Engine supercharging device

1. A supercharging device for an engine, which comprises:

a supercharger for pressurizing air to be introduced into the engine;

an air cleaner for purifying an ambient air;

a purified air supply passage for supplying a purified air from the air cleaner towards the supercharger;

a supercharged air passage for supplying the pressurized air from the supercharger towards the engine; and

a relief valve for adjusting an air pressure within the supercharged air passage,

in which the relief valve is disposed in a portion of the supercharged air passage, which portion is enclosed by at least one of the air cleaner and the purified air supply passage, and

in which a discharge port portion of the relief valve is accommodated within at least one of the air cleaner and the purified air supply passage for releasing supercharged air above a predetermined pressure.

2. The supercharging device for the engine as claimed in claim 1 , in which the engine has a plurality of engine cylinders;

further comprising an intake air chamber, forming a downstream portion of the supercharged air passage, for supplying an intake air into a plurality of air intake passages of the plural engine cylinders,

wherein the relief valve is operable to adjust the air pressure within the intake air chamber.

3. The supercharging device for the engine as claimed in claim 2 , in which:

a rotary shaft of the engine extends in a direction laterally of an engine body;

a casing for the air cleaner and the intake air chamber are arranged above the engine body;

the supercharger is arranged below the casing for the air cleaner and the intake air chamber;

the purified air supply passage is arranged at a location adjacent one lateral side of the engine body; and

the supercharged air passage is arranged at a location laterally intermediate of the engine body with respect to the purified air supply passage.

4. The supercharging device for the engine as claimed in claim 2 , further comprising an additional fuel supply device for supplying an additional fuel towards an inlet of the air intake passage, the additional fuel supply being mounted on the intake air chamber.

5. The supercharging device for the engine as claimed in claim 2 , in which:

respective inlets of the intake passages leading to the associated engine cylinders are arranged in a row extending in a predetermined direction; and

relative to an outlet of the supercharged passage opening into the intake air chamber, the larger the distance from the outlet to the inlet of the air intake passage, the smaller the passage cross sectional area at a portion where the inlet of the air intake passage confronts within the intake air chamber to balance the amount of supercharged air flow for each of the engine cylinders.

6. The supercharging device for the engine as claimed in claim 1 , in which the engine is adapted to be mounted on a motorcycle, and at least one of the air cleaner and the purified air supply passage covers an area above at least a portion of the supercharged air passage.

7. The supercharging device for the engine as claimed in claim 6 , in which a transmission is arranged rearwardly of the engine body and the supercharger is arranged rearwardly of the forwardly tilted cylinder block, and

the intake air chamber is arranged above the supercharger and the air cleaner is arranged above the cylinder block.

8. The supercharging device for the engine as claimed in claim 7 , in which the supercharger is driven by a rotary shaft of the engine through a chain.

9. The supercharging device as claimed in claim 2 , in which the air cleaner covers an area above a portion of the intake air chamber and the relief valve is arranged in that portion.

10. The supercharging device as claimed in claim 1 , in which the air cleaner or the purified air supply passage and the supercharged air passage are arranged one above the other, and

the relief valve is arranged between the air cleaner or the purified air supply passage and the supercharged air passage with respect to a vertical direction.

11. The supercharging device as claimed in claim 1 , in which a portion of the relief valve is accommodated within at least the air cleaner and the purified air supply passage.

12. In a motorcycle, the improvement of a supercharging device for a motorcycle engine, which comprises:

a supercharger, positioned rearwardly adjacent the engine, for pressurizing air to be introduced into the engine;

an air cleaner with a casing for purifying an ambient air;

a purified air supply passage for supplying a purified air from the air cleaner casing positioned above the engine and extending across the engine to the supercharger;

a supercharged air passage for supplying the pressurized air from the supercharger towards the engine; and

a relief valve for adjusting an air pressure within the supercharged air passage,

wherein the relief valve is in a direct fluid connection between a portion of the supercharged air passage and the air cleaner, in which a discharge port portion of the relief valve is accommodated within the casing of the air cleaner for releasing supercharged air above a predetermined pressure from the supercharged air passage.