Analytical comparison of 2 and 3 way digital valves for use on direct needle control fuel injectors
For years, diesel Common Rail Systems (CRS) have utilized 2-way valves and orifices for needle control in the injectors. Although this approach has been quite successful, challenges to improving injection performance still remain. The design of a reliable 3-way high pressure control valve is challenging, but its implementation for needle control could represent a natural path to further improvements in injection performance. The rational is that a 3-way valve would provide more independent control of needle opening and closing rates than the 2-way design. An efficient way of comparing 2-way and 3-way needle control methods is through a numerical simulation study. To that end, numerical simulation models of injectors with both needle control concepts were built. These numerical models were very detailed, and accounted for compressible line dynamics of the working fluids, cavitation in the working fluids, compression of the injector parts under pressure load, and bouncing of injector parts due to impacts. Detailed modeling of the electromagnetics and the electronic controls used to actuate the spool valves was also included. The platform for the injector models was the validated technology of hydraulic intensification and very rapid electromagnetic valve actuation applied to a unit diesel injector currently in production. Verification of the simulation models was achieved by comparing the results of simulation with actual measured data. The 2- and 3-way valve direct needle control concepts were then evaluated with simulation at specific fuel system application points for performance parameters such as small quantity control capability, close coupling of multiple events, and parasitic losses. The numerical results show that the 3-way needle control valve offers distinct advantages over a 2-way valve in certain areas of injection performance. Other factors important for fuel system selection, such as cost, manufacturability and injector-to-injector variability were considered, but are not presented in detail in this study. Copyright © 2004 SAE International.