Research on internal combustion engine using gaseous fuel is getting popular. In order to reduce exhaust gas emissions including CO₂, heavy oil is replaced by gas fuel, especially for ships and stationary generators. Demand of natural gas is rapidly increased due to the clean exhaust gas emissions and the price. Moreover, combustion study of hydrogen and ammonia is also getting active as both fuels can be made from green energy. In this article, combustion technologies using natural gas are mainly focused, such as abnormal combustion, ignition method, methane slip and combustion analogy. Many research papers are reviewed and discussion from both experimental and numerical viewpoints are briefly described.

With a view to limiting global warming, a growing volume of research is being conducted on the use of natural gas, hydrogen, ammonia, biogas and other gaseous fuels in internal combustion engines. The efficient use of gaseous fuels calls for appropriate control of the combustion process, taking account of fuel characteristics. Osaka Gas is also conducting studies with a range of different gas fuels. This report summarizes the results of our work, citing recent studies on engines fueled by natural gas, especially dual fuel and pre-chamber ignition engines. For dual fuel engines, the advantages and disadvantages of dual fuel combustion were examined, and tests indicated that performance could potentially be improved by selecting the appropriate diesel injection conditions. For pre-chamber ignition engines, pre-chamber gas pressure, main chamber gas pressure, OH* and CH* were measured with a view to suggest for pre-chamber design. In future, it is likely that gaseous fuels will be put to effective use, with the most appropriate fuels being selected on the basis of a range of criteria, including social environment, fuel costs and degree of infrastructure provision, as well as applications.

This paper introduces fundamental research work for the marine gas engines. First, fundamental research on diffusive combustion by direct injection methane gas was conducted. In this experiment, a diffusive flame was visualized using a rapid compression expansion machine. Methane gas was injected into the cylinder at a high pressure of 30 MPa. The gas is ignited by micro pilot injection. The combustion images, rate of heat release, and exhaust gas analysis results were compared with conventional diesel combustion. As a result, it was found that combustion by gas injection is almost the same as diesel combustion. NOx emissions were slightly reduced. In lean pre-mixed combustion, the excess air ratio is important. The combustion rate decreases as the excess air ratio increases. An increase in the excess air rate reduces the peak value of the in-cylinder pressure. Lean premixed combustion is ignited not only by the flame of micro pilot but also by the flame from droplets of lubricant oil. Increasing the oxygen concentration in the atmospheric air dramatically reduces methane slip.

In order to meet stringent exhaust emission regulation for marine engine, the alternative fuel is growing in usage and popularity. In particular, the market demand of gaseous fuel as typified by natural gas is high thanks to its high environmental adaptability and worldwide falling prices. According to this circumstance, the dual fuel engine with advanced transient performance and thermal efficiency was developed. The engine is developed to apply to marine engine which directly drive the fixed pitch propeller. The reason comes from the fact that the effect of greenhouse gas reduction from marine gear direct drive is larger in comparison with electric propulsion which is affected by generator efficiency. In case of the direct drive propulsion, it is difficult to ensure the sufficient air delivery and the knocking might be occurred since the engine load is changed along the propeller law. Therefore, the combustion control technology and air/fuel control technology as countermeasure for this difficulty is required. This developed engine has superior transient performance which allows load acceptance within fifteen second or so due to high performance control technology. In this paper, the engine performance, estimation of greenhouse gas reduction by comparison with existing diesel engine performance and the adaptability of this developed engine are described.

It is strictly required to use carbon-free fuels for the existing gas turbines, gas engines, and diesel engines which currently use fossil fuels and natural gas to generate heat and power. Hydrogen and ammonia are expected not only to become energy carriers storing renewable electricity generated by solar and wind power but also to be alternative fuels used for decarbonization in the sectors of heat and power generation and transportation. This report explains fundamental combustion characteristics of hydrogen and ammonia which are extremely different from those of conventional hydrocarbon fuels. In addition, recent research and development of hydrogen engine and hydrogen/diesel co-firing engine are introduced in this report.

Temperature is one of the important parameters for understanding the combustion phenomena, and various laser diagnostics for quantitative measurement of temperature have been developed. However, there are physical factors which cause difficulties of laser diagnostics at high pressure conditions. Laser Induced Thermal Grating Spectroscopy (LITGS) is expected as a potential non-intrusive laser diagnostics for high pressure combustion because it is considered that the LITGS signal increases with an increase in pressure. LITGS can derive quantitative temperature at the crossing point of two incident pump lasers. Rapid excitation and subsequent quenching create a thermal grating, and the density perturbation is measured as the LITGS signal. Temperature can be evaluated from the oscillation frequency of the LITGS signal. In this paper, the fundamental theory of LITGS, applications for temperature measurement, and recent studies of LITGS are explained. In the final part of this paper, an example of LITGS application to oxygen enriched methane/oxygen/nitrogen premixed flame at high pressures is introduced.

In quantitative measurements and investigations, "equivalence ratio" and "refractive index" are the most significant parameters in combustion flow fields. Experimental and analytical studies were performed to understand the relation between equivalence ratio and refractive index in flow fields. A "universal relationship" between the refractive index of a premixed gas of fuel and air and its equivalence ratio has been established in this study. Through this relationship, we can calculate the equivalence ratio of any premixed gas (except those of light fuels) from its refractive index using a unique coefficient. The uniqueness of the relation coefficient enables us to obtain the equivalence ratio of even a multi-fuel premixed gas from its refractive index. In this study, the novel relationship is applied to a non-scanning 3D-CT (computerized tomography) technique using a multi(20)-directional quantitative schlieren optical system with a flashlight source. Instantaneous equivalence ratio distributions of a dual-fuel premixed gas flow with two jets of ethanol/air and propane/air mixtures are measured by CT-reconstruction. Good agreement was observed between the CT-reconstructed data and the proposed comprehensive relation.