Abstract
Hydrogen with low spark-energy requirement, wide flammability range and high burning velocity is an important candidate for being used as fuel in spark-ignition engines. It also offers CO2 and HC free combustion and lean operation resulting in lower NOx emissions. However, well examined external mixing of hydrogen with intake air causes backfire and knock especially at higher engine loads. In addition, low heating value per unit of volume of hydrogen limits the maximum output power. In this study, attention was paid to full usage of hydrogen advantage employing internal mixing method. Hydrogen was directly injected into cylinder of a single-cylinder test engine using a high-pressure gas injector and effects of injection strategy, compression ratio, combustion-chamber geometry and nitrogen dilution on engine performance and NOx emission were investigated under wide engine loads. The results reveal that: (a)Increase in compression ratio; expands the low load operation by combustion enhancement; however intense combustion limits the high load operation and spark advance, therefore limits increase of the thermal efficiency. (b)Split injection is effective to control the intense combustion of hydrogen mixture. However, hydrogen injection into combustion gas with high kinetic viscosity weakens the mixing rate and therefore gives lower thermal efficiency. (c)Change of piston geometry from flat to shallow dish improves mixture preparation in periphery of the spark plug. Therefore, under late fuel injection timing improves the thermal efficiency. Implementation of deep-dish combustion chamber results in an intense combustion therefore limits the thermal efficiency. (d)Nitrogen dilution of intake gas is effective to control the intense combustion of hydrogen by lowering the oxygen concentration. Therefore, stoichiometric operation of engine becomes possible. In addition, inert gas addition (nitrogen) to intake gas lowers the flame temperature therefore, drastically reduces the NOx emissions. Optimization of nitrogen dilution offers low NOx emissions under a wide engine operation range with no penalty in thermal efficiency.
Keywords: Internal combustion engines, hydrogen engine, direct-injection, combustion, engine performance and emissions