Autoignition of H ₂/CO at elevated pressures in a rapid compression machine

Autoignition of H ₂/O ₂ and H ₂/CO/O ₂ mixtures has been studied in a rapid compression machine at pressures from 15 to 50 bar. temperatures from 950 to 1100 K. and equivalence ratios from 0.36 to 1.6. In addition, the effect of change in relative concentrations of H ₂ and CO is investigated by replacing H ₂ with CO. while keeping the total fuel mole fraction of the combined H ₂/CO fuel constant. Under the experimental conditions of intermediate temperature and high pressure, reactions involving formation and consumption of HO ₂ and H ₂O ₂ are important. Results show that for H ₂ autoignition. the mechanism of O'Conaire et al. agrees well with the experimental data, although some improvements can still be made. At all the pressures investigated, replacement of some amount of H ₂ by CO in a reacting mixture leads to an increase in ignition delay, even with small amounts of CO addition. The inhibition effect of CO addition is also observed to be much more pronounced with increasing pressure. For H ₂/CO mixtures, the existing mechanisms of Li et al.. Davis et al., and GRI-Mech 3.0 fail to describe the experimental trend of ignition delay. At a pressure of 50 bar. ignition delays are seen to decrease monotonically with O ₂ addition, although such an effect is rather weak. Kinetic analysis further demonstrates that under the present experimental conditions. CO + HO ₂ = CO ₂ + OH is the primary reaction responsible for the mismatch of experimental and calculated ignition delays. Significant improvements in the ignition delay predictions can be made by reducing this reaction rate.