Comparison of hydrogen port injection and direct injection (DI) in a single-cylinder dual-fuel diesel engine

Hydrogen direct injection (DI) in a dual-fuel diesel engine is a new technology that can resolve two major issues of its port injection counterpart – knocking and NOx emissions. Compared to widely studied hydrogen port injection in a diesel engine, the hydrogen DI concept executes a near topdead centre (TDC) injection to cause hydrogen mixingcontrolled combustion. The slower burning rate is expected to hinder a rapid pressure rise and subsequent pressure ringing (i.e. knocking) and to reduce NOx emissions, which are problematic in premixed combustion dominant, hydrogen port injection dual-fuel diesel engines. This study directly compares the in-cylinder pressure, efficiency and engine-out emissions of port injected and direct injected hydrogen-diesel dual-fuel combustion in the same engine. The tests were performed in a single-cylinder engine equipped with three injection systems including a hydrogen port injector, a hydrogen direct injector and a common-rail diesel direct injector. The engine was operated at intermediate load using a fixed total energy input of 820 J with hydrogen energy fraction of 50%. The results show that mixing-controlled combustion of the hydrogen in direct injection mode leads to lower in-cylinder pressure and thus lower engine efficiency. However, the severe pressure ringing observed for the hydrogen port injection is avoided and engine-out NOx emission is reduced, indicating the hydrogen DI operation is more stable, its combustion is cleaner and a higher hydrogen utilisation can be achieved.