Air-intake structure of an engine

1. An air-intake structure of an engine comprising:

a duct structure configured to take in air from outside;

an air cleaner including an air filter configured to clean the air guided to the air cleaner through the duct structure;

a throttle device including a throttle valve for controlling a flow rate of the air guided to the throttle device through the air cleaner;

an engine including an intake port configured to suction the air guided to the engine through the throttle device; and

an on-off valve provided in the duct structure;

the duct structure including:

a main inlet configured to take in air therethrough from outside by utilizing a ram pressure;

a main passage extending from the main inlet to the air filter;

a sub-inlet configured to take in air therethrough from outside; and

a joint section located upstream of the air filter, the air flowing from the sub-inlet being joined to the air in the main passage, at the joint section;

wherein the on-off valve is configured to permit and inhibit air communication between the sub-inlet and the joint section;

wherein the on-off valve includes a valve body positioned to permit and inhibit air flow through the sub-inlet of the duct structure; and

wherein the valve body is configured to open by an air pressure directly applied to the valve body during running of the engine.

2. The air-intake structure of the engine according to claim 1 , wherein the on-off valve is a one-way valve configured to permit only air flowing from the sub-inlet toward the joint section.

3. The air-intake structure of the engine according to claim 1 ,

wherein the main inlet is disposed to receive a higher ram pressure than the sub-inlet; and

wherein the on-off valve is at least a two-way valve configured to permit the air flowing from the sub-inlet toward the joint section and the air flowing from the joint section toward the sub-inlet.

4. The air-intake structure of the engine according to claim 3 ,

wherein the on-off valve includes a first counter member configured to apply a first counter force against a first operation of the valve body in which the valve body opens by a pressure of the air flowing from the sub-inlet toward the joint section; and a second counter member configured to apply a second counter force against a second operation of the valve body in which the valve body opens by a pressure of the air flowing from the joint section toward the sub-inlet, the second counter force being larger than the first counter force; and

wherein in a state where the engine is in a stopped state, the valve body is in a closed position in which the valve body inhibits air communication between the sub-inlet and the joint section.

5. The air-intake structure of the engine according to claim 4 , wherein at least one of the first counter member and the second counter member includes an elastic member which is elastically deformed by at least one of the first operation and the second operation, and the first counter force or the second counter force applied to the valve body is a restoring force of the elastic member.

6. The air-intake structure of the engine according to claim 1 ,

wherein the duct structure includes a main duct component constituting at least a portion of the main passage and a sub-duct component constituting a sub-passage extending from the sub-inlet to the joint section;

wherein the main duct component includes an upstream duct member having the main inlet and a downstream duct member provided downstream of the upstream duct member in an air flow direction and upstream of the air cleaner in the air flow direction; and

wherein the sub-duct component is provided integrally with the downstream duct member.

7. The air-intake structure of the engine according to claim 1 , wherein a passage extending from the sub-inlet to the air filter through the joint section has a length smaller than a length of the main passage.

8. An air-intake structure of an engine comprising:

a duct structure configured to take in air from outside;

an air cleaner including an air filter configured to clean the air guided to the air cleaner through the duct structure;

a throttle device including a throttle valve for controlling a flow rate of the air guided to the throttle device through the air cleaner; and

an engine including an intake port configured to suction the air guided to the engine through the throttle device;

the duct structure including:

a main inlet configured to take in air therethrough from outside by utilizing a ram pressure;

a main passage extending from the main inlet to the air filter;

a sub-inlet configured to take in air therethrough from outside; and

a joint section located upstream of the air filter, the air flowing from the sub-inlet being joined to the air in the main passage, at the joint section;

wherein the main inlet is disposed to be applied with a higher ram pressure than the sub-inlet; and

wherein a passage extending from the sub-inlet to the air filter through the joint section has a length smaller than a length of the main passage.

9. The air-intake structure of the engine according to claim 8 ,

wherein the sub-inlet is configured to take in the air from forward in a driving direction of a vehicle including the air-intake structure.

10. The air-intake structure of the engine according to claim 9 ,

wherein the sub-inlet is disposed in a space above a radiator.

11. The air-intake structure of the engine according to claim 9 ,

wherein the air-intake structure is used in a motorcycle;

wherein the main inlet is disposed at a front end portion of the motorcycle; and

wherein the sub-inlet is disposed behind a head pipe of the motorcycle.

12. The air-intake structure of the engine according to claim 8 ,

wherein the sub-inlet is disposed below the main passage.

13. The air-intake structure of the engine according to claim 8 ,

wherein a passage resistance in a passage extending from the sub-inlet to the air filter through the joint section is smaller than a passage resistance in the main passage.