As an exhaust apparatus of an internal combustion engine to be used by an automotive vehicle, there has so far been known an exhaust apparatus as shown in FIG. 49 (for example see Patent Document 1). The known exhaust apparatus 4 is constructed in FIG. 49 to allow an exhaust gas to be introduced therein after the exhaust gas is exhausted from an engine 1 serving as an internal combustion engine and then passes through an exhaust manifold 2 and a catalytic converter 3 where the exhaust gas is purified.
The exhaust apparatus 4 is constituted by a front pipe 5 connected to the catalytic converter 3, a center pipe 6 connected to the front pipe 5, a main muffler 7 connected to the center pipe 6 and serving as a sound deadening device, a tail pipe 8 connected to the main muffler 7, and a sub-muffler 9 connected to the tail pipe 8.
The main muffler 7 has an expansion chamber for introducing therein and expanding the exhaust gas to mute the sound of the exhaust gas, and a resonance chamber for muting the sound of the exhaust gas having a specified frequency by Helmholtz resonator effect. More specifically, the resonance chamber is designed to enable its resonance frequency to be tuned to the low frequency side by increasing the volume of the resonance chamber or otherwise by lengthening the length of the center pipe 6 projecting into the resonance chamber, while enabling its resonance frequency to be tuned to the high frequency side by decreasing the volume of the resonance chamber or otherwise by shortening the length of the center pipe 6 projecting into the resonance chamber.
The sub-muffler 9 is adapted to suppress the sound pressure level of the air column resonance from being increased when the air column resonance is generated in response to the pipe length of the tail pipe 8 in the tail pipe 8 by the pulsation of the exhaust gas during the operation of the engine 1.
In general, the tail pipe 8 having an upstream opening end and a downstream opening end at the respective upstream and downstream sides of the exhaustion direction of the exhaust gas is subjected to incident waves caused by the pulsation of the exhaust gas during the operation of the engine 1 at the upstream opening end and the downstream opening end, thereby generating an air column resonance. The air column resonance has a frequency as a basic component with a half wavelength equal to the pipe length of the tail pipe, and thus has a frequency wavelength several times that of the half wavelength.
For example, taking an example in which the tail pipe 8 having no sub-muffler 9 extends backwardly from the main muffler 7, as shown in FIG. 50, the wavelength λ1 of the air column resonance of a basic vibration (primary component) is roughly double the pipe length L of the tail pipe 8, while the wavelength λ2 of the air column resonance of the secondary component is roughly one time the pipe length L of the tail pipe 8. The wavelength λ3 of the air column resonance of the third component is ⅔ times the pipe length L of the tail pipe 8. Therefore, the tail pipe 8 has therein standing waves having respective nodes of sound pressure distributions at the upstream opening end and the downstream opening end.
The air column resonance frequency “fm” of the tail pipe 8 is given by the following equation (1)fm=(c/2L)·m  (1)
c: sound speed, L: pipe length of tail pipe, m: degree
As it is obvious from the above equation (1), it is known that the longer the pipe length L of the tail pipe 8, the more the air column resonance frequency “fm” is transferred to the low frequency area where the rotation number of engine 1 is low.
It is further known that as shown in FIG. 51, the frequency of the exhaust gas pulsation of the engine 1 is increased as the rotation number of the engine 1 is increased, and the sound pressure levels (dB) of the exhaust gas sounds are raised with the primary component f1 and the second component f2 of the exhaust gas sounds caused by the air column resonance in response to the rotation number of the engine 1.
Therefore, in the case of using a tail pipe 8 having a long pipe length (for example, the pipe length of the tail pipe 8 is equal to or more than 1.5 m), there is occasionally generated such an air column resonance in the normal rotation area having a low engine rotation number Ne, thereby causing exhaust gas noises to be deteriorated and giving unpleasant feelings to a driver.
In particular, as shown in FIG. 51, the peak (the width of the antinode portion of the sound pressure distribution) of the sound pressure for the secondary component f2 of the air column resonance is larger than the peak of the sound pressure for the primary component f1 of the air column resonance, so that there is generated in the normal rotation area of the engine unpleasant noises called muffled sounds which are a cause for the exhaust gas noises to be deteriorated.
