Cooling water passage structure of outboard motor

An outboard motor includes a cooling water passage structure, in which a combustion chamber periphery water jacket through which the cooling water flows around the combustion chamber and an exhaust port periphery water jacket through which the cooling water flows around the exhaust port are formed in the cylinder head, a cylinder periphery water jacket through which the cooling water flows around the cylinder is formed in the cylinder block, and an exhaust passage periphery water jacket through which the cooling water flows around the exhaust passage is formed around the exhaust passage. The water jackets are connected such that the cooling water from the water passage will flow through the combustion chamber periphery water jacket, the exhaust port periphery water jacket, the cylinder periphery water jacket, and the exhaust passage periphery water jacket in order.

CROSS REFERENCE TO RELATED APPLICATION

This application is based upon and claims the benefit of priority of the prior Japanese Patent Application No. 2015-026188, filed Feb. 13, 2015, the entire contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention relates to a cooling water passage structure of an outboard motor for cooling an engine mounted on the outboard motor by using cooling water.

Description of the Related Art

As shown inFIG. 9, a cooling water passage structure of an outboard motor disclosed in Patent Document 1 (Japanese Patent Laid-Open No. 2013-124592) includes a cooling water passage which causes sea water, river water, or the like taken in through an intake101of the outboard motor and led to a water passage103by a water pump102to flow as cooling water through a cylinder head water jacket104of a cylinder head, exhaust passage periphery water jacket105of an exhaust passage containing an exhaust purification catalyst, and a cylinder periphery water jacket106of a cylinder block in order.

In the cylinder head water jacket104, a first combustion chamber periphery water jacket107A and a second combustion chamber periphery water jacket107B located on an upstream side are communicated with an exhaust manifold periphery water jacket108located on a downstream side. The first combustion chamber periphery water jacket107A, second combustion chamber periphery water jacket107B, and exhaust manifold periphery water jacket108are so designed as to pass cooling water upward from a lower side.

In the case of the cooling water passage structure of an outboard motor described in Patent Document 1, in a peripheral portion of a combustion chamber in the cylinder head, a temperature of a portion in the first combustion chamber periphery water jacket107A and second combustion chamber periphery water jacket107B which corresponds to a lowermost cylinder and into which cooling water flows first is lower than that of portions corresponding to the other cylinders, and hence, is overcooled.

Furthermore, in a peripheral portion of an exhaust manifold of the cylinder head, cooling water yet to be cooled in the first combustion chamber periphery water jacket107A and second combustion chamber periphery water jacket107B flows into that peripheral portion of the exhaust manifold periphery water jacket108which corresponds to the lowermost cylinder, and accordingly, a temperature of such portion is lower than portions corresponding to the other cylinders, and hence, is overcooled.

Consequently, exhaust gas flowing through the exhaust manifold is cooled excessively, steam in the exhaust gas condenses, and droplets are produced in the exhaust manifold.

SUMMARY OF THE INVENTION

The present invention was conceived in consideration of the circumstances mentioned above, and an object thereof is to provide a cooling water passage structure of an outboard motor, capable of preventing steam in exhaust gas from being condensed by excessive cooling of the exhaust gas flowing through an exhaust passage, preventing an oxygen sensor installed in the exhaust passage from getting wet, and thereby improving durability of the oxygen sensor.

The above and other objects can be achieved according to the present invention by providing, in one preferred embodiment, a cooling water passage structure of an outboard motor which includes a four-stroke engine, an intake unit having an intake port configured to take in water from an underwater, and a water passage configured to supply the water taken in through the intake unit to the four-stroke engine as cooling water, wherein the four-stroke engine includes a cylinder block in which a cylinder is formed by extending in a horizontal direction, a cylinder head fixed to the cylinder block so as to cover the cylinder, configured to form a combustion chamber together with the cylinder, and provided with an exhaust port configured to discharge exhaust gas in communication with the combustion chamber, and an exhaust passage connected to the exhaust port so as to lead the exhaust gas to outside the engine, wherein a combustion chamber periphery water jacket through which the cooling water flows around the combustion chamber and an exhaust port periphery water jacket through which the cooling water flows around the exhaust port are formed in the cylinder head, a cylinder periphery water jacket through which the cooling water flows around the cylinder is formed in the cylinder block, and an exhaust passage periphery water jacket through which the cooling water flows around the exhaust passage is formed around the exhaust passage, and wherein the water jackets are connected such that the cooling water from the water passage will flow through the combustion chamber periphery water jacket, the exhaust port periphery water jacket, the cylinder periphery water jacket, and the exhaust passage periphery water jacket in order.

