Fuel Consumption Optimization and Noise Reduction in a Spark-Ignition Turbocharged VVA Engine

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.