Autoignition, knock, detonation and the octane rating of hydrogen

Abstract

This paper presents an experimental and numerical study of hydrogen’s combustion, autoignition and octane rating. The experiments are conducted in a standard engine that is compliant for rating liquid fuels to the ASTM’s Research Octane Number (RON) method, with minor modifications to enable hydrogen fueling. The numerical analysis is undertaken using a combination of calibrated, two-zone combustion and kinetic modeling of the end-gas. Application of the standard RON method first shows that hydrogen has a RON of 62–64, which is significantly lower than that of standard gasolines. However, this standard RON method requires conditions that are not relevant to practical, hydrogen-fueled engines and, indeed, do not appear to feature autoignition and knock. A set of modified RON tests are therefore undertaken at more practical conditions. These indicate that hydrogen at standard knock intensity and λ=1,1.5 and 2 has a modified RON of 93.7,117 and greater than 120 respectively, spanning the range of λ that match the energy delivered by common SI engine fuels.Together, these results show that hydrogen is significantly more knock-resistant than standard gasolines when providing similar energy to the premixture. Indeed, it should have comparable knock resistance to some high-octane fuels. However, care must be taken when examining hydrogen autoignition, knock and abnormal combustion more generally. Hydrogen’s high flame speeds near stoichiometric conditions can produce high rates of pressure rise at lower compression ratios that can be mistakenly identified as autoignition, even with contemporary in-cylinder pressure measurement, while higher compression ratios near stoichiometric conditions can cause detonation. The identification and avoidance of conventional knock and other forms of abnormal combustion is therefore particularly important for hydrogen fueled engines. This also has significant implications for the development of standard test methods for determining the octane and methane numbers of hydrogen-rich fuel blends.