Effect of ignition strategy on combustion and NOx formation characteristics of an Ammonia/hydrogen Wankel rotary engine

Ammonia‑hydrogen blended fuel represents a promising zero‑carbon fuel pathway for Wankel rotary engines (WREs). In this study, a 3D numerical model of a dual direct-injection (DDI) ammonia/hydrogen WRE was established using CONVERGE software and validated against experimental data. The effects of dual-spark synchronous and asynchronous ignition strategies and ignition timing (IT) on combustion and NOx formation characteristics were systematically investigated. Results show that as the leading spark plug (LSP) IT is gradually advanced from 25 °CA before top dead center (BTDC) to 40 °CA BTDC, both peak in-cylinder pressure and peak heat release rate (HRR) first increase and then decrease, reaching an optimum near 30 °CA BTDC. The asynchronous ignition case L-30 T-25 delivers the best combustion performance. It achieves a peak in-cylinder pressure of 2.5 MPa, a peak HRR of 5.8 J/°CA, an indicated mean effective pressure (IMEP) of 0.430 MPa, and an indicated thermal efficiency (ITE) of 28.1%. When the IT is further advanced beyond 35 °CA BTDC, negative compression work increases significantly, causing both expansion work and IMEP to decline. Regarding nitrogen oxides (NOx) formation, the differences in the simulated final in-cylinder NOx content among the investigated cases are relatively small. Case L-30 T-25 achieves the highest IMEP and ITE, while its simulated final NOx level remains comparable to those of the other cases. The results indicate that an appropriate asynchronous ignition interval, with the LSP firing at 30 °CA BTDC and the trailing spark plug (TSP) at 25 °CA BTDC, can improve the combustion phasing and the effective work output in ammonia/hydrogen WREs, while keeping the NOx formation at a comparable level under the present operating conditions. It therefore strikes a balance between efficient work output and in-cylinder NOx formation control.