The above values were held fixed for the calculation
of any operating condition of the engine. In addition, they
are very similar to the ones identified for the simulation
of a very different research engine [5]. This practically
means that the model description of the kernel duration,
initial flame development, turbulent flame propagation
and combustion termination, is able to reproduce the
underlying physics in a satisfactory way.
Fig. 5-Fig. 7 report the mean performance
parameters of the engine at WOT. Agreement with the
experimental data is satisfactory all over the engine
speed range. An increased volumetric efficiency can be
observed at about 3000 rpm (Fig. 5). The 1D
schematization of the intake and exhaust pipe network
probably determines an overestimation of the gas-
dynamic tuning at this particular engine speed. The latter
inaccuracy also reflects in the IMEP, BMEP, Power and
BSFC profiles in Fig. 6 and Fig. 7.
A more detailed comparison is presented in Fig. 8-
Fig. 13, in terms of instantaneous pressure cycles at
WOT. The figures also report the pressure cycles
computed with the base version of the model (eq. (2)).
The correction proposed (eq. (16)) enhances, as
expected, the burning rate at high speeds, and
considerably improves the prediction of the in-cylinder
pressure peak, especially in the medium-speed range. In
each operating condition, combustion start, maximum
pressure and expansion phase are well reproduced by
the improved fractal model.
In order to check the model accuracy at part-load
too, some analyses were carried out at fixed rotational
speed (2000 rpm) and for different load levels (WOT and
2.3 bars BMEP). Two different VVT positions (0° and 25°
cam angles) were also analyzed. The results obtained
are summarized in Fig. 14-Fig. 16. The prediction of the
pressure cycle is satisfactory also in these more critical
operating conditions on both high pressure cycle (Fig.
14) and mass exchange phase (Fig. 15). Fig. 15
particularly puts into evidence the strong reduction of the
pumping work achieved through a delayed camshaft
position, reflecting in a relevant BSFC improvement. The
coupled effects of the spark advance, residual fraction
level at intake valve closure, and valve timing really
determine a very different development of the
combustion process, as shown in Fig. 16.
In the tested operating conditions, a maximum EGR
level of about 24% was reached (Fig. 16). A further
validation of the combustion model with a percentage of
residual gases greater than 30-40% is however required
to fully asses the model accuracy.
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