Page 296 - Air pollution and greenhouse gases from basic concepts to engineering applications for air emission control

9.6 Engine Exhaust Gas Recirculation 273

9.6 Engine Exhaust Gas Recirculation

The major air emissions from an engine are CO, total hydrogen carbon (HC), NO x ,
and soot, which take up only 1 % of the exhaust gas. The rest are CO 2 ,H 2 O, N 2 ,
and O 2 . The motivation of engine EGR is to further reduce these trace level
emissions with minimal compensation in power and thermal efficiency.
Similar to FGR approach for stationary combustion processes, engine exhaust
can also be recirculated for the reduction of NO x emission from engines [1, 23].
With more and more stringent limits for NO x emissions, further reductions in NO x
emissions from mobile sources without notable sacrifice to engine power have
become more and more challenging. These challenges have led to recent growing
R&D projects in engine EGR systems. EGR has become one of the essential
techniques for engine emission reduction.
When EGR system is installed the engine intake consists of fresh air and
recirculated exhaust. The percentage EGR can be defined as

EGR
r EGR ¼ ð9:12Þ
Air þ Fuel þ EGR
where EGR, Air and Fuel are the amount, mostly in terms of mass, of EGR, air and
fuel, respectively. They can be measured in mass or volume. By mass up to 30 % of
the engine exhaust can be recirculated whereas the ratio can reach 50 % by volume.
A more practical EGR ratio is based on the CO 2 concentration.
c CO 2 ;in
r EGR ¼ ð9:13Þ
c CO 2 ;ex

where c CO 2 ;in is the intake CO 2 concentration and c CO 2 ;ex is the exhaust CO 2
concentration.
Fresh air contains negligible amount of CO 2 compared to that in the engine
exhaust. Since CO 2 is primarily a combustion product. The recycled engine exhaust
carries a substantial amount of CO 2 that increases with EGR flow rate and engine
loads.
CO, HC, NO x and soot account for less than 1 % of the engine exhaust and their
relative abundance depends on the engine load. The corresponding oxygen level
varies from 5 % at full load to 20 % during idling. Therefore, the effectiveness of
NO x reduction by EGR also depends on engine load.
Diesel engines can operate with a high EGR ratio because the exhaust contains a
high concentration of O 2 and low concentrations of CO 2 and H 2 O. In addition, test
results showed that cooled EGR reduces NO x more effectively than hot EGR [20].
Unlike the exhaust from other engines with premixture aiming at certain air-fuel
ratios, the relative amount of gases in a diesel exhaust depends very much on the
diesel engine load. A diesel engine adjusts its fuel injection rate according to the
engine load. As a result, the oxygen concentration in the diesel engine exhaust
varies with the engine load. The oxygen concentration may vary from 5 to 20 %