Transportation - Energy and the Environment - page 218

to low values as the extra oxygen finds and oxidizes them. On the other hand, NO is formed by
the reaction of N 2 with O 2 at the high temperature behind the flame front, but only in very small
quantities, and doesn't revert completely to N 2 and O 2 at the much lower exhaust temperature, as
it should if thermochemical equilibrium prevailed. It is highest at or close to the stoichiometric
mixture where the flame temperature is highest. The mass of HC and NO can be of the order of 1
% of the fuel mass (about 0.1% of the exhaust gas mass), but the CO mass is 10 times larger. Such
values are 10 or more times higher than allowed by future U.S. exhaust emission standards.
For pollutants to reach the very low levels now being required of road vehicle exhaust streams,
two steps must be undertaken simultaneously. The first is to reduce as much as possible the pollutant
concentrations in the exhaust gas as it leaves the engine (engine-out emissions); the second is to
reduce these emissions even further by exhaust gas treatment systems located between the engine
and the tailpipe. Neither of these systems by itself can suffice to clean up the emissions to the
required low levels.
8.7.2.1 Reducing Engine-Out Emissions
There are several features of modern SI engines that are nearly universally used to improve engine-
out emissions.
Precise Control of Air/Fuel Ratio. Low values of the three principal pollutants—HC, CO, and
NO—can be maintained if the air/fuel ratio is kept close to its stoichiometric value under all
operating conditions. Fuel injection permits close control over fuel flow to each cylinder, and it
can be computer-controlled to be proportionate to the intake air flow. An oxygen detector placed
downstream of the exhaust ports provides a sensitive signal used to correct the fuel flow so as to
home in on the desired air/fuel ratio. A further benefit of this control system is that it can provide
optimum conditions for subsequent exhaust gas processing.
Exhaust Gas Recirculation. At the end of the exhaust stroke, when the exhaust valve has closed
and the intake valve opens to admit a fresh charge of air-fuel mixture, the residual volume of
the cylinder is filled with exhaust gas. This mixes with the incoming fresh charge, diluting it
and reducing the temperature and pressure that is reached when that charge is fully burned at the
beginning of the power stroke. Because the amount of NO formed is very sensitive to the peak
temperature reached during combustion, we can reduce engine-out NO by diluting the fresh charge
with even more exhaust gas than is normally encountered. This can be done by varying the exhaust
and inlet valve timing or pumping exhaust gas from the exhaust system into the intake system.
This is done at part load so the maximum engine torque and power are not compromised, but is
acceptable because these maximum values are seldom utilized in standard driving cycles.
8.7.2.2 Catalytic Converters for Exhaust
Gas Treatment
The exhaust gas pollutants—HC, CO, and NO—are not in thermochemical equilibrium with the
rest of the exhaust gas. It should be possible to oxidize both HC and CO to CO 2 and H 2 Oif
enough oxygen is present, and to reduce NO to N 2 and O 2 , because these are thermodynamically
favored. To make this happen quickly enough, these molecules must attach themselves to a solid