The Effects of EGR Composition on Combustion Performance and Efficiency

Because of the thermodynamic relationship of pressure, temperature and volume for processes which occur in an internal-combustion engine (ICE), and their relationship to ideal efficiency and efficiency-limiting phenomena e.g. knock in spark-ignition engines, changing the thermo-chemical properties of the in-cylinder charge should be considered as an increment in the development of the ICE engine for future efficiency improvements. Exhaust gas recirculation (EGR) in spark-ignited gasoline engines is one increment that has been made to alter the in-cylinder charge. EGR gives proven thermal efficiency benefits for SI engines which improve vehicle fuel economy, as demonstrated through literature and production applications. The thermal efficiency benefit of EGR is due to lower in-cylinder temperatures, reduced heat transfer and reduced pumping losses. The next major increment could be modifying the constituents of the EGR stream, potentially through the means of a membrane. By doing so, this can change both the chemical and thermodynamic properties of the in-cylinder gas mixture and therefore have an impact on combustion and efficiency. This study evaluated conventional EGR, CO₂-only and N₂-only EGR on a single-cylinder engine under non-knocking and knock limited conditions. Overall, this study demonstrated the impacts of CO₂-only and CO₂-reduced EGR on efficiency, combustion stability, burn rate, knock abatement and compression ratio on a single-cylinder engine. Through analysis with Chemkin and GT-Power, it also sought to explain the behaviors observed.