Abstract Nowadays different technical solutions have been proposed to improve the performance of internal combustion engines, especially in terms of Brake Speciic Fuel Consumption (BSFC). Its reduction of course contributes to comply with the CO 2 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 eficiency 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 eficiency beneits 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 deines a complete map of the VCR engine. Fixed and variable CR strategies, with two or multiple CR stages, are inally compared in terms of CO 2 emission over a WLTP driving cycle, with reference to a segment A vehicle, denoting interesting advantages for VCR solution. Introduction Automotive Internal Combustion Engines (ICEs) are characterized by an increasing architectural complexity aiming to reduce the Brake Speciic Fuel Consumption (BSFC), without penalizing power and torque performance, and to comply with the more and more stringent European law concerning the pollutants and CO 2 emissions [1]. In particular, since more highly loaded homologation driving cycles are going to be stated, such as World-wide harmonized Light vehicles Test Procedure (WLTP), the reduction of total emitted g(CO 2 )/km requires an improved fuel consumption also at medium-high load operation. This further demand also addresses the research of more advanced knock mitigation methods for Spark Ignition (SI) ICEs. Actually, many technical solutions have been already developed, or are currently under study, to reduce the fuel consumption. As an example, the “downsizing” concept, consisting in the design of boosted engines of reduced displacement [2], is widely adopted. It, in fact, allows for BSFC beneits, due to less part load throttling and mechanical frictions. Further fuel consumption improvements can be realized through the minimization of pumping losses, by means of Variable Valve Actuation (VVA) systems [3,4]. On the other hand, for a boosted downsized engine, knock or even pre-ignition phenomena [5,6] are strongly promoted. A lower knock propensity can be obtained by using fuels of high octane number [7,8,9,10,11] and/or anti-knock additives [12]. Another option is the adoption of external cooled exhaust gas recirculation [13,14,15,16,17,18,19,20,21], although it presents the drawbacks of dificult control during fast transients, higher cyclic variability and lower power output. Another interesting knock mitigation technique is represented by the injection of liquid water within the cylinders or in the intake ports [22,23,24]. The related evaporation heat, subtracted to the working luid, reduces the gas temperature during the compression stroke [22], hence substantially improving the knock resistance. Numerical Study of the Potential of a Variable Compression Ratio Concept Applied to a Downsized Turbocharged VVA Spark Ignition Engine 2017-24-0015 Published 09/04/2017 Luigi Teodosio, Vincenzo De Bellis, Fabio Bozza, and Daniela Tufano University of Naples CITATION: Teodosio, L., De Bellis, V., Bozza, F., and Tufano, D., "Numerical Study of the Potential of a Variable Compression Ratio Concept Applied to a Downsized Turbocharged VVA Spark Ignition Engine," SAE Technical Paper 2017-24-0015, 2017, doi:10.4271/2017-24-0015. Copyright © 2017 SAE International Downloaded from SAE International by Luigi Teodosio, Wednesday, July 19, 2017