Exhaust system for outboard motor

An outboard motor includes a housing unit mounted on an associated watercraft. An engine is disposed above the housing unit. The engine defines a first exhaust passage communicating with a combustion chamber of the engine. The housing unit defines a second exhaust passage communicating with the first exhaust passage. The second exhaust passage communicates with outside through at least an underwater exhaust discharge port formed at a portion of the housing unit. An air intake device communicates with either the first exhaust passage or the second exhaust passage. The air intake device includes a one-way valve that allows air to enter the first or second exhaust passage and inhibits exhaust gases from moving beyond the one-way valve.

PRIORITY INFORMATION

This application is based on and claims priority to Japanese Patent Application No. 2001-150288, filed May 21, 2001, the entire contents of which is hereby expressly incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to an exhaust system for an outboard motor, and more particularly to an improved exhaust system for an outboard motor that has an exhaust discharge port at a portion of a housing unit of the outboard motor.

2. Description of Related Art

An outboard motor typically includes a housing unit that can be mounted on an associated watercraft and an engine disposed above the housing unit. The outboard motor also includes an exhaust system to discharge exhaust gases from one or more combustion chambers of the engine to a location outside of the motor. Typically, an underwater exhaust discharge port is formed at a lowermost section of the housing unit so that the exhaust gases are discharged to a body of water surrounding the outboard motor when the outboard motor is mounted to an associated watercraft. An above-water exhaust discharge port also is formed at a higher section of the housing unit to discharge exhaust gases under idle condition of the engine.

The outboard motor normally employs a propeller as a propulsion device powered by the engine. A crankshaft of the engine drives a driveshaft and a propulsion shaft coupled with the driveshaft. The propulsion shaft then drives the propeller. A transmission also is employed to change a rotational direction of the propeller among forward, neutral and reverse.

When an operator of the outboard motor shifts the transmission, for example, to the reverse direction from the forward direction, the inertia of water flow by the propeller can cause the impeller to continue to rotate in the forward direction even after the transmission has been shifted into reverse. As such, the impeller can rotate the crankshaft inversely through the driveshaft and the propeller shaft. An engine control device such as, for example, an ECU (electronic control unit) recognizes the reverse rotation of the crankshaft and controls the engine to stop. However, for a moment before the engine stops, the exhaust system can generate negative pressure. For example, if the crankshaft is rotated in the reverse direction, driving a piston downwardly while an exhaust valve is open, air will be drawn into the engine through the exhaust system. Because of this negative pressure, the underwater and above-water ports can draw water or air containing water, respectively, into the exhaust system. The water can reach the engine and can cause rust or corrosion of the engine. Particularly, if the water contains salt, the corrosion can ruin the engine faster.

SUMMARY OF THE INVENTION

A need therefore exists for an improved exhaust system for an outboard motor that can inhibit negative pressure from being generated in the exhaust system at least when the crankshaft is driven in a reverse direction.

In accordance with one aspect of the present invention, an outboard motor comprises a housing unit adapted to be mounted on an associated watercraft. An internal combustion engine is disposed above the housing unit. The engine defines a first exhaust passage communicating with a combustion chamber of the engine. The housing unit defines a second exhaust passage communicating with the first exhaust passage. The second exhaust passage communicates with outside through an exhaust discharge port formed at a portion of the housing unit. An air intake device communicates with either the first exhaust passage or the second exhaust passage. The air intake device includes a one-way valve that allows air to enter the first or second exhaust passage and inhibits exhaust gases from moving beyond the one-way valve.

In accordance with another aspect of the present invention, an outboard motor comprises a housing unit adapted to be mounted on an associated watercraft. An internal combustion engine is disposed above the housing unit. The engine includes an engine body defining a combustion chamber. An air induction system is arranged to introduce air to the combustion chamber. The air induction system includes a plenum chamber. An exhaust system is arranged to discharge exhaust gases from the combustion chamber to outside through an exhaust discharge port formed at a portion of the housing unit. An air intake device communicates with the exhaust system. The air intake device includes a one-way valve that allows air to enter the exhaust system and inhibits exhaust gases from moving beyond the one-way valve. The air intake device draws the air from the plenum chamber.

