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Combustion in a lean atmosphere diluted with a large amount of air can greatly improve fuel efficiency by reducing cooling loss [1, 2]. On the other hand, when air-fuel mixture in cylinder becomes lean, the turbulent combustion speed will decrease, resulting in problems such as the generation of unburned hydrocarbon (HC) and combustion instability [3, 4]. In order to solve these problems, it is important to increase the turbulence intensity and combustion speed [5, 6, 7, 8, 9, 10]. When designing combustion in cylinder by using Computational Fluid Dynamics (CFD), K-epsilon model is widely used for a turbulence model, and the calculated turbulence energy k or turbulence intensity u’ have been used as important indices of combustion velocity [11, 12].

To prevent global warming, many countries are making efforts to reduce CO2 emissions toward achieving 2050 carbon neutrality. In order to reduce CO2 concentration quickly, in addition to spread of renewable energy and expansion of BEV, it is also important to reduce CO2 emissions by improving thermal efficiency of ICE (internal combustion engine) and utilizing carbon neutral fuels such as synthetic fuels and biofuels. It is well known that lean burn is an effective technology to increase thermal efficiency of engine highly. However, since NOx emission from lean burn engine cannot be reduced with three-way catalyst, there have been issues such as complicated system configuration due to the addition of NOx reduction catalyst or limiting lean operation to narrow engine speed and load in order to meet emission regulation of each country.

A new method to measure in-cylinder flame propagation and mixture distribution has been developed. The distribution is derived from analyzing the temporal history of flame spectra of CH* and C2*, which are detected by a spark plug type sensor with multi-optical fibers. The validity of this method was confirmed by verifying that the measurement results corresponded with the results of high speed flame visualization and laser induced fluorescence (LIF) measurement. This method was also applied to analysis of cyclic combustion fluctuation on start-up in a direct injection spark ignition (DISI) engine, and its applicability was confirmed.