Thermodynamic analysis for improving the efficiency of SI engine at part-load conditions

In this study, a new thermodynamic model was developed to specifically investigate part-load conditions, where efficiency typically declines in spark-ignition (SI) engines. The model calculates pumping and net works, volumetric and indicated efficiencies, with a particular focus on throttling losses during the exhaust and intake, enabling realistic prediction of an actual engine behavior. After validation with experimental results, five commonly used methods were analyzed for their potential to improve part-load efficiency: Variable Valve Timing (VVT), Variable Compression Ratio (VCR), Variable Stroke Volume (VSV), Stratified Charge (StC), and Turbocharging (TC). While each method offers distinct advantages in reducing pumping losses and enhancing efficiency, this study also evaluates their combined effects. Results show that triple-method combinations yield significant efficiency improvements, with the combination of StC, VSV, and VCR achieving the highest indicated and volumetric efficiencies with reduced pumping losses. Adding a fourth strategy or integrating all methods did not produce substantial additional gains. These findings highlight the potential of combined strategies for part-load efficiency improvements. At 0.3 MPa IMEP, indicated power of 9 kW, the combined methods can achieve up to a 55 % reduction in fuel consumption and corresponding CO₂ emissions compared to the reference engine. The indicated efficiency improvement rises to approximately 62 % at 0.2 MPa IMEP, and decreases to around 50 % at 0.4 MPa IMEP, corresponding to indicated powers of 6 and 12 kW, respectively.