In diesel engines the optimization of engine-out emissions,
combustion noise and fuel consumption requires the experimental
investigation of the effects of different injection strategies as
well as of a large number of engine operating variables, such as
scheduling of pilot and after pulses, rail pressure, EGR rate and
swirl level. Due to the high number of testing conditions involved
full factorial approaches are not viable, whereas Design of
Experiment techniques have demonstrated to be a valid methodology.
However, the results obtained with such techniques require a
subsequent critical analysis, so as to investigate the cause and
effect relationships between the set of engine operating variables
and the combustion process characteristics that affect pollutant
formation, noise of combustion and engine efficiency.
To this purpose, the zero-dimensional multizone diagnostic
combustion model developed at ICEAL was applied for the combustion
and emission formation analysis in two different diesel engines,
for various sets of injection strategies and engine operating
parameters. The experimental data were acquired at the highly
dynamic test rig of ICEAL, both in a EURO V low compression ratio
diesel engine with a twin-stage turbocharger, equipped with
piezoelectric injectors, and in a PCCI low compression ratio diesel
engine equipped with solenoid injectors. The model results were
discussed and reported in the well-known ᵩ-T diagrams, which give a
synthetic representation of the local thermodynamic charge
conditions during the mixture formation and premixed-diffusion
combustion processes. The rail pressure increase was found to be an
effective means to improve fuel-charge premixing and to lower the
average local equivalence ratio of the charge during premixed
combustion, so leading to a decrease in soot formation. As regards
the effects of cooled high-pressure EGR on combustion, it was shown
that an increase of its rate does not significantly affect the
average equivalence ratio during premixed combustion, which is
associated to the soot formation rate. Finally, a proper
calibration of the dwell-time between pilot and main injection, so
as to have the main injection pulse center-of-gravity phased in
correspondence to the start of pilot burning, resulted to produce a
reduction of CO and soot emissions higher than 50% with respect to
baseline case, without any deterioration of NOx
emissions.