Reducing combustion engine exhaust emissions is a continued object of research and development, driven both by awareness of environmental effects and increased government regulation. Some of the most effective and cost-efficient emissions controls involve the use of downstream chemical catalysts（化学催化剂） that further oxygenate incompletely combusted compounds. Sometimes exhaust is directed sequentially through multiple catalyst beds. It is generally understood that higher catalyst temperatures provide more effective emissions control. Much exhaust catalysis development has been focused on developing catalytic converters for automotive applications, in which engine speed varies substantially with vehicle speed and gear selection.Marine engines and generators are subjected to specific regulations, both for emissions and for safety concerns. For example, exposed engine suce temperatures (including exhaust system suce temperatures) must be kept low to avoid increased risk of fire hazard. Seawater is injected into many marine engine exhaust flows so as to cool exiting exhaust gases, and seawater is also frequently circulated through exhaust system components so as to maintain low suce temperatures.Further improvements in exhaust emissions controls for constant and variable speed engine applications are desired, particularly improvements suitable for marine use.

Which method is used commonly to reduce exhaust emissions?