In accordance with a further aspect of the present invention, an outboard motor comprises a housing unit adapted to be mounted on an associated watercraft. An internal combustion engine is disposed above the housing unit. The engine defines a combustion chamber therein. An exhaust system is arranged to discharge exhaust gases from the combustion chamber to outside through an exhaust discharge port formed at a portion of the housing unit. Means are provided for delivering air to the exhaust system when the exhaust system generates negative pressure.

With particular reference toFIG. 1, an overall construction of an outboard motor30configured in accordance with certain features, aspects and advantages of the present invention is described below.

In the illustrated arrangement, the outboard motor30comprises a drive unit34and a bracket assembly36. The bracket assembly36supports the drive unit34on a transom38of an associated watercraft40and places a marine propulsion device in a submerged position with the watercraft40resting on the surface of a body of water. The bracket assembly36preferably comprises a swivel bracket42, a clamping bracket44, a steering shaft46and a pivot pin48.

The steering shaft46typically extends through the swivel bracket42and is affixed to the drive unit34with upper and lower mount assemblies. The steering shaft46is pivotally journaled for steering movement about a generally vertically extending steering axis defined within the swivel bracket42. A steering handle stay50extends forwardly atop the steering shaft46so that the operator can operate the steering shaft46.

The clamping bracket44comprises a pair of bracket arms that are spaced apart from each other and that are affixed to the watercraft transom38. The pivot pin48completes a hinge coupling between the swivel bracket42and the clamping bracket44. The pivot pin48extends through the bracket arms so that the clamping bracket44supports the swivel bracket42for pivotal movement about a generally horizontally extending tilt axis defined by the pivot pin48. The drive unit34thus can be tilted or trimmed about the tilt axis.

As used through this description, the terms “forward,” “forwardly” and “front” mean at or to the side where the bracket assembly36is located, and the terms “rear,” “reverse,” “backwardly” and “rearwardly” mean at or to the opposite side of the front side, unless indicated otherwise or otherwise readily apparent from the context use.

A hydraulic tilt and trim adjustment system not shown preferably is provided between the swivel bracket42and the clamping bracket44to tilt (raise or lower) the swivel bracket42and the drive unit34relative to the clamping bracket44. Otherwise, the outboard motor30can have a manually operated system for tilting the drive unit34. Typically, the term “tilt movement,” when used in a broad sense, comprises both a tilt movement and a trim adjustment movement. The outboard motor30can be in a propelling position of the watercraft40when the drive unit34is in a relatively lower tilt range including the trim adjustment range with the propulsion device submerged.

The illustrated drive unit34comprises a power head52and a housing unit54which includes a driveshaft housing56and a lower unit58. The power head52is disposed atop the drive unit34and houses an internal combustion engine59that is positioned within a protective cowling60.

Preferably, the protective cowling60defines a generally closed cavity61in which the engine59is disposed. The protective cowling60preferably comprises a top cowling member62and a bottom cowling member64. The top cowling member62preferably is detachably affixed to the bottom cowling member64by a coupling mechanism so that a user, operator, mechanic or repair person can access the engine59for maintenance or for other purposes.

The top cowling member62preferably defines at least one air intake opening68and at least one air duct disposed on its rear and top portion. Ambient air is drawn into the closed cavity61through the opening68and then through the duct. Typically, the top cowling member60tapers in girth toward its top surface, which is in the general proximity of the air intake opening68.

The bottom cowling member64preferably has an opening at its bottom portion through which an upper portion of an exhaust guide member72extends. The exhaust guide member72preferably is made of an aluminum based alloy and is affixed atop the driveshaft housing56. The bottom cowling member64and the exhaust guide member72together generally form a tray. The engine59is placed onto this tray and is affixed to the exhaust guide member72. The exhaust guide member72also defines an exhaust passage74through which burnt charges (e.g., exhaust gases) discharged from the engine59moves to a next stage.

The engine59in the illustrated embodiment preferably operates on a four-cycle combustion principle. With continued reference to FIG.1and with additional reference toFIGS. 2-5, the presently preferred engine59has a cylinder block78configured as a V shape. The cylinder block78thus defines two cylinder banks which extend side by side with each other. In the illustrated arrangement, each cylinder bank has three cylinder bores80such that the cylinder block78has six cylinder bores80in total. The cylinder bores80of each bank extend generally horizontally and are generally vertically spaced from one another.

