JPH0681901B2 - Exhaust system for 2-cycle engine - Google Patents

Description

TECHNICAL FIELD The present invention relates to an exhaust system for a two-cycle engine.

[Prior art]

In a two-cycle engine, in order to improve the scavenging efficiency, a positive pressure wave is applied to the vicinity of the exhaust port between the closing of the scavenging port and the closing of the exhaust port, so that the fresh air blown out from the exhaust port is removed. Pushing back into the combustion chamber.

By the way, the timing at which this pressure wave returns to the exhaust port is greatly affected by the pipe length and diameter of the exhaust pipe, and conventionally, the shape and size of this exhaust pipe have been determined in accordance with the rotation region where the maximum output of the engine is obtained. Many.

[Problems to be solved by the invention]

However, in this way, the rotation region in which the pressure wave of the exhaust gas can be effectively used for gas exchange is narrowed, and in other rotation regions, before the exhaust port is closed, the negative pressure around the exhaust port is reduced. The waves sometimes reached and the fresh air filled in the combustion chamber was sucked into the exhaust pipe.

For this reason, there is a problem that gas exchange in the combustion chamber is difficult and the engine output is adversely affected.

[Means for solving problems]

Therefore, in the present invention, at the downstream end of the exhaust pipe connected to the exhaust port, a tapered inner pipe whose diameter gradually increases as it goes to the exhaust downstream side is housed, and the exhaust downstream end is closed inside the inner pipe. And a second resonance chamber whose exhaust downstream end is similarly closed between the inner pipe and the exhaust pipe, and the second resonance chamber is formed in the first resonance chamber. While the tail pipe that penetrates the closed end on the side and opens to the outside is inserted and arranged, the upstream open end of the tail pipe is led out to the exhaust upstream side with respect to the first and second resonance chambers. Characterize.

[Action]

According to this configuration, when the exhaust port is opened, it becomes a positive pressure wave of the exhaust gas and propagates through the exhaust pipe to reach the tail pipe and the two resonance chambers. In this case, since the tail pipe is opened to the outside, the pressure wave is reflected back at the upstream opening end of the tail pipe and returns to the exhaust port as a negative pressure wave.

On the other hand, since the downstream ends of the first and second resonance chambers are closed, the positive pressure wave is specularly reflected and returns to the exhaust port. In this case, the pressure wave arriving at the first resonance chamber is reflected at the downstream end of the inner pipe, while the pressure wave arriving at the second resonance chamber is reflected at the outer peripheral surface of the inner pipe. In between,
A difference occurs in the position where the pressure wave from the exhaust port is reflected, and the timing at which the positive pressure wave returns also differs. Therefore, in addition to the negative pressure wave, two types of positive pressure waves having different periods exist in the exhaust pipe.

Therefore, when the engine speed is such that the periods of the positive pressure wave and the negative pressure wave exactly overlap at the upstream opening end of the tail pipe,
These positive pressure wave and negative pressure wave resonate with each other and cancel each other out.
Since the negative pressure wave is attenuated, the shape and dimensions of the inner pipe must be adjusted so that the engine speed at which this resonance occurs matches the rotational range where negative pressure acts on the exhaust port before the exhaust port is closed. If determined, the adverse effect of negative pressure on the exhaust port can be reduced.

Moreover, since the cycles of the two positive pressure waves are different, the engine speeds at which the respective positive pressure waves resonate with the negative pressure waves are naturally different, and the range of tuning speeds for attenuating the negative pressure waves is expanded. be able to.

Further, since the upstream opening end of the tail pipe is located on the exhaust upstream side of the resonance chamber, the negative pressure wave reaches the vicinity of the exhaust port earlier than the positive pressure wave in the high-speed rotation region, and the influence of the positive pressure wave Will be less.

Therefore, in the high-speed rotation region, the negative pressure wave reaches the exhaust port during the scavenging period in which both the exhaust port and the scavenging port are open, and promotes the scavenging.

Example of Invention

A first embodiment of the present invention will be described below with reference to FIGS. 1 and 2.