According to the preferred embodiment of the present invention, the cooling water from the water passage flows through the combustion chamber periphery water jacket, the exhaust port periphery water jacket, the cylinder periphery water jacket, and the exhaust passage periphery water jacket in order. Accordingly, the cooling water heated in the combustion chamber periphery water jacket and exhaust port periphery water jacket in sequence flows through the exhaust passage periphery water jacket. Therefore, the exhaust gas flowing through the exhaust passage is not cooled excessively by the cooling water, which makes it possible to prevent condensation of the steam contained in the exhaust gas, and hence, makes it possible to prevent an oxygen sensor installed in the exhaust passage from getting wet, thereby improving the durability of the oxygen sensor.

The nature and further characteristic features of the present invention will be made clearer from the following description made with reference to the accompanying drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENT

An embodiment of the present invention will be described hereunder with reference to the accompanying drawings. It is to be noted that, in the following descriptions, terms “upper”, “lower”, “right”, “left”, “forward”, “rearward” and the like terms indicating directions are used with reference to the illustrated state of the drawings or a state mounted to a hull.

FIG. 1is a left side view showing an outboard motor to which an embodiment of a cooling water passage structure of the outboard motor according to the present invention is applied. The outboard motor10shown inFIG. 1is equipped with an engine holder12, on which an engine11is mounted. The engine11is a vertical engine in which a crankshaft26(described hereinlater) is mounted substantially perpendicularly (i.e., vertically). A drive shaft housing13and a gear case14are assembled in sequence under the engine holder12.

InFIG. 1, an oil pan15is located under the engine holder12in which a lubricating oil is reserved. A vertically dividable engine cover9includes a lower engine cover9A and an upper engine cover9B so as to cover the engine11and engine holder12.

The outboard motor10is supported pivotally in a horizontal direction by means of a pilot shaft16pivotally supported on a swivel bracket17. The swivel bracket17is supported on a swivel shaft18pivotally in a vertical direction with respect to a clamp bracket19, which is attached to a stern (transom)20A of a hull20. Consequently, the outboard motor10is mounted on the hull20swingably in a horizontal direction (steering direction) and vertical direction (trim and tilt direction).

A driving force generated on the crankshaft26of the engine11is transmitted through reduction gears21A and21B to a drive shaft22disposed so as to extend substantially vertically in the drive shaft housing13and gear case14and is then transmitted through a shift mechanism23and propeller shaft24disposed in the gear case14to a propeller25, thereby turning the propeller25in a forward or reverse direction. According to such arrangement, the outboard motor10causes the hull20to move forward or backward.

As shown inFIGS. 1 and 2, the engine11is a V-type four-stroke-cycle engine which includes the crankshaft26extending in a vertical direction, a left bank27extending diagonally left rearward, and a right bank28extending diagonally right rearward. In such V-type engine, the left bank27is composed of a cylinder head31and a cylinder head cover38placed in sequence behind a left bank portion30A of a cylinder block30, and the right bank28is composed of a cylinder head31and a cylinder head cover38placed in sequence behind a right bank portion30B of the cylinder block30. Further, a crankcase32is placed in front of the cylinder block30.

As shown inFIG. 2, cylinders33are formed in a horizontal direction inside the left bank portion30A of the cylinder block30, extending diagonally left rearward. Cylinders33are also formed in a horizontal direction inside the right bank portion30B of the cylinder block30, extending diagonally right rearward. Pistons29are reciprocally located in the cylinders33and coupled to the crankshaft26via connection rods, not shown.