A piston84reciprocates within each cylinder bore80. Because the cylinder block78is split into the two cylinder banks, each cylinder bank extends outward at an angle to an independent first end in the illustrated arrangement. Cylinder head members86are affixed to the respective cylinder banks to close those ends of the cylinder bores80. The cylinder head members86, together with the associated pistons84and cylinder bores80, preferably define six combustion chambers94. Cylinder head cover members96are affixed to the cylinder head members oppositely to the cylinder block78.

A crankcase member100closes the other ends of the cylinder bores80and, together with the cylinder block78, defines a crankcase chamber. A crankshaft104extends generally vertically through the crankcase chamber and can be journaled for rotation by several bearing blocks. Connecting rods106couple the crankshaft104with the respective pistons84in any suitable manner. Thus, the reciprocal movement of the pistons84rotates the crankshaft104. A crankcase cover member108is affixed to the crankcase member100oppositely to the cylinder block78.

In the illustrated arrangement, generally, the cylinder block78, the cylinder head members86, the cylinder head cover members96, the crankcase member100and the crankcase cover member108together define an engine body110. Preferably, at least these major engine portions78,86,96,100,108are made of aluminum alloy.

The engine59also comprises an air induction system114. The air induction system114draws air from within the cavity61to the combustion chambers94. The air induction system114preferably comprises six intake passages116and a pair of plenum chambers118. In the illustrated arrangement, each cylinder bank is allotted with three intake passages116and one plenum chamber118.

The most-downstream portions of the intake passages116are defined within the cylinder head members86as inner intake passages120. The inner intake passages120communicate with the combustion chambers94through intake ports, which are formed at inner surfaces of the cylinder head members86. Typically, each of the combustion chambers94has one or more intake ports. Intake valves124are slideably disposed at each cylinder head member86to move between an open position and a closed position. When each intake valve124is in the open position, the inner intake passage120that is associated with the intake port communicates with the associated combustion chamber94.

Outer portions of the intake passages116, which are disposed outside of the cylinder head members86, preferably are defined with intake manifolds128, throttle bodies130and intake runners132. Those members128,130,132extend forwardly along respective side surfaces of the engine body110.

Each throttle body130preferably includes a throttle valve. The operator can control the opening degree of the throttle valves through a control linkage. The throttle valves regulate amounts of air that flow through the intake passages116to the combustion chambers94in accordance with the opening degree. Normally, the greater the opening degree, the higher the rate of airflow and the greater the power output from the engine59.

The respective plenum chambers118preferably are defined with plenum chamber units134which are disposed side by side in front of the crankcase cover member108. Both the plenum chamber units134are coupled with each other with connecting pipes136. Each plenum chamber unit134defines an air inlet (not shown) through which the air in the cavity61is drawn into the plenum chamber118. The plenum chambers118coordinate air delivered to each intake passage116and also act as silencers to reduce intake noise. In other words, the chambers118act to reduce the pulsation energy within the intake system and to smooth the airflow being introduced to the engine.

In the illustrated embodiment, the throttle valves are substantially closed to bring the engine59to idle speed and to maintain this speed. Preferably, the valves are not fully closed such that the likelihood of throttle valve sticking can be reduced. As used throughout the description, the term “idle speed” generally means a low engine speed that is achieved when the throttle valves are closed but also includes a state in which the valves are slightly opened to allow a small level of airflow through the intake passages116. Also, the outboard motor30is often used for trolling, which is a very low speed, generally forward movement of the watercraft. Thus, when trolling, a shift mechanism, which will be described later, is in a forward position and the engine59operates in the idle speed.

With particular reference toFIGS. 4 and 5, the illustrated air induction system114preferably includes a secondary air delivery unit or idle speed control (ISC) mechanism140that can deliver idle air to the combustion chambers94when the throttle valves are substantially closed. In this arrangement, the intake passages116and the plenum chambers118together define a primary air delivery unit.

The secondary unit or ISC mechanism140preferably comprises a secondary plenum chamber member144, an upstream conduit146, an ISC device148and a pair of downstream conduits150.

Preferably, the secondary plenum chamber member144is generally disposed atop a recessed portion defined by the two banks and affixed to the cylinder head cover member96on the bank located on the port side and an outer exhaust cover member151.FIG. 5schematically illustrates a location of the secondary plenum chamber member144rather than an actual location thereof. The secondary plenum chamber member144defines a secondary plenum chamber152that acts as an air coordinator and a silencer similarly to the primary plenum chambers118. An air inlet153is formed to draw the air in the cavity61to the secondary plenum chamber152.

