704 MOBILE SOURCE POLLUTION
EMISSION CONTROL DEVICESAdditional CombustionAn afterburner is an additional baffl ed tubular reactor in
which the gases are reignited and burned to completion. An
air pump provides the necessary oxygen-rich mixture and
the heat of reaction maintains a high temperature to speed
its completion.
It is not necessary in all cases to have a separate after-
burner. The aforementioned report^5 states, “An injection sys-
tems decrease exhaust CO and HC emissions by injecting air
at a controlled rate and at low pressure into each exhaust port.
Here, the oxygen in the air reacts with the hot exhaust gases,
resulting in further combustion of the unburned hydrocarbons
and CO that would otherwise be exhausted to the atmosphere.
Optimum reduction of emissions by this method depends on
proper air injection rates over a wide range of engine operating
conditions, carefully tailored air-fuel mixture ratios and spark
advance characteristics, and in some cases the use of heated
carburetor air. Some engines also provide for retarded ignition
timing during closed-throttle operation.
All air injection systems use essentially the same basic
air pump, a positive displacement rotary-vane type.
To guard against excessive temperature and back pres-
sure in the exhaust system resulting from high air delivery
rates at full throttle and high speeds, a pressure-relief valve is
installed in the pump housing. The valve opens to bleed off
some of the pump fl ow at a predetermined pressure setting.
Output from the air pump is directed through hoses and
an air distribution manifold (or two manifolds—one for each
bank on V-8 engines) to the air injection tubes located in
each exhaust port. A check valve between the air distribu-
tion manifold and the air pump prevents reverse fl ow of hot
exhaust gases in the event that pump output is interrupted.
A vacuum-controlled antibackfi re valve is used to pre-
vent fl ow of air to the exhaust ports during the initial stage
of closed-throttle acceleration. The high vacuum that occurs
during deceleration causes rapid evaporation of liquid fuel
from the intake manifold walls. The resulting rich mixture
creates a potentially explosive vapor in the exhaust manifold
if injected air is present.
As with engine modifi cation systems, most air injec-
tion systems also employ spark retard during idle or idle
and deceleration through use of ‘ported’ vacuum sources of
dual-diaphragm distributor-vacuum-advance mechanisms.”
Besides residual gas dilution and wall quenching, engine
variables have the most effect on the amount of hydrocarbons
in exhaust gases. Over sixty privately owned and operated
automobiles fueled with commercial leaded gasoline have
been tested and seven main engine variables were found
which changed the hydrocarbon concentration in the exhaust
gases. These results can be briefl y summarized as:1) Air-fuel ratio: Low value for gas mixture results
in higher hydrocarbon concentration in exhaust
gases.
2) Ignition timing.3) Speed: Increase in speed of engine decreases the
amount of hydrocarbon.
4) Air-flow rate: The effect of air-flow rate on the
total hydrocarbon concentration depends on the
air-flow ratio, ignition timing combination.
5) Compressor-ratio: Increasing the compression
ratio, by decreasing head-to-piston distance,
increases the total hydrocarbon concentration.
6) Exhaust Back Pressure: The amount of hydrocar-
bon in exhaust decreases with increasing exhaust
pressure.
7) Coolant Temperature: Increasing the coolant tem-
perature decreases the hydrocarbon concentration.Catalytic ReactionsThe engine exhaust gases may be passed through a
cylindrical shaped canister packed with catalytic particles.
Although this method has great potential, two problems
may arise. The long term stability of the catalyst (50,000
miles) is diffi cult to maintain since lead and other chemi-
cals in trace amounts poison the catalyst. The catalyst
structural stability is diffi cult to maintain under the infl u-
ence of varying gas fl ow rates and fairly high temperature.
Also, removing three or four pollutants simultaneously can
prove diffi cult for any single type of catalyst. The removal
of NO x , for instance, requires a reduction catalyst, whereas
CO requires an oxidation catalyst. For this reason dual stage
catalytic reactors have been proposed. The technical prob-
lems for this method are greater but the potential advantages
are even greater.
Nitric Oxide Removal A review of some chemical reac-
tion data which might be useful in automobile pollution con-
trol work has been presented by Shelef and Kummer.^7
One approach to solving the stability problems has been
to avoid leaded fuel in automobiles containing converters.
The catalytic approach to conversion for American Motor
cars is to use a pellet-type of catalyst with a monolithic-type
warm-up feature for California and high altitude cars. The
warm-up converter is separately mounted ahead of the cata-
lytic converter. Chrysler Corporation and Chevrolet use a cat-
alyst support coated with platinum, palladium and rhodium.
Hydrocarbons, CO and NO x are all reduced by this three
component catalyst. An extensive literature exists on more
economical active phases 11,13,16,22 which are not as effective
converters. The air to exhaust ratio in catalytic converters is
computer controlled in American Motor cars.
For this reason, reactions which involve combination
with rather than decomposition of NO 2 are being studied
very carefully. The equilibrium constants in terms of par-
tial pressure are given in Table 3 for NO combination with
hydrogen, CO and methane at various exhaust temperatures.
The thermodynamic conditions (large K p values) are gener-
ally favorable for conversion (reduction). A Monel (nickel–
copper alloy) catalyst has been found reasonably successful
for removing NO by combining it with residual CO in the
exhaust stream. The Monel dissociates the oxygen from NO
and then oxides CO to the harmless dioxide.C013_005_r03.indd 704C013_005_r03.indd 704 11/18/2005 10:42:27 AM11/18/2005 10:42:27 AM