Mercury 100 FourStroke, Руководство по обслуживанию

Страница: 205 / 574

FUEL SYSTEM - 3D-1 90-830234R3 DECEMBER 1997
Exhaust Emissions
Standards
Through the Environmental Protection Agency
(EPA), the federal government has established ex-
haust emissions standards for all new marine en-
gines 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 combus-
tion were complete, the mixture of air and gasoline
would result in these emissions: water, carbon diox-
ide and nitrogen, which are not harmful to the envi-
ronment. 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 cer-
tain pollutants, or potentially harmful gases, in the ex-
haust 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 oxy-
gen atom). This forms carbon monoxide, CO. Car-
bon monoxide is the product of incomplete combus-
tion 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 temper-
atures it combines with oxygen to form oxides of ni-
trogen (NOx). This happens in the engine’s combus-
tion 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 seri-
ous problem in California as well as many other
heavily populated areas of the United States.
Controlling Emissions
There are two principle methods of reducing emis-
sions 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 con-
trol the time when this air/fuel mixture enters the com-
bustion 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 ex-
haust 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 in-
fluenced 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 rapid-
ly. It would seem that controlling HC and CO by them-
selves might not be such a difficult task; the air/fuel
ratio only needs to be kept leaner than 14.7:1. How-
ever, there is also NOx to consider.