Along cylinder axes P of the cylinders33in the left bank portion30A and right bank portion30B of the cylinder block30, the cylinder heads31are fixed, respectively, to the left bank portion30A and right bank portion30B so as to cover the cylinders33, and concurrently, to form combustion chambers34in conjunction with respective cylinders33in the left bank portion30A and right bank portion30B.

Moreover, in the left bank portion30A and right bank portion30B of the cylinder block30, intake ports35communicated with the combustion chambers34are formed in the cylinder heads31inwardly of the cylinder axes P of the cylinders33in a width direction of the outboard motor. Furthermore, in the cylinder heads31, exhaust ports36communicated with the combustion chambers34are formed in the cylinder heads31outwardly of the cylinder axes P of the cylinders33in the left bank portion30A and right bank portion30B of the cylinder block30in the width direction of the outboard motor.

The crankcase32is coupled to the cylinder block30, thereby forming a crank chamber37in conjunction with the cylinder block30, and the crankshaft26is housed in the crank chamber37. Herein, in each of the above-mentioned left bank27and right bank28, plural cylinder assemblies (i.e., piston-cylinder assemblies)40each equipped with a cylinder33, a combustion chamber34, an intake port35, and an exhaust port36are arranged side by side in a vertical direction as shown inFIGS. 3 and 4. According to the present embodiment, three cylinder assemblies40are arranged side by side in the vertical direction in each of the right bank28and left bank27, thus constituting the engine11as a V-type six-cylinder four-stroke engine.

As shown inFIGS. 2 and 3, the respective exhaust ports36of the plural cylinder assemblies40in the left bank27are connected with a left exhaust passage41adapted to lead the exhaust gas from the exhaust ports36out of the engine11. The left exhaust passage41is formed integrally with the left bank portion30A of the cylinder block30. Further, the respective exhaust ports36of the plural cylinder assemblies40in the right bank28are connected with a right exhaust passage42adapted to lead the exhaust gas from the exhaust ports36out of the engine11. The right exhaust passage42is formed integrally with the right bank portion30B of the cylinder block30. Each of the left exhaust passage41and the right exhaust passage42includes an exhaust manifold43serving as a first exhaust passage section and a catalyst storage space44serving as a second exhaust passage section.

The exhaust manifold43is mounted on at least one of both the lateral sides, in the present embodiment, on both sides, of the cylinder block30in the width direction. That is, the exhaust manifold43of the left exhaust passage41is provided in lateral part of the cylinder block30on the left side in the width direction (left bank portion30A), corresponding to the left bank27, while the exhaust manifold43of the right exhaust passage42is provided in lateral part of the cylinder block30on the right side in the width direction (right bank portion30B), corresponding to the right bank28.

Furthermore, as shown inFIG. 3, in particular, the exhaust manifolds43collect the exhaust gas discharged from the respective exhaust ports36of the plural cylinder assemblies40.

In addition, a plurality of exhaust guiding portions46are provided for the respective exhaust manifolds43of the left exhaust passage41and right exhaust passage42so as to guide the exhaust gas discharged from the respective exhaust ports36of the plural cylinder assemblies40to connecting portions45between the exhaust manifolds43and catalyst storage chambers44. Each of the exhaust guiding portions46is formed as a vertical plane opposed to a joint surface (parting plane)47between the cylinder block30and the cylinder head31. The exhaust gas flowing in the exhaust port36is guided upward by the exhaust guiding portion46in the exhaust manifold43, and the exhaust gas then reaches the connecting portion45between the exhaust manifold43and the catalyst storage chamber44.

As shown inFIG. 2, the catalyst storage chamber44in the left exhaust passage41is formed integrally on the left bank portion30A of the cylinder block30and the catalyst storage chamber44in the right exhaust passage42is formed integrally on the right bank portion30B of the cylinder block30, both being, for example, substantially circular in passage section.

As shown inFIG. 3, the catalyst storage chambers44are communicated with both the connecting portions45of the exhaust manifolds43and an exhaust passage51of the engine holder12, thereby connecting exhaust manifolds43with an exhaust silencing chamber (i.e., muffler), not shown, inside the drive shaft housing13installed outside the engine11. Then, catalytic converters53having, for example, a circular shape in section for purifying the exhaust gas are installed and housed in the catalyst storage chambers44.

