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Temperature distributions in n-heptane and n-tridecane sprays were measured by the planar laser induced fluorescence (PLIF) method. The spray was formed by injecting fuel into high temperature and high pressure ambient, which was formed by compression of rapid compression and expansion machine (RCEM). In this PLIF method, acetone was used as a fluorescence tracer and was mixed with ambient gas. The fluorescence tracer was excited by 266 nm (the 4th harmonic of a Nd:YAG laser). The fluorescence intensity was measured by an ICCD camera. The temperature and concentration were estimated based on temperature dependency of the fluorescence intensity and the assumption of adiabatic mixing. Based on the measurement results, The difference of mixture distributions in n-heptane and n-tridecane sprays are discussed.

As a result of the excavation of unconventional sources of natural gas, which has rich reserves, has attracted attention as a fuel for use in natural gas engines for power generation. From the viewpoints of efficient resource utilization and environmental protection, lean burn is an attractive technique for realizing a higher thermal efficiency with lower NOx emissions. However, ignition systems have to be improved for lean-burn operations. Laser ignition, which is expected to serve as an alternative to spark plug ignition, can decrease the heat loss and has no restriction on the ignition location because of the absence of an electrode. Consequently, an extension of the lean-burn limit by laser ignition has been demonstrated. In this study, we investigated the effects of the location and number of laser ignition points on engine performance and exhaust emissions. Laser ignition was also compared with conventional spark plug ignition.

Dielectric barrier discharge (DBD) was applied to control the pressure-rise rate of homogeneous compression ignition, which is an important obstacle for homogeneous charge combustion engines. DBD can produce nonthermal plasmas and has been generated in air/fuel mixtures to reform some of the fuel molecules found in such mixtures. This generally shortens the ignition delay of compression ignition of the air/fuel premixture. Stratification of the reformed premixture in the combustion chamber was achieved by pulsed DBD irradiation during the induction process. The formation of inhomogeneous distribution of the reformed premixture is expected by the formation of discharge at the end of the intake processes. A demonstrative experiment was conducted by using a rapid compression and expansion machine. A simple plasma reactor was developed and installed at the intake tube. High-voltage, high-frequency pulses were applied to form plasmas. n-Heptane was used as fuel.

Single-excite dual-fluorescence PLIF was applied to a diesel spray of a two-component fuel, the components of which have different boiling points. The spray was formed by injecting fuel into a constant-volume vessel under high-temperature, high-pressure conditions. The fluorescence emitted from the two tracers for the fuel was optically separated to measure the concentration of each component. Mixture formation was investigated based on the concentration distributions of each fuel component. The fuel concentration was derived based on the change in fluorescence intensity due to temperature and the assumption of adiabatic mixing of fuel and the surrounding fluid. The variation in the mixture distribution due to differences in the vaporization characteristics was investigated, and the results revealed that the two components have similar distribution. The concentration of the high-boiling-point component increased upstream region in a spray.

Numerical calculations were performed to investigate the mixture formation, ignition, and combustion processes in a diesel spray. The spray was formed by injecting n-heptane into a constant volume vessel under high-temperature and high-pressure conditions. The fuel droplets were described by a discrete droplet model (DDM). Numerical calculations for the flow and turbulent diffusion processes were performed on the basis of large eddy simulation (LES) to describe the processes of local non-homogeneous mixture formation and heat release. The oxidation processes in the mixture were calculated by Schreiber's five-step mechanism for n-heptane. Calculations were performed for sprays formed by single-stage injection and pilot/main two-stage injection. The flame structure in a diesel spray and its temporal change were discussed using a flame index proposed by Yamashita et al.

A diesel combustion model for CFD simulation is established taking into account the auto-ignition process of a non-homogeneous mixture. In a previous paper, the authors revealed that the non-homogeneity of a fuel-air mixture has a more significant effect on the auto-ignition process with respect to, for example, ignition delay or combustion duration, as compared to the turbulent mixing rate. Based on these results, a novel diesel combustion model is proposed in the present study. The transport calculation for the local variation of the fuel-air PDF is introduced, and the chemical reaction rate is obtained based on the local non-homogeneity. Furthermore, this model incorporates RANS-based CFD simulation of the spray combustion in a constant-volume vessel under a high-temperature, high-pressure condition. The results show that the combustion process is well described for a wide range of temperature and pressure conditions.