Abstract:
A two-cycle, multi-cylinder engine maintains roughly equal temperatures between cylinders, despite differing exhaust efficiencies between the cylinders. Several ways of reducing the temperature of one of the cylinders in comparison to the other are disclosed. One embodiment involves supplying more coolant to the cooling passages around the hotter running cylinder. Other ways involve reducing the compression ratio or retarding ignition timing in the hotter running cylinder in comparison to the other cylinder.

Description:
RELATED CASE 
     This present application is a continuation of U.S. application Ser. No. 08/717,763, filed Sep. 23, 1996, now U.S. Pat. No. 6,109,220. issued on Aug. 29, 2000. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a method and device for reducing the temperature in a hotter running cylinder of a multi-cylinder marine engine. 
     2. Description of Related Art 
     Personal watercrafts have become popular in recent years. This type of watercraft is quite sporting in nature and is designed to carry a rider and possibly one or two passengers. A relatively small hull of the personal watercraft commonly defines a rider&#39;s area above an engine compartment. 
     An internal combustion engine frequently powers a jet propulsion unit which propels the watercraft. The engine lies within the engine compartment in front of a tunnel formed on the underside of the watercraft hull. The jet propulsion unit is located within the tunnel and is driven by a drive shaft. The drive shaft commonly extends between the engine and the jet propulsion device, through a wall of the hull that forms a front gullet portion of the tunnel. 
     Personal watercrafts often employ an in-line, multi-cylinder, crankcase compression, two-cycle engine, usually including two or three cylinders. The engine conventionally lies within the engine compartment with the in-line cylinders aligned along a longitudinal axis of watercraft hull (in the bow-stem direction). 
     An exhaust manifold typically couples the exhaust ports of the engine cylinders to an exhaust system. The exhaust manifold usually includes several of runners. Each runner communicates with an exhaust port of one of the engine cylinders. The runners merge together at a downstream point and communicate with an exhaust pipe of the exhaust system at a common end. The length of the runners commonly differ in prior exhaust manifolds. 
     The exhaust system discharges exhaust byproducts from the watercraft. The exhaust system commonly includes a water jacket which cools at least a portion of the exhaust system. At least a portion of the cooling water usually is introduced into the exhaust stream after an expansion chamber of the exhaust system to further silence exhaust noise and for discharge from the watercraft. 
     One of the cylinders often runs hotter than the other cylinders in prior marine engines. For instance, in a two cylinder engine, the first cylinder usually runs hotter than the second cylinder. This occurs in part because of exhaust gas flow differences between the cylinders. The exhaust flow differences between the cylinders is largely attributable to the differences in length and shape of the exhaust manifold runners. 
     The first cylinder, which exhausts into the exhaust manifold upstream of the second cylinder, usually has better scavenging. As a result, more complete combustion occurs first cylinder than in the second cylinder. The first cylinder thus produces more power and heat. A temperature difference consequently results between the cylinders. 
     Elevated operating temperature of one cylinder commonly causes the engine to knock. The high temperature also tends to deteriorate rubber products which are located near the engine or exhaust manifold. For example, vibration-attenuating rubber engine mounts, which are usually located near the exhaust manifold, may deteriorate due to high operating temperature of the engine and the exhaust manifold. 
     Pressure fluctuations, which normally occur in the exhaust system, may cause the exhaust gas to carry some cooling water upstream and into the cylinders. The backflow water tends to evaporate in a cylinder which has a higher operating temperature that the other cylinder. Evaporation can prove problematic where precipitates (e.g., salt) remain and cylinder. Under server condition, salt can corrode the piston and even lead to seizure of the piston within the cylinder. 
     SUMMARY OF THE INVENTION 
     A need therefore exists for a way of maintaining a generally uniform temperature between the cylinders even where exhaust gas flow differs between the cylinders. 
