Dual nature of hydrogen combustion knock

• A hydrogen fueled spark ignited CFR engine was applied for investigation.
• Study on hydrogen knock was conducted vs. variable engine compression ratio.
• Light and heavy knock were recognized and examined.
• Intensity of the light knock depends on temperature of hydrogen–air mixture.
• Intensity of the heavy knock is associated with mass of hydrogen self-ignited.

Combustion knock is abnormal combustion taking place in an internal combustion spark ignited engine. It might be particularly observed in the engine at the end of combustion when the air–fuel mixture residue can be self-ignited due to exceeding auto-ignition temperature of this mixture. However, while hydrogen is combusted the knock can also occur as a result of non-auto-ignited combustion events. Investigation on knock, presented in the manuscript, was conducted in a hydrogen fueled spark ignited single cylinder engine with variable compression ratio. To express in numbers intensity of the combustion knock the in-cylinder pressure pulsations were used as a credible metrics. On the basis of analysis of these pulsations the hydrogen knock was distinguished as light and heavy one depending on its origin. The light knock is generated by combustion instabilities, which are a source for generating pressure waves inside the engine cylinder. The heavy knock results from hydrogen auto-ignition at the end of combustion. Its intensity is several times higher in comparison to the light knock. These observations were additionally confirmed by analysis of heat release rate. Finally, the light and the heavy knock were characterized by average amplitude of the pulsations from the entire test series of hundreds and several thousands kPa, respectively.