1. Field of the Invention
This invention relates to an intake device for an internal combustion engine, and more particularly to an intake device for an internal combustion engine in which the air-column vibration of the intake system is synchronized with the intake period to obtain so-called intake inertia supercharging effect, thereby improving the engine output power.
2. Description of the Prior Art
In the internal combustion engine, as the piston moves up and down in the cylinder, a pressure wave is created in the intake passage and transmits through the intake passage up to the surge tank, where it is reflected and then returns to the cylinder. This is repeated and accordingly results in periodical pressure change in the intake passage at a frequency determined by the such operating condition of the engine as the engine speed. On the other hand, every engine intake system has its natural frequency determined by the volume of the intake passage and the cylinder. It is known that the volumetric efficiency is improved by virtue of the intake inertia effect and that the engine output power can be enhanced by tuning the two frequencies with each other.
More particularly, the natural frequency of the intake system is determined by the length and cross-sectional area of the intake passage and the average volume of the cylinder during the intake period. In order to enlarge the engine speed range over which the pressure change frequency can be tuned with the natural frequency of the intake system and to widen the engine speed range over which the engine output power can be enhanced by virtue of the intake inertia effect, it has been proposed to make the effective length or the effective cross-sectional area of the intake passage according to the engine speed. See Japanese Unexamined Patent Publication Nos. 48(1973)-58214, 56(1981)-115819 and 58(1983)-119919, for example.
However, in accordance with the prior art, the engine speed range over which the engine output power can be enhanced by virtue of the intake inertia effect cannot be sufficiently widened and comparatively complicated and large equipment is required to change the effective length of the intake passage by a sufficient amount. That is, practically, the effective length of the intake passage can be changed only by a limited amount and therefore said engine speed range over which the engine output power can be enhanced by virtue of the inertia effect cannot be sufficiently widened by only changing the effective length of the intake passage. Further, even in the engine speed range in which the effective length of the intake passage can be practically changed to tune said two frequencies with each other, an optimal intake inertia effect cannot be obtained unless the intake valve timing is suitable.