Suppression of Aromatic Volatility in SI-Engines via Low-Molecular-Weight Oxygenates

Gasoline direct-injection (GDI) engines are increasing market penetration. They are attractive because they substantially decrease CO₂ emissions and can increase fuel economy. However, due to their design, GDI engines are prone to increases in soot production. Blends of alcohols with gasoline have been observed to decrease soot production in GDI engines. However, results have been mixed, with publications suggesting either a decrease or an increase in soot production. Recent publications have indicated that increases in soot production are tied to fuel impingement onto the cylinder head during high-load engine periods. The work presented here utilizes an equation of state (EoS) model to understand the volatility characteristics of oxygenate-surrogate fuel blends, focusing on the volatility of aromatics. EoS calculations are rapid, and allow for the simulation of a broad range of temperatures and pressures. Results with low-molecular-weight alcohols (methanol and ethanol) reveal two major regions of compositional space: low-volume and moderate-volume blends. In the low-volume blend region, these alcohols decrease the potential of fuel impingement, but in the moderate-blend region (>20–30 vol%) fuel impingement becomes more likely. In general, this work finds that blends with highly volatile oxygenates increase the likelihood of fuel impingement. These blends can potentially increase soot production, especially at high engine speeds, where volatility has the greatest impact. Additionally, results suggest that low-molecular-weight oxygenates have higher rates of volatility. Thus, they preferentially volatilize relative to aromatic components in the fuel leaving behind droplets with low volatility, that are enriched in aromatics.