1. Field of the Invention
This invention relates to an exhaust system for a personal watercraft and more particularly to a catalytic exhaust system for such a watercraft.
2. Description of Related Art
Personal watercraft have become very popular in recent years. This type of watercraft is quite sporting in nature and carries a rider and possibly one or two passengers. A relatively small hull of the personal watercraft commonly defines a riders' area above an engine compartment. A two-cycle 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 usually extends between the engine and the jet propulsion device, through a wall of the hull that forms a front gullet portion of the tunnel.
An exhaust system of the personal watercraft discharges engine exhaust to the atmosphere either through or close to the body of water in which the watercraft is operating. Although submerged discharge of engine exhaust silences exhaust noise, environmental concerns arise. These concerns are particularly acute in connection with two-cycle engines because engine exhaust from two-cycle engines often contains lubricants and other hydrocarbons.
Such environmental concerns have raised a desire to reduce exhaustion of hydrocarbons and other exhaust byproducts (e.g., carbon monoxide and oxides of nitrogen) and thus reduce pollution of the atmosphere and the body of water in which the watercraft is operated. In response to the increased concerns regarding exhaust emissions, it has been proposed to equip personal watercraft with a catalyst to convert exhaust byproducts to harmless gases.
The nature of watercraft exhaust systems has a tendency to impair the efficiency of or cause damage to the catalyst. Catalysts must operate at a relatively high temperature in order to produce the necessary thermal reaction and burning of exhaust byproducts. Peak catalyst efficiency is achieved within a range of temperatures. Operation at either too high or too low of a temperature can cause the catalyst to lose efficiency and operation at too high of a temperature can even damage the catalyst. In certain circumstances, for instance when an engine fails to fire the cylinders, the catalyst is often excessively heated by unburned fuel that is discharged to the exhaust pipe and then burned in the catalyst. This excessive heat could cause inefficiency in the operation of the catalyst or damage the catalyst. Often, a watercraft exhaust pipe will have a water jacket disposed thereon to cool the exhaust system and catalyst. However, this water jacket only indirectly cools the catalyst. This indirect cooling is may not be sufficient to keep the catalyst from overheating under some operating conditions.
Also, the sporting nature of a personal watercraft subjects the catalyst to damage. It is likely that water will occasionally flow upstream through the exhaust system of the watercraft, especially if the watercraft becomes inverted and is subsequently righted. If a sudden rush of water comes in contact with the catalyst, the catalyst bed can become polluted, particularly when operating in marine environments, and can even shatter.