Carbon Monoxide Emissions from Marine Outboard Engines 2004-32-0011
Carbon Monoxide (CO) has become a pressing issue for the recreational marine industry. An increasing number of boating incidents have been linked to CO poisoning caused by emissions from gasoline-powered marine engines. Measurements by the National Institute for Occupational Safety and Health (NIOSH) and the U.S. Coast Guard have confirmed potentially hazardous CO concentrations near many of these engines. The measurements have also shown much lower CO concentrations for Evinrude® two-stroke direct-injected engines.
This paper reviews national and international CO emission regulations for marine engines and discusses CO formation and reduction mechanisms. The differences between homogeneous- and stratified-charge combustion systems on CO formation, resulting from design and calibration criteria, are analyzed. The primary driving factors for CO formation under high-load homogeneous operation are driven by a desire to maximize power output and a necessity to control the thermal loading of internal engine components. Part throttle and low-load operation are largely driven by emissions and run-quality requirements.
Several distinct advantages for direct-injected engines lead to the formation of significantly lower CO concentrations in the exhaust. The unavoidable over-scavenging of air aids in diluting cylinder-out CO levels. This, combined with high exhaust gas temperatures, can generate a significant post-oxidation effect. The post-oxidation effect was investigated in detail and some engine test results are presented. Stratified operation at low loads with high air-fuel ratios result in CO concentrations up to 100 times less than homogeneous-charge engines.