EP0795684B1

EP0795684B1 - marine outboard engine

Info

Publication number: EP0795684B1
Application number: EP19970104073
Authority: EP
Inventors: Masanori Takahashi; Hiroshi Oishi
Original assignee: Sanshin Kogyo KK
Current assignee: Yamaha Marine Co Ltd
Priority date: 1996-03-11
Filing date: 1997-03-11
Publication date: 2002-06-19
Anticipated expiration: 2017-03-11
Also published as: JP3685416B2; EP0795684A1; DE69713420T2; DE69713420D1; JPH09240590A

Description

The present invention relates to a marine outboard engine having an engine unit being vertically arranged with respect to a driving direction of a boat, said engine unit comprising a cylinder block having at least one cylinder, and a cylinder head connected to said cylinder block and defining at least one combustion chamber, said cylinder block further comprising an exhaust passage for collecting exhaust gases from the combustion chamber and for expelling said exhaust gases downward beneath the engine unit, said marine outboard engine further having a cooling structure for cooling said at least one cylinder and said at least one combustion chamber by means of a fresh coolant water, whereas said coolant water first is directed to the exhaust passage for firstly cooling said exhaust passage and then is directed to said at least one cylinder and respective combustion chamber.
Such an engine is known from GB-A-2 055 422.
The general structure for marine outboard engines that are mounted in small boats that is known to the art is one whereby, in order for the crankshaft to be disposed in the vertical direction, the engine unit in which the cylinders are arrayed in the vertical direction has been housed inside the top cowling at the top of the outboard engine. In such engines, in order to house such engines compactly, an exhaust passage is integrally formed in the cylinder block of the engine unit in order to convey the exhaust gases collected from the combustion chambers of the cylinders to the area beneath the engine unit.
In this type of marine outboard engine that has an exhaust passage integrally formed in the engine unit, there are additionally a coolant passage around the cylinders and a coolant passage around the exhaust passages in order to cool the areas around the cylinders and combustion chambers of the engine unit and around the exhaust passage using coolant water drawn up from below.
Figure 11 shows a conventional arrangement of these respective coolant passages in the engine unit. First, after the coolant water has been introduced through the inlet that opens on the lower end of the coolant passage 71 and around the exhaust passage on the cylinder block side, it branches off [severally] midway into the coolant passage 72 running around the exhaust passage and also continues on to cause the coolant to flow in from the bottom coolant passage 71 around the exhaust passage upward, from where it is directly sent to the bottom of the coolant passage 74 around the cylinders. The coolant flows upward from the bottom of said coolant passage 74, and it subsequently flows upward where the coolant is then [severally] branched off, and in addition, the coolant that is flowing through the coolant passage 71 around the exhaust passage in the cylinder block is gradually diverted midway, and then the coolant flowing through the coolant passage 72 proceeds through the coolant passage 73 around the combustion chambers and merges with the coolant passage 74 around the cylinders, and then finally, the various flows of coolant water around the cylinders are expelled through the outlet at the top of coolant passage 74 to the outside of the cooling circuit
With regard to the overall cooling circuit for marine outboard engines, normally it is the case that outside water introduced into the lower case is used as the coolant, and this coolant water is drawn up by means of a water pump that is linked to the rotational drive of the engine. Midway in the coolant transmission piping, while cooling inside the upper case, the coolant is transmitted to the engine unit inside the top cowling. After the engine unit has been cooled by means of coolant passage through the above described cooling circuit, it is returned to the upper case where, after cooling the oil pan and muffler, the coolant is sent through the lower case and expelled to the outside of the unit.
However, with regard to marine outboard engine cooling structures such as described above for the coolant passages of the engine, the coolant is circulated in the coolant passage around the cylinders, applying a direct coolant flow without first circulating around the exhaust passage. In addition, since this coolant is merged with a flow of coolant that was heated by passing around the exhaust passage, in some areas, a temperature variation will develop in the coolant temperature flowing around the cylinders; this can cause uneven temperatures around the cylinders.
The result is the development of a significant deformation of the cylinders while the engine is running, which increases the resistance on the sliding motion of the pistons, and which can cause them to seize, increase the amount of blowby gas, promote higher oil consumption, and in addition, impair the combustion stability during low speed operations due to the coolant that is directly circulated around the cylinders and combustion chambers lowering the temperature of those combustion chambers.
