HCCIエンジンにおけるアンモニアの燃焼特性と水素添加の影響

抄録

Ammonia possesses excellent properties such as high energy density and easy liquefaction conditions, making it an attractive carbon-free fuel. However, its low laminar burning velocity makes it difficult to use in spark-ignition engines, and its use in HCCI engines is also being considered. We have confirmed that ammonia HCCI combustion can be achieved by heating the intake air at compression ratios of 22-26. This study investigates the effect of onboard hydrogen addition, obtained through reforming, on improving ignition properties while using an ammonia-only fuel system through simulations. Simulations were performed by varying intake temperature and ammount of hydrogen addition, using a zero-dimensional model constructed with Cantera and reaction schemes which has shown good agreement with pressure traces of an ammonia-hydrogen dual-fuel HCCI engine. The results showed that adding about 5 mol% hydrogen significantly improved the ignitability of the ammonia-air premixed gas, and reduced the required intake temperature for ignition by 60 K. Additionally, a sensitivity analysis of ammonia oxidation was conducted for both pure ammonia and ammonia-hydrogen combustion, revealing that hydrogen addition did not significantly affect the ammonia oxidation mechanism. The improvement in ignitability was concluded to be due to the heat release from hydrogen reactions at relatively low ignition temperatures, raising the mixture temperature.