EGR Tolerance of Synthetic Gasoline Fuels in a DISI Engine: Fuel Effects on Performance and Emissions

Drop-in synthetic gasoline fuels are an attractive alternative to traditional fossil fuels for transportation due to their high energy density, compatibility with the existing fleet and potential to decrease carbon intensity. Despite of meeting gasoline standards, the composition of these fuels can vary depending on the feedstock used for production and production process, which has shown to affect engine performance and emissions. This study investigated the effects of synthetic fuel composition on combustion in a direct-injection spark-ignition engine. Spark timing sweeps from the stability limit to the knock limit were performed with three different bio-fuels, methanol-to-gasoline, ethanol-to-gasoline and hydrotreated-biomass gasoline, at different loads, speeds and exhaust gas recirculation (EGR) rates, and results were compared against a research-grade E10 (10%vol ethanol) regular gasoline representative of petroleum gasoline available in the US. Octane index analyses showed that knock resistance differences between fuels can be explained by their octane rating at low and medium loads and, partially, at different engine speeds. However, octane rating cannot explain differences observed at high load and, especially, when EGR is added. Results demonstrated that adding EGR at medium loads is a very effective approach to increase efficiency despite of increasing burn duration because higher EGR rates led to lower pumping loses and lower heat transfer, while keeping combustion efficiency constant. The impact of EGR on combustion has shown to be very sensitive to fuel composition, and the knock resistance of fuels with strong low-temperature chemistry increased more with EGR addition that that of fuels with mild low-temperature chemistry. Similarly, the early flame propagation of fuels with strong low-temperature chemistry is more affected by EGR, limiting retardability and EGR tolerance. Results from this study indicated that, despite of being considered drop-in, composition variability of synthetic fuels can be leveraged to improve engine performance and emissions.