Accordingly, it is an objective of the present invention to provide an improved marine outboard engine comprising a cooling system facilitating an always reliable cooling of the engine and simultaneously enhances the longevity of this marine outboard engine.
According to the present invention, this objective is solved for a marine outboard engine as indicated above in that a pressure valve being provided close to a downstream end of a coolant passage around the exhaust passage.
According to a preferred embodiment of the present invention, said coolant water is fed into the engine unit from its lower area and first is passing through a coolant passage around the exhaust passage and then is passing through a further coolant passage around the at least one cylinder and the respective combustion chamber.
In some cases, it is advantageous to control the temperature dependent amount of coolant expelled. In these cases, a thermostatic valve may be provided at the top of the engine unit close to the end of said coolant passage.
Furthermore, with regard to the above described marine outboard engines of the prior art that employed an exhaust passage integrally formed in the cylinder block, and that had a water jacket formed around the cylinders, because the heat from the exhaust gases was transmitted to the area around the cylinders on the exhaust passage side, a very large temperature difference would develop between the exhaust side and the air intake side of the engine.
Increasing the size of the water jacket around the cylinders on the exhaust passage side could be considered a way to insulate them from the heat from the exhaust gases, but if the size of the water jacket were varied between the intake side and the exhaust side, the effect of the coolant would differ between the exhaust side and the air intake side. In the final analysis, temperature differences would develop between the exhaust side and air intake side of the engine unit in the region around the cylinders.
As a result, when the temperature is not uniform around the cylinders in the cylinder block, problems would arise from the resulting deformation of the cylinders during engine operation, such as increased sliding resistance of the pistons that could result in seizing or increases in blow-by gases, and additionally, increased oil consumption.
Another solution of the afore-mentioned objective, according to the present application, is given by providing a heat insulating space between the exhaust passage and the coolant passage surrounding said at least one cylinder. This solution is an independent one but it can also be applied simultaneously with the solution of claim 1.
According to a further preferred embodiment of the present invention, four coolant passages are provided, a first coolant passage around the exhaust passage in the cylinder block, a second coolant passage around the exhaust passage in the cylinder head, a third coolant passage around the respective combustion chamber of the cylinder, and a fourth coolant passage around the cylinder in the cylinder block.
In that case, it may be advantageous that the first and second coolant passages are connected in series and that the third and fourth coolant passages are both connected in parallel to the second coolant passage or that the second and first coolant passages are connected in series and that third and fourth coolant passage are both connected in parallel to the first coolant passage.
Other preferred embodiments of the present invention are laid down in further dependent claims.
In the following, the present invention is explained in greater detail with respect to several embodiments thereof in conjunction with the accompanying drawings, wherein:
Figure 1 is a diagram seen from the starboard side of a marine outboard engine having the cooling structure of this invention,
Figure 2 is a partial sectional diagram for purposes of explanation of the internal structure of the marine outboard engine shown in Figure 1,
Figure 3 is a partial sectional diagram for purposes of explanation of the internal structure of the engine in the top cowling area,
Figure 4 is a side view for purposes of explanation of the upper case area showing the internal structure of the marine outboard engine shown in Figure 2,
Figure 5 is a top sectional view for purposes of explanation of the top cowling area internal structure of the marine outboard engine shown in Figure 2,
Figure 6 is an enlarged, top sectional view for purposes of explanation of the engine shown in Figure 5,
Figure 7 is a rear surface view of the cylinder block when viewed from the direction of arrow A in Figure 6,
Figure 8 is a block diagram showing an embodiment of the cooling circuit for a marine outboard engine cooling structure according to this invention,
Figure 9 is a block diagram showing another embodiment of the cooling circuit for a marine outboard engine cooling structure according to this invention,
Figure 10 is a block diagram showing yet another embodiment of the cooling circuit for a marine outboard engine cooling structure according to this invention, and
Figure 11 is a block diagram showing an example of a conventional cooling circuit for marine outboard engines.
Implementation embodiments of cooling structures for marine outboard engines will be described below for marine outboard engines according to the present invention.
Figure 1 is a starboard side external view of a marine outboard engine equipped with the cooling structure of this invention. The various parts of the marine engine 1 are housed inside a housing composed of a top cowling 2, upper case 3 and lower case 4. The top cowling 2 is composed of the upper cowling 2a, a lower cowling 2b and the air duct cover 2c, while the upper case 3 is covered at its top by the apron 5, and the propeller 6 is attached opposite the lower case 4. The engine is attached to the stern of the boat 8 by an attachment member 7 which allows it to be raised or lowered and turned left and right.
