Page 185 - 05. Subyek Teknik Mesin - Automobile Mechanical and Electrical Systems Automotive Technology Vehicle Maintenance and Repair (Vehicle Maintenance Repr Nv2) by Tom Denton

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Engine systems 169

aquatic life in streams, rivers and lakes. Fuel suppliers now remove, or signifi cantly
reduce, the amount of sulphur during the refi ning process.
Air consists of about 80% nitrogen which, under normal circumstances, is an inert
gas. An inert substance is one that has very little chemical reactivity and does not Defi nition
burn, or mix easily with other chemicals. Nitrogen, however, will mix with oxygen
at high temperatures to form nitrogen oxides (NO x ). These combine in exceptional Inert substance
A substance with very low chemical
geographical and meteorological conditions to form smog, acids and increases in
reactivity.
low-level ozone. This serves to make a very unpleasant atmosphere in which to
live. Many respiratory and asthmatic fatalities occur under these conditions.
The combustion of fuel inside the engine is a chemical process that combines the
carbon and hydrogen in the fuel with oxygen to release energy. Slightly less than 20%
of air is made up of oxygen. Complete combustion produces carbon dioxide (CO )
2
and water (H O). Neither of these is directly harmful. Both are naturally occurring
2
substances in the atmosphere (but note the comments in the previous section).
Incomplete combustion leaves some of the carbon and oxygen not fully
combined. The product of this is carbon monoxide (CO), which is toxic. Small
quantities of carbon monoxide molecules are dangerous because they attach
themselves to red blood cells. This reduces the oxygen that the cells normally
carry around the body, resulting in oxygen deprivation, brain damage and fatality.

Another product of incomplete combustion is particles of fuel that have not
been burnt. These are carried, with the exhaust gases, into the atmosphere and
are called unburnt hydrocarbons. Very small amounts of hydrocarbons in the
atmosphere can cause respiratory problems.
Engine oil drawn into the combustion chamber, either from the inlet valve stem
or by bypassing the pistons, can also be a source of hydrocarbon pollution. Oil
vapours form in the engine crankcase and can escape into the atmosphere. A
positive crankcase ventilation system is now used to draw the vapours into the
engine so that they are burnt to form water and carbon dioxide.
On old vehicles, vapour in the fuel tank was directly vented to the atmosphere. This

is no longer the case, but the fuel tank must still be vented in some way to allow
air to fl ow into the tank as fuel is used. A charcoal fi lter is now used to prevent the
loss of fuel vapour and for the expansion of the fuel when the weather is hot. The
fuel vapour in the charcoal canister is drawn into the engine and burnt.
Good fuel economy is obtained with a lean air-to-fuel mixture. However, this

mixture produces higher combustion temperatures and greater risks of NO x
being formed. In order to prevent, or reduce to a minimum, the formation of NO , Key fact
x
the combustion temperature has to be kept as cool as possible and the amount Good fuel economy is obtained with
of oxygen limited to match the quantity of fuel delivered. To reduce the amount a lean air-to-fuel mixture.
of oxygen in the air charge, a gas that is low in oxygen can be introduced.
This maintains the total air-charge mass to give good compression pressures
and effi cient operation of the engine. The available gas is the exhaust gas that
has already used up its oxygen content during combustion. The addition of a
regulated charge of exhaust gas reduces the oxygen content of the new charge
to suit the amount of fuel delivered. This, in turn, reduces the combustion
temperature and limits the formation of NO x . Catalytic conversion of any
remaining harmful gases can result in a much cleaner exhaust gas.

2.4.2 Reducing pollution

Vehicle engine and component manufacturers have put a great deal of effort into
reducing pollution. For example, lead is no longer needed in petrol (gasoline)