Numerical Study of the Potential of a Variable Compression Ratio Concept Applied to a Downsized Turbocharged VVA Spark Ignition Engine 2017-24-0015

Nowadays different technical solutions have been proposed to improve the performance of internal combustion engines, especially in terms of Brake Specific Fuel Consumption (BSFC). Its reduction of course contributes to comply with the CO₂ emissions legislation for vehicle homologation. Concerning the spark ignition engines, the downsizing coupled to turbocharging demonstrated a proper effectiveness to improve the BSFC at part load. On the other hand, at high load, the above solution highly penalizes the fuel consumption mainly because of knock onset, that obliges to degrade the combustion phasing and/or enrich the air/fuel mixture. A promising technique to cope with the above drawbacks consists in the Variable Compression Ratio (VCR) concept. An optimal Compression Ratio (CR) selection, in fact, allows for further improvements of the thermodynamic efficiency at part load, while at high load, it permits to mitigate knock propensity, resulting in more optimized combustions. Of course, the VCR implementation involves increased costs and mechanical complexity, which can be only accepted if actual and relevant efficiency benefits are achieved. In this work, the potential advantages of VCR technique are numerically investigated with reference to a small turbocharged SI engine. First, a 1D model of the tested engine is implemented in GT-Power™ framework and is integrated with “in-house developed” sub-models for the description of in-cylinder phenomena. The engine model with the standard CR, selected by the manufacturer, is validated against the experimental data over the complete range of speed and load levels. In a second stage, an engine calibration strategy is proposed, aiming to automatically identify, for each operating point, the optimal spark timing, throttle valve opening, intake valve strategy, air-to-fuel ratio and turbocharger setting, complying with proper limitations on allowable levels of boost pressure, in-cylinder peak pressure, turbine inlet temperature, and knock intensity. This effort is hence considered to numerically reproduce the actual engine calibration process, resulting in a realistic prediction of the performance maps, at various CRs. The calibration strategy, allowing to select the CR realizing the minimum BSFC for each operating condition, also defines a complete map of the VCR engine. Fixed and variable CR strategies, with two or multiple CR stages, are finally compared in terms of CO₂ emission over a WLTP driving cycle, with reference to a segment A vehicle, denoting interesting advantages for VCR solution.