In this work different internal and external EGR strategies,
combined with extreme Miller cycles, were analyzed by means of a
one-dimensional CFD simulation code for a Wärtsilä 6-cylinder,
4-strokes, medium-speed marine diesel engine, to evaluate their
potential in order to reach the IMO Tier 3 NOx emissions
target.
By means of extreme Miller cycles, with Early Intake Valve
Closures (up to 100 crank angle degrees before BDC), a shorter
compression stroke and lower charge temperatures inside the
cylinder can be achieved and thanks to the cooler combustion
process, the NOx-specific emissions can be effectively reduced.
EIVC strategies can also be combined with reductions of the
scavenging period (valve overlap) to increase the amount of exhaust
gases in the combustion chamber. However, the remarkably high boost
pressure levels needed for such extreme Miller cycles, require
mandatorily the use of two-stage turbocharging systems. Despite two
stages turbocharging, combined with extreme Miller timings, may
allow up to 50% NOx reduction compared to a conventional,
single-stage turbocharger architecture, further NOx emissions
reductions are necessary to meet the IMO Tier 3 NOx limit. Higher
EGR percentages, which could be achieved by means of external
circuits, were therefore also evaluated. However, it should be
pointed out that, although the external EGR technology is well
established for automotive and heavy-duty diesel engines, it is not
yet state of the art for marine diesel engines, and its application
to a highly boosted engine using extreme Miller timings is not
straightforward.
Several different complex EGR routes were thus investigated, and
for the preliminary assessment of their NOx emissions abatement
potentialities, the use of numerical simulation allowed a detailed
and extensive evaluation of the effects on engine performance, fuel
consumption, NOx emissions and thermal and mechanical loads on
engine components of the combination of different intake valve
profiles, intake valve closure timings and scavenging periods with
different external exhaust gas recirculation solutions. Percentages
of exhaust gases recirculated in the combustion chamber up to 20%
were evaluated that combined with extreme Miller timings (up to 100
crank angle degrees before BDC) allowed up to 90% NOx reduction
compared to a conventional, single-stage turbocharger architecture,
with only moderate BSFC increase.