Mercury 200 OptiMax Jet Drive, Service Manual, Page: 263 / 391

Emissions
Page 3D-2 90-8M0050731 MAY 2011
Exhaust Emission Standards
Through the Environmental Protection Agency (EPA), the federal government has established exhaust emissions standards for
all new marine engines sold in the U.S.
What Are Emissions?
Emissions are what comes out of the exhaust system in the exhaust gas when the engine is running. They are formed as a result
of the process of combustion or incomplete combustion. To understand exhaust gas emissions, remember that both air and fuel
are made of several elements. Air contains oxygen and nitrogen among other elements; gasoline contains mainly hydrogen and
carbon. These four elements combine chemically during combustion. If combustion were complete, the mixture of air and gasoline
would result in these emissions: water, carbon dioxide and nitrogen, which are not harmful to the environment. But combustion is
not usually complete. Also, potentially harmful gases can be formed during and after combustion.
All marine engines must reduce the emission of certain pollutants, or potentially harmful gases, in the exhaust to conform with
levels legislated by the EPA. Emissions standards become more stringent each year. Standards are set primarily with regard to
three emissions: hydrocarbons (HC), carbon monoxide (CO), and oxides of nitrogen (NOx).
Hydrocarbons – HC
Gasoline is a hydrocarbon fuel. The two elements of hydrogen and carbon are burned during combustion in combination with
oxygen. But they are not totally consumed. Some pass through the combustion chamber and exit the exhaust system as unburned
gases known as hydrocarbons.
Carbon Monoxide – CO
Carbon is one of the elements that make up the fuel burned in the engine along with oxygen during the combustion process. If
the carbon in the gasoline could combine with enough oxygen (one carbon atom with two oxygen atoms), it would come out of
the engine in the form of carbon dioxide (CO
2
). CO
2
is a harmless gas. But carbon often combines with insufficient oxygen (one
carbon atom with one oxygen atom). This forms carbon monoxide, CO. Carbon monoxide is the product of incomplete combustion
and is a dangerous, potentially lethal gas.
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 two‑stroke‑cycle 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
Models Affected
Models Covered Serial Number or Year
Mercury/Mariner/Force/Sport Jet 2.5–250 HP 1998 and Newer