Thermally stratified compression ignition enabled by wet ethanol with a split injection strategy: A CFD simulation study

Abstract

The potential application of partially distilled alcohol biofuels in a new, advanced, low temperature combustion (LTC) concept called Thermally Stratified Compression Ignition (TSCI) was studied in this work. TSCI, which was recently conceived, attempts to intentionally stratify the temperature distribution in the cylinder prior to ignition in a lean, premixed, compression-ignited combustion mode. Preliminary experiments used the latent heat of vaporization from the direct injection of water to stratify the temperatures prior to ignition and therefore provide control over the start and rate of combustion in LTC and expand the operating range. While these preliminary results are very encouraging, the requirement of a separate direct injector for water somewhat limits the commercial applications of this variant of TSCI (i.e. employing direct water injection). Instead, this work investigates using the latent heat of vaporization of wet ethanol to introduce the forced thermal stratification of TSCI to control the combustion process.A 3D CFD model was implemented and validated in CONVERGE with the SAGE detailed chemistry solver. This model was then used to simulate TSCI with wet ethanol employing a split injection strategy. The results indicate that TSCI with split direct injection of wet ethanol has an overall lower heat release rate and longer burn duration compared to HCCI due to the higher level of thermal stratification prior to ignition achieved through the evaporative cooling effect of the wet ethanol and its high latent heat of vaporization. Additionally, the ethanol/water blend ratio was varied to observe its effect on TSCI combustion. The results show that a higher percentage of water reduces the peak heat release rate and elongates combustion. Finally, different split injection mass fractions were simulated and the results indicated that injecting a higher mass fraction of wet ethanol in the second injection reduces the peak heat release rates and can increase the burn duration up to 68.8%. This way, TSCI with wet ethanol provides cycle-cycle control over the combustion process through adjusting the mass fraction of the first and second injection without requiring any engine hardware changes.