Internal-combustion engines – Two-cycle – Whirl through piston-controlled ports
Reexamination Certificate
2001-06-06
2002-07-09
McMahon, Marguerite (Department: 3747)
Internal-combustion engines
Two-cycle
Whirl through piston-controlled ports
Reexamination Certificate
active
06415748
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an exhaust control system provided with an exhaust control valve for controlling the opening/closing timing of an exhaust port in accordance with an operating condition of a two-cycle internal combustion engine.
2. Description of Background Art
In a conventional two-cycle internal combustion engine an exhaust control system is provided which has an exhaust control valve which changes the opening/closing timing of an exhaust port for preventing blow-by of fresh gas while the engine is operating under a condition where there is a small intake volume; for example, when the engine is operating at a low revolution. In addition, the opening/closing timing of the exhaust port has been controlled to ensure a high output while the engine is operating under a condition where there is a large intake volume; for example, when the engine is operating at a high revolution. An example of such an exhaust control valve will be described below with reference to FIG.
6
.
This exhaust control valve, indicated at a, is swingably supported by a cylinder e, the cylinder e being formed with an exhaust passage having a pair of exhaust ports c and d which are open to a peripheral wall surface of a cylinder bore b with a piston fitted reciprocatably therein. The pair of exhaust ports c and d are positioned on both sides in the circumferential direction of a reinforcing wall f which extends axially on the circumferential wall of the cylinder bore b. The exhaust control valve a has a pair of valve elements g and h which are disposed correspondingly to the exhaust ports c and d.
At a predetermined number of revolutions in a low speed region of the internal combustion engine, the pair of valve elements g and h assume respective first positions indicated with solid lines in
FIG. 6
, in which control surfaces k and m of the valve elements g and h close upper port portions of the exhaust ports c and d so that an opening timing of the exhaust ports c and d is made at a latest point and a closing timing thereof is made at an earliest point, to suppress blow-by of fresh gas when the engine is at a small intake volume and improve the combustion stability. On the other hand, at a predetermined number of revolutions in a high speed region of the internal combustion engine, the control surfaces k and m of the valve elements g and h assume respective second positions indicated with dash-double dot lines in FIG.
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and the control surfaces k and m open the upper port portions, so that the opening timing of the exhaust ports c and d is made at an earliest point and the closing timing thereof is made at a latest point, whereby the exchange of gases between burned gas and fresh gas by exhaust and scavenging when the engine is operating at a large intake volume is performed smoothly and positively and there is obtained a high engine output.
Lower edge portions n and p of the control surfaces k and m are formed in a shape that substantially coincides with the shape of upper edge portions of the exhaust ports c and d when the valve elements g and h assume their second positions. Therefore, when the valve elements g and h assume their first positions, the lower edge portions n and p of the control surfaces k and m are inclined downward from portions close to the reinforcing wall f to circumferential ends thereof. The reason why the lower edge portions n and p are formed so as to coincide with the shape of the upper edge portions of the exhaust ports c and d at the second positions of the valve elements g and h is that if the lower edge portions n and p of the control surfaces k and m are positioned above the upper edge portions of the exhaust ports c and d and a dead space is formed therebetween, output characteristics in a high speed region of the engine are deteriorated and a desired high output cannot be obtained. Although this has been made sure by experiment, the cause is presumed to be because of the fact that a smooth exhaust of burned gas does not occur under the influence of vortices created in the dead space.
When the piston moves down to a position lower than the center of the lower edge portions n and p, the exhaust ports c and d begin to be opened by the piston, the valve elements g and h being provided with the control surfaces k and m which include the lower edge portions n and p. In this state, circumferential ends of the exhaust ports c and d are still closed by the control surfaces k and m. With subsequent descent of the piston, the exhaust ports c and d are opened gradually from the center of the lower edge portions n and p toward end portions thereof.
Having made an experiment in connection with the above background art, the inventors have obtained such results as indicated with broken lines in FIG.
5
. The graph of
FIG. 5
illustrates changes in internal pressure of a combustion chamber during the lapse of time just after the start of opening of a throttle valve, which is in an idle opening condition in a two-cycle internal combustion engine provided with an exhaust control valve and assuming an idling state. In the same figure, in a period T′ which starts just after the start of opening of the throttle valve, the internal pressure of the combustion chamber varies largely at a variation width H′ and it can be seen that unstable combustion involving alternate occurrence of misfire and combustion in the next cycle of gas left unburned by the misfire, i.e., irregular combustion, occurs over a relatively long period. The misfire is caused by retention of an excess of burned gas in the cylinder bore b. At least during this period T′, the number of revolutions of the internal combustion engine is smaller than a predetermined number of revolutions in the foregoing low speed region, so that the valve elements g and h of the exhaust control valve occupy their first positions. Thus, the reason why such irregular combustion occurs when the valve elements g and h of the exhaust control valve occupy their first positions is presumed to be because the exhaust ports c and d are opened later at circumferential ends than at the center thereof. This is attributable to the shape of the lower edge portions n and p of the control surfaces k and m, in the vicinity of the late opening portions in the cylinder bore b. Accordingly, exhaust and scavenging are not conducted to a satisfactory extent, resulting in excess burned gas remaining within the cylinder bore b, thus facilitating the occurrence of irregular combustion.
In order to improve the exhaust and scavenging functions, if downwardly extending portions from both circumferential ends of the control surfaces k and m are shortened to quicken the timing at which circumferential ends of the exhaust ports c and d are opened, the period of occurrence of irregular combustion decreases. However, when the valve elements g and h assume their second positions in the high speed region, the foregoing dead space occurs, thus making it impossible to obtain a desired high output of the internal combustion engine throughout the entire operating range.
SUMMARY OF THE INVENTION
The present invention has been accomplished in view of the above-mentioned circumstances and it is an object of the invention to provide an exhaust control system in a two-cycle internal combustion engine capable of suppressing the occurrence of irregular combustion when the opening timing of the exhaust ports is set to a delay timing including a maximum delay timing, to improve the combustion stability. Furthermore, it is an object of the present invention to afford a high engine output when the opening timing of exhaust ports is set to a maximum advance timing.
A first aspect of the present application resides in an exhaust control system in a two-cycle internal combustion engine which includes an exhaust control valve disposed in a cylinder thereof to control the opening/closing timing of an exhaust port. The cylinder is formed with an exhaust passage having an exhaust port, the exhaust port being formed
Hara Shigeyuki
Sasamura Yasuo
Uchida Hiroyuki
Ugai Masaharu
Ali Hyder
Birch & Stewart Kolasch & Birch, LLP
Honda Giken Kogyo Kabushiki Kaisha
McMahon Marguerite
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