Reference numeral 1 in FIG. 1 denotes a two-cycle engine, and an intake port opened and closed by a piston 3 is provided on an inner surface of a cylinder 2 thereof.
4, the exhaust port 5 and the scavenging port 6 are opened.

The scavenging port 6 communicates with the crank chamber via a scavenging passage (not shown).

An exhaust pipe 7 is connected to the exhaust port 5 of the cylinder 2. The exhaust pipe 7 is composed of a tapered pipe portion 8 whose diameter gradually increases toward the exhaust downstream side, and a straight pipe portion 9 which extends substantially straight from the tapered portion 8 at the downstream end of the straight pipe portion 9. Is closed by the end plate 10. Inside the straight pipe portion 9 which is the downstream end of the exhaust pipe 7, a tapered inner pipe whose diameter gradually increases as it goes to the exhaust downstream side.
12 are housed. The upstream open end of the inner pipe 12 is the straight pipe portion 9
The diameter of the inner pipe 12 is smaller than that of the inner pipe 12, and the downstream opening end of the inner pipe 12 is welded to the inner peripheral surface of the straight pipe portion 9 over the entire circumference. Therefore, the space inside the inner pipe 12 forms the first resonance chamber 13 whose exhaust downstream end is closed, and the space between the inner pipe 12 and the outer straight pipe portion 9 is also the same. The second resonance chamber 14 is closed at the exhaust downstream end.

Further, the tail pipe 11 is coaxially inserted and arranged in the first resonance chamber 13. The upstream opening end of the tail pipe 11 passes through the upstream opening end of the inner pipe 12 and is led out to the exhaust gas upstream side of the resonance chambers 13 and 14, and the downstream opening end penetrates the end plate 10. The tail pipe 11 is led out to the outside, and the downstream opening end of the tail pipe 11 forms an opening 15 that opens to the outside.

The shape and size of the exhaust pipe 7 in this embodiment is a high-speed type in which the timing at which the positive pressure wave returns to the exhaust port 5 is synchronized in the high-speed rotation region from R ₃ to R _{4 in} FIG.

In such a configuration, when the exhaust port 5 is opened by the lowering of the piston 3, the pressure wave of the exhaust gas becomes a positive pressure and propagates in the exhaust pipe 7, and the upstream open end of the tail pipe 11 and the first and second exhaust pipes. Reach the resonance chambers 13 and 14.

In this case, since the tail pipe 11 is opened outward, the pressure wave is reflected back at the upstream opening end of the tail pipe 11 and returns to the exhaust port 5 as a negative pressure wave. On the other hand, since the downstream ends of the first and second resonance chambers 13 and 14 are closed, the pressure wave is specularly reflected and returns to the exhaust port 5. In this case, the pressure wave reaching the first resonance chamber 13 is reflected by the end plate 10, while the pressure wave reaching the second resonance chamber 14 is reflected by the outer peripheral surface of the inner pipe 12 upstream of the end plate 10. Since they are reflected, a difference occurs in the position where the pressure wave is regularly reflected between the resonance chambers 13 and 14, and the cycle of this positive pressure wave is also different.

Therefore, when the engine speed is such that the periods of the positive pressure wave and the negative pressure wave exactly overlap at the upstream opening end portion of the tail pipe 11, the positive pressure wave and the negative pressure wave resonate with each other to cancel each other out, and the negative pressure wave is attenuated. , The engine speed at which this resonance phenomenon occurs is conventionally defined as a low / medium speed rotation region (indicated by the rotation speeds R ₁ to R ₃ in FIG. 2) in which a negative pressure wave returns during the period until the exhaust port 5 is closed. If the shape and size of the inner pipe 12 are determined so as to coincide with each other, the negative influence of the negative pressure on the exhaust port 5 can be suppressed to a small level.

