Modeling study of hydrogen or syngas addition on combustion and emission characteristics of HCCI engine operating on iso-octane

Abstract

Numerical study of energetic and emission characteristics of HCCI engine operating on iso-octane based fuel blends comprising hydrogen or syngas as additives is conducted on the basis of 2D CFD simulation for two values of fuel-to-air equivalence ratio ϕ = 0.4 and 0.2. The features of combustion are analyzed for the case when the maximal mass-average temperature (Tev)max during the combustion of different fuels is realized at an identical crank angle, which can be implemented at adequate intake temperature for every fuel blend. It is shown that the addition of H2 or syngas retards the ignition and decreases the combustion duration. This effect is more pronounced for leaner mixture with ϕ = 0.2. The admixture of H2 to iso-octane leads to the increase of the specific indicated work both for ϕ = 0.2 and ϕ = 0.4. The opposite tendency is detected when syngas H2/CO = 2/1 is added to iso-octane. The total emission index EINOx + EICO is determined mostly by the emission of NOx at ϕ = 0.4 and by the emission of CO at ϕ = 0.2. At ϕ = 0.4, the replacement of iso-octane by iso-octane/hydrogen(syngas) blend increases the NOx emission index (by 86% for iso-C8H18/H2 = 20/80 blend), which occurs due to the growth of maximal temperature in the cylinder. At ϕ = 0.2, the addition of H2 or syngas decreases the emission indices of CO, unburned hydrocarbons, and organics. The higher is the H2 amount in the blend, the more significant is the effect. The emission indices of NO2 and N2O, in this case, are essentially higher than that of NO. As N2O is a very harmful gas, its emission should be taken into consideration when evaluating the total emission of the engine at low load regimes.