     An aspect of the present invention involves a multi-cylinder engine for a small watercraft. The watercraft comprises a first variable-volume combustion chamber and a second variable-volume combustion chamber. An exhaust manifold communicates with both the first and second combustion chamber. Means are provided for reducing the temperature of first variable-volume combustion chamber in comparison to the second variable-volume combustion chamber. 
     In accordance with another aspect of the present invention, a multi-cylinder engine for a small watercraft is provided. The engine comprises a first variable-volume combustion chamber and a second variable-volume combustion chamber. An exhaust manifold communicates with both the first and second combustion chambers. The first variable-volume combustion chamber exhausts into the exhaust manifold upstream of the point where the second variable-volume combustion chamber exhausts into the exhaust manifold. A cooling system includes at least a first coolant passage near the first variable-volume combustion chamber and a second coolant passage near the second variable-volume combustion chamber. A flow regulator of the cooling system produces a larger coolant flow rate through the first coolant passage than through the second coolant passage. 
     A preferred method of maintaining generally equal temperatures within at least two cylinders of a multi-cylinder engine involves cyclically providing a fuel/air charge to first and second cylinders of the engine and burning the fuel/air charge in the first cylinder and in the second cylinder. Combustion byproducts are ported to an exhaust manifold such that the first cylinder exhausts upstream of the second cylinder. The operating temperature in the first cylinder is reduced to generally match or to be lower than the temperature of the second cylinder during the cyclic operation of the a engine. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other features of the invention will now be described with reference to the drawings of a preferred embodiment which is intended to illustrate and not to limit the invention, and in which: 
     FIG. 1 is a partial sectional, side elevational view of a personal watercraft which employs a marine engine which is configured in accordance with a preferred embodiment of the present invention; 
     FIG. 2 is a partial, sectional side elevational view of an engine of the watercraft of FIG. 1, illustrating a portion of an exhaust system in section; 
     FIG. 3 is a front elevational view of the engine of FIG. 2; 
     FIG. 4 is a top plan view of the engine of FIG. 2, 
     FIG. 5 is a side elevational view of the engine of FIG. 3 taken in direction V; 
     FIG. 6 is an enlarged side view of an exhaust manifold of the exhaust system illustrated in FIG. 2; 
     FIG. 7 is a rear elevational view of the exhaust manifold of FIG. 6 taken in direction VII of FIG. 6; 
     FIG. 8 is an opposite side view of the exhaust manifold of FIG. 6 taken in direction VIII of FIG. 7; 
     FIG. 9 is a cross-sectional view of the exhaust manifold of FIG. 6 taken along line  9 — 9 ; 
     FIG. 10 is a cross-sectional view of the exhaust manifold of FIG. 7 taken along line  10 — 10 ; and 
     FIG. 11 is a cross-sectional view of the exhaust manifold of FIG. 7 taken along line  11 — 11 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1 illustrates a personal watercraft  10  which includes a marine engine  12  configured in accordance with a preferred embodiment of the present invention. Although the present engine  12  is illustrated in connection with a personal watercraft, the engine  12  can be used with other types of watercraft as well, such as, for example, but without limitation, small jet boats and the like. 
     Before describing the engine  12 , an exemplary personal watercraft  10  will first be described in general details to assist the reader&#39;s understanding of the environment of use and the operation of the engine  12 . The watercraft  10  includes a hull  14  formed by a lower hull section  16  and an upper deck section  18 ; The hull sections  16 ,  18  are formed from a suitable material such as, for example, a molded fiberglass reinforced resin. The lower hull section  16  and the upper deck section  18  are fixed to each other around the peripheral edges  20  in any suitable manner. 
