Backfire mechanism in a carburetted hydrogen fuelled engine

A 4 cylinder diesel engine is modified for hydrogen use by the installation of a gas carburettor (external mixture formation system). The pressure development in the combustion chamber of one cylinder and in the inlet manifold is stored prior to backfire (during 32 successive cycles), as well as at the moment of and after backfire. The pressure traces are carefully examined (enlargement of the signals around TDC and during the compression period). The pressure traces are also used for heat release analyses. From these the start of combustion, the combustion duration, the maximum combustion pressure and the occurrence of knock for each of the successive cycles is analysed. It is observed that prior to backfire (some 14 cycles before) suddenly preignition and knock appears at the same time. This results in a run-away preignition. First the maximum combustion pressure increases, combustion duration decreases and heavy knock is measured. But with earlier and earlier preignition, knock disappears, till the mixture is ignited before the closure of the inlet valve, which results in backfire. The appearance of knock and the resulting backfire is related to the air-fuel ratio of the mixture, thus to the power output of the engine (the load is regulated by the richness of the inlet mixture in the same manner as for diesel engines - completely wide open throttle). The time between knock occurrence and backfire is too short to react for a carburetted engine. With fuel injection systems actions can be taken to prevent the transition from preignition to backfire.