Mercury FourStroke 2.5 Service Manual page 77

Emissions
Exhaust Emission Standards
Through the Environmental Protection Agency (EPA), the federal government has established exhaust emission 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. Emission 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
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
The reduction of exhaust emissions is accomplished by controlling the air/fuel ratio that goes into the combustion chamber and
with adjusting the timing curve of the spark ignition.
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.
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). CO is a harmless gas. Carbon often combines with insufficient oxygen (one
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90-8M0065421 NOVEMBER 2012