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The only "passive" emission control device is the Catalytic Convertor.
It does not
require any external controls to make it operate
and has no moving parts to service or
replace.
PCV - Positive Crankcase Ventilation
This system is controlled by engine vacuum. When the engine is running
under light
load, vapours are drawn from the crankcase
and recycled to the intake. As vapours are
drawn out of the crankcase, fresh filtered
air is drawn in. Under load, there is no available
vacuum and the PCV valve closes. Any
vapours are then drawn back through the fresh air
passageway into the intake air stream.
When the engine is shut off, the PCV valve closes
and any excess pressure vents to the air cleaner
intake. There are no computer controls
over this system.
Thermactor Air Pump
Belt driven air pumps run all the time, therefore the pump output must
be controlled.
On older systems, a vacuum controlled valve
allowed air to be pumped into the exhaust
system or bypassed back to the atmosphere.
Today's vehicles that are equipped with air
pumps will use a solenoid vacuum control valve
to switch the air flow from the exhaust to
the atmosphere - or in the case of a three
way convertor - to the center of the convertor.
Some systems use an electric pump that is
switched on and off by the engine control
computer.
ECS - Evaporative Control System
When the vehicle is not running, fuel vapours from the tank pass through
a line
to the vapour canister. When the engine
is started, fumes are drawn from the canister.
A vacuum operated valve was used on older
vehicles in the line running from the
canister to the intake. Today's vehicles
use an electrically controlled "canister purge
solenoid valve" that is switched on at the
appropriate time by the computer.
EGR - Exhaust Gas Recirculation Valve
The EGR valve on older vehicles used a vacuum control that was connected
to
the intake manifold by a "ported vacuum switch"
(PVS). This PVS was located in
the engine cooling system so that no vacuum
was supplied to the EGR valve when
the engine was cold, as no EGR control is
required until the engine is hot enough
to produce NOx. Newer systems use a
solenoid vacuum valve that is operated by
the computer to only allow vacuum under programmed
operating conditions. In
addition, some newer vehicles use electrically
operated EGR valves rather than
the vacuum type.
Maintaining a Balance
One of the problems associated with air/fuel mixtures
is illustrated by this graph. As mixtures become leaner
(more air, less fuel) - CO and HC decrease, but NOx
increases because of the increased burning temperature.
The best balance of economy, performance and most
importantly - emissions - occurs at the
stoichiometric air/fuel ratio of 14.7:1.
OBD 2
In order to enhance emission controls, a new set of regulations took effect
with
1996 model year of all vehicles sold in the
United States - and in turn Canada as well.
These were a stringent set of standards -
both in terms of emissions and in how the
computerized information was handled.
Use the following link to find out more about
the OBD2 system and how it has affected vehicle
design and technical support. When
you are done, return here to move on to the
Unit Six Assignment.