EMISSIONS
Page 3B-2 90-828883R2 JUNE 1998
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 cham-
ber. 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 discov-
ered 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 combus-
tion 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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