The upstream conduit146defines an upstream passage connecting the secondary plenum chamber152with the ISC device148. The ISC device148contains an ISC valve that is controlled by an ECU (not shown) to open when the throttle valves in the primary unit are closed or almost closed. The downstream conduits150define downstream passages connecting the ISC device148with the respective intake manifolds128which locate downstream of the throttle valves. The upstream and downstream conduits146,150together define a bypass conduit assembly154because air under idle condition can bypass the throttle valves to the combustion chambers94through the bypass conduit assembly154. The air drawn into the secondary plenum chamber152moves to the intake manifolds128through the bypass conduit assembly154and the ISC device148as indicated by the arrows156ofFIGS. 4 and 5.

The secondary air delivery unit140is disclosed in, for example, a co-pending U.S. application filed Jul. 16, 2001, titled AIR INDUCTION SYSTEM FOR ENGINE, which Ser. No. is 09/906,570, the entire contents of which is hereby expressly incorporated by reference.

The engine59comprises an exhaust system160that routes burnt charges, i.e., exhaust gases, to a location outside of the outboard motor30. Each cylinder head member86defines a set of inner exhaust passages162(FIG. 3) that communicate with the combustion chambers94through one or more exhaust ports163, which may be defined at the inner surfaces of the respective cylinder head members86. Exhaust valves164are slideably disposed at each cylinder head member86to move between an open position and a closed position. When each exhaust valve164is in the open position, the inner exhaust passage162that is associated with the exhaust port163communicates with the associated combustion chamber94.

Exhaust manifold passages166preferably are defined generally vertically by the respective cylinder head members86with inner exhaust cover members167. In other words, exhaust manifolds166min this arrangement are unitarily formed with the cylinder head members86.FIGS. 1 and 2schematically illustrate the exhaust manifold passages166in phantom line and part thereof is out of the cylinder head members86. The exhaust manifold passages166communicate with the combustion chambers94through the inner exhaust passages162and the exhaust ports163to collect exhaust gases therefrom. Two of the exhaust manifold passages166define one exhaust manifold passage unit168. The exhaust manifold passage unit168is unified together within the cylinder block78to form a single exhaust passage section170. The exhaust passage section170in turn is coupled with the exhaust passage74of the exhaust guide member72. Thus, when the exhaust ports163are opened, the combustion chambers94communicate with the exhaust passage74through the exhaust manifold passages166, i.e., exhaust manifold passage unit168, and the exhaust passage section170.

A valve cam mechanism preferably is provided for actuating the intake and exhaust valves124,164in each cylinder bank. Preferably, the valve cam mechanism includes two camshafts174per cylinder bank. The camshafts174extend generally vertically and are journaled for rotation relative to the cylinder head members86. The camshafts174have cam lobes176to push valve lifters that are affixed to the respective ends of the intake and exhaust valves124,164in any suitable manner. The cam lobes176repeatedly push the valve lifters in a timed manner, which is in proportion to the engine speed. The movement of the lifters generally is timed by rotation of the camshafts174to appropriately actuate the intake and exhaust valves124,164.

A camshaft drive mechanism (not shown) preferably is provided for driving the valve cam mechanism. Thus, the intake and exhaust camshafts174comprise intake and exhaust driven sprockets positioned atop the intake and exhaust camshafts174, respectively, while the crankshaft104has a drive sprocket positioned atop thereof. A timing chain or belt is wound around the driven sprockets and the drive sprocket. The crankshaft104thus drives the respective camshafts174through the timing chain in the timed relationship. Because the camshafts174must rotate at half of the speed of the rotation of the crankshaft104in a four-cycle engine, a diameter of the driven sprockets is twice as large as a diameter of the drive sprocket.

The engine59preferably has indirect, port or intake passage fuel injection system. The fuel injection system preferably comprises six fuel injectors180with one fuel injector allotted for each one of the respective combustion chambers94. The fuel injectors180preferably are mounted on the throttle bodies130and a pair of fuel rails connects the respective fuel injectors180with each other on each cylinder bank. The fuel rails also define portions of the fuel conduits to deliver fuel to the injectors180. In this arrangement, the fuel injectors and the fuel rails are positioned in spaces182formed between the engine body110and the throttle bodies130.

