Numerical investigation of combustion system design for knock mitigation and thermal efficiency enhancement in a carbon-neutral methanol engine

Methanol, a promising carbon-neutral fuel, enables engines operating stoichiometrically with a three-way catalytic converter (TWCC) to achieve high power output while maintaining low emissions. However, engine performance under high-load conditions is limited by knock and maximum in-cylinder pressure (Pmax). Combustion system plays a crucial role, yet research in this field remains scarce. This study performs a numerical investigation of a port-fuel-injection (PFI) heavy-duty spark-ignition (SI) methanol engine, with the objective of guiding combustion system design through enhanced in-cylinder flow, knock mitigation, and thermal efficiency improvement. The results indicate that designing chamber geometry for tumble ratio (TR) enhances turbulent kinetic energy (TKE) more effectively than adjusting swirl or squish flows. The tumble-optimized cylindrical chamber improves indicated thermal efficiency (ITE) by 0.5% over the original chamber. The diameter-depth ratio critically influences flow patterns—larger ratios increase TR during compression but reduce swirl ratio (SR). The high-TR (D86) and high-SR (D70) chambers, featuring large and small diameter-depth ratios respectively, significantly enhance combustion performance, while the medium diameter-depth ratio design (D78) exhibits intermediate performance. Compared to the D78 chamber, the D70 and D86 chambers achieve 0.3% and 0.5% higher ITE, representing improvements of 0.8% and 1.0% over the original chamber, respectively. Furthermore, the diameter-depth ratio significantly affects engine knock characteristics. The knock intensity (KI) initially decreases and then increases with increasing diameter-depth ratio. The D78 chamber exhibits the lowest knock tendency, indicating an optimal diameter-depth ratio near 2.5. This design achieves the highest ITE under both Pmax and knock-limited conditions, reaching 46.92% and 47.21%, respectively.