An increase in the temperature of charge in an engine combustion chamber is now more and more attractive due to its advantages in energy savings and environmental control. However, this will affect the design of engine elements, since a higher temperature will result in considerable thermal stresses and distortion, material and lubricant degradation, or even seizure and scuffing failures. Actually even for currently designed diesel engines, engine piston assembly failure, particularly piston ring/cylinder liner interface failure due to heat accumulation, is a very serious problem. Effective means of carrying heat away from this area are crucial for the prevention of scuffing. However, due to the reciprocal motion of the mechanism, efficient piston cooling is difficult to achieve using conventional cooling methods.A newly developed engine piston incorporating heat pipe cooling technology aims at efficiently transferring excessive heat from the piston ring region to a location where cooling is sufficient, so that the temperature in the ring region can be reduced. Heat pipes can be implanted close to the ring grooves and move together with the piston assembly. This design utilizes the high heat conductance of heat pipes due to the two-phase heat transfer to achieve this heat transfer process, and employs the piston reciprocating motion as the means for the liquid phase return. Therefore, impingement of fluid droplets inside the heat pipe is critical for the new piston design. Dynamic analysis has been conducted and the results indicate that for a given fluid and pipe combination, the motion of the liquid droplets is a function of the piston stroke, cranking speed, heat pipe length, as well as drag and viscous resistances. Experimental dynamic observations have also been performed on an engine/heat pipe apparatus constructed based on a single-stroke internal combustion engine. Full scale liquid impingement on the entire inner wall of the pipe can be achieved when the angular cranking frequency was around and above 6-7 Hertz.