     A passenger seat  22  is provided proximate to the stern of the hull  14 . The passenger seat  22  is mounted longitudinally along the center of the watercraft  10 . In the illustrated embodiment, the seat  22  has a longitudinally extended straddle-type shape which may be straddled by an operator and by at least one or two passengers. A forward end  24  of the seat  22  lies proximate to the controls  26  of the watercraft  10  which generally lie at about the longitudinal center of the watercraft  10 . This position of the operator on the watercraft  10  gives the watercraft fore and aft balance when the operator rides alone. A rear portion  28  of the seat  22  is configured to allow one or two passengers to be comfortably seated behind the operator of the watercraft  10 . The seat  22  desirably includes a removable seat cushion to increase the comfort of the operator and the passengers. 
     The upper deck section  18  of the hull  14  advantageously includes foot areas. The foot areas extend generally longitudinally and parallel to the sides of the elongated seat  22  so that the operator and any passengers sitting on the seat  22  can place their feet in the foot areas. A non-slip surface (not shown) is located in the foot areas to provide increased grip and traction for the operator and the passengers. 
     The lower hull section  16  of the personal watercraft  10  includes a forward compartment  32  and a rear compartment  34 . In the exemplary watercraft depicted in FIG. 1, a fuel tank  36  and a buoyant block (not illustrated) are located in the forward compartment  32 . The buoyant block affords additional buoyancy to the watercraft  10 . 
     The engine  12  is contained within the rear compartment  34  and is mounted primarily beneath the forward portion of the seat  22 . Vibration-absorbing engine mounts  38  secure the engine  12  to the hull lower portion  16  in a known manner. The engine  12  is mounted in approximately a central position in the watercraft  10 . 
     As seen in FIG. 1, a coupling  40  interconnects an engine output shaft  42  to an impeller shaft  44 . If the engine output shaft  42  is vertically disposed, the impeller shaft  44  will be driven through a bevel gear transmission or a similar transmission. 
     The propeller shaft  44  extends rearwardly through a bulkhead  46 , and a protective sleeve  48 , to a jet propulsion unit  50  and drives an impeller  52  of the unit  50 . A bearing assembly  54 , which is secured to the bulkhead  46 , supports the impeller shaft  44  behind the shaft coupling  40 . 
     The jet propulsion unit  50  is positioned in a tunnel  56  in the rear center of the lower hull section  16  located behind the bulkhead  46 . The propulsion unit  50  includes a gullet  58  having an inlet opening  60  formed on the bottom side of the lower hull section  16 . The gullet  58  extends from the inlet opening  60  to a pressurization chamber  62 . The pressurization chamber  62  in turn communicates with a nozzle section  64  of the propulsion unit  50 . A ride plate  66  covers a portion of the tunnel  56  behind the gullet inlet  60  to enclose the pump chamber  62  and the nozzle  64  within the tunnel  56 . In this manner, the lower opening of the tunnel  56  is closed by the front edge of the pump gullet  58  and the ride plate  66 . 
     The impeller  52  is located toward the front end of the chamber  62 . A central support  68  supports the rear end of the impeller shaft  44  behind the impeller  52  and generally at the center of the pressurization chamber  62 . A bearing assembly journals the rear end of the impeller shaft  44  within the support  64 . 
     The rotating impeller  52 , which the impeller shaft  44  drives, pressurizes the water within the chamber  62  and forces the pressurized water through the nozzle section  64  of the propulsion unit  50 . A steering nozzle  70  directs the exit direction of the water stream exiting the jet propulsion unit  50 . The steering nozzle  70  is pivotally supported at the rear of the jet propulsion unit  50  to change the thrust angle on the watercraft  10  for steering purposes as is known in the art. 
     The steering nozzle  70  is connected to a steering handle  72 . The steering handle  72  forms -part of the operator controls  26  which are mounted in front of the operator seat  22  as noted above. The steering handle  72  also can include a throttle control for controlling the speed of the engine  12 . 
     The personal watercraft  10  so far described is conventional and represents only an exemplary watercraft on which the present engine  12  can be employed. A further description of the personal watercraft  10  therefore is not believed necessary for an understanding and an appreciation of the present invention. The details of the engine  12 , including its exhaust system, will now be described in detail. 
