This study summarizes the results of the AOI project, which aims to clarify liquid fuel recipes suitable for future high-efficiency engines and to reduce CO₂ emissions. Through the basic research phase from 2020 to 2022 and the subsequent demonstration phase starting in 2023, fuel combustion characteristics and emission reduction potentials were evaluated. Adjustments in fuel composition confirmed improvements in laminar combustion velocity, ignition delay time, and the expansion of lean combustion limits, leading to the identification of fuel recipes optimal for high-efficiency combustion. In particular, oxygen-containing fuels including ethanol and ETBE, as well as mixed fuels with light olefins, demonstrated superior combustion speed and ignition properties, contributing to increased lean combustion limits and improved thermal efficiency. The behavior and impact of exhaust gases and evaporative emissions associated with the prior introduction of oxygenates were also assessed to understand related challenges. Based on these findings, the feasibility of large-scale demonstration for social implementation will be determined in the future.

This study investigates the properties of premixed laminar and turbulent flames of light olefins, oxygenated fuels and gasoline containing these fuels. The analysis focuses on the influence of thermo-diffusive effects under stoichiometric and lean conditions. The results indicate that the laminar burning velocity, u_l, of 1-pentene, a light olefin, is larger than that of ethanol, an oxygenated fuel, and iso-octane, a paraffin. However, a different trend is observed in the turbulent regime. The turbulent burning velocity of the stochiometric 1-pentene flame is not larger than that of ethanol flame at a strong turbulence intensity, u’. Furthermore, a significant difference in quenching behavior was observed under lean conditions. The lean 1-pentene flame does not quench and continues to propagate even at a strong turbulence intensity, whereas the ethanol flame is observed to quench. These findings suggest that the properties of turbulent flames are not solely dependent on the laminar burning velocity. They are also significantly influenced by the relative turbulence intensity, u’/u_l, and the Lewis and Markstein numbers, both of which are related to thermo-diffusive effects.

This article introduces several research topics obtained by the author's laboratory in the AOI project, which has been ongoing since 2020. First, we experimentally evaluated the autoignition characteristics of 17 hydrocarbons, including oxygenated compounds, using a long-heating duration high-pressure shock tube capable of conducting ignition (combustion) experiments at high pressures and low temperatures simulating internal combustion engines. Of these, four light olefin isomers with five carbon atoms were compared to examine the differences in knock resistance and flame propagation characteristics under lean combustion. The differences in the chemical structure of light olefins significantly affected knock resistance. In contrast, no differences were observed among the four isomers in terms of flame propagation assisting properties, demonstrating that all isomers exhibited favorable properties. Based on the knowledge from these single-component ignition (combustion) experiments, screening evaluations were performed to develop new recipes. The results confirmed the importance of light olefins and bio-based components in next-generation gasoline, and the blending effects of these components with base gasolines were subsequently evaluated. The low-temperature ignition suppression by ethanol, which is one of bio-based components, is non-linear with the amount added to the base fuel, and even small amounts have been shown to have a significant knock suppression. However, this suppression by ethanol was suggested to depend on the components of base fuels, and light olefins and aromatics were found to weaken this effect. Such knowledge of the interactions between fuel components is extremely important for creating next-generation gasoline recipes, and further research in more complex components is required.

During the period of transition to renewable energy, not only the use of biofuels and synthetic e-fuels is necessary but also the improvement of the thermal efficiency of the internal combustion engines is inevitable. At the same time, the use of new fuels for engines implies the possibility of co-optimizing the combustion in engines with new fuels. It depends on how well we know about the combustion properties of fuels. The effect of modifying or changing fuels on the engine combustion can be investigated in terms of the fundamental combustion properties. The role of the author in this project is the construction of the detailed kinetic mechanisms of newly proposed fuels and analyze the effect of mixing fuels. For this purpose, the elementary reactions for alkene and alcohol oxidation has been precisely investigated. The rate rules for the Waddington reactions and H-abstraction reactions by HO₂ from alcohols have been established. The resultant mechanisms well reproduced ignition delay times of and laminar flame speeds of alkene/air mixtures for 1-pentene, 2-pentene, 2-methyl-1-butene, and 2-methyl-2-butene. The kinetic analysis showed that the non-linear effect of ethanol addition is caused by the transformation of OH radicals to HO₂ radicals and the non-monotonic effect can be explained by the inhibited accumulation of H₂O₂ via HO₂. Also, the base fuel dependences of ethanol addition has been explained in terms of the reactivity of added fuel components toward HO₂. That is, compared to alkane (the base component), the higher HO₂-reactivity of 1-pentene and toluene results in the antagonization effect of alcohols.

