An Experimental Based Investigation of Oxycombustion in an SI Engine
This work investigates oxycombustion in a spark-ignited (SI), piston engine. Oxycombustion is the process in which nearly pure oxygen and fuel are burned in a diluent of exhaust gas which is recirculated from the exhaust to the intake, referred to as exhaust gas recirculation (EGR). In typical air burning engines, EGR is used to reduce combustion temperatures, thereby reducing the formation of the pollutant nitric oxides (NOx). Conversely, the motivation for power generation using oxycombustion with EGR is the ease with which carbon dioxide (CO2) can be isolated in the exhaust for sequestration through a carbon capture and sequestration (CCS) process. At similar operating points the negative effect of EGR on the thermal efficiency of traditional air engines is magnified in oxycombustion due to the very high EGR levels. Therefore, it is the objective of this study to determine where the operating limits can be extended beyond the normal limits of air systems, regaining lost thermal efficiency.
Two main EGR diluents are considered: dry EGR which is primarily CO2, and wet EGR, which includes both CO2 and water. Dry EGR has been found to have significantly higher knock resistance than air, allowing operation at higher compression ratios (CR) which can ultimately produce higher thermal efficiencies than air operation when combusting a low octane fuel. In fact, dry EGR produced higher thermal efficiencies when operating with low octane fuels (which have very poor knock resistance) than it did with high octane fuels (which have very high knock resistance). The primary reason for this anomaly was determined to be minor auto-ignition events, which significantly decrease combustion duration but do not produce significant knock, occurring with the low octane fuel. When operation is limited by engine configuration rather than knocking, as with methane, the performance of dry EGR is very low relative to air, roughly proportional to the decrease in theoretical thermal efficiency due to the decrease in the ratio of specific heats. In order to take advantage of the improved knock resistance it is necessary to operate in a region with a high enough CR and low enough oxygen concentration that minor auto-ignition events are common. Wet EGR produces similar trends as dry EGR, but cannot attain the high CRs which dry EGR operated at due to the elevated intake temperature required to prevent water condensation. Ultimately, despite the chemical and thermal advantages of water compared to CO2 in regards to laminar flame speeds, wet EGR with high water content is not a reasonable choice when operating in an IC engine. Despite its limitations in an IC engine, wet EGR has several advantages which should be carefully considered in applications where high initial temperatures are not propagated negatively.