<div class="section abstract"><div class="htmlview paragraph">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. This paper introduces super lean burn engine with over lambda 2.5 that achieves both high thermal efficiency and significantly low NOx emission in order to solve the issues of conventional lean burn engine. Key technologies for combustion such as ignition, flame propagation, and unburned HC reduction are described firstly, and then the possibility of further improvement of thermal efficiency and combustion effect by applying ethanol as CN (Carbon neural) fuel to the super lean burn engine is indicated.</div></div>

<div class="section abstract"><div class="htmlview paragraph">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].</div><div class="htmlview paragraph">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 <i>k</i> or turbulence intensity <i>u’</i> have been used as important indices of combustion velocity [11, 12]. However, it has been confirmed by measurements that the flow will conversely weaken near the top dead center and the combustion duration will become longer when the air flow in the cylinder is extremely strengthened by improved intake port. This phenomenon can be expressed by using Large Eddy Simulation (LES) instead of K-epsilon model as a turbulence model, so K-epsilon model cannot be used for future combustion development including lean burn, and LES is critical. This paper proposes a model-based development method for designing engine combustion by using LES and Kolmogorov scale.</div></div>