Abstract

Hydrogen-fueled engine is considered as an optimal alternative to fossil fuel engine due to non-carbon emissions after in-cylinder combustion. Compared with application of H2 fuel-cell, the structure of hydrogen-fueled engine is more like IC engine and durability of it is stronger. Therefore practical use of hydrogen can be visible in the short term. In addition, use of homogeneous charge compression ignition (HCCI) can achieve improvement on thermal efficiency by increased combustion speed and homogeneous fuel-air mixing. However, relatively high self-ignition temperature of hydrogen means a high compression ratio required for realizing spontaneous ignition. It will cause steep in-cylinder pressure rise, which is the reason of knock occurrence and narrow operation region. Therefore, decrease of pressure rise rate is important for achieving practical use of hydrogen-fueled HCCI engine. In this research, cold GH2 injection is considered as one of countermeasures expected to attain both of suppressing pressure rise rate and increasing engine power. There are two main advantages of using cold GH2 injection for improving a HCCI engine performance. One is higher density of cold hydrogen gas that contributes to increase energy supplied in a constant combustion volume. This determines that the higher compression pressure which increases work done on piston top. Hence engine output can be improved. Another one is cooling characteristics of cold hydrogen gas. Low initial compression temperature results in self-ignition timing retarded, which contributes to decrease rapid rate of pressure rise, meanwhile combustion temperature trends to be lower. Therefore, knock occurrence is possible to be controlled. Further, owing to low cylinder-wall temperature and cold H2-air mixture, cold GH2 injection with use of cold-start is difficult to realize self-ignition. Therefore hot-start is adopted in this research.
As the results, because of increased supply energy after using cold GH2 injection, the in-cylinder wall temperature can be increased. It will contribute to achieve possibility of decreasing compression ratio. Meanwhile, knock restricted effect by using cold GH2 injection increases possibility of engine operated at richer equivalence ratio. Therefore, in this research minimum compression ratio in firing can be decreased to ε=22 when hydrogen cooled to -30℃. Moreover, at this compression ratio the operation region can be extended by 40% compared with room-temperature hydrogen condition. In addition, it is found that both of engine power and thermal efficiency can be increased by using cold GH2 injection because of increased supply energy and decreased cooling loss, and the effect on reduction of NOx emissions is also confirmed due to decreased combustion temperature.