Abstract
Exhaust gas recirculation (EGR) is effective to reduce nitrogen oxides (NOx) from Diesel engines because
it lowers the flame temperature and the oxygen concentration of the working fluid in the combustion
chamber. However, as NOx reduces, particulate matter (PM) increases, resulting from the lowered oxygen
concentration. When EGR further increases, the engine operation reaches zones with higher instabilities,
increased carbonaceous emissions and even power losses. In this research, the paths and limits to reduce
NOx emissions from Diesel engines are briefly reviewed, and the inevitable uses of EGR are highlighted.
The impact of EGR on Diesel operations is analyzed and a variety of ways to implement EGR are outlined.
Thereafter, new concepts regarding EGR stream treatment and EGR hydrogen reforming are proposed.

2003 Elsevier Ltd. All rights reserved.
Keywords: Diesel engine; EGR; NOx; Lean burn; Gaseous fuel; Energy efficiency; Aftertreatment
1. Introduction
Diesel engines have inherently high thermal efficiencies, resulting from their high compression
ratio and fuel lean operation. The high compression ratio produces the high temperatures required
to achieve auto-ignition, and the resulting high expansion ratio makes the engine discharge
less thermal energy in the exhaust. The extra oxygen in the cylinders is necessary to facilitate
complete combustion and to compensate for non-homogeneity in the fuel distribution. However high flame temperatures predominate because locally stoichiometric air–fuel ratios prevail in such
heterogeneous combustion processes [1]. Consequently, Diesel engine combustion generates large
amounts of NOx because of the high flame temperature in the presence of abundant oxygen and
nitrogen [2,3].
Diesel engines are lean burn systems when overall air–fuel ratios are considered, commonly
with an air excess ratio k ¼ 1:5–1.8 on full loads and higher k values as load reduces. During
idling, for instance, the air to fuel ratio of a modern Diesel engine can be 10-fold higher than that
of stoichiometric engines (k > 10). However, diffusion controlled Diesel combustion is predominately
stoichiometric burn, in a microscopic sense, because the flames are prone to localize at
approximately stoichiometric regions within the overall fuel lean but heterogeneous mixture. The
prevailing flame temperature can be estimated with adiabatic stoichiometric flame temperature
calculations [1,4]. For a given engine speed, it is obvious that the NOx generation rate is closely
related to the fueling rate, the engine load level. On a power generation basis, therefore, the decrease
in overall mixture strength will not drastically reduce the specific rate of NOx generation.
Unlike Diesel engines, homogeneously charged engines, such as spark ignited gasoline engines
or other gaseous fuel engines, can actually use k control to reduce NOx effectively. To a homogeneous
charge, the weakening in mixture strength can effectively reduce the flame temperature
and propagation speed. An excessively fuel lean mixture, k > 1:2–1.4 (depending on the type of
fuel), could produce substantially lowered NOx emissions [4–8]. The trend in NOx reduction
enhances with further weakening of the cylinder charge until sustainable flame propagation becomes
unreliable and unburned combustibles intolerable. When an extremely lean mixture is used,
for instance when k
1:8, a homogeneous charge compression ignition (HCCI) concept could be
applied, where the engine operation improves fuel economy through nearly instantaneous

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