Impact of Exhaust Gas Recirculation on Performance and Emissions of Free-Piston Electrical Generator Fueled by DME

Abstract

This study presents a novel and interesting investigation regarding the use of exhaust gas recirculation (EGR) in free-piston engines by exploring EGR impact on both engine performance and emissions. A single-cylinder, two-stroke free-piston engine with an electrical generator model was investigated under the influence of EGR. A new dimethyl ether (DME) reaction mechanism was modified and used to simulate the DME combustion process under homogenous charge compression ignition conditions. The combustion process was described using the one-dimensional approach in which the in-cylinder gas mixture was treated as a single-zone reactor. Chemical kinetics approach was applied to calculate the combustion heat releasing rate and to include EGR effects. Many engine parameters were investigated by applying EGR in different ratios and in different temperatures. The results showed that EGR influences the combustion process by shifting the top dead center location toward the cylinder head, which yields higher compression ratio but shorter ignition delay. The engine ran with less generator power output when EGR was applied but with higher overall efficiency, meaning that the engine could convert more energy out of the combustion process at constant or low generator power demands. The study includes EGR impact on engine emissions in which both oxides of nitrogen (NOx) and carbon monoxide (CO) emissions were investigated. Exhaust gas recirculation reduced the maximum bulk mean temperature of the gas mixture, which decreased the reaction rate of NOx emissions. When running under stoichiometric conditions, EGR seemed to decrease CO emission by reducing the dissociation rate of carbon dioxide. Increasing the intake temperature by applying hot EGR affected both the engine power and NOx emissions. A reduction in the engine efficiency and generator power output was observed as hot EGR was applied; additionally, high intake temperatures decreased EGR’s ability to reduce NOx.