ABSTRACT A system level analysis was carried out on the effect of flow forces on a flow control variable force solenoid (VFS) used in automatic transmissions. Classic flow force model was reviewed as a function of the pressure difference and the solenoid current. A force balance analysis was conducted on the spool valve in the VFS, in order to study the relationship among the control current, flow forces, spring forces, and flow area. Flow bench testing was used to characterize a specific flow control VFS by both the pressure drop and solenoid current, in forward and reverse flow directions. The behavior of flow control VFS valve is significantly affected by flow forces. A sub-system level model was thus created to predict the steady-state and dynamic behavior of the flow VFS valve, which can be used in a transmission system level analysis. The modeling results were compared against experimental data to show the validity of the methodology. INTRODUCTION A flow control variable force solenoid (VFS) is an electro- hydraulic valve that controls flow rate in proportion to an input signal. It is widely used in automotive hydraulic control systems [ 1], especially in automated manual transmissions and dual clutch transmissions. A flow VFS is essentially a direct acting spool valve, where the electromagnetic field generated by input current going through coils exerts electromagnetic force on an armature, which then comes into contact with a spool valve and acts on it. The spool valve moves inside a sleeve to regulate flow area in the hydraulic ports of the sleeve. The coils, armature, spool valve, and sleeve are enclosed in the same package as a flow VFS. Therefore, the tolerance between the sleeve and spool valve can be made small and thus greatly reduce leakages. In the mean time, due to its direct acting feature, a flow VFSresponds to signal change faster than a conventional two- stage control system with a solenoid and a spool valve. Moreover, the compactness of flow VFS can simplify the transmission hardware design and packaging [ 2]. Therefore, it is important to investigate the hydraulic dynamics and flow response characteristics of flow VFS valves. Among the critical characteristics of flow VFS valves, the flow force is an important factor. Flow forces are also referred to as flow induced forces, Bernoulli forces, or hydraulic reaction forces [ 3]. These forces are a result of the flow momentum change going through an orifice. They act on the spool valve in the sleeve of VFS in the direction of reducing the valve opening flow area. The flow forces are present in all hydraulic spool valves, including the pressure regulation valves in the conventional two-stage system. However, the flow force is not an important factor affecting the characteristics of pressure regulation valves other than direct acting VFS valves, because the magnitude of flow forces is much less significant than the magnitude of signal forces and feedback forces. In contrast, the signal force in a direct acting VFS valve is the magnetic force directly applied on the spool, which is in the same order of magnitude of the flow force. Therefore, the flow force has to be considered in the overall force balance of VFS valves. Theoretical analysis of the flow force was performed in classical hydraulic control literatures [ 2, 3] in the past. With the advance of computer technology, Three-dimensional numerical approaches have been taken to simulate the flow force and how it affects the hydraulic valve performance [ 1, 4, 5]. However, most previous studies have dealt with pressure regulation valves. There have been few studies of the flow force on flow control VFS valves and how it affects the flow response characteristics. Effect of Flow Forces on a Flow Control Variable Force Solenoid2011-01-0394 Published 04/12/2011 Zhe Xie General Motors Company Copyright © 2011 SAE International doi:10.4271/2011-01-0394Downloaded from SAE International by Univ of California Berk