Lean-burn stability and thermal efficiency of a high-compression SI engine using late direct water injection and multi-coil ignition

Lean operation in high-compression spark-ignition engines is constrained by the competing requirements of knock suppression and combustion stability. Direct water injection (DWI) provides an effective means of increasing knock tolerance, but its influence on combustion depends strongly on the dilution level and water-injection conditions. In this study, a single-cylinder direct-injection spark-ignition engine with a compression ratio of 17:1 was employed to investigate the λ–WFR response under a fixed late-DWI and multi-coil ignition configuration. The excess air coefficient was varied from λ = 1.0 to 1.7 and the water-to-fuel ratio (WFR) from 0.1 to 0.5, while the water-injection pressure and start of injection were maintained constant. The results show that the stabilizing effect associated with increasing WFR becomes increasingly evident as the mixture approaches the lean operating limit. At WFR = 0.5, stable operation was maintained up to λ = 1.7, achieving a peak indicated thermal efficiency of 46.7% with CoVIMEP below 4%. A multivariate CA05–Pmax–MAPO analysis revealed the coexistence of knock-dominated and delayed-combustion/misfire-like events under highly diluted operation. The severe-knock cycles examined were more closely associated with rapid early combustion than with a simple preceding-cycle effect. PV-based analysis showed that higher WFR was primarily associated with increased expansion work through a larger spark-advance margin and more favorable combustion phasing, whereas the reduction in gas-exchange pumping loss was mainly associated with the λ-dependent intake-pressure adjustment. Meanwhile, increasing WFR prolonged ignition delay and reduced combustion completeness, as reflected by higher CO and THC emissions. These results provide a process-level characterization of the λ–WFR response under the investigated late-DWI and multi-coil ignition configuration and highlight the need to balance knock suppression, combustion robustness, and combustion completeness when extending lean operation in high-compression SI engines.