Each catalytic converter53is configured such that a catalyst carrier54formed into, for example, a columnar shape and equipped with an exhaust purification function is housed in a catalyst tube55, having a cylindrical shape, for example. When the catalyst carrier54comes into contact with exhaust gas, it chemically changes toxic substances such as carbon monoxide, hydrocarbon, nitrogen oxides, and the like contained in the exhaust gas into water, carbon dioxide, nitrogen or the like via oxidation-reduction reactions to thereby purify the exhaust gas.

Accordingly, the exhaust gas produced in the combustion chambers34of the plural cylinder assemblies40in the left bank27and right bank28of the engine11shown inFIG. 2, flows through the exhaust ports36of the cylinder assemblies40in the left bank27and right bank28and into the respective exhaust manifolds43of the left exhaust passage41and right exhaust passage42.

As shown inFIG. 3, the exhaust gas flowing into each exhaust manifold43ascends by being guided by the exhaust guiding portion46, and then reaches the connecting portion45between the exhaust manifold43and the catalyst storage chamber44. The exhaust gas flows downward in the connecting portion45into the catalytic converter53in the catalyst storage chamber44in order to be purified.

The exhaust gas purified by the catalytic converters53flows downward into the exhaust silencing chamber of the drive shaft housing13, thereby being expanded and silenced (muffled) therein. Subsequently, the exhaust gas flows in an exhaust passage, not shown, formed around the propeller shaft24in the gear case14shown inFIG. 1and is discharged into water from a center of the propeller25.

InFIG. 2, reference numeral57denotes an intake manifold connected to the intake port35of the engine11and adapted to introduce fuel/air mixture into the combustion chamber34through the intake port35.

Herein, as shown inFIG. 4, the outboard motor10shown inFIG. 1is provided with a cooling water passage structure60to cool the engine11by leading water, as cooling water, to the engine11from a sea or a river on which the hull20with the outboard motor10installed thereon navigates. The cooling water passage structure60includes an intake port61formed in the gear case14, a water passage62installed in the drive shaft housing13and provided with a water pump63, a combustion chamber periphery water jackets65and an exhaust port periphery water jackets66formed in the cylinder heads31on the left bank27and right bank28of the engine11, and cylinder periphery water jackets67and exhaust passage periphery water jackets68formed in the left bank portion30A and right bank portion30B of the cylinder block30.

As the gear case14is located in water during the use and operation of the outboard motor10, the intake port61is formed in the gear case14so as to be able to take in water by being located underwater. Furthermore, the water passage62installed in the drive shaft housing13includes the water pump63and has its lower end and upper end connected to the intake port61and a cooling water passage64of the engine holder12, respectively. The water pump63is installed in the drive shaft housing13near a mating surface between the drive shaft housing13and the gear case14and driven by the drive shaft22.

The water passage62takes in water through the intake port61when the water pump63operates, and supplies the water as cooling water to cooling water inlet ports69formed in the left bank portion30A and right bank portion30B of the cylinder block30through the cooling water passage64of the engine holder12.

The cooling water supplied to the cooling water inlet ports69in the left bank portion30A and right bank portion30B flows first through the combustion chamber periphery water jackets65in the cylinder heads31on the left bank27and right bank28as shown inFIGS. 4 and 5without cooling the cylinder block30to thereby cool the peripheries of the combustion chambers34of the plural cylinder assemblies40in the cylinder heads31on the left bank27and right bank28. Then, the cooling water flows through the exhaust port periphery water jackets66in the cylinder heads31on the left bank27and the right bank28to thereby cool the peripheries of the exhaust ports36of the plural cylinder assemblies40in the cylinder heads31on the left bank27and the right bank28.

Then, as shown inFIGS. 4 and 7, the cooling water flows simultaneously through the cylinder periphery water jackets67and the exhaust passage periphery water jackets68in the left bank portion30A and the right bank portion30B of the cylinder block30in parallel to thereby cool the peripheries of the cylinders33of the plural cylinder assemblies40in the left bank portion30A and the right bank portion30B of the cylinder block30as well as the peripheries of the left exhaust passage41and the right exhaust passage42(especially, peripheries of the exhaust manifolds43in the left exhaust passage41and the right exhaust passage42as well as the catalyst53).

