Modern VVA systems offer new potentialities in improving the
fuel consumption for spark-ignition engines at low and medium load,
meanwhile they grant a higher volumetric efficiency and performance
at high load. Recently introduced systems enhance this concept
through the possibility of concurrently modifying the intake valve
opening, closing and lift leading to the development of almost
"throttle-less" engines. However, at very low loads, the
control of the air-flow motion and the turbulence intensity inside
the cylinder may require to select a proper combination of the
butterfly throttling and the intake valve control, to get the
highest BSFC (Brake Specific Fuel Consumption) reduction. Moreover,
a low throttling, while improving the fuel consumption, may also
produce an increased gas-dynamic noise at the intake mouth.
In highly "downsized" engines, the intake valve
control is also linked to the turbocharger operating point, which
may be changed by acting on the waste-gate valve. Depending on the
valve lift and the actual exhaust pressure, different internal-EGR
levels can be achieved, requiring a variation in the spark advance
as well. Of course, the introduction of new degrees of freedom to
the engine control poses new problems in terms of engine
calibration and tuning. To best exploit the potential offered by
these systems, new methodologies and development tools must be
utilized as well.
In this paper, an optimization procedure is presented, aiming to
select the best combination of four control parameters (intake
valve closure angle, butterfly valve opening, waste-gate opening
and spark advance) of a twin-cylinder turbocharged engine in
different operating conditions. A detailed 1D simulation model of
the whole engine is firstly developed and validated against the
experimental data in WOT conditions and in predefined low load
operating points. The model is developed within GT-Power™
commercial code environment, but employs an in-house routine that
is implemented to simulate the combustion process. This routine
takes into account the effects on the heat release induced by the
variations of the in-cylinder flow field, turbulence and
internal-EGR. These, in fact, are substantially modified by the
actual strategy specified for the engine control. An external
module is implemented to evaluate the intake valve lift profile as
a function of the closure angle and the engine speed. The 1D
prediction of the gas-dynamic noise at the intake mouth is further
validated through the comparison with the results obtained by the
unsteady 3D CFD analysis of the flow field within the air-box
device. The 1D computed pressure profile downstream the device is
utilized as boundary condition for the 3D model.
The validated 1D model of the turbocharged engine is then
coupled to a multipurpose commercial optimizer (ModeFRONTIER™).
The optimization procedure identifies the best combinations of the
previously selected control parameters with the goal of minimizing
both the fuel consumption and the gas-dynamic noise, for defined
low-load low-speed operating points. The procedure is validated
against the experimental data obtained by a calibration of the
engine at the test-bed. The values of the control parameters
selected by the optimizer well agree with the experimentally
identified ones. Alternative settings are also proposed with the
aim to substantially reducing the radiated noise with limited or no
penalty for the BSFC. The optimization results are also verified by
a 3D CFD analysis.
Therefore, the procedure allows for the pre-calibration of a VVA
turbocharged SI ICE on completely theoretical basis and proves to
be very helpful in reducing the experimental costs and the engine
time-to-market.