A retrofit spark plug fuel injector (SPFI) for direct injection natural gas engines: Effects of ignition and injection timing on combustion and performance

A retrofit-capable spark plug fuel injector (SPFI) enabling direct injection of compressed natural gas (CNG) in spark ignition engines is developed and experimentally assessed. The novelty lies in integrating injection and ignition within a single device, eliminating cylinder head modification while enhancing in-cylinder thermofluid control. Experiments were conducted on a 1.5 L engine at 1500–2500 rpm with 6 MPa injection pressure. The effects of ignition and injection timings on combustion phasing, heat release, performance, and emissions were examined under part-load and wide-open throttle conditions. Optimal ignition timing was identified at 19 °CA BTDC, resulting in increase in BMEP by 20–40% relative to the tested ignition-timing extremes (16–24 °CA BTDC), and 31% superior to the port injection operation on the same engine. Injection timing governs combustion phasing, with optimal timing advancing under high load and retarding with increasing engine speed. Peak cylinder pressure and heat release rate decrease at higher speeds due to slower flame propagation of natural gas. Under injection-timing-optimized operation, BMEP increased by 2.1–2.9% from 50% throttle to wide-open throttle across the tested speed range (628.98 → 645.02 kPa at 1500 rpm, 656.28 → 675.33 kPa at 2000 rpm, and 652.66 → 666.13 kPa at 2500 rpm). The SPFI system improves volumetric efficiency and mixture preparation, leading to enhanced BMEP. CO and CO₂ emissions show limited sensitivity to ignition timing but align with optimal combustion during injection timing variation. These results demonstrate that SPFI provides an effective and practical strategy for improving combustion control and efficiency in direct injection of natural gas engines.