Page 477 - Air Pollution Control Engineering

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Emerging Pollution Control Technologies 449

and dual-catalyst beds are both used for this. The most common catalyst consists of the
noble metals mixture of 70% platinum and 30% palladium in the shape of either pellets
or a monolith (honeycomblike structure). Typical automobile converters are about 160
3
in. in size.
The following subsections describe the various alternative engines under continuous
evaluation by the automobile industry, one or more of which may provide the answer to
reduced automotive air pollution and adequate transportation.

3.4.1. Internal Combustion Engines
Most of the current development centers on the reciprocating ICE. The catalytic
converter is a piece of plumbing added after emissions have been produced. Catalysts
in converters require that lead and other heavy metals be eliminated from the emissions
to reduce poisoning of the noble metal catalysts. Accomplishing this, however, requires
lowering the engine compression ratios so that the lower-octane fuel made without lead
additives can be burned without causing knocking in the engine. Unfortunately, this also
reduces the engine’s efficiency.
New systems are continually being developed to improve combustion efficiency and
lower emission. Most of these, in contrast to the catalytic or “after the emissions are
produced” method, are accounted for by changes in the engine itself. Examples of these
include the stratified charge systems, tapered cams, ultrasonic fuel atomizing, catalytic
fuel cracking, and engines that operate on a variable number of cylinders.
The German Porsche’s and the Japanese Honda’s stratified charge systems both appear
to be useful engine innovations for reducing emissions and improving fuel economy. A
potential advantage of these systems is that they enable the engine to accept a wide range
of fuels, which could make it possible to obtain better utilization of fuels from crude oil,
coal, or solid-waste sources. The Porsche engine inlet as shown in Fig. 1 has two combus-
tion chambers: main and auxiliary. A very weak mixture is supplied to the main chamber
through the conventional inlet valve. A rich yet combustible mixture is injected into the
auxiliary chamber where spark ignition occurs. The flame rapidly spreads from the calm
auxiliary chamber into and throughout the turbulent weak mixture in the main chamber.
Combustion occurs regularly and consistently over a wide range of speed and load con-
ditions. This combustion procedure results in low emissions and high fuel economy.
The stoichiometric combustion of gasoline with air results in an air-to-fuel ratio of
about 14.7 lb of air per pound of fuel. The Porsche engine runs well on air-to-fuel ratios
ranging from 0.8 to 2.2 times the stoichiometric rates. Maximum power is actually
attained at 0.9 times the stoichiometric ratio (3).
Honda’s compound vortex controlled combustion (CVCC) stratified charge engine is
similar to the Porsche system in that a precombustion chamber is used. The CVCC unit
uses three valves (two standard valves plus one valve for the precombustion chamber)
to cause the rich fuel–air mixture to ignite then swirl with minimum turbulence into the
main chamber to complete the combustion.
The use of tapered cams can help to reduce emissions. Normal engine valves are
opened and closed by cams that do not allow for any change in valve timing or lift with
changes in engine speed. At higher speeds, it is desirable to open the valves earlier and
keep them open longer to ensure the complete filling and emptying of the combustion