The effects of hydrogen addition on the auto-ignition delay of homogeneous primary reference fuel/air mixtures in a rapid compression machine

• The effect of H2 addition on the combustion of the primary reference fuel/air mixture was investigated.
• Factors affecting the ignition delay were found by varying initial condition.
• OH mole fraction profiles explain the effect of H2 addition on the first and the second stage reaction.

The ignition delay time with mixtures of primary reference fuels (PRFs) and hydrogen were measured using a rapid compression machine (RCM), where the concentration of hydrogen was systematically varied. The experiments were performed for compressed temperatures in the range 735–847 K. The fuel/air equivalence ratio was varied to investigate its effects on the ignition delay of n-heptane and iso-octane. Hydrogen was added with hydrogen-energy-share ratios in the range of 0–10%, meaning that the energy of the hydrogen replaced 0–10% of that of either n-heptane or iso-octane. When hydrogen was added to n-heptane, there was a significant increase in the ignition delay, especially during the first-stage reaction region. This was explained using the chemical kinetic analysis software package CHEMKIN PRO; hydrogen consumed radical species related to the ignition of hydrocarbon fuels. With iso-octane, there was a significant increase in the ignition delay not only in the fist-stage reaction region, but also in second-stage reaction region. The effects of the hydrogen concentration were found to depend on the equivalence ratio and temperature for both fuels.