     With reference to FIGS. 1 through 5, the engine  12  desirably is a multi-cylinder internal combustion engine. In the illustrated embodiment, the engine  12  includes two in-line cylinders and operates on a two-stroke, crankcase compression principle. For ease of discussion, the cylinders are generally reference by reference numeral  73 , with a first cylinder of the cylinders  73  being referenced by reference numeral  73   a  and a second cylinder being referenced by reference numeral  73   b.  The engine  12  is positioned such that the row of cylinders  73  lies parallel to a longitudinal axis of the watercraft  10 , running from bow to stem. This engine type, however, is merely exemplary. Those skilled in the art will readily appreciate that the present engine principals can be used with any of a variety of engine types having other number of cylinders, having other cylinder arrangements and operating on other combustion principles (e.g., four-stroke principle). 
     The engine  12  includes a cylinder block assembly  74  that defines a pair of parallel cylinder bores formed by cylinder liners. Each cylinder liner is cast or pressed in place in a cylinder block. Pistons (not shown) reciprocate within the bores and are rotatably journaled about the small ends of connecting rods by means of piston pins. The big ends of the connecting rods in turn are journaled about throws of the crankshaft  42 . 
     A crankcase member  76  is attached to the lower end of the cylinder block assembly  74  and forms a two crankcase chambers at the lower ends of the cylinder bores. The crankshaft  42  is rotatably journaled within crankcase chambers. 
     As has been noted, the engine  12  operates on a two-cycle crankcase compression principle. As is typical with such engines, the crankcase chambers associated with each of the cylinder bores are sealed relative to each. For this purpose, the crankshaft  42  can include sealing disks. These disks are disposed on the throws of the crankshaft  42  and separate the big ends of adjacent connecting rods. 
     Within the cylinder block  74 , an exhaust passage is formed which communicates with each cylinder  73 . Each exhaust passage extends from an exhaust port formed in the side of the cylinder wall to an exhaust discharge port located on the side of the engine block  74 . 
     One or more scavenge passages (not shown) also are formed within each cylinder. Each passage includes an inlet port which is disposed in the lower end of the bore and opens to the crankcase chamber, and an outlet port which is disposed at a longitudinal position along the bores that is slightly below and on the opposite side of the exhaust passage and opens to each of the bores. 
     A cylinder head assembly  78  is affixed in closing relation to the upper ends of the cylinder bores by any suitable means. The cylinder head assembly  78  defines a pair of recess which cooperates with the bores and heads of the pistons to form combustion chambers, whose volume varies cyclically with the motion of the pistons. 
     A spark plug  80  is mounted atop each of the recesses in the cylinder head assembly  78  and has its gap extending into the combustion chamber. The spark plugs  80  are fired by an ignition control circuit (not shown) that is controlled by the ECU. 
     A fuel/air charge is delivered to the crankcase chambers by an induction system  82 . In the illustrated embodiment, the induction system  82  is located on a side of the engine  12 . An air intake silencer  84  is located above the engine  12  and includes a downwardly-facing inlet opening  86 . The inlet opening  86  opens into at least one plenum chamber within the silencer  84 . 
     The plenum chamber of the silencer  84  communicates with a plurality of charge formers  88 . The engine  12  desirably includes a number of charger formers  88  equal to the number of cylinders  73  of the engine  12 . 
     In the illustrated embodiment, the charger formers  88  are floatless-type carburetors; however, it is understood that other types of charge formers, such as, for example, fuel injectors, also can be used with the engine  12 . 
     Each carburetor  88  includes a throat in which a throttle valve is disposed. A throttle shaft supports each throttle valves in the respective carburetor throat. The throttle shafts are coupled to a throttle operator in a known manner. 
     A venturi is located upstream of the throttle valve within the carburetor throat. A floatless fuel metering device supplies fuel to a fuel opening within the venturi. The air flow through the venturi draws the fuel through the hole to form the fuel/air charge delivered to the respective combustion chambers. 
