Reliable hydrogen combustion aero engines will be needed for future aircrafts including UAVs (Unmanned Aerial Vehicles). In our previous study, we established how to supply a lubricant into a micro reciprocating engine (FG-11, originally a 4-stroke gasoline engine) and succeeded in its continuous operations with hydrogen. To maximize the power output in a micro reciprocation hydrogen engine, it was important to spark in the narrow range near the TDC due to the short ignition delay and high burning velocity of hydrogen. However, it was found that increasing the hydrogen flow rate caused a rapid decrease in the power output even with optimal ignition timings. High concentrations of unburned hydrogen were measured in its exhaust gas at these hydrogen flow rates. The reason was considered that the combustion occurred immediately after ignition in the extremely narrow space at the top dead center, causing the flame quenching partially. The engine was operated at higher rotating speed with a lower load propeller to reduce the reaction rate relatively to the space increasing rate. The increased power outputs were successfully obtained with increasing hydrogen flow rates with this approach.
MINAKAWA et al. (Wed,) studied this question.