Emission and Combustion Characteristics of a Hydrogen Direct-Injection Spark Ignition Engine using EGR

Hydrogen is recognized as a promising alternative fuel for internal combustion engines as it contains no carbon, but in addition to NOx emissions during hydrogen combustion, unburned hydrogen emissions have recently emerged as an important environmental issue. In this study, a 400 cc single-cylinder direct-injection hydrogen engine based on a 1.6 L Hyundai Kappa engine was used to analyze the effects of injection and ignition timing on combustion characteristics and engine emissions. Experimental results showed that the formation state of the air-fuel mixture varied with injection and ignition timing. Under conditions where locally fuel-rich mixtures were formed, NOx emissions increased while unburned hydrogen decreased. In contrast, when the mixture was relatively homogeneous, NOx decreased but unburned hydrogen increased. These findings clearly confirmed the trade-off relationship between NOx and unburned hydrogen emissions. To address this issue, exhaust gas recirculation (EGR) was applied, which diluted the oxygen concentration and increased the heat capacity of the in-cylinder mixture, effectively lowering combustion temperature and suppressing NOx formation. At the same time, an appropriate EGR rate suppressed additional increases in unburned hydrogen, maintaining a balance between the two emissions. Therefore, this study demonstrates that the combined control of injection and ignition timing with EGR application is a key technology for achieving high-efficiency and low-emission operation in direct-injection hydrogen engines.