702 MOBILE SOURCE POLLUTION
industry researchers.^4 However, the U.S. government took
the initiative for the welfare of the public in not waiting for
all the details to be perfectly established. Certainly there are
those among us (aged, with heart conditions, etc.) who will
be more severely affected than the average by typical ambi-
ent pollutant concentrations and will need protection.
At least two approaches to the removal of internal
combustion engine pollutants have come into wide accep-
tance. One is the improvement of the combustion process
itself. This includes reducing manifold vacuum (and hence
dilution of charge) between exhaust and intake steps;
increasing the cylinder wall temperature; and designing
for less maximum horsepower by minimizing the surface
to volume ratio.
The other approach is controlling exhaust emissions
by further reaction, either after-burning or catalytic. After-
burning can be accomplished slightly downstream from the
exhaust valve by additional oxygen injection converting HC
and CO to CO 2 and water vapor. The after-burner has the dis-
advantage of not being able to remove NO x. In fact, because
of its high temperature, still more NO x is formed. The cata-
lytic approach is to fi nd a catalyst or set of catalysts and
temperature or set of temperatures which will completely
oxidize both CO and HC, but also will reduce the NO x. These
two approaches will be discussed in detail below.
Meteorological and vehicle persistence factors have been
developed for estimation of 1 hour carbon monoxide concen-
trations defi ned as worst-case total persistence factors.^33IMPROVEMENT OF THE COMBUSTION PROCESSAs mentioned above the explosion of fuel accounts for
chemical pollutant formation. In the Rankine cycle engines,
liquids may continually be vaporized and recondensed, and
in the Sterling cycle engines a gas is repeatedly heated and
cooled. Both cycles are accomplished in a sealed container
and the heat used for the process comes from an open fl ame
external to the engine. Very little pollution is generated in
such a controlled rather than explosive fl ame.Exhaust gas recirculation (EGR) is used on American
Motors, Chevrolet and Chrysler automobiles. This includes
a diaphragm-actuated fl ow control valve located between
the exhaust and intake manifolds. The valve is operated by
ported vacuum directed through hoses and a coolant temper-
ature override. Permitting metered amounts of exhaust gases
to enter the intake manifold, which are mixed with incoming
fuel mixtures, lowers the combustion temperatures within
the cylinders. Reducing maximum cylinder combustion
temperatures minimizes the creation of Oxide of Nitrogen
(NO x ). EGR operation does not take place until engine oper-
ating temperature has reached a preset level and engine load
is suffi cient to permit proper EGR operation.
Almost all autos now contain a PVC system which
directs fi ltered air into the crankcase and channels vapors
but toward the manifold leading to the combustion chamber.
Fuel tank vapors are also concentrated by charcoal canisters
in American Motors vehicles for recycled combustion.
An H.E.W. report^5 discusses engine modifi cation sys-
tems, “Features shared by essentially all versions of the
engine modifi cation system include calibrated carburetors
that provide (a) relatively lean air–fuel mixtures for idle
and cruise operation and (b) higher engine idle speeds.
Refi ned control of spark timing is also used, and, in some
cases, regarded spark timing at idle is employed. In addition,
many engines are fi tted with special air cleaners and ducting
designed to supply heated air at nearly constant temperature
to the carburetor, to permit even leaner mixture settings.
Most versions also incorporate high-temperature radiator
thermostats to raise coolant temperatures, and thus improve
mixture distribution and promote complete combustion. In
some cases, higher capacity cooling systems are used to
handle the additional cooling load at idle that results from
wider throttle openings and retarded ignition timing during
this operating condition. In addition, combustion chamber
design attempts to avoid fl ame quenching zones where com-
bustion might otherwise be incomplete, and result in high
hydrocarbon emissions.”
Hydrocarbon and CO emissions are reduced by adjust-
ing the carburetor to a fuel-lean mixture during part throttleTABLE 1
Federal short cycleMode
Average acceleration
No. Type Speed range Time in mode (sec) Average speed (mph) rate (mph/sec)1 Accel 0–16 6.0 8.00 2.67
2 Accel 16–29 23.0 22.50 0.57
3 Cruise 29 10.0 29.00 0.00
4 Accel 29–37 18.0 33.00 0.44
5 Accel 37–42 4.5 39.50 1.11
6 Decel 42–37 2.5 39.50 2.00
7 Decel 37–20 32.0 28.50 -0.53
8 Decel 20–0 7.5 10.00 -2.67
9 Idle 0 21.5 0.00 0.00C013_005_r03.indd 702C013_005_r03.indd 702 11/18/2005 10:42:27 AM11/18/2005 10:42:27 AM