Air filter structure in general purpose engine

1. An air filter structure for a general purpose engine, which comprises:

a fan casing to cover an air cooling fan for the engine;

a cleaner casing fitted to the fan casing to define a cleaner chamber therebetween;

a filter element of a cylindrical configuration accommodated within the cleaner chamber and configured to purify air which will be used as an engine intake air; and

a swirling passage defined in an outer periphery of the filter element within the cleaner chamber and configured to pass the air into a hollow portion of the filter element while the air is swirled around the filter element, wherein a suction opening is provided for introducing air outside the fan casing into the swirling passage, wherein the fan casing includes a fan housing portion for covering the air cooling fan, and a rear wall that divides the cleaner chamber from an interior space in the fan housing portion of the fan casing and that prevents air induced by the air cooling fan from flowing into the cleaner chamber.

2. The air filter structure for the general purpose engine as claimed in claim 1 , wherein

the engine is a vertical two cylinder engine, and

the filter element, when viewed in a planar view, is disposed between a pair of cylinder heads.

3. The air filter structure for the general purpose engine as claimed in claim 1 , wherein the suction opening is oriented downwardly.

4. The air filter structure for the general purpose engine as claimed in claim 3 , further comprising:

a screen fitted to the suction opening and configured to remove foreign matter; and

an introducing passage formed outside the cleaner chamber and configured to introduce the air, which has passed through the screen, into one end portion of the cleaner chamber and then providing the air with swirling flow.

5. The air filter structure for the general purpose engine as claimed in claim 1 , further comprising:

a first retainer having a retaining portion configured to retain a first end portion of the filter element from radially outwards to form a portion of the swirling passage in an outer periphery of the first end portion of the filter element and;

a second retainer having a pair of guide pieces configured to retain a second end portion of the filter element from radially outwards to form a portion of the swirling passage in an outer periphery of the second end portion of the filter element.

6. The air filter structure for the general purpose engine as claimed in claim 5 , wherein:

the first retainer comprises a pipe formed integrally with one of the fan casing and the cleaner casing, the first end portion of the filter element being inserted into the pipe in an axial direction of the filter element; and

the pair of guide pieces fit respectively to the fan casing and the cleaner casing, the guide pieces supporting the second end portion of the filter element from a radial direction of the filter element.

7. The air filter structure for the general purpose engine as claimed in claim 1 , wherein:

the fan casing has a fan casing side mating surface defined therein and the cleaner casing has a cleaner casing side mating surface defined therein, the fan casing and the cleaner casing contacting with each other at a butting area between the fan casing side mating surface and the cleaner casing side mating surface, and;

a projection is formed in one of the fan casing side mating surface and the cleaner casing side mating surface, and a groove engageable with the projection is formed in the other of the fan casing side mating surface and the cleaner casing side mating surface.

8. The air filter structure for the general purpose engine as claimed in claim 1 , wherein:

the air flows within the cleaner chamber from one end portion towards another end portion; and

a dust discharge opening for discharging dust within the cleaner chamber into a discharge passage is formed axially outwardly of the filter element at the other end portion of the cleaner chamber.

9. The air filter structure for the general purpose engine as claimed in claim 8 , wherein:

the dust discharge opening has an upstream edge, which is positioned on an upstream side of the air discharged from the dust discharge opening, and a downstream edge positioned on a downstream side thereof;

the upstream edge and the downstream edge both extend in an axial direction of the filter element; and

the downstream edge is positioned above the upstream edge.

10. The air filter structure for the general purpose engine as claimed in claim 8 , further comprising a dust discharge pipe fluid connected with the dust discharge opening, the dust discharge pipe defining the discharge passage therein; and

a one-way valve fitted to an outlet of the dust discharge pipe.

11. The air filter structure for the general purpose engine as claimed in claim 10 , wherein the one-way valve has an outlet opening oriented downwardly.

12. The air filter structure for the general purpose engine as claimed in claim 7 , further comprising a stripe sealing member mounted in the groove.

13. The air filter structure for the general purpose engine as claimed in claim 1 , further comprising:

an introducing passage formed between the suction opening and the cleaner chamber;

a communicating port is formed at one end portion of the introducing passage, the communicating port being communicated with one end portion the cleaner chamber; and

a slope is provided in the communicating port, the slope serving as a guide member for guiding the intake air and bring about a swirling force on the intake air, wherein

the slope has an upstream edge and a downstream edge which have different angles of inclination for the guidance of the intake air.

14. The air filter structure for the general purpose engine as claimed in claim 8 , wherein the dust discharge passage has a passage surface area that is larger than an opening surface area of the dust discharge opening.