This study was conducted to determine the macroscopic and fuel atomization characteristics of a piezo-driven injection system and to compare it with a solenoid-driven injection system in a common-rail diesel engine. In order to obtain the macroscopic characteristics of fuel spray, the spray development process was visualized using the spray visualization system composed of a Nd:YAG laser and an ICCD camera. The injection characteristics of the fuel spray such as injection rate and injection delay profile were investigated under the various injection conditions. The injection and atomization characteristics were investigated in terms of the droplet mean axial and radial velocities, droplet diameter, and droplet distributions obtained using a phase Doppler particle analyzer (PDPA) system. It was found that the piezo-driven injection system has a short injection delay and quickly reaches the maximum injection rate compared to the solenoid-driven injection system. Spray tip penetration makes little difference; however, the spray cone angle of the piezodriven injector is larger than that of the solenoid-driven injector. The atomization performance of the piezo-driven injection system is superior to that of the solenoid-driven injection system due to a faster response time and higher injection rate. (c) 2007 Elsevier Ltd. All rights reserved.