How far can we take advantage of combustion characteristics of fuels? It depends on how well we know about the combustion properties of fuels. The effect of modifying or changing fuels on the SI combustion has been investigated in terms of the fundamental combustion properties calculated by detailed kinetic models. Several previous studies suggested that the lean limit of SI combustion is significantly improved by the additives like nitromethane and by using high octane sensitivity fuels. The effects of the addition of nitromethane to methane and butane have been investigated based on the extinction flame stretch rate as well as the well-known properties including ignition delay time and laminar flame propagation velocities. The results suggested that the addition of nitromethane or the increase of octane sensitivity expands the lean and stretch extinction limit of early flame propagation. Also, recent experimental works have identified ethanol as a fuel with extended lean limit for SI combustion due to improved anti-knock and initial flame propagation. However, a part of reason may be in the smaller molecular weight and size which decrease Lewis number. For the elucidation of the effect of this Lewis number on the flame stretch-extinction behavior, numerical calculations have been performed by artificially modifying the collision diameters of the fuel molecules. The results indicate that both Lewis number and chemistry affects the extinction limit in the same order of magnitude.

Reactivity evaluation of gasoline components were conducted using a micro flow reactor with a controlled temperature profile. Weak flame images of n-heptane/air and iso-octane/air mixtures at equivalence ratios of 0.5, 0.75 and 1.0 were taken and equivalence ratio dependence on the hot flame position was examined. Rate-of-production and sensitivity analysis identified the importance of HO₂ radical formation and consumption of H₂/O₂ reactions. Weak flame responses to alkanes with various research octane numbers were investigated and reactivity indexes using heat release rate profiles and CH₂O formation from cool flames were proposed. Effects of N₂, CO₂ and H₂O dilutions on fuel reactivity were investigated. N₂ and CO₂ dilution decreased reactivity, while H₂O dilution increased reactivity.

In this study, the combustion characteristics of furan and nitromethane-added fuels were studied to clarify the thermal efficiency improvement mechanism in engine experiments. Firstly, the laminar combustion characteristics were studied using a constant volume chamber. The laminar burning velocity of the furan was found to be high, but the effect of the furan addition to iso-octane on the laminar burning velocity was small. Alternatively, the addition of furan to iso-octane was found to increase the lean flammability limit, which can be related to the thermal efficiency improvement in engine experiments. In addition, the effects of furan and nitromethane additions on the turbulent combustion characteristics of isooctane were studied, showing that the addition of furan to isooctane promotes turbulent burning velocity, and the addition of nitromethane is effective in improving turbulent combustion characteristics near the lean combustible limit. Finally, extinction characteristics were evaluated for premixed flames using a counterflow burner. Consequently, it was found that the addition of furan to isooctane improved the extinction characteristics, and the addition of nitromethane further improved the extinction characteristics in the leaner condition, whose results are consistent with the experimental results from the constant volume chamber.

Shock tubes have been used to ignition research for a long time, because they can heat a combustible fuel mixture instantaneously to high temperatures and pressures. However, conventional shock tubes have been limited to high-temperature ignition research due to their short heating duration. In recent years, large-scale equipment improvements and advanced experimental techniques have been succeeded in extending ignition research to lower-temperature region. As a result, the shock tubes have come to be applied as an effective tool for solving various problems in internal combustion engines, such as knocking phenomenon. In the present paper, I introduce our high-pressure shock tube with a long heating duration that can observe low-temperature ignition. Using this device, I report on the latest research results on fuel optimization for the “Super-lean burn engine”, which is attracting attention as a high thermal efficiency automobile engine for reducing CO₂. Furthermore, the future role and possibilities of the shock tubes for realizing a carbon-neutral society are also mentioned.

Biofuels are attracting attention as carbon-neutral fuels and are often blended with existing fuels for use in spark-ignition engines. In order to further reduce carbon dioxide emissions from spark-ignition engines, the thermal efficiency is required to be further improved, even if the biofuels are used. For the improvement of the thermal efficiency, it is important to understand the ignition characteristics of biofuels and biofuel-gasoline blends. In addition to practically used ethanol and ethyl-tert butyl ether (ETBE) biofuels, butanol, furan, and furan's derivatives, generated from biomass, have been investigated as fuels for spark-ignition engines. This article summarizes the ignition characteristics of ethanol, butanol, ETBE, and furan's derivatives (2-methylfuran) used in spark-ignition engines.

