Effect of Exhaust Gas Recirculation and Intake Air E-Boosting on Gasoline Compression Ignition Combustion

This experimental study aims to evaluate the engine performance and emissions when exhaust gas recirculation (EGR) and e-boosting are used in a gasoline compression ignition (GCI) engine operating at 2000 rpm and 800-900 kPa indicated mean effective pressure (IMEP) conditions. In an automotive size common-rail diesel engine architecture, a partially premixed charge-based GCI combustion was realized implementing triple injections with a split ratio of 50%, 10%, and 40% and injection timings of 170, 40, and 9-6 crank angle degrees (°CA) before top dead center (bTDC). The previous tests performed in the same engine suggested this injection strategy could achieve further nitrogen oxides (NO_x) reduction if EGR is utilized with the help of intake air boosting to compensate for the loss in power output and engine efficiency. In the present study, the GCI engine is set up with a conventional EGR system and a supercharger driven by an electric motor (or an e-booster). Each EGR and e-boosting effect was systematically evaluated, and the tests were repeated not only for GCI combustion but also diesel combustion, as a reference case. This study found that the charge dilution and reduced combustion temperature due to 16% EGR achieve over 50% NO_x reduction and 5% noise reduction, but only at the expense of reduced engine efficiency and increased emissions of unburnt hydrocarbon (uHC), carbon monoxide (CO), and smoke. The test on e-boosting showed that the efficiency loss caused by EGR could be fully recovered with only 10 kPa of intake air boosting. At higher boosting pressure of 30 kPa, GCI combustion achieved not only 11% efficiency improvement but also 75% NO_x and 5.6% noise reduction compared with no EGR and no e-boosting baseline condition. This was significant because uHC and CO emissions were also reduced due to enhanced oxidation. It was found that the sensitivity to charge dilution with EGR and e-boosting is much higher for GCI combustion than that for diesel combustion. This was due to a higher mixture homogeneity expected for GCI combustion with the EGR diluted and higher-density air being mixed better with fuel. The results were a higher NO_x reduction rate for the same EGR ratio and a much more effective power/efficiency recovery using e-boosting.