Editing Continuously variable transmission (section)

=== Hydrostatic/hydraulic ===


A hydrostatic CVT uses an engine-driven, [[positive-displacement pump#positive-displacement_pumps|positive-displacement pump]] to deliver oil under pressure to one or more [[hydraulic motor]]s, the latter creating the torque that is applied to the vehicle's driving wheel(s).  The name "hydrostatic CVT", which misuses the term "[[hydrostatics|hydrostatic]]", differentiates this type of transmission from one that incorporates a [[torque converter|hydrodynamic torque multiplier]] ("torque converter") into its design.

In a hydrostatic CVT, the effective "gear ratio" between the engine and the driving wheel(s) is the result of a difference between the pump's displacement—expressed as cubic inches or cubic centimeters per revolution—and the motor's displacement.  In a closed system, that is, a system in which all of the pump's output is delivered to the motor(s), this ratio is given by the equation '''{{mono|1=GR = Dm ÷ Dp}}''', where '''{{mono|Dp}}''' is the pump's effective displacement, '''{{mono|Dm}}''' is the motor's displacement, and '''{{mono|GR}}''' is the "gear ratio".

In a hydrostatic CVT, the effective "gear ratio" is varied by varying effective displacement of the pump, which will vary the volume of oil delivered to the motor(s) at a given engine speed (RPM).  There are several ways in which this may be accomplished, one being to divert some of the pump's output back to the reservoir through an adjustable valve.  With such an arrangement, as more oil is diverted by opening the valve, the effective displacement of the pump is reduced and less oil is delivered to the motor, causing it to turn more slowly.  Conversely, closing the valve will reduce the volume of oil being diverted, increasing the effective displacement of the pump and causing the motor to turn more rapidly.

Another method is to employ a [[variable displacement pump]].  When the pump is configured for low displacement, it produces a low volume of oil flow, causing the [[Hydraulics|hydraulic]] motor(s) to turn more slowly.  As the pump's displacement is increased, a greater volume of oil flow is produced for any given engine RPM, causing the motor(s) to turn faster.

Advantages of a hydrostatic CVT include:
* Capacity scalability.  A hydrostatic CVT's power-transmission capacity is readily adapted to the application by using a correctly-sized pump and matching hydraulic motor(s).
* Flexibility.  As power transfer from the engine-driven pump to the hydraulic motor(s) is through the medium of flowing oil, the motor(s) can be mounted in otherwise-inconvenient locations by using hoses to convey oil from the pump to the motor(s), thus simplifying the design of [[all-wheel drive]] [[articulated vehicle]]s.
* Smoothness.  As the effective "gear ratio" of a hydrostatic CVT is infinitely-variable, there are no distinct transitions in torque multiplication, such as produced with conventional, geared transmissions.
* Simplified control.  Operation through the full range of forward and reverse speeds can be controlled using a single lever or a foot pedal to actuate a diversion valve or variable-displacement pump.
* Arbitrarily slow crawl speeds.  The potential for high torque multiplication at very low speeds allows for precise vehicle movement while under load.

Disadvantages of a hydrostatic CVT include:
* Reduced efficiency.  Gears are one of the most efficient methods of mechanical power transmission, with efficiencies as high as 90 percent in many cases.  In contrast, few hydrostatic transmission systems achieve more than about 65 percent efficiency.  This is due to a combination of internal losses in the pump and motor(s), and losses in the piping and valves.
* Higher cost.  For a given level of power-transmitting capacity, a hydrostatic CVT will be more expensive to produce than an equivalent geared transmission.  In addition to the pump and motor(s), a hydrostatic system requires the use of an oil reservoir, piping and in many applications, an oil cooler, this last item being necessary to dissipate the waste heat that results from hydrostatic power transmission's relatively low efficiency.
* Greater weight.  Due to the high oil pressure at which a hydrostatic CVT operates, the pump and motor(s) are under considerable mechanical stress, especially when maximum power and loading is being applied.  Hence these items must be very robust in construction, typically resulting in heavy components.  Additional weight will be found in the oil reservoir and its oil load, as well as the piping and valving.

Uses of hydrostatic CVTs include [[forage harvester]]s, [[combine harvester]]s, small wheeled/tracked/skid-steer [[Loader (equipment)|loaders]], crawler [[tractor]]s, and [[road roller]]s.  One agricultural example, produced by [[AGCO]], splits power between hydrostatic and mechanical transfer to the output shaft via a planetary gear in the forward direction of travel (in reverse, the power transfer is fully hydrostatic).  This arrangement reduces the load on the hydrostatic portion of the transmission when in the forward direction by transmitting a significant portion of the torque through more efficient fixed gears.<ref>{{cite web |url=https://www.youtube.com/watch?v=dgtIKMAjvFI | archive-url=https://ghostarchive.org/varchive/youtube/20211107/dgtIKMAjvFI| archive-date=2021-11-07 | url-status=live|title=AGCO's Continuously Variable Transmission (CVT) Explained |website=YouTube | date=2 August 2010|access-date=26 October 2012}}{{cbignore}}</ref>

A variant called the ''Integrated Hydrostatic Transaxle'' (IHT) uses a single housing for both hydraulic elements and gear-reducing elements and is used in some [[mini-tractor]]s and ride-on [[lawn mower]]s.

The 2008–2010 [[Honda DN-01]] [[Cruiser (motorcycle)|cruiser motorcycle]] used a hydrostatic CVT in the form of a variable-displacement axial piston pump with a variable-angle [[swashplate]].