Abstract: A hydraulic control system provides lubrication fluid to a gearbox via two flow paths in parallel. One of the flow paths includes a passive thermal valve in series with a heat exchanger. When the fluid temperature is elevated, the thermal valve opens such that fluid flows through the heat exchanger for cooling. The thermal valve restricts flow to the heat exchanger when transmission fluid is in a normal operating temperature range, reducing flow demands. When the flow demands are reduced, a variable displacement pump requires less torque improving fuel economy. The thermal valve may also open when the fluid is below the normal operating temperature such that the heat exchanger heats the fluid. The parallel path may include a regulator valve. When the pressure of fluid in the lubrication circuit is elevated, the regulator valve reduces flow through the parallel path and may also divert flow to a sump.

Abstract: A hydraulic control system provides lubrication fluid to a gearbox via two flow paths in parallel. One of the flow paths includes a passive thermal valve in series with a heat exchanger. When the fluid temperature is elevated, the thermal valve opens such that fluid flows through the heat exchanger for cooling. The thermal valve restricts flow to the heat exchanger when transmission fluid is in a normal operating temperature range, reducing flow demands. When the flow demands are reduced, a variable displacement pump requires less torque improving fuel economy. The thermal valve may also open when the fluid is below the normal operating temperature such that the heat exchanger heats the fluid. The parallel path may include a regulator valve. When the pressure of fluid in the lubrication circuit is elevated, the regulator valve reduces flow through the parallel path and may also divert flow to a sump.