Subsequently, the cooling water is discharged out of the engine11through a thermostat case70of the engine11. In order for the cooling water to flow as described above, the combustion chamber periphery water jacket65, the exhaust port periphery water jackets66, the cylinder periphery water jacket67, and the exhaust passage periphery water jacket68are connected in sequence.

The combustion chamber periphery water jackets65formed in the cylinder heads31on the left bank27and the right bank28are formed around the combustion chambers34of the plural cylinder assemblies40in the cylinder heads31by being communicated with each other as shown inFIGS. 4 and 5. In the combustion chamber periphery water jacket65, as indicated by arrow “A”, the cooling water flows in from the side of the lowermost cylinder assembly40, ascends, and flows around the combustion chambers34in sequence to the side of an uppermost cylinder assembly40.

Consequently, the peripheries of the combustion chambers34in the cylinder heads31are cooled in sequence from the lowermost cylinder assembly40to the uppermost cylinder assembly40.

The exhaust port periphery water jackets66formed in the cylinder heads31on the left bank27and the right bank28are formed around the exhaust ports36of the plural cylinder assemblies40in the cylinder heads31by being communicated with each other as shown inFIGS. 3, 4, and 5. As indicated by arrow “B”, on the side of the uppermost cylinder unit40, the cooling water from the combustion chamber periphery water jacket65flows into the exhaust port periphery water jacket66, descends, and flows around the exhaust ports36in sequence to the side of the lowermost cylinder assembly40.

Consequently, the peripheries of the exhaust ports36in the cylinder heads31are cooled in sequence from the uppermost cylinder assembly40to the lowermost cylinder assembly40.

As shown inFIGS. 3, 4, and 6, a lowermost portion (on the side of the lowermost cylinder unit40) of the exhaust port periphery water jacket66formed in the cylinder head31on each of the left bank27and the right bank28is communicated with a first exhaust manifold periphery water jacket73A (described hereinlater) of the exhaust passage periphery water jacket68in the corresponding one of the left bank portion30A and the right bank portion30B of the cylinder block30.

The first exhaust manifold periphery water jackets73A are communicated with lowermost portions of the cylinder periphery water jackets67in the left bank portion30A and the right bank portion30B of the cylinder block30through a communicating path71between the left bank portion30A and the right bank portion30B and communicated with catalyst periphery water jackets74(described hereinlater) of the exhaust passage periphery water jackets68in the left bank portion30A and the right bank portion30B of the cylinder block30through communicating paths72of the left bank portion30A and the right bank portion30B.

Further, as shown inFIGS. 4, 6, and 7, the cylinder periphery water jackets67formed in the left bank portion30A and the right bank portion30B of the cylinder block30are formed around the cylinders33of the plural cylinder assemblies40in the left bank portion30A and the right bank portion30B of the cylinder block30by being communicated with each other. The cooling water flowing into the cylinder periphery water jackets67from the lowermost portions (on the side of the lowermost cylinder units40) of the exhaust port periphery water jackets66through the first exhaust manifold periphery water jackets73A and communicating path71flows into the side of the lowermost cylinder assemblies40, ascends, and flows around the cylinders33in sequence to the side of the uppermost cylinder assembly40, as indicated by arrow “C”.

Consequently, the peripheries of the cylinders33in the left bank portion30A and the right bank portion30B of the cylinder block30, are cooled in sequence from the lowermost cylinder assembly40to the uppermost cylinder assembly40.

As shown inFIGS. 2 to 4, 6, and 7, the exhaust passage periphery water jacket68formed in each of the left bank portion30A and the right bank portion30B of the cylinder block30includes the first exhaust manifold periphery water jacket73A, a second exhaust manifold periphery water jacket73B, a third exhaust manifold periphery water jacket73C, and the catalyst periphery water jacket74, which are communicated with one another.

