Method and apparatus for maintaining hydraulic pressure when a vehicle is stopped
A system for maintaining hydraulic pressure in a vehicle hydraulic system when the vehicle is stopped or idling. A hydraulic pressure reserve system includes a primary hydraulic circuit that includes a hydraulic pump and a hydraulic sump and that provides hydraulic pressure to at least a hydraulically actuated transmission and an accumulator tank and control valve connected from the primary hydraulic circuit for storing a reserve of hydraulic fluid at an accumulator pressure. An accumulator control valve, such as a bi-directional valve, allows a flow of hydraulic fluid between the primary circuit and the accumulator tank according to the primary circuit pressure to raise the primary circuit pressure towards a desired pressure. The hydraulic pressure reserve system may include a primary gear connected from a primary drive and driving an auxiliary secondary gear with a primary drive clutch for connecting the secondary gear to drive the hydraulic pump and an auxiliary drive clutch for connecting the auxiliary secondary gear or a drive chain to drive the hydraulic pump.
1. A hydraulic pressure reserve system for a vehicle hydraulic system including a primary hydraulic circuit including a hydraulic pump and a hydraulic sump and providing hydraulic pressure to at least a hydraulically actuated transmission, the hydraulic pressure reserve system comprising:
an accumulator tank connected from the primary hydraulic circuit for storing a reserve of hydraulic fluid at an accumulator pressure, and
an accumulator control valve for controlling a flow of hydraulic fluid between the primary hydraulic circuit and the accumulator tank, and
an accumulator controller responsive to a primary hydraulic circuit pressure and to the accumulator pressure for controlling the accumulator control valve, wherein
the accumulator controller actuates the accumulator control valve to allow a flow of the hydraulic fluid from the primary hydraulic circuit and into the accumulator tank when the primary hydraulic circuit pressure is greater than the accumulator pressure, and
the accumulator controller actuates the accumulator control valve to allow a flow of the hydraulic fluid from the accumulator tank to the primary hydraulic circuit when the primary hydraulic circuit pressure is lower than a desired minimum primary hydraulic circuit pressure, thereby raising the primary hydraulic circuit pressure towards the desired primary hydraulic circuit pressure.
2. The hydraulic pressure reserve system for a vehicle hydraulic system of claim 1 wherein:
the accumulator control valve includes a bidirectional valve allowing a bidirectional flow of hydraulic fluid between the accumulator tank and the primary circuit, and
the accumulator controller includes a spring biased bidirectional valve controller actuating the bidirectional valve dependent upon a differential pressure across the valve controller between the primary circuit and the accumulator tank, such that
when there is a positive differential pressure across the bidirectional valve controller wherein the primary circuit hydraulic pressure is greater than the accumulator pressure by greater than a selected positive actuation pressure, the bidirectional valve is opened by the bidirectional valve controller to allow the hydraulic fluid to flow into the accumulator tank and the accumulator pressure to rise accordingly, and
when there is a negative differential pressure across the bidirectional valve controller wherein the primary circuit pressure is less than the accumulator pressure by greater than a selected negative actuation pressure, the bidirectional valve is opened by the bidirectional valve controller to allow the hydraulic fluid to flow from the accumulator tank to the primary circuit, thereby raising the primary hydraulic circuit pressure towards a desired primary hydraulic circuit pressure.
3. The hydraulic pressure reserve system for a vehicle hydraulic system of claim 2 wherein:
the magnitude of the positive actuation pressure is less than the magnitude of the negative actuation pressure.
4. The hydraulic pressure reserve system for a vehicle hydraulic system of claim 2 wherein:
the bidirectional valve controller includes positive and negative adjustable bias springs acting upon the bidirectional valve.
5. The hydraulic pressure reserve system for a vehicle hydraulic system of claim 4 wherein:
the positive and negative actuation pressures are selected to provide hysteresis in opening and closing of the bidirectional valve.
6. The hydraulic pressure reserve system for a vehicle hydraulic system of claim 1 wherein:
a first accumulator control valve and a corresponding first accumulator controller connected from the primary circuit and to the accumulator tank controlling the flow of hydraulic fluid from the primary hydraulic circuit and to accumulator tank, and
a second accumulator control valve and a corresponding second accumulator controller connected from the primary circuit and to the accumulator controller for in parallel with the first accumulator control valve and the first accumulator controller for controlling the flow of hydraulic fluid from the accumulator tank and to the primary hydraulic circuit.