Each fuel injector180preferably has an injection nozzle directed downstream within the associated intake passage116, which is downstream of the throttle valve and within the intake manifold128. The fuel injectors180spray fuel into the intake passages116under control of the ECU. The ECU controls both the initiation timing and the duration of the fuel injection cycle of the fuel injectors180so that the nozzles spray a proper amount of fuel each combustion cycle.

Typically, a fuel supply tank disposed on a hull of the associated watercraft40contains the fuel. The fuel is delivered to the fuel rails through the fuel conduits and at least one fuel pump, which is arranged along the conduits. The fuel pump pressurizes the fuel to the fuel rails and finally to the fuel injectors180. A vapor separator184preferably is disposed in a space186formed between the engine body110and the intake runners132on the port side. The vapor separator184separates vapor from the fuel therein and sends the vapor to the plenum chambers118through a vapor delivery conduit188. The vapor thus can be delivered to the combustion chambers94through the plenum chambers118together with the air for combustion. A direct fuel injection system that sprays fuel directly into the combustion chambers can replace the indirect fuel injection system described above. Moreover, other charge forming devices, such as carburetors, can be used instead of the fuel injection systems.

The engine59further comprises an ignition or firing system (not shown). Each combustion chamber94is provided with a spark plug (not shown) which preferably is disposed between the intake and exhaust valves124,164. Each spark plug has electrodes that are exposed into the associated combustion chamber94and that are spaced apart from each other with a small gap. The spark plugs generate a spark between the electrodes to ignite an air/fuel charge in the combustion chamber94at selected ignition timing under control of the ECU.

In the illustrated engine59, the pistons84reciprocate between top dead center and bottom dead center. When the crankshaft104makes two rotations, the pistons84generally move from the top dead center position to the bottom dead center position (the intake stroke), from the bottom dead center position to the top dead center position (the compression stroke), from the top dead center position to the bottom dead center position (the power stroke) and from the bottom dead center position to the top dead center position (the exhaust stroke). During the four strokes of the pistons84, the camshafts174make one rotation and actuate the intake and exhaust valves124,164to open the intake ports and the exhaust ports163during the intake stroke and the exhaust stroke, respectively.

Generally, during the intake stroke, air is drawn into the combustion chambers94through the air intake passages116and fuel is injected into the intake passages116by the fuel injectors180. The air and the fuel thus are mixed to form the air/fuel charge in the combustion chambers94. Slightly before or during the power stroke, the respective spark plugs ignite the compressed air/fuel charge in the respective combustion chambers94. The air/fuel charge thus rapidly bums during the power stroke to move the pistons84. The burnt charge, i.e., exhaust gases, then are discharged from the combustion chambers94during the exhaust stroke.

The engine59may comprise a cooling system, a lubrication system and other systems, mechanisms or devices in addition to the systems described above. For example, water jackets192of the cooling system are formed within the cylinder head members86and the inner and outer exhaust cover members167,151in proximity to the exhaust manifold passages166.

A flywheel assembly196preferably is positioned atop the crankshaft104and is mounted for rotation with the crankshaft104. The flywheel assembly198comprises a flywheel magneto or AC generator that supplies electric power to various electrical components, such as the fuel injection system, the ignition system and the ECU. A protector198covers at least the engine body110, the flywheel assembly196and the camshaft drive mechanism.

With particular reference toFIG. 1, the driveshaft housing56is positioned below the exhaust guide member72. A driveshaft202preferably extends generally vertically through an opening formed at forward portions of the engine body110, the exhaust guide member72and the driveshaft housing56to be coupled with the crankshaft104at a bottom portion of the engine body110. The driveshaft202is journaled for rotation in the driveshaft housing56and is driven by the crankshaft104.

A top portion of the driveshaft housing56preferably defines a lubricant reservoir206together with the lower surface of the exhaust guide member72for the lubrication system. The illustrated reservoir206is unitarily formed with internal wall portions208of the driveshaft housing56.

The illustrated driveshaft housing56also defines internal exhaust sections with the internal wall portions208and an exhaust conduit210. The exhaust conduit210depends from the exhaust guide member72to form an exhaust passage communicating with the exhaust passage74of the exhaust guide member72. The illustrated exhaust conduit210extends generally vertically through the lubricant reservoir206. Below the lubricant reservoir206, the internal wall portions208forms a first expansion chamber212communicating with the exhaust passage of the exhaust conduit210. The exhaust passage74of the exhaust guide member72, the exhaust passage of the exhaust conduit210and the expansion chamber12together define a first section214of a primary exhaust pathway in this arrangement.