Furthermore, the exhaust passage periphery water jacket68(actually, the second exhaust manifold periphery water jacket73B, the third exhaust manifold periphery water jacket73C, and the catalyst periphery water jacket74) is configured into a separate circuit by being connected in parallel with the cylinder periphery water jacket67.

That is, as shown inFIGS. 3 and 6, in particular, the first exhaust manifold periphery water jacket73A is formed around a lower portion of the exhaust manifold43formed in each of the left bank portion30A and the right bank portion30B of the cylinder block30and is communicated with the catalyst periphery water jacket74through the communicating path72as described above. The catalyst periphery water jacket74is formed by a gap59provided between an inner wall surface of the catalyst storage chamber44in each of the left bank portion30A and the right bank portion30B of the cylinder block30and an outer lateral surface of the catalyst tube55of the catalytic converter53and is provided around the catalytic converter53.

As shown inFIGS. 2, 3, and 7, in particular, the second exhaust manifold periphery water jacket73B is formed around that part of the exhaust manifold43formed in each of the left bank portion30A and right bank portion30B of the cylinder block30which is located on the side of the cylinders33, with a lower part of the water jacket73B being communicated with the catalyst periphery water jacket74through a communicating path75and an upper part of the water jacket73B being communicated with the third exhaust manifold periphery water jacket73C through a communicating path76.

As shown inFIGS. 3 and 7, in particular, the third exhaust manifold periphery water jacket73C is formed around an upper portion (connecting portion45) of the exhaust manifold43formed in each of the left bank portion30A and the right bank portion30B of the cylinder block30. The third exhaust manifold periphery water jacket73C is communicated with the second exhaust manifold periphery water jacket73B through the communicating path76as described above as well as communicated with the catalyst periphery water jacket74through a communicating path77, a communicating path80between an exhaust passage lid78and the water jacket lid79, and a communicating path81as shown inFIG. 8.

Thus, in the exhaust passage periphery water jacket68described above, as indicated by arrow “D” inFIGS. 4, 6, 7, and 8, the cooling water from the lowermost portions of the exhaust port periphery water jackets66flows into the first exhaust manifold periphery water jacket73A, and then flows into the catalyst periphery water jacket74through the communicating path72and ascends therein while flowing into and then ascending in the second exhaust manifold periphery water jacket73B through the communicating path75in parallel to the flow in the catalyst periphery water jacket74. The cooling water in the catalyst periphery water jacket74flows into the third exhaust manifold periphery water jacket73C through the communicating paths77,80, and81.

The cooling water in the second exhaust manifold periphery water jacket73B also flows into the third exhaust manifold periphery water jacket73C through the communicating path76. The cooling water flowing through the exhaust passage periphery water jacket68in this way cools the periphery of the exhaust manifold43in each of the left exhaust passage41and the right exhaust passage42as well as the catalyst53in each of the left exhaust passage41and the right exhaust passage42.

As shown inFIGS. 4 and 8, the cooling water flowing into the third exhaust manifold periphery water jacket73C flows through a communicating path82formed in each of the left bank portion30A and the right bank portion30B of the cylinder block30and joins with the cooling water flowing through the uppermost portion (on the side of the uppermost cylinder unit40) of the cylinder periphery water jacket67. The joined cooling water flows into the thermostat case70as indicated by arrow “E” inFIG. 4and is discharged out of the engine11when a thermostat contained in the thermostat case70, not shown, opens.

Then, as shown inFIG. 3, a portion of the exhaust manifold43in each of the left exhaust passage41and the right exhaust passage42, i.e., a portion of the exhaust manifold43which is close to the exhaust guiding portion46, is positioned adjacent to the catalyst periphery water jacket74of the exhaust passage periphery water jacket68.

Consequently, the cooling water flowing through the catalyst periphery water jacket74has a function to cool not only the catalytic converter53, but also that portion, mentioned above, of the exhaust manifold43which is close to the exhaust guiding portion46.