As part of global efforts to reduce GHG emissions, there is a strong demand to reduce CO₂ emissions in the transportation sector by improving the thermal efficiency of internal combustion engines. As a technology for improving the thermal efficiency of gasoline engines, super lean-burn is known, in which the air-fuel mixture in the cylinders of the engine is burned with a leaner air-fuel ratio than the stoichiometric air-fuel ratio. By reducing energy loss such as cooling loss and improving the specific heat ratio, thermal efficiency is improved, leading to a reduction in CO₂ emissions. However, there is a limit point (lean limit) at which combustion becomes unstable as the air-fuel mixture becomes leaner. If the lean limit is exceeded, misfiring occurs in the engine and stable operation becomes impossible. This makes it difficult to obtain sufficient output, and adversely affects the running of the vehicle. To further improve thermal efficiency, it is necessary to expand the lean limit, and various methods have been proposed as engine technologies, such as increasing ignition energy and improving in-cylinder flow. In this study, as a fuel technology, we tried to expand lean limit by changing fuel composition and molecular structure. As a result, the fuel which have a lower molecular weight and high reactivity at around 1000K showed good lean limit expansion property. Specifically, it can be said that olefins and the like are excellent molecular structures.

The paper overviews low-carbon technology based on the electrification of chemical processes via nonthermal plasma and heterogeneous catalyst coupling. First, the current status of electrification technology is overviewed, followed by the introduction of nonthermal plasma technology used in this field. Then the principle of dielectric barrier discharge and the evaluation method of energy efficiency are introduced. Dry methane reforming (DMR: CH₄ + CO₂ = 2CO + 2H₂) by a packed-bed dielectric barrier discharge (DBD) reactor is kinetically analyzed using La-Ni/Al₂O₃, showing the drastic decrease of activation energy of 91 kJ/mol in thermal case to 45 kJ/mol in DBD 100 kHz case. Vibrationally excited CH₄ is the key reactive species which is also confirmed by the state-specific kinetic analysis by the molecular beam study. Moreover, overall CH₄ conversion behavior was further promoted by fluidized-bed DBD where heat and mass transfer was promoted efficiently. Reverse water gas shift reaction (CO₂ + H₂ = CO + H₂O) was also studied using Pd₂Ga/SiO₂ alloy catalyst. Similar to DMR, remarkable reaction enhancement by CO₂ vibrational excitation was confirmed which was validated by kinetic analysis using fluidized-bed DBD and DFT calculation. Finally, concluding remarks and future prospects are described briefly.

This article introduces a gas reforming using atmospheric pressure plasma technique, which has attracted much attention in recent years, and also describes a spouted bed plasma reactor technology being developed by the authors. Plasma-based gas reforming has been the subject of rapid research and technological development in recent years, driven by global warming and the efficient use of surplus electricity in the future. In addition, plasma gas reforming has the advantage of being able to produce a wide variety of gases, allowing the process to meet local needs. Although research and development of gas reforming using atmospheric pressure plasma has just begun, it is attractive in that it is possible to construct a process that matches the future social situation. Therefore, this article introduces the current status of technological development and discusses issues in plasma gas reforming.

Carbon-neutral fuels have been studied in order to reduce the Well to Wheel CO₂ emissions from vehicles equipped with ICEs. In order to apply carbon-neutral fuels to existing engines, it is necessary to control the combustion pattern of carbon-neutral fuels. In this study, a detailed chemical reaction model of dimethyl carbonate (DMC), one of the carbon-neutral fuels, was developed. We obtained a laminar burning velocity model which can simulate the combustion characteristics in the initial stage of combustion in an engine cylinder. This combustion model was introduced into a three-dimensional combustion simulation, and the calculated cylinder pressures were validated by comparison with experimental data.

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