Figures 2 through 5 show the positioning of the various parts of the marine engine 1 inside the housing. Figure 2 is a view of the internals of the engine 1 from the starboard side; Figure 3 shows the region under the top cowling 2; Figure 4 shows the upper casing region; and Figure 5 shows a top view of the inside of the top cowling 2.
The various cylinders of the marine engine 1 are arrayed vertically in the 4-cycle, 4-cylinder, L-type engine 10, to which an exhaust guide 11 is affixed between the bottom cowling 2b and the upper case 3. A cover member 12 is affixed to and covers the upper surface of the exhaust guide and is contained within the top cowling 2.
The air intake and exhaust valves 17, 18 for the various cylinders are attached to the cylinder head 14, and the camshafts 20, 21 that drive the respective valves 17, 18 are positioned with their respective rotational axes disposed in the vertical direction; they are axially supported by the cam caps 22 that cover the head cover and by the bearing area of the cylinder head 14. Cam pulleys 23, 24 are attached to the respective top ends of each camshaft 20, 21.
The crankshaft 26 is axially supported with its rotational axis in the vertical direction in the crank chamber 25, which is the space bounded by the front of the cylinder block 15 and the crankcase 16. A flywheel 27 is attached to the upper end of the crankshaft 26 and a timing pulley 30 is attached under the flywheel 27. The timing belt 28 spans the timing pulley 30 and the cam pulleys 23, 24 in a manner such that the rotation of the crankshaft is transmitted to the camshafts 20, 21 to drive the air intake and exhaust valves 17, 18.
A tensioner 29 engages the timing belt 28 to prevent it from slackening. This tensioner 29 constantly pushes the timing belt 28 toward the inside by means of a gas filled cylinder (not shown).
The surge tank 31 is positioned on the front left side of the engine and it integrally connects with the front face of the crankcase 16. The air intake passage 32 that extends from the surge tank 31 connects, via the throttle, with the air intake ports 34 of the cylinders at vertically disposed intervals on the left side of the cylinder head 14.
The exhaust ports 35 on the right side of the engine connect at vertically disposed intervals with the combustion chambers of the cylinders. An exhaust passage 36, which is integrally formed in the cylinder block at the right of the cylinders 37, collects the exhaust gases expelled from the exhaust ports 35 and expels that gas to the area beneath the engine unit. This exhaust passage 36 that extends vertically inside the cylinder block 15 is connected at its lower end to the exhaust passage 11a of the exhaust guide 11.
The foregoing engine 10 is further equipped with an oil pump located near the lower end of the crankshaft 26 and drawing-in lubricating oil from an oil pan located beneath the exhaust guide 11 and delivering it via an oil passage inside the engine unit. A thermostat valve 41 controls the outflow of the coolant according to the temperature around the cylinders and combustion chambers, and a pressure valve 42 is further mounted inside the coolant passage that allows coolant to escape when the pressure in said coolant passage exceeds a certain level.
There are also a fuel tank (not shown) mounted in the boat and a fuel pump 43 that is located on the air intake side which pumps fuel to the engine 10. Also, attached to the exhaust side of the engine unit is a pilot water pipe to which a part of the coolant from the coolant passage in the engine is diverted and discharged from the water discharge opening 45 of the top cowling as pilot water.
As is shown in Figure 4, the coolant returning from the engine 10 is temporarily pooled in a water reservoir unit 50 that is positioned under the top cowling 2 in the upper case 3. Said water reservoir 50 is isolated from the vertically disposed drive shaft 51 that connects to the top end of the crankshaft 26 of the engine 10, and its bottom end extends to the inside of the lower case 4.
There is a water pump 52 that is driven by the drive shaft 51 and located near the lower case 4 crossed by the drive shaft 51 that passes through the upper case 3. While not shown in the Figures, the water intake of said water pump 51 is connected with a coolant water intake pipe that extends downward inside the lower case and with a coolant transport pipe 53 that extends upward to the engine 10 through the upper case 3.