Furthermore, as shown inFIGS. 4 and 6, the cylinder periphery water jacket67and the exhaust passage periphery water jacket68are formed in each of the left bank portion30A and the right bank portion30B of the cylinder block30, and in these water jackets, only the catalyst periphery water jacket74of the exhaust passage periphery water jacket68is formed and the cylinder periphery water jacket67is not formed in a region outside the cylinders33in the cylinder block30in the width direction of the outboard motor, i.e., in a portion “M” corresponding to the largest diameter portion of the catalytic converter53along a radial direction of the cylinders33.

Consequently, in each of the left bank portion30A and the right bank portion30B of the cylinder block30, the catalyst storage chamber44is formed close to the cylinders33.

Furthermore, as shown inFIG. 3, in each of the left bank portion30A and the right bank portion30B of the cylinder block30, an oxygen sensor83adapted to measure oxygen concentration in the exhaust gas is installed at that position, mentioned above, of the exhaust manifold43in each of the left exhaust passage41and the right exhaust passage42which faces a neighborhood of the connecting portion45.

The oxygen sensor83is intended to measure the oxygen concentration in the exhaust gas to detect whether the engine11achieves a theoretical air fuel ratio needed for the catalytic converter53to efficiently oxidize or reduce nitrogen oxides, hydrocarbons, and carbon monoxide. However, the oxygen sensor83is made of ceramics, and thus could be broken if the oxygen sensor83gets wet repeatedly at a time of being activated under high temperature conditions.

According to the structure and arrangement of the embodiment of the present invention described above, the following advantageous features (1) to (7) can be achieved.

(1) As shown inFIG. 4, since the cooling water taken in through the intake61and led to the water passage62flows through the combustion chamber periphery water jacket65, the exhaust port periphery water jackets66, the cylinder periphery water jacket67, and the exhaust passage periphery water jacket68in sequence, the cooling water heated in the combustion chamber periphery water jacket65and the exhaust port periphery water jackets67in sequence flows through the exhaust passage periphery water jacket68. Because of this reason, as shown inFIG. 3, the exhaust gas flowing through the exhaust manifold43in each of the left exhaust passage41and the right exhaust passage42is not cooled excessively by the cooling water, and as a result, it becomes possible to prevent condensation of the steam contained in the exhaust gas in the exhaust manifold43, and this fact in turn makes it possible to prevent the oxygen sensor83installed in the exhaust manifold43from getting wet and to thereby improve the durability of the oxygen sensor83.

(2) As shown inFIG. 4, in each of the left bank portion30A and the right bank portion30B of the cylinder block30, the cylinder periphery water jacket67and the exhaust passage periphery water jacket68(the second exhaust manifold periphery water jacket73B, the third exhaust manifold periphery water jacket73C, and the catalyst periphery water jacket74) are connected in parallel and constitute a separate circuit. Consequently, as shown inFIG. 7, even in a case when the required temperature characteristics are different between a periphery of the cylinders33and peripheries of the left exhaust passage41and the right exhaust passage42(peripheries of the exhaust manifolds43in the left exhaust passage41and the right exhaust passage42as well as the catalyst53), if the cooling water flow rate is adjusted by changing flow channel diameters of, for example, the cylinder periphery water jacket67and the exhaust passage periphery water jacket68according to the required temperature characteristics, the temperatures around the cylinders33as well as around the left exhaust passage41and the right exhaust passage42can be managed optimally.

(3) As shown inFIGS. 4 and 5, in the combustion chamber periphery water jacket65formed in the cylinder head31, when the cooling water flows in from the side of the lowermost cylinder unit40, ascends, and flows to the side of the uppermost cylinder unit40, the temperature of the portion of the combustion chamber periphery water jacket65which corresponds to the cylinder head31located on the side of the lowermost cylinder unit40into which the cooling water flows first, the periphery of the exhaust port36becomes lower than the temperature of the other cylinder assemblies40because the periphery of the combustion chamber34is cooled by the cooling water having low temperature. At this time, in the exhaust port periphery water jacket66formed in the cylinder head31, the cooling water from the combustion chamber periphery water jacket65on the side of the uppermost cylinder assembly40flows into the side of the uppermost cylinder assembly40and flows downward to the side of the lowermost cylinder assembly40, making it possible to prevent the temperature form falling around the exhaust port36on the side of the lowermost cylinder assembly40in the cylinder head31, and as a result, the cylinder head31can be cooled uniformly.

