Environmental Engineering Reference
In-Depth Information
8.7.2.4 Reducing CI Engine Emissions
Diesel (CI) engines generally emit lesser amounts of CO and HC than SI engines, but more of
NO and particulate matter. The overall air/fuel ratio in a CI engine is always lean, the more so
at partial power levels, providing excess oxygen to oxidize HC and CO to CO
2
and H
2
O, and
N
2
to NO. But the evaporation and mixing of the fuel droplets in a CI engine with surrounding
air is uneven, so that some of the fuel burns in an oxygen-deficient atmosphere, giving rise to
tiny solid elemental carbon (soot) particles. Most of this particulate matter eventually is oxidized,
but some does not and remains unburned in the engine exhaust stream. Soot particles may be
coated with low-volatility hydrocarbon molecules (polycyclic aromatic compounds, PAH) which
are toxic to humans. Both NO and PM emissions are more difficult to control in CI engines than
in SI engines.
Various modifications to direct injection diesel engine combustion are being tried to reduce
both NO and PM engine-out emissions. One approach is to increase the fuel injection pressure and
control its timing so as to provide more uniform fuel-air mixing and thereby better combustion
conditions. Four-valve cylinders and exhaust gas recycling also help to control the combustion
process so as to reduce emissions. If the mixture of fuel and air within the cylinder can be made
nearly uniform (called homogeneous charge compression ignition), emissions are significantly
reduced, at least at part load.
The use of catalytic converters to reduce NO molecules in the engine exhaust is less successful
for CI engines than for SI engines because there are fewer hydrogen-containing molecules needed
for catalytic reduction of NO. Injecting small amounts of fuel into the exhaust improves NO
catalytic reduction. Nevertheless, substantial catalytic conversion will be necessary to meet Tier 2
NO
x
emissions standards for CI-powered vehicles.
Catalytic converters oxidize some of the engine-out PM, but not enough to meet future PM
emissions standards. Particle filters can further reduce tailpipe PM emissions, but periodic cleaning
of the filters by catalytic combustion or other means is necessary to ensure reliable PM reduction.
Particle filters have not yet reached the level of development of catalytic converters, but are likely
to be necessary for future diesel-powered light duty vehicles in the United States.
CI engine emissions are affected somewhat by the composition of diesel fuel. The greatest
effect is due to fuel sulfur, which burns to SO
2
and hampers NO reduction in the catalytic converter.
The sulfur content of vehicle fuels is currently regulated by the U.S. EPA to maintain good lifetime
performance of catalytic converters.
The superior fuel efficiency of direct injection CI engines is a strong incentive for their use
where consumer fuel cost is high. In the United States, where fuel prices are low, the fuel efficiency
incentive resides with the manufacturer, who must meet CAFE standards, especially difficult for
the current light-duty truck market. Should reduction of vehicle carbon dioxide emissions become
a public policy goal in the United States, thereby encouraging greater use of CI engines in the
passenger vehicle market, more intense development of CI engine emission control technology
will be necessary to meet expected emission standards.
8.7.2.5 Fuel Quality and Its Regulation
We have already noted that fuel anti-knock lead additives and sulfur have been restricted to ensure
the successful operation of exhaust gas catalytic converters. Other regulation of fuel properties
have been directed at both exhaust and evaporative emissions.
