The control valves in the middle (the open center valves) direct the flow from the pump to different destinations depending on whether or not the motors are running. If no motors are running, the valves are designed to direct the pump flow to a reservoir (tank4) instead of having the pump run at the relief valve setting.
The motor model with open center valves as seen in the diagram view in Wolfram SystemModeler.
The simulations of the model (seen below) clearly show that the pump torque is reduced as the valves redirect the fluid to the reservoir. In other words, the suggested model represents a system design that fulfills the original goal of pump torque reduction.
The plots visualize the correlation between motors running and the flow from the open center valves to the reservoir. The left plot shows that no motors are running in the approximate intervals 0–1 s, 2–3 s, 4–5 s, and 6–7 s. The right plot of the valve activity shows that during these intervals, the different valves redirect their outflow to the reservoir. The negative flow indicates flow away from the valves. The flow into the reservoir is shown in red in the right plot.
A simulation plot of the pump torque. Comparing this plot to the motor plot above, it is clear that the torque is significantly reduced in the time intervals when no motors are running.