1. Field of the Invention
The present invention relates to an intake system of a multi-cylinder engine.
2. Description of the Related Art
In a multi-cylinder engine provided with two cylinder banks each comprised of a plurality of cylinders, for example, a V-engine, it is necessary to arrange the surge tank above the engine for structural reasons. This is because with a V-engine, the intake ports are provided above the center of the structure and because the surge tank has to be arranged near the intake ports from the viewpoint of the intake efficiency or the layout of the intake pipe and exhaust pipe, etc.
In this way, since the surge tank is arranged above a V-engine, when placing the engine and the surge tank in the engine compartment, the position of the engine hood inevitably ends up becoming high. In particular, if also considering the need to provide a cushion material etc. between the engine hood and the tops of the engine body and the surge tank from the viewpoint of protecting pedestrians, the engine hood has to be positioned higher, and thus the possibility of changes in vehicle design is limited.
In general, from the viewpoint of securing the field of vision of the vehicle driver, the engine hood usually has to be made to incline downward toward the front of the vehicle. To make the height of the engine hood as a whole lower, it is necessary to make the height of the engine hood lower at the front region of the engine hood. In the intake system of the V-engine disclosed in Japanese Unexamined Patent Publication (Kokai) No. 4-121224, the surge tank is provided above the center of the two cylinder banks of the V-engine and facing throttle valves are provided at the two sides of the rear end of the surge tank. Due to this, it becomes possible to arrange the surge tank further to the rear. Further, the surge tank is formed so that its top surface is inclined downward toward the front of the vehicle when the surge tank is arranged above the center of the V-engine. By forming the surge tank in this way, the height of the engine hood is effectively made lower at the front region of the engine hood.
However, there is a surge tank which is divided in its internal space into a top part and a bottom part and provided with a partition between the top part and the bottom part. Further, in such a surge tank, the intake tubes connected to one cylinder bank of the V-engine are connected to the top part, while the intake tubes connected to the other cylinder bank are connected to the bottom part. Further, an opening communicating the top part and bottom part is provided at part of the partition in the surge tank. A valve for opening/closing the opening is provided in the opening. By operating this valve, the effective intake pipe length, which has an effect on the period of the intake pulsation occurring in the intake passage, is changed. It is possible to use this to raise the charging efficiency by the pulsation effect.
In this way, even when using a surge tank with an internal space divided into a top part and bottom part, the height of the engine hood has to be made lower. If however forming a surge tank so that its top surface inclines downward toward the front of the vehicle as described in Japanese Unexamined Patent Publication (Kokai) No. 4-121224, at the front region of the surge tank, the top part and bottom part of the surge tank end up becoming extremely thin. Therefore, in this case, it is difficult to make intake tubes branch from the sides of the top part and bottom part in the front region of the surge tank.