BACKGROUND

This disclosure is directed toward power machines. More particularly, this disclosure relates to hydraulic systems, such as hydrostatic drive systems and hydraulic charge systems for power machines. Power machines, for the purposes of this disclosure, include any type of machine that generates power to accomplish a particular task or a variety of tasks. One type of power machine is a work vehicle. Work vehicles are generally self-propelled vehicles that have a work device, such as a lift arm (although some work vehicles can have other work devices) that can be manipulated to perform a work function. Work vehicles include loaders, excavators, utility vehicles, tractors, and trenchers, to name a few examples.

Conventional hydrostatic drive systems are used to provide tractive power for power machines for travel over terrain. To ensure a proper supply of hydraulic fluid at inlets to drive pumps of a hydrostatic drive system, and to counteract leakage of hydraulic fluid, a hydraulic charge pump can be provided. A hydraulic charge pump, as driven by an engine of a power machine, can pump hydraulic fluid from a reservoir into a hydrostatic drive circuit to make up for leakage and to prime a drive pump, and can also power other equipment, such as a fan motor.

The discussion above is merely provided for general background information and is not intended to be used as an aid in determining the scope of the claimed subject matter.

SUMMARY

Some embodiments of the present disclosure provide systems (and corresponding methods) to control the flow of hydraulic fluid for a charge circuit of a hydrostatic drive system. More specifically, a controllable bypass value can be included within a charge circuit for a hydrostatic drive system to selectively divert hydraulic flow back to a tank, depending on current and historical operating conditions. In some implementations, this can allow hydraulic flow from a hydraulic charge pump to be temporarily diverted directly to the tank, rather than passing through a hydraulic load or charging a hydrostatic drive circuit and can thereby temporarily reduce engine load during a starting operation (i.e., during “startup” of the power machine). For example, a bypass valve can be activated to bypass a hydraulic load based on operational parameters that indicate low ambient or system temperatures, low engine speeds, large amounts of elapsed time since a previous run cycle, or different permutations of these or other decision inputs.

Some of the disclosed embodiments provide a power machine that includes an engine. A hydrostatic drive system can include a hydraulic charge pump circuit that includes a hydraulic load and a tank. A hydraulic charge pump can be in fluid communication with the hydraulic charge pump circuit. A bypass line can extend to the tank from a junction between the hydraulic charge pump and the hydraulic load. A charge bypass valve can be arranged to control flow through the bypass line. A control system can include a control device configured to determine an engine speed for the engine and to control the charge bypass valve to allow flow through the bypass line, from the hydraulic charge pump to the tank, based on the engine speed being below a speed threshold.

Some of the disclosed embodiments provide a control system for a hydrostatic drive system, for use in a power machine with an engine. The hydrostatic drive system can include a hydrostatic drive pump and a hydraulic charge pump circuit that includes a hydraulic load and a tank, a hydraulic charge pump in fluid communication with the hydraulic charge pump circuit, a bypass line extending to the tank from a junction between the hydraulic charge pump and the hydraulic load, and a charge bypass valve arranged to control flow through the bypass line. The control system can include a control device that is configured to determine one or more operational parameters for the power machine. The control device can be further configured to, during a startup of the engine, control the charge bypass valve to allow flow through the bypass line, to bypass the hydraulic load and the hydrostatic drive pump, for a flow from the hydraulic charge pump to the tank, based on the determined one or more operational parameters.

Some of the disclosed embodiments provide a method for controlling a bypass flow for a hydraulic charge pump circuit of a power machine. Using a control device, one or more operational parameters for the power machine can be determined. During a startup of the power machine, a charge bypass valve can be controlled, based on the one or more operational parameters, to temporarily route a hydraulic flow from a hydraulic charge pump through a bypass line to a tank, to reduce hydraulic flow from the hydraulic charge pump to a hydrostatic drive system of the power machine.

This Summary and the Abstract are provided to introduce a selection of concepts in a simplified form that are further described below in the Detailed Description. This Summary is not intended to identify key features or essential features of the claimed subject matter, nor are they intended to be used as an aid in determining the scope of the claimed subject matter.

