. The ratio is affected by the compression ratio at low speed rotation, which is disadvantageous because it cannot be increased much.

Further, the upper edge of the exhaust port is exposed to the burnt gas ejected immediately after the exhaust port opens, and is the portion of the exhaust port that has the highest temperature. If the lubricating oil contained in the fresh air adheres to the upper edge of the hot exhaust port, it will easily become carbonized. Since the above-mentioned conventional control valve is disposed at the upper edge of the exhaust port and forms the upper edge of the exhaust port, the control valve becomes high in temperature, requiring the use of heat-resistant materials or measures against thermal expansion. Furthermore, if this carbon builds up on the movable parts of the control valve, there is a risk of the valve sticking, so consideration must be given to moving the control valve once when the engine switch is turned on to prevent sticking. There are problems such as design difficulties.

This problem was solved in view of the problems of the conventional technology, and the object is to provide a blow-through prevention device for a two-stroke engine in which there is no fluctuation in the compression ratio and less carbon buildup on the movable parts of the control valve. That is.

[Means for Solving the Problems] In order to achieve the above object, the blow-through prevention device for a two-stroke engine according to the present invention includes a control valve disposed along the lower edge of the exhaust port, and a control valve provided at the exhaust port of the control valve. The tip portion located at the lower edge is positioned toward the upper edge side from the lower edge of the exhaust port when the engine speed is low, and is higher than the outer circumferential position of the upper surface of the piston at the bottom dead center. be.

[Function] When the engine rotates at low speed, the tip of the control valve is positioned from the lower edge of the exhaust port toward the upper edge to reduce the area of the exhaust port, and the exhaust port is raised higher than the outer circumferential position of the top surface of the piston. The lower edge part acts like a weir in the waterway, and the lower edge part of the exhaust port prevents fresh air from blowing through. Furthermore, when the engine rotates at high speed, the tip of the control valve is positioned at the lower edge of the exhaust port to increase the area of the exhaust port, so that fresh air and the aforementioned gas can be easily discharged out of the cylinder during scavenging.

Since the opening area of the exhaust port is adjusted by the control valve at the lower edge of the exhaust port, the compression ratio of the engine is constant regardless of the rotational speed of the engine.

In addition, since this control valve is placed along the lower edge of the exhaust port, the control valve is not exposed to burnt gas immediately after the exhaust port opens, and the control valve part is located at the exhaust port. The temperature is lower than that at the upper edge, and there is less carbon buildup due to carbonization of the lubricating oil.

[Example] An example of the present invention will be described below with reference to the drawings.

In the figure, 1 is a cylinder and 2 is a piston. An intake port 3, an exhaust port 4, and an exhaust port 5 are opened in the inner peripheral wall of the cylinder 1, respectively.

FIG. 1 shows a state in which the piston 2 is located at the bottom dead center, and as the piston 2 reciprocates up to the top dead center indicated by the imaginary line X in the figure, these openings open on the side surface of the piston 2. It is opened and closed by The intake port 3 is an opening at the end of the intake passage passing through the carburetor 6, and
Supply air-fuel mixture (fresh air) to the engine. In this embodiment, when the piston 2 moves upward from the bottom dead center position shown and opens, it flows into the crank chamber (not shown) through the space below the piston 2, and as the piston 2 moves downward, the crankshaft Fresh air in the room is precompressed.

As the piston 2 moves downward from the top dead center, the exhaust port 4 first opens, and high-pressure burnt gas begins to be discharged from there. Next, the exhaust port 5 opens, and fresh air precompressed in a crank chamber (not shown) flows into the cylinder 1 from the scavenging port 5 via the scavenging passage 7 (arrow A in the figure), and this fresh air flows into the cylinder 1 from the burned air. Push the gas out of the cylinder. In this case, a portion of the fresh air that has flowed in flows out of the cylinder from the widely opened exhaust port 4 as shown by arrow B in FIG. 1, resulting in so-called blow-through. In particular, when the engine rotation speed is low, the time during which the exhaust port 4 is open becomes longer, so that the ratio of the amount of blow-through to the fresh air that has flowed into the cylinder 1 increases.

Therefore, in this embodiment, the blow-through prevention device is constructed as described below based on the technical idea of the present invention.

Reference numeral 8 denotes a swinging valve, which is arranged along the lower edge 4a of the exhaust port 4, and the above-mentioned blow-through is reduced by the upward protrusion of the swinging valve 8 from the lower edge 4a of the exhaust port. This corresponds to the control valve in the present invention. The swing valve 8 is driven as follows, and the amount of protrusion from the lower edge 4a of the exhaust port (L in the drawing) is adjusted to prevent fresh air from blowing through.

