The relevance of this study is subject to the fact that the intricate design of the valve assembly's control system shows the insufficiently highperformance characteristics of the pump-injector units. The purpose of the study is to develop a modernization model for a pump-injector unit to increase its functional performance and develop its control valve. The principal method to be used to investigate the problem is to assess the influence of the design parameters of the control valve on the quality of the operation of the pump-injector unit. The authors developed a method to upgrade a diesel pump-injector unit using a ring control valve, as well as a mathematical model for the operation of the ring control valve of the pump-injector unit, which allows for the establishment of parameters affecting the quality of its operation. Studies have shown the effectiveness of the control ring valve on the pump-injector unit. The ring valve performs the function of both the force and locking elements. The absence in it of a particular power element (spring) can significantly reduce the volume of the intra-valve cavity. The ring functions as a freeedge mechanical multiplier built into it with a multiplication factor π. There is no spring, discharge volume decreases, and the pump-injector unit speed increases.