(4) As shown inFIG. 3, a portion of the exhaust manifold43in each of the left exhaust passage41and right exhaust passage42, i.e., a portion of the exhaust manifold43which is close to the exhaust guiding portion46, is positioned adjacent to the catalyst periphery water jacket74of the exhaust passage periphery water jacket68. Accordingly, the cooling water flowing through the catalyst periphery water jacket74cools not only the catalytic converter53, but also the portion, mentioned above, of the exhaust manifold43which is close to the exhaust guiding portion46, thereby eliminating the need for a water jacket used to cool that portion of the exhaust manifold43which is close to the exhaust guiding portion46, and therefore, an opening diameter of the catalytic converter53can be expanded, thereby decreasing pressure loss of the exhaust gas, and improving the power of the engine11.

Furthermore, since the exhaust gas flowing along the exhaust guiding portion46in the exhaust manifold43is cooled by the cooling water in the catalyst periphery water jacket74warmed by the catalytic converter53, the temperature of the exhaust gas flowing through the exhaust manifold43can be suppressed more securely from excessively falling down, which makes it possible to prevent the oxygen sensor83installed in the exhaust manifold43from getting wet and thus, further improving the durability of the oxygen sensor83.

(5) As shown inFIGS. 3 and 6, the catalyst periphery water jacket74of the exhaust passage periphery water jacket68is formed by the gap59between the inner wall surface of the catalyst storage space44in each of the left bank portion30A and the right bank portion30B of the cylinder block30and the outer lateral surface of the catalyst tube55of the catalytic converter53and is provided around the catalytic converter53. Accordingly, since the catalyst tube55of the catalytic converter53comes into direct contact with the cooling water in the catalyst periphery water jacket74, it becomes possible to improve the cooling efficiency of the catalytic converter53, and the volume of the cooling water flowing through the catalyst periphery water jacket74can be hence reduced. It also becomes possible to reduce a cross sectional area of a flow channel of the catalyst periphery water jacket74to thereby downsize the cylinder block30in which the catalyst periphery water jacket74is formed.

(6) The catalytic converter53is made up of the catalyst carrier54contained in the catalyst tube55. For example, as described in Patent Document 2 (Japanese Patent Laid-Open No. 2010-242744), when a catalyst storage portion is formed integrally with a housing which makes up an exhaust passage, the catalyst storage portion has a thick-walled structure. In contrast, according to the present embodiment, since the catalyst carrier54is contained in the catalyst tube55of the thin-walled structure, the catalytic converter53can be downsized accordingly. Thus, if the catalytic converter53is not downsized, the opening diameter of the catalyst carrier54can be increased and the pressure loss of the exhaust gas flowing through the catalyst carrier54decreases. As a result, the power of the engine11can be improved.

(7) As shown inFIG. 6, in a region outside the cylinders33in the cylinder block30in the width direction of the outboard motor, i.e., in the portion “M” corresponding to the largest diameter portion of the catalytic converter53along the radial direction of the cylinders33, only the catalyst periphery water jacket74of the exhaust passage periphery water jacket68is formed and the cylinder periphery water jacket67is not formed. As a result, the catalyst storage chamber44can be formed close to the cylinders33in each of the left bank portion30A and the right bank portion30B of the cylinder block30, thereby reducing the size of the outboard motor10in the width direction. Further, when a plurality of the outboard motors10are installed side by side on the transom20A of the hull20, the plural outboard motors10can be clustered in a center of the hull20in the width direction by reducing installation intervals of the plural outboard motors10.

It is to be noted that the present invention is not limited to the embodiments described above as preferred examples, and many other changes, modifications, and alternations may be made without departing from the sprits of the present invention and scope of the appended claims.

For example, although in the embodiment described above, the engine11mounted on the outboard motor10is a V-type multi-cylinder four-stroke engine, an in-line multi-cylinder four-stroke type or single-cylinder four-stroke type may be adopted as the engine11.