On the other hand, an oil pan 54 bottom member is tightly attached to the lower surface of the exhaust guide 11 at the water reservoir 50 of the upper case 3 in a manner such that its upper opening is closed off by the bottom surface of the exhaust guide 11. Integrally attached at the center of the bottom member of said oil pan 54 is an exhaust pipe insert member 54a that is pipe-shaped and passes above and below the air space inside the oil pan 54.
Further, a muffler (exhaust gas expansion chamber) 55 is attached to the exhaust gas outlet 55a located in the lower wall of the oil pan 54 and covers the exhaust gas pipe insert member 54a of the bottom surface of the oil pan 54 beneath the water reservoir 50. The space inside said muffler 55 connects by means of an exhaust gas outlet 55a to an exhaust passage formed inside the lower case (not shown).
An exhaust pipe 56 is attached to the exhaust gas insert pipe member 54a of the oil pan 54 which connects it to the exhaust passage 11a of the exhaust guide 11. The bottom end of the exhaust pipe 56 projects downward from the oil pan 54 and opens inside the muffler 55. The exhaust passage 11a formed by the exhaust pipe 56 and the exhaust guide 11 connect the space inside the muffler 55 to the exhaust passage 36 of the engine.
There is a suction pipe 57 that projects downward from the bottom surface of the exhaust guide 11, drawing oil residing in the oil pan 54 into the oil pump 38 for the engine 10.
There is a water pipe 58 integrally formed in the perimeter wall of the oil pan 54 that permits the overflow of water from the water reservoir 50 surrounding the oil pan 54. The water passage composed of the water pipe 58 fitted with a water inlet opening inside the water reservoir 50 connects via the water passage 55b, that is integrally formed on the top outer wall of the muffler 55, to the water drain opening 3a of the upper case 3.
The water drain passage 59 that continues from the water drain opening 3a in the upper case to the lower case divides at its lower end and further divides above the idle expansion chamber that passes through the muffler 55 and opens into the drain hole 61 and the drain hole 62 which connect with the outside of the engine at the idle expansion chamber 60.
While not shown in the figures, the drive shaft 51, which passes through the upper case, is axially supported in the lower case 4 beneath the upper case 3. The lower end of said drive shaft 51 is attached to the rotating shaft of the propeller 6 via a shiftable gear unit.
Also omitted from the figures is a cold water inlet mounted on the lower case 4 that draws in cold water from the water pump, and, in addition, an exhaust passage that is attached to the muffler 55 and allows expelling exhaust gases underwater in conjunction with the rotation of the propeller 6, and this passage is partitioned into a coolant drain passage allowing to expel to the engine-outside the coolant water that drops down from the upper case.
With regard to the exhaust route for the marine outboard engine 1 with the various members located as described above, the exhaust gases, which are expelled downward from the exhaust passage 36 formed in the engine unit, pass through the exhaust passage 11a of the exhaust guide 11 and the exhaust pipe 56 into the muffler 55, and then pass through the idle expansion chamber 60 through the exhaust hole and 61 are expelled into the atmosphere, while in addition, when the engine is running and the propeller 6 is turning, the majority of the exhaust gases pass from the exhaust outlet 55a in the muffler 55 and pass through the exhaust passage in the lower casing 4, and are expelled underwater in conjunction with the rotation of the propeller 6.
With regard to the cooling circuit of the marine outboard engine 1, the coolant water drawn up by the water pump 52 though the water inlet of the lower case 4 cools inside the upper case 3 while flowing through the coolant conduit 53, and is then sent to the engine 10 via the exhaust guide 11. Then the coolant cools the engine 10, with the pressure valve 42 returning excess coolant to the upper case 3 to prevent the coolant pressure from rising above a certain level, and next the coolant passes through the thermostat valve 41 and is returned to the upper case 3, from where it is circulated to cool around the oil pan 54 and the muffler 55, after which it drops down into the lower case and is expelled from the engine from the coolant expulsion area.
With a marine outboard engine as described above, the water pump 52 transmits the coolant through the coolant conduit 53 into the engine 10 fitted with an integrally formed exhaust passage 36 in the cylinder block 15. As is shown in Figure 6, formed in the cylinder head 14 are a coolant passage (water jacket) 73 which runs around the combustion chambers, and a coolant passage (water jacket) 72 which runs around the exhaust passage (exhaust ports 29); and in addition, formed in the cylinder block, are a coolant passage (water jacket) 74 around the cylinders and a coolant passage (water jacket) 71 around the exhaust passage 36, respectively.
