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.
