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Emission Control Systems
 

When gasoline and air are mixed together and
burned, there are a number of bi-products that
are produced.  Some are relatively harmless,
but some are harmful to the environment.  Gasoline
is a Hydrocarbon (HC) - a mixture of Hydrogen and
Carbon molecules.  The air contains two main
elements - Nitrogen and Oxygen (NO) - with
Nitrogen making up almost 80% of the air.
When these are combined under heat and
pressure, a number of new compounds are
formed in the process:

Harmful Compounds

Harmless Compounds

One of the problems that occurs is when HC (unburned Hydrocarbons)
and NOx (oxides of Nitrogen) combine in the presence of sunlight
to form Photochemical Smog - a brownish gas that can cause
respiratory problems, especially in small children and the
elderly.  This is most common in large urban areas where
the traffic is heavier.

Controlling Emissions

        There are a number of ways in which emissions are controlled.  Some are designed to
prevent them from forming in the first place - others are designed to control them once they
are formed and reduce them to less harmful compounds.

HC - Unburned Hydrocarbons

    There are three places where unburned hydrocarbons (HC) will be present - oil and
fuel vapours in the bottom of the engine (crankcase), from evaporation from the fuel tank,
and from the exhaust system when fuel is not completely burned.

Crankcase Vapours

The PCV (Positive Crankcase Ventilation) system is
designed to draw fumes from the engine crankcase
and recirculate them back into the intake where they
can be burned with the air/fuel mixture and prevent
them from escaping to the atmosphere.

Fuel Tank Vapours

The ECS (evaporative control system) canister is
located in the front of the vehicle.  It traps vapours
from the fuel tank (and float bowl in carburetor systems)
in a layer of activated carbon.  When fresh air is
drawn through the bottom of the canister by engine
vacuum - the air will pick up the vapours and carry
them to the intake manifold to be burned with the
air/fuel mixture

Catalytic Convertor

The catalytic convertor is located in the exhaust system
just behind the engine.  It contains a ceramic element
coated with platinum and palladium.  These two elements
act as a catalyst and cause any oxygen (O2) still remaining
in the exhaust gases to combine more completely (burn
more) and reduce the HC to H2O and CO2 (water
vapour and carbon dioxide).

Thermactor Air Pump

Some vehicles use an air pump driven by a belt or
an electric motor to force additional air into the
exhaust system to provide more air (and Oxygen)
for more complete burning to change the HC to
H2O and CO2.

Carbon Monoxide

        Carbon monoxide is formed through incomplete combustion.  In order to change CO to
the less harmful CO2 - more Oxygen must be combined with it.  This is done through the
use of two devices already mentioned:
 


NOx - Oxides of Nitrogen

        Oxides of nitrogen are formed when HC combines with the Nitrogen in the air under
two conditions:

        At the beginning of the 1970's - manufacturers tried to reduce NOx emissions by reducing
engine compression ratios.  Although this helped to reduce NOx, it also reduced engine power
output and increased fuel consumption.  As a result, engine compression ratios are now usually
no less than 8:1 or 8.5:1.
        The other alternative was to reduce temperatures in the engine cylinder
itself.  The solution was to add something to the cylinder that would occupy space, but not burn
and therefore reduce the amount of heat produced.  The obvious choice was exhaust gases - they
were already burned and also were hot, which made them combine more readily with the air/fuel
mixture in the cylinder.

EGR (Exhaust Gas Recirculation) Valve

The EGR valve is controlled by engine vacuum
(although some newer ones are solenoid controlled)
and often have their vacuum supply controlled by
the engine computer system.  The first EGR valves
had their vacuum supply controlled by a temperature
activated vacuum switch - because the EGR valve
is not needed when the engine is cold and engine
combustion temperatures are lower.  The EGR valve
will also not be open under heavy load - as there is
no vacuum at full throttle.  Some of the first owners
of EGR equipped vehicles disconnected the EGR
vacuum supply in the belief that it reduced engine
power (which it doesn't because there is no vacuum
and therefore no EGR operation under load).  The
result was often cylinder overheating and
burned valves.

Three Way Convertor

Some vehicles do not use an EGR valve.  Instead
they use a special two-compartment convertor.
The front section contains platinum and rhodium,
which help to reduce NOx.  The rear section contains
platinum and palladium to control HC and CO.  Note
the inlet from the air pump that goes into the
center of the convertor to provide additional
Oxygen to help change HC and CO to H2O and
CO2.  If air was introduced into the front section
of the convertor - the Nitrogen in the air would
prevent it from controlling the NOx

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