Strategies for Improving Fuel Consumption at Part-Load in a Downsized Turbocharged SI Engine: a Comparative Study

摘要

It is commonly recognized that the paths for improving fuel consumption (BSFC) in a spark-ignition engine at part-load require more advanced valve actuation strategies, which largely affect the pumping work. Since several years, many different solutions have been proposed, characterized by different levels of complexity, effectiveness, and cost. Valve systems currently available on the market allow for variable phasing (VVT - Variable Valve Timing), and/or lift (VVA - Variable Valve Actuation). Usually VVT devices are applied on intake and exhaust camshafts, in the "phased" or "unphased" configuration, as well. VVA devices are instead commonly mounted on the intake camshaft. More recent VVA systems also allow for a double intake valve lift during a single engine cycle (multi-lift), or may include a small intake pre-lift during the exhaust stroke. The latter solutions may determine further BSFC reductions. Alternatively, an external-EGR circuit can be considered, as well. Each system introduces additional parameters to the standard engine control variables (throttle opening, spark advance, waste-gate valve opening in turbocharged engines). In order to quantitatively compare the potentialities of the above systems, proper numerical models can be employed. In this paper, a comparative study is performed on a small-size turbocharged engine to estimate the maximum BSFC improvement which can be obtained with the employment of the above mentioned advanced techniques. Firstly, a 1D model of a commercial engine, equipped with a VVA device, is validated at full and part-load; then, different valve and EGR strategies are investigated starting from the base engine architecture. Analyses are carried out with the commercial 1D thermo fluid-dynamic code GT-Power, provided with user routines for the modeling of the combustion process and the handling of different valve strategies. The 1D model is coupled to an external optimizer (modeFRONTIER) which selects the various engine control parameters to minimize the fuel consumption at a prescribed part load and speed condition.