In addition to the coolant passages 71, 74, there is also an insulating space 76 formed in the cylinder block 15, which prevents exhaust heat from the exhaust passage 26 from being transmitted to the side of the cylinders 37, while a coolant return passage formed in the cylinder block returns coolant that has passed through the thermostat valve 41 and was expelled through the cooling circuit back to inside of the upper case 3.
As is shown in Figure 7, because of the vertical array of the cylinders, the various coolant passages 71, 72, 73 and 74 are all vertically disposed coolant passages, and coolant water that passes from the coolant supply pipe 53 through the exhaust guide 7, passes through the coolant passages 71, 72, 73 and 74 , and because these coolant passages 71, 72, 73 and 74 all connect to form one cooling circuit, the coolant thereby may be expelled from the top of the engine unit (cylinder block 15).
Figure 8 shows the cooling structure for the present embodiment with its constituent coolant passages 71, 72, 73 and 74 that were described above. In this engine cooling circuit formed by these coolant passages 71, 72, 73 and 74, the water pump 52 pumps water through the coolant transmission pipe 53 and the exhaust guide 11 in a manner such that it first flows from the bottom to the cylinder block 15 side, and enters the coolant passage 71 that lies around the exhaust passage.
Then the coolant is circulated in the vertical direction from the coolant pipe 72 around the exhaust passage in the cylinder head 14 through the cooling circuit, in the cylinder block 15, formed around the exhaust passage, and then it circulates from bottom to top respectively through the coolant passage 73 around the combustion chambers in the cylinder head 14 and through the coolant passage 74 around the cylinders in the cylinder block 15, and then it is expelled from the cooling circuit at the top of the coolant passage 74 around the cylinders in the cylinder block 15.
To wit, the engine cooling structure in the present embodiment connects the coolant passages 71, 72 around the exhaust passages in series with the coolant passages 73, 74 around the cylinders and combustion chambers in forming the cooling circuit for the engine unit.
Furthermore, with regard to the cooling circuit in the present embodiment, there is a thermostat valve 41 to control the amount of coolant expelled according to the temperature around the cylinders and combustion chambers; it is located at the top of the engine unit at the outlet for the coolant in the cooling circuit. In addition, there is also a pressure valve 42 that allows the coolant to escape from said cooling circuit when the water pressure inside the cooling circuit rises above a certain level, and it is located in the downstream end of the coolant passages 71, 72 formed around the exhaust passage.
The coolant that is expelled through the thermostat valve 41 passes through the coolant return passage 77 in the engine unit and the exhaust guide 11 and returns to the inside the upper case 3; the coolant that escapes from the pressure valve 42 passes through the coolant return pipe 78, the exhaust guide 11, and is returned to inside of the upper case 3.
Comparing the above described cooling structure for marine outboard engines of the present embodiment and forming a cooling circuit as described above, with the engine cooling structure of the prior art, that has the cooling circuit as shown in Figure 11, because in this invention, the coolant is sent only through the circuit composed of the coolant passages 73, 74 around the combustion chambers and the cylinders, and the coolant passages 71, 72 around the exhaust passage, no localized variations develop in the coolant temperature around the cylinders 37, thereby maintaining temperature uniformity around the cylinders, further making it possible to inhibit the deformation of the cylinders 37 during engine operations, to prevent piston-seizing or increases in the amount of blowby gases and, at the same time, to hold down oil consumption.
Furthermore, since all the coolant taken into the engine unit 10 has first been circulated through the coolant passages 71, 72 around the exhaust passage without being branched off, there is an enhanced cooling efficiency in the high temperature exhaust passage, and then the coolant that has been adequately heated around the exhaust passage is supplied to the coolant passages 73, 74 around the combustion chambers and cylinders, thereby improving the combustion stability of the engine during low speed operations.
Further, as shown for the prior art engine cooling structure in Figure 11, the pressure valve 42 for returning the coolant is positioned near the cooling circuit inlet, but in the present embodiment, that pressure valve 42 is located near the downstream end of the coolant passages 71, 72 around the exhaust passages, so even when the thermostat valve 41 is closed, coolant is circulated through the coolant passages 71, 72 around the exhaust passages, thereby assuring cooling the high temperature exhaust passages 35, 36.
