Application of field synergy principle to analysis of flow field in underhood of LPG bus

•The thermal performance in the LPG bus underhood is examined.•The formation mechanism of flow field is studied to account for high local temperature.•Significant reduction in temperature is achieved in the improved underhood layout.•A decrease in synergy angle between the air velocity and heat flow was obtained.•By help of field synergy principle, underhood heat transfer capability is improved.