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 TM 9-8000
c. Special Considerations. Vehicles equipped with
catalytic converters require special considerations and
more complete burning will reduce carbon monoxide and
generally are made to work in conjunction with other
hydrocarbon emissions greatly.
emission systems.
(2) Air injection is vital to ensure an adequate
(1) The use of gasoline containing lead is
supply of oxygen in the exhaust stream on vehicles
destructive to a catalytic converter. in use, the lead will
equipped with catalytic converters.
coat the catalyst as the exhaust passes through the
converter. This will halt catalytic converter operation
b. Air Pump System (Fig. 7-9), The air pump
completely.
system uses an engine-driven pump to force air into the
exhaust.
(2) The use of gasoline with a high sulphur
content will cause considerable amounts of sulphur
(1) The pump usually is a vane-type pump that
dioxide to be produced in the converter and emitted to
operates exactly like the vane-type oil pump (para 20-6).
the atmosphere.
The pump is driven by a belt. A relief valve is built into
the pump to prevent it from building too much pressure.
(3) A heat shield must be installed between the
converter and the vehicle floor because the converter
(2) The air from the pump is directed through
can, at times, produces enough heat to ignite the interior
rubber hoses to the air manifold. The air manifold
floor covering. A heat shield also is installed under the
distributes the air to each exhaust port. The point where
converter to minimize the possibility of igniting objects
the air is fed in may be located at the exhaust manifold or
such as grass and leaves.
This is of particular
directly into the cylinder head at the exhaust port. The air
importance to military vehicles during cross-country
is fed in through nozzles called injection tubes.
operation.
(3) A check valve is installed between the air
(4) An overly rich air-fuel mixture is disastrous
manifold and the air pump feed hose to prevent hot
to a catalytic converter. Excessive carbon monoxide and
exhaust from feeding back to the pump.
hydrocarbons result in such a high rate of oxidation in the
converter that it can overheat to the point where its outer
(4) Whenever the throttle is closed suddenly, a
shell actually can melt. Because of this, the engine
temporary overrich air-fuel mixture will result. The rich
always must be kept in the proper state of tune.
mixture will leave the engine with a large percentage of it
unburned. When an engine is equipped with an air
(5) An adequate amount of oxygen must be
pump, the rich mixture will flare up and explode as it
present in the exhaust stream for the catalytic converter
enters the exhaust and contacts the injected fresh air,
to operate. Therefore, a supporting system such as an
resulting in a backfire condition. To correct this situation,
air injection system (para 7-9) usually is placed on
an antibackfire valve is installed in series in the air pump
catalytic converter equipped engines to dilute the exhaust
feed hose. The antibackfire valve prevents the overrich
stream with fresh air.
mixture from occurring by injecting a short burst of air
into the intake manifold whenever the throttle is released,
thus preventing a backfire. Some models use a diverter
7-9. Air Injection Systems.
valve.
The diverter valve eliminates backfiring
a. Purpose. Air injection systems mix fresh air with
temporarily by diverting the air pump delivery to the
the vehicle exhaust. There are two purposes for air
atmosphere whenever the throttle is released suddenly,
injection systems:
allowing the rich mixture to pass through.
(1) The exhaust gases still are burning as they
c. Naturally Aspirated System (Fig. 7-10). The
are pushed out of the combustion chamber through the
naturally aspirated system uses the negative pulses of
exhaust valve. The burning will be prolonged and
the exhaust system to draw
intensified by injecting fresh air
7-9
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