To reduce CO₂ emissions from spark-ignition engines, improving thermal efficiency is essential. One approach is to explore fuels that contribute to higher thermal efficiency. The present study aimed to evaluate the lean-limit extension effects of simultaneously blending light olefins, ethanol, or ETBE under a wide range of operating conditions, and to clarify the underlying mechanisms. The results revealed that blending light olefins extended the lean limit, and further extension was achieved when ethanol was also blended. Across various operating conditions, it was found that under conditions where MBT could be obtained, blending light olefins with ethanol was effective in extending the lean limit. Even under knocking conditions, the combined use of light olefins and ethanol increased the octane number and expanded the operating limit. From the correlation between fundamental combustion characteristics of fuels and the lean limit, it was demonstrated that laminar burning velocity, Lewis number, and RON are key factors in identifying fuels effective for extending the lean limit across wide operating conditions.

The authors evaluated olefin-based fuels and bio fuels using a single-cylinder engine. EGR and excess air were added from 0 to their maximum values, and thermal efficiency was evaluated within the range where stable combustion was possible. The results showed that thermal efficiency varies depending on fuel characteristics and can be quantitatively expressed using characteristic parameters such as laminar combustion velocity, fuel Lewis number, and Karlovitz number (chemical reaction time). Next, fuel efficiency was predicted for three types of passenger vehicles (K-car N/A, K-car T/C, and series HV) when the fuel was changed. The results showed that fuel efficiency could be improved by adding olefin-based fuels that maintain stable combustion even with increased EGR or airflow, or by adding oxygen-containing biofuels that enhance octane rating. In particular, a significant improvement in fuel efficiency was confirmed for series hybrid vehicles (HV). These fuels were shown to effectively utilize the optimal thermal efficiency range in many cases.

Ammonia is a carbon-free fuel that plays an important role in achieving carbon neutrality. Partial cracking of ammonia into hydrogen and nitrogen provides an effective and economical strategy to overcome its low reactivity. This article summarizes the combustion characteristics of partially cracked ammonia in non-premixed combustion, including laminar burning velocity, NO formation, extinction strain rate, and differential molecular diffusion. Subsequently, recent advances in physics models for turbulent combustion simulations are reviewed, with a particular focus on the description of differential diffusion, NO formation, and localized extinction. The need for further development of reduced-order modeling framework is also discussed, especially regarding the treatment of differential diffusion and subgrid-scale closures.

In order to further investigate the discrepancy between the numerical and experimental outcomes of ion current trends of stagnated ammonia-air flames, the measurement technique of the ion current was altered from the flame being scanned by a single vertically moving cathode probe, to both the anode and the cathode moving in tandem horizontally through the flame. As with the vertically scanning cathode method, horizontal scanning results show that, as the equivalence ratio increases from 0.89 to 1.19, the peak value of ion current generally decreases as progress is made into the rich zone. The horizontal scan technique results in a peak ion current at φ = 1.01, whereas the vertical scanning method gives a peak at φ = 0.95. Elementary reaction calculations performed using an expanded ionic mechanism and the burner-stabilized stagnation flame model, predicts the dominant positive ion NO⁺ to peak at φ = 0.98. Furthermore, the new expanded mechanism predicts the existence of high concentrations of OH^- ions in the burned gasses peaking at φ = 0.95. These negative ions could possibly explain the variance in equivalence ratios where the peak ion current occurs with different measurement techniques. This paper attempts to create a model to explain the phenomena based on these findings. It is concluded that in stagnated ammonia-air flames, and possibly in ammonia-air flames in general, the technique used to measure the ion current must be taken into consideration when used as a diagnostic tool.