The foregoing explanation has described but one embodiment of the engine cooling structure for marine outboard engines according to this invention, but the present invention is not confined to this embodiment. Figures 9 and 10, for example, show other possible cooling circuits, and it goes without saying that the design can be modified appropriately.
As described above, the present invention's cooling structure for marine outboard engines provides, through the same circuit, coolant at a uniform temperature that has flowed around the exhaust passage, to the bottom of the coolant passages around the cylinders and combustion chambers, thereby minimizing the temperature difference between the top and bottom cylinders and making it possible to maintain a uniform temperature distribution around the cylinders, to maximize the cooling provided to around the exhaust passage, and to allow the efficient absorption of heat around the exhaust passage.
As a result, it is possible to decrease the amount of cylinder deformation during engine operations, to prevent seizing, to improve or reduce the amount of blowby gases, and to inhibit increases in oil consumption, and, at the same time, to improve combustion stability during low speed operations.

Claims

Marine outboard engine (1) having an engine unit (10) being vertically arranged with respect to a driving direction of a boat (8), said engine unit (10) comprising a cylinder block (15) having at least one cylinder (37), and a cylinder head (14) connected to said cylinder block (15) and defining at least one combustion chamber, said cylinder block (15) further comprising an exhaust passage (36) for collecting exhaust gases from the combustion chamber and for expelling said exhaust gases downward beneath the engine unit (10), said marine outboard engine (1) further having a cooling structure for cooling said at least one cylinder (37) and said at least one combustion chamber by means of a fresh coolant water, whereas said coolant water first is directed to the exhaust passage (36) for firstly cooling said exhaust passage (36) and then is directed to said at least one cylinder (37) and respective combustion chamber, characterized in that a pressure valve (42) being provided close to a downstream end of a coolant passage (71,72) around the exhaust passage (36).
Marine outboard engine (1) as claimed in claim 1, characterized in that said coolant water is fed into the engine unit (10) from its lower area and first is passing through a coolant passage (71,72) around the exhaust passage (36) and then is passing through a further coolant passage (73,74) around the at least one cylinder (37) and the respective combustion chamber.
Marine outboard engine as claimed in claims 1 or 2, characterized in that the coolant in the coolant passage (73,74) and the cylinder (37) and respective combustion chamber is fed to flow from the bottom to the top of said engine unit (10) and is expelled out of the coolant passage (73,74) at the top of the engine unit (10).
Marine outboard engine as claimed in at least one of claims 1 to 3, characterized in that a thermostatic valve (41) being provided at the top of the engine unit (10) close to the end of said coolant passage (73,74) for controlling the temperature dependent amount of coolant expelled.
Marine outboard engine, as claimed in at least one of claims 1 to 4, characterised by a heat insulating space (76) between the exhaust passage (36) and the coolant passage (74) surrounding said at least one cylinder (37).
Marine outboard engine as claimed in at least one of claims 2 to 5, characterized in that four coolant passages (71,74) are provided, a first coolant passage (71) around the exhaust passage (36) in the cylinder block (15), a second coolant passage (72) around the exhaust passage (35) in the cylinder head (14), a third coolant passage (73) around the respective combustion chamber of the cylinder (37), and a fourth coolant passage (74) around the cylinder (37) in the cylinder block (14).
Marine outboard engine as claimed in claim 6, characterized in that the first and second coolant passages (71,72) are connected in series and that the third and fourth coolant passages (73,74) are both connected in parallel to the second coolant passage (72) or that the second and first coolant passages (72,71) are connected in series and that third and fourth coolant passage (73,74) are both connected in parallel to the first coolant passage (71).
Marine outboard engine as claimed in claim 7, characterized in that a waterpump (52) is connected to the first or to the second coolant passage (71,72).
Marine outboard engine as claimed in claims 7 or 8, characterized in that the third and fourth coolant passages (73,74) comprising passages interconnecting the respective combustion chamber coolant passage with the respective cylinder (37) coolant passage.
Marine outboard engine as claimed in one of claims 1 to 9, characterized in that the coolant expelled at the top of the engine unit (10) is returned via a first coolant return pipe (27).
Marine outboard engine as claimed in one of claims 1 to 10, characterized in that the coolant expelled at the pressure valve (42) is returned via a second coolant return pipe (78).
Marine outboard engine as claimed in one of claims 1 to 11, characterized in that said exhaust passage (36) being integrally formed in the cylinder block (15).