Knock and Combustion Rate Interaction in a Hydrogen Fuelled Combustion Engine

The paper describes correlation between combustion knock intensity and combustion rate calculated as the heat release rate from combustion pressure traces of a hydrogen fuelled spark ignited engine. Unlike a gasoline spark ignited (SI) engine, the hydrogen fuelled engine can easily generate knock during combustion at working conditions similar to a gasoline engine. However, the hydrogen knock does not necessarily come from hydrogen auto-ignition at the end phase of spark-controlled combustion process as it is typical at the gasoline fuelled engine. The phenomenon of hydrogen knock significantly differs from the gasoline knock due to different combustion mechanisms and different fuel thermo-chemical properties. The knock can be generated during hydrogen combustion itself as result of combustion instabilities. Intensity of this knock, expressed here by intensity of combustion pressure fluctuations, is several times lower in comparison with the combustion knock by fuel self-ignition process. This „light knock“ is a matter of this paper. The tests of hydrogen combustion in the IC engine has been conducted at air to hydrogen stoichiometric ratio at various compression ratios with spark timing sweep from -10 to 4 crank angle degrees referring to top dead centre of the engine piston. Obtained results show, that there is a positive correlation between the knock intensity and the combustion rate. This correlation is particularly observed at tests taken on the engine with compression ratio of 10. The conclusions should provide good premises for combustion knock modelling and its prediction.