Combined effects of cooled EGR and a higher geometric compression ratio on thermal efficiency improvement of a downsized boosted spark-ignition direct-injection engine

• Experiments for the effects of cooled EGR and two compression ratios (CR) on fuel efficiency were conducted.
• The mechanism for the observed fuel efficiency behaviors by cooled EGR and high CR was clarified.
• Cooled EGR offers more fuel efficiency improvement than elevating CR from 9.3 to 10.9.
• Combining 18–25% cooled EGR with 10.9 CR lead to 2.1–3.5% brake thermal efficiency improvements.

The downsized boosted spark-ignition direct-injection (SIDI) engine has proven to be one of the most promising concepts to improve vehicle fuel economy. However, the boosted engine is typically designed at a lower geometric compression ratio (CR) due to the increased knock tendency in comparison to naturally aspirated engines, limiting the potential of improving fuel economy. On the other hand, cooled exhaust gas recirculation (EGR) has drawn attention due to the potential to suppress knock and improve fuel economy. Combing the effects of boosting, increased CR and cooled EGR to further improve fuel economy within acceptable knock tolerance has been investigated using a 2.0 L downsized boosted SIDI engine over a wide range of engine operating conditions from 1000 rpm to 3000 rpm at low to high loads. To clarify the mechanism of this complicated effects, the first law of thermodynamics analysis was conducted with the inputs from GT-Power® engine simulation.Experiment results indicate that cooled EGR provides more brake thermal efficiency improvement than increasing geometric CR from 9.3 to 10.9. The benefit of brake thermal efficiency from the higher CR is limited to low load conditions. The attributes for improving brake thermal efficiency by cooled EGR include reduced heat transfer loss, reduced pumping work and increased ratio of specific heats for all the engine operating conditions, as well as higher degree of constant volume heat release only for the knock-limited high load conditions. The combined effects of 18–25% cooled EGR and 10.9 CR lead to 2.1–3.5% improvements in the brake thermal efficiency (6–9% improvements in the fuel economy) compared to the baseline (9.3 CR without EGR). Among several effects contributing to the fuel economy improvement, the theoretical thermal efficiency improvement is primary which is caused by increased ratio of specific heats and increased geometric CR. The reduction in heat transfer loss due to lower combustion temperature with EGR is the secondary effect.