According to high demands of reduction on CO2 gas and fuel consumption,clean diesel vehicle has been emerged in automotive industry.Clean diesel vehicles make the best use of cutting edge technology such as common-rail system,after-treatment system,and electronic control mechanism technique to meet the stringent emission regulations and to increase its efficiency.In a diesel CR engine,high-pressure fuel injection has been widely adapted for better combustion and emission performance.So its characteristics are highly dependent on the air-fuel charged mixture quality inside combustion chamber.By these reasons,fuel injection rate is the major factor to control air-fuel mixing rate and diffusion combustion in a diesel CR engine.Among FIE(Fuel Injection Equipment)systems,there are two types of mechanisms for operating a diesel high-pressure fuel injector.Firstly,a servo-hydraulic type fuel injector has hydraulic circuit inside injector body,and by making hydraulic inner force,the force generates the lift of injector needle.Another method for generating driving force of needle lift is that the actuating force is directly utilized to produce needle displacement.Those two methods require an actuator such as a solenoid coil or a piezo stack to produce unbalanced hydraulic force or the needle lift directly.In this study,direct and indirect acting piezo-driven injectors were used to evaluate the effects of fuel injection rate on diesel combustion performance.In order to measure injection rates of direct and indirect acting piezo injector,Bosch tube apparatus was built and adapted for this study.The experimental results show that direct acting type piezo-driven injector has a higher injection flow rate by a fast needle response time and high potential for accurate control capacity in diesel diffusion combustion.