A new engine piston incorporating heat pipe cooling technology is under development for improved thermal-tribological. performance. Due to the particular structure of the engine piston, curved heat pipes should be employed. The objective of this paper is to study fluid impingement inside curved pipes for curved heat pipe design. Experimental studies of fluid impingement using different curved transparent pipes are performed on an engine-heat pipe apparatus to determine the correlation between the heat pipe design parameters and minimum cranking frequency, or critical impingement frequency. Experimental results show that pipe curvature has little influence on the critical impingement frequency if the non-dimensional offset value of curved pipes is less than 0.3. The critical impingement frequency is higher for the pipe with a longer length and smaller diameter. The liquid charge level should be more than 30% of the pipe volume to avoid any negative effect of the pipe curvature on the heat pipe performance.