In the illustrated arrangement, the exhaust guide member72and the internal wall portions208of the driveshaft housing56also define an idle exhaust pathway216. The idle exhaust pathway216is branched off from the first section214of the primary exhaust pathway at an idle exhaust inlet218formed within the exhaust guide member72and communicates with the atmosphere through an above-water “aerial” or exhaust discharge port220formed at an upper rear portion of the driveshaft housing56. One or more expansion chambers can be formed between the idle exhaust inlet218and the aerial discharge port220. The aerial exhaust discharge port220is, because of its own location, not submerged regardless of any positions of the drive unit34.

With continued reference toFIG. 1, the lower unit58depends from the driveshaft housing56and journals a propulsion shaft224, which is driven by the driveshaft202. The propulsion shaft224extends generally horizontally through the lower unit58. A propulsion device is attached to the propulsion shaft224to be driven by the propulsion shaft224. In the illustrated arrangement, the propulsion device includes a propeller226affixed to an outer end of the propulsion shaft224. The propulsion device, however, can take the form of a dual counter-rotating system, a hydrodynamic jet, or any of a number of other suitable propulsion devices.

A transmission230preferably is provided between the driveshaft202and the propulsion shaft224. The transmission230couples together the two shafts202,224which lie generally normal to each other (i.e., at a 90° shaft angle) with bevel gears. A switchover clutch232allows the transmission230to change the rotational direction of the propeller226among forward, neutral or reverse. A shifter shaft234extends upwardly from the switchover clutch232through the steering shaft46. A shifter cable236is coupled with the shifter shaft234via a slider238and extends forwardly. The operator can operate the switchover clutch232through the shifter cable236and the shifter shaft234to shifth the transmission230among the forward, neutral and reverse positions.

The lower unit58and the propeller226together define a second section240of the primary exhaust pathway. A second expansion chamber242occupies major volume of the section240and is formed above a space where the propulsion shaft224extends. The second expansion chamber242communicates with the first expansion chamber212and with an underwater exhaust discharge port244defined at the hub246of the propeller226as part of the second section240. The primary exhaust pathway comprising the first and second sections214,240thus is submerged when the outboard motor30is in a propelling position of the watercraft40.

At engine speeds above idle, the exhaust gases coming from the engine59descend the exhaust passage74of the exhaust guide member72, the exhaust passage of the exhaust conduit210, the first and second expansion chambers212,242and then go out to the body of water through the discharge port244of the propeller226. Because the gases expand and contract twice within the first and second expansion chambers212,242, exhaust noise is sufficiently reduced.

At idle speed, the exhaust gases go to the idle exhaust pathway216through the idle exhaust inlet218and are discharged through the aerial discharge port220. The difference in the locations of the discharges accounts for the differences in pressure at locations above the waterline and below the waterline. Because the opening above the waterline, i.e., the aerial discharge port220, is smaller, pressure develops within the lower unit58. When the pressure exceeds the higher pressure found below the waterline, the exhaust gases exit through the underwater discharge port244. If the pressure remains below the pressure found below the waterline, the exhaust gases exit through the idle exhaust pathway216above the waterline.

With reference toFIGS. 1-5, an air intake device250is described below.100641When the operator shifts the transmission230, for example, to the reverse direction from the forward direction with switchover clutch232, the inertia of water flow by the propeller226can rotate the crankshaft104inversely through the driveshaft202and the propeller shaft224. The ECU recognizes the inverse rotation of the crankshaft104and ceases the engine operation by stopping fuel injection or by stopping the ignition. However, as the crankshaft rotates in the reverse direction, due to the downward movement of a piston during what would otherwise be an “exhaust stake,” the exhaust system160can generate negative pressure. Because of this negative pressure, the underwater and aerial discharge ports244,220can draw water or air containing water, respectively, into the exhaust system160. The air intake device250is provided to overcome the negative pressure within the exhaust system160and preferably is formed and arranged to guide air from the cavity61of the protective cowling60into the exhaust system160.

The illustrated air intake device250employs the secondary plenum chamber member144as an air inlet. Alternatively, one of the primary plenum chamber members134can replace the secondary plenum chamber member144. Otherwise, the top cowling member62can define the air inlet at any portion thereof to directly intake ambient air out of the protective cowling60.

