Mariner Mercury 105/140 JET, Руководство по обслуживанию

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FUEL SYSTEMS - 3F-1 90-824052R3 JUNE 2002
Exhaust Emissions
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; gasolene 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 in-
sufficient 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 byproduct 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.