Visualization Analysis of Hydrogen Combustion Triggered by Non-Controlled Hotspots Using a Rapid Compression Expansion Machine

Research on hydrogen-fueled internal combustion engines has gained growing attention as a carbon-neutral solution to reducing emissions in the transport sector. However, challenges remain, with the risk of abnormal combustion being one of the major criticalities. This paper aims to clarify the pre-ignition process of a hydrogen-air mixture caused by lubricant oil droplets and soot deposition. To achieve this, high-speed imaging methods were applied with a Rapid Compression Expansion Machine under engine-like conditions. Direct imaging and OH* chemiluminescence were captured simultaneously on the engine head to visualize the ignition point and flame propagation. Different operating conditions were tested to evaluate the influence of lambda, intake pressure, and soot quantity on pre-ignition occurrence. For each test bench configuration, ten successive tests were conducted to assess the probability of preignition occurrence. The presence of soot was ensured through a preliminary run with diesel injection. The presence of oil, instead, naturally increases inside the cylinder with each successive run due to the functioning of the modified test bench’s lubricating system. Three typologies of combustion modes were identified - standard pre-ignition, weak preignition, and late ignition. These are analyzed using chamber dynamic pressure and heat release rate trends, along with optical analysis. Direct imaging enables the identification of the droplet responsible for ignition, and OH* chemiluminescence allows visualization of the flame front propagation from the ignition point. As expected, an increase in intake pressure, a decrease in lambda, and the presence of soot all raise the probability of pre-ignition, with lambda having the most significant effect.