Investigating Fuel Condensation Processes in Low Temperature Combustion Engines

Abstract

Condensation of gaseous fuel is investigated in a low temperature combustion (LTC) engine fueled with double direct-injected diesel and premixed gasoline at two load conditions. Possible condensation is examined by considering real gas effects with the Peng–Robinson (PR) equation of state (EOS) and assuming thermodynamic equilibrium of the two fuels. The simulations show that three representative condensation events are observed. The first two condensations are found in the spray some time after the two direct injections (DI), when the evaporative cooling reduces the local temperature until phase separation occurs. The third condensation event occurs during the late stages of the expansion stroke, during which the continuous expansion sends the local fluid into the two-phase region again. Condensation was not found to greatly affect global parameters, such as the average cylinder pressure and temperature mainly because, before the main combustion event, the condensed phase was converted back to the vapor phase due to compression and/or first stage heat release. However, condensed fuel is shown to affect the emission predictions, including engine-out particulate matter (PM) and unburned hydrocarbons (UHCs). Specifically, it was shown that the condensed fuel comprised more than 95% of the PM in the low load condition, while its contribution was significantly reduced at the high load condition due to higher temperature and pressure conditions.