     The floatless fuel metering device of each carburetor desirably includes a diaphragm pump  90  which is driven by pressure fluctuations in the associated crankcase chamber. The carburetors  90  are arranged such that the diaphragm pumps  90  lie on a side of the carburetors  88  opposite of the engine block  74 . This arrangement provides a simplified fuel piping layout. Conduits thus connect the diaphragm pumps  90  with the associated crankcase chamber to convey pressure pulses within the chamber to the respective pump  90 . 
     The diaphragm pump  90  draws fuel into a pump chamber through a check valve. The fuel is then metered through a second check valve through a second fuel line to a deliver chamber (not shown) located on the opposite side of the throttle throat. The pump  90  controls the movement of fuel in to and out of the pump chamber, in a known manner. 
     The rate at which fuel is delivered into the delivery chamber is controlled, at least in part by a needle valve operated by a conventional throttle control. Fuel delivered to the delivery chamber is subsequently introduced into the incoming air stream so as to create a fuel/air mixture. In particular, a diaphragm is mounted in the delivery chamber and divides the chamber into an atmospheric area and a fuel storage area. When the second check valve of the pump chamber closes, the diaphragm moves toward the throttle throat to force the fuel through a delivery tube that opens at the fuel opening in the venturi. 
     A fuel supply system delivers a continuous flow of fuel to the pump chamber of the diaphragm pump  90 . The fuel supply system also removes excess fuel from the carburetors. For this purpose, the fuel supply system includes a fuel supply line  92  and a fuel return line  94 . Both the fuel supply line and the fuel return line communicate with the fuel tank  36  and with parallel branches of a fuel circuit that lie between the supply line  92  and the return line  94 . The carburetors  82  are positioned within these parallel branches. 
     The fuel/air charge formed within the carburetor  82  is delivered to the corresponding crankcase chamber through an intake passage of an intake manifold  96 . In the illustrated embodiment, the intake manifold  96  lies below the carburetors  82 . Each intake passage of the intake manifold  96  communicates with the outlet of one of the carburetors  82 . 
     Upward motion of the piston of each cylinder  73  draws atmospheric air and fuel from the respective carburetor  82  through the induction passage and into the crankcase chamber, past the reed valve. The reed valve is open at this point, because the pressure in the induction passage is greater than the pressure in the crankcase chamber. 
     Sometime after the piston passes top dead center (TDC), the pressure in the crankcase chamber exceeds the induction passage pressure, and the reed valve closes. The air-fuel mixture in the crankcase chamber is then compressed by the piston during its down stroke until the outlet port of the scavenge passage is exposed to the combustion chamber. At this point the compressed air-fuel mixture enters the combustion chamber through the scavenge passage and is further compressed by the ensuing compression stroke of the piston. 
     At some point before top dead center (TDC), the spark plug is fired by the ECU, and the air-fuel mixture ignites, burns, and expands. This forces the piston downwardly, and thus drives the crankshaft  42 . Continued downward motion of the piston exposes the exhaust passage to the combustion chamber, and thus permits the combustion gases to be expelled from the combustion chamber through the exhaust passage. 
     A conventional magneto-flywheel assembly  98  desirably triggers the ignition timing. The magneto-flywheel assembly  98  is connected to the crankshaft  42  on the front side of the engine  12 . A pulsar coil used with the magneto-flywheel assembly  98  produces a signal indicative of a particular crankshaft angle. The signal pulse desirably is received and processed by the ECU to determine the specific crankshaft angle at any given time. 
     The ECU uses this information to control ignition timing. For this purpose, the ECU includes an ignition controller which cooperates with a capacitor discharge ignition circuit (CDI). A charging coil used with the magneto-flywheel assembly  98  desirably charges the CDI circuit. The discharge of a CDI capacitor generates a voltage in an ignition coil associated with each spark plug. The ECU normally instructs the ignition system to fire the spark plugs at a certain degree before top-dead-center (e.g., 20 degrees); however, the ignition timing can be retarded or advanced. 
