Lu Qiu

Development of an n-heptane/toluene/polyaromatic hydrocarbon mechanism and its application for combustion and soot prediction

A chemical reaction mechanism has been developed for modeling the combustion process and polyaromatic hydrocarbon formation of diesel and n-heptane/toluene fuels. A reduced n-heptane/polyaromatic hydrocarbon mechanism was applied and updated to better predict the formation of polyaromatic hydrocarbon up to four rings (A4) in ethylene and n-heptane premixed flames. In addition, a reduced toluene mechanism was updated and combined with the n-heptane/polyaromatic hydrocarbon mechanism to predict the combustion and polyaromatic hydrocarbon formation of diesel and n-heptane/toluene fuels. The final mechanism consists of 71 species and 360 reactions. This mechanism was validated with experimental ignition delay data in shock tubes, premixed flame species concentration profiles, homogeneous charge compression ignition combustion and direct injection spray combustion data. A practical multistep soot model was integrated with the polyaromatic hydrocarbon kinetic model to predict soot emissions of diesel and n-heptane/toluene direct injection engine data. Constant-volume combustion vessel simulations were also conducted and the effects of combustion parameters, such as temperature and equivalence ratio, together with the n-heptane/toluene ratio on polyaromatic hydrocarbon and soot formation are discussed. The results show that the present mechanism provides promising agreement in terms of polyaromatic hydrocarbon prediction for various fuels in premixed flames and highlights the importance of aromatics on the polyaromatic hydrocarbon formation and soot emissions. Homogeneous charge compression ignition combustion and direct injection spray combustion simulation results confirm that the present mechanism gives reliable predictions of combustion and soot emissions for both diesel and n-heptane/toluene fuels under various conditions.

Investigating Fuel Condensation Processes in Low Temperature Combustion Engines

Condensation of gaseous fuel is investigated in a low temperature combustion 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 equation of state 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, 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 and unburned hydrocarbons.Copyright