Predictive modeling of total pressure loss in a high-speed SI engine exhaust manifold: Influence of penetration length and divergence angle

Stricter global emission standards necessitate the optimization of internal combustion engine exhaust systems to enhance energy efficiency. Penetrating exhaust manifolds (PEMs) are widely adopted for their energy-conservation potential. However, their configuration for high-speed, small-displacement engines remains limited. This study integrates computational fluid dynamics (CFD) simulations with experimental testing to evaluate flow losses and optimize PEM geometry for a 115-cc single-cylinder spark ignition (SI) engine. The CFD model was validated against experimental temperature and velocity data at 5000–8000 rpm, achieving a maximum error of 9.17%. Flow loss was characterized by entropy production as an indicator of energy degradation within the muffler first expansion chamber. A derived multiple linear regression model predicted the total pressure loss coefficient (K) with a maximum error of 7.8%. Statistical analysis identified the divergence angle (∅) as the dominant parameter, accounting for 47.3% of the model variance. Reducing the penetration length (L) to 0.03 m decreased frictional losses, resulting in an absolute loss reduction of 1.63 (14.82%). Among the tested configurations, ∅ = 12° achieved the highest improvement, reducing K by 2.61 (26.38%). Entropy analysis further confirmed that the 12° configuration minimized system irreversibility by redirecting exhaust flow away from the chamber end surface. These findings provide valuable insights for optimizing PEM geometry to minimize pressure losses in small, high-speed SI engines.