     An exhaust manifold  100  is attached to the opposite side of the engine  12  and communicates with the exhaust discharge ports associates with each cylinder  73 . The exhaust manifold  100  delivers exhaust byproducts to an exhaust system  102  for discharge, as described below. 
     With reference to FIGS. 6 through 11, the exhaust manifold  100  desirably includes a numeral of runners  104  equal to the number of cylinders. In the illustrated embodiment, the exhaust manifold includes two runners  104 . For ease of description, the runner associated with the first cylinder will be referenced by reference numeral  104   a  and the runner associated with the second cylinder will be referenced by reference numeral  104   b.    
     Each runner extends from a mounting flange  106 . The mounting flanges  106  are attached to the side of the engine block  74  by conventional means opposite of the induction system  82 . In this position, the exhaust ports of the first and second cylinders  73  communicate with the passages  108 ,  110  through the first and second runners  104   a,    104   b,  respectively. 
     The exhaust passage  108  defined by the first runner  104   a  has an elongated shape with a gradual  90 E bend which smoothly transitions into a straight section  112 . The exhaust passage  110  terminates at an outlet  114 . An attachment flange  116  is formed about the outlet  114 . 
     The exhaust passage  110  defined by the second runner  104   b  merges into the exhaust passage  108  through the first runner  104   a.  The second cylinder  73   b  thus ports into the exhaust manifold  100  downstream of the point where the first cylinder  73   a  ports into the exhaust manifold  100 . The second runner passage  110  is significantly shorter than the first runner passage  108 . The flow axis through the second runner passage  110  also takes an abrupt directional change where the two runners merge. 
     A coolant jacket surrounds the exhaust manifold  100 . The coolant jacket includes a plurality of passages  118 ,  120  which are formed in the walls  121  of the exhaust manifold  100  adjacent to the runner passages  108 ,  110 . As best seen in FIGS. 9 through 11, the coolant passages  118  about the first runner  104   a  have a significantly greater volume that the coolant passages  120  about the second runner  104   b.  The coolant passage  118  extends about an entire initial segment  122  of the first passage runner  104   a  and communicates with a larger passage  118  that lies about the outer radius of the first runner passage  108 . 
     The coolant passages  118  about the first runner passage  108  extend up to outlet opening  124  formed in the mounting flange  106 . The outlet openings  124  communicate with coolant passages in the cylinder block  74  that lie near the first cylinder  73   a.    
     The coolant passages  120  about the second runner passage  110  also extend up to outlet passages  126  formed in the mounting flange  106  of the second runner  104   b.  These outlet openings  126  communicate with coolant passages in the cylinder block  74  that extend next to the second cylinder  73   b.    
     A coolant inlet port  128  desirably is formed on the lower rear end of the exhaust manifold  100 . The inlet desirably communicates with the coolant passages  118 ,  120  that surround the first runner passage  108  as well as those that surround the second coolant passage  110 . 
     A flow regulator  130  operates between the two sets of coolant passages  118 ,  120  to restrict coolant flow into the coolant passages  120  that extend along a portion of the second runner passage  110 . That is, the flow regulator  130  directs more coolant flow into the coolant passages  118  that surround the first runner passage  104   a.    
     In the illustrated embodiment, the flow regulator  130  includes a restriction in the passage  132  from the inlet to the coolant passages  120  of the second runner  104   b.  The restriction desirably has a smaller diameter than the passage  134  that leads to the coolant passages  118  of the first runner  104   a.  In this manner, more coolant flows through the passages that lie near the first cylinder  73   a  than through the coolant passages that lie near the second cylinder  73   b.    
