Installation manual

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1-50

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

(CO2). CO2 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 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.

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.

Notes

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