Two-stage ignition and NTC phenomenon in diesel engines

• 3-D, transient, turbulent spray simulations performed.
• Two-stage ignition in diesel sprays examined from the evolving QOOH (alkyl hydroperoxy) and OH fields.
• NTC (negative temperature coefficient) behavior investigated in sprays and homogeneous mixtures.
• Effect of methane on the ignition of n-heptane sprays in dual-fuel diesel engine characterized.

Two-stage ignition and NTC phenomenon in diesel sprays is investigated by performing 3-D two-phase reacting flow simulations in a dual-fuel engine. Spray processes modeled include fuel atomization, droplet distortion, droplet drag, turbulent dispersion, droplet interactions in terms of collision and coalescence, vaporization, and spray–wall interaction. A validated reaction mechanism is implemented in the CFD solver, which has previously been validated for both evaporating and reacting sprays. For single-fuel cases, the effect of temperature on two-stage ignition is examined by varying the start of injection (SOI). While results indicate global similarities between the two-stage ignition processes in diesel sprays and spatially homogeneous mixtures, there are also noticeable differences between them due to temporally and spatially evolving temperature and species fields in the spray case. For instance, both the first- and second-stage ignition delays are higher for the spray cases compared to homogeneous mixtures. Second, while ignition delay for homogeneous mixtures exhibits a NTC region, that for sprays indicate a ZTC region. Moreover, the first- and second-stage ignitions for the spray occur over a wide φ range and at multiple locations in the spray, implying a spatially wide ignition kernel. Additionally, while the chemical ignition delays are strongly influenced by the injection timing, the physical delays are essentially independent of this parameter. Results with dual fuel indicate that the two-stage ignition behavior remains intact even at high molar fractions of methane. The addition of methane increases ignition delays for both sprays and homogeneous mixtures, and can be attributed to the reduction in O2 and the chemical effect of methane. The sensitivity analysis indicated that the chemical effect is primarily due to reaction CH4 + OH = CH3 + H2O.