     With reference to FIG. 4, the coolant passages in the engine block that lie near the first cylinder  73   a  terminate at a common first outlet port  136 . Likewise, the coolant passages in the engine block that lie near the second cylinder terminate at a common second outlet port  138 . 
     As best understood in reference to FIGS. 1 and 2, the exhaust system  102  is provided to discharge exhaust byproducts from the engine to the atmosphere and/or to the body of water in which the watercraft  10  is operated. The exhaust system includes a C-shaped pipe section  140 . This C-pipe  140  includes an inner tube  142  that communicates directly with the discharge end  114  of the exhaust manifold  100 . An outer tube  144  surrounds the inner tube  142  to form a coolant jacket  146  between the inner and outer tubes  142 ,  144 . Although not illustrated, the C-pipe  140  includes an inlet port positioned near its inlet end. The inlet port communicates with the water jacket  146 . 
     The outlet end of the C-pipe  140  communicates with an expansion chamber  148 . In the illustrated embodiment, the expansion chamber  148  has a tubular shape in which an expansion volume is defined within an annular, thick wall  150 . Coolant jacket passages  152  extend through the expansion chamber wall  150  and communicate with the coolant jacket  146  of the C-pipe  140 . 
     A flexible coupling  154  connects the outlet end of the C-pipe  140  to the inlet end of the expansion chamber  148 . The flexible coupling  154  also includes an outlet port  156  which communicates with an internal coolant passage  157  within the flexible coupling  154 . The coolant passage  154  places the coolant jacket  146  and the coolant passages  152  in communication. 
     The outlet end of the expansion chamber  148  is fixed to reducer pipe  158  which tapers in diameter toward its outlet  160 . The pipe  158  has a dual shell construction formed by an inner shell  162  which defines an exhaust flow passage. The expansion volume communicates with this passage. 
     An outer shell  164  is connected to the inner shell  162  and defines a cooling jacket  166  about the inner shell  162 . The coolant jacket passages  152  of the expansion chamber  148  communicate with the coolant jacket  166  of the pipe  158  to discharge a portion of the coolant with the exhaust gases. 
     The lower section of the reducer pipe  158  includes a downwardly turned portion that terminates at the discharge end  160 . The inner shell  162  stops short of the outer shell  164  such that the water flow through the water jacket  166  merges with the exhaust gas flow through the exhaust passage at the discharge end  160 . 
     A flexible pipe  168  is connected to the discharge end  160  of the reducer pipe  158  and extends rearwardly along one side of the watercraft hull tunnel  55 . The flexible conduit  168  connects to an inlet section of a water trap device  170 . The water trap device  170  also lies within the watercraft hull  16  on the same side of the tunnel  56 . 
     The water trap device  170  has a sufficient volume to retain water and to preclude the back flow of water to the expansion chamber  148  and the engine  12 . Internal baffles within the water trap device  170  help control water flow through the exhaust system  102 . 
     An exhaust pipe  172  extends from an outlet section of the water trap device  170  and wraps over the top of the tunnel  56  to a discharge end  174 . The discharge end  174  desirably opens into the tunnel  56  at an area that is close to or actually below the water level with the watercraft  10  floating at rest on the body of water. 
     An engine and exhaust cooling system is provided for cooling the engine and the exhaust system. The cooling system is formed in part by coolant passages and jackets described above in connection with the exhaust manifold and the exhaust system. 
     The cooling system supplies fresh cooling water to the inlet port  128  of the exhaust manifold  100 . In the illustrated embodiment, the propulsion unit  50  supplies cooling water through a conduit  176  to an exhaust manifold cooling jacket  118 ,  120 . 
     The cooling water passing through the exhaust manifold coolant passages  118 ,  120  flows into the cooling passages within the engine  12 , as described above. The cooling water for the passages near the first cylinder  73   a  is then discharged through the first discharge port  136 , and the cooling water for the passages near the second cylinder  73   b  is discharged through the second discharge port  138  on the cylinder head  78 . A small amount of the cooling water passes through the second port  138  and into a telltale line  178 . As telltale water, the water is discharged from a plate on the port side of the watercraft  10  in a position visible to the rider. 
