Product Manual

Oxides of Nitrogen – NOx
NOx is a slightly different by‑product of combustion. Nitrogen is one of the elements that makes up the air going into the
engine. Under extremely high temperatures it combines with oxygen to form oxides of nitrogen (NOx). This happens in the
engine's combustion chambers when temperatures are too high. NOx itself is not harmful, but when exposed to sunlight it
combines with unburned hydrocarbons to create the visible air pollutant known as smog. Smog is a serious problem in
California as well as many other heavily populated areas of the United States.
Controlling Emissions
There are two principle methods of reducing emissions from a marine engine. The first method is to control the air/fuel ratio that
goes into the combustion chamber. The second is to control the time when this air/fuel mixture enters the combustion chamber.
Timing is important, to prevent any unburned mixture from escaping out of the exhaust port.
Stoichiometric (14.7:1) Air/Fuel Ratio
In the search to control pollutants and reduce exhaust emissions, engineers have discovered that they can be reduced
effectively if a gasoline engine operates at an air/fuel ratio of 14.7:1. The technical term for this ideal ratio is stoichiometric. An
air/fuel ratio of 14.7:1 provides the best control of all three elements in the exhaust under almost all conditions. The HC and CO
content of the exhaust gas is influenced significantly by the air/fuel ratio. At an air/fuel ratio leaner than 14.7:1, HC and CO
levels are low, but with a ratio richer than 14.7:1 they rise rapidly. It would seem that controlling HC and CO by themselves
might not be such a difficult task; the air/fuel ratio only needs to be kept leaner than 14.7:1. However, there is also NOx to
consider.
As the air/fuel ratio becomes leaner, combustion temperatures increase. Higher combustion temperatures raise the NOx
content of the exhaust. But, enrichening the air/fuel ratio to decrease combustion temperatures and reduce NOx also increases
HC and CO, as well as lowering fuel economy. So the solution to controlling NOx ‑ as well as HC and CO ‑ is to keep the air/
fuel ratio as close to 14.7:1 as possible.
Emissions Information
Manufacturer's Responsibility
Beginning with 1998 model year engines, manufacturers of all marine propulsion engines must determine the exhaust emission
levels for each engine horsepower family and certify these engines with the United States Environmental Protection Agency
(EPA). A certification decal/emissions control information label, showing emission levels and engine specifications directly
related to emissions, must be placed on each engine at the time of manufacture.
Dealer Responsibility
When performing service on all 1998 and newer outboards that carry a certification, attention must be given to any adjustments
that are made that affect emission levels.
Adjustments must be kept within published factory specifications.
Replacement or repair of any emission related component must be executed in a manner that maintains emission levels within
the prescribed certification standards.
Dealers are not to modify the engine in any manner that would alter the horsepower or allow emission levels to exceed their
predetermined factory specifications.
Exceptions include manufacturer's prescribed changes, such as that for altitude adjustments.
Owner Responsibility
The owner/operator is required to have engine maintenance performed to maintain emission levels within prescribed
certification standards.
The owner/operator is not to modify the engine in any manner that would alter the horsepower or allow emission levels to
exceed their predetermined factory specifications.
Exceptions
•
Carburetor jets may be changed for high altitude use in accordance with factory recommendations.
• Single engine exceptions may be allowed with permission from the EPA for racing and testing.
EPA Emission Regulations
All new 1998 and newer outboards manufactured by Mercury Marine for operation in the USA, are certified to the United States
Environmental Protection Agency as conforming to the requirements of the regulations for the control of air pollution from new
outboard motors. This certification is contingent on certain adjustments being set to factory standards. For this reason, the
factory procedure for servicing the product must be strictly followed and, whenever practicable, returned to the original intent of
the design.
General Information
90-8M0125265 eng NOVEMBER 2017 © 2018 Mercury Marine Page 1C-15