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

Paper #:
  • 2017-24-0015

  • 2017-09-04
Nowadays different technical solutions have been proposed to improve the performance of internal combustion engines, especially in terms of Brake Specific Fuel Consumption (BSFC). As known, the latter has to be reduced 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 fuel economy 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 CR selection, in fact, allows for further improvements of the thermodynamic efficiency at part load, while at high loads, it permits to mitigate knock propensity, resulting in an enhanced fuel economy. 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, 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. Any effort is hence considered to numerically reproduce the actual engine calibration process, resulting in a realistic prediction of the performance maps, at various Compression Ratios. 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.
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