     The majority of the cooling water flows through the first port  136  and into a conduit  180  which delivers the cooling water to water jackets  146  surrounding the exhaust pipe sections  140 ,  148 ,  158 . The conduit  180  connects to the inlet port (not shown) of the C-pipe  140 , located near the outlet end  114  of the exhaust manifold  100 . The cooling water flow through the water jacket  146  of the C-pipe  140  and into the jacket  157  of the flexible coupling  154 . 
     A portion of the cooling water is discharged through the outlet port  156  because too much cooling water in the exhaust stream tends to cause flow resistance. A conduit  182  cl s the cooling water that is discharged tough the outlet port  156  to the outlet end  174  of the exhaust pipe  172 . 
     The balance of the cooling water flows through the jackets within the expansion chamber  148  and the reducer pipe  158 . The cooling water merges into the exhaust gas stream at the discharge end  160  of the pipe  158 , and flow into the flexible hose  168  toward the water trap  170 . The cooling water is eventually discharged with the exhaust gases through the outlet end  174  of the exhaust pipe  172 . 
     The first cylinder  73   a  tends to operate at a lower temperature than in prior engine due to the increased flow of coolant through the passages about the first cylinder  73   a.  The temperature of the first cylinder desirably matches that of the second cylinder  73   a  within at least 10 to 15 percent of the temperature of the second cylinder  73   b.  As a result, deterioration of rubber engine components (e.g., the engine mounts  38 ) about the engine block  74  is diminished, and the engine  12  is less susceptible to knocking. 
     The engine  12  can employ one or more of the following mechanism to reduce the temperature of the first cylinder  73   a,  in addition or in the alternative to supplying more coolant to the passages about the first cylinder  73   a.  The first cylinder  73   a  can be designed with a lower compression ratio than the second cylinder  73   b.  That is the ratio of the volumetric sizes of the variable-volume chamber of the first cylinder  73   a  when the associated piston in a bottom-dead-center (BDC) position and in a top-dead-center (TDC) position is less than the ratio of the volumetric sizes of the variable-volume chamber of the second chamber when the associated piston in a BDC position and in a TDC position. As a result, the power output by the first cylinder is. reduced and less heat is generated by the first cylinder. 
     The ignition timing of the first cylinder  73   a  also can be retarded. That is, the ECU can instruct the ignition controller to fire the spark plug  80  of the first cylinder  73   a  after the ignition controller causes the spark plug  80  of the second cylinder  73   b  to fire. For instance, the ignition controller can fire the second cylinder  73   b  when the associated piston is at a  20 E before top-dead-center position and can fire the first cylinder  73   a  when the associated piston is at an  18 E before top-dead-center position. The retardation of the ignition in the first cylinder  73   a  provides less time for the combustion to occur and consequently complete combustion of the fuel charge does not happen. As a result, the power output by the first cylinder  73   a  is reduced and less heat is generated by the first cylinder  73   a.    
     Each of these methods of evening the temperatures in the two cylinders effectively reduces the temperature of the first cylinder in reference to the second cylinder, without meaningfully affecting the operation of the second cylinder. That is, the methods do not alter in any substantial way the performance of the second cylinder. The temperature and the power output of the second cylinder remains generally the same as that in a similar engine without a temperature evening means. The latter two methods, however, reduce the overall power of the engine by affecting the performance of the first cylinder. It is contemplated that those skilled in the art will be able to readily employ these means of reducing cylinder temperature, either separately or together, in order to obtain desired operating characteristics for the engine. 
     Although this invention has been described in terms of certain preferred embodiments, other embodiments apparent to those of ordinary skill in the art are also within the scope of this invention. Accordingly, the scope of the invention is intended to be defined only by the claims that follow.