To achieve the desired fuel switch from natural gas to hydrogen in internal combustion engines for combined heat and power units, it is necessary to make some adjustments to the fuel supply system. External gas mixers increase the probability of backfiring when natural gas is replaced by hydrogen. In addition, the low density of hydrogen results in a loss of power. Therefore, direct gas injection is preferred when using hydrogen. A drawback of direct injection is the requirement of higher injection pressures to achieve the desired fuel mass and mixture homogeneity as well as the additional access to the combustion chamber for the direct gas injector in the cylinder head. This paper proposes an alternative approach that does not necessitate the implementation of a high-pressure direct injection system nor additional access to the combustion chamber via the cylinder head. A combined injection and ignition unit, called HydroFit, was developed which uses a sleeve inside the spark plug bore to supply hydrogen to the combustion chamber. The hydrogen is injected into the combustion chamber via capillaries integrated into the sleeve with a moderate pressure below 1.5 MPa. Hydrogen quantity is controlled by PFI solenoid valves which are protected from combustion chamber pressure by check valves. The spark plug is mounted inside the sleeve.
The HydroFit prototype was tested on a naturally aspirated single cylinder gas engine at an engine speed of 1500 rpm. A parameter variation was conducted to analyse the influence of the combustion phasing as well as the start and the end of the injection on performance and NOx emissions. The results were compared to operation with port fuel injection.
The results confirm that the general concept of the HydroFit unit is functional, achieving 75% of the targeted hydrogen mass flow. However, hydrogen slip caused by the low hydrogen mass flow, combined with the high volume inside the sleeve, results in decreased engine power. Compared to port fuel injection, the NOx emissions are slightly higher, presumably due to poorer mixture homogenisation.