11 is an engine rotational speed sensor such as a tachometer, 12 is a control device such as a microcomputer, and 13 is a servo motor. The engine rotational speed signal obtained from the rotational speed sensor 11 is input to the control device 12 and calculated, and the control device 12 sends an output signal to the servo motor 13.

This output signal follows the relationship between the engine rotational speed and the preferred protrusion amount L, which is stored in advance in the microcomputer that is the control device 12. When the servo motor 13 is appropriately driven by the output signal of the control device 12, the swing valve 8 is connected to the servo motor 13 by the Bowden wire 14, so the swing valve 8 swings. The protrusion amount L is adjusted according to the engine rotation speed.

In other words, when the engine speed is low,
By increasing the protrusion amount L, the lower edge of the exhaust port of the swing valve 8 is located at a higher position than the outer circumferential position of the upper surface of the piston 2 at the bottom dead center, and the opening area of the exhaust port 4 is reduced. At the same time, the lower edge of the exhaust port, which is higher than the outer circumferential position of the upper surface of the piston 2, acts like a weir in a waterway, and after air is blown up from the scavenging passage 7 into the cylinder 1, it is blown up by the cylinder head at the top of the engine. This effectively prevents fresh air from blowing through, which is directed downward and goes along the upper surface of the piston 2 and toward the exhaust port. Furthermore, when the engine rotational speed is high, the protrusion amount L is reduced to enlarge the opening area of the exhaust port 4, and the scavenging efficiency is improved by opening the exhaust port for a short time.

At this time, since the swing valve 8 is arranged along the lower edge 4a of the exhaust port 4 and is not provided on the upper edge 4b, the change in the protrusion amount L of the swing valve 8 is caused by the compression of the engine. It does not affect the ratio, and
The swing valve 8 is not exposed to burnt gas immediately after opening the exhaust port 4, and there is less risk of carbon depositing on the movable part of the swing valve 8 and causing the valve 8 to become stuck.

In the embodiment described above, the swing valve 8 is used as the control valve, but the present invention is not limited to this, and other types such as a slide valve may be used.

[Effects of the Invention] In this invention, the control valve is arranged along the lower edge of the exhaust port, and the tip portion of the control valve located at the lower edge of the exhaust port is moved from the lower edge of the exhaust port when the engine speed is low. (1) Adjustment of the opening area of the exhaust port by this control valve is as follows:
This is done without causing any fluctuation in the upper edge position of the exhaust port, which determines the engine's compression ratio. Therefore, it eliminates the factors that cause fluctuations in the compression ratio and improves engine output from low to high engine speeds while allowing fresh air to blow through. It can be prevented.

(2) Even when the engine rotates at low speed and the exhaust port is open for a long time, the area of the exhaust port becomes smaller, and the lower edge of the exhaust port, which is higher than the outer circumferential position of the top surface of the piston, is placed in the waterway. The exhaust port acts as a dam, allowing fresh air to blow up from the scavenging passage into the cylinder, then be directed downward by the cylinder head at the top of the engine, and then flow along the top of the piston toward the exhaust port. Effectively prevented at the lower edge. Therefore, it is possible to prevent the output from decreasing during low speed rotation.

(3) In addition, even if the engine rotates at high speed and the time that the exhaust port is open is shortened, the compression ratio does not decrease and the area of the exhaust port becomes larger, allowing fresh air to flow out of the cylinder through scavenging. The resistance to exhaust air is reduced, improving scavenging efficiency and engine output.

(4) Furthermore, since the control valve is arranged along the lower edge of the exhaust port, the lower edge portion is not exposed to the burnt gas immediately after the exhaust port is opened;
Since the temperature is lower than that at the upper edge and the lubricating oil is less likely to be carbonized, it is much easier to design than the conventional example.

[Brief explanation of the drawing]

FIG. 1 is a longitudinal sectional view of a two-stroke engine with precompression in the crank chamber to which an embodiment of the present invention is applied;
FIG. 2 is a view in the direction of the arrow, and FIG. 3 is a view in the direction of the arrow. 1... Cylinder, 2... Piston, 4... Exhaust port, 4a... Lower edge of exhaust port, 8... Control valve.

Claims (1)

[Claims] 1. In a cylinder having an exhaust port opened and closed by a piston on the inner peripheral wall of the cylinder, and a control valve for adjusting the opening area of the exhaust port provided, the control valve is placed along the lower edge of the exhaust port. The tip portion of the control valve located at the lower edge of the exhaust port is positioned toward the upper edge side from the lower edge of the exhaust port when the engine speed is low, and is higher than the outer circumferential position of the top surface of the piston at bottom dead center. This is a blow-through prevention device for a two-stroke engine, which is characterized by: