Optimization of Compression Ratio of a Boosted PFI SI Engine with Cooled EGR

This paper studies the effect of cooled EGR on fuel consumption and anti-knock performance of a boosted port fuel injection (PFI) SI engine. Experimental results show that the cooled EGR increases the thermal efficiency by 2%∼18% depending on the operation conditions. Compared to low load operations, more improvements of the thermal efficiency are obtained at higher loads, primarily owing to the enhanced anti-knock performance, advanced combustion phasing, elimination of fuel-rich operations as well as reduced heat transfer loss with cooled EGR. The anti-knock performance of cooled EGR provides further potential to improve the thermal efficiency by increasing the compression ratio. To this end, a 1-D thermodynamic model of the engine is built and calibrated using the GT-Power code. A knock prediction correlation considering EGR is developed and validated with the experimental data. The geometric compression ratio of the engine with EGR is optimized by a strategy combining artificial neural networks (ANNs) with genetic algorithm (GA) with the 1-D engine cycle simulations. A further improvement of the thermal efficiency is obtained without significant penalty of the engine torque output performance.