Environmental Engineering Reference
In-Depth Information
of nitrogen (NO
x
), hydrocarbons (HC), sulfur dioxide (SO
2
), and particulate matter (PM), are
distributed geographically in proportion to vehicle usage, which is concentrated in urban regions.
But the secondary pollutants photochemically formed from the direct emissions of NO
x
and HC
can reach elevated levels downwind from the vehicular sources and outside the urban region. As
a consequence, all primary emissions are (or soon will be) regulated to ensure that primary and
secondary air pollutant levels do not exceed harmful levels, either locally or regionally.
In this chapter we discuss the technology of the automobile as it affects the efficiency of fuel
use and the emission of atmospheric pollutants. The chapter emphasizes the cause and amelioration
of ordinary air pollutant emissions as well as the reduction of CO
2
emissions by improvements
in vehicle fuel efficiency. We summarize the current state of development of alternative vehicle
systems that show promise of significant emission reductions, including electric drive vehicles
(battery-powered or fuel-cell-powered) and hybrid electric/ICE drive vehicles.
8.2
INTERNAL COMBUSTION ENGINES FOR
HIGHWAY VEHICLES
The most common engine in road vehicles is the gasoline-fueled spark ignition (SI) reciprocat-
ing engine. It is economical to manufacture and maintain, provides ample power per unit weight,
and has a useful life that equals that of the typical passenger vehicle. The less common alterna-
tive, the diesel-fueled compression ignition (CI) reciprocating engine, is more durable and more
fuel efficient, but is more expensive to manufacture and provides less power per unit weight (un-
less turbocharged).
5
Each engine type has different air pollutant emissions and requires different
technology to reduce them.
The SI and CI engines, developed at a time when the dominant steam engine was a reciprocating
device, utilize the same mechanism of a movable piston within a closed-end cylinder linked by
a connecting rod to a rotating crankshaft that converts the reciprocating motion of the piston to
the rotary motion of the crankshaft [see Figure 8.2(a)]. High-pressure gas in the cylinder exerts
an outward force on the piston, doing mechanical work on the crankshaft as the piston recedes on
the power stroke. On the return inward stroke, where the cylinder is filled initially with low-pressure
gas, less work is done by the crankshaft on the piston than was done on the crankshaft during the
outward stroke, so the crankshaft delivers net positive work to the mechanism the engine is turning.
A flywheel is required to smooth out the rotational speed of the crankshaft while the piston exerts
variable amounts of torque during its reciprocating motion.
As previously explained in Section 3.10 on the Otto cycle, a high pressure is generated in
the cylinder just at the beginning of the outward expansion stroke by burning a fuel with air
in the cylinder. In the SI engine, a fuel-air mixture, prepared outside the cylinder, flows into
it prior to the inward compression stroke, eventually being ignited by a spark when the piston
reaches its innermost position (called top center, TC). On the other hand, for the CI engine, pure
air is ingested prior to the compression stroke and the fuel is sprayed into the air at the end of
5
The CI engine, in contrast to the SI engine, does not lend itself to light, low-power uses such as motorcycles,
chain saws, lawn mowers, and so on, or for use in small aircraft, where engine weight is important. On the other
hand, large reciprocating engines for construction equipment, ships, and railroad locomotives are invariably
CI engines.
