To study the knock phenomenon in a direct injection gasoline engine, n-heptane/air mixtures were fed stratified into a rapid compression machine (RCM) to simulate the engine combustion process. The auto-ignition took place in a rapidly compressed mixture. The pressure history in the cylinder was recorded with a pressure transducer. To separate the low frequency component from the high one, the pressure signal was processed numerically by 1kHz and 5kHz frequency filters, and the ignition delay, the rate of pressure rise, the maximum pressure amplitude and the knocking intensity factors were defined. To analyze the knocking intensity, the relationship between the maximum pressure amplitude, knocking intensity factor and the rate of pressure rise was studied. The fuel concentration and the concentration gradient were found which has a high knocking probability. By means of direct flame photography, the difference between the flame onsets of the uniform mixture and the stratified mixture was studied to analyze the effect of the concentration gradient on the behavior of the flame. To numerically predict the occurrence of the knocking phenomenon at stratified charge mixture condition, a reduced n-heptane oxidization reaction mechanism was employed. Both the experimental and numerical simulation results show the same tendency.
This paper explores an explanation of soot track formation adopted in visualization of detonation propagation, compared against previous speculations of formation mechanism. Focusing on the role of shear stress in transporting soot along the surface, we investigated the non-reactive Mach reflections experimentally with soot foil records and numerically with three-dimensional compressible Navier-Stokes simulations. Numerical results were compared with experimental results and used to interpret the effect of shear stress spatial and temporal variations on soot redistributions. The motions of soot due to surface shear stress were numerically examined with treating soot as both particles and fluid parcel. Numerical soot foils indicated the same features as those in experiments. The result clearly showed that the surface shear stress was strongly related to soot track generation.
The premixed combustion characteristics of the fuel mixture of carbon monoxide, hydrogen and nitrogen (CO-H2-N2) were studied by numerical simulations with detailed chemistry, and were compared with those of the mixture of hydrogen and nitrogen (H2-N2), and the mixture of methane and nitrogen (CH4-N2). The compositions of the latter two mixtures were set to have the same adiabatic flame temperatures as the CO-H2-N2 flame at the stoichiometric condition. One-dimensional flames, coflow premixed flames and conterflow premixed (twin) flames were adopted as the objects of this study. The dependence of the burning velocity on the equivalence ratio and the pressure, and the flame structures of the one-dimensional flames and the coflow flames were investigated in detail. In addition, the response of the maximum temperature of counterflow flames to the injection velocity was investigated to check the effect of a flame stretch. As a result, it was found that the absolute value of the burning velocity and the flammability limits of CO-H2-N2 flames are similar to those of H2-N2 flames and much different from those of CH4-N2 flames. It was also found that H2 is much more actively consumed than CO at the flame surface of the rich CO-H2-N2 premixed flame, which causes a large unbalance of remaining concentrations between CO and H2 in the region beyond the flame. In the rich coflow CO-H2-N2 flame, conversion from CO to H2 occurs in the region between the inner flame and the outer flame. In the counterflow premixed CO-H2-N2 flame, the thermal-diffusive unbalance occurs similarly to the H2-N2 flame, due to the fast diffusion of H2 toward the flame surface.