A one-way valve unit252preferably is disposed between the cylinder banks and is affixed to the outer exhaust cover member151. A single upstream air conduit254defines an air passage255(FIG. 2) connecting the secondary plenum chamber152and an inner cavity of the one-way valve unit252. A pair of downstream air conduits256, in turn, define air passages257connecting the one-way valve unit252and inner air passages258which are formed within the outer and inner exhaust cover members151,167and communicate with the exhaust manifold passages166. Joints260preferably are used for coupling the downstream air conduits256with the outer exhaust cover members151. The upstream air conduit254and the downstream air conduits256preferably are made of an elastic or flexible material such as rubber.

The one-way valve unit252preferably contains a reed valve264(FIG.2). The reed valve264is positioned within the unit252to allow air from the secondary plenum chamber152to enter the exhaust manifold passages166and to inhibit the exhaust gases within the exhaust manifold passages166from going out.

Part of the air in the secondary plenum chamber152thus moves to the exhaust manifold passages166, i.e., the exhaust system160, as indicated by the arrows268ofFIGS. 1,2and4. Meanwhile, the exhaust gases in the exhaust manifold passages166, i.e., the exhaust system160, are blocked from moving beyond the one-way valve unit252. Accordingly, the negative pressure, even if generated in the exhaust system160, is overcome by the entering air. Because no exhaust gases go out to the closed cavity61, the air in this cavity61can be kept clean.

The air intake device250draws air from the plenum chamber152(the plenum chamber118in an alternative arrangement). Moisture, oily air, or dust within the closed cavity61, if any, is prevented from directly entering the exhaust system160.

Because of being connected to the exhaust manifold passages166, the downstream air passages257are located relatively adjacent to the respective exhaust ports163in comparison with other locations such as connected to the exhaust passage section170of the cylinder block78or the exhaust passage74of the exhaust guide member72. That is, the air intake device250is positioned in the close proximity to the respective combustion chambers94which are source of the negative pressure. Response speed thus is faster than those in other arrangements.

With reference toFIGS. 6 and 7, a modified arrangement of the air intake device250will be described. The same members, components and systems as those described above will be assigned with the same reference numerals and will not be described repeatedly.

In this modified arrangement, the upstream air passage255is not connected to the exhaust manifold passages166. Instead, the upstream air conduit254defining the passage255extends downwardly through the bottom cowling member64and forwardly toward the idle exhaust inlet218of the exhaust guide member72. A bracket portion278of the bottom cowling member64defines an aperture through which the upstream air conduit254passes. A grommet280is fitted into the aperture to support the conduit254. The one-way valve unit252in this arrangement is positioned at the idle exhaust inlet218and is affixed to the exhaust guide member72. The illustrated upstream air conduit254is coupled with the one-way valve252.

In this arrangement, no downstream conduits256are necessary and only one conduit254can complete a passage connecting the secondary plenum chamber152and the exhaust system160. Accordingly, the construction is quite simple. In addition, the one-way valve unit252is not exposed to the first section214of the primary exhaust pathway where majority of the heated exhaust gases flows. The one-way valve unit252thus is protected from the heat of the primary exhaust pathway.

With reference toFIGS. 8 and 9, another modified arrangement of the air intake device250will be described. Again, the same members, components and systems as those described above will be assigned with the same reference numerals and will not be described repeatedly.

This arrangement is modified from both the first arrangement shown inFIGS. 1-5and the second arrangement shown inFIGS. 6 and 7. That is, a single downstream air conduit256is coupled with the idle exhaust inlet218with the one-way valve unit252being positioned between the banks and on the outer exhaust cover member151.

This arrangement needs only one downstream air conduit256. Additionally, the exhaust guide member72is not necessitated to change greatly because the conduit256, not the valve unit252, is coupled herewith. The construction thus is simple and is not affected by the heat of the exhaust gases passing through the primary exhaust pathway.

Of course, the foregoing description is that of a preferred construction having certain features, aspects and advantages in accordance with the present invention. For instance, the downstream conduit of the air intake device can be connected to either the exhaust passage section of the cylinder block or the exhaust passage of the exhaust guide member in some arrangements. Accordingly, various changes and modifications may be made to the above-described arrangements without departing from the spirit and scope of the invention, as defined by the appended claims.