Ottimizzazione del Consumo Specifico e del Rumore Gasdinamico di un Motore VVA Bicilindrico Sovralimentato

The development of modern VVA systems recently offered new potentialities in improving fuel consumption for sparkignition engines at low and medium load, while granting higher volumetric efficiencies and performance at high load. The recently introduced ‘Multi-Air’ system, by Fiat Powertrain Group, enhanced this concept through the possibility of contemporary modify the intake valve opening, closing and lift, leading to the development of ‘throttle-less’ engines. However, at very low loads, the control of air-flow motion inside the cylinder may require to select a proper combination of butterfly throttling and intake valve control, to get the best BSFC improvement. Moreover, a reduced throttling, while improving the fuel consumption, may also produce a higher gasdynamic noise at the intake mouth. To this aim, a ‘Multi-lift’ strategy, along a single intake stroke can be analyzed, too. Of course, the introduction of new degrees of freedom for the engine control also 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, too. In this paper, an optimization procedure is presented, aiming to select the best combination of control parameters of a twincylinder turbocharged engine in different operating conditions. A detailed 1D simulation model of the whole engine is firstly developed and validated against experimental data in WOT conditions and in predefined low load operating points. The model is developed within GT-Power, commercial code environment, but employs a in-house developed routine to simulate the combustion process. The latter takes into account the effects on heat release induced by variations on in-cylinder flow field and gas composition. Both of them, in fact, are substantially modified by the actual strategy specified for the intake valve control. The 1D model is then coupled to an external optimizer (ModeFRONTIER) to perform the minimization of the fuel consumption at defined low-load operating points, with imposed constraints on the intake mouth radiated gasdynamic noise. An external module is employed to predict the overall intake valve lift profile as a function of two control parameters of the system (φ1 and φ2). The optimization procedure then selects the best combination of the above parameters together with the spark advance and WG / butterfly valve opening degree. The procedure validated against experimental data gives the possibility to perform a pre-calibration of the engine on a completely theoretical basis, and proves to be very helpful in reducing the experimental costs and time-to-market of new engine prototypes.