For this reason, in the case of the pipe length of the tail pipe 8 being long, the sub-muffler 9 smaller in capacity than the main muffler 7 is provided at the optimum position among the antinode portion of the standing wave high in the sound pressure level as shown in FIG. 50, and the respective antinode portions of the primary component f1 and the secondary component f2 of the exhaust gas sound caused by the air column resonance, so that the exhaust gas noises are suppressed in the normal rotation area of the engine 1 to prevent the unpleasant feeling from being given to the driver.
On the other hand, it may be considered that the resonance frequency of the resonance chamber of the main muffler 7 to be connected with the upstream opening end of the tail pipe 8 is tuned to the air column resonance frequency of the tail pipe 8, thereby muting the air column resonance of the tail pipe 8 in the resonance chamber of the main muffler 7.
It is considered that by increasing the volume of the resonance chamber and by lengthening the projecting portion of the center pipe 6, the resonance frequency of the resonance chamber is tuned to the low frequency side, thereby preliminarily muting in the resonance chamber the air column resonance generated in the tail pipe 8 in the normal rotation number area of the engine.
However, only the exhaust gas flow is left with the gas amount discharged from the engine 1 to the exhaust apparatus 4 being drastically reduced because the throttle valve is opened at the deceleration time of the vehicle, so that the air pressure to be introduced into the resonance chamber comes to be small.
As a result, the sufficient amount of air cannot be obtained to cause the Helmholtz resonance effect in the resonance chamber, thereby making it difficult to suppress the air column resonance in the tail pipe 8. Especially at the deceleration time of the vehicle, the rotation number of the engine 1 is drastically decreased, so that the primary component f1 of the exhaust gas sound by the air column resonance enters the normal rotation number area, thereby occasionally causing muffled sounds in the vehicle cabin at the low rotation number of the engine 1 and thus giving unpleasant feelings to the driver.
As one of the conventional exhaust apparatuses for suppressing the noises at the deceleration time of the vehicle, there has so far been known an exhaust apparatus which comprises a valve for opening and closing an exhaust gas discharging pipe, and a control unit for controlling the opening and closing operations of the valve (for example see Patent Document 2).
As shown in FIGS. 52 and 53, the previously mentioned conventional exhaust apparatus further comprises a sound muting valve 10 provided at the downstream opening end 8b of the tail pipe 8 occupied by a node of the sound pressure of the standing wave of the air column resonance. The sound muting valve 10 is constituted by a valve case 11 and a butterfly type of valve body 12 mounted on the downstream opening end 8b of the tail pipe 8. The valve body 12 has a central portion formed with an orifice 13 for throttling the passage cross-sectional area of the tail pipe 8.
The valve body 12 is provided with a driving shaft 14 which is provided to extend in a direction perpendicular to the center axis of the tail pipe 8. The driving shaft 14 is connected with an electromagnet actuator 17 through a drum 15 and a wire 16. The electromagnet actuator 17 is on-off controlled by a control unit 19.
The control unit 19 is adapted to output to the electromagnet actuator 17 a command signal for on-off controlling electromagnet actuator 17 on the basis of the detection signal of a throttle sensor 18 for detecting the opening degree of the throttle valve not shown.
More specifically, the control unit 19 is adapted to allow the electromagnet actuator 17 to have the valve body 12 held in the open state by outputting the off-signal to the electromagnet actuator 17 in a usual case. The control unit 19 is adapted to output the on-signal to the electromagnet actuator 17 based on the detection information from the throttle sensor 18 at the deceleration time of the vehicle to have the valve body 12 perform the closing operation with the action of the electromagnet actuator 17.
For this reason, the sound muting valve 10 can serve to prevent the discharge of the exhaust gas from being hindered at the times of the normal cruising and the acceleration of the vehicle. Because of the exhaust gas passing through only the orifice 13 at the deceleration time of the vehicle, the motion of the particle of the exhaust gas is given resistance at the node of the sound pressure of the standing wave of the air column resonance at which the particle speed of the exhaust gas is at a maximum level, thereby making it possible to suppress the sound pressure level caused by the air column resonance of the tail pipe 8 from being increased.