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Penetrating exhaust manifold systems are commonly used in low-speed two-stroke marine diesel engines owing to their potential for energy conservation. However, few studies in the literature have reported on this type of manifold system, and there are few guidelines for optimizing the configuration or predicting the performance via a mean-line model. This study investigates the loss characteristics and flow mechanism of the penetrating exhaust manifold of a low-speed two-stroke marine diesel engine using experimental and numerical methods. The loss is characterized by the entropy production, which indicates a decrease in the energy grade. The results show that the minimum loss can be achieved when the penetration depth and diffusion angle of the manifold are approximately 0.5 and 6°, respectively. Moreover, the loss coefficient is maximally reduced by 14.0 % with penetrating exhaust manifold. The flow field analysis shows that the trend of the flow loss is 'decrease-increase' owing to the interaction among three factors: the mixing loss due to the shearing of the injection flow with the main flow, the momentum loss due to the impingement on the bottom of the main pipe, and the separation loss in the branch pipe owing to the flow divergence. This investigation can provide guidance for the configuration optimization and loss modeling of the exhaust manifold for low-speed two-stroke marine engines. • Loss characteristics of penetrating exhaust manifold in low-speed engine is studied. • Loss performance of PEM shows 'decrease-increase' trend with change of key geometry. • 'V-shape' trend is due to trade-off among losses of mixing, impinging and separation. [ABSTRACT FROM AUTHOR]