At the same time, the two positive pressure waves have different periods, so the engine speed when these positive pressure waves and negative pressure waves resonate will naturally be different, and the range of the tuning speed for damping the negative pressure should be expanded. You can

As a result, it is possible to prevent fresh air from being sucked out of the combustion chamber over a wide range from the low-speed rotation region to the high-speed rotation region, improve the scavenging efficiency, and, as shown by the solid line in FIG. The output in the low and medium speed rotation range is improved compared to

Also, the upstream open end of the tail pipe 11 has two resonance chambers 1
Since it is located on the upstream side of the exhaust gas with respect to 3 and 14, the negative pressure wave reaches the vicinity of the exhaust port 5 earlier than the positive pressure wave in the high speed rotation region, and the influence of the positive pressure wave is reduced.

Therefore, in the high-speed rotation region, the exhaust port 5 and the scavenging port 6
Since the negative pressure wave reaches the exhaust port 5 to help push out the combustion gas and fill the fresh air during the scavenging period when both are open, there is no adverse effect due to the provision of the resonance chambers 13 and 14. As shown in FIG. 2, high output can be obtained as before.

It should be noted that the present invention is not limited to the above-described first embodiment. For example, as in the second embodiment of the present invention shown in FIG. 3, the first pipe is provided at the downstream end of the inner pipe 12 having a large surface area. Resonance chamber
A large number of small holes 21 ... Which communicate the 13 with the second resonance chamber 14 may be opened.

According to this configuration, in the low speed rotation region where the exhaust gas flow rate is small, the exhaust gas has a small resistance when passing through the small holes 21 ...
The loss in the small holes 21 is small, and it can be considered that the two resonance chambers 13 and 14 are communicated with each other and the volume is increased.

On the other hand, in the medium and high speed rotation range where the exhaust flow rate increases, the small holes 21
Since the equivalent diameter of ... becomes small and the resistance when exhaust gas passes increases, these small holes 21 ... are substantially closed.

Therefore, it is apparent that the resonance chamber depends on the engine speed.
Since the volumes of 13, 14 change, the period of the reflected waves reflected by the resonance chambers 13, 14 fluctuates every moment depending on the engine speed, and it is possible to further widen the range of the tuning speed at which the negative pressure wave is resonantly attenuated. it can.

〔The invention's effect〕

According to the present invention described in detail above, the negative pressure acting on the exhaust port in the rotation region where the negative pressure acts on the periphery of the exhaust port during the period from the closing of the scavenging port to the closing of the exhaust port. Can be attenuated. At the same time, it is possible to widen the range of the tuned rotational speed that attenuates this negative pressure, and thus it is possible to prevent blow-through of fresh air over a wide rotational range, which improves scavenging efficiency and improves engine output. To be enhanced.

Further, in the high speed rotation region, the negative pressure wave reaches the exhaust port earlier than the positive pressure wave to assist the scavenging, so that the influence of the positive pressure wave is small and a high output can be obtained as before.

[Brief description of drawings]

1 and 2 show a first embodiment of the present invention.
The drawing is a sectional view, FIG. 2 is a characteristic view, and FIG. 3 is a sectional view showing a second embodiment of the present invention. 5 ... Exhaust port, 7 ... Exhaust pipe, 11 ... Tail pipe, 12 ... Inner pipe, 13 ... First resonance chamber, 14 ... Second resonance chamber, 15 ... Opening part.

Claims (2)

[Claims] 1. A downstream end portion of an exhaust pipe connected to an exhaust port accommodates a tapered inner pipe whose diameter gradually increases as it goes to the exhaust downstream side, and the exhaust downstream end is closed inside the inner pipe. Forming a first resonance chamber and forming a second resonance chamber in which the exhaust downstream end is closed between the inner pipe and the exhaust pipe,
In the first resonance chamber, a tail pipe that penetrates the closed end on the exhaust gas downstream side and opens to the outside is inserted and arranged,
The upstream open end of the tail pipe is connected to the first and second
The exhaust system for a two-cycle engine, characterized in that the exhaust system is led upstream of the resonance chamber. 2. The two cycles according to claim 1, wherein a large number of small holes for communicating the first and second resonance chambers are formed on the peripheral surface of the inner pipe. Exhaust system of the engine.