Replacing conventional fuels with ammonia (NH₃) reduce greenhouse gas emissions from internal combustion engines, but incomplete fuel oxidation will release unburned NH₃ and nitrous oxide (N₂O). This study numerically investigated the effects of crevice-temperature history and equivalence ratio on NH₃ conversion and N₂O formation in a heavy-duty compression-ignition engine converted to spark-ignition operation. The analysis combined experimental in-cylinder pressure data with a Rassweiler-derived mass fraction burned (MFB) and applied a multi-Wiebe formulation to separate the combustion stages. The final Wiebe component, which correlated to the late release and oxidation of the crevice mixture (~4.6–6.0% of total), was simulated with a closed, fixed-mass, zero-dimensional batch-reactor that tracked the mixture chemistry until exhaust valve opening. The analysis evaluated equivalence ratios (ϕ) from 0.7 to 1. The model also defined five crevice-temperature profiles through a temperature-weighting factor, , extending from the wall-temperature limit ( = 0) to the burned-zone-temperature limit ( = 1). Detailed chemical kinetics and an integrated rate-of-production analysis quantified the final NH₃ and N₂O concentrations and the dominant reaction pathways. The mixture remained nearly nonreactive for  of 0 and 0.25, producing negligible NH₃ conversion and N₂O formation. NH₃ conversion remained limited at =0.5, while N₂O increased up to 642 ppm. The  = 0,75 condition represented a transition region in which ϕ strongly influenced the chemical response. The highest N₂O concentration (~1640 ppm) occurred at ϕ = 0.7, whereas the ϕ = 0.9 and ϕ = 1.0 produced less than 1 ppm. The simulations for  = 1, predicted nearly complete NH₃conversion and final N₂O concentrations of 0.07–0.17 ppm. Reaction NH₃+OH H₂O+ NH₂ dominated NH₃ consumption for ϕ = 0.8, while NH₂+NO₂ N₂O+H₂O dominated N₂O formation at  = 0.5. Increasing temperature suppressed this formation pathway and strengthened the reactions responsible for N₂O consumption. These results identify an intermediate-temperature window that promotes crevice-associated N₂O accumulation and show that equivalence ratio shifts this window by changing the mixture composition and thermal conditions.