An experimental study has been completed which evaluated the influence of structure parameters of a high pressure common rail injector on the response ability and injection rate.In this study,structure parameters of hydraulic pressure balance valve were optimized which was newly designed for the common rail injector.Optimization of the control chamber in the common rail injector indicates that injection rate can be shaped flexibly by changing the parameters related to the control chamber,as a result we called the common rail injector as